Lithofacies in Twowells Tongue of Dakota Sandstone, Southern San Juan Basin, New Mexico: ABSTRACT

The Upper Cretaceous Twowells Tongue forms the uppermost part of the Dakota Sandstone in the southern San Juan basin and represents the last minor regressive pulse in the regional Cenomanian transgression of the Late Cretaceous seaway. This widespread marine sandstone ranges in thickness from less than 15 to 98 ft in the study area between Gallup and Laguna, New Mexico. In outcrop, four major lithofacies are distinguished on the basis of sedimentologic structures and ichnofossils: (1) very silty, (2) horizontally bedded, (3) bioturbated structureless, and (4) cross-bedded sandstone. The distribution of these lithofacies suggests that depositional environments varied significantly from west to east. The very silty sandstone facies is the lowest unit in the coarsening-upward sequence of the Twowells. To the east, this facies also occurs in the middle of the tongue, indicating fluctuating depositional conditions. The horizontally bedded sandstone and bioturbated structureless sandstone facies usually occur above the very silty sandstone but randomly alternate with each other. These two facies are thinner to the east and are replaced by the cross-bedded sandstone and very silty sandstone facies. The cross-bedded sandstone facies occurs at the top of the sequence. It is scarce in the west because of erosion or non-deposition.more » To the east, it occurs within, and at the top of, the tongue; at one locality it composes the entire tongue. These four lithofacies formed under a wide range of energy conditions, show similarities to deposits in both shoreface and offshore marine environments, and indicate a complex depositional history for the Twowells Tongue.« less