Abstract

The Lower Cretaceous Woburn Sands (Lower Greensand Group) in southern England constitutes one of the most intensively studied tidal sandstone outcrops for sedimentological and reservoir analog studies. Most recent workers have interpreted the whole 30–60-m (100–200-ft)-thick succession around Leighton Buzzard as representing an ancient tide-dominated estuary. However, unequivocal estuary characteristics are limited to the lowermost part (about 15–20 m [50–66 ft]). We suggest that a significant portion of the Woburn Sands, and most of the middle part, was formed in a tide-dominated marine embayment. Hence, the vertical facies change from the lower to middle part of the Woburn Sands is interpreted as a change from (1) a narrow estuary to (2) a broad marine embayment. The Wash embayment in eastern England is a striking modern analog; it receives most of its sediments and waters from marine sources and is largely filled with nondiluted seawater. Moreover, the Holocene transgressive history of The Wash is remarkably similar to the transgressive evolution of the Woburn Sands.

Early estuarine sequence models predict landward translation of facies zones along the valley thalweg during transgression, but eventual facies translation in the strike direction has not been fully documented or discussed. An embayment facies that is not commonly confined in the early incised valley can occur vertically between the estuarine valley fill and marine shelf deposits and is probably underrepresented in current models. However, this segment of the transgression, comprising along-strike bay expansion and the development of a broad marine embayment, may be more important than previously thought.

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