LITHOSTRATIGRAPHY AND TECTONISM AT THE PENNSYLVANIAN-PERMIAN BOUNDARY, ANCESTRAL ROCKY MOUNTAIN FORELAND, CENTRAL NEW MEXICO

Across much of New Mexico, strata of the Pennsylvanian-Permian transition are the Bursum Formation, which is as much as 330 m thick, but generally less than 100 m thick, and composed of interbedded siliciclastic red beds and marine limestone and shale. The Bursum is distinguished from underlying Pennsylvanian strata by its substantial content of red-bed shale and mudstone and some beds of limestone-pebble conglomerate and trough-crossbedded sandstone. Unlike immediately overlying Permian strata, the Bursum contains beds of marine limestone and calcareous shale. Thus, the Bursum is transitional between shallow marine carbonate facies of the Pennsylvanian and the continental red bed facies of the Lower Permian. Regional and local variation in Bursum lithofacies is best expressed by recognizing four members of the formation: (1) Bruton Member, a moderately thick (< 85 m), mostly marine lithofacies present in the Joyita Hills (Sierra de la Cruz), Cooke's Range and the Oscura, northern San Andres, Fra Cristobal (Hellion Canyon, Red Gap) and Caballo Mountains; (2) Laborcita Member, a thick (< 330 m) unit mostly of nonmarine red-beds, basement-cobble conglomerates, bedded fossiliferous limestones and algal bioherms in the Sacramento Mountains; (3) Oso Ridge Member, a thin (< 12 m) unit containing much reworked local basement in the Zuni Mountains; and (4) 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 (Arroyo Tinajas). The Red Tanks Member in the Lucero uplift of central New Mexico thickens southward, and its upper part becomes more marine southward. However, south of the Lucero uplift there are substantial thickness and facies changes in the Bursum Formation. In the Joyita Hills east of Socorro the Bursum Formation is characterized by considerable lateral changes in facies and thickness within a relatively small area. Regional synsedimentary tectonic movements of the Ancestral Rocky Mountain orogeny strongly influenced Bursum sedimentation, resulting in such conspicuous lateral variations in lithofacies and thickness. The widespread Bursum Formation thus represents a significant tectonic pulse in the Ancestral Rocky Mountain orogeny.