Abstract

Two Upper Jurassic sandstone formations (the Gres de Chatillon and Gres de la Creche) of the Boulonnais of northern France record shoreline conditions on the eastern margin of a large epicontinental sea--the Wessex Basin. The interpreted range of bedforms is more diverse than has been recorded from other shorelines, either ancient or modern. The lower shoreface to transition zone was characterized by small lunate dunes, oscillatory ripples, mudstone drapes, and discrete burrowed beds, indicating the alternation of moderate currents, oscillatory flow, and prolonged quiescent intervals. The shoreface itself was dominated by dunes that ranged in height from a few decimeters to more than a meter. The larger sizes are twice the height of any shoreface bedforms previously described. Paleocurrent indicators within the shoreface deposits are highly variable from bed to bed and within individual cosets, a feature that may indicate deposition on a barred shoreface. Evidence for tides is care (occasional herringbone cross-stratification). In contrast to many other ancient shoreface sandstone units, storms left little sedimentary record (rare, large, shell gravel bars in the upper offshore zone), although they generated numerous erosion surfaces in the lower shoreface. The unusual aspects of the Boulonnais sandstone units may relate in part to the particular paleogeographic location of the region (an open embayment). Regardless, this is a rare example of a fair-weather wave-dominated, medium sand shoreline from the geological record.

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