BIOTURBATED-CARBONATE-CAPPED PARASEQUENCES: A KEY TO SHORT TERM CLIMATE/SEA LEVEL CHANGE ON CARBONATE PLATFORMS CONTROLLED BY CLIMATE AND PLANT AND ANIMAL BIOTURBATION

Asymmetric parasequences with rare transgressive laminites, and highly bioturbated, poorly laminited tops to cycles are reminiscent of Fischer's Triassic "Lofer cycles", and are an important but commonly overlooked cycle (parasequence) type in many carbonate platform sequences, compared with the more common asymmetric laminite capped shallowing-up parasequences and symmetrical transgressive-regressive parasequences. Such parasequences are common in the Early Cretaceous of Croatia and consist of an emergence surface, thin clay, followed by microbial or fenestral laminite, a subtidal carbonate mudstone with marine biota (algae, microbial lumps, forams, mollusks) and pellet filled burrows, along with green dolomite filled root-like structures, capped by an overlying emergence surface. The parasequences probably formed by a combination of Milankovitch-driven, high-frequency changes in sea level; with lesser autocyclic processes and jerky subsidence. The main dilemma with these Lofer-style cycles is why transgressive laminites developed, and why regressive laminite deposition was inhibited, which is the opposite of what we typically see with assymetric shallowing up parasequences. We propose that such parasequences may be a response to lowstand early transgressive drier climates (which favor hypersalinity and laminite preservation) and wetter highstand climates that are more humid (favoring depressed or normal marine salinities in the coasal zone, macrophytes, and intense bioturbation) suppressing development of laminite caps. Such parasequences appear to typify the Holocene Andros tidal flats, parts of the Alpine Middle Triassic, the Late Triassic of Hungary, the Late Jurassic of Portugal, and the Cretaceous Hauterivian-Early Barremian, Croatia. Their recognition may be important in refining our understanding of sea level and climate fluctuations on carbonate platforms.