Middle Pennsylvanian composite stratigraphic section of Holman Grade records cyclic fluvio-δic sedimentation within the Taos trough axis

The Taos trough is a structurally-controlled basin formed from stresses related to the ancestral Rocky Mountain orogeny. Pennsylvanian sediments derived locally from Precambrian-cored uplifts fill the basin. However, structural models proposed for the development of the basin conflict. One model proposes flexure from emplacement of crustal load via a thrust fault to the west of the basin. The other model invokes intrabasinal strain manifested in thrusts with Precambrian-cored hanging walls and a strike-slip bounding fault to the west.

To test these competing models, we have accumulated detailed sedimentologic and stratigraphic data from a location within the axis of the Taos trough. Specifically, we construct a detailed composite stratigraphic column for the Holman grade area using a series of road cuts along Highway 518. Construction of the column accounts for along strike duplication of strata, covered section and broad folding in the area. The composite section is ~950 m thick and is primarily composed of shallowing upward cycles that grade upward from offshore marine mudstone to a cycle cap of lower shoreface facies, upper shoreface facies, or nonmarine facies. These cycles are best characterized as fluvio-deltaic cycles. The persistent occurrence of shallow marine and nonmarine facies at the base of the section are difficult to reconcile with a flexural basin model, which often predicts a long-term, gradual shallowing as flexural accommodation that is progressively filled. Our analysis appears to show the opposite trend, albeit inconclusive, as the interval studied does not represent the entire section.