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A Hierarchy of Current-Produced Bedforms in a Source Rock from the Eastern Carpathians Points to Predominant Bedload Deposition of an Organic-Rich Mudstone


The Oligocene age Bituminous Marl Formation (BMF) of the East Carpathians is a laterally extensive source rock succession in the Moldavide Basin of Romania, and historically was thought of as a rather monotonous lithostratigraphic unit of largely pelagic origin. It contains, however, a wide spectrum of lithofacies that are characterized by sedimentary structures such as intraformational slump folds, clastic dykes, cross-bedding, hummocky cross-stratification, current and wave ripples, and mm-scale fine lamination. Some of these structures are consistent with a slope and deeper water depositional setting, but wave generated features suggest that intermittently the seabed was within the reach of storm waves. On the basis of flume experiments, the pervasive mm-scale fine laminae suggest that a substantial portion of the BMF was deposited and redistributed via floccule ripples by bottom currents, an interpretation that is supported by the presence of ripples wherever sand size particles are available. The BMF contains fish debris, occasional bivalves and isopods on bedding planes, and macroscopic bioturbation features are rare. The most remarkable feature of the BMF are true cross-beds with sigmoidal foresets (10-15 degrees dip). Cross-bedded marls occur either as thin (5-10 cm) or thick to very thick (1-1.2 m) bedsets. Differential compaction features indicate that surficial sediments had a water content of approximately 70%. When this is taken into account, the decompacted foreset slopes of above bedforms dipped at 30-40 degrees, and the bedforms themselves should have produced from a few dm to as much as 4-5 meters relief respectively. The BMF shows a full suite of traction-produced muddy bedforms, including cm-scale ripples, dm-scale megaripples, and m-scale mudwaves. Although part of the sediment probably arrived at the seafloor via pelagic settling, reworking and transport of flocculated muds in bedload appears to have been a major factor in producing bedding features from the mm- to m-scale. The observed level of bottom current activity seems incompatible with previous scenarios of anoxic bottom waters as the cause for preservation of lamination and organic matter. Instead, frequent bedload transport of flocculated muds may have prevented the establishment of a burrowing infauna and through formation and rapid burial of organo-mineral aggregates may have promoted organic matter preservation and enhanced source rock potential.