SYNDEPOSITIONAL ANCESTRAL ROCKY MOUNTAIN TECTONICS AND THE LATE PENNSYLVANIAN BURSUM FORMATION, JOYITA HILLS, CENTRAL NEW MEXICO

Pennsylanian Bursum Formation is a transitional facies between underlying shallow marine limestones (Madera Group) and overlying continental red beds (Abo Formation). In the Joyita Hills, Socorro County, the Bursum is a mixed siliciclastic-carbonate succession of continental and shallow marine sediments deposited in the Lucero basin, on the eastern flank of the Joyita uplift. We measured three Bursum sections--Sierra de la Cruz, Arroyo Tenajas and Loma de la Cañas--that differ significantly in thickness (53 m to 95 m) and facies. The Late The main Bursum lithotypes are red and purple, and subordinate gray and greenish gray, mudstone with intercalations of marine and rare pedogenic limestone, sandstone and conglomerate. Most limestones are bioclastic wackestones and bioclastic mudstones; rare fusulinid wackestones and packstones are in the lower part of the sections. The three sections display a strong lateral facies change over relatively short distances. At Sierra de la Cruz, limestones are more abundant (18.5%) than at the other sections, and an approximately 10 m thick, fossiliferous limestone horizon in the middle part contains a diverse invertebrate fauna. Sandstones and conglomerates are rare (2.5%). At Loma de las Cañas, coarse clastics (immature arkosic sandstones and conglomerates) are the dominant lithofacies intercalated in mudstone and shale. Limestone intercalations are rare and restricted to thin nodular beds. At Arroyo Tenajas, marine limestone beds are developed in the middle part, and sandstones and conglomerates are rare. The only horizon that can be correlated across all three sections are the limestones in their middle parts that represent a major transgressive event. The strong lateral and vertical facies changes over relatively short distances indicate sedimentation was strongly influenced by synsedimentary tectonics. This is underscored by N/NW-striking faults that were active during the ancestral Rocky Mountain deformation and are truncated by the Abo Formation.