--> Grain Assemblages in Organic-Rich Mudstones Dominated by Extrabasinal Sediment Sources, Yanchang Formation (Triassic), Ordos Basin, China

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Grain Assemblages in Organic-Rich Mudstones Dominated by Extrabasinal Sediment Sources, Yanchang Formation (Triassic), Ordos Basin, China

Abstract

The primary composition of grain assemblages in mudrocks is a major control on post-depositional processes that affect the evolution of bulk rock properties such as porosity, sonic velocity, and mechanical moduli. Grain assemblages in a suite of mudstones from the lacustrine Yanchang Formation have been examined by field-emission scanning electron microscopy. Energy-dispersive X-ray mapping in combination with cathodoluminescence imaging reveals that the grain assemblages are dominated (>90 percent by volume) by grains derived from outside the basin of deposition, and hence, these mudstone are ‘tarls’ (terrigenous argillaceous rocks). Major extrabasinal grains include K-rich clay, quartz, plagioclase, K-feldspar, volcanic rock fragments, and micas. In terms of the quartz-feldspar-lithic grain compositions these samples are classified as arkoses. Intrabasinal grains include particulate organic matter and phosphatic debris. Organic matter content in particular is positively correlated with detrital clay content, denoting a reduction in the abundance of coarser extrabasinal detrital components in organic-rich samples. Authigenic components that can be observed using the imaging methodologies employed are restricted to localized cone-in-cone structures, grain replacements, pore-filling precipitates within anomalously large pores hosted by dissolved feldspars and other grains, and very minor quartz overgrowths associated with local packing flaws around detrital quartz grains. Matrix-dispersed cement crystals are not observed, suggesting that mechanical compaction and pore-filling by migrated hydrocarbon have been the dominant causes of porosity decline. Creation of brittle rock properties through early cementation related to alteration of abundant and unstable grains of intrabasinal derivation, analogous to other well-known organic-rich mudstones such at the Barnett Shale or the Eagle Ford Formation, is unlikely to have played a role in the diagenesis of these particular lacustrine mudstones.