New Mexico Geological Society Annual Spring Meeting — Abstracts

The dominant depositional basin across much of New Mexico during the Pennsylvanian-Permian transition was the Orogrande basin of the southern and central parts of the state. Strata that encompass that transition have been given several names: Bruton (1944), Bursum (1956), Aqua Torres (1946), Red Tanks (1946), Laborcita 1957) and Oso Ridge (1994) formations or members. In other areas, the transitional strata have either been assigned to the Abo Formation or to the Madera (Magdalena) Group Formation without a separate name. A single formation name, Bursum Formation, best describes these strata which constitute a mappable lithostratigraphic unit throughout their outcrop area. The Bursum Fonnation thus is as much as 330 m thick and is interbedded siliciclastic red beds and marine limestone and shale. The Bursum is distinguished from underlying Madera Group strata by its substantial content of red-bed shale and mudstone and some beds of limestone-pebble conglomerate and trough-crossbedded sandstone. Unlike the overlying Abo Fomlation, the Bursum contains beds of marine limestone and calcareous shale. Thus, the Bursum represents a transitional facies between the dominantly shallow marine carbonate facies of the underlying Madera Group and the continental red bed facies of the overlying Abo Formation. In the keel of the Orogrande basin (southern Oscura Mountains-northern San Andres Mountains), the Bursum grades laterally into the
upper part of the Panther Seep Formation and the lower part of the overlying Hueco Group.

Regional and local variation in Bursum lithofacies is best expressed by recognizing four members of the formation: (1) Oso Ridge Member, a thin (<12 m) unit containing much reworked local Proterozoic basement in the Zuni Mountains representing very proximal facies; (2) Red Tanks Member, moderately thick (<100 m) unit dominated by nonmarine shale and mudstone present in the Lucero uplift, Sandia, Manzano and Los Pinos Mountains and locally in the Joyita Hills; (3) Bruton Member, a moderately thick (< 85 m), dominant marine lithofacies present in the Joyita Hills, Cooke's Range and the Oscura, northern San Andres, Fra Cristobal and Caballo Mountains (= upper member of Bar-B Formation of previous usage); and (4) Laborcita Member, a thick (< 330 m) unit dominated by nonmarine red-beds, basement-cobb!e conglomerates and algal bioherms in the Sacramento Mountains. Sedimentation of the Bursum was strongly influenced by synsedimentary tectonic movements of the Ancestral Rocky Mountain orogeny, resulting in conspicuous lateral variations in lithofacies and thickness. The widespread Bursum Formation, of Virgilian and early Wolfcampian age, thus represents a significant tectonic pulse in the Ancestral Rocky Mountain orogeny.