Shelf Sandstones of Twowells Tongue, Dakota Sandstone, Northwestern New Mexico

Neils R. Wolter, Dag Nummedal

The Dakota Sandstone of northwestern New Mexico is composed of basal continental strata and three marine sandstone tongues, which intertongue with the Mancos Shale. The late Cenomanian Twowells tongue was the last tongue deposited in the Dakota transgressive systems tract. This tongue is most commonly gradationally underlain by the Whitewater Arroyo shale tongue and abruptly overlain by the Rio Salado tongue of the Mancos Shale.

Data collected from 85 outcrop sections and 180 electric well logs, from the San Juan, Acoma, and Zuni basins, indicates that the Twowells tongue represents three phases of marine deposition. The Whitewater Arroyo shale tongue, the muddy burrowed facies, and the horizontally bedded facies of the Twowells tongue represent a shoaling-upward sequence (regressive phase) of shelf and shoreface deposition. The regressive phase is sharply overlain by an inferred transgressive cross-bedded facies. Erosional scour and an extensive pebble lag mark the contact between the regressive and the transgressive facies. In the Acoma basin, the transgressive cross-bedded facies describes a north-south oriented shelf-sand ridge 32 km long, 18 km wide, and 32 m thick. Bimodal current directions and rare re ctivation surfaces and mud drapes suggest deposition by tidally influenced currents. The Twowells tongue is commonly capped by the bioturbated sandstone facies and a Pycnodonte oyster bed; representing a phase of nondeposition. Deposition of the Twowells tongue is thought to have been controlled by a third-order relative sea level cycle. The erosional contact, between the regressive and the transgressive phases, is thought to be a type 2 unconformity (sequence boundary).

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.