Stratigraphy and sedimentology of the Santa Fe Group adjoining the Little San Pascual Mountains – implications for footwall exhumation and evolution of the Little San Pascual Mountain fault zone
— Daniel J. Koning and Matthew T. Heizler

Detailed stratigraphic and sedimentologic investigation of the Santa Fe Group adjoining the Little San Pascual Mountains (LSPM) allows reconstruction of an exhumational history for these mountains during the middle Miocene–Quaternary. One can also interpret the evolution of the two main strands of the 035°-striking Little San Pascual Mountain fault zone (LSPMFZ) that bounds the west side of the mountains; this fault is the master fault for the east-tilted, Bosque del Apache half graben (Cikoski, 2010). The Santa Fe Group between the two strands of the LSPMFZ consists of the >50 m thick, early Pleistocene–age Sierra Ladrones Formation unconformably overlying (with angularity) the 700–800 m thick Popotosa Formation. In the Popotosa Formation, an unroofing sequence is recognized by up-section changes in the composition and texture of the conglomerate. Lower strata are characterized by conglomerate clasts dominated by rounded-subrounded limestone with minor, variable percentages of reddish Permian-derived clasts and 1–5% rounded chert. This gravel assemblage closely resembles that of the Eocene Baca Formation. About 50–100 m up-section, Glorieta-derived, reworked sandstone clasts appear, and the gravel becomes slightly more heterolithic; also, intermediate volcanic clasts increase in abundance northwards. In the next overlying 400 stratigraphic meters, red and yellow Permian-derived clasts progressively increase up-section relative to gray limestone clasts, and there is an upward decrease and disappearance of rounded chert and Glorieta Sandstone clasts. Conglomerate in the uppermost 300 m of preserved Popotosa strata is dominated by red and yellow, relatively angular, Permian-derived siltstones and very fine sandstones, which are mixed with minor dolomite and limestone. Age control is provided by a detrital sanidine age of 1.641±0.004 Ma from a pumiceous pebbly sandstone bed in the middle of the Sierra Ladrones Formation and correlation of a >100 m thick basin-floor facies, containing thickly cross-stratified eolianites, to 12–9 Ma strata 6–10 km to the northwest (Cikoski, 2010). We interpret the following exhumational history of the LSPM. In the middle Miocene, the LSPM were low hills capped by the limestone-dominated San Andres Formation, and sediment shed westward into the adjoining Bosque del Apache graben was predominately derived from erosion of the Baca Formation to the east of the LSPM. At 12–8 Ma, displacement rates increased along the east strand of the LSPMFZ, initiating footwall uplift of the LSPM and an eastward shift of basin floor facies toward the immediate hanging wall of that fault. As uplift progressed, lowermost San Andres Formation limestone and Glorieta Sandstone, as well as the underlying Yeso Group, were eroded from these mountains. At the end of the studied Popotosa stratigraphic record, inferred to be 9–7 Ma (but poorly constrained), exposed bedrock in the LSPM was mostly Permian. If one assumes that the readily erodible Permian did not produce notable topographic relief, as is evident by the low topography of Permian strata cropping out in the southern LSPM, then one can use modern topography and the bedrock stratigraphic record to calculate an uplift rate of 53–64 m/ my from ca. 7 Ma to 1 Ma. During the Pliocene, extensional strain increasingly shifted to the west strand of the LSPMFZ, resulting in an angular unconformity in the Popotosa Formation between the west and east strands of the LSPMFZ. Based on 15 m of observed vertical displacement of lower-middle Sierra Ladrones Formation strata (2.0–1.0 Ma) and its basal unconformity, throw rates for the east strand of the LSPMFZ were 7.5–15.0 m/my during the Quaternary.