Genesis of stromatactis in an upper jurassic carbonate ... - Springer Link

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ERLANGEN 1993

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Genesis of Stromatactis in an Upper Jurassic Carbonate Buildup (Mlynka, Cracow Region, Southern Poland): Internal Reworking and Erosion of Organic Growth Cavities Jacek Matyszkiewicz, Cracow KEYWORDS:

STROMATACTIS - CYANOBACTERIAL- SPONGE BUILDUP- INITIAL FRAMEWORK- GROWTH CAVITY - INTERNAL REWORKING - MASS FLOW- SOUTHERN POLAND- UPPER JURASSIC

SUMMARY The Middle Oxfordian strata in the southern part of the Cracow-Wielun Upland consist of platy and bedded limestones ('normal facies~),of massive limestones as well as locally of mass flow sediments. Massive limestones, prevailing in the Upper Oxfordian, form commonly carbonate buildups, which are made up predominantly of cyanobacterial allochems and to a minor amount of siliceous sponges. Stxomatactiscan be best observed in the Mlynkaquarry. They occurs in the uppermost part of slope sediments close to a cyanobacterial-sponge buildup. The bedding-plane of the slope sediments is directly overlain by debris-flow and grain-flow sediments. Fragments of a primary laminar framework rich in growth-cavities occur in the uppermost part of the slope sediments as precondition for the formation of stromatactis. The stromatactis cavities were formed by internal reworking and erosion within these organic growth cavities, caused by strong bottom currents due to mass transport from higher parts of the buildup. ~TRODUCTION Stromatactis cavities are problematic structures, which were described t-n'st in Palaeozoic carbonate buildups (Duporcr, 1881, 1882; LECOMm~, 1937). According to BAa'mrgsa"(1982:167) stromatactis cavities are described as 'irregularly shaped sheet- like to globose mass of sparry calcite, dominated by radiaxial fibrous calcite or internal sediment, irregular in plan, with a thickness from about 1 mm to 3 cm, though, larger examples are known'. Their formation is interpreted to be organic (BATmmST,1959; BECHSTAt)T, 1974; BOURQUE& GIGNAC, 1983; CORON& TEx'rogls, 1974; LEES, 1964; PmlxZOX,1963; PRATt, 1982; Slamy, 1968; TsmN, 1985) or inorganic (Bna'mmST, 1982; I-IEcr,EL, 1972; LOtAN& SzMz~atm,1976; Ross et al., 1975; SCnWARZ~CI~R,1961; WALLACE,1987).

In the Middle- and Upper-Oxfordian carbonate buildups from the Cracow area are isolated cavities, similar to stromatactis described by MAT,ZSZKrm~CZ(1990), but according to BA77almST(1982; p. 167) - 'single isolated cavities cannot be diagnostic'. Therefore these cavities cannot be called 'stromatactis cavities' according to the original description. Abundant stromatactis cavities, which are interconnected, occurin Middle-Oxfordian limestones (Perisphinctesplicatilis and Gregoryceras transversarium zone) of the Mlynka quarry (Fig.l) directly overlying a cyanobacterial-sponge buildup (HOFFMANN& MA~SZXlZWKX,1989). The morphology of the stromatactis cavities and their position within the outcropping sequence allows to interpret their origin to beexclusively inorganic. It can be well compared with the model presented by Wm_z~c~(1987) for the formation of stromatactis. GEOLOGICAL SETTING The Middle-Oxfordian sediments have a thickness of about 100 m in the southern part of the Cracow-Wielun Upland. They can be characterized as shelf sediments deposited in a shallow epicontinental sea. The lowermost part of the sequence (P. plicatilis and base of the G. transversarium zone) consists of platy limestones (micritic, lightcolouredlimestones of Plattenkalk-type) with intercalations of marls. Locally small sponge-bioherms occur which have a size of some meters only. In the proximity of the sponge bioherm, the platy limestones change their originally horizontal arrangement. The higher-lying strata ascend above the sponge bioherm, while the lower-lying strata bend downward. The beds are rich in ammonites. In upppermost part of the sequence (G. transversarium and P erisphinctes bifurcatus zone) platy limestones, bedded limestones with cherts and massive limestones (unbedded, without cherts limestones of Felsenkalk type) occur. Mass flow deposits occur only locally. Massive limestones which are prevailing in the Upper Oxfordian, consist of carbonate buildups predominantly. The buildups reveal cyanobacterial products mainly (=components of

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

88 outcrops in the Cracow area (DzuLWSrd, 1952; HOFFMANN& MArVSZrdEw~cz, 1989; MA~SZrdEW~CZ,1990). The diameter of the quarry is about 350 m, with the biggest wall of about 25 m in height. In the quarry platy limestones, massive limestones and mass flow sediments can be observed (Fig. 2). Micritic, platy limestones with marly intercalations occur only in the lowermost part of the profde and are overlain by massive limestones without any bedding features. The base of the platy limestones is not exposed. The micritic, platy limestones reveal an irregular surface (Fig.2). This phenomen was produced by diagenetic compaction, because the massive limestones were less compressible that the sediment from which the platy limestones were formed. The massive limestones consist of irregularly distributed carbonate buildups (bioherms and biostroms). The most extended bioherms overtop the wails of the quarry (> 25 m). The Fig. 1. Location of the 'Mlynka' quarry in the Cracow region, southern Poland. massive limestones predominantly consist ofboundstones (algal-sponge cyanobacterial origin) and siliceous sponges only locally crust and stromatolite/thrombolite boundstone) and (MAzvSZKmWICZ,1989; 1990). Ammonites occur only rarely wackestones (tuberolitic/peloid wackestone and peloid in the carbonate buildups. Therefore an exact biostratigraphic wackestones). Sections through some bioherms reveal a position cannot be given. well-developed vertical facies-differentiation. Siliceous The Mlynka-quarry is situated about 10 km to the west of sponges predominate in the lower part. Cyanobacterial Cracow (Fig. 1) and is considered to be one of the classical products (crusts, stromatolites and thrombolites), Tubiphytes, Plate Fig. 1. Fig. 2, 4.

Fig. 3.

Fig. 5.

Fig. 6.

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Stromatactis in Upper Jurassic cyanobacterial-sponge buildup fi'om Mlynka (Cracow Upland; Poland) Stromatactis cavities in mberolitic/peloid wackestones are often interconnected and arranged in distinct layers within the vertical sequence. Fig. 4 documents a detail of this microphotograph.//Nic.; x 4.5 Detail of PI. 24/1. The upper part of the stromatactis cavities are filled by varying carbonate cements. Thin isopachous cement seams are lining the walls of the cavity. The center of the cavity is filled with large aequigranular calcite crystals. The colour of the uppermost layers of the internal sediment is lighter, compared to the surrounding sediment and to the lower part of the internal sediment. Therefore the boundary between the bottom of the internal sediment and the rock itself often cannot be defined.//Nic; + Nic.; x 16.5 Detail of PI. 24/1. Stromatactis cavity with 'double floor'. The peloidal internal sediment, surrounded by equant calcite, was probably Idled in the cavity through a narrow upper part. The relative large peloids were eroded from the roof of the cavity.//Nic.; x 25 Stromatactis cavities within algal-sponge crusts. The roof of the stromatactis is formed by lower side of a siliceous sponge and therefore exhibits a different morphology. The sponge was partly eroded from bottom to top (arrow). The larger components in the lower part of this cavity are tuberoids which were partly formed due to the erosion of this sponge. Stromatactis cavities in the lower part of the photograph, occuring within peloid crusts, reveal very irregular roofs. This is caused by the selective winnowing of only weakly lithified parts of the sediment.//Nic.; x 4.3 The stromatactis cavities depicted exhibit,cery irregularroofs and smooth, horizontal floors. Weak larniantions can be always recognized. These are relics of the linfmm'ylaminationoccuring in the sediment throughout, now partly preserved between the stromatactis r The undolous walls of the cavities therefore resulted from different erodability of laminae with v a r y i ~ g r e e s of cementation and organic matter content.//Nic.; x 4.2

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90 DESCRIPTION OF STROMATACTIS

Fig. 2. Distribution of rocks-typos within the NW-waU of the quarry at Mlynka. abundant worm-tubes (Terebella or Thartharella) and nubeculinellid foraminifera are characteristic for the upper part, where siliceous sponges occur in very minor amounts. Some crabs (Dromioidea), isolated hermatypic corals (Stylosmilia; PI. 26/5) and local enrichments of ammonites (Perisphinctes) can be observed in the uppermost parts of the carbonate buildups. In the uppermost massive limestones of the quarry, vertical neptunian dikes (0.4 m) occur, which are t'tlled by a brachiopod-coquina. Directly on the beddingplane (erosional surface ?) of the slope sediments of the biggest carbonate buildup, debris-flow and grain-flow deposits occur (Figs. 2, 3). They contain fragments of massive limestones including hermatypic corals (Stylosmilia) among others components. Locallya weak downward grading of the sediments is obvious (HOFFMANN& MATVSZr,.mWICZ, 1989). Stxomatactis cavities in the Mlynka quarry occur in a zone of about 1.5 m in thickness in the uppermost part of the slope sediments covering the greatest cyanobacterial-sponge buildup (Fig. 2).

Plate Fig. 1.

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

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Slxomatactis cavities occur in the algal-sponge crust (bindstone-framestone), stromatolite/thrombolite boundstone (bindstone) and in tuberolitic/peloid wackestone. They can be described as three-dimensional cavities with a width of up to 2 cm connected among each other vertically and laterally (P1.24/1, 2, 4, 6; Fig. 4). The stromatactis cavities generally exhibit irregular roofs (P1. 24/1-6; 25/1-3). If the roofs are formed by biogenic components (especially by sponges) different morphologies of the cavities can be 9observed (P1. 24/5; 25/1; 261-3, 5). Bioclasts smaller than the cavities, can only form one wall of the cavity which is now expanding vertically to a size much greater than the bioclast. The upper part of stromatactis is filled mainly by two types of carbonate cements: 1. The walls are lined by thin isopachous cements (P1. 24/2, 4; 26/1,4) of up to 0.1 mm thickness. The center of the pore is Idled with large (up to 1 mm) aequigranular crystals (PI. 24/2, 4; 26/1,4). This type of cement occurs predominantly in the uppermost part of the slope sediments. 2. Radiaxial fibrous cements (up to 0.8 mm size) arecovering the walls (PI. 25/4; 26/2). In the center of the pore space aequigranular cements can be observed, as described above.

Fig. 3. Debrisflow in the upper part and grain flow deposits (lower) at the top of the NW-wall of the Mlynka quarry. Flat structures within the grain flow are calcified siliceous sponges (arrow). (After HOF~t~SN& MAaXSZ~EWICZ,1989)

Stromatactis in an Upper Jurassic cyanobacterial-sponge buildup from Mlynka (Cracow Upland; Poland) Sl~omatactis cavities within algal-sponge crusts. The smooth roof of the upper cavity is formed by the lower side of a silica sponge. The lower cavity exhibits an irregular roof because it was developed within cyanobacterial crusts.//Nic.; x 5.5 Stromatactis cavities occuring within stromatolite boundstones exhibit extremely irregular roofs. Relics of the primary laminar framework are well preserved between the cavities.//Nic.; x 6.0 Stromatactis cavities in tuberolitic/peloid wackestone show characteristic irregular roofs and smooth, horizontal floors. + Nic.; x 4.0 Detail of Pl. 25/3. Radiaxial fibrous calcite cements are lining the walls of the cavity whereas aequigranular cements are occurring in the center. + Nic.; x 6.0

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(Fig. 4; PI. 24/1-4). Peloid crusts are completely absent in the internal sediments. The uppermost part of the internal sediment is much lighter in colour than the surrounding sediments. In the basal part the internal sediments become more and more dark, finallyexhibiting the same colour as the surrounding sediment. Therefore the baseofthe stromatactis often cannot be well defined (Fig. 4).

P R O P O S E D ORIGIN OF S T R O M A T A C T I S

Fig. 4. Stromatactis in the upper part of the cyanobacterial-sponge buildup within the tuberolitic/peloid wackestone microfacies. Carbonatecements (radiaxialfibrous,isopachousandequant calcite cement) are white; skeletal components and tuberoids are black. The differently stippled fields show the difference between peloid wackestone (densely stippled) and peloid packstone (poorly stippled). Only these parts of the internal sediments at the boundary to carbonate cement and, occasionaly, at the boundary to the host rock consist of peloid packstone. The transitions between peloid packstone and peloid wackestone are gradual within the internal sedirnem.The boundarybetweenpeloid wackestonewithininternal sediment and tuberolitie/peloid wackestone within host rock is distinct only sporadically (upper part of the figure). This cementation pattern is found predominantly in the lower part of the 'stromatactis zone' (Fig. 2), in cavities of up to 1 cm size. The floor of the stromatactis is generally formed by the horizontal surface of internal sediments, which consist pedominantly of a peloid wackestone (locally containing small tuberoids; P1.26/1, 3). Only the uppermost parts of the internal sediment can be described locally as peloid packstone

Plate Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

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The original sediment at the slope of cyanobacterialsponge buildups in the Mlynka quarry exhibits peloidal textures and is mainly composed of peloid-cyanobacterial crusts (sensu SUN & WRIam', 1989). The genetic model presented by the author (Fig. 5) stipulates the existence of incipient fragments of a laminar framework' (PRAVr, 1982), containing primary growth cavities ofcyanobacterial origin. This laminar framework forms only at the slope of the buildup whereas the centre of the buildup itself is consists of a 'reticulate framework' (PRArr, 1982), both of them being products ofcyanobacteria ('micropeloidal framework'; l:J.T.r.~ et al., 1985). The 'reticulate' or 'micropeloidal' framework forms an intermediate category between microbial reefmounds and framebuilt reefs (SuN & Wpaorrr, 1989). Early lithification of similar thrombolite-stromatolite buildups (stromatolite mounds, sensu C~-vmax~ & HARRIS, 1984) from other areas is evident because of extensive brecciation of the framework (Etzas et al., 1985). The described internal structure of the cyanobacterial- sponge buildups is characteristic for their occurrences found in Middle- and Upper Oxfordian sediments of the Cracow area (MAa'Ysz~au~cz, 1989, 1990; HOFFMANN& MAaa'sTxaramcz, 1989; MAr'tszraEVaCZ& FEtaSL~a~,1992). Carbonate buildups of the cyanobacterial-sponge type visible in Mlynka were originally formed at a moderate water depth below the wave base. The peloidal fabric was developed best in zones with relatively low growth rates of

Stromatactis in an Upper Jurassic cyanobacterial- sponge buildup from Mlynka (Cracow Upland; Poland) The roof of the stromatactis cavity depicted is formed by the lower side of a calcified siliceous sponge. The irregular morphology of the'sponge-roof' is probably caused by different degrees oflithification of the sponge at the time ofreworking within the cavity. The internal erosion, prograding upwards, only affected selectively weakly lithified parts of the sponge. Thin rims of isopachous cements are lining the walls of the cavity and aequigranular cements occur in the center. The internal sediment predominatly consists of small tuberoids. //Nic.; x 26 This stromatactis cavity below a calcified siliceous sponge exhibits poroslromate algae as epifauna. The walls of the cavity are lined with radiaxial fibrous calcite whereas aequigranular cements occur in the center.//Nic.; x 25 Stromatac tis cavity below a calcified siliceous sponge. The tuberoids occuring in the internal sediment derived from the eroded 'sponge-roof of the cavity. In the lower part of the photograph peloid crusts can be observed. //Nic.; x 11 The detail of the upper part of a cement-f'dled stromatactis cavity exhibits well-developed isopachous cements lining the walls and large aequigranular cements in the center.//Nic.; x 96 Hermatype coral (Stylosmilia) can be found only very locally within the algal-sponge crusts.//Nic.; x 25

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Fig. 5. Modell for the origin of stromatactis in 'Mlynka'. A partly lithified slope of the cyanobactedal-sponge buildup with initial 'laminar framework' is modified by internal erosion of the sediment.This may be caused by an intensive flow of pore waters through the sediment initiated by a of mass flow (grain and debris flow). Laminar and reticulate framework are very enlarged; not adequate for to scale of the buildup. the framework, slow sedimentation and a limited water energy (SuN & Wmorrr, 1989). The carbonate buildups in Mlynka grew during active synsedimentary tectonics (HowMA~'S& MA~SZKmWtCZ,1989), proved by the presence of a neptunian dyke (cf. GARClA-I-I~RNArCD~Zet al., 1989; KErvrxR & CAMI'BEt.L,1991) and thick mass-flow deposits. Owing to the local tectonic uplift, the uppermost parts of the Mlynka buildup were gradually elevated into a shallow water environment, as shown by a marked alteration in the fauna e.g. by appearance of hermatypic corals. Continuous water movement should have, after some time, either brought about a complete change of the fauna assemblage and formation of a coral-reef or brought to a halt growth of the buildup through levelling rates of sedimentation and wave erosion. The intensive tectonic uplift resulted in a distinct relief of the seafloor and caused, in addition, a relatively fast shift of the topmost parts of the buildup above the wave base. The uplift was too fast to allow growth of a typical reefal fauna and resulted in intensive destruction of the original carbonate buildup. The process progressed, most probably, in several episodes, caused by strong sea storms or earthquakes. Fragments of the buildup were next transported downwards as mass flows (Fig. 5) along the slope of the buildup; initially as a grain flow, later as a debris flow. A significant inclination (more than 30 ~) of the slope, along which underwater slides were flowing, can be deduced from the position of the grain flow below the debris flow in the vertical proFde in Mlynka (Fig. 3). At that time the slope sediments of the buidup were already partly lithified ('laminar framework') but still containing some primary porosity in the form of growth cavities. Strong turbulence, caused by the mass flow, influenced the bottom layers of the seawater as well as the water present in the growth cavities of the laminar framework, resulting in their internal erosion. Part of weakly lithified peloid crusts and their surrounding unlithified sediment was

winnowed selectively and deposited at the bottom of the cavities (Fig. 5). Roofs of the cavities began to collapse during this process and every stromatactis cavity was 'migrating' generally upwards (cf. WALLACE,1987), maintaining a constant inclination angle with the slope of the buildup (Fig. 5). This translocation of the primary growth cavities occurred in the sediment until turbulences ceased or the internal erosion was stopped by amore strongly lithified peloid crust or a skeletal component (a sponge in most of the studied stromatactis) bigger than the width of a cavern. The stromatac tis cavities are, sometimes, arranged within the vertical profile one above another. This sequence is not a result of reworking of one and the same bed (cf. WAia~c~, 1987) but was caused by complicated 'migration' paths of individual stromatactis cavities through the weakly lithified sediment during an isolated mass flow evenL Repeated reworking of the unlithified sediment could not lead to the formation of cavities as found in the Mlynka quarry. Lamination, occurring locally between the stromatactis, is interpreted as relics of the primary laminar framework (PI. 24/6; 25/2, 3). Characteristic cement types (isopachous and radiaxial fibrous calcite cements) indicate a normal marine phreatic environment (PREz~INDOWSrd, 1985; K2.rOAI.L, 1985). An aequigranular cement of secondary origin was probably formed during burial diagenesis or in a meteoric environment (PRRznlN-OOWSIa,1985). The lighter colour of the upper parts of the internal sediment (.peloid packstone) within the stromatactis cavities (Fig. 4; P1. 24/2, 4) is caused by local winnowing of the micritic matrix. Stromatactis cavities are commonly found in similar sedimentary sequences within slope sediments of carbonate buildups underlying directly mass-flow deposits (cf. STEIOa~ & J~sA, 1985; ET.T.Iset al., 1985; ELItlK& LEVESQUE,1989). Therefore it can be expected that the model presented here is valid for other Jurassic stromatactis.

95 ACKNOWLEDGMENTS The author is grateful to Prof. R. Koch (Erlangen) for his support and valuable discussions. I would like to thank Prof. E. Fliigel (Erlangen) for useful and helpful comments, Prof. R. G. C. Bathurst (Liverpool) and Prof. G. Flajs (Aachen) for their very helpful reviews o f the paper. This paper has been critically read by Dr. B. Senowbari-Daryan, Dr. D. Wurm (Erlangen), Prof. A. Kostecka and Dr. A. Skowronski (Cracow). Their suggestions are gratefully acknowledged. The colleagues of the Institute o f Paleontology of the Erlangen -Niirnberg University (especially Dipl. Biol. D. Kaiser) kindly helped during m y research scholarship in Erlangen supported by Deutscher Akademischer Austauschdienst. I also thank the German Research Foundation for the invitation to the Neustadt meeting (1992) o f the Schwerpunkt'Globale und regionale Steuerungsprozesse biogener Sedimentation' and the interesting discussion with the colleagues of the 'Maim group'.

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Manuscript received August 10, 1992 Revised manuscript accepted December 15, 1992