New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologists have long believed that Lower Permian nonmarine red beds of the Abo Formation in the Caballo Mountains are laterally equivalent, at least in part, to a mixed nonmarine (Abo) and marine (Hueco) stratigraphic interval 50 or more km to the south in the Dona Ana and Robledo Mountains. Stated another way, current data suggest that in the Rio Grande valley, the northernmost outcrops of marine strata of the Hueco Group are in the Dona Ana Mountains. Our recent fieldwork, however, documents outcrops of marine and estuarine equivalents of the Hueco Group in the Abo Formation of the Derry Hills and southern Caballo Mountains.

The outcrops are: (1) in the Derry Hills near Garfield (sec. 33, T17S, R4W), ~30 m above the Abo base is a 4.5-m thick interval of pink shale and marine limestone (contains phylloid algae and marine gastropods) that is a tongue of the HHHhasdfaso;dfho;sadhfoasdhfoasdhfo[sadhifoasdifhoiasdhfoasdhfnuHueco; (2) in the southern Caballos, at Red Hill Tank (sec. 15, T17S, R4W), ~40 m above the abo base is an 8-m thick interval of green shale with marine bivalves (myalinids and pectinaceans), the brachiopod Lingula, palaeoniscoid fish and fossil plants; (3) at McLeod Draw (sec. 22, T17S, R3W), a similar shale bed in the Abo yields myalinid bivalves and fossil plants; and (4) in the McLeod Hills, as far north as sec. 3, T17S, R3W, we traced two intervals of green shale, limestone and limestone-pebble conglomerate that yield fossil plants and myalinid bivalves. One interval is ~87 m and the other is ~112 m above the Abo base, the the northern limit of these beds in the Caballo Mountains indicate brackish to marine water, and the paleoflora from Red Hill Tank includes two forms of walchian conifer, the walchian cone scale Gomphostrobus, two types of pteridophylls and a variety of seeds. We interpret these shales as estuarine deposits based in their stratigraphic architecture (shales and intercalated thin sandstones with current ripples represent deposits of a tidal flat, the thicker, crossbedded sandstones and conglomerates, frequently displaying an erosive base, are tidal bars and estuarine channel fills) and the association of terrestrial plants with marine bivalves.

Clearly, a tongue of the Hueco Group is present in the Derry Hills, and estuarine deposits that formed marginal to the Hueco seaway are complexly interbedded with the Abo Formation in the southern Caballo Mountains. These recently identified outcrops thus force us to redraw the Early Permian paleogeography of the Orogrande basin in south-central New Mexico to extend the Hueco shoreline at least 50 km northward.