THE VIRGILIAN-WOLFCAMPIAN BURSUM FORMATION AND ANCESTRAL ROCKY MOUNTAIN TECTONICS IN NEW MEXICO

Across much of New Mexico, strata of the Pennsylvanian-Permian transition are best given a single formation name, Bursum Formation, which 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 Formation, the Bursum contains beds of marine limestone and calcareous shale. Thus, the Bursum is transitional between the dominantly shallow marine carbonate facies of the Madera Group and the continental red bed facies of the Abo Formation. In the keel of the Orogrande basin (southern Oscura Mountains-northern San Andres Mountains), the Bursum grades laterally into the upper Panther Seep Formation and the lower 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 basement in the Zuni Mountains; (2) Red Tanks Member, a moderately thick (<100 m) unit of mostly 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), mostly marine lithofacies present in the Joyita Hills, Cooke's Range and the Oscura, northern San Andres, Fra Cristobal and Caballo Mountains; and (4) Laborcita Member, a thick (< 330 m) unit mostly of nonmarine red-beds, basement-cobble conglomerates and algal bioherms in the Sacramento Mountains. Synsedimentary tectonic movements of the Ancestral Rocky Mountain orogeny strongly influenced Bursum sedimentation, resulting in conspicuous lateral variations in lithofacies and thickness. The widespread Bursum Formation (Virgilian-early Wolfcampian) thus records a significant tectonic pulse in the Ancestral Rocky Mountain orogeny.