Circular 192 — Eocene tectonics and depositional setting of west-central New Mexico and eastern Arizona

By S. M. Cather and B. D. Johnson, 1984, 32 pp., 1 table, 27 figs.

Discussed are the Carthage–La Joya Basin between south of Albuquerque and southeast of Socorro, the much larger Baca Basin reaching from northwest of Socorro to west of Showlow, Arizona, and the seven uplifts that border and in part delineate the two basins. Analysis includes the structural styles and geometries of the uplifts, depositional environments and facies geometrics in the basins, and the influence of intrabasinal structures and adjacent uplifts on the basinal deposition.

The deposits of the Baca and Carthage–La Joya basins record late Laramide (Eocene) sedimentation in the west-central New Mexico and eastern Arizona. Sedimentation in these basins was influenced by tectonism (both intrabasinal and in surrounding uplifts) and by the prevailing semiarid climate.

A well-exposed facies tract through the central part of the Baca basin reveals a broad spectrum of depositional paleoenvironments. The braided alluvial-plain system, consisting of the deposits of coalesced humid alluvial fans, is widespread throughout the basin. Sedimentary structures present in these deposits indicate that discharge was increasingly flashy toward the basin center. Meanderbelt sedimentation was dominantly restricted to an actively subsiding, fault-bounded block in the eastern area. Lacustrine sedimentation occurred in small, floodplain lakes and in a large, shallow, closed lake near the eastern end of the basin. Cyclic progradation, abandonment, and subsidence of fine-grained deltas and fan deltas well out into the lake basins prevented development of extensive lacustrine basin-center deposits.

Only rocks of the braided alluvial-plain system are present in the poorly preserved deposits of the Carthage–La Joya Basin. Updip retreat of facies tracts in both the Carthage–La Joya and Baca Basins reflects waning Laramide deformation in adjacent uplifts during late Eocene time.