The role of event beds in the preservation of organic carbon in fine-grained sediments: Analyses of the sedimentological processes operating during deposition of the Whitby Mudstone Formation (Toarcian, Lower Jurassic) preserved in northeast England

Abstract Geologists have recently observed that source rocks commonly contain a wide variety of microfabrics, including diminutive ripples and burrows. The existence of these fabrics challenges the assumption that persistent bottom-water anoxia and low-energy conditions are necessary prerequisites for enhanced organic carbon preservation in these units. In order to enhance our understanding of the depositional and diagenetic processes responsible for organic carbon preservation in these strata, the lower Jurassic Whitby Mudstone Formation, which contains mudstones with up to 14.2% total organic carbon, has been investigated using optical, electron optical and geochemical methods. Four thinly-bedded mudstone lithofacies were identified: (a) silt-bearing, clay-rich mudstones; (b) clay-rich mudstones; (c) clay-, calcareous nannoplankton-, and organic carbon-bearing mudstones; and (d) cement-rich mudstones. Internally individual beds are typically sharp-based and, normally-graded. They are either laminated, partially bioturbated or homogenized. Where depositional fabrics are preserved, pellets, silt lags, ripple lamination, shell pavements, wave enhanced sediment gravity flows of fluid mud, and organo-minerallic aggregates have been observed. The presence of up to 14.2% TOC in units that are sharp based and normally-graded suggests that mass flows, possibly generated by storms likely influenced sedimentation during deposition of these mudstones. The abundant organo-minerallic fabrics in these units indicates that at least some of the organic carbon was delivered to the seafloor as marine snow aggregates, that likely formed in the water column following phytoplankton blooms. Homogenization of bed tops indicates that between sediment delivery episodes, an infauna was able to colonize the seafloor, implying that the bottom waters were at least partly oxygenated during these intervals. These fabrics suggest that during deposition of the Whitby Mudstone Formation, organic carbon was being preferentially preserved below regions of high primary organic production. The close association between sharp-based, graded beds and organo-minerallic aggregates indicates that large volumes of organic carbon were delivered episodically to the seafloor, and that conditions at the sediment-water interface were episodically dynamic. Here organic carbon is likely to have been preferentially preserved because the frequency of depositional events was sufficiently high to minimize both oxidant diffusion into the sediment and organic carbon mineralization. While bottom water anoxia may have developed during the phytoplankton blooms, it was unlikely to have been persistent, because the microfabrics present indicate that there was sufficient time and oxygen available for an infauna to disrupt the sediment between deposition events.