ABSTRACT: Sandstone Stromatolites from the Yates Formation, New Mexico, United States

Gill Harwood

Although stromatolites commonly are reported from carbonate depositional environments, they are rare in siliciclastics. Sand-dominated intervals of the Yates Formation, the shoreward equivalent of the Middle Capitan reef, contain several developments of sandy algal stromatolites, in some places stacking to heights in excess of 2 m.

The depositional environments of Yates sandstones are disputed, largely due to the lack therein of definitive bed forms; proposed depositional systems range from eolian through fluvial to submarine. The sandstone stromatolites occur at several localities a few hundred meters shoreward of the Capitan reef. In Walnut Canyon one sandstone unit thickens basinwards to nearly 4 m and contains bed forms indicating shallow (less than 2 m) low-energy submarine deposition. This unit contains both vertical and lateral transitions between sandstones and carbonates. Vertical sandstone-to-carbonate transitions are interspersed with numerous minor erosion surfaces, and it is toward the upper parts of these individual transitions that the sandstone stromatolites are best developed. In the lower parts of the sandstone unit, algal heads rarely are higher than 20 cm, although laterally they continue for over several meters. Higher in the sandstone unit, individual stromatolite heads reach over 1 m in vertical and lateral dimension. Nonalgal sandstones and mixed carbonate-sandstone storm beds onlap the stromatolite heads.

The presence of sandstone stromatolites may explain the basinward disappearance of sandstone units within a few hundred meters of the Capitan reef. The stromatolites formed an effective trap for sand grains, preventing lowstand sand transport from back-reef eolian and interdune deposits into the nearby reef environments.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990