Memoir 3—Middle Pennsylvanain Brachiopods from the Mud Springs Mountains and Derry Hills,
New Mexico

By J. L. Gehrig, 1958, 38 pp., 1 table, 1 fig., 6 plates, 1 index.

A brachiopod fauna from carefully measured sections of the lower part of the Pennsylvanian in Sierra County, New Mexico, was studied to determine if the megafossils could be useful in designating and correlating the various units of the Pennsylvanian in this area.

It was found that strata belonging to the Derry and the Des Moines series could be distinguished on the basis of the contained brachiopod species. Several characteristic species from the Derry, which is believed to be younger than the Morrow of the mid-continent region, are similar to upper Mississippian forms. Brachiopods of the Bolander Group in the upper part of the Desmoinesian show close affinities with those of the Marmaton Group of Kansas. It thus appears that both megafossils and microfossils can be used successfully to distinguish groups and series in the Pennsylvanian of New Mexico.

Sedimentary rocks of Pennsylvanian age are widespread in New Mexico both on the surface and in the subsurface. They are particularly important as a source of petroleum and natural gas, and in the western part of the State they contain mineral deposits. They vary greatly from place to place in the proportions of sandstone and shale to carbonates; yet they are commonly more than 1,000 ft in total thickness, so that the distinction of units which can be recognized in outcrops or in drill holes is of economic importance.

Thompson measures a number of sections and divided the strata into series, groups, and formations. The age determinations and the correlations of these units with the standard section of the mid-continent region were based in large part upon the microfossil content. In many localities, mega fossils, particularly brachiopods, are abundant. The purpose of this study was to determine whether or not the brachiopods could be used to distinguish units within the Pennsylvanian.

In the summer of 1953, Thompson's sections in the vicinity of Derry on the Rio Grande, 30 mi south of Truth or Consequences, and at Whiskey Canyon in the Mud Springs Mountains, 7 mi northwest of Truth or Consequences, were remeasured, and both megafaunas and microfaunas collected. These collections represent the Green Canyon and Mud Springs Groups of the Derry series and the Armendaris and Bolander Groups of the Des Moines series. The brachiopods are described and illustrated in this report, and their stratigraphic relationship to the Fusulinidae is recorded. All figured specimens are in the collection of the New Mexico Bureau of Mines and Mineral Resources at Socorro; the remainder were left at the University of Wisconsin, where the laboratory studies were completed.