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