Petrographic analyses of organo-mineral relationships: depositional conditions of the Oxford Clay Formation (Jurassic), UK

Abstract: Five samples representing the range of variability of palynofacies and organic geochemical parameters of the organic-rich facies of the Peterborough Member (formerly Lower Oxford Clay) and one sample from the Stewartby Member (Middle Oxford Clay) were studied using optical microscopy (transmitted light and fluorescence) and scanning electron microscopy (backscattered electron mode). The organic matter assemblage of the Oxford Clay Formation is dominated by amorphous organic matter of marine origin (75 to 95%). Whatever their organic content, the sediments display a highly microbioturbated texture, with edge-to-face disposition of clay minerals and numerous microburrows, signifying that the sea floor was never completely anoxic. The presence of abundant organic matter in the sediment has enhanced diagenetic transformations, and in particular is related to the formation of pyrite, calcite and authigenic quartz. The nature of diagenetic changes, particularly those involving silica species, provides information on the interstitial environment. The sediment displays, at the micrometric scale, numerous microenvironments with varying reducing conditions, resulting in different diagenetic products. Pyrite displays two habits, framboidal and euhedral. Euhedral pyrite, which characterizes more anoxic interstitial environments than framboids, is found in the most organic-rich sediments. The abundance of large coccolith-rich faecal pellets in the most organic-ric