HOW FAR WEST DOES THE ANCESTRAL ROCKY MOUNTAINS OROGEN EXTEND? TECTONIC ANALYSIS OF THE LATE PALEOZOIC ELY-BIRD SPRING BASIN OF NEVADA AND UTAH

During Pennsylvanian to early Permian time, much of western Laurasia experienced broad intraplate deformation. Most of this deformation involved basement-cored uplifts and associated basins of the Ancestral Rocky Mountains (ARM) orogeny. Due to poor exposure and post-Paleozoic tectonics, driving mechanisms for many of these basins are not well understood. At that time on the western Laurasian margin a series of stacked, unconformity-bounded, tectonically-generated basins formed as part of the Antler Overlap sequence. One of these basins, the Ely-Bird Spring basin (EBSB) initiated in mid-Carboniferous time, synchronous with many of the ARM basins. Though coeval with the ARM basins, the EBSB is generally not considered as part of the ARM orogeny.

In part, this study tests whether the EBSB has anything in common with ARM-style basins. EBSB tectonic evolution was elucidated by a combination of 1-D geohistory analyses and detailed carbon isotope chemostratigraphy of six stratigraphic sections throughout the EBSB. We evaluated the tectonic driving mechanism of basin formation by comparing tectonic subsidence curves to plots from basins with known tectonic settings (cf. Xie and Heller, 2009). Geohistory plots from the EBSB were compared to existing analyses from four western ARM basins (Oquirrh, Paradox, and Eagle basins and the Wyoming Shelf; Johnson et al., 1992) The geohistory analyses for the EBSB and Wyoming Shelf are compatible with foreland basin systems, whereas the other ARM basins are similar to strike-slip basins. Carbon isotope chemostratigraphy in the EBSB shows the most rapid sediment accumulation rates in the northwestern part of the basin. This, combined with the geohistory analyses, is consistent with flexural response to loading from the west. Therefore, the EBSB seems to have formed due to proximal loading at the northwest corner of the basin, not as a far-field ARM basin. However, the combination of far-field stresses from tectonic loading in western Laurasia and in southern/southeastern Laurasia may help explain the rapid subsidence in the Oquirrh basin, the westernmost ARM basin.