Fundamental Controls on the preservation of organic matter in fine-grained sediments: an example from Lower Jurassic Cleveland Basin, North Yorkshire Coast, England

Source rocks are typically interpreted as being the products of deposition in anoxic / dysoxic, low energy basins, where rates of organic-carbon production were relatively high. The controls on organic-carbon preservation and lithofacies variability are investigated in ancient, organic-rich mud-dominated strata (Lower-Jurassic, Whitby Mudstone Formation, UK). Using a combination of field, optical, electron-optical, and geochemical methods, three lithofacies types were recognized to contain > 2% organic carbon: (a) Thin-bedded, partially-bioturbatedsilt-bearing, clay-rich mudstones; (b) Thin-bedded clay-rich mudstones; and (c) Thin-bedded and pelleted, clay-,calcareous nannoplankton-, organic carbon-bearing mudstones. The presence of micro-textures including normally-graded beds, micro-scours, ripples, «wave-enhanced sediment gravity flows of fluid mud» in these lithofacies suggests that sediment was commonly delivered to sites of deposition by advective, sediment transport mechanism sand that deposition was not always a continuous-rain out of a low-energy water column. Where suspension settling was the dominant mode, it is likely that organic-matter was delivered as organo-minerallic aggregates rapidly to the sea floor. Here enhanced organic carbon preservation was likely due to enhanced primary production, coupled to episodic delivery to the sea floor and rapid burial. The implications of these data are profound for existing source rock paradigms that assume predominantly low energy depositional conditions.