LATERAL FACIES CHANGES OF THE UPPER PENNSYLVANIAN BURSUM FORMATION, NEW MEXICO, AS A RESPONSE TO ANCESTRAL ROCKY MOUNTAIN DEFORMATION

The Bursum Formation of central New Mexico represents alternating nonmarine and shallow marine sedimentary rocks of latest Pennsylvanian (Virgilian) age transitional from the underlying, dominantly calcareous, shallow marine Atrasado Formation to the overlying continental red beds of the Abo Formation. Due to strong lateral facies variation, the Bursum Formation is divided into four members: (1) Oso Ridge Member, representing dominantly nonmarine, coarse-grained, proximal facies, grading laterally into the (2) Red Tanks Member, representing dominantly nonmarine, fine-grained, distal facies, grading into the (3) Bruton Member, a dominantly shallow marine, mixed carbonate-siliciclastic facies, and the (4) Laborcita Member, representing the shelf facies between the Pedernal uplift and the Orogrande basin. Facies and thickness variation within the Bursum Formation are observed over short distances, for example, in the Joyita Hills, Lucero uplift and Fra Cristobal Mountains. These variations preclude correlation of individual marine horizons over short distances greater than a few km, indicating that sedimentation was strongly influenced by local tectonic movements of the Ancestral Rocky Mountain deformation and less by global eustatic sea-level fluctuations. Tectonic movements are reflected by the common occurrence of carbonate conglomerates within the Bursum Formation composed of reworked Bursum limestones. The successions of the Bursum Formation also indicate that tectonic activity increased towards the C/P boundary and that the strongest tectonic event occurs at the Bursum/Abo contact, with locally strong reworking of the underlying Bursum Formation (e.g., in the Joyita Hills). It seems reasonable that the Ancestral Rocky Mountain orogeny of New Mexico and adjacent areas is the result of intraplate stress caused by the collision zone along the southwestern margin of North America. Intraplate stress produced basins and uplifts and particularly controlled basin architecture and sedimentation of the Bursum Formation, including the formation of sedimentary cycles, although glacioeustatic influences cannot be completely excluded.