New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1900, C. L. Herrick named the “Sandia Series” for a “series of shale, sandstones, and conglomerate with occasional brands of limestone” about 50 m thick in the Sandia, Manzano and San Andres Mountains. Although no type section has ever been designated from the Sandia formation, the unit has long been recognized and mapped in central and northern New Mexico as the basal, clastic-dominated interval of the Pennsylvanian section. We designate a lectostratotype section (principal reference section) of the Sandia Formation at Doc Long Campground in the Sandia Mountains (SE ¼ sec. T11N, R5E). This well-exposed, fossiliferous and readily accessed section well represents Herrick’s and subsequent workers concept of the Sandia Formation.

At the lectostratotype section, the Sandia Formation rests with nonconformity on Proterozoic granite and is disconformably overlain by cherty limestone at the base of the Gray Mesa Formation. The Sandia Formation is ~ 124 m think and consists of shale (41% of the measured section), sandstone/ conglomerate (36%) and limestone (23%) and can be divided into lower (0-52 m) and upper (53-124 m) parts. The lower part is mostly gray to black shale, one layer yielding fossil plants. Sandstone and conglomerate beds are relatively thin (<4.2 m thick), arkosic and ripple laminated or crossbedded. Limestone beds are mostly gray, fossiliferous bioclastic wackestone; some beds contain abundant siliciclastic material. The upper part of the lectostratosphere section of the Sandia Formation consists of less shale with thicker sandstones and limestones similar on lithology to beds in the lower part. Some of the limestone beds have abundant bryozooans and chert nodules. We interpret deposition of the lectostratotype Sandia Formation to have taken place in brackish to marine costal facies (dark shales with marine fossils), fluvial channels and upper shoreface and sand bodies (sandstones/conglomerates) and shallow marine shelf facies (limestones). Irregular stacking of different lithotyoes and interbedding of coarse nonmarine lithofacies with fine marine lithofacies indicate syndepositional tectonics heavily influenced sedimentation. The Sandia Formation at the lectostratotype section is thus largely a synorogenic deposit of the first pulse at the Ancestral Rocky Mountain orogeny in the central New Mexico foreland.