Bulletin—Geology of the Fort Sumner sheet, New Mexico

By V. C. Kelley, 1972, reprinted 1987, 55 pp, 8 figs., 2 sheets, 1 index.

The work was done as part of a program by the NM Bureau of Mines and Mineral Resources to make available basic geologic data bearing upon the mineral, land, and water resources in the state. Rocks in the Fort Sumner quadrangle range from Precambrian to Holocene. The oldest outcropping sedimentary rocks are the Abo Formation which occurs as only very small inliers in the Gallinas Mountains. At the surface the Yeso Formation lies on the Precambrian of the ancestral Pedernal Mountains in the western part of the map area. However, in the subsurface an eastward-thickening wedge of Abo and Magdalena Formations intervenes between the Yeso and the Precambrian basement. The major part of the area is surfaced by broad, irregular, north-trending outcrop belts which are, from west to east, the Yeso, San Andres, and Artesian Formations of Permian age, and the Santa Rosa and Chinle Formations of Upper Triassic age. In the extreme northeastern corner of the map area is a small area of Jurassic and Cretaceous formations which is more widespread to the northeast.

Considerable Ogallala is mapped west of the Pecos River at levels of 50–150 ft above the extensive pediment gravels on the widespread surfaces around Yeso and less-defined levels around Vaughn and Duran. The well known Pedernal Hills, the original or type exposures of the Late Pennsylvanian to early Permian buried ancestral mountains, are also a Laramide to late Tertiary uplift with distinctive fault development on the western side between the uplift and the late Tertiary Estancia Basin. A new north-south alignment of gentle folds and small faults, described as the Vaughn trend, extends across most of the area near Vaughn; these structures align also with the western front of the Rockies near Las Vegas and with the Tinnie fold belt and Dunken uplift for to the south.

The Fort Sumner sheet includes an unusual number of physiographic features which bear importantly on the interpreta- tion of the erosional history of eastern NM. Some attention is given to several of the geomorphic problems including hypothes- es for the origin of the Pecos Valley and the western extent of the Ogallala surface. The drainage system at overflow levels of old Estancia, Pinos Wells, and Encino lakes is traced into the Vaughn sag, and is concluded that surface drainage from the lakes did not reach the Pecos River. This bulletin covers the 8,000 mi2 area of the Army Map Service 1º x 2º Fort Sumner sheet. Five small towns, Santa Rosa, Fort Sumner, Vaughn, Corona, and Encino are within the area. Vaughn, near the center, is served by three U.S. highways and two transcontinental railways. U.S. Highway 66 traverses the northern boundary of the quadrangle. Santa Rosa and Fort Sumner are county seats of Guadalupe and DeBaca Counties, respectively. The county seats of Lincoln and Torrance are outside the area. A small portion of Quay County is included in the northeastern corner of the quadrangle.