LOAD-INDUCED SUBSIDENCE MODEL OF THE CORE ANCESTRAL ROCKY MOUNTAINS: THE HYPOTHESIS AND PRELIMINARY TESTS

The Ancestral Rocky Mountains (ARM) formed a collage of intraplate Precambrian-cored uplifts and intervening late Paleozoic basins, extending from Oklahoma—Utah—southern New Mexico. Soreghan et al. (2012) proposed that the ARM core, defined as Wichita, Front Range and Uncompahgre uplifts and adjacent basins, subsided in early Permian from isostatic adjustment of a high-density upper crustal root after horizontal compressional stresses relaxed. This hypothesis has testable consequences on ARM basins i.e. two distinct phases of subsidence: 1) Pennsylvanian stratigraphy should record ARM uplift history resulting from horizontal stresses and 2) Early Permian stratigraphy should record broad regional flexural subsidence spatially controlled by the location of the upper crustal root.

Preliminary research in the late Paleozoic Taos trough of northern New Mexico reveals hints that two distinct styles of subsidence occur. Commonly, this region is portrayed has a broad trough receiving sediment only from the westerly-located southern Uncompahgre uplift. However, the Pennsylvanian record suggests otherwise. Stratigraphic and soft-sedimentary fold analysis indicates that at least periodically delta fronts dipped to the northwest, necessitating an eastern source. Moreover, sedimentary facies range from fluvial to prodelta in the center of the trough in part controlled by varying rates of tectonic subsidence. These data suggest that in the Pennsylvanian the Taos trough experienced multiple episodes of renewed subsidence and sedimentary dispersal patterns were largely controlled by basement uplifts, all consistent with the Pennsylvanian history predicted by the load-induced subsidence model. However, the Permian section exhibits quite different characteristics. Strata of the region progressively thicken northward, strike east-westerly, and bury intrabasinal uplifts. Sediment dispersal was largely northward from southerly exposed localities. These data are consistent with flexural subsidence to the north of the Taos trough as predicted for the Permian in the load-induced model.

Soreghan, G.S., Keller, G.R., Gilbert, M.C., Chase, C.G., and Sweet, D.E., 2012, Load-induced subsidence of the Ancestral Rocky Mountains recorded by preservation of Permian landscapes: Geosphere, v. 8, p. 654-668.