graphic correlation of the lower-middle Cambrian ...

Geosciences Journal Vol. 9, No. 2, p. 00

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000, June 2005

Major geodynamic and sedimentary constraints on the chronostratigraphic correlation of the lower-middle Cambrian transition in the western Mediterranean region J. Javier Álvaro* Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009-Zaragoza, Spain Sébastien Clausen Laboratoire de Paléontologie et Paléogéographie du Paléozoïque, UMR 8014 CNRS, Université des Sciences et Technologies de Lille, 59655-Villeneuve d'Ascq, France

ABSTRACT: The sedimentary rocks of the lower-middle Cambrian transition in the western Mediterranean region have recorded a superposition of extensive tectonic, volcanic, epeirogenic, and eustatic events that led to a complex sequence framework that, in some cases, makes detailed chronostratigraphic correlations difficult. This paper summarizes and updates the relationships between event stratigraphy, fluctuations of relative sea level, setting of major stratigraphic discontinuities, unconformities, and condensed levels, and succession of benthic community replacements displayed by outcrops located in the Iberian Peninsula (Iberian, Cantabrian and Córdoba platforms), Moroccan Atlas (Souss Basin), and the Montagne-Noire and Sardinian platforms. The resulting mosaic of inter-related geodynamic processes is correlated by trilobite, archaeocyath- and acritarch-based chronostratigraphic scales, taking as reference for the base of the Middle Cambrian in West Gondwana the immigration of paradoxidid trilobites. Key words: stratigraphy, discontinuities, series boundary, W Gondwana, Cambrian

1. INTRODUCTION

During the last two decades, active geological and paleontological research has focused on the sedimentary rocks of the lower-middle Cambrian transition (LMCt) dispersed in the western Mediterranean region (the Iberian Peninsula, Morocco, southern France, and Sardinia; Fig. 1). New multidisciplinary data has yielded evidence of a complex of sedimentary events, geodynamic conditions, and evolutionary and biodiversity patterns along the western Gondwanan margin (e.g., Geyer and Landing, 1995; Loi et al., 1995; Liñán et al., 1996; Álvaro et al., 2000a, 2003; Vizcaïno and Álvaro, 2001; Geyer and Landing, 2004). Although broadly speaking, it is relatively easy to distinguish between the strata bearing olenellid (lower Cambrian s.l.) and paradoxidid (middle Cambrian s.l.) trilobite assemblages, according to the rules of the International Commission of Stratigraphy (ICS), chronostratigraphic boundaries should be based on FADs of taxa when possible (Remane, 2003). This is the problem that the ISCS has begun to try to solve by forming the Lower-Middle Cambrian GSSP Working Group. In Baltica (Scandinavia and the eastern European plat-

*Corresponding author: [email protected]

form) and on the western Gondwanan margin [Avalonia (e.g., Newfoundland and Wales), and in western and central Europe (Spain, Sardinia, Germany, and Bohemia), and Turkey] traditionally the occurrence of trilobite paradoxidids has been used as the base of the middle Cambrian. This proposal was advanced by Brøgger (1886), and the suggestion predated Walcott’s (1891) concept of a Lower Cambrian Series based on the stratigraphic range of olenellids. Different paradoxidid species in different regions have been selected to define the oldest middle Cambrian zones, e.g., the Eccaparadoxides oelandicus Stage (with the lowermost Eccaparadoxides insularis zone) in the Baltoscandian and east-European platforms (see recent syntheses in Ahlberg, 1998; Moczyd owska, 1998), and the Acadoparadoxides mureroensis zone in the Iberian Peninsula, Sardinia and Turkey (Álvaro et al., 1993b; Liñán et al., 1993b; Dean and Özgül, 1994; Loi et al., 1995). Obviously, the FADs of these paradoxidid species are diachronous, although correlations based on other taxa allow estimates of their relative diachroneity. In Morocco the problem of the LMCt is more complex because it includes an overlap of typical lower Cambrian trilobites with middle Cambrian paradoxidids (a coincidence first pointed out by Neltner, 1938). The paradoxidids of Morocco seem to appear earlier in that area than in the platforms of Baltica and West Gondwana (Geyer and Landing, 2004). Although other bioevents have been selected to define other lower-middle Cambrian boundaries in the western Mediterranean region (such as a trilobite replacement from antatlasiine-strenuelline-saukiandine-dominated to protolenine-ellipsocephaline-dominated trilobites in Morocco, and a characteristic acritarch replacement in the Iberian Chains correlatable with Baltica; Geyer and Landing, 2004 and Palacios and Moczyd owska, 1998, respectively) in this paper we associate the LMCt with the paradoxidid immigration event recorded both in Baltica and the western Gondwanan margin. The aim of this paper is three-fold: (1) to update a synthesis of the major geodynamic and sedimentary factors that controlled the LMCt sedimentary record in the western Mediterranean region; (2) to locate these factors within the chrono- and event stratigraphic charts currently used in the l

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J. Javier Álvaro and Sébastien Clausen

Paleogeographic reconstruction of the Mediterranean region of the western Gondwana margin during Cambrian times (modified from Álvaro et al., 2000a). Fig. 1.

Cantabro–Iberian Basin (Carls, 1983; Álvaro et al., 2000a) (Fig. 1). Boundaries of the Iberian platform and determination of its shape and proximity to other neighboring platforms are key matters in discussions about improvements to early Paleozoic paleogeographic models. The original dimensions of the platform are unknown because of the lack of palinspastic reconstructions due to the interplay of Hercynian and Alpine deformations. In addition, the NW–SE trending structural framework of the Cambrian outcrops and the coincidence of the main Hercynian and Alpine directions of compression have favored a dominant trend of paleogeographic proximal-distal gradients: paleocurrents and basinward shift of facies are either SE–NW (parallel to the structural framework of outcrops) or NE–SW (transverse). As explained above, two lower-middle Cambrian boundaries have been proposed in the Iberian Chains based respectively on acritarchs and trilobites. The first boundary (Palacios and Moczyd owska, 1998) is located at the base of the Eliasum llaniscum+Celtiberium dedalinum Assemblage-zone, within the homogeneous and distal shales of the Daroca Formation, and ca. 60 m below the boundary based on trilobites. The latter was defined in the shale-dominated outcrops of the Valdemiedes Formation in the Murero 2. THE IBERIAN PLATFORM (IBERIAN CHAINS, Lagerstätte (Liñán et al., 1993a, b), which represents, from NE SPAIN) a paleogeographic point of view, a graben or tectonically The Iberian platform, reconstructed from Paleozoic out- induced depression (named Villafeliche graben by Álvaro crops of the Iberian Chains (NE Spain), is a part of the and Vennin, 1996a). This boundary has been subsequently region; and (3) to discuss if the disagreements envisaged in the chronostratigraphic correlation of the western Mediterranean stratigraphic charts are related to FAD diachroneities or based on different sedimentary and geodynamic conditions recorded in the platforms of the western Mediterranean region. In contrast to one chronostratigraphic proposal recently made by Geyer and Landing (2004) for the Cambrian of West Gondwana, any regional chronostratigraphic chart is selected preferentially here. The proper definition of a chronostratigraphic chart (both global and regional, e.g., Mediterranean, in character) must follow the ICS guidelines (Salvador, 1994; Remane et al., 1996; Murphy and Salvador, 1999; Remane, 2003) and take into account the presence of stratigraphic discontinuities, gaps and condensed levels. The selection of stratotypes should be made in complete sections with uniform sedimentation rates and monofacial lithologies. The vertical succession of the FAD of different species does not necessarily correspond to phylogenetic FADs, but in some cases to sedimentary and taphonomic biases. Both the Iberian and Moroccan Cambrian charts are used below: see Figure 2 for correlation.

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Chronostratigraphic correlation of the lower-middle Cambrian transition in the western Mediterranean region

Cambrian chronostratigraphic chart of the western Mediterranean region (modified from Álvaro et al., 2000a). Fig. 2.

recognized in other outcrops of the Iberian Chains that characterize different sedimentary conditions (Álvaro et al., 1993a; Gozalo et al., 1993; Álvaro, 1994), the Cantabrian Mountains and Sardinia (see below), and Turkey (Dean and Özgül, 1994).

2.1. Stratigraphic and Sedimentary Patterns The sedimentary and sequence trends involved in the LMCt of the Iberian Chains are recorded in the Daroca Formation and the Mesones Group (subdivided into the Valdemiedes,

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Mansilla and Murero Formations). Environmental interpretations of the LMCt are based on platform-scale lateral variations of facies associations constrained by trilobite-based zones. The early-Bilbilian Daroca Formation (90–250 m thick) consists of feldspathic to subfeldspathic graywackes, green shales and isolated conglomeratic layers. Álvaro and Vennin (1998) recognized three environmentally significant facies associations displaying a broad progradational trend: (1) parallel to low-angle cross-laminated and plane-bedded sandstones interrupted by trough cross-stratified beds interpreted as deposited in foreshore and beach environments episodically interrupted by foreshore ridges and nearshore bars; (2) tabular cross-bedded sandstones, conglomerate channels and shales representing nearshore/shallow-marine environments; and (3) shales with wave- and hummocky-cross-laminated siltstones deposited in offshore areas. The overlying Valdemiedes Formation, in which the LMCt is located, consists of an alternation of green marly shales and carbonates, 20–150 m thick. The carbonate intercalations are centimeter- to decimeter-thick beds of white limestones and yellow dolostones, stromatolitic and bioclastic in character. Álvaro and Vennin (1997) distinguished six major facies associations, from proximal to distal: (1) alternating stromatolitic limestones and shales, interpreted as deposited in restricted peritidal environments with episodic development of centimeter-thick microbial boundstones (Fig. 3A); (2) alternating bioclastic limestones and shales that suggest deposition in open-sea low-energy, shallow subtidal environments under normal conditions of salinity; (3) crosslaminated, very fine-grained sandstones to coarse-grained siltstones interbedded with centimeter-thick shales, locally rich in wavy-to-lenticular structures (Fig. 3B) that were deposited as tabular sets, bearing monotypic concentrations

(A) Detail of a stromatolitic dolostone exhibiting crinkled structures and mudcracked ‘V’ sections likely related to subaerial exposure; upper Bilbilian Valdemiedes Formation, Mesones section. (B) Wavy-tolenticular textures in the Valdemiedes event; Valdemiedes Formation, Jarque section (scale bar=1 cm); top arrowed. (C) Low-angle, laminated sandstones truncating the coastal deposits of the lower-Bilbilian Daroca Formation; unconformity at the San Martin section. (D) Bioturbated sediments of the Valdemiedes event from the Jarque section (scale bar=5 cm). Fig. 3.

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(A). Field aspect of the LMCt on the Esla nappe between the localities of Crémenes and Valdoré: w, white bedded limestones; g, grey lenticular limestones; Be, ‘Beleño’ facies; Ba, ‘Barrios’ or ‘griotte’ facies; G, Genestosa Member. (B) Breccia limestone of the ‘Beleño’-facies base in the Crémenes section (scale=0.5 cm). (C) Encrinitic packstone with unsorted quartz grains, Los Villares Formation, Fuente Bernardo section (scale bar=0.5 cm). (D) Well-rounded, volcanigenic, quartz grains in a poorly sorted greywacke rich in undetermined skeletons; Los Villares Formation, Fuente Bernardo section (scale bar=0.5 cm). Fig. 4.

of either brachiopods or hyoliths, which represent episodes of migrating bedforms in a shoreface environment that recorded stressful ecological conditions (identified at the Valdemiedes event; see below); (4) storm-induced bioclastic limestones; and (5) open-platform offshore-dominated shales. During late Bilbilian times the Daroca prograding plain-coastal system was suddenly replaced by a mixed (carbonate-siliciclastic) platform (Valdemiedes Formation): terrigenous input was low, permitting the episodic development of widespread microbial colonies in peritidal environments. During this episode, some storm-induced limestones were episodically deposited in the central part of the platform; their distribution, both vertically and laterally was associated with stromatolitic mats, and suggests the record of tempestites that interrupted the restricted sedimentation of peritidal environments. In the Villafeliche graben, this interval is composed of deep- and open-sea shales; the paleogeographic position of this graben (surrounded by shallower deposits) suggests that the seafloor was becoming differentiated into a mosaic of topographic highs and lows. The Leonian Mansilla Formation, 10–70 m thick, consists of reddish to purple, alternating limestones and shales changing both upward and laterally into shales with carbonate nodules. The unit can be subdivided into three members, from bottom to top: (1) a basal stromatolitic dolostone (0–40 cm thick) with millimeter-scale intercalations of purple shales; (2) a middle member (up to 40 m thick) composed of alternating beds of shales and bioclastic limestones; and (3) an upper member (2–30 m thick) of shales with bioclastic carbonate nodules paralleling stratification. The second and third members represent the so-called Cambrian ‘griotte facies’ of southwestern Europe. The thicker limestone beds are dominated by packstone-to-wackestone tex-

tures whereas the thinner ones show packstone-to-mudstone textures including disarticulated to broken eocrinoids, carbonate- and phosphate-shelled brachiopods and trilobites and, locally, abundant siliceous sponge spicules and chancelloriid sclerites. Interbedded shales contain mainly trilobites, cinctan plates, hyoliths and linguliformean brachiopods. The thicker limestones represent a shoreface environment. They grade upward into an alternation of thinner carbonate and shale beds that indicate storm-influenced offshore conditions. This carbonate/shale alternation was caused by periodic fluctuations in carbonate productivity and/or supply of fine terrigenous sediments. As explained below this episodic carbonate productivity, which displays sharp lateral changes in thickness and facies types, took place on tectonically induced paleohighs. Finally, the Caesaraugustian-early Languedocian Murero Formation is a monotonous succession, 50–250 m thick, composed of green shales with scarce carbonate nodules paralleling stratification. The biota, diverse and well preserved, includes complete to disarticulated trilobites, cinctans, calcite- and phosphate-shell brachiopods and hyoliths, rarer nonmineralized fossils, and complete sponges and eocrinoids. The fossil record is preserved as alternating storm-induced coquinas and pavements and sparse undisturbed fossiliferous offshore sediments.

2.2. Benthic Community Replacements As summarized above, after an early Bilbilian progradation of coastal systems, the late Bilbilian terrigenous input sharply decreased, permitting the episodic development of stromatolitic mats in peritidal environments. Some areas display bioclastic limestones developed under the influence


of storms. These limestones are rich in trilobites, calciteshelled brachiopods, hexactinellid and demospongean sponges, chancelloriid sclerites, and echinoderm ossicles (Álvaro and Vennin, 1996b). They indicate distal development of chancelloriid-echinoderm-sponge meadows (the CES non-reefal benthic community). This community is preserved as disarticulated to broken coquinas and pavements influenced by the action of waves, storms, and burrowing. Some silicified limestones rich in thecal and basal ossicles and columnals of eocrinoids indicate widespread development of pelmatozoan eocrinoid mat-stickers inserted in soft substrates partly stabilized by microbial films (Clausen, 2003). The CES progressively spread across the LMCt on the Iberian platform, whereas the stromatolitic boundstones were restricted to more proximal areas, disappearing finally at the base of the second member of the Mansilla Formation (Álvaro and Vennin, 2001). The CES continued developing over tectonically induced paleohorsts during late Leonian times (Mansilla Formation). By contrast, depressed areas recorded deposition of shale substrates rich in trilobite-dominated communities, suggesting a sharp decrease in carbonate productivity. The biotic components of the CES show a high paleontological diversity, with strong dominance of suspension feeders, such as eocrinoids and other echinoderms, hexactinellid and demospongean sponges, and less abundant chancelloriids, and phosphate- and calcite-shelled brachiopods. Trilobites and hyoliths occupied the mobile carnivorous, deposit-feeding and grazing niches in the CES, where infaunal metazoans do not seem to be abundant, although they can be recognized by development of burrowed limestones. High-energy pulses caused extensive winnowing, and provided a firm, shelly or hard substrate necessary for attachment of encrusting microepibenthic suspension-feeders composed of agglutinated foraminifera (psammosphaerids) and serpulid tubes (Clausen and Álvaro, 2002). These organisms exploited parasitic and/or commensal ecological niches. Trilobite-dominated communities are extremely fossiliferous and occur in the monotonous shales of the Villafeliche graben (Valdemiedes Formation) and the Murero Formation. The shales were deposited under open-platform conditions, with a very low topographic gradient, and represent a uniform, offshore regime over a large area that buried the paleotopography. The biota includes complete to disarticulated, miomerid and polymerid trilobites, echinoderms (rare eocrinoids and abundant cinctans), calcite- and phosphate-shelled brachiopods, hyoliths, nonmineralized metazoans, sponges, and chancelloriids, among others. Skeletons occur as articulated multielement skeletons, or as lenticular to discontinuous shell pavements reflecting alternation of enriched shelly debris, and sparse undisturbed fossiliferous sediments. Two rates of deposition are indicated: a background or continuous accumulation below fair-weather conditions, and an episodic storm-induced deposition. The reduced rate of

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background sedimentation favored the colonization of the muddy bottom by an epifaunal and infaunal community dominated by deposit-feeders and carnivores. The scarcity of framebuilding organisms and the lack of archaeocyaths across the LMCt in the Iberian platform seems to be directly related to (1) a high terrigenous input during the early Bilbilian that inhibited carbonate productivity, and (2) the widespread development of peritidal environments (Valdemiedes Formation) with episodic growth of stromatolitic mats under stressful (restricted) paleoecological conditions. In the more proximal quiet environments of the Iberian platform, stromatolites persisted during early Leonian times and formed centimeter-scale microbial boundstones (youngest relics preserved in the lower member of the Mansilla Formation). They ended their development as a result of a tectonic breakdown phase of the platform associated with development of a paleotopography and record of the ‘griotte’ facies.

2.3. Event and Cyclostratigraphic Patterns across the Valdemiedes Event In the Iberian platform a lower Bilbilian shoreline system was composed of shoreface environments sandwiched between proximal coastal sandflat and distal storm-dominated environments. After maximum progradation the resulting depositional system was capped by an erosive unconformity (D1a; Fig. 3C), recognizable by a truncating surface that can be flat or gently curved to trough-like where a significant hiatus is not indicated. As a whole, the Daroca Formation underlying D1a forms a third-order highstand systems tract (HST) topped by a major unconformity only recorded in the more proximal outcrops of the Iberian platform (Álvaro and Vennin, 1998). The sedimentary conditions across the overlying LMCt (Valdemiedes and Mansilla Formations) were controlled by the interplay of two variable factors: an episodic tectonic activity and a cyclic Milankovitch-like orbital forcing. A quantitative analysis of the tectonically induced subsidence recorded in the Iberian platform (Álvaro and Vennin, 1996a) revealed that two main geodynamic disturbances occurred at the times of the Bilbilian-Leonian and middle-upper Caesaraugustian transitions, whereas other minor pulses were only recorded on some parts of the platform. The successive tectonically induced disturbances produced a major rearrangement in the intraplatform patterns of differential subsidence, in which the Villafeliche graben can be considered as the hinge point of geodynamic rearrangement. In addition, a multifold cyclicity has been reported from the outcrops of the Valdemiedes Formation related to peritidal environments and to those of the Mansilla Formation associated with tectonically induced paleohorsts. Spectral analyses of the high-frequency cycle-stacking patterns offer an interplay of a two-fold hierarchy, which results in a 1.44/1

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ratio between the two significant peaks (Álvaro et al., 2000c). Although it is not possible to compare these spectra with the geochronological time scale, as Cambrian numerical ages are poorly constrained in the Iberian Chains, after assuming that a small-scale sedimentary cycle corresponds to the precessional cycle (8.7 couplets/cycle), the second significant peak may correspond to the obliquity cycle with a period of 12.5 couplets/cycle, according to the obliquity/ precessional ratio predicted by Berger and Loutre (1994) for early Paleozoic time. As a result, the carbonate/shale couplets reflect, at least partly, periodic fluctuations in supply of terrigenous sediments and carbonate productivity. Biogenic carbonate production was inhibited and diluted by fluctuating input of the detrital phase, probably driven by climate cycles. During dry episodes, when continental input was at a minimum, carbonate sedimentation would dominate on the platform, whereas during wetter episodes, continental runoff would greatly increase, resulting in deposition of fine-grained siliciclastic sediments. In the Valdemiedes Formation the superposition of small-scale, shallowing-upward peritidal cycles forms medium-scale cycles, in which the upper parts of stacked cycles become richer in stromatolitic limestones, at the expense of the shale parts. This peritidal multifold cyclicity forms as a whole one aggradational depositional sequence. The peritidal Iberian platform is characterized by low energy settings on the inner platform, very low slope gradients, low rates of sea-level rise allowing upward-shallowing by sediments to fill the available accommodation space, and absence of omission surfaces. In contrast, the multifold cyclicity recognized in the Mansilla Formation is arranged into medium-scale, deepening-upward cycles, beginning with mainly carbonate deposits and passing upward to mainly fine-grained siliciclastic deposits, leading to the final flooding of the platform and burial of the paleotopography by the Murero shales. The Mansilla and Murero Formations represent a transgressive systems tract (TST) in which the setting of the maximum flooding surface is poorly constrained due to the uniform character of the Murero shales. Eustatic sea level changes and subsidence controlled the accommodation space (low-frequency, medium- and largescale cycles), whereas high-frequency fluctuations (smallscale cycles) were mainly controlled by carbonate productivity and terrigenous input. Tectonic pulses in the Iberian platform had lower frequencies than obliquity and precession cycles, and could mask the expression of eccentricity cycles. If the LMCt was a tectonically active episode in the Iberian platform, if the record includes climatically enhanced astronomical forcing, and if there were no significant benthic replacement (in fact, both the CES and stromatolitic communities crossed the series boundary), what is the meaning of the Valdemiedes event (whose top is the series boundary; Liñán et al., 1993a)? The marker bed is 0.4 to 5

m thick, characterized by an increase in terrigenous material in both siliciclastic and carbonate substrates, and contains numerous examples of monospecific ‘crisis taxa’ present in scattered coquinas (e.g., coquinas of brachiopods, hyoliths, ichnofossil concentrations, etc.; Fig. 3C) reflecting elevated environmental stresses. Serpulids experienced a population bloom during the ecosystem recovery interval after the community turnover directly overlying the Valdemiedes event, and disappeared quickly during the ensuing return to background conditions. Psammosphaerid foraminiferans were relatively abundant across the event (Clausen and Álvaro, 2002). As the top of the event defines the Bilbilian-Leonian boundary, the event may coincide in time with the tectonic breakdown of the Iberian platform documented above. During the succeeding A. mureroensis Chron the northeastern (distal) area of the Iberian platform displayed a widespread colonization of the CES, whereas in southern (proximal) areas, deposition of stromatolitic mats continued into this interval, and shale units still predominated in the Villafeliche graben. Assemblages of protolenid and ellipsocephalid trilobites were replaced in stepwise fashion by relatively cosmopolitan assemblages, dominated in order of appearance, by paradoxidids, conocoryphids, and solenopleurids (Álvaro et al., 1999). As a result, the Valdemiedes event characterizes a protolenid-to-paradoxidid trilobite replacement associated with the immigration of relatively cosmopolitan trilobite taxa, but is not related to any significant benthic community turnover as the CES and stromatolitic mats crossed the series boundary. The record of the Valdemiedes event is currently recognized only in the Iberian Chains.

3. THE CANTABRIAN PLATFORM (CANTABRIAN MOUNTAINS, NORTHERN SPAIN) The Cantabrian platform constitutes another subdivision of the Cantabro-Iberian Basin. Its history is reconstructed from Paleozoic outcrops of the Cantabrian Zone (Fig. 1). This section summarizes studies of the LMCt in the Esla nappe, which occurs on the southeasternmost part of the foreland thrust and fold belt area of the Cantabrian Zone. This nappe has been selected due to the presence of the youngest archaeocyathan-microbial reefs in the Iberian Peninsula.

3.1. Stratigraphic and Sedimentary Features Zamarreño (1972) established a two-fold subdivision of the Láncara Formation. The lower member (100–225 m thick) is composed of yellow dolostones, commonly rich in ooids, fenestrae, pellets, and stromatolites. In the Esla nappe, archaeocyathan-microbial patch reefs are also present. The upper member (1–40 m thick) consists of gray and pink bioclastic limestones rich in glauconite (‘Beleño facies;’ Zama-


rreño, 1972), overlain by an alternation of reddish to purple nodular limestones and shales, 0–30 m thick (Cambrian ‘griotte’ or ‘Barrios facies;’ Zamarreño, 1972). The archaeocyathan fauna of the Esla nappe is Bilbilian in age (Perejón and Moreno-Eiris, 2003), which is considered equivalent to the Siberian Toyonian Stage (Spizharski et al., 1986). By contrast, the base of the upper member of the Láncara Formation contains the trilobite guide fossil Acadoparadoxides mureroensis (Álvaro et al., 1993b). Álvaro et al. (2000b) recognized nine major sedimentary facies associations across the LMCt on the Esla nappe. They are subdivided by two discontinuities into three paleogeographic suites (Fig. 4A), from bottom to top: (1) a white bedded limestone suite with fenestral, peloidal, and microbial grainstones (representing peritidal environments with common subaerial exposure surfaces), ooidal grainstones (high-energy shoals that reworked the allochems of the previous peritidal facies association), and distal bioclastic wackestones to mudstones deposited under offshore, essentially calm-water conditions; (2) a Bilbilian gray lenticular limestone suite comprising sandy prograding and amalgamating channels composed of ooidal to bioclastic grainstones (prograding ooidal and bioclastic shoals on an open-marine platform suggesting deposition by storms and waves from upper shoreface to foreshore, and back-shoal archaeocyathan-microbial reefs); and (3) a Leonian (‘Beleño’) pink encrinitic limestone suite deposited in relatively high energy, subtidal environments characterized by the development of low relief, bioclastic shoals. Finally, a mixed (siliciclastic-carbonate) unit was deposited on tectonically induced paleohighs and lows over the platform. It represents a major breakdown phase of the Cantabrian platform. The final drowning of the platform is characterized by deposition of offshore green shales (Oville Formation), as previously documented by Zamarreño (1972) and Aramburu et al. (1992).

3.2. Benthic Community Replacements The regional unconformity located at the top of the ooidal-bioclastic shoals that crop out in the Esla nappe is the contact between the lower and upper members of the Láncara Formation and the Iberian lower-middle Cambrian boundary. The Bilbilian sedimentary rocks represent a ramp with ooidal-bioclastic shoals that favored the development of protected archaeocyathan-microbial reefs. The shoals have yielded abundant debris of tube-shelled fossils, such as hyoliths (abundant in thin-section but incompletely phosphatized, so they are badly preserved after etching) and hyolithelminths. Above the erosive unconformity, a different benthic community developed, as recorded in the overlying glauconitic limestones. It is associated with development of widespread low relief bioclastic shoals. Their lowermost part is rich in hyoliths and heteractinid and hexactinellid

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sponge spicules, chancelloriid sclerites, cambroclavids, and probable eoconchariids, and sclerites of uncertain affinity. Although both bioclastic shoals represent similar highenergy conditions, the Bilbilian–Leonian boundary is marked by a drastic replacement of microfossil communities, with a change from a hyolith-hyolithelminth community to CES meadows dominated by suspension feeders (spiculate sponges, chancelloriids, calcite-shelled brachiopods, and echinoderms). Sediment-feeders, carnivores and grazers are represented by cambroclavids, eoconchariids?, hyoliths, and trilobites. This community replacement is not only represented by an increase in diversity, but also by an increase in the number of preserved individuals. Obviously, interpretation of community replacement is taphonomically biased, as originally phosphatic and secondarily phosphatized microfossils are only available after etching.

3.3. Stratigraphic Discontinuities and Sequence Framework The LMCt of the Esla platform can be divided into two sequence intervals: (1) a lower highstand systems tract that comprises both the white bedded and gray lenticular limestone suites separated by a distinct discontinuity (D1b), and the entire HST topped by a major unconformity (D2b); and (2) an upper transgressive systems tract (TST). D1b is recognized as an erosive contact and marks a sharp change from a peritidal homoclinal ramp to a ramp with ooidal and bioclastic shoals. A tectonic origin is proposed for this surface due to (1) the absence of D1b and the overlying gray lenticular limestones in other areas of the Cantabrian Zone; (2) the sharp increase in accommodation space; and (3) the development of higher slopes in the intrashelf ramp due to the distinct progradation of shoreface-to-foreshore shoals and sandy channels (Álvaro et al., 2000b). A second large-scale, shallowing-upward trend characterizes the upper part of the lower Láncara Member: the succession from D1b to D2b is interpreted as a progradational, shallowing-upward sequence reflecting a sharp increase of accommodation space, possibly tectonically enhanced. At the end of this episode, a sandy prograding channel wedge of suspected delta affinity caps the previous sequences in the more proximal outcrops. This wedge extended seaward cutting the underlying facies. A second discontinuity (D2b) is placed at the top of the gray lenticular limestone and is the contact between the lower and upper members of the Láncara Formation. This contact can be traced throughout the Esla nappe and the entire Cantabrian Zone (Zamarreño, 1972), and shows clear evidence of erosion. A discontinuous ferruginous level or hardground (up to 3 cm thick) marks the boundary in the Esla nappe. However, the latter level is commonly eroded and its reworked clasts are present in the first centimeters of the overlying pink glauconitic limestones. Immediately overlying D2b, an accumulation of glauconitic pellets is widely

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present. Also present are reworked breccia lenses from the underlying white bedded and gray lenticular limestones. D2b marks the Bilbilian–Leonian boundary and is interpreted as a major unconformity because of the sharp erosive contact and reworking of underlying and previously cemented facies. The early Leonian slow rise in relative sea-level combined with a moderate subsidence, leading to the creation of new accommodation space, controlled the formation of widespread low relief, bioclastic shoals. During most of the Leonian stage, shallow water, high energy conditions prevailed, where storm-induced processes are indicated by erosive truncations. The lack of a significant, vertical tendency in the pink encrinites allows us to envisage an aggradational trend for the whole sedimentary succession between discontinuities D2b and D3b, the latter one representing a tectonically induced discontinuity. D3b is placed at the base of the ‘Barrios’ facies. It marks a major tectonic pulse and the input of fine-grained siliciclastics. In some areas, this contact is not sharp but gradual. D3b is a diachronous boundary, progressively younger from SE to NW as documented by biostratigraphic relationships in some areas of the Cantabrian Zone (Sdzuy, 1968; Sdzuy and Liñán, 1993). D3b marks the end of the aggradational trend and the beginning of platform drowning. The rhythmic sedimentation of the ‘Barrios’ facies represents a gradual deepening of the platforms culminating with flooding of the platform and deposition of fine-grained terrigenous material.

4. THE CORDOBA PLATFORM (OSSA-MORENA, SOUTHERN IBERIAN PENINSULA) Despite the lack of Bilbilian trilobites and the guide fossil trilobite A. mureroensis, the LMCt is roughly constrained on the Córdoba platform by lower Cambrian archaeocyaths and assemblages of earliest middle Cambrian trilobites and acritarchs. The importance of this area for study of the LMCt is related to its special sedimentary conditions, characterized by an episodic development of volcanic activity geochemically similar to those recorded in Morocco (Ait Ayad et al., 1998). This volcanic activity reflects rifting processes (Liñán and Quesada, 1990) developed under extensional conditions that controlled remarkable facies and thickness variations. The area summarized below is located in the CórdobaAlanís tectonostratigraphic domain, which represents a paleogeographic subdivision of the Córdoba platform (Fig. 1). The LMCt is located overlying a siliciclastic unit, named the Castellar Formation, a barren interval composed of sandstones and conglomerates. The LMCt lies within the lower part of the fine-grained siliciclastic Los Villares Formation (Liñán et al., 1995). The formation (more than 300 m thick) is composed of green shales with interbedded sandstones and conglomerates, partly volcaniclastic in ori-

gin. The top of the Los Villares Formation is unknown as current faults occur in the area of definition. Only the lower member of the Los Villares Formation (ca. 70 m thick) is described below. Some interbedded, centimeter- to decimeter-thick encrinitic packstones occur episodically in the lower member and contain common echinoderm ossicles, trilobites and brachiopods (Fig. 4C). The member is bounded at the top by millimeter-thick alternations of claystones and siltstones. The sedimentary environment represented by these noncyclic sediments was interpreted by Liñán et al. (1995) as offshore and episodically subjected to the action of storms and turbidity currents. The mixing of allochthonous (fossils, polyphasic and intraformational clasts) and autochthonous (fossils) components was due to turbidity currents. Sedimentation took place close to a slope whose episodic turbidite currents (recorded as microconglomeratic levels) interrupted background clay sedimentation. The abundance of dispersed monocrystalline grains of quartz, well rounded, with unit extinction and relatively large size evinces a volcanic influence (Fig. 4D).

5. THE SOUSS BASIN (MOROCCAN ATLAS) The axis of the late Proterozoic to early Paleozoic Souss Basin roughly coincides with the modern trend of the AntiAtlas (SW–NE), in which lateral facies changes throughout the Cambrian successions reflect the eastern setting of proximal areas (Destombes et al., 1985).

5.1. Stratigraphic and Sedimentary Features As the guide fossil trilobites of the early-middle Cambrian transition in Morocco (Hupeolenus termierelloides) and the Iberian Peninsula (Acadoparadoxides mureroensis) occur both in Morocco and Spain, both species can be used as key fossils for regional correlation. They appear in the Moroccan Atlas across the Asrir and laterally equivalent formations (‘grès terminaux’) and the Jbel Wawrmast Formation, which are documented below. The Asrir Formation (30–180 m thick) is a heterolithic succession of sandstones, conglomerates, and tuffaceous strata recognizable along the margin of the Anti-Atlas and in the High Atlas. A major episode of volcanic activity took place during deposition of this unit (Choubert and FaureMuret, 1956), with a source likely lying in the western High Atlas (Boudda et al., 1979; Buggish and Siegert, 1988). The Asrir Formation and its laterally equivalent formations (see nomenclature and characteristics in Geyer, 1990b) have been interpreted as deposited under the influence of a prograding delta front complex with littoral and lagoonal intervals (Siegert, 1986; Bernekert and Geyer, 1990). The overlying Micmacca Breccia (5–26 m thick in the Lemdad syncline), proposed as a member of both the Jbel


Wawrmast and Tamanart Formations (Geyer, 1990b), is composed of volcaniclastic and bioclastic, bedded and nodular limestones that alternate with shales and sandstones, in some cases tuffaceous in character. Thin (up to 5 cm thick) volcanic ashes (frequently K-bentonites in the High Atlas, central and western Anti-Atlas, and Jbel Sahro) occur interbedded, and pillowed flows lie locally at the base of the member in the Jbel Sarho (Viland, 1972; Boudda and Choubert, 1972; Destombes et al., 1985). The member crops out from the eastern Adrar n’Dren massif of the High Atlas to the western Jbel Sarho and El Graara massif. This volcanism is associated with an unconformable onlap of the Jbel Wawrmast shales on the Jbel Sarho and eastern High Atlas (Hupé, 1955), subaerial exposures and erosion of the platform (Boudda et al., 1979: p. 75), and epeirogenic basin reorganization (Geyer and Landing, 1995). Finally, the upper part of the Jbel Wawrmast Formation (100–300 m thick), a possible senior synonym of the Tamanart Formation (Geyer et al., 1995: p. 31), consists of finegrained sandstones and green (and minor purple) shales, with isolated ash intercalations and storm-induced bioclastic limestone beds and nodules. Álvaro (2002) reported a selection of three facies associations and sedimentary patterns recorded in the Micmacca Breccia of the Lemdad syncline (in the Lemdad syncline there is exposed an archaeocyathan-microbial reef complex named the ‘bioherme de la cascade’ underlying the Micmacca Breccia): (1) clast-supported, polymictic microconglomerates deposited as volcaniclastic channels and shoals with clasts derived from laterally related pillow lavas; (2) bioclastic-intra/extraclastic packstones, grainstones, and breccias, interbedded with shales displaying erosive and irregular, intrabedded surfaces and lower and upper contacts, and derived from washing and reworking of bioclasts and other allochems from the surrounding seafloor, generating low-angle shoals under high-energy conditions; and (3) offshore-related green and purple shales containing interbedded tuffs.

5.2. Biodivesity and Geodynamic Patterns Sharp changes in lithology and facies across the Hupeolenus-K. arenosa zones (Fig. 2) observed in the Lemdad

syncline not only reflects active synsedimentary tectonism, but also indicates carbonate factories that became increasingly restricted and less productive with time. Carbonate factories were drastically transformed from productive reefrelated factories (‘bioherme de la cascade’) to weak, aerially restricted substrates (carbonates of the Micmacca Breccia), disappearing in the K. arenosa Chron. Related to this carbonate productivity, three major benthic communities succeeded across the LMCt in the Lemdad area. The first one (lithostratigraphically recognized as the Issafen Formation) is characterized by the installation of

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microbial-archaeocyathan reefs. The second benthic community, illustrated by the Micmacca Breccia, exhibits high diversity patterns, but is sharply different from the older reefal community: echinoderms (such as ceratocystids, ctenocystids, edrioasteroids, eocrinoidal thecal plates and columnals, and encrusting pelmatozoan holdfasts; Clausen, 2004) and trilobites dominate in volume, but chancelloriids, heteractinid and hexactinellid sponge spicules, brachiopods, and helcionellids occur in extreme abundance at many levels. The fossils are well preserved, always disarticulated but not necessarily fragmented, so that there are no signs of transport over long distances. The sediment resulted episodically in sheets of loose bioclastic sediment that covered the open seafloor. They were sites of accumulation and reworking of skeletons partially carried in from neighboring areas, and partially produced in situ from local assemblages. The limestones represent intensive current and storm washing and winnowing. The CES meadows were subjected to ‘highenergy’ conditions that destroyed them almost as quickly as they formed. The coarsest fraction of the skeletons was left in place or transported short distances, while the smallest grains were redistributed by currents across the platforms, with most moving downslope into deeper-water settings. Finally, the definitive flooding of the platform led to the establishment of a benthic community dominated by trilobites that colonized muddy substrates.

5.3. Major Stratigraphic Discontinuities Two major stratigraphic discontinuities are recognizable in the Lemdad syncline (Álvaro, 2002). D1c is an ironstone bed located in the uppermost part of the Asrir Formation and overlain by offshore shales, where hematite is chiefly present as thin hematite-impregnated litho- and bioclasts, and primary porosity was occluded with hematite crystals. This hardground level marks the succeeding flooding of the platform and the input of carbonate breccia levels from dispersed centers of carbonate productivity. The limestone intercalations of the Micmacca Breccia contain a mixture of skeletons, intraclasts, and volcanigenic extraclasts, some of them exhibiting polyphasic reworking. These bioclasts and allochems are in some cases covered and replaced by iron oxides. The selective precipitation of iron oxides (mainly hematite) suggests multiple depositional, cementation, and erosive phases, allowing differentiation between autochthonous (uncoated) and allochthnous/ polyphasic (secondarily coated and replaced) skeletons partly carried in from neighboring areas (Fig. 5A). These limestone strata record a stratigraphic condensation (compared with the laterally equivalent thickness of the center of the Souss Basin, e.g., the Tazlaft syncline; Geyer et al., 1995: p. 91), but do these limestones represent taphonomic condensation levels? The problem of the Lemdad syncline is to determine whether the trilobites that co-occur in a

10


(A) Volcaniclastic grainstone shoals with darker hematite-impregnated, reworked allochems and bioclasts; Lemdad syncline, High Atlas (scale bar=0.5 cm). (B) Karstic cavity infill by hematite-impregnated bioclasts surrounded by a bioclastic-extraclastic grainstone; Lemdad syncline, High Atlas (scale bar=0.5 cm). (C) ‘Griotte’ alternations in the Ferrals-lesMontagnes section. (D) Birdseye-rich (fenestral) limestones of the top of the S. Giovanni Formation covered by the ‘griotte’-type Campo Pisano Formation; Corovau section; top arrowed (scale bar=4 cm); top arrowed. Fig. 5.

same limestone bed of the Micmacca Breccia are mixed (diachronous) or not because their last appearances (LADs) are incompletely known due to the lack of published stratigraphic ranges (only fossil lists and no Figures with stratigraphic ranges are documented in Geyer et al., 1995). As a result, the FAD of a biostratigraphically significant species is used to identify the trilobite-based zones of Geyer (1990a) but it is not possible, however, to establish if the possible taphonomic condensation recorded in these limestones is beyond the resolution capability of the trilobitebased zones. However, another taxon can offer a better resolution to solving this problem: the archaeocyaths. Buggish et al. (1978) mentioned the presence of reworked archaeocyathan fragments in these middle Cambrian limestones; the study of more material is in progress. In addition, the presence of pitted truncation surfaces at the top of the youngest breccia level of the Micmacca Breccia permits identification of a major regionally correlatable discontinuity (D . Millimeter-thick cavities are easily recognizable in cross section, and consist of a series of bowland funnel-shaped pits and vugs. Vugs and interconnected pores offer distinct cross-cutting relationships, truncating both skeletal grains and sparry cements (Fig. 5B). Microkarst voids were commonly filled by hematite and iron-coated skeletons, although some of them previously recorded microbial colonizations, composed of stromatolitic crusts. Contrasts in depositional environment and diagenetic history across pitted surfaces indicate that the truncated top of the Micmacca Breccia was subaerially exposed prior to goethite infill. The microkarst structures described at the top of the Micmacca Breccia reflect a major fall in relative sea-level resulting in dissolution. As this time span coincides with 2c)

shale deposition in other areas of the Souss Basin, this shallowing-upward tendency related to active volcaniclastic input topped by subaerial exposure and local karstification should be interpreted as a consequence of a forced regression related to epeirogenic uplift. The platform was subsequently drowned during the succeeding rise (Jbel Wawrmast shales), and subsequently accommodation space became available across the O. frequens-K. arenosa Chron transition. A major basin reorganization, which featured the development of local volcanic flows and deposition of numerous of K-bentonites was considered epeirogenic and not eustatic in origin by Geyer and Landing (1995), and is associated with episodic carbonate productivity, intervals of condensation and final subaerial exposure (discontinuity D ). Sequence stratigraphy concepts are difficult to apply in this case study considering the episodic input of great volcaniclastic volumes, tectonic activity, and the drastic subsidence that kept accommodation space associated with volcanism. Eruptions were followed by tilting and tectonically induced subsidence, so that the relative sea-level fluctuations reported above are not necessarily induced by eustasy, but by epeirogenic rebound related to volcanism. Subsidence was probably due to evacuation of voluminous magma chambers, isostatic adjustment due to thick lava pile, and regional extension. 2c

6. THE MONTAGNE-NOIRE PLATFORM (SOUTHERN FRANCE) To the north of the Cantabro-Ebroan Land area (Fig. 1), a mosaic of marine platforms is postulated, dispersed at present between the lower Paleozoic outcrops of the Pyrenees and the Montagne Noire (southern France). As the


Cambrian stratigraphic and biogeographic patterns of southwestern Sardinia reveal close similarities to the Montagne Noire this platform is represented as neighboring the Montagne Noire segment in paleogeographic reconstructions. The LMCt is not biostratigraphically constrained with precision in the southern Montagne Noire, but is interpolated across the Pont-de-Poussarou and La Tanque Formations, due to the presence of Marianian–earliest Bilbilian? and Caesaraugustian trilobites in the underlying Lastours and the overlying Coulouma Formations, respectively (see last synthesis in Vizcaïno and Álvaro, 1998). The Pont-dePoussarou Formation consists of massive white limestones (20–80 m thick), locally dolomitized, rich in bioclastic debris, which locally exhibit a gradual transition into the overlying La Tanque Formation. Its age is probably Bilbilian–Leonian but the FAD of A. mureroensis is not recognizable due to the important diagenetic processes recorded in the limestones. The overlying Leonian–lower Caesaraugustian La Tanque Formation is composed of a centimeterthick alternation of bioclastic limestones and shales (up to 60 m in thickness), characterized by reddish and purple colors (Fig. 5C). The formation represents the CES- bearing ‘griotte’ facies described in southwestern Europe across the LMCt. Cambrian outcrops are also known from both the Axial (central) Zone and the northern flank of the Montagne Noire. Although their lithostratigraphy is relatively well established based on lithologic correlations with the southern Montagne Noire, their lower Paleozoic chronostratigraphy is incompletely known due to their poor fossil record and the scarcity of radiometric ages. Numerous magmatic and plutonic phases are recorded in the lower Paleozoic (Guérangé-Lozes and Burg, 1990), but the Mont-Merdellou thrust (Lacaune Mountains) is the single tectonostratigraphic unit in which volcanic and ash beds, and tuffs occur sporadically interbedded across the laterally equivalent strata of the Pardailhan, Lastours and Coulouma formations. This volcanic episode is also recorded across the LMCt of the Moroccan Atlas and the Iberian Ossa-Morena Zone.

7. THE SARDINIAN PLATFORM As in the Cantabrian and West-Asturian Leonese Zones and the Montagne Noire, the LMCt is also recorded in a carbonate platform in SW Sardinia. The special paleogeographic setting of the Sardinian isolated platform has encouraged a Cambrian lithostratigraphic subdivision into proximal (platform-interior) and distal (platform-marginal) strata. As a result, the Marianian-Bilbilian (or Botoman-Toyonian) Gonnesa Group has recently been subdivided (Perejón et al., 2000) into the inner Sta. Barbara and S. Giovanni formations. Both of them change laterally into the marginal Planu-Sartu Formation. The Sta. Barbara Formation, 100–600 m thick,

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consists of peritidal carbonates rich in stromatolites and thrombolites, ooidal shoals and archaeocyathan-microbial reefs (Bechstädt and Boni, 1994). The overlying S. Giovanni Formation, 300–600 m thick, contains peloidal to intraclastic black limestones (Schledding, 1985) locally covered by sparry limestones (ceroid or ‘waxy’ limestones; Fig. 5D) that pass laterally into ooidal grainstones/packstones bearing archaeocyathan debris of Bilbilian–Toyonian age (Perejón et al., 2000). The slope-to-basin Planu Sartu Formation, 100–600 m thick, consists of millimeter- to centimeter-thick laminated limestones with common interbedded seismites, slumping, debris-flow and local megrabreccias. The top of the Gonnesa Group is considered as a major stratigraphic discontinuity in which Gandin (1987) has reported microkarstic features. The FAD of A. mureroensis (base of fossil assemblage CP1 of Loi et al., 1995) is located within the overlying ‘griotte’-type Campo Pisano Formation (15–80 m thick). Finally, the lower part of the Cabitza Formation (400–600 m thick) is an uniform, fine-grained siliciclastic succession of middlelate Cambrian and Tremadocian age.

8. CORRELATION PROBLEMS The cooling associated with the southward drifting of the western Gondwanan margin during Cambrian times can be considered as primarily responsible for (1) the north-eastward migration of the trilobite-based biodiversity center, and (2) the same migration displayed by the early Cambrian evaporitic precipitation and the early-middle Cambrian carbonate productivity (Álvaro et al., 2000a, 2003). However, although each platform has recorded its own relative sealevel fluctuations due to variable subsidence and sedimentation-rate patterns, the LMCt in the western Mediterranean region displays numerous similarities in sequence stratigraphy and setting of major unconformities. The LMCt is underlain by a common early Bilbilian regression and, after a diachronous breakdown of platforms that favored the record of the ‘griotte’ facies, is overlain by an uniform Caesaraugustian transgression that led to deposition of the green shales classically known as the Barrande’s (1846) primordial-fauna or Paradoxides Beds. Identifying and correlating the unconformities and stratigraphic gaps included within the stratigraphic interval sandwiched between the early Bilbilian regression and the late Leonian-Caesaraugustian transgression is more complicated. A regionally correlatable, circum-Iapetus regression (named the Hawke Bay event) has been defined in western Newfoundland during latest early Cambrian times (Palmer and James, 1980), also correlated and reported in the eastern United States (the Rome Formation of the Appalachians; Barnaby and Read, 1990), NW Scotland (the Salterella Grit; McKie, 1993), and northern Greenland (the Buen Formation; Ineson and Peel, 1997). An unconformity is pos-

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tulated in Baltoscandia above terminal lower Cambrian sedimentary rocks (see a recent synthesis in Ahlberg, 1998), in which a stratigraphic gap (correlated with the Hawke Bay event) extended through the earliest middle Cambrian. Other regressive trends related to similar time spans are recognized as a disconformity within the Toppigarre Shale in Spitzsbergen (Kidder and Swett, 1989), the Toyonian regression of the Siberian Platform (Rowland and Gangloff, 1988), the end-Comleyan regression of Wales (Brasier, 1985), and other discontinuities located around the LMCt of the northern China Platform (within the Mao Zhuang Formation; Meng and Tucker, 1992), the Georgina Basin in Australia (within the Beetle Creek Formation; Shergold and Brasier, 1986), and the Lesser Himalaya (Shah, 1992). In the western Mediterranean region an early Bilbilian regression (ca. Hupeolenus zone of Geyer, 1990a) was named the Daroca regression by Álvaro and Vennin (1998). The latter was recognized after progradational highstand systems tracts in the Daroca (Iberian Chains), Riocabado (Demanda Ranges), Los Cortijos (Galician-Castilian Zone), and the Spanish Castellar and Portuguese Barra Quartzítica (Ossa-Morena Zone) Formations, whereas peritidal-dominated carbonate productivity took place in the platforms preserved in the NW of the Iberian Peninsula, e.g. the Láncara and Vegadeo Formations of the Cantabrian and West-Asturian Leonese Zones, respectively. The correlation of the sequence framework overlying the above-summarized regression and the following middle Cambrian transgression is problematical due to the superimposed co-occurrence of geodynamic, volcanic, and tectonic processes (see Fig. 6). (1) In SW Sardinia, the middle Cambrian TST directly overlies the Lower Cambrian HST; (2) in the Iberian Chains an aggradational sequence (reflecting equilibrium between accommodation space and sedimentation rate) bounds the lower HST and the upper TST; (3) in the Esla nappe of the Cantabrian Mountains a similar tendency is recorded, but the HST is interrupted by a stratigraphic discontinuity (D2a) and the Bilbilian–Leonian boundary coincides with the succeeding erosive unconformity (D2b); and (4) in the Lemdad syncline of the High Atlas there is no record of this aggradational sequence. A significant epeirogenic adjustment, related to volcanic activity and episodic uplift of neighboring areas of the platform, recorded a forced regression (HST) and subsequent erosion of lithified sediments, influencing both stratigraphic and likely taphonomic condensation in the limestone strata of the Micmacca Breccia. All this was finally topped by a karstic surface related to subaerial exposure (D2c). Discontinuities D3a–c are easily recognisable in southwestern Europe as they mark the base of the ‘griotte’ facies and represent a tectonically induced contact reflecting the diachronous and progressive, tectonic breakdown of the platforms that bordered the western Gondwanan margin. The ‘griotte’ has been defined, from a lithostratigraphic point of

view, as the Mansilla Formation in the Iberian Chains, the ‘Barrios facies’ of the upper member of the Láncara Formation in the Cantabrian Mountains (Zamarreño, 1972; Aramburu et al., 1992), the La Tanque Formation in the southern Montagne Noire (Vizcaïno and Álvaro, 2001), and the Campo Pisano Formation in southwestern Sardinia (Loi et al., 1995). The formations overlying these ‘griotte’-type units are similar in lithology and sedimentary environments, consisting of fine-grained, transgressive sediments. They are identified as the Caesaraugustian–lower Languedocian Murero Formation in the Iberian Chains, the Genestosa Member in the Cantabrian Zone (Spain), the Coulouma Formation in the southern Montagne Noire, and the lower parts of the Cabitza Formation in Sardinia and the Jince Formation in Bohemia. A peak in diversity of trilobites and cinctans is associated with development of open-sea substrates brought in by this trangression, which coincided with the widest biogeographic distribution of trilobites characterizing an open connection from the Avalonian to the Mediterranean regions. 9. CONCLUSIONS

This paper offers an updated synthesis of the sedimentary and paleoecological processes recorded across the LMCt, an interval related here to the immigration of paradoxidid trilobites into a region of the western Gondwanan margin represented here by the Iberian Peninsula, the southern Montagne Noire (France), Sardinia and the Moroccan Atlas. Several geodynamic and sedimentary processes coincided in space and time, affecting directly the replacement of benthic communities. Although it is difficult to differentiate between ‘causes’ and ‘effects’ in some of them, as they are interrelated and acted more as feedbacks than as opposite factors, three orders of factors can be broadly distinguished: (1) The major influence in the western Gondwanan margin was, probably, its southward drifting leaving subtropical arid conditions and entering into temperate waters that affected directly the type of facies as well as the carbonate productivity. The LMCt is the hinge timespan to understand this mosaic of climatically induced processes. (2) Second-order factors can be separated into relative sea-level fluctuations and tectonic activity. A synthesis of the third-order sequences is approached in Figure 6, in which the LMCt reflects a composite regressive-(aggradational)- transgressive tendency, punctuated by at least one regional unconformity. The latter is diachronous in character as a result of the regional geodynamic (subsidence) and sedimentation rates, and varies in age from latest Bilbilian to the Bilbilian–Leonian transition. The unconformity is identified as local diastems in coastal deposits (Iberian Chains), erosive surfaces locally covered by breccia levels (Esla nappe), and a sharp lithological and facies shift (SW


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tC M L e h t f o s i s e h t n y s c i h p a r g i t a r t s t n e v e d n a e c n e u q e S . 6 . g i

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Sardinia). Both in the Córdoba platform (Ossa-Morena) and the southern Montagne Noire, the unconformity is not recognized: in the former, the stratigraphic succession has a distal and deep-basin character, recording a significant volcaniclastic input, whereas in the latter the special diagenetic conditions recorded across the Pont-de-Poussarou and La Tanque Formations do not allow us to understand properly its sedimentological significance. Finally, in the Lemdad syncline (High Atlas) the interaction of epeirogenic rebound associated with a volcanic episode favored influence of a forced regression where sediments are topped by a karstic surface (top of the youngest Micmacca-Breccia limestone), which represents the end of successive condensed and shallowing-upward trends. Numerous local-to-regional stratigraphic discontinuities are described in the text, indicating the coincidence of tectonic activity. This represents the episodic breakdown of the Mediterranean marine platforms, which favored local tilting, development of a characteristic facies for this time span (named ‘griotte’), and a major reorganization of the basins across the LMCt. The paleotopographies played a key role in both carbonate productivity, and the immigration and colonization of new benthic communities, because the relative paleohighs were episodically protected from the external influence of siliciclastic sediments. (3) A third-order factor can be related to the capacity of benthic communities to modify their ecosystems. This time span represents a major reorganization in bio cœnoses, changing from (a) communities rich in ooidal/ oncoidal shoals and archaeocyathan-microbial reef systems directly controlled by the activity of microbial colonies through; (b) the progressive reduction of carbonate productivity leading to hydrodynamic CES pavements that led to preservation of reworked skeletons in mixed (carbonate-siliciclastic) substrates; and finally to (c) the widespread establishment of muddy, high-turbidity substrates colonized by trilobites and cinctans during transgressive middle Cambrian episodes, alternating with impoverished siliciclastic shoals and barriers bearing linguliformean communities in regressive trends. As a consequence of these paleogeographic conditions, the trilobites exhibit distinct biogeographic patterns, in which the southward drifting of the western Gondwana margin is related to an opposite-directed migration of the center of the biodiversity peak, which migrated from Morocco to the Iberian Peninsula and the Montagne Noire across the LMCt. The transgressive episodes are associated with peaks in the geographic distribution of trilobites (relatively cosmopolitan trilobites during the lower-middle Caesaraugustian) punctuated by regressions that isolated the populations, which lost their reproductive communication.

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