DECIPHERING EARLY PENNSYLVANIAN PALEOGEOGRAPHY OF THE ANCESTRAL ROCKY MOUNTAINS IN THE TAOS TROUGH REGION THROUGH SOFT-SEDIMENTARY FOLD ANALYSIS: SANGRE DE CRISTO MOUNTAINS, NEW MEXICO

The Ancestral Rocky Mountains (ARM) formed a collage of uplifted Precambrian basement rocks (highlands) with intervening sedimentary basins (troughs). Paleogeography of the ARM has been ever evolving. In northern New Mexico, the Taos trough is most commonly portrayed as a single, north-south trending basin that was filled with sediment derived from the westerly located Uncompahgre uplift. Here we present stratigraphic, structural, and sedimentologic data that indicates that portions of the earliest basin fill (early Pennsylvanian Sandia Formation) was derived from intrabasinal highs located to the east of the Taos trough depocenter.

Facies analysis of a measured stratigraphic section indicates two distinct facies. The mudstone facies (Ms) is comprised of both organic-rich shales and siltstones that vary in coloration from black, gray-brown, or dark gray. These sediments are extremely fine grained, thinly laminated, and fissile. The sandstone facies (Sm) is distinguished by the appearance of sharp based, tan to buff colored sandstone beds of varying thicknesses. Sm facies is found sporadically throughout the measured section intercalated with the Ms facies. It is common to find fragments of brachiopods, crinoids, or plant material in either of these facies. Taken together, these stratigraphic data are inferred to record delta front sedimentation.

Fold axes and hinge line data were measured in thirteen syn-sedimentary folds occurring within the Ms facies. These data show a wide variance of axial plane orientations, but in general, the fold axes have a preferred eastern dip orientation (i.e. westward vergence). The unrestored data was then corrected to early Pennsylvanian orientations by restoring nearby, coeval fluvial bed attitudes to horizontal. Final paleo-corrected fold orientations were found to be similar to field measurements though slightly less consistently eastward dipping. This syn-sedimentary fold vergence indicates that the delta slope dipped to the northwest. Thus, sediments were being sourced from the east, likely near present day Mora, New Mexico. An eastern source contradicts an exclusive western source shown in most paleogeographic reconstructions.