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CATHER, STEVEN M., New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining & Technology, Socorro, New Mexico

ABSTRACT: Preliminary Analysis of the Fresnal Fault System--a Major Dextral Normal Fault of Late Pennsylvanian Age in SouthCentral New Mexico

The Fresnal fault system was a bounding structure along the east side of the Late Paleozoic Orogrande Basin in south-central New Mexico. The Fresnal system in most places strikes NNE; maximum stratigraphic separation is about 500 m. Where best exposed, it consists of two strands that in most places are steep (70° to 85°), west-dipping normal faults. At Salado Canyon, however, the western strand (Salada fault of Otte, 1959) bends sinstrally to the NNW and there becomes a steep east-dipping reverse fault that juxtaposes over-turned bedding both in the footwall and hanging wall. The fault-bend relationship at Salado Canyon, together with drag folds and the presence of numerous en echelon NW-plunging folds between and adjacent to the fault strands argues for dextral slip on the Fresnal fault system. Principal episodes of deformation occurred on the Fresnal fault system in late Virgilian and early Wolfcampian time as shown by: (1) Local westward divergence of Holder Fm limestone beds in the eastern limb of the Dry Canyon syncline; (2) as much as 150 of local angular discordance between basal beds of the Bursum and Abo Fms and underlying units; (3) restriction of the Bursum Fm to the western (downthrown) side of the Fresnal fault system; (4) truncation of folded beds of the Holder, Beeman and Gobbler Fms beneath weakly deformed beds of the Abo Fm east of the fault system (Pray, 1961). Possibly significant post-Abo, W-down normal slip (rift related?) on the Fresnal system is suggested by incomplete exposures in the La Luz Canyon area. The dextral component of slip on the fault, however, appears to be largely pre-Abo, as shown by the burial of en echelon folds by the Abo Fm.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico