The Nature and Origin of Organic-Rich (Source) Strata in Deepwater Rocks of the Neoproterozoic Windermere Supergroup

Abstract

Petroleum source rocks generally occur in Phanerozoic strata as fine-grained, organic-rich lithologies. Organic-rich deep marine sedimentary rocks of the Neoproterozoic Windermere Supergroup are enriched not only in the thin mudstone caps of thin-bedded turbidites deposited on the levees of slope channel, but also in intercalated thick- and very thick-bedded turbidites. Organic-rich sediment associated with thin-bedded turbidites represents fine suspended sediment that was carried high above the channel bottom and overspilled onto the levee. Organic-rich sediment in thick- and very thick-bedded turbidites, on the other hand, occurs in the thin, fine-grained cap to the bed, but significantly also as discrete layers within the medium-grained, typically parallel-laminated sandstone that comprises most of the bed. The intercalated organic-rich and quartz-rich layers indicate that the organic material particles were hydraulically equivalent to medium quartz sand grains that were either originally in-situ bound or partly lithified and preferentially sorted on the bed just prior to deposition. Moreover the sedimentation style in these thicker beds suggests deposition by more energetic flow stripping processes at channel bends. Collectively the preferential organic matter preservation in levee deposits indicates higher local sedimentation rates and more rapid burial, which together would have enhanced source rock preservation potential. Contemporaneously, common bed scour preferentially removed fine-grained bed tops, and exposed buried organic material to oxidation in the adjacent channels. Additionally, the abundance of mineral matter (grains) in channel fills would have diluted and therefore further reduced the organic content. We conclude that organic-rich Windermere Supergroup deep marine strata were re-sedimented from an upflow source area and that they are unrelated to either water column anoxic events or a quasi-permanent rain of hemipelagic or pelagic organic matter to the sea floor. Rather, the vertically spaced and paleogeographically controlled occurrence of organic-rich beds is due to periodic episodes of high organic productivity in the upflow source region subsequently enriched by sediment sorting processes during transport and deposition on the deep sea fan.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016