Carbonate facies evolution from the late albian to Middle Cenomanian in Southern Istria (Croatia): influence of synsedimentary tectonics and extensive organic carbonate production

Summary During the Late Albian, Early and Middle Cenomanian in the NW part of the Adriatic Carbonate Platform (presentday Istria) specific depositional systems characterised by frequent lateral and vertical facies variations were established within a formerly homogeneous area, ranging from peritidal and barrier bars to the offshore-transition zone. In southern Istria this period is represented by the following succession: thin-bedded peritidal peloidal and stromatolitic limestones (Upper Albian); well-bedded foreshore to shoreface packstones/grainstones with synsedimentary dliding and slumping (Vraconian-lowermost Cenomanian); shoreface to off-shore storm-generated limestones (Lower Cenomanian); massive off-shore to shoreface carbonate sand bodies (Lower Cenomanian); prograding rudist bioclastic subaqueous dunes (Lower to Middle Cenomanian); rudist biostromes (Lower to Middle Cenomanian), and high-energy rudist and ostreid coquina beds within skeletal wackestones/packstones (Middle Cenomanian). Rapid changes of depositional systems near the Albian/Cenomanian transition in Istria are mainly the result of synsedimentary tectonics and the establishment of extensive rudist colonies producing enormous quantities of bioclastic material rather than the influence of eustatic changes. Tectonism is evidenced by the occurrence of sliding scars, slumps, small-scale synsedimentary faults and conspicuous bathymetric changes in formerly corresponding environments. Consequently, during the Early Cenomanian in the region of southern Istria, a deepening of the sedimentary environments occurred towards the SE, resulting in the establishment of a carbonate ramp system. Deeper parts of the ramp were below fair-weather wave base (FWWB), while the shallower parts were characterised by high-energy environments with extensive rudist colonies, and high organic production leading to the progradation of bioclastic subaqueous dunes. This resulted in numerous shallowing- and coarsening-upwards clinostratified sequences completely infilling formerly deeper environments, and the final re-establishment of the shallow-water environments over the entire area during the Middle Cenomanian.     

Storm-dominated deposition of the Lower Jurassic crinoidal limestones in the Krížna unit, Western Tatra Mountains, Poland

The Lower-Middle Jurassic of the Krína unit in the Western Tatra Mts. (southern Poland) shows considerable facies variation. Crinoidal grainstones of variable thickness (up to 12 m) are one of characteristic facies. They occur above spiculites which were deposited below storm wave base on the slopes of elevated horsts. First single beds of crinoidal limestones occur within spiculites. They were deposited as event beds, generated probably by storms. The overlying thick complex of well-sorted grainstones composed almost entirely of crinoidal ossicles display widespread erosional bed amalgamation, hummocks and locally wave-formed ripples. The above characteristics are the effects of multiple reworking and winnowing of fine crinoidal material by oscillatory currents related to storms. This proves that the sedimentation of the crinoidal grainstones took place between the storm and the fair-weather wave bases. The vertical transition from spiculites with intercalations of crinoidal limestone beds towards the crinoidal grainstones is interpreted as the result of a shallowing upward trend. Since this trend is opposite to the global Early Toarcian transgression, this could be related to an uplift due to local block tectonic activity. The evidence for that is diversification of facies in the Krína Basin, as well as submarine slumps recorded in spiculites. The location of Krína Basin on relatively low northern latitude and on western edges of great Tethys Ocean during Early Jurassic enabled the formation of the crinoidal tempestites.     

A seaward prograding siliciclastic sequence from upper tidal flats to salt marsh facies (Southern North Sea)

Summary The uppermost part of the seaward prograding sequence in quartz sandy areas of the southern North Sea coast starts at the base with tidal flat layers, often strongly perturbated by burrowing endobenthic animals. This section is overlain by microbial mats intercalated by tempestites, and in part bioturbated by shaped burrows. The top consists of salt marsh facies strongly penetrated by roots. The seaward prograding sequence is characterized by numerous vertical changes in small-scale facies. The facies development and changes are caused by both organisms (halophytes, mat building microbes, benthic animals) and primary physical processes (erosion, transport and sedimentation, or sedimentation alone). The small-scale facies includes (i) bioturbation structures, (ii) biogenic growth structures caused by microbial mats, (iii) subaerial and subaqueous tempestites. Typical is the sudden breakdown of the biocoenosis by fast sedimentation due to tempestites or wind-blown sand sheets. Typical is also the renewal of the former biocoenosis in the new top section. Such sequences may be preserved by wind-blown sand sheets or by washover fans. The sequence is thus an important and unique tool for the identification of fossilized sequences of the intertidal-supratidal transition. It is also a useful mark for estimating the mean sea level and the level of the lower supratidal horizon within vertical sections. Finally the sequence indicates progress in island development governed by the interaction of biological components with physical processes.

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Zusammenfassung Die Verlandungssequenz sandiger Küsten der Nordsee besteht aus einer Abfolge, die von Wattenschichten mit überwiegend ungestalteter Verwühlung im Liegenden über Mikrobenmatten, in die Tempestite mit vorwiegend vertikal angelegten G?ngen eingeschaltet sind, zu supralitoralen Salzwiesen mit starker Durchwurzelung führt. Sie wird durch zahlreiche, oft nur zentimeterm?chtige Fazieswechsel charakterisiert. An der Entwicklung und am Wechsel der Fazies sind Organismen (benthische Fauna, mattenbildende Mikroorganismen und Halophyten) sowie prim?r physikalische, subaquatische wie subaerische Transport-und Sedimentationsprozesse beteiligt. Bezeichnend für die Verlandungssequenz sind wiederholte Zusammenbrüche der lokalen Bioz?nosen durch eingeschaltete Tempestite mit nachfolgender Entwicklung der gleichen Bioz?nose bei hinreichender sediment?rer Ruhezeit. Die beschriebene Sequenz ist ein diagnostisches Mittel zur Identifizierung flachmariner Verlandungsabfolgen. Sie erlaubt, die H?he der mittleren Tidehochwasserlinie festzulegen. Solche Sequenzen k?nnen durch Windsanddecken oder sturmflutbedingte Durchbruchsf?cher überdeckt und erhalten werden.