EARLY PENNSYLVANIAN PALEOGEOGRAPHIC ANALYSIS OF THE TAOS TROUGH, NEW MEXICO

The Taos trough is recognized by the accumulation of Pennsylvanian-early Permian sediments that were derived from nearby Precambrian basement-cored uplifts of the Ancestral Rocky Mountains. Two basin models have been proposed for the Taos trough: 1) a north-south trending flexural basin that was sourced solely from the westerly-located ancestral Brazos uplift; and 2) an intrabasinal Precambrian uplift model whose geodynamic and structural mechanism isn’t fully realized. Here we present new stratigraphic and sedimentologic data from the early Pennsylvanian eastern half of the Taos trough that bear on these two models.

The exposed Sandia Formation is ~ 930 m thick along the Holman grade of state road 518. This section is near the axis of most paleogeographic reconstructions of the Taos trough. Cycles typical of the lower ~170 m are characterized by shallowing upward cycles of marine dark shale through a beach succession and upward into fluvial channel tops. Cycles in the next ~ 360 m are entirely marine and characterized by a progressive shallowing of facies from dark shale, to increasing more sandstone upward with cycle tops of hummocky cross-stratified sandstone. Cycles within the upper 400 m are difficult to define owing to abundance of marine shale. However, the last ~ 50 m of section represents a transition to shallow water carbonates. Although lobe switching, sediment supply changes and glacio-eustatic likely affected each cycle, we posit that variations in subsidence is the strongest control. Thus, the overall trend in cycles from partially continental in the lower part of the section to entirely marine shale and back to shallow water carbonates may indicate a large pulse of subsidence that outpaced sediment supply in the middle of the section. Late Atokan conodonts recovered from the base of the section suggest that this pulse of subsidence likely began in the Desmoinesian.

In the same section, sediment transport indicators, such as soft-sediment folding on delta fronts, fluvial paleocurrents, clinoform progradation and dips in foreshore facies, all generally indicate transport directions from the east-southeast to west-northwest. These data indicate that sediment was sourced at least partially from the east and seem to corroborate the presence of intra-basinal Precambrian-cored uplifts within the Taos trough.