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.