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Microfacies and Diagenesis of an Upper Oxfordian Carbonate Buildup in Mydlniki (Cracow Area, Southern Poland). Jacek Matyszkiewicz and Ireneusz Felisiak, ...
FACIES

27 179-190

PI. 38-40

5 Figs.

ERLANGEN 1992

Microfacies and Diagenesis of an Upper Oxfordian Carbonate Buildup in Mydlniki (Cracow Area, Southern Poland) Jacek Matyszkiewicz and Ireneusz Felisiak, Cracow KEYWORDS: MICROFACIES- CARBONATEBUILDUP - TUBIPHYTESREEF - DIAGENESIS- SOUTHERN POLANDUPPER JURASSIC SUMMARY A carbonate buildup near the top of the Upper Jurassic limestone sequence in the Cracow area with a rigid frameworkbuilt of Tubiphytes and thrombolites, and some fragments of encrusted siliceous sponges and serpules is described. The limestones form a dome-like elevation at the eastern wall ofa 15 m high quarry flanked on both sides by stratified limestones with cherts. Six microfacies have been distinguished within the buildup: (1) Tubiphytes[ thrombolite boundstone and (2) bioclastic Tubiphytes/ thrombolite wackestone dominate in the central and bottom part of the buildup. They gradually replace the cyanobacterial crusts and siliceous sponges (3. sponge-algal boundstone), which are sporadically the rock-forming elements in the basal part of the buildup as well as the top. Serpules randomly distributed within the buildup also form small cm-sized structures with a rigid framework (4. serpula-peloid boundstone). (5) Tuberoid-peloid wackestone/floatstone and (6) ooid intraclastic grainstone exhibit no significant distributional pattern. Bioclasticpeloidal packstone comprising material derived from the destruction of the buildup occurs in the highest part of the outcrop, overlying the buildup. The sediments of the buildup were subject to rapid lithification, evidenced by borings and neptunian microdykes filled with internal sediments, as well as by fractured Tubiphytes. Numerous petrographic features indicate probable episodic emergence of the buildup during its growth; these include asymmetric dissolution textures, asymmetric cements, vadose crystal silt and calcite pseudomorphs after gypsum. Upper Oxfordian carbonate buildups in the Cracow area display various stages of evolution. The carbonate buildup in Mydlniki most closely resembles classical Upper Jurassic reefs. I INTRODUCTION Carbonate buildups are a characteristic feature of the Upper Jurassic limestones in the epicontinental seas

bordering the Tethyan Ocean. They were described in Europe from Portugal, Spain, France, Germany (Ktaa,P et al., 1990 with references), Poland (MArvszram~t~, 1989a, b, c, 1990; T~R, 1989; I-IELtASZ,1990) and Roumania. Up to the Idoceras planula time the Cracow area was occupied by a shallowing epicontinental sea open southwards to the Tethyan Ocean (Fig. 1). The massive and bedded limestones (DZtJLVNSra, 1952) were the main facies types in this area during the Middle and Late Oxfordian. The massive limestones are carbonate buildups with the rigid framework forming elevations within the basin. The internal construction of the buildups changed over time cyanobacteria becoming increasingly important (MArVSZraLWXCZ,1990): the initialspongecyanobacterial buildups evolved to cyanobacterial-sponge buildups, and these in turn to stromatolite mounds (sensu CREVELLO& HARm,1984). The maximum development of the cyanobacterial structures in the Cracow area is marked by the so-called Tubiphytes reefs. The carbonate buildup in Mydlniki was sampled by I. Felisiak in 1986 and 1990. It has been mentioned in several publications (Rtri~owsra, 1986, 1989; F~JsxAr, 1988; MAavszraEWICZ,1990). The buildup is an example ofa Tubiphytes reef, very similar to that described from the Swabian Jura (PoMo~a-PAPAiOgr,rNou et al., 1989). 2 GEOLOGICAL SETTING The Cracow area is situated at the boundary of three major geological structural units: the Silesian-Cracovian monocline, the Nida depression and the Carpathian foredeep (Fig. 2). The Mesozoic strata form a separate Early Alpine structural storey. The Upper Jurassic sediments are exposed along the SE margin of the Silesian-Cracovian monocline. The Oxfordian sediments in the Cracow area (up to 235 m thick) are underlain by thin Callovian strata. The presence of numerous ammonites penn itted a biostratigraphic subdivision of the Lower Oxfordian and the lower part of the Middle Oxfordian (TARKOWSra,1983). AS ammonites are scarse in the higher part of the section, the lithostratigraphical scheme of the Oxfordian sediments in the Cracow area (Fig. 3) is based on the vertical distance separating the strata from the top of the

Address: Dr. J. Matyszkiewicz, Dr. I. Felisiak, Institute of Geology and Mineral Resources, Academy of Mining and Metallurgy, AI. Mickiewicza 30, PI-30-059 Cracow, Poland

180 respond to the I. planula zone ( G ~ & WnntzBowsra, 1972). The Jurassic sequence is unconformably overlain by only locally preserved Cretaceous or Miocene strata. 3 FORM OF THE BUILDUP AND METHODS

The carbonate buildup is exposed at the eastern wall of a 15 m high quarry (Fig. 4; PI. 38/1). The lowermost strata at the base of the wall are horizontally bedded limestones with cherts. In the central part of the wall they are discontinously overlain by indistinct stratified limestones withoutcherts.These limestones form the top of the domelike elevation. To the NE and SW the buildup laterally grades into the bedded facies dipping away up to 30~. The beds are 0.3-1.8 m thick and are separated by bedding joints up to 1-2 cm wide. Samples from the outcrop were collected along vertical lines at intervals of ca. 1.5 m and at various points (Fig. 4). 48 thin sections were prepared. Fig. 1. Paleogeographic position of the Upper Jurassic facies in Europe. The area shown within the rectangle was investigated (after MATYJA,1976). CaUovian (Rtrmows~a, 1986). The biostratigraphic subdivision of the upper part of the prof'de (Fig. 3) is speculative. The sediments situated higher than 130-140 m above the bottom of the Oxfordian are considered Upper Oxfordian (Epipeltoceras bimammatum and ldoceras planula zones). Their lower and middle parts comprise mainly the facies described as the massive and bedded limestones (DzuLVNSm,1952), probably representing the E. bimammatum zone The sediments exposed in Mydlniki are situated about 200 m above the base of the Oxfordian and they belong to the youngest Jurassic strata exposed in the Cracow area. The strata vary strongly in lithology. Platy limestones (wackestone, packstone and grainstone) with intercalations of marls, and unstratified nodular limestones, as well as synsedimentary breccias occur along with the massive and bedded limestones. Carbonate turbidites (allodapic limestones; MArvszxmwlcz, 1989c, 1990) can be found in the upper part of the section. The youngest beds of of this sequence cor-

4 FOSSILS

The mesoscopically recognizable fossils: sponges, brachiopods, so-urchin spine and bryozoans fragments are sporadically observed. The fossils in thin sections include in order of frequencies: foraminifers, ostracods, Tubiphytes, echinoderms, polychaetes, siliceous and calcareous sponges, bryozoans, brachiopods, gastropods, ammonites and forms attributable to calcareous algae. Most frequent foraminifers are the encrusfing forms of Nubeculariidae (Nubeculinella, Nodophthalmidium). The specimens Nodophthalmidium are most commonly found within Tubiphytes cf. morronensis CRESCE~rn0al. 38/2, 3; FLt~GEL, 1981). The specimens NubeculineUa mainly encrust complex oncoids, as well as some thrombolites and Tubiphytes. The sediments contain Nodosariidae (Lenticulina), Involutinidae (Trocholina,P aalzowella),Lituolidae (Ammobaculites) and Hormosinidae (Rheophax). Distinctive foraminifers (Bullopora) occur within canals of unpreserved siliceous sponges. Textularia occurs within the rare ooid-intraclast grainstones. A rich fauna (Fig. 5) occurs in various microfacies types, but they are most frequent in the Tubiphytes/ thrombolite boundstones and bioclastic Tubiphytes/thrombolite wackestone. Thin and thick-shelled forms are sometimes covered with cyanobacterial crusts. The polychaetes are represented by" (1) serpules, mainly

Fig.2. Locationof the outcrop (geology after FEUsIAr., 1988). 1-pre-Jurassic strata, 2-Jurassic, 3-Cretaceous, 4Cretaceous and Palaeogene of the Carpathians, 5-Tertiary, 6-quarries, 7cliffs.

181 surrounding the buildups (Nrrzopota~os, 1974). Juvenile forms of ammonites, gastropods and probable calcareous algae (Globochaete sp. PI. 39/2) occur in sediments with asymmetricdissolution textures, asymmetric cements, vadose crystal silt and calcite pseudomorphs after gypsum (Fig. 5). 5 FACIES TYPES Seven microfacies types were found; six occur within the carbonate buildup and one represents the overlying sediment. The transition between the facies types is gradational. 5.1 Bounflstones 5.1.1 Tubiphytes/thrombolite boundstone (bindstoneframestone) (PI. 38/2, 4)

Fig. 3. Geological sequence of the Jurassic strata in the environs of Cracow (after M^'r,tsz~azw:cz, 1990). Arrow indicates the approximate position of the carbonate buildup in Mydlniki.

Serpula cf. gordialis and (2) agglutinated micritic tubes, referred to TerebeUa lapilloides (Kta~m~, 1985) or Thartarella sp. (JAr~S^et al., 1989). Sporadically the serpula are frame-builders (serpula-peloid boundstone; PI. 38/4). Single serpules and agglutinated forms of polychaetes occur in all microfacies. Tubiphytes (mainly Tubiphytes cf. morronensis CRESCENT:)and thrombolites are the main framebuilders (Tubiphytes/thrombolite boundstone; PI. 38/2, 4). Siliceous Hexactinellida and Lithistida and calcareous sponges acquire a rock-forming role in the lower part of the buildup, where they form (together with cyanobacterial crusts) typical sedimentary sequences (sponge-algal boundstone; G~n L~a~I),1983), common in the Oxfordian strata of the Cracow area ( M a ~ s ~ c z , 1989b, 1990). Bryozoans, brachiopods, and echinoderms can be observed in all microfacies, but are never rock-forming. Especially noteworthy is the presence of juvenile forms of ammonites in the buildup. Ammonites are usually not preserved in the cyanobacterial-sponge buildups (also in the Cracow area), though they can be found in the sediments

The dominant components (up to 60%) are thrombolites and Tubiphytes cf. morronensis, visible on fresh fractures as white elliptical or irregular spots, a few millimeters in size. Nodophtalmidium or Nubeculinella can be seen in some longitudinal sections of the Tubiphytes. Nodophtalmidium usually occupies the axial part of Tubiphytes, while the Nubeculinella usually encrusts the outer micritic envelopes. Structures corresponding to the composed oncolites (type IVC; DArtANAYAr~, 1977) are frequent. Some Tubiphytes display borings or fractures. Their walls are covered with isopachous cement. Thrombolites include irregular nonlaminated cyanobacterial structures, which tend to form slrongly branching forms (cf. Arrm~, 1967; ~-r,rNARD& JAMES,1986). Despite the fact that they occur in situ, these structures neither develop on the substrate or a sedimentary discontinuity (cf. MATYSZ~EWlCZ, 1989b) nor do they form typical domeshaped structures. The thrombolites bind Tubiphytes, thus producing a rigid framework in which the growth cavities are filled with vadose crystal silt or micritic mud (PI. 38/2). The walls of the cavities are lined with isopachous rim cement (PI. 40/4).

Fig. 4. Carbonate buildup in Mydlnikiwith location of samples.

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Fig. 5. Distribution and general characteristics of microfacies in the carbonate buildup in Mydlrtiki.Sample N exhibits characteristics of a probable emersion as asymmetric dissolution texture,asymmetric cernents,vadose crystal silts, and pseudomorphs of calcite after gypsum. Location of samples see Fig. 4. The space between the cyanobacterial structures is occupied by wackestone with numerous small foraminifers and fragments ofbryozoans, polychaetes, brachiopods, siliceous and calcareous sponges and echinoderms. Numerous ostracods occasionally encrust the thrombolites and Tubiphytes. An initial stage of colonization ofa detrital sediment surface (tuberoid-peloid wackestone) by Tubiphytesandthrombolites Plate Fig. 1. Fig. 2.

Fig. 3. Fig. 4.

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forming a rigid framework was observed in sample Z (P1.38/ 4). The boundary between tuberoid-peloid wackestone and Tubiphytes/thromboliteboundstone is accentuated atpresent by a stylolitic seam.Organogenic structures of unclear origin, probably related to bacteria (? Spherulites; cf. A~oR~ws, 1986), occur near the base of the Tubiphyteslthrombofite boundstone (PI. 39/3).

Carbonate buildup in Mydlniki (Upper Oxfordian, South Poland). General view and microfacies rich in Tubiphytes. Quarry in Mydlniki. General view of the wall with the carbonate buildup. Bioclastic Tubiphytes/thrombolite wackestone. Numerous cyanobacterial structures are visible, mainly Tubiphytes cf. morronensis, joined locally with irregulare shaped thrombolites. In the bottom center cyanobacterial structures form a fragment of rigid framework (Tubiphytes/thrombolite boundstone). The growth cavities are filled with internal sediments (arrows), and the cavity walls are lined with isopachous cement. Sample 11. x 8 Bioclastic Tubiphytes/thrombolite wackestone. In addition to Tubiphytes, thrombolites and bioclasts numerous tuberoids occur; abundant peloids in the matrix. Sample P. x 13 Tuberoid-peloid wackestone (lower part) passing upwards to Tubiphytes/thrombolite boundstone. The boundary between microfacies is accentuated by a stylolitic seam (arrows). Sample Z. x 9

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5.1.2 Sponge-algal boundstone (bindstone-framestone) The main components are cyanobacterial crusts (up to 50%) and calcified siliceous sponges (Hexactinellida and Lithistida). Cyanobacterial crusts consist of peloids (< 0. I ram). They also include ooids and microoncoids up to 0.2 mm in diameter. The cyanobacterial crusts in Mydlniki are similar to the peloidal crusts, common in the Upper Jurassic reef limestones of South England (Sty & WeacFrr, 1989), and typical of an initial framework, a low sedimentation rate,and a moderate energy environment. The cyanobacterial crusts are usually developed on the upper surfaces of siliceous sponges, where they form domeshaped structures occasionally encrusted by Nubecularia. At some places they were boreal, and the locally voids are filled with inversly graded pellet silt. The sponges are usually preserved as the Xalkmumien' (Fear-z, 1958). Some of them arc bored. Epifaunal bryozoans and polychaetes can be found on the undersides. Bullopora occurs sporadically within the sponge canals. Sporadic bioclasts (foraminiferms, echinoderms, polychaetes, brachiopods, Tubiphytes) are present within the micritic matrix. 5.1.3 Serpula-peloid boundstone (framestone) (Pl. 39/4) The dominant component is Serpula cf. gordialis growing in colonies up to 1.5 cm thick. The serpules occur together with peloidal crusts and both build a rigid framework. Numerous growth cavities are present within this structure; their walls are lined with isopachous rim cement, and filled with crystal silt (P1. 39/4). This type of internal sediment consists of fine (up to ca 0.015 mm) crystals of yellowish calcite and it lacks bioclasts similar to "vadose crystal silt" described by Dt.rNnAM(1%9). Zones of cementation with brown iron oxides are present in the upper parts of vadose crystal silt infillings. The peloidal crusts consists ofpeloids (up to 0.3 ram) and of aggregate grains. The texture is compact in places and thc peloidal crusts pass to micrite crusts.

5.2 Bioclastic Tubiphytes/thrombolite wackestone (Pl. 38/2-4; 39/1; 40/I, 2) This microfacies is a transitional type between the

Tubiphytes/thrombolite boundstone and the tuberoid-peloid wackestone/floatstone. It is the most common microfacies Plate Fig. 1.

Fig. 2. Fig. 3. Fig. 4 .

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in the carbonate buildup in Mydlniki. Bioclasts (mainly polychaetes, brachiopods, foraminifers, ostracods) form up to 20% of the rock. Fragments of gastropods,juvenile ammonites and questionable calcareous algae (Pl. 39/2) are less frequent. Some bioclasts are bored. Tuberoids (< 1 mm) comprise up to 10% of the rock, isolated intraclasts of similar size occur. Cyanobacterial structures (Tubiphytes, thrombolites) constitute up to 15% of the rock. The Tubiphytes occur in biopelmicrite matrix, either separated or connected with distinctive thrombolitic 'bridges' (LAN~, 1989), thus forming an initial rigid-framework. Typical growth cavities are present within these structures, filled with geopetal vadose crystal silt or micritic mud (PI. 39/1 ;40/1,2). The internal sediments fill borings and small neptunian dykes (P1.40/2). Calcite pseudomorps after gypsum were found in sample N within the geopctal vadose crystal silt filling voids (PI. 40/ 1,5-7). Asymmetric dissolution textures can be found in the lower parts of the tuberoids in sample N (PI. 40/1). The thin section from sample N reveal a variety of cements (Fig. 5). In addition to blocky and isopachous cements, common in all microfacies types, an asymmetric cement (PI. 40/5), a dog-tooth and a questionable meniscus cement were found.

5.3 Tuberoid-peloid wackestone/fioatstone (Pl. 38/4; 40/1) The main component are peloids (0.2-0.3 mm) comprising up to 40% of the rock volume. Occasional microoncoids and aggregate grains accompany the peloids. Tuberoids (up to 20% of the rock) with an average diameter of 0.6 mm are important. In sample F 15% of the grains were larger than 2 mm (floatstone). The tuberoids are usually poorly sorted; smaller grains (0.6 mm) are better rounded, and the larger ones are bored. Tuberoids can be mainly derived from the disintegration of Hexactinellida and Lithistida. Tubiphytes are sporadically present. Sponge spicules, brachiopods, echinoderms, foraminifers, polychaetes, ostracods and rare calcitic spheres of unknown origin occur.

5.40oid-intraclast grainstone (PI. 40/3) This microfacies occurs sporadically in various parts of the buildup. It is composed ofooids, intraclasts, microoncoids and peloids. The diameters of the ooids are usually 0.2-0.3

Carbonate buildup in Mydlniki (Upper Oxfordian, South Poland). Biota Coryozoans, serpules, probable algal structures) and microproblematicum. Bioclastic Tubiphytes/thrombolite wackestone. Note the abundance of bioclasts (bryozoans, serpules, brachiopods). Numerous voids with geopetal fills (arrows) are due to the activity of boring organisms. Sample F. x l 3 Fragment of calcareous algae (? Globochaete sp.). Bioclastic Tubiphytes/thrombolite wackestone. Sample N. xl00 Microproblematicum (bacterial structures type of spherulites?). Tubiphytes/thrombolite boundstone. Sample Z. xS0 Serpula-peloid boundstone. Colonies of Serpula cf. gordiatis form the rigid framework. Growth cavities are filled with vadose crystal silt; zones of Fe-oxide cementation (arrows) are visible at the top of the fill. Sample M. x l 3

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mm, up to 0.5 mm. The cortex layers are indistinct, often completely micritized. Some grains have only one micritic envelope (microoncoids). Intraclasts 0.2-0.4 mm in diameter are composed of pelmicrite (wackestone). The transitions between the ooids, microoncoids and peloids are gradational due to advanced micritization of the grains. Most grains are rimmed by isopachous cement. The relicts of the intergranular pores space are filled with blocky calcite cement. The scarce fauna is represented by single polychaetes Terebella cf. lapilloides, ostracods, foraminifers, fragments o fpelecypods and brachiopods.

5.5 Bioclastic-peloid packstone This microfacies is strictly limited to the topmost part of the outcrop. The main components are peloids (0.2-0.3 mm) and small (0.5 mm) bioclasts, mainly calcified spicules of siliceous sponges and fragments of planctonic Saccocoma. The fragments of Saccocoma are covered with syntaxial cement. The components are well sorted. 6 DIAGENESIS 6.1 Cements Six main types of cements can be observed in the buildup studied (Fig. 5). 1. Isopachous cement (P1.40/3,4) as rims up to 0.2 mm thick on tuberoids, intraclasts and ooids. Very thin (up to 0,1 mm) cortices of isopachous cement also occur on walls of growth cavities and neptunian microdykes. The isopachous cement is locally strongly recrystallized and it grades into granular cement. Relics of primary rim cements with needle textures are sporadically visible in the granular rims. The isopachous cement is generally interpreted as typical of a marine phreatic environment (P~zBIr,VOOWSKI,1985). Recrystallization of the isopachous cement and its transition to the granular cement-rims can occur in an early diagenetic

Plate Fig. 1.

Fig. 2. Fig. 3.

Fig. 4. Fig. 5.

Fig. 6. Fig. 7.

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freshwater phreatic environment (PoMoNI-PAPAIOA,~OUet al., 1989) and also in late stage cementation under burial conditions. 2. Blocky calcite cement (PI. 40/3) consists of large irregular crystals which tend to widen towards the center of the void. It also fills ther largest interparticles pores or recrystallized bioclasts (mainly gastropods) as well as the upper parts of any voids filled with geopetal internal sediments. Under meteoric conditions the aragonitic shells of gastropods were partially dissolved and filled with blocky calcite cement. This type of cement forms in the active freshwater phreatic zone (LoucKs, 1977; LONGMAN,1980), or under burial condition (PP,EZne~oowsr,J, 1985). 3. Syntaxial overgrowth is represented by large (up to 0.5 ram) crystals growing coaxially on echinoderm fragments. This cement may originate in various marine and freshwater environments, as well as under burial conditions (MALwA, 1985; EVAMV& Stn~.ARM~N,1969). 4. Dog-tooth cement consists of short, anhedral crystals up to 0.3 mm in diameter. It occurs usually on roofs of voids filled with geopetal vadose crystal silt, growing towards the centres of the voids. The dog-tooth cement may form in meteoric phreatic zone (MooRE, 1989) or under burial conditions (PI~ZB~CDOWS~, 1985). 5. Asymmetric and 'meniscus' cement (PI. 40/5). The asymmetrical cement occurs exclusively on roofs of voids filled with geopetal crystal silt. It consists of irregular crystals of various size hanging downwards from the roof of the void. Relics ofisopachous cement are occasionally foundbetween the asymmetric cement and its subswate. It is very similar to the gravitational cement described by MtYLLER(1971) and interpreted as of marine vadose origin. The asymmetrical cement is always found in carbonates which have undergone extensive early dissolution (Pm~zBt~DOWSra,1985). Only questionable examples of meniscus cement were recognized during petrographic studies of these car-

Carbonate buildup in Mydlniki (Upper Oxfordian, South Poland). Diagenetic features (calcite pseudomorphs after gypsum and asymmetrical cement). Bioclastic Tubiphytes/thrombolite wackestone passing to tuberoid-peloid wackestone (upper right). At the center a void is filled with geopetal vadose crystal silt, including numerous calcite pseudomorphs after gypsum. Asymmetric dissolution textures (arrows) occur above the cavern on the undersides of greater tuberoids. Sample N. x 13 Bioclastic Tubiphytes/thrombolite wackestone. Distinct fractures filled with internal sediment (arrows) are neptunian microdykes. Their walls are lined with a layer of isopachous cement. Sample C. x 13 Micritized ooids, intraclasts and coated grains. Rims of isopachous cement visible on grain margins. The relicts of the intergranular pores space filled with blocky calcite cement. Ooid-intraclast grainstone. Sample E.x 100. Growth cavity filled with internal sediment of vadose crystal silt type. The cavity walls are lined with isopachous rim cement. Tubiphytes/thrombolite boundstone. Sample 11. x 90 Void with geopetal filling. In lower part vadose crystal silt with calcite pseudomorphs after gypsum (arrow). Asymmetrical cement (double arrow) on top of the void. Bioclastic Tubiphytes/thrombolite wackestone. Sample N. x 90 Pseudomorphs of calcite after gypsum exhibiting a pronounced primary dove-tailed fabric within vadose crystal silt. Bioclastic Tubiphytes/thrombolite wackestone. Sample N. x 100 Calcite pseudomorphs after gypsum within vadose crystal silt. B ioclastic Tubiphytes/thrombolite wackestone. Sample N. x 90

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188 bonates.The questionable meniscus cement was observed in a void filled with geopetal vadose crystal silt at the contact between two micritic components, but it lacks most of the diagnostic features typical of the meniscus cement (RIOrrER, 1976). 6. The zones of cementation with brown Fe-oxides, up to 0.4 mm thick, occur in the top parts of growth voids with geopetal crystal silt filling (P1.39/4). The zones of Fe-oxides cementation commonly mark the position of local fossil groundwater levels in the void (FLt~OEI, 1982; VALFrON, 1983).

6.2 Asymmetric dissolution textures Asymmetric dissolution textures occur on lower purls of tuberoids or other grains in thin sections which display asymmetric cements and calcite pseudomorphs after gypsum. These textures seem to be good indicators ofdiagenesis in vadose environments in non-lithified or weakly cemented sediments ~ z ~ , o o w s K I & TApp, 1989).

6.3 Pseudomorphs of calcite after gypsum Thin sections from sample N reveal numerous roughly lenticular crystals of calcite up to 1 mm long. They are only developed within voids with geopetal vadose crystal silt (PI. 40/1,5-7). Crystals exhibiting a pronounced primary 'dovetailed' fabric occur, too (PI. 40/6). The angle between crystals in the dove-tailed fabric is approx. 105 ~ and 120~. These values and the lenticular form suggest that they are pseudomorphs of calcite after gypsum. The formation of gypsum crystals may be explained by sulphide oxidation in contact with oxidative groundwaters rich in calcium (BAxY, 1990). There is no evidence of a sabkha environment for the Mydlniki buildup (cf. Kocll & Sotw~i~R, 1986). 7 CONCLUSIONS As compared to the underlying cyanobacterial-sponge buildup (MAxYSZmZW~, 1989b, 1990), the limestone buildup in Mydlniki displays an abundance of forms capable of providing a rigid framework. Thrombolites and Tubiphytes dominate in the central and bottom part of the buildup (Fig. 5). They gradually replace the cyanobacterial crust and siliceous sponges which are sporadically the rock forming elements in the bottom part of the buildup and also at its top. Serpules randomly distributed within the buildup also form small structures with a rigid framework. The rich fauna includes especially ostracods, and also polychaetes, brachiopods, gastropods, juvenile forms of ammonites and calcareous algae which are almost absent in the underlying cyanobacterial-sponge buildups. The fauna thus reflects a distinct change in conditions of sedimentation in the upper pan of the Oxfordian sequence in the Cracow area.This change relates mainly to an increase in the energy of the environment, indicated by the presence of ooids (ooidintraclast grainstone). The presence of the calcareous algae and abundance of fauna suggest that the buildup was growing at shallower depths than the underlying cyanobacterial-

sponge buildup. Especially the abundance of the ostracods might be explained by the decreasing salinity. The accumulating .sediment was subjected to a rapid iithification of the boundstones and detritic sediments (wackestones). The high rate of the early lithification is proved by numerous borings, neptunian microdykes and fractures in

Tubiphytes. The presence of asymmeWic dissolution textures as well as calcite pseudomorphs after gypsum in voids with geopetal vadose crystal silt suggest a high intertidal to supratidal environmental setting and occasional emergence of the buildup during its growth (see Fig. 4, 5). It should be noted that almost all of the features indicating meteoric environment in early diagenesis occur in sample N (Fig. 5). The dogtooth cement, asymmetric cement and the zone of Fe-oxide cementation were found in samples which also display other indications related to an early diagenetic meteoric or marine vadose zone. It may be concluded that these cements were formed in similar conditions. The bioclastic-peloid packstone at the top of the exposure reflects the termination of the the growth of the buildup and demise in the deepening basin. Similar deposits in the highest part of the Oxfordian sequence in the Cracow area are represented by Saccocoma-peioidpackstone forming part of a turbidite sequence (MATYSZVaZWI~, 1989C, 1990). The described carbonate buildup in Mydlniki is a stratigraphic reef (scnsu DtmnAM, 1970) but includes also the ecologic stages (cf. JAMES, 1983). This buildup is the endmember closest to an ecological reef within the spectrum of Upper Oxfordian carbonate buildups described from the Cracow area. ACKNOWLEDGMENTS The authors wish to express their thanks to Prof. Dr. R. Koch (Erlangen), Dozent A. Kostecka (Cracow) and Dr. B. Senowbari-Daryan (Erlangen) for comments and discussion. Thanks also go to Mrs. M. Kusmierek for drawing the figures. The work was clone as a part of the research project 'Sedimentation and diagenesis of the sediments in the highest part of the Jurassic section in the environs of Cracow', funded by the Ministery of National Education and the Deutscher Akademischer Austauschdienst.

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Manuscript received March 3, 1991 Revised manuscript accepted January 24, 1992