Injection clastic dykes in the Lower Oxford Clay (Jurassic) of central England: relationship to compaction and concretion formation

Abstract

ABSTRACTAn injection dyke of fine‐grained sandstone derived from the Kellaways Sand Formation intrudes overlying organic‐rich shales and shell beds of the Lower Oxford Clay. The dyke shows cross‐cutting relationships with early carbonate concretions, and fills uncompacted kosmoceratid ammonite shells both within the concretions and surrounding shales. Internally the dyke displays flow‐like features, and the walls show lobate flow structures. Clasts of uncompacted Lower Oxford Clay and fragments of pyrite‐rich concretions occur within the sandstone intrusions. The sandstone of the dyke was cemented by calcite identical to that precipitated in septarian cracks in the concretions. This cementation took place prior to final compaction of the Oxford Clay. The dyke has a sub‐parallel relationship to the nearby Tinwell‐Marholm fault suggesting that the dyke may be related to local tectonic events during the Middle Jurassic.