Sequence Stratigraphy in the Upper Cretaceous Series of the Anglo-Paris Basin<subtitle>Exemplified by the Cenomanian Stage</subtitle>

Abstract

Five Cenomanian successions, situated in contrasting positions within the Anglo-Paris Basin and on its margins, are described in detail, and their sequence stratigraphies analyzed and compared. The Cenomanian Stage was chosen specifically for this study because of the high biostratigraphical resolution (about 0.5 my/zone/subzone) achievable for this stage by the use of ammonites. The successions in Kent (south east England) and the Boulonnais (north east France) are the most basinward in position and comprise rhythmically bedded argillaceous micrites (chalks and marls) with few major hiatuses. The thinner succession of coarser carbonates in Normandy (north west France) contains larger gaps. The succession in Maine (north west France) was deposited in relatively shallow water near the basin margin and includes several sand bodies and well-developed hardgrounds. The highly condensed succession of sandy limestones, containing major hardground-bounded hiatuses in Devon (south west England) represents deposition closest to the basin margin. Systems tracts are characterized and defined by diverse criteria, including geometry of sediment bodies, basin margin onlap, discontinuity surfaces (omission surfaces and hardgrounds) and lithological characters, including variations in quartz sand content, clay:carbonate ratios and the presence of authigenic glauconite and phosphate. Detailed comparison of the successions enables us to demonstrate the existence of five basinwide sequences and the lower part of a sixth.