Sequential Development of an Valley Fill: the Twowells Tongue of the Dakota Sandstone, Acoma Basin, New Mexico

Abstract

ABSTRACT The late Cenomanian Twowells Tongue of the Dakota Sandstone in the Acoma Basin (northwest New Mexico, USA) is a basinwide sandbody, 30 m thick, deposited during a period of relative sea-level fall and subsequent rise. For most of its extent, the Twowells Tongue consists of two markedly different sandstone units: shoreface and estuarine, respectively. The shoreface sediments were deposited during a period of relative sea-level highstand. The shoreface profile is incomplete, having its uppermost part (upper shoreface and foreshore) removed during subaerial valley incision. In places, the lower shoreface deposits are overlain by paleosols. The erosional surface is of regional extent; it depicts a complex valley morphology, in places 30 m deep, and is locally overlain by pebbles and shell lags. The valley, created during a relative sea-level lowstand, was filled by the second sandbody, consisting of a thick package (20-35 m) of transgressive, cross-bedded medium sandstones probably deposited in a middle to outer estuarine setting. The transgressive deposits of the Twowells Tongue are capped by a horizon of Pycnodonte oysters underlying black shales interpreted to represent abrupt marine deepening. The cross-bedded unit, sharply overlying shales or lower shoreface sediments, resembles other sandbodies of the Cretaceous Western Interior Seaway, traditionally interpreted as offshore ridges, or more recently as lowstand shorelines. However, the investigations in the Acoma Basin show that both the shelf-ridge and the lowstand shoreline models are inadequate explanations, because the cross-bedded lithosome of the Twowells Tongue has the internal characteristics of tidally dominated estuarine deposits.