Mississippian rocks have been studied in detail over all of their area of exposure in southwestern New Mexico except in the Peloncillo and Little Hatchet mountain areas. Sections have been diagrammed and correlation charts prepared for all pertinent areas.

Rocks representing parts of all four standard Mississippi Valley series—Kinder-hook, Osage, Meramec, and Chester—are present in New Mexico.

The Kinderhook is represented by the Caballero formation, a correlative of the Chouteau limestone (Chouteau = Compton of Moore, 1928), consisting of soft, nodular, fossiliferous, shaly limestone, resting unconformably on various parts of the Devonian and overlain by Osage beds.

The Osage series is represented by the Lake Valley, an early Osage formation, and the Kelly formation tentatively assigned to the Osage because of lithologic characteristics and stratigraphic position. The Lake Valley formation is divided into six members: Andrecito, Alamogordo, Nunn, Tierra Blanca, Arcente, and Dona Ana. Andrecito, Nunn, and Tierra Blanca are new names. The Andrecito consists of a variable series of soft, shaly limestone beds; the Alamogordo of hard, black, cherty, cliff-forming limestone; the Nunn of soft, blue-gray crinoidal marl beds; the Tierra Blanca of massive cherty criquina (closely packed, fragmental, crinoid skeletal parts in which even the separate columnals are broken (Tester, 1941, p. 6). This term is used throughout this paper for highly fragmental crinoidal limestone as opposed to normal crinoidal limestone in which the skeletal parts are relatively intact. The Arcente consists of thin-bedded, soft, relatively unfossiliferous siltstone; and the Dona Ana of massive, very cherty criquina. The Kelly formation consists of massive, gray, slightly crinoidal limestone of lighter color than the Lake Valley and is developed only in the western part of the area. Its relation to the underlying Lake Valley formation has not been accurately determined. The Lake Valley and Kelly formations are absent from the Hueco and Franklin mountain areas.

The Meramec series is represented by two new formations, the Las Cruces and the Rancheria. Both were originally included in the Helms formation. The former formation consists of medium-bedded, dense, black, gray-weathering, chert-free, poorly fossiliferous limestone. It rests unconformably on Devonian beds and is overlain unconformably by the Rancheria formation. The Las Cruces formation is developed only in the Franklin Mountains and in the southern San Andres Range. The Rancheria formation consists of medium-bedded, very cherty, black, silty limestone. It rests with marked unconformity on both Devonian and Mississippian beds and transgressively overlaps northward in both Sacramento and San Andres mountain areas. In the Franklin and Hueco mountains, it is unconformably overlain by the Helms formation (restricted) of late Chester age. The Leiorhynchus carboniferum fauna occurs in the Rancheria formation making possible a direct correlation with the Moorefield formation of the Ozark region.

The Chester series is represented by the Helms formation (restricted) which consists of soft, green, sandy shales with nodular, fossiliferous limestone beds near the top and containing upper Chester fossils.

Mississippian rocks are unconformably overlain by Pennsylvanian strata. Evidence of marked relief on the erosion surface is common. In the San Andres Range, thick accumulations of reworked Mississippian cherts in basal Pennsylvanian beds offer strong evidence suggesting post-Mississippian orogenics.

The strange biohermal structures that characterize the Lake Valley formation in the Sacramento Mountains are locally, but less spectacularly, developed throughout the area of exposure of the Lake Valley formation.

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