PETROGRAPHIC ANALYSES OF ORGANO-MINERAL RELATIONSHIPS - DEPOSITIONAL CONDITIONS OF THE OXFORD CLAY FORMATION (JURASSIC), UK

Five samples representing the range of variability of palynofacies and organic geochemical parameters of the organic-rich facies of the Pete rborough Member (formerly Lower Oxford Clay) and one sample from the S tewartby Member (Middle Oxford Clay) were studied using optical micros copy (transmitted light and fluorescence) and scanning electron micros copy (backscattered electron mode). The organic matter assemblage of t he Oxford Clay Formation is dominated by amorphous organic matter of m arine origin (75 to 95%). Whatever their organic content, the sediment s display a highly microbioturbated texture, with edge-to-face disposi tion of clay minerals and numerous microburrows, signifying that the s ea floor was never completely anoxic. The presence of abundant organic matter in the sediment has enhanced diagenetic transformations, and i n particular is related to the formation of pyrite, calcite and authig enic quartz. The nature of diagenetic changes, particularly those invo lving silica species, provides information on the interstitial environ ment. The sediment displays, at the micrometric scale, numerous microe nvironments with varying reducing conditions, resulting in different d iagenetic products. Pyrite displays two habits, framboidal and euhedra l. Euhedral pyrite, which characterizes more anoxic interstitial envir onments than framboids, is found in the most organic-rich sediments. T he abundance of large coccolith-rich faecal pellets in the most organi c-rich sediments is correlated with chemical evidence of intense troph ic activity.