A lithofacies study of the Peterborough Member, Oxford Clay Formation (Jurassic), UK: an example of sediment bypass in a mudstone succession

Thirteen samples from the Peterborough Member (Oxford Clay Formation, Middle Jurassic), from Quest Pit, Stewartby, central England, were examined in detail using a combination of optical microscopy and backscattered electron imagery. The majority of the samples were clay-rich mudstones, two were silt-rich mudstones and one was a muddy sandstone. The dominant components of these samples are fine-grained clays (illite, mixed layer illite/smectite, kaolinite) and amorphous organic matter; minor components include authigenic pyrite (framboidal and euhedral), silt-sized quartz grains, calcareous nannoplankton (mainly disarticulated coccolith plates) and a variety of early carbonate cements. Some of the mudstones were found also to contain abundant foraminifera. Temporal trends between samples in grain size, foraminiferal abundance and total organic carbon contents were observed. These mudstones were deposited in a shallow epeiric sea, where productivity was high and the bottom waters were oxic. Relative sea-level change, coupled to a lack of available accommodation space within the basin at low sea-level stands is invoked to explain the observed mudstone stacking patterns.

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