SEQUENTIAL DEVELOPMENT OF AN ESTUARINE VALLEY FILL - THE TWOWELLS-TONGUE OF THE DAKOTA SANDSTONE, ACOMA BASIN, NEW-MEXICO

The late Cenomanian Twowells Tongue of the Dakota Sandstone in the Aco ma Basin (northwest New Mexico, USA) is a basinwide sandbody, 30 m thi ck, deposited during a period of relative sea-level fall and subsequen t rise. For most of its extent, the Twowells Tongue consists of two ma rkedly different sandstone units: shoreface and estuarine, respectivel y. The shoreface sediments were deposited during a period of relative sea-level highstand. The shoreface profile is incomplete, having its u ppermost 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 over lain 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 sandsto nes probably deposited in a middle to outer estuarine setting. The tra nsgressive deposits of the Twowells Tongue are capped by a horizon of Pycnodonte oysters underlying black offshore shales interpreted to rep resent abrupt marine deepening. The cross-bedded unit, sharply overlyi ng offshore shales or lower shoreface sediments, resembles other sandb odies of the Cretaceous Western Interior Seaway, traditionally interpr eted as ''offshore ridges'', or more recently as lowstand shorelines. However, the investigations in the Acoma Basin show that both the ''sh elf-ridge'' and the lowstand shoreline models are inadequate explanati ons, because the cross-bedded lithosome of the Twowells Tongue has the internal characteristics of tidally dominated estuarine deposits.