COMPARATIVE STRATIGRAPHY OF THE DAKOTA SANDSTONE ACROSS THE PICURIS–PECOS FAULT SYSTEM SOUTH OF LAMY, NEW MEXICO: DEFINITIVE EVIDENCE OF LARAMIDE STRIKE-SLIP

Abstract

The Picuris–Pecos fault of northern New Mexico is the largest known fault in the Rocky Mountain region with 37 km of dextral strike separation of Proterozoic lithotypes. The timing of dextral slip is disputed. The Picuris-Pecos fault system continues southward from Lamy, New Mexico, as a complex zone of faults that cuts strata of Mesozoic age and is intruded by the unfaulted, 27 Ma Galisteo dike. On the San Cristobal Ranch, ~20 km south of Lamy, a ~2 km dextral step in the fault system is characterized by numerous, steep NNE-striking faults that exhibit normal separation and form an en echelon array (A. Lisenbee, 2000, NMBMMR OF-GM-39). Many of the fault blocks in this en echelon array include outcrops of the Dakota Sandstone (Upper Cretaceous, ~95 Ma), thus affording a unique opportunity to stratigraphically evaluate Laramide strike-slip across much of the Picuris–Pecos fault system. On the San Cristobal Ranch, eight detailed measured stratigraphic sections (separated from each other by 0.1 to 2.0 km) of the Oak Canyon and Cubero Members of the Dakota Sandstone display significant stratigraphic differences between adjacent fault blocks. Comparison to six control sections (separated from each other by 0.6 to 3.5 km) measured in unfaulted areas west of the Picuris–Pecos fault system (two sections near Lamy; four near Galisteo Dam) indicates the across-fault stratigraphic differences observed on the San Cristobal Ranch are too great to be attributed simply to lateral facies variation, but instead require strike-slip juxtaposition of dissimilar