Mixed Regressive-Transgressive Coastal Deposits in Upper Cretaceous Prograding Sequence, Southwest Raton Basin, New Mexico

Steven R. Haymes, Romeo M. Flores

A record of mixed regressive-transgressive deposits of an overall prograding barrier of a deltaic-interdeltaic system is found in the Upper Cretaceous Pierre Shale, Trinidad Sandstone, and Vermejo Formation in the southwest Raton basin, New Mexico. The upper Pierre Shale consists of bioturbated and rippled shales, siltstones, and sandstones that coarsen upward into the Trinidad Sandstone. The Trinidad Sandstone consists of an eastward-pinching lower unit and a westward-pinching upper unit separated by an eastward-pinching tongue of the Vermejo Formation. The lower Trinidad consists of coarsening-upward, bioturbated, hummocky, and planar to trough cross-bedded sandstone in the lower part and fining-upward, planar to trough cross-bedded, parallel-ripple laminated, and roote sandstone in the upper part. This sandstone represents a regressive wave-influenced, barred beach to shoreface complex. Siltstone, mudstone, carbonaceous shale, and coal beds of the overlying Vermejo tongue represent back-beach swamp and estuarine environments.

The upper Trinidad Sandstone is basally erosional and coarsens upward. It consists of multiple scoured bodies of trough and planar cross-bedded and convolute-bedded sandstone. The lower sandstone bodies of this unit are bioturbated and contain Toredo-bored wood. The upper Trinidad Sandstone records a transgressive (backstepping) pulse that resulted in tidal-channel inlet and estuarine environments. This unit merges westward into sandstone, siltstone, mudstone, carbonaceous shale, and coal beds of the Vermejo Formation. Sandstones of the Vermejo are interbedded with and grade into carbonaceous shale and coal, mudstone, and siltstone. Some sandstone beds are lenticular, erosional based, and trough cross-bedded; they fine upward and have parallel and ripple laminations. Other sandstone b ds are tabular and sharp based, and have parallel and ripple laminations. These Vermejo deposits represent inland fluvial environments.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988