Stratigraphic and structural development of the southern Winston graben, Rio Grande rift, southwestern New Mexico
Colin T. Cikoski and Richard W. Harrison


Recent field work near the southern terminus of the north-trending, late Oligocene to early Pliocene Winston graben (WG) provides details into the interrelated stratigraphic and structural development of the graben. A 5-10° angular unconformity between the 28.9 Ma Vicks Peak Tuff (Tvp) and the 29.4 Ma tuff of Little Mineral Creek (Tlmc) suggests extensional fault block tilting and graben development began circa 29 Ma. Overlying Tvp and Tlmc are interbedded conglomerates and tuffs, including one tentatively correlated to the 27.7 Ma South Canyon Tuff that are assigned to the lower Santa Fe Group (SFG). The irregular, erosive basal contact of the lower SFG suggests initial graben subsidence was minimal. Conformably overlying the tuff-bearing conglomerates is the widespread middle SFG unit consisting of a sandstone to mudstone fine-grained facies and a conglomeratic coarse facies. The distribution of facies and the directions of clast imbrications in this unit suggest derivation from both eastern and western highlands, and also suggest the presence of a southward-flowing, fine-grained axial drainage that terminated at a lake or playa. The middle SFG is capped by the 18 Ma andesite of the Winston graben (Twa) on the east side of the basin, and by an angular unconformity on the west, with the dividing line between these two domains being the Cuchillo Negro uplift (CNU), a moderately east-tilted intrabasinal horst. This horst may have acted as a topographic barrier to limit the extent of Twa. Above Twa and the angular unconformity, the upper SFG also shows evidence for derivation from both eastern and western highlands, and is truncated by a pediment that is locally capped by the 5 Ma basalt of Tabletop Mountain (Ttb). Bound by large faults on both the east and west, the WG generally resembles a symmetric graben. In the southern WG, however, SFG deposits dip westward on the west side of the basin and eastward on the east side, suggesting the graben developed at least in part as a pair of narrow asymmetric half-graben of opposing polarity. The divide between the two dip domains lay along the west side of the CNU. Offset along the eastern graben-bounding fault is apparent in deposits as young as Ttb, suggesting graben subsidence continued into at least the early Pliocene. No deformation is apparent in inset Pleistocene terrace deposits.


  1. Cikoski, Colin T.; Harrison, Richard W., 2012, Stratigraphic and structural development of the southern Winston graben, Rio Grande rift, southwestern New Mexico, in: Geology of the Warm Springs region, Lucas, Spencer G.; McLemore, Virginia T.; Lueth, Virgil W.; Spielmann, Justin A.; Krainer, Karl, New Mexico Geological Society, Guidebook, 63rd Field Conference, pp. 447-456.

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