Print this pageYOSHIDA, SHUJI, and HOWARD D. JOHNSON
T. H. Huxley School, Imperial College, London, UK
Abstract: Sequence and Facies Architecture of Tidal Sand Bars within the Lower Cretaceous Vectis Formation in the Isle of Wight, South England
The Vectis Formation (Lower Cretaceous) in the Isle of Wight, south England, has been traditionally interpreted as a single lagoonal complex. Based on detailed outcrop mapping which adapts architectural element analysis methodology, we found that it consists of at least two juxtaposed tidal depositional systems separated by an erosional, regionally extensive sequence boundary. The lower tidal system (Cowleaze Chine Member) is characterized by a muddy succession containing brackish- and fresh-water fossils. This muddy zone, with few lateral facies variations, overlies fluvial strata of the Wessex Formation and is interpreted as a broad lagoonal complex.
The upper tidal system Barnes High Sandstone and Shepherd's Chine Members exhibits considerable lateral facies variations over a short distance. In the central and eastern parts of the island, the basal part of the system is characterized by a few, vertically-stacked coarsening-upward units containing abundant tidal sedimentary structures, and is interpreted as a composite tidal bar formed within the deeper part of a broad estuary (incised valley). In the western part of the island, the system is characterized by a series of coarser-grained sandstones with blocky to fining-upward grain-size profiles which are interpreted as upper-estuarine channels. The tidal bar complex is sharply overlain by a muddy succession with fresh- and brackish-water fossils and evidence of storm action, which is interpreted as a bayfill within a broad embayment.
AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas