Cycle stacking pattern, diagenesis and reservoir geology of peritidal dolostones, Trigonodus-Dolomite, Upper Muschelkalk (Middle Triassic, SW-Germany)

Summary Peritidal dolostones (Trigonodus-Dolomite) characterize the back-bank environment of the Upper Muschelkalk (Middle Triassic) carbonate ramp of SW-Germany. These deposits represent the Late Highstand Systems Tract (HST) of the ‘Third-Order’ Middle to Upper Muschelkalk depositional sequence. The HST forms an overall shallowing-upward trend and is build by a progradational stack of 1–2 m thick shallowing-upward cycles. The latter vary from subtidal-to-intertidal cycles at the base of the investigated section to intertidal-to-supratidal cycles at the top of the section. Six major facies types can be recognized: subtidal associations are characterized by oolithic grainstones, lagoonal oncolithic wackestones and peloidal mudstones. Intertidal associations are characterized by ostracod wackestones and laminated mudstones, supratidal facies consist of laminated mudstones with tepee horizons and flat pebble conglomerates as well as paleosol horizons. Thin section petrography, cathodoluminescence-microscopy and stable isotope geochemistry reveal a complex dolomitization history (evaporative dolomitization; burial dolomitization). The strong negative oxygen isotope signatures(−3.28 to−5.85‰) point out burial dolomitization as the dominant stage. The Trigonodus-Dolomite shows intercrystalline porosity and some vuggy porosity. Subtidal dolo-grainstones with idiotopic texture at the base of the investigated section have fair permeabilities (5–30 mD) and high porosities (14–32%). Inter-to supratidal dolo-wackestones and dolo-mudstones with xenotopic texture at the top of the section have very low permeabilities (0.3–1.0 mD) and lower porosities (11–16%). The reservoir characteristics with lateral continuity of porous and permeable zones at the base of the section and less porous and impermeable zones at the top again reflect the stacking pattern of shallowing-upward cycles within the overall shallowing-upward trend of the HST. Primary facies and dolomitization processes thus control the distribution of porosity and permeability.     

Upper jurassic to lower cretaceous carbonate facies of african affinities in a Peri-European area: Chalkidiki Peninsula, Greece

Summary The Epanomi-New Iraklia area (West coast of the Chalkidiki peninsula) is considered to belong to the Prepeonias subzone (or Gevgeli unit), with a palaeogeographic position near the European margin, represented by the Serbo-Macedonian massif, and at a considerable distance from the fragmented African plate, the marginal block of which is here the Pelagonian Domain. In some boreholes in the area an Upper Jurassic to Lowei Cretaceous limestone sequence has been observed, ending with an unconformity and followed by an Upper Middle-Lower Upper Eocene transgressive bioclastic limestone, an Upper Eocene to Lower Oligocene clastic series and Neogene deposits. This Upper Jurassic to Lower Cretaceous carbonate platform sequence and probably the Upper Jurassic limestones with bauxites of the nearby Mt. Katsika, show African affinities, viz: the presence of the essentially Aptian algal speciesSalpingoporella dinarica, an African plate marker; the chlorozoan type association and the bauxite formation during the Late Jurassic indicating tropical conditions; finally, the chloralgal type association and the sporadic presence of radial-fibrous ooids during the Early Cretaceous indicating peritropical conditions. Lower Cretaceous limestones are apparently missing in the innermost Hellenides. In the Pelagonian Domain s.l., on the other hand, Upper Jurassic to Lower Cretaceous limestones are found in some places, with same characteristics as in the Epanomi-New Iraklia boreholes. On the contrary, the Upper Eocene to Lower Oligocene clastic series of the boreholes can be correlated with the Axios (=Vardar) molassic basin, inline with its present situation. During the Mesozoic, the Epanomi area therefore belonged to a micro-block, next to the NE margin of the Pelagonian Domain, in contrast to earlier interpretations. Its present time position results from Early Cenozoic tectonic phases.     

Microfacies and diagenesis of an upper oxfordian carbonate buildup in Mydlniki (Cracow Area,Southern Poland)

Summary A carbonate buildup near the top of the Upper Jurassic limestone sequence in the Cracow area with a rigid framework built ofTubiphytes and thrombolites, and some fragments of encrusted siliceous sponges and serpules is described. The limestones form a dome-like elevation at the eastern wall of a 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) bioclasticTubiphytes/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. Bioclastic-peloidal 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, evidence by borings and neptunian microdykes filled with internal sediments, as well as by fracturedTubiphytes. 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.     

Analysis of Late Jurassic to Early Cretaceous algal debris-facies of the Plassen carbonate platform in the Northern Calcareous Alps (Germany,Austria) and in the Kurbnesh area of the Mirdita zone (Albania): a tool to reconstruct tectonics and palaeogeography of eroded platforms

A characteristic microfacies of the Late Jurassic to Early Cretaceous allodapic Barmstein Limestone of the Northern Calcareous Alps are clasts of wackestones with numerous fragments of calcareous algae (“algal debris-facies”). According to dasycladale palaeocoenoses, several subtypes comprising different associations can be distinguished. One association is characterized by the debris of an unknown large dasycladalean alga reported as dasycladalean alga indet. sp. 1 from different localities in the Northern Calcareous Alps, typically forming a monospecific assemblage. Another microfacies type contains star-like calcitic bodies tentatively referred to the morphospecies Coptocampylodon pantici Ljubović-Obradović and Radoičić, originally described as being from the Turonian of NW-Serbia. Other Coptocampylodon-like bodies represent the calcified tufts of the laterals of Selliporella neocomiensis (Radoičić). The occurrence of Coptocampylodon pantici-like microfossils in the Late Tithonian to Early Berriasian, shows that obviously different species of dasycladaleans display identical to similar shaped tufts of laterals in transverse sections when becoming fragmented. Coptocampylodon pantici Ljubović-Obradović and Radoičić was observed only from different occurrences of Barmstein Limestone, but not from the autochthonous platform carbonates of the Plassen carbonate platform. The Coptocampylodon algal debris-facies is also reported from the Late Jurassic of Albania, Mirdita zone. Occurrences of different types of algal debris-facies in components of mass-flow deposits can be used as a tool to reconstruct eroded carbonate platforms and tectonics, as demonstrated in the Northern Calcareous Alps and the Albanides. Finally, the general occurrences of algal debris-facies in both settings—intra-Tethyan mostly isolated platforms (Alps, Albanides) vs. extended epeiric platforms (Middle East)—are compared and discussed.