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Fossil reefs of Permian age in West Texas and southern New Mexico are remarkably well developed and are ideally exposed. This study is an attempt to interpret the environmental conditions under which some of these interesting structures were formed.

Three adjacent geologic provinces are characterized by stratigraphically equivalent rocks and fossils of strongly contrasting fades: (1) the Delaware basin with laminated detrital drab-to-black limestones and quartz sandstones and a pelagic fauna; (2) the basin margin, occupied by light-colored very fossiliferous massive reefs and by banks of detrital limestone and dolomite; and (3) a shelf area covered by comparatively thin-bedded unfossiliferous, light-colored rocks, evaporites, dolomite, and quartz sandstone. Regional analysis leads to the following paleoecologic conclusions:

The land around the Permian seaway was very low after earliest Permian time, and the climate was warm and dry. The marine faunas are most similar to contemporaneous faunas at low latitudes in the Eastern Hemisphere (Tethys). This probably is a reflection of circumequatorial conditions. The lithologic, paleontologic and structural characteristics of the Delaware basin suggest deposition in quiet waters which at times were at least 1800 feet deep. On the other hand reefs and banks at the basin rim were formed near the surface where wave attack and occasional collapse resulted in a succession of wedges of detrital limesand and talus seaward and lagoonward from the basin rim. These marginal deposits, unlike those of the basin and shelf, are mainly composed of skeletal material reflecting relatively greater organic productivity here of calcareous deposits. The lagoonal, or shelf, deposits are relatively poor in skeletal material. This and other characteristics of the rocks show that environmental conditions on the shelf were unfavorable for many kinds of organisms. Waters of the shelf area probably were generally deeper than at the rim, but it is unlikely that they exceeded a few tens of feet.

The regional relationships suggest a shelfward flow of surface waters over evaporating pans where hypersaline waters were trapped behind the low barrier of slightly elevated banks and reefs. Basin waters of nearly normal salinity evidently were continuously renewed through one or more shallow inlets, probably at the south side of the Delaware basin. The depths of the Delaware basin below inlet threshold were little disturbed by this flow, and because of mild winter temperatures there was only limited seasonal turnover. Hence, the deeper waters were generally stagnant.

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