The Upper Triassic Nayband

Sonderdruck aus

Beringeria Wurzburger geowissenschaftliche Mitteilungen

Heft 35 2005

The Upper Triassic Nayband and Darkuh formations of east-central Iran: Stratigraphy, facies patterns and biota of extensional basins on an accreted terrane FRANZ

T.

DARYAN

FORSICH, MICHAELHAUTMANN, SABA SENOWBARI-

& KAZEM

SEYED-EMAMI

WOrzburg 2005

53-133

The Upper Triassic Nayband and Darkuh formations of east-central Iran: Stratigraphy, facies patterns and biota of extensional basins on an accreted terrane F.T.

FORSICH,

M.

HAUTMANN,

B.

SENOWBARI-DARYAN

& K.

SEYED-EMAMI

FDRSICH, F.T., Hi\UTMANN, M., SENOWBARI-DARYAN, B. & SEYED-EMAMI, K. 2005. The Upper Triassic Nayband and Darkuh formations of east-central Iran: Stratigraphy, facies patterns and biota of extensional basins on an accreted terrane. Beringeria 35: 53-133, 11 text-figs., 13 pis; 7 Appendix-figs.; Wurzburg.

Abstract. Seven sections through the Upper Triassic strata of the Tabas Block, part of the Central-East Iranian Microcontinent, were measured in detail in order to reconstruct their facies pattern, biota, and environments. The sediments post-date the Early Cimmerian tectonic event (i.e., the collision of the Iran Plate with Eurasia) and overlie Middle Triassic carbonates of the Shotori Formation with an erosional unconformity. The Nayband Formation reaches a thickness of up to 3,000 m in the Naybandan area and records the infilling of a strongly subsiding marine basin with sediment from a nearby source area. Sediments include both siliciclastics and carbonates, and environments ranged from an outer ramp well below storm wave base to marginal marine, deltaic settings with coal beds and rare fluvial interludes. The rich benthic fauna and flora is restricted to certain levels within the succession and is dominated by bivalves, the hydrozoan Heterastridium, calcareous algae, corals and coralline sponges, the latter two groups forming patch reefs that occupied positions on the middle ramp. The sediments form highly asymmetric thickening- and/or coarsening-upward cycles, which are interpreted as high frequency transgressive-regressive (TST-RST) sequences reflecting eustatic sea level changes. Retro- and progradational stacking ofTST-RST sequences is only partly developed and some sequences strongly differ in thickness. This is thought to indicate the influence of additional factors, e.g. tectonic events and climatic fluctuations, on the cycle pattern. South of the central area of the Tabas Block, in the Kuhbanan-Buhabad area, Upper Triassic strata are represented by the non-marine Darkuh Formation (new). It either represents < 100 m thick carbonaceous marls and marly limestones with unionid bivalves, gastropods and ostracod shell beds of eutrophic lakes (Kuhbanan Carbonaceous Limestone Member, new), several tens of metres of red clays with caliche of playa origin (Kuh-e-Eshkeli Variegated Marlstone Member, new), or else more than 100 m of gypsum (the new informal Buhabad Gypsum member) accumulating in hypersaline lakes. In the southern part of the Tabas Block, around Kerman, the marine Nayband Formation reappears, albeit strongly reduced in thickness «200 m). In the Late Triassic, sedimentation on the Tabas Block took place in a morphologically richly structured area, produced by the warping of the Middle Triassic carbonate platforms in connection with collision of the Central-East Iranian Microcontinent and the Turan Plate and subsequent fracturing as a result of an extensional regime. Regional differences in subsidence and synsedimentary block movements produced several basins that were separated by uplifted areas. Thus, the distribution pattern and thickness variations of Upper Triassic rocks of the Tabas Block do not correspond to that of a simple, asymmetric foreland basin, even though their post-orogenic position strongly suggests so. III Upper Triassic, Iran. Central-East Iranian Microcontinent, Tabas Block, basin analysis. sequence stratigraphy. palaeogeography. biota. Nayband FOr/nation, Darkuh Formation (new)

Zusammenfassung: Sieben Profile durch die obertriassische Schichtenfolge (Nayband-Formation, Darkuh-Formation) des Tabas-Blocks, cines Teils des Zentral-Ostiranischen Mikrokontinents, wurden detailliert gernessen und auf Fossilien beprobt, um das Faziesmuster, die Lebenwelt und die Ablagerungsraurne zu rekonstruieren. Die Sedimente liegen mit einer Erosionsdiskordanz auf den mitteltriassischen Karbonaten der Shotori-Forrnation und wurden nach del' fruhkimmerischen Orogenese abgelagert, d.h. nach der Kollision des Zentral-Ostiranischen Mikrokontinents mit Eurasien. Die Nayband Formation erreicht cine Machtigkeit von bis zu 3000 m im Gebiet von Naybandan und stellt die vorwiegend marinen Sedimente cines rasch absinkenden Beckens dar, das uberwiegend mit Material aus einem nahe gelegenen Liefergebiet verfullt wurde. Die Sedimente bestehen uberwiegend aus meist unreifen Siliziklastika, in die in mehreren Niveaus Karbonate eingeschaltet sind. Die Ablagerungsraume erstreckten sich von einer aufseren Rampe unterhalb der Sturmwellenbasis bis hin zu randlich marinen Buchten von Delta-Ebenen mit Kohlelagen und seltenen Mundungsarmen,

54

FRANZ

T. FURSICH et a1.

In bestimmten Horizonten tritt cine reichhaltige Benthosfauna und Flora auf, die von Muscheln, del' Hydrozoe Heterastridium, Kalkalgen, Korallen und corallinen Schwammen dominiert wird. Kora11en und Schwamme, gelegentlich auch Muscheln, waren am Aufbau von Fleckenriffen beteiligt, die vorwiegend auf del' mittleren Rampe wuchsen. Die Sedimente del' Nayband-Forrnation bilden asymmetrische "thickening-upward"- und "coarsening-upward"-Zyklen, die als hochfrequente transgressiv-regressive (TST-RST) Sequenzen interpretiert werden, die auf eustatische Meeresspiegelschwankungen zuruckgehen, Nul' teilweise bilden die TST-RST-Sequenzen retrograde und prograde Bundel, und die einzelnen Sequenzen schwanken zum Teil enorm in ihrer Machtigkeit. Hier auliert sich del' Einfluss zusatzlicher Faktoren auf das Sedimentationsgeschehen, vor a11em tektonische Ereignisse und Klimaschwankungen. Sudlich des zentralen Tabas-Blocks, im Gebiet von Kuhbanan und Buhabad, sind obertriassische Schichten durch die nicht-rnarine Darkuh-Formation (neu) vertreten, die ortlich aus .

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not as Liassic. Moreover, the Howz-e-Khan and Qadir members of the Nayband and Parvadeh sections contain a bivalve fauna that is Triassic in character with several taxa that became extinxt before the Jurassic (HAUTMANN 2001 a). This is true of taxa such as Indopecten, Newaagia, Serania, Costatoria, and Myophoricardium, to name but a few. Moreover the algae, benthic foraminifera, corals, and sponges of the Howz-e-Khan Member are also distinctly Late Triassic in character and do not support a Jurassic age of the member. Elsewhere in Central Iran, ammonoids, yielding a Middle Norian (Alaunian) age (SEYED-EMAMI 1975, 2003; REPIN 1987), have been recovered from the lower part of the Nayband Formation. Thus, the Nayband Formation between Kuh-e-Nayband and the Parvadeh area very likely corresponds to the Norian and Rhaetian, with the position of the boundary to the Jurassic being far from clear but lying above the Howz-e-Khan Member, most likely at the top of the Qadir member, because the bivalve fauna of the latter still is Triassic in character (HAUTMANN 2001 a). It is also obvious that the thin, and often non-marine Upper Triassic successions of the Nayband Formation in the southern part of the Tabas Block, e.g. in the Kuhbanan-Kerman area, probably record only a tiny fraction of the Norian-Rhaetian time interval. Lack of index fossils in these beds do not allow more precise statements. At its type section SW of Kuh-e-Nayband and 20 km W of Naybandan village, the Nayband Formation consists chiefly of silty to fine sandy siliciclastics, which

are interrupted twice by partly calcareous units, allowing the lithostratigraphical subdivision in five members (synthesis of ST()CKLlN 1961, MOSIHAGHIAN 1970, BR0NNIMANN et al. 1971, KLUYVER et al. 1983a and own observations) (Text-fig. 2): The basal Gelkan Member (915 m) is composed of dark grey-green, recessiveweathering siltstones with rhythmic intercalations of sandstones, both of which are poor in body fossils. In contrast, the calcareous beds of the mixed siliciclasticcarbonate Bidestan Member (450 m) are highly fossiliferous. Although well preserved ammonoids were not found, the presence of Heterastridium throughout this member allows confining its age to the Late Alaunian - Late Sevatian. The following Howz-e-Sheikh Member (365 m) is lithologically similar to the Gelkan Member and likewise poor in body fossils, while the Howz-eKhan Member (465 m) is characterized by conspicuous coral-sponge reefs. The occurrence of Rhaetavicula contorta and of other macro- and microfossils indicates a Rhaetian age of this member (HAUTMANN 2001 a). Succeeding the Howz-e-Khan Member are coal-bearing siliciclastics with rare intercalations of shell beds, designated as Qadir member by BRAGIN et al. (1981). This unit, up to 1,000 m thick, is followed by the quarzitic sandstones and shales of the Lower Jurassic (Abe Haji Formation). Interregional tracing of the different members developed at the type section is complicated by rapid lateral facies changes and strongly varying thicknesses. However, it is possible to apply the lithostratigraphic

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subdivision of the type section to most parts of the area of the Lakar Kuh Quadrangle Map 1:250000 [sheet J9] (KLUYVER et al. 1983b), which lies approximately between 45-155 km south of the Nayband Quadrangle Map 1:250000 [sheet J8]. The thickness of the "classical" Nayband Formation continuously decreases in a southern direction, varying between 2300 m and 450 m within the limits of the Lakar Kuh Quadrangle Map (KLUYVER et ai. 1983b). Around the city of Kerman it does not exceed 200 m, but contains typical elements of the N ayband fauna (HUCKRlEDE et al. 1962 and own observations). Correlation of parts of these sections with particular members of the type section is not possible. A very different kind of facies is developed (1) near Kuhbanan, approximately 150 km northwest of Kerman, where lacustrine marlstones and limestones with ostracods and unionid bivalves occur (PI. 5, Figs. 1, 36), (2) at nearby Kuh-e-Eshkeli, where the Triassic platform carbonates are overlain by a few tens of metres of playa sediments, and (3) around Buhabad, where about 150 m of gypsum occur (HUCKRlEDE et al. 1962). The overall thickness of the Nayband Formation also decreases from the type section towards the north. Northwest ofAliabad, the thickness of the Howz-e-Khan Member is already reduced to approximately 200 m (see below). Somewhat complicated is the correlation of a section southwest of Parvadeh, around 75 km northwest of Nayband. This section was described by SHARIAT NIA (1994), obviously partly based on an unpublished study of REPIN, and correlated with the type section. However, re-studying this section revealed that the unit, which has

S9

been designated as Howz-e-Khan Member by SHARlAr NIA (1994), commonly contains Heterastridium (SHARlAT NIA 1994: Fig. 5), which clearly points to its Norian age. Yet, in the type section the occurrence ofHeterastridiwn is confined to the BidestanMember, while the occurrence of Rhaetavicula contorta in the Howz-e-Khan Member of the Aliabad section indicates at least in part a Rhaetian age of this unit. According to KLUYVER et al. (1983a: 25), the Howz-e-Khan Member wedges out towards northwest, represented by "a single coral blanket" south of Cheshmeh Araqi at the north-western limit of the Nayband Quadrangle Map. During re-studying the Parvadeh section, some coral-bearing beds were found within the "submember 2" of what was designated as "Qadir" Member by SHARIAT NIA (1994). HAUTMANN (2001 a) regarded this part of the section as the lateral equivalent of the Howz-e-Khan Member, and, in consequence, the underlying "submember 1" as Howze-Sheikh Member and the Heterastridium bearing unit as BidestanMember (Text-fig. 2). North-east of Parvadeh, near Kamar-e Machekuh (Boshruyeh Quadrangle Map 1:250000, sheet 17), the thickness of the Nayband Formation is less than 800 m, and the lithostratigraphic subdivision from the type section is only partly transferable to this area (STOCKLIN et al. 1965, STOCKLIN & NABAVI 1971). Further north, sediments of the Nayband Formation are disappearing within the Shotori range, but reappear southwest of Ferdows with a thickness of up to 500 m (STOCKLIN & NABAVI 1971).

The sections Altogether seven sections were investigated (Text-figs. 3-4; see also Appendix-Figs. 1-7). The thickest was measured south of Kuh-e-Nayband (base at N 32° 24' 27", E 57° 29' 9"; top at N 32° 22' 46", E 57° 24' 14") and is situated a few km east of the type section of BRONNIMANN et al. (1971) and approximately 18 km ESE of the section measured by KLUYVER et al. (1983a) (Textfig. 1). It is 2,350 m thick, starts on top of the Shotori Formation and spans the four formal members of the Nayband Formation and the base of the Qadir member (Appendix-Fig. 1). Characteristic are alternations of purely siliciclastic units (Gelkan Member, Howz-eSheikh Member) with mixed carbonate-siliciclastic units (Bidestan Member, Howz-e-Khan Member). All four members record fully marine environments that differ in depth and sediment input. Due to faulting, the position of the measured section had to be changed several times (Text- fig. 1) in order to obtain a continuous record.

A second section (480 m) was measured N ofKuh-eNayband, west of the village Aliabad (N 32° 30' 30", E 57° 21' 21 "). It starts in the top part of the Howz-eSheikh Member and extends into the coal-bearing Nayband member 5, the latter now included in the Qadir member of BRAGIN et al. (1981) (Appendix-Fig. 2). The Howz-e-KhanMember shows great similarity to its development south of Kuh-e-Nayband, except that the thickness decreased to less than half (206 m as opposed to 433 m). The northernmost section was measured about 19 km ESE of the Parvadeh Mine (see also HAUTMANN 2001a). The 830 m thick section ranges from the upper part of the Bidestan Member into the coal-bearing Qadir member (Appendix-Fig. 3; PI. 1, Figs. 6, 8) and records a further thinning of the formations in a northward direction.

60

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T. FORSICH et al.

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The remaining four sections are f1"0111 the "~~"Ul'-lll The Nayband Formation of the Bolbolu section Kerman is a fully marine, mixed sediment package, a mere 84 m in thickness Fig. 4, PI. 2, Figs. I-2; see also et al. 1962). 5, Similarly, the section NE of Kuh-e-Tizi 2) records fully marine environments, carbonates, which dominate at the base, are gradually upsection by fine-grained siliciclastics. The total thickness of the Nayband Formation at Kuh-e- Tizi is 169 m (Appendix-Fig. 5).

sections at Kuhbanan NW of Ravar (50 m) and I-L-".~u.,u~~,,> N of Kerman (34 m; Appendix-Fig. 6) 1-III'-UU'~ in nature and record lake (Kuhbanan)

to semi-arid coastal plain environments (Kuhfollowing, the main facies associations of these sections are briefly decribed and interpreted in terms of their palaeoenvironments.

Facies analysis (1) Low energy, middle to outer siliciclastic ramp

Features. Dark- to medium-grey, structureless, soft clay, argillaceous silt, silt, fine-sandy silt, and marly silt are the most widespread facies of the Nayband Formation. They reach their greatest thickness (nearly 40 in the Howz-e-Khan Member of the Nayband section (Appendix-Fig. 1 at 1952 m). Usually, their thickness comprises several metres. They are underlain and followed by coarser-grained units. In the Howz-e-Sheikh and Howz-e-Khan members of the Nayband section and in the Qadir member of the Aliabad section layers of ironstone concretions are occasionally associated. Intercalations of 1-3 em-thick ripple-bedded or bioturbated fine-grained sandstones are rare. In the Gelkan Member of the Nayband section, isolated, distorted nodules and folded layers of fine-grained sandstone occur (e.g. Appendix-Fig. 1 at 23-26 m, 81, 360 and 440 m; PI. 3, Fig. 7). The facies are, for the most part, totally unfossiliferous except for palynomorphs and dinoflagellates (DRILL! et al., 2005). In the Bidestan Member, the globular colonial hydrozoan Heterastridium is common at several horizons. Interpretation. Grain size and lack of primary sedimentary structures indicate that the sediments were deposited in a low energy environment below storm wave base. The sediment appears to have been thoroughly bioturbated, but discrete trace fossils have not been recorded. Fully marine, offshore conditions can be reconstructed from the associated facies types and the fossil Heterastridium in the case of the Nayband section and the formal members at the Aliabad and Parvadeh sections. The occasional intercalations of distorted nodules and folded sandstone layers are due to slumping and are evidence of a certain depositional slope. In the Qadir member, such sediments usually represent shallower parts (protected embayments) of the basin as is demonstrated by its close associated with carbonaceous silt and coal seams (e.g. Appendix-Fig. 2 at 433 m;

Appendix-Fig. 3 between 485 and 497 m). They are described under (9).

(2) Storm-influenced siliciclastic middle ramp

Features. Heterolithic sediments, consisting of coarsely interlayed silt or fine-sandy silt and fine-grained sandstone, are a widespread facies association. The finegrained units, corresponding in composition to facies association (1), range between 1 em and several decimetres in thickness, the interlayered sandstones are for the most part 2-20 em thick, rarely their thickness exceeds 1 m. The sandstones invariably exhibit a sharp base and some of them are graded. On the lower surfaces often load casts and flute casts are seen (e.g., PI. 6, Fig. 6). Rarely gutter casts (e.g., in the Gelkan Member of the Nayband section at 540 m; PI. 6, Fig. 5) are observed. Sedimentary structures include small ripple lamination, parallel lamination, rarely large-scale trough crossbedding, and occasionally convolute bedding (e.g., Appendix-Fig. 1 from 384-393 m, 597-620 m, and 15871653 m). The surfaces of some sandstones are covered with oscillation ripples. At some levels, in particular within the Bidestan Member of the Nayband section, some of the sandstone interbeds exhibit slumping (e.g., PI. 2, Fig. 3). Very rarely, 1-3 em thick shell beds are intercalated. Equally rare are scattered bivalve shells. Trace fossils are slightly more common and include Teichichnus, Diplocraterion parallelum, Thalassinoides suevicus, C.vrochorte, Rhizocorallium irregulare, and as yet undescribed conical vertical burrows. interpretation. The interlayered bedding signals differences in the energy level and rate of sedimentation. The silt units can be interpreted as background sedimentation, corresponding more or less to a low energy environment and moderate to low rates of sedimentation, which allowed ample time for


bioturbation. The sandstone intercalations, in contrast, are event beds, which were rapidly deposited by currents. Rapid deposition is corroborated by convolute bedding, scarcity of bioturbation, and sedimentary structures such as horizontal lamination, load casts and flute casts. These features of the sandstones, as well as their sharp base, which in some cases cuts into the underlying sediment, identify them as deposits of mainly unidirectional currents, most likely produced by storm events (e.g. AIGNER 1985). The combination of horizontal lamination, small current ripple lamination, and oscillation ripple surfaces can be explained by the interplay of geostrophic currents and wave oscillations (e.g. MONACO 1992, MYROW & SOUTHARD 1996). As oscillation ripple surfaces are usually rare except at the top of coarsening and thickening upward sequences, the sea floor apparently was below storm wave base for most of the time, but was subject to storm-induced currents (distal tempestites; AIGNER & REINECK 1982). Some of the sandstones carry features also characteristic of turbidites such as convolute bedding, grading, ripple bedding, horizontal lamination, and flute casts. However, gutter casts and wave ripples as well as the scarcity of typical Bouma sequences show that they are the product of storm events rather than of deep sea turbidity currents.

(3) Shoreface and submarine dune and megaripple fields of the inner ramp

63

Interpretation. The sandstone units represent several subenvironments within the siliciclastic shelf regime. Large-scale low angle crossbedded sandstone units that coarsen upwards from ripple-laminated silt are interpreted as shoreface sequences (e.g., REINECK & SINGH 1975) recording increasing water energy. Alternations of bioturbated and laminated sandstone is characteristic of the lower shoreface, where the effects of storminduced currents alternate with quiet episodes during which the substrate becomes thoroughly bioturbated. Sandstone packages in which large-scale cross-beds or ripple-bedding prevails record upper shoreface conditions, permanently above fair-weather wave base. Large hummocky cross-stratification in combination with horizontal lamination, and oscillation ripples indicates deposition by combined flow, storm-generated waves and wind-driven geostrophic flows (M YROW & SOUHIARD 1996). Thinner (0.5-2 m) sandstone units with large-scale cross-beds and sharp, but only occasionally erosional base that generally are found at the top of small parasequences (see below) are interpreted as representing megaripple fields. Thicker units with identical features may represent submarine dunes. As will be argued below, the thinner units partly represent the tops of small shallowing cycles, partly the transgressively reworked base of such cycles. The thicker sandstone units, in contrast, are interpreted as lowstand deposits. (4) Distributary channels

Features. Fine-grained arkosic sandstone units, ranging from 1 m to more than 10m in thickness are occasionally intercalated between the more fine-grained siliciclastics (e.g., Nayband section at 127 m, 240 m, 721 m, 920 m, 1052 m). Most of them exhibit large-scale trough crossbedding (e.g., PI. 1, Fig. 3), less commonly small ripple bedding; rarely, they are bioturbated or structureless. Hummocky cross-stratification was encountered in the Gelkan Member of the Nayband section at 170 m. The base of the sandstone units is either sharp or gradational, but generally not erosional. Rarely, the sandstones are topped by megaripple surfaces. Commonly, the top is bioturbated. Trace fossils (Skolithos, Arenicolites, Diplocraterion) are rare, except at the top of sandstone units that have been reworked. Alternations of horizontal laminated beds with beds bioturbated by Rhizocorallium irregulare and Thalassinoides occur in the Nayband section at 546 m (Appendix-Fig. 1). Rarely, the grain size exceeds that of fine-sand. Siltstone pebbles are found in channeled cross-bedded fine-grained sandstone of the Howz-e-Sheikh Member of the Nayband section at 1736 m and medium- to coarse-grained sandstones occur, for example, in the same section at around 2,300 m (top part of Howz-e-KhanMember).

Features. A 40 m thick package in the Bidestan Member of the Nayband section (Appendix-Fig. I at 1152 m) consists of two sharp-based fining-upward units that grade from medium- to fine-grained arkosic sandstone and silt, respectively. Load structures are developed at the base of each sequence. Sedimentary structures are large-scale trough-crossbedding and small ripplebedding. The two sets are unusual in that they are the only units within the "classical" Nayband Formation of the study area that contain larger plant fragments (e.g., leaf impressions). The two unfossiliferous sandstone bodies are separated by a few metres of bioturbated silt and fine-grained sandstone with shells of marine bivalves and the trace fossil Thalassinoides. A similar sandstone body, 4 m in thickness, occurs in the Howz-e-Sheikh Member of the same section (Appendix-Fig. I at 1734 m). The fine-grained arkosic sandstone, cutting into the underlying sandy silt, exhibits large-scale trough cross-bedding and carries siltstone pebbles. Interpretation. The sharp-based sandstone units in the Bidestan Member most likely represent the fills of two

64

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T. FORSICH et al.

distributary channels of a deltaic system, which are separated in time by a marine flooding event (delta abandonment). The sandstones record sudden lowering of the relative sea level ("forced regression"; e.g., POSAMENTIER et al. 1992, POSAMENTIER & MORRIS 2000). Their base corresponds to a sequence boundary in the classical sequence stratigraphic concept. This explains the lack of associated prodelta and delta front sediments that would be expected in a prograding deltaic system. (5) Submarine channels

Features. Laterally discontinuous sandstone bodies up to 3 m thick, with sharp, erosional base, are rare within the Gelkan and Howz-e-Sheik members of the Nayband section (e.g., at 596 m, 1446 m, and 1450 m). The grain size is usually fine sand, in one case ferruginous pebbles up to 3 em in diameter are common. The sandstones exhibit large-scale trough crossbedding, at 596 m the bedding is strongly distorted. Huge flute casts may be seen at the base. Interpretation. The discontinuous nature and the strongly erosional base clearly identify these sandstones as channel deposits. The ferruginous pebbles probably were eroded from the underlying silts. Cross-bedding and flute casts indicate deposition of the sediment under high energy conditions, distorted bedding may be evidence of loading due to rapid deposition or of slumping. From the associated sediments a submarine shelf origin of the channel sandstones, below fair weather wave-base, is highly likely. They are here interpreted as having been deposited by storm-generated, offshore directed currents that transported large amounts of sediment, possibly liquefied shoreface sands (WALKER 1984) offshore. The fact that the transport took place in channels suggests a depositional slope, a view corroborated by the existence of slumpings at several levels within the Gelkan and basal Bidestan members.

(6) Low energy, middle to outer carbonate ramp

Features. The fine-grained, commonly bioturbated sediments (mudstone, bio-wackestone, bio- floatstone) are the carbonate equivalents of the low energy siliciclastic middle to outer ramp. Between these two end members numerous transitional mixed carbonatesiliciclastic facies types exist (e.g., silty wackestone, finesandy bio-floatstone). The fossil content varies considerable. In most cases, scattered bivalve shells and, in the Bidestan Member, the hydrozoan Heterastridium are present. In the upper part of the Howz-e-Khan Member of the Nayband and Aliabad sections nuculids are the dominant elements of a well preserved, autochthonous benthic fauna of small bivalves. Very conspicuous are two nodular limestone units (foraminifera-rich wacke- to packstone with dasycladacean algae), each several metres in thickness, at the top of the Bidestan Member in the Nayband section (at 1381 m and 1394 m), which contain concentrations oflarge megalodonte bivalves and occasionally dendroid coral colonies (the Lower and Upper Mega1odon Bed; PI. 6, Fig. 8). The up to 7 em high bivalves occur scattered, but in the upper Megalodon Bed also form a 100 em thick parautochthonous layer of articulated and disarticulated shells. Reef debris and scattered coral heads occur in some of the floatstones as do scattered ooids, oncoids, or sand grains. Some faunal elements such as the bivalve Pinna and coral heads are found in life position. Rarely, thin intercalations occur, in which the biofabric is clastsupported (shelly rudstones). Most mud- and wackestones of the Howz-e-Khan Member are bedded and exhibit a nodular texture (e.g., PI. 1, Fig. 5, PI. 3, Fig. 1). They laterally replace coralsponge reefs and may contain thin intercalations of bioclastic packstone, grainstone and rudstone, full of reef debris. Such intercalations also occur in the Bidestan Member. Occasionally, they exhibit a sharp base and contain a variable amount of ooids.

EXPLANATION OF PLATE 1 Fig. I. Bidestan Member of the Nayband Formation, south of Kuh-e-Nayband. Fig. 2. Bidestan and Howz-e-Sheikh members west of Kuh-e-Nayband (N 32° 28', E 57° 30'). Fig. 3. Large-scale trough cross-bedded sandstone, Bidestan Member, section south of Parvadeh. Scale: 20 em. Fig. 4. Reef in the Bidestan Member, section south of Parvadeh at 43 m. Scale: 20 em. Fig. 5. Nodular carbonates, Howz-e-Khan Member, section south of Kuh-e-Nayband. Fig. 6. Coarsening-upward cycles, Bidestan Member, about 1 km west of the measured section south of Parvadeh (see Parvadeh section II of Hautmann 2001a at 76 m). Fig. 7. Informal member of the Nayband Formation, overlain by Jurassic oo-grainstone, northwest of Kuh-e-Nayband. Fig. 8. ?Qadir member capped by basalt, several km SE of measured section south of Parvadeh.


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Interpretation. The fine-grained nature of most of the carbonates, the lack of primary sedimentary structures, and the well preserved autochthonous fauna at some levels are evidence of a low energy environment mostly below storm wave base. Sporadic components such as ooids and fine-grained siliciclastic material most likely were brought in by distal storm-induced currents but subsequently became mixed with the micritic carbonate mud. The grain- and rudstone intercalations between the nodular mud- to wackestones report short-lived highenergy events that swept skeletal debris from the reefs into neighbouring areas. From a sequence stratigraphic point of view (see below), these carbonates are interpreted as TST deposits, recording a reduced influx of siliciclastics. Thus, the carbonates can be interpreted as having been deposited on a ramp system, the middle to outer part of which was carbonate-dominated whereas the inner part was influenced by siliciclastics. Megalodontids are generally regarded as characteristic faunal elements of carbonate lagoons in the Triassic of the Alps (e.g., ZAPFE 1957). However, their sequence stratigraphic context identifies the two Megalodon beds of the Nayband section as part of transgressive systems tracts. They probably formed in protected settings of the middle ramp, shielded from influx of siliciclastic material and only rarely disturbed by storms (see also VEGJ-INEUBRANDT 1982). A relatively shallow water environment within the photic zone is supported by the presence of dasycladacean algae. Most other fossil concentrations, when not representing transported reef debris, occur in thin beds, are autochthonous, and apparently formed during times of reduced sediment input. Usually, they are strongly bioturbated. In sequence stratigraphic terms they correspond to sediments of the late TST (maximum flooding zone: MFZ; e.g. ABBOTT 1997). (7) High energy inner carbonate ramp

Features. Oo-grainstones and -packstones, b iorudstones, and, more rarely, onco-bio-rudstones, often with large-scale trough cross-bedding, ripple surfaces,

and a sharp erosional base, occasionally occur in the Bidestan Member of the Nayband section and in the section south ofParvadeh (e.g., PI. 4, Figs. 2,4-6). They are usually less than 1 111 thick. In some cases, their top is bioturbated.

Interpretation. Most of these carbonates overlie siliciclastic sediments with a sharp base, heralding a new depositional regime. They are interpreted here as having formed on the shallower, inner part of the ramp system during early TST, when reduced input of siliciclastics and high energy conditions favoured ooid formation. Due to the dominance of siliciclastics in shallow water, nearshore environments of the Nayband-Parvadeh area, the high energy carbonate ramp sediments are much rarer than their siliciclastic equivalents. (8) Reefs

Features. In the central and northern part of the outcrop belt of the Nayband Formation biostromes and bioherms built by sponges, corals or bivalves are restricted to the Bidestan and Howz-e-Khan members (e.g., PI. 1, Fig. 4, PI. 2, Fig. 4, PI. 4, Fig. 1). In the strongly reduced succession at Bolbolu in the south small coral-sponge reefs occur near the top. The build-ups are generally between 5-10 m thick, rarely they reach 20 m. The typical thickness of bivalve build-ups is 1-2 m. As is generally the case with shallow water reefs, they are associated with abundant reef debris, part of it filling the reef framework, the other part extending as thin aprons around the reefs. In the Howze-Khan Member of the Aliabad section the topography of the reefs is preserved in several cases as an undulating surface. At 269 m of the Aliabad section (Appendix-Fig. 2) the top surface of a 1-2 m thick biostrome consists of parallel ridges and troughs. The matrix of the reefs ranges from bio-wackestone to -floatstone. A more detailed description ofthe composition of the reefs is found under the heading "Benthic macrofauna" below.

EXPLANATION OF PLATE 2 Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.

I. 2. 3. 4. 5. 6. 7. 8.

Shotori Formation (left) and lower part of Nayband Formation, Bolbolu section. Nayband and Ab-e-Haji formations overlying the Shotori Formation at Bolbolu. Slumping in the Bidestan Member, section south of Kuh-e-Nayband at 960 m. Hammer for scale. Patch reef in Bidestan Member, section south of Kuh-e-Nayband at 1,220 m. Boundary between the Shotori Formation (right) and the Gelkan Member (left), section south of Kuh-e-Nayband. Bidestan Member, section south of Kuh-e-Nayband at around 1,300-1,320 m. Qadir member, section south of Parvadeh. Ripple-bedded sandstone, Howz-e-Sheikh Member, 29 km west of Alia bad. Diameter of coin: 3 em.

Upper Triassic Nayband and Darkuh formations of east-central Iran

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Interpretation. Although the reefs are associated with a fair amount of reef debris, much of this debris may have been produced by the activity of organisms rather than waves. Still, the presence of some rudstone beds extending laterally from the reefs points to at least occasional impact of strong waves, most likely of storm origin. Also, most of the reefs occur within bioturbated silty marl or nodular wackestone units, which indicate low energy conditions. Thus, most Nayband reefs, although growing in shallow water, did not grow in the high energy zone but below the fair weather wave base. Termination of reef growth was usually brought about by renewed influx of fine-grained siliciclastic material. This is particularly well illustrated by the reefs in the Aliabad section, the undulating topography of which has been smothered by silt or marl. The parallel ridges and troughs of the last reef within the Howz-e-Khan Member at Aliabad superficially resembles the groove and spur system of modern shallow water reefs but the amplitude is much less (around 50 em) and the sediment does not correspond to a high energy facies. The parallel elongated ridges more likely reflect the growth of the reefs rather than erosional features, even though their origin remains, at present, enigmatic. In sequence stratigraphic terms most reef growth occurred during late TST, but some reef growth was initiated already during early TST. However, this does not imply a great water depth, as coral growth most likely took place within the photic zone.

energy conditions and probably deposition in a protected coastal setting. A marginal marine setting is likely because of the close associations of clay and silt with coal or highly carbonaceous beds. As the coal is never associated with rootlets, it is most likely allochthonous and the product of plant material that was transported from densely vegetated coastal plains and swamps into a marginal marine setting. The fine-grained siliciclastics never contain any fossils and may have accumulated in low salinity bays. The thicker sandstone units are interpreted as small delta lobes that extended into the embayments. Where the change in grain size is very pronounced and sharp, thin sandstone beds may record breaching of levees of distributaries and crevasse splay deposits (e.g., ELLIOTT 1974). The sharp-based thicker sandstone units such as in the Parvadeh section at 509.5 m (Appendix-Fig. 3) probably are small distributary channels. Switching of such distributaries or small-scale sea-level fluctuations terminated sediment supply and led to the establishment of fully marine conditions, which are evidenced by a thin bed of bioturbated sandstone with bivalves that is found at the top of several of the cycles. The alternative explanation, that we are dealing with a non-marine coastal plain, the sandstones representing fluvial channel deposits and the coal and fine-grained siliciclastics mires on the flood plain, is rejected not only because of the lack of root structures, but also because the sandstones typically do not represent fining-upward sequences as one would expect of fluvial channel fills.

(9) Coastal swamps and protected bays: delta top environments

(10) Flood plain

Features. In the upper part of the Nayband Formation (Qadir member) at Kuh-e-Nayband and south of Parvadeh (PI. 1, Fig. 8), layers of coal and carbonaceous fine-grained siliciclastics occur at or close to the base of small coarsening upward cycles that are often topped by thin sandstones with marine fossils. Facies include coal, coal with variable amounts of clay or silt, black carbonaceous clay and silt; dark-grey clay or silt with 510 em thick intercalations of ripple-bedded, fine-grained sandstone beds with ferruginous concretions; and finegrained, ripple-bedded, more rarely trough-crossbedded sandstone. At some levels, thicker, medium-grained, large-scale trough-crossbedded sandstones overlie, with a sharp base, clay. In several cases, the tops of the sandstones are bioturbated and contain shells of marine bivalves. Interpretation. The fine-grained, structureless nature of the bulk of the sediment suggests a prevalence of low

Features. At Bolbolu, the basal bauxite of the Nayband Formation is overlain by an 8 m thick unit consisting of dark red silt and alternations of fine- to coarse-grained cross-bedded sandstone beds, 10-30 em in thickness, and red clay with caliche nodules. The quartz grains are well rounded and display hematitic coats. The base of some of the sandstone beds is erosional. Interpretation. The association of red silt and clay, partly with caliche nodules, and sharp-based sandstones is interpreted as representing a flood plain setting and a semi-arid climate. The sandstones most likely correspond to deposits of crevasse splays that distributed coarser material across the incipient soils of the flood plain. The very mature sandstones indicate transport for considerable distances or reworking from older sandy deposits.

Upper Triassic Nayband and Darkuh formations of east-central [ran

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(l1)Freshwater Lakes

(12) Hypersaline lakes and playa deposits

Features. In the Kuhbanan area in the south-central part of the Tabas Block (Appendix-Fig. 6, Pi. 5, Fig. 1) the Upper Triassic sediments reach only 50 m in thickness. The typical platform carbonates of the Shotori Formation (birdseye mudstones, algal limestones, mudstones with pseudomorphs after gypsum) grade into 25 m of bituminous mud- and wackestones (Pi. 5, Fig. 6), which are followed, without noticable break in sedimentation, by a mixed fine-grained siliciclastics/carbonate unit. Sediments range from black carbonaceous clay and silty clay with ironstone nodules to bituminous marlstone and mudstone. Depending on their carbonate content they turn yellowish in colour when weathered. At some horizons, the clay and silty clay is reddish in colour. Coarser-grained siliciclastics (silt, sand) are rare and usually only occur as mm- or cm thick interbeds. Most sediments are finely laminated (PI. 5, Fig. 5). Plant debris is common, as are fish scales, unionid bivalves, and ostracods. The latter form mm- to em-thick packstone or grainstone intercalations. The unionids are usually disarticulated and occur either scattered, as pavements in which the shells are mainly convex-up oriented, or as shells beds up to 40 em thick (Pi. 5, Figs. 3-4, 8). Some of these shell beds are sharp-based and laterally continuous, others pinch out laterally. Another characteristic feature are numerous rootlet horizons. In contrast, bioturbation is nearly absent except for rare vertical tubes. Similar, albeit very thin, intercalations ofgrey to olivecoloured marly silt, marl, and marlstone with plant debris occur at Kuh-e-Eshkeli (Appendix-Fig. 7). They contain monospecific layers of ostracods. Root horizons are rare.

Features. In the Kuh-e-Eshkeli section (Appendix-Fig. 7) reddish to varicoloured layers of gypsum are interbedded with black bituminous micrite, grey marly clay, and red to violet-beige micrite, marl, clay, and silty clay with caliche nodules. The only faunal elements are ostracods and small gastropods found at a single level in black bituminous mudstone.

Interpretation. The sediments described above clearly represent various sub-environments of freshwater lakes. This assumption is supported by the rootlet horizons and the numerous horizons with unionid bivalves. The black, often laminated bituminous sediments and the abundant plant debris suggest that, at Kuhbanan, the lake system was eutrophic and that conditions at the lake floor were anoxic for much of the time. The unionid bivalves are never found in life position but appear to have been transported into deeper parts of the lake by currents that were most likely storm-induced. Similarly, the thin ostracod shell beds appear to represent physical concentrations, caused by currents. Beds with rootlets can be interpreted as shallow parts of the lake that were densely vegetated, whereas black laminated clays and mudstones without rootlets probably correspond to the deepest part of the lake system.

Interpretation. At Kuh-e-Eshkeli, the gypsum layers probably were primary precipitates within a hypersaline lake, which repeatedly dried out and turned into a playa. Red marls and clays with caliche nodules formed in the centre of the playa at times when the water table in the sediment was relatively high. The interpretation as flood plain deposits is less likely due to the fine-grained and, in part, calcareous nature of the sediment. Two-thirds up the section at Kuh-e-Eshkeli these sediments are intimately associated with freshwater lake deposits, probably reflecting a shift in climate. In the Golianar anticline NW of Buhabad, Upper Triassic sediments are represented by approximately 150 m of gypsum with thin intercalations of carbonate beds (HUCKRIEDE et al. 1962), which are interpreted as deposits of a relatively large hypersaline lake. (13) Karst and soil horizons

Features. Features indicative of karstification and extensive soil formation are commonly encountered at the base of the Nayband and Darkuh formations, at the contact to the underlying Shotori Formation (e.g., at Kuhe-Nayband, Bo1bo1u, and Kuh-e-Eshkeli). A particular well developed karst horizon is developed at Kuh-eEshkeli (Appendix-Fig. 7), where fissures in the limestone, several decimetres wide and filled with red bauxitic clay, extend down for 7 m. The surface of the dolomite/limestone is generally very irregular and coated with a blackish-brown iron crust. At the Nayband section, only traces of red-brown bauxite are seen between the Shotori Dolomite and the olive-green sediments of the typical Nayband Formation (Pi. 2, Fig. 5). At Bolbolu (Appendix-Fig. 4; Pi. 2, Fig. 1), in contrast, the dolomite is overlain by 8.5 m of dark red bauxite with a level of caliche nodules. In the Kuhbanan section (Appendix-Fig. 6, at 100 m) a 1.5 m thick hematitic ironstone, partly developed as glaskopf, occurs in the lower part, but not at the base of the Darkuh Formation. Interpretation. The strongly irregular surface of the Shotori Formation suggest extensive weathering and

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leaching of carbonate which produced a karst topography. The bauxitic clay filling this topography and, at some localities, forming several metres thick deposits, is interpreted as lateritic soil, the hematitic ironstone as

ferricrete. All these pedogenic features point to a warm, humid to seasonally wet climate and probably represent a prolonged time of subaerial exposure and weathering of platform carbonates of the Shotori Formation.

Lithostratigraphic consequences The strongly differing facies encountered in the various sections investigated cannot all be subsumed under the Nayband Formation as was the case in the past, but partly must be accommodated in new lithostratigraphic units. In the following, these units are formally erected and briefly defined.

Darkuh Formation (new) The Darkuh Formation belongs to the Triassic part of the Shemshak Group and includes relatively thin packages of non-marine sediments that overlie the Triassic platform carbonates (Shotori Formation) and are followed by siliciclastics of the Liassic Ab-e-Haji Formation. The type section (Appendix-Fig. 6) is situated about 5 km east of the edge of the town of Kuhbanan (Text-fig. 1) and was measured on the inverse flank of a large syncline, the western flank of which borders a mountain called Darkuh. At its type locality, the formation measures 51 m. The boundary with the underlying carbonates of the Espahak Limestone Member of the Shotori Formation is drawn at the last thick carbonate unit that is followed by black, strongly carbonaceous clay. The boundary with the overlying AbeHaji Formation is defined by the first bed of quarzitic sandstone, overlying black silty clay of the topmost Darkuh Formation. At the type locality, the Darkuh Formation is represented by the non-marine Kuhbanan Carbonaceous Limestone Member (see below), which consists of sediments ranging from carbonaceous clay, black laminated micrite and marlstone to rare intercalations of siltstones. Root horizons and plant fragments are common. Fossils consist of unionid bivalves, ostracods, fish scales and rare gastropods.

Conspicuous is as m thick unit of blackish to redbrown hematitic ironstone 14 m above the base of the Formation. The Darkuh Formation differs from the classical Nayband Formation by its strongly reduced thickness and by the totally different facies, which is non-marine and represents, at the type section, a lacustrine environment. Moreover the Darkuh Formation occurs in a different geographic area. Two formal members are recognised within the Darkuh Formation: The Kuhbanan Carbonaceous Limestone Member and the Kuh-e-Eshkeli Variegated Marl Member. A third member, the Buhabad Gypsum member, is kept informal, because the locality was not visited by us and we did not investigate the facies characterising the member. The information presented by HUCKRIEDE et al. (1962: 80) is too scant for formally defining the member.

Kuhbanan Carbonaceous Limestone Member (new) The Kuhbanan Carbonaceous Limestone Member corresponds to the development of the Darkuh Formation at the type section. It occurs also at a number of other localities in the Kuhbanan area (HUCKRIEDE et al. 1962) and overlies the Buhabad Gypsum member at the Golianar anticline NW of Buhabad.

Kuh-e-Eshkeli Variegated Marl Member (new) The Kuh-e-Eshkeli Marlstone Member is exposed on the slopes of the Kuh-e-Eshkeli (2.5 km NE ofBidou; Textfig. 1). At the type section (Appendix-Fig. 7) the member is 28.5 m thick. The lower boundary with the Espahak Limestone Member of the Shotori Formation is sharp

EXPLANATION OF PLATE 3 Fig. 1. Nodular biomicrite passing upward into coral reef, Howz-e-Khan Member, south of Kuh-e-Nayband. Fig. 2. Progradational cycle, coarsening from silt to sandstone, within the Nayband Formation near Gazo, SW of Dehuk. Fig. 3. Bidestan Member, section south of Kuh-e-Nayband. Fig. 4. Qadir member at Shurabi near Parvadeh. Fig. 5. Howz-e-Khan Member, Aliabad section. The light-coloured silty marl and marlstones in the centre of the photograph are the lateral equivalents of coral reefs. Fig. 6. Coarsening-upward progradational cycles within siliciclastic sediments of the Nayband Formation near Gazo, SW of Dehuk. Fig. 7. Slump structure in the Gelkan Member, section south of Kuh-e-Nayband at 596 m.


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and represented by a prominent palaeokarst, covered with an iron crust. The highly irregular surface is filled with red clay. Red to variegated clay, marl and silty marl, often in association with caliche nodules are the characteristic facies of the member. Root horizons and plant fragments are rare and confined to the upper part of the member where also thin layers of gypsum are intercalated. The boundary with the Liassic Ab-e-Haji Formation is drawn at the first intercalation of quartzitic sandstones and a change to grey and olive colours. Fossils are rare and confined to layers of monospecific ostracods and tiny gastropods. The Kuh-e-Eshkeli Marl Member represents nonmarine environments in an arid setting, in particular playa deposits and hypersaline lakes.

Buhabad Gypsum member (new, informal) From the Golianar anticline NW of Buhabad (Text-fig. l ) HUCKRIEDE et aI. (1962: 80) describe an Upper Triassic succession that consists of approximately] 50 m of lightcoloured gypsum with thin carbonate intercalations followed by roughly 60 m of dark marly limestones, limestones and dolomites with unionid bivalves in the upper part. Whereas this upper unit apparently belongs to the Kuhbanan Carbonaceous Limestone Member and represents freshwater lake environments, the thick gypsum deposits are interpreted to have formed in an hypersaline lake under arid climate conditions and thus may be coeval with the Kuh-e-Eshkeli Variegated Marl Member that, in its upper part, also contains thin gypsum layers. As argued above, the status of this member is kept informal, as not sufficient information is available at present to formally erect a lithostratigraphic unit.

Qadir member The Qadir member, proposed in an unpublished report of the National Iranian Steel Cooperation by BRAGIN et

aI. (1976), has never been formalized. It corresponds to the Nayband member 5 of KLUYVER et aI. (1983b) who published a reference section, measured 30 km W of Naybandan village. Although the 592 m thick section has been measured in some detail, it is not suitable as a type section, because its top is erosionally truncated and overlain by the Jurassic Badamu Formation (e.g. PI. l , Fig. 7). Similarly, the section of the member given in Appendix-Fig. 3 is far from complete. For this reason, the informal status of the member is retained until the description of a complete section is published.

Chronostratigraphic relationships and correlation It is fairly obvious that the three members of the Darkuh Formation are not contemporaneous, having been deposited under distinctly differing climatic regimes. At present, it is not possible to tie any age to the new lithostratigraphic units, except that they lie between the Triassic platform carbonates and the Lower Jurassic Abee-Haji Formation and are thus time-equivalent to the Late Triassic Nayband Formation. However, judging from their limited thickness it is highly likely that they represent only a very small fraction of that epoch. The observation that, at the Golianar anticline, the Buhabad Gypsum member is followed by the Kuhbanan Carbonaceous Limestone Member may indicate that the latter is generally younger. This is corroborated by the occurrence of thin intercalations of freshwater lake sediments (Kuhbanan Carbonaceous Limestone Member) in the upper part of the Kuh-e-Eshkeli section (Kuh-e-Eshkcli Variegated Marl Member), which indicates an intertonguing of the two formal members of the Darkuh Formation. Although the two sections of the Nayband Formation measured at Kuh-e-Tizi and at Bolbolu (Appendix-Figs. 4-5) are fully marine in nature and consist offacies types that are characteristic of the Nayband Formation in its type area, they cannot be correlated with any of the

EXPLANATION OF PLATE 4 Fig. 1. Bivalve framestone, Bidestan Member, section south of Parvadeh at 100.5 m; acetate peel, x 3. Fig. 2. Oo-grainstone, Bidestan Member, section Parvadeh II of Hautmann (200] a) at 138 m; thin-section, x 50. Fig. 3. Onco-floatstone, Bidestan Member, section Parvadeh II of Hautmann (2001a) at 89.5 m; thin-section, x 12.5. Fig. 4. Bio-rudstone, Bidestan Member, section Parvadeh II of Hautmann (200la) at 192 m; thin-section, x 12.5. Fig. 5. Onco-bio-rudstone, Bidestan Member, section south of Kuh-e-Nayband at 1,134.5 m; thin-section, x ]2.5. Fig. 6. Well sorted oo-grainstone, Bidestan Member, section Parvadeh II of Hautmann (2001 a) at 138 m (see also Fig. 2); thin-section, x 12.5. Fig. 7. Well sorted, fine-grained sandstone, Bidestan Member, section south of Kuh-e-Nayband at 1,] 07 m; thinsection, x 12.5. Fig. 8. Poorly sorted bio- wackestone, basal Howz-e-Khan Member, section south of Kuh-e-Nayband at ] ,894 m; thin-section, x 12.5.


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members of the formation because their thickness is greatly reduced (l 69 m and 84 m, respectively) and the succession of facies does not resemble that of the type

area. The lack of any Heterastridium specimens in both sections may suggest that they are not time-equivalent to the Bidestan Member

Trace fossils Trace fossils are common at certain levels within the Nayband Formation, even though they are of limited diversity. They are most commonly encountered as positive hyporeliefs within interbedded silt-sandstone successions, more rarely as full reliefs. As a more detailed description of the ichnofauna will be published elsewhere, only a briefreview of the more common forms and their facies context is given here. Long, horizontal U-shaped spreiten burrows of Rhizocorallium irregulare (PI. 6, Fig. 3) are widespread in the Nayband section. Together with Thalassinoides suevicus, Teichichnus rectus, and an yet undescribed vertical, conical burrow (PI. 6, Fig. 4) they form a characteristic assemblage occurring mainly at the base of transgressive-regressive sequences. Usually they were dug into firm, consolidated sediment of the preceeding cycle. The substrate consistency can be inferred from scratch marks found on the various ichnotaxa. They point to erosional truncation (removal of soft substrate) of the top beds of the sequences taking place during the following transgression. The assemblage consists of domichnia (Th.alassinoides. conical burrows, rare Skolithos) and fodinichnia (Rhizocorallium irregulare, Teichichnus rectus), and is a typical example of the Cruziana ichnofacies OfSEILACHER (1967), characterising the open shelf below the fair weather wave-base. Long, narrow vertical U-tubes with spreite (Diplocraterion parallelumi occur at some levels in the Nayband section (e.g., PI. 6, Fig. 7). They differ from

most other D. parallelum described in the literature by being very narrow, but extremely long. A specimen within the Gelkan Member of the Nayband section at 401.6 m was seen to extend for more than ] 50 ern, whereby the distance between the two arms of the Utube was merely 2 em. Such specimens strongly resemble those illustrated by FORSICH & WENDT (J 977) from the Triassic Cassian Formation of the Southern Alps. Whereas Diplocraterion is generally interpreted to represent domichnia of suspension-feeders, the present burrows more likely correspond to the paradigm of deposit-feeders. This view is corroborated by the occurrence of the traces within low energy sediments, usually in the middle part of transgressive-regressive sequences. Of particular interest is the occurrence of the graphoglyptid Paleodictyon (PI. 6, Fig. 2), a typical member of the deep-sea Nereites ichnofacies, within the Gelkan Member of the Nayband section. Unfortunately, the two specimens were found in the scree of a wadi and could not be placed in the section. Based on sedimentological features (see below) it is quite clear that the Gelkan Member does not represent deep sea conditions, but corresponds to a storm-flow influenced middle to outer ramp setting. Thus it appears that the producers of Paleodictyon were not restricted to the deep sea, but occasionally ventured into shallower water (see also ERNST & ZANDER 1993).

Flora and benthic fauna Palynomorphs In the Nayband section, palynomorphs were studied by

CIRILLl et aI. (2005), who recognised four palynological assemblages: A Lower Norian assemblage characterised

EXPLANATION OF PLATE 5 Fig. I. Type section of the non-marine Darkuh Formation ENE of Kuhbanan. The Darkuh Formation, sandwiched between the limestones and dolomites of the Triassic Shotori Formation forming the flank of the mountain and sandstones of the Liassic AbeHaj i Formation at the bottom of the valley, is part of the overturned flank of the Kuhbanan syncline. Fig. 2. Nayband Formation at Kuh-Payeh (Tizi Anticline), NE of Kerman. Fig. 3. Lower surface ofunionid shell bed, Darkuh Formation, ENE of Kuhbanan. Hammer for scale. Fig. 4. Cross-section through unionid shell bed, Darkuh Formation, ENE of Kuhbanan. Polished section, scale: I em. Fig. 5. Cross-section through laminated, Corg-rich lacustrine limestone with scattered unionid shells, type section of the Darkuh Formation ENE of Kuhbanan at 34. I m. Lens cap for scale. Fig. 6. Overturned black, laminated lacustrine limestone of the Darkuh Formation, type section ENE of Kuhbanan. Hammer for scale. Fig. 7. Marl overlying coral reefs, Howz-c-Khan Member, Aliabad section. Fig. 8. Pavement of unionid bivalves, type section of the Darkuh Formation, ENE of Kuhbanan.


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by the presence of Annulispora folliculosa and A. microannulata, a Middle to Upper Norian assemblage marked by the first occurrence of Polycingulatosporites mooniensis, a Rhaetian assemblage with Classopollis chateaunovi in association with Retitriletes austroclavatidites, Gliscopollis meyeriana, Limbosporites lundbladii, and Rugaletes awakinoensis, and, finally, a Lower Jurassic assemblage characterised by Callialasporites dampieri, Striatellajurassica and the bloom of Araucariacites australis. Biogeographically the palynomorphs are mixed in character, containing both Eurasian and Gondwanan elements. The Gondwanan element is stronger in the Norian and diminishes towards the end of the Triassic. This probably reflects the increasing length of time the terrane was situated at the southern margin ofEurasia. The palynomorphs also allow some statement about the palaeoclimate ofthe area during the Late Triassic: The dominance of a hygrophytic microflora in the Norian assemblages as compared to xerophytic elements within the Rhaetian assemblages suggest a change to dryer and warmer conditions towards the end of the Triassic (ClRILLl et aI. 2005).

Reefs At the type locality of the Nayband Formation and at the north flank of Kuh-e-Nayband, several levels with reefs, mainly biostromal and subordinately biohermal in nature (usually smaller than 50 m in diameter and less than 20 m high) occur in the Bidestan and Howz-e- Khan members (SENOWBARI-DARYAN 1996). The reefs are either dominated by sponges with corals being subordinate constituents or vice versa. Generally, the bioconstructions within the Bidestan Member are sponge-dominated, those in the Howz-e-Khan Member are dominated by corals. A characteristic feature of the Nayband Formation is the occurrence of bivalve reefs, which were observed in both the Bidestan and the Howz-e-Khan Member. Typically, their height varies between 1-2 m, while their lateral extension usually does not exceed 5 m. These bivalve reefs are maybe the most remarkable

bioconstructions of the Nayband Formation, because they coincide with the first acme in the development of the cementing habit in the Bivalvia (HAUTMANN 2001 a, b). In the following, the main reef builders and reef dwellers are briefly reviewed. Some characteristic elements of the reefbiota are documented on Plates 7-9.

Sponges. Sponges are the main reef building organisms in the bioconstructions of the Bidestan Member. The sponge fauna is represented mainly by coralline sponges ("Sphinctozoida" or "Thalamida", "Inozoida" and "Chaetetida"). Hexactinellida are extremely rare. Spongiomorphids are treated here as inozoid sponges. The "Sphinctozoida" or chambered sponges (PI. 7, Figs. 3-5) are represented by approximately 15 genera and about 30 species within the reefs around Kuh-eNayband. Nevadathalamia (PI. 7, Fig. 5) is the most abundant genus, followed usually by Paradeningeria, but locally other genera may dominate. Sphinctozoid sponges of the Nayband Formation have been described from south of Abadeh (central Iran) by SENOWBARIDARYAN & HAMEDANI (1999a). The non-chambered .Jnozoida" are almost as diverse (about 10 genera and 20 species) within the reefs. The most abundant genus in the reefs around Kuh-e-Nayband, especially within the reefs on the northern flank of the mountain, near the village of Ali-Abad, is a sheet-like sponge (PI. 7, Figs. 6-7). In a small reef near the town of Naybandan the small species Peronidella iranica (SENOWBARI-DARYAN, 2003a) (PI. 7, Figs. 1-2) is very abundant. Some inozoid sponges of the N ayband Formation from Kuh-e-Nayband and other localities in central Iran were described by SENOWBARI-DARYAN et aI. (1997). Chaetetid sponges are the third group of coralline sponges occurring within the reefs. According to the diameter of the tubes and based on other features several taxa may be differentiated (PI. 8, Figs. 1C-D, 4-5). Chambered and non-chambered hexactinellid sponges are extremely rare within the reefs around Kuh-eNayband. Spongiomorphids occur in two morphotypes: bush-

EXPLANATION OF PLATE 6 Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.

1. Shell bed, Bidestan Member, section south of Kuh-e-Nayband. Lens cap for scale. 2. Paleodictyon isp., Gelkan Member, section south of Kuh-e-Nayband; x 2. 3. Branched Rhizocorallium irregulare, Gelkan Member, section south of Kuh-e-Nayband at 547 m. Scale: 20 em. 4. Cone-shaped burrow, Bidestan Member, section south of Kuh-e-Nayband; x 1. 5. Gutter cast, Howz-e-Sheikh Member, 29 km west of Aliabad. Hammer for scale. 6. Flute easts on lower surface of sandstone bed, Gelkan Member, section south of Kuh-e-Nayband at 169.5 m. 7. Long Diplocraterion parallelum, Gelkan Member, section south of Kuh-e-Nayband at 408 m. Hammer for scale. 8. Lower Megalodon Bed, top Bidestan Member, section south of Kuh-e-Nayband at 1382 m. Scale in em.


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like and multi-branched growth forms (Spongiomorpha ssp.) and sheet-like and multi-layered growth forms (StratomOlpha ssp.). The latter constructed layers up to 1 m in thickness (PI. 8, Fig. 2).

Corals. The corals are represented by three morphotypes: solitary, dendroid, and cerioid. Solitary corals are not as abundant as the other two types. According to the calyx diameter and the number of septa several taxa can be distinguished. Distichophyllium (PI. ] 0, Figs. 4-5) appears to be the most abundant genus. The several species of dendroid corals belong to at least three genera. Most dendroid corals belong to the genus Retiophyllia (PI. 9, Figs. ] -2). Cerioid corals dominate within the bioconstructions. Several genera may be distinguished. Astraeomorpha (PI. 9, Fig. 4) is the most abundant genus around the Kuh-e Nayband, followed by the genus Pamiroseris (PI. 9, Fig. 8). Generally, cerioid corals are more common in the Bidestan Member, while dendroid corals are abundant in reefs within the Howz-e-Khan Member. Some corals from the N ayband Formation near the town of BagherAbad (N of Esfahan) have been described by KRISTANTOLLMANN et aI. (I 980) and MELNIKOVA (! 989). Bryozoans. Bryozoans are extremely rare within the bioconstructions around the Kuh-e-Nayband. However, at some other localities (e.g., in the reefs and reefal limestones south of Abadeh; SCHAFER et aI. 2003) a variety of bryozoans were found. Algae. Solenoporacean red algae (Solenopora, Parachaetetes) occur in moderate numbers within the bioconstructions (PI. Ll , Fig. I) in contrast to diploporid green algae, of which only few were found. However, in the bedded limestones surrounding the bioconstructions, especially in the Howz-e-Khan Member, a variety of dasycladacean green algae occurs (PI. ] 0, Figs. 6-9). Some dasycladacean and udoteacean green algae from other localities ofthe Nayband Formation in central Iran have been described by SENOWBARI-DARYAN & HAMEDANI (1999, 2000).

Brachiopods. Brachiopods are important epifaunal elements within the bioconstructions and surrounding bedded limestones. In particular, Gosaukamerella eomesozoica (FLOGEL) is abundant in the reefs around Kuh-e Nayband and at other localities (SENOWBARIDARYAN & FLOGEL 1996). Bivalves. Bivalves (PIs. 12-] 3) were both dwellers within the sponge and coral reefs and framework constructors on their own. The habitats of the sponge-coral reefs were entered by means of three different strategies: byssal attachment, cementation and chemically boring. The most conspicuous, byssally attached bivalve that lived within these reefs was Cultriopsis canalis HAUTMANN (PI. ] 2, Figs. 9-] 0), which is characterized by a huge byssal tube below terminal beaks of very thick, sabre-shaped valves. The enormous size of the byssal tube and the large scar of a corresponding byssal retractor muscle points to an extreme firm attachment that probably was an adaptation to a relatively high water energy and was a competitive advantage in the struggle for space with other reef organisms. The most common cementing bivalve species that settled within the sponge-coral reefs was Newaagia stocklini (REPIN). It is characterized by its large size, a very high ligament area, and by spines which were advantageous in supporting the attachment as well as in defence and in competition for space. An inconspicuous, but pervasive element of the reef habitats was Atreta subrichthofeni (KRUMBECK). This small bivalve cemented its right valve to all kinds of hard substrates available, which were chiefly the skeletons of living and dead animals. Apart from contributing to reef growth by cementing forms, bivalves also played a destructive role by a chemically boring species that chiefly lived in cerioid corals and spongiomorphid sponges. Although borings are quite common and destroyed locally up to 50 % of skeletal volume (e.g., PI. 9, Fig. 3), the producer of these borings could not been identified with certainty. One small specimen was found in situ (HAUTMANN 200] a: pI. 3, fig. ] 6), but it cannot be ruled out with certainty that it

EXPLANATION OF PLATE 7 Figs. 1-2. Peronidella iranica SENOWBARI-DARYAN. 1. Oblique section through a branched colony, x 2.3; 2. Cross-section showing the cicular outline of the sponge, the thick wall and the narrow spongocoel with a distinct wall pierced by large openings, x 7.5. Small reef near the village Naybandan, Bidestan Member. Fig. 3. Sphinctozoan sponge gen. et sp. indet. This sponge is very abundant within the reefs of the Howz-e-Khan Member outcropping on the northern flank of the Kuh-e Nayband, west of the village of Ali-Abad, x 1. Fig. 4. Sphinctozoan sponge gen. et sp. indet., x 5. Locality and stratigraphic position as in Fig. 3. Fig. 5. Nevadathalamia sp.; two longitudinal sections and some cross-sections. x 1.2. Locality and stratigraphic position as in Fig. 3. Figs. 6-7. lnozoid sponge gen. et sp. indet. 6. A sheet-like specimen with pores oriented in lines, x 1.2. Locality and stratigraphic position as in Fig. 3. 7. Cross-sections through specimens at different angles, x 0.8. Locality and stratigraphic position as in Fig. 3.


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was a nestler rather than the producer of the boring. Morphologically, it resembles Lithophaga, but it shows some radial ribs on the posterior side, a feature that has never been observed elsewhere in this genus. In addition to dwelling within reefs, the framework of which was built chiefly by other organisms, bivalves also formed bioconstructions on their own. Although small in comparison with the coral-sponge reefs, these bivalve reefs are worth noting because they are among the geologically earliest true bivalve reefs. Their main constituent is the oyster Umbrostrea emamii HAUTMANN. However, in contrast to most of the later oyster reefs, other bivalves and other classes of organisms also contributed to these reefs. Apart from bivalves such as Umbrostrea iranica HAUTMANN, Newaagia stocklini (REPIN), Persia monstrosa REPIN (PI. 13, Fig. 2), Eoplicatula p arv adehens is HAUTMANN, and Pseudoplacunopsis asymmetrica HAUTMANN, also corals (Thomnasteria sp.) and terebratulid brachiopods were observed (HAUTMANN 2001a: figs. 7b, 9). This may indicate that these reefs grew in a normal marine environment and not in brackish bays, lagoons or nearshore stressed environments, in which most later oyster reefs occurred (HAUTMANN 2001 b).

Worm tubes. Serpulid worm tubes belong to the most abundant framework encrusters within the reefs (PI. 8, Figs. 6-7). Agglutinated worm tubes occur only in moderate numbers (PI. 11, Fig. 4). Foraminifera. Generally, foraminifera are not abundant within the bioconstructions. Vagile (Ophthalmidium spp., Galeanella sp.) and sessile miliolids (Planiinvoluta sp.; PI. 11, Fig. 11), which are so abundant in NorianRhaetian reefs of the western Tethys, are very rare. Some agglutinated types (e. g. Alpinophragmium, Kaeveria, Palaeolituonella, Duostominidae) are exceedingly rare. However, in some bedded limestones surrounding the bioconstructions, very abundant aulotortid, involutinid, trocholinid and other foraminifers were found (PI. 11, Figs. 6-9). Some of the foraminifers occurring in the type section have been described by BRC)NNIMANN et aI. (1971). However, we found a number of additional taxa in the Nayband section. They form part of a detailed description of foraminifera from the Nayband Formation (SENOWBARI-DARYAN, in prep.). Other organisms. Various problematic organisms (Microtubus communis Radimura cautica SENOWBARI-DARYAN & Lith oco dium sp., Baccinella irregularis RADOICIC), known from NorianRhaetian reefs of the western Tethys, occur also in the carbonates (reefs and bedded facies) of the Nayband Formation, albeit in lower numbers. Microbial crusts

("Spongiostromata"), so abundant in Norian-Rhaetian reefs of the western Tethys, are extremely rare in bioconstructions of the Nayband Formation and are usually found around dendroid corals. Level bottom fauna

Foram inifera. Foraminifera are generally not abundant in the N ayband Formation apart from the reef facies, but may form rich assemblages mainly of involutinid and aulortid taxa (PI. 11, Figs. 6-11) at certain levels within the Bidestan and Howz-e-Khan members, where they occur in bedded limestones surrounding the reefs. In the nodular limestones of the Howz-e-Khan Member at the southern slopes of Kuh-e-Nayband, involutinid foraminifers form a diverse association together with dasycladacean green algae (PI. 11, Figs. 6-9). Hydrozoans. Heterastridium (PlIO, Figs. 1-3) with a spherical calcareous skeleton, known as "pelagic hydrozoan", is the most peculiar fossil within the Nayband Formation. It occurs abundantly in marly to silty sediments, more rarely in pure carbonates. Heterastridium seems to be limited to the Bidestan Member. The diameter of the specimens varies usually between 0.5 mm and 100 mm. The largest specimen, found at the southern slope of Kuh-e-Nayband, has a diameter of 220 mm. SENOWBARI-DARYAN & HAMEDANI (2000) reported the largest specimen with a diameter of 34 em, found near the town Bagher-Abad, north of Esfahan. Based on the spine-like structures of the outer surface more than 25 "species" have been described in the literature. A detailed monographic treatment of the taxon is in preparation (SENOWBARI-DARYAN in prep.). Recently, WILMSEN (2003) interpreted ball-like structures such as Parkeria and Heterastridium as "benthic drifters" that were constantly moved across the sea floor by currents and waves. Bivalves. Both in terms of abundance and diversity, bivalves strongly dominated the level-bottom communities of the Nayband Formation. The following description of their palaeobiology is based on HAlJTMANN (2001 a, b). 104 bivalve species, assigned to the subclasses Palaeotaxodonta (7), Pteriomorphia (51), Palaeoheterodonta (12), Heterodonta (22) and Anomalodesmata (12), have been found in the Nayband Formation. Apart from the frame-builders and reefdwellers described above, the vast majority of them belonged to level-bottom communities. Probably the most conspicuous epifaunal bivalve genus of the Nayband Fm. is lndopecten DOUGLAS, a genus with numerous species in the Norian and Rhaetian of the eastern Tethys. In the Nayband Formation six


species of Indopecten are known, which were either epibyssate, or free living and able to swim. The paradigm of a swimming pectinid is particularly well illustrated by the large, circular disc of l. glaber (PI. 13, Fig. 4), which reaches a diameter up to IS cm. The epifaunal habit of Indopecten was shared by numerous other pteriomorph bivalves as well as by a few heterodonts. Entolium incognitum (BITTNER), E. cf. quotidianum (HEALEY) and Prop eamus sium (Parvamussium) schafhaeutli (WINKLER) represent free living species, which were probably also able to swim. Common epibyssate species include Primahinnites iranica REPIN (PI. 13, Fig. 1) with a pleurothetic mode of life, and Mysidiella imago HAUTMANN (PI. 12, Figs. 5, 7), Iso gn omon rep ini HAUTMANN (PI. 12, Fig. 14), Antiquilima hians HAUTMANN (PI. 13, Fig. 3) and Coelopis aurea HAUTMANN (PI. 13, Fig. 7) with an orthothetic attitude. The various, but comparatively rare species of Parallelodon (PI. 12, Fig. 8) and Grammatodon probably lived epibyssally attached both within reefs and on the sea floor. Examples of unattached but immobile epifaunal bivalve species are Cass ianella inaequiradiata (SCHAFHAUTL) (PI. 12, Fig. 13, 15), Schafhaeutlia sphaerioides (BOETTGER), Opis (Trigonopis?) eumorpha HAUTMANN (PI. 12, Fig. 6) and 0. (Trigonopis?) douglasi HAUTMANN. The transition zone between epifaunal and infaunal habitats was successfully occupied by various semiinfaunal bivalves. The abundant bakevelliids Hoernesia shaniorum (HEALEY) (PI. 12, Figs. 11-12) and Gervillaria inflata (SCHAFHAUTL) lived with their left valves slightly sunken into the substrate, additionally anchored by a byssus. More deeply embedded were the permophorid Healeya gonoides (HEALEY) (PI. 13, Fig. 8), the mytilid Modiolusfrugi (HEALEY), and three different species of Pinna. Palaeocardita stoecklini HAUTMANN (PI. 13, Fig. 12) and P carinataHAuTMANN may be cited as rare examples of semi-infaunal, byssally anchored heterodonts. Among infaunal bivalves, deposit-feeding nuculoids characteristically occur in situ within fine grained, marly sediments that were probably deposited in protected low energy settings. Out of seven documented species, Palaeonucula sundaica (KRUMBECK) (PI. 12, Fig. 1) and Nuculana naibandensis HAUTMANN (PI. 12, Fig. 3) are by far most common, but two others are also worth mentioning explicitly: Mesosaccella sub z eli m a (KRUMI3ECK) (PI. 12, Fig. 4) as one of the oldest records of the genus Mesosaccella, and Trigonucula goniocostata HAUTMANN (PI. 12, Fig. 2) with its well-developed divaricate ornamentation on the anterior side of the valves, that probably aided in burrowing. Divaricate ribbing is also found in the mytilid Inoperna (Triasoperna) schafhaeutli (STUR) (PI. 12, Fig. 6), which

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might represent a unique, but in the long run unsuccessful attempt of the order Mytiloida to gain a burrowing habit. Some of the eight trigonoid species that are found in the Nayband Formation show adaptations for fast burrowing such as divaricate ribs and strong pedal muscles, but all of them were unable to invade deep-infaunal habitats as is shown by their lack of siphons. This lack was probably also shared by the heterodonts of the Nayband Fm. Most of them, such as Myophoricardium subquadratum HAUTMANN (PI. 13, Fig. 9), Praeconia matura HAUTMANN (PI. 13, Fig. 10), Tutcheria cloacina QUENSTEDT (PI. 13, Fig. 14), Palaeocardita iranica HAUTMANN (PI. 13, Fig. 13), andP stoeckliniHAuTMANN (PI. 13, Fig. 12), belong to the generally non-siphonate Crassatelloidea and Carditoidea. Cardiids are represented by four species assigned to Vietnamicardium Vu KHUC (e.g., V nequam; PI. 13, Fig. 11) and Protocardia BEYRICH. In these earliest genera of the Cardiidae, the sharp posterior truncation of the valves contradicts the presence of well developed siphons. The frequently cited claim of STANLEY (1968) that siphon formation played a central role in the initial post-Palaeozoic radiation of the Bivalvia may therefore need revision (HAUTMANN, in prep.). Deep-infaunal bivalves are represented in the Nayband Formation only by a few taxa of siphonate Anomalodesmata, Homomya sublariana KRUMI3ECK (PI. 13, Fig. 16) being the most abundant one. Although no quantitative palaeoecological studies were carried out, bed by bed sampling allow to distinguish different bivalve-dominated assemblages that were tied to particular habitats. In most cases, sedimentological data suggest that the different assemblages were chiefly controlled by water energy, substrate consistency, and grain size. Marly soft substrate assemblage. - The fauna of this low energy setting is chiefly autochthonous. It is strongly dominated by the deposit-feeding nuculoids Palaeonucula sundaica (PI. 12, Fig. 1) and Nuculana naibandensis (PI. 12, Fig. 3) as well as by the endobyssate suspension-feeder Palaeocardita iranica (PI. 13, Fig. 13). Minor faunal elements are the shallow infaunal suspension-feeder Unionites griesbachi BITTNER and the epifaunal suspension-feeder Cassianella inaequiradiata (PI. 12, Figs. 13, 15). Examples of this assemblage occur in the Aliabad section at 271 m and in the Howz-e-Khan Member of the Nayband section at 2075 m. Carbonaceous clay assemblage. - This assemblage occurs in shell beds within carbonaceous clays of the Parvadeh section (e.g., Bidestan member at 50 m, Appendix-Fig. 3), which have been interpreted (see above) as sediments of protected bays. The high content of plant debris of the sediment indicates a nearshore environment subject

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to freshwater influx. The fauna is dominated by endobyssate suspension-feeding bivalves [e.g., Healeya gonoides (PI. 13, Fig. 8), Palaeocardita stoecklini (PI. 13, Fig. 12), Pinna sp. A, and occasionally Hoernesia shaniorum (PI. 12, Figs. 11-12)]. Shallow infaunal suspension-feeders are chiefly represented by Gruenewaldia magna and G iranica (PI. 13, Fig. 5). Deposit-feeders are absent. Usually, the faunal elements of this assemblage are preserved in situ. The marginal marine setting suggests some salinity control of the assemblage, but there is no indication of this, as all members of the assemblage belong to fully marine taxa. Most likely, the assemblage formed during transgressive interludes and therefore marks the base of small transgressive-regressive sequences. Sandy substrate assemblage below fair weather wavebase. - This assemblage commonly occurs at the base of thickening-upward cycles, mostly in orange-coloured, calcareous, bioturbated, shell-rich sandstones. The fauna is relatively diverse, with epifaunal and shallow-infaunal bivalves dominated by various species of Indopecten, Chlamys, Plagiostoma, lnoperna, Costatoria, and Myophoricardium. A typical example of this assemblage is represented by sample (4) ofHAuTMANN (2001a), which comes from an widely exposed horizon of the upper Qadir member of the Parvadeh area. In most cases, this assemblage characterises the transgressive systems tracts of the parasequences and is indicative of open marine, offshore environments. High energy, medium-sandy substrate assemblage. Because of frequent reworking, deep burrowing species such as Homomya sublariana (PI. 13, Fig. 16) are the only autochthonous faunal elements of this facies. However, the relatively abundant Trigonia (Modestella) zlamb achensis HAAs, Myophoricardium lineatum WOHRMANN, and M. subquadratum HAUTMANN (PI. 13,

Fig. 9) suggest that these fast-burrowing species also lived in this high energy setting. Low-energy megalodontid assemblage. - Megalodontids form a characteristic assemblage in soft, fine-grained carbonates that most likely accumulated on an middle carbonate ramp. The megalodontids lived with the anterior side of their shell slightly buried in the soft substrate (ZAPFE 1957). Examples of this assemblage are found on top of the Bidestan Member of the Nayband section at 1384 m and 1400 m.

Gastropods. DOUGLAS (1929) reported 17 taxa of gastropods from the Upper Triassic of Iran. Later on, FALLAHI et al. (1983) described five species, two of them new, from the Bagherabad area north ofEsfahan. NOTZEL et al. (2003) described seven species, two of them new, from the Zefreh area northeast of Esfahan. The gastropod fauna from the Nayband Formation of the Tabas Block has been documented by NOTZEL & SENOWBARI-DARYAN (1999). Among the 29 taxa (11 of them are new) there are representatives of the Archaeogastropoda (5 species), Caenogastropoda (18 species), Heterostropha (3 species), and Neritimorpha (3 species). The gastropods show a strong relationship to the late Triassic gastropod fauna of the Alps, especially to that of the Carnian Cassian Formation, and considerable affinities to the gastropod fauna of southeast Asia (southern China and Myanmar). Brachiopods. Compared with bivalves and gastropods, the brachiopod fauna of the Nayband formation is sparse. Our sampling yielded 5 species, all of them also known from the Late Triassic of the Alps: Rhaetinia gregaria (SUESS), Zeilleria norica (SUESS), Triadithyris gregariaeformis (ZUGMAYER), Fissirhynchiafissicostata (SUESS), and Zugmayerella uncinata (SCHAFHA.UTL).

EXPLANATION OF PLATE 8 Fig. 1. Chaetetid sponge gen. et sp. indet. 1 (C) and 2 (D), the latter showing large circular openings distributed across the whole sponge; inozoid sponge gen. et sp. indet. (1), and marginal section through Peronidella iranica SENOWBARI-DARYAN (P), x 4. Small reef, about 6 km W of Aliabad, Howz-e-Khan Member. Fig. 2. Stromatomorpha sp., building sheet-like crusts with several layer up to 10 em thick and 10 em long, x 2. Northern flank of Kuh-e-Nayband, Howz-e-Khan Member. Fig. 3. Sections through two specimens ofthe brachiopod Gosakammerella eomesozoica (FLUGEL) and a cross-section of Peronidella iranica SENOWBARI-DARYAN, x5. Southern flank of Kuh-e-Nayband, Bidestan Member. Figs. 4-5. Chaetetid sponge gen. et sp. indet. 4. Top view, x 2. 5. Side view, x 2. Locality and stratigraphic position as in Fig. 1. Fig. 6. Sections through numerous serpulid tubes, x, 4. Southern flank of Kuh-e-Nayband, Bidestan Member. Fig. 7. Section trough a serpulid tube showing the laminated aragonitic wall. A small articulated bivalve is imbedded wihin the tube, x 5. Southern flank of Kuh-e-Nayband, Bidestan Member.


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Sedimentation patterns The Nayband Formation of the three northern sections (at Nayband, Aliabad, and Parvadeh) exhibits a pronounced cyclicity. The cycles, although varying considerably in thickness, correspond to parasequences, which are here described as transgressive-regressive (TR) sequences (EMBRY 2002). Commonly they are strongly asymmetric and separated from each other by distinct erosional surfaces. In the following, the cyclic development in each of the members of the Nayband Formation is described and interpreted in terms of sequence stratigraphy.

Cycles in the Gelkan Member (Text-fig. 5, AppendixFig. 1)

Features. In the 790 m thick Gelkan Member, the cycles are nearly always strongly asymmetric and range in thickness from 2 to 27 m, most of them being between 5 and 15 m thick. The cycles usually are developed as coarsening-upward, in several cases in addition also as thickening-upward cycles, starting with silt or sandy silt and ending with fine-grained sandstone. Commonly, 15 em thick sandstone beds are intercalated in the upper part of the cycles and may thicken towards the top (Textfig. 5a). The change in grain size may be gradational or discontinuous. The top bed, which is often better cemented than the remaining beds, commonly exhibits a sharp, erosional base. Concomitant with the change in grain size there is also a change in the fabric of the sediment: The basal part of the cycles is usually structureless, which can be taken as having been caused by intensive bioturbation. Towards the top sedimentary structures are common. They range from ripple bedding, bedding surfaces covered with oscillation or current ripples, to parallel lamination, and to large-scale trough cross-bedding and megaripple surfaces. Rarely, hummocky cross-bedding has been observed. Convolute bedding is a common feature. The lower surfaces of the thin sandstone intercalations often exhibit flute casts. Slumping occurs at several levels and is not restricted to certain parts of the cycles. Discrete trace fossils are usually restricted to the top bed of the cycle where they usually are found at the base (in the case of thin sandstones) or at the top of the bed. Most of them are characteristic of the Cruziana ichnofacies (see above) and indicate moderate to low energy settings. The topmost few cm of the cross-bedded top bed may also be thoroughly bioturbated. Interpretation. The sediments of the Gelkan Member were deposited on a storm-influenced outer to inner ramp, but mostly below storm wave base (i.e., on the outer ramp). Occasional slumping indicates a depositional

slope and a relatively high sediment input. The coarsening- to thickening-upward cycles represent shallowing cycles, but the cycle boundary as defined by a distinct change to more fine-grained sediment usually does not correspond to the shallowest phase of the cycle. Instead, the top bed either records both, the shallowest part of the cycle as well as the deepening phase, or especially where the top bed is fairly thin, only the deepening phase. In the latter case the top sediments that were deposited during the preceding shallowing phase were reworked during transgression. Evidence in support of this interpretation is the presence of an internal erosional surface within the top bed, often associated with a change in texture. For example, large-scale trough cross-bedding below this internal erosional surface is replaced by ripple bedding above, or primary sedimentary structures below were obliterated by strong bioturbation above. The higher degree of cementation of the top bed is most likely due to its higher structural and compositional maturity (e.g. better sorting, less arkosic). This feature is a characteristic product of repeated phases of reworking that generally go hand in hand with early transgression. The change from the regressive to the transgressive phase of a cycle often goes hand in hand with considerable erosion. This can be demonstrated by the nature of the trace fossils that extend either from the base or from the top of the topmost sandstone of a cycle. These trace fossils were generally excavated in fairly cohesive sediment, which is shown by the presence of scratch marks found on Thalassinoides, Teichichnus and on cone-shaped vertical burrows (Text-fig. 9f). Obviously, the soft top layers of sediment were removed exposing already compacted, cohesive, deeper strata for colonisation.

Cycles in the Bidestan Member (Text-fig. 6, AppendixFigs. 1 and 3, PI. 1, Fig. 1, PI. 3, Fig. 3)

Features. In the Bidestan Member of the Nayband and Parvadeh sections sediments are mixed carbonate and siliciclastic in nature, whereby silt and sandstones strongly dominate. A lower rate of subsidence and a lower input of siliciclastic material generally led to deposition of carbonates which range from oo-grainstones to biograinstones and -rudstones, but also wackestones and silty marl occur. Most sediments accumulated on a storminfluenced ramp, but the spectrum ranges from environments below storm influence at the deep end to distributary channels at the shallow end (see below). Five levels with slumpings in the Nayband section indicate the presence of a depositional slope, whereas at Parvadeh no slump structures were encountered. At Parvadeh, the


85

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FRANZ

120

T. FURSICH et al.

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129

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FRANZ

132

T. FORSICH et al.

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