Revised Paleozoic depositional history of the central Rocky Mountains (Utah and Colorado)

Abstract

Precise sedimentological, stratigraphic, paleontological, and geochronological data collected from Paleozoic strata of the Dinosaur National Monument (DNM) area (NW Colorado and NE Utah) allow for comprehensive revision of the depositional history of that part of the central Rocky Mountains.Recovery of multiple species of the trilobite genus Elrathiella from sandstone beds in the middle of the Lodore Formation assigns those deposits to the mid-Miaolingian Ehmaniella Zone near the top of the Wuliuan Stage. This is significantly older than the previous “Dresbachian” age estimate for those coarse clastics, which attributed them to deposition during the late Miaolingian (latest Drumian or Guzhangian). This age revision re-positions the southernmost boundary of widespread Miaolingian inner detrital belt deposits of Wyoming to within 90 km of an extensive area of central Colorado where strata of this age are absent, and middle Furongian successions rest directly on basement. The boundary between these two regions, one with and one without Miaolingian strata, is nearly coincident with the Cheyenne belt, an east–west oriented Precambrian continental suture zone at the northern margin of the Yavapai Terrane. This association supports the idea that deep-seated crustal-scale features controlled the Cambrian paleotopography of the Rocky Mountain region and, in turn, the regional pattern of Cambrian transgression across western Laurentia.Early cursory paleontological work suggested a Devonian age for a thick sandstone interval near the top of the Lodore Formation, based on phosphatic fossils. We demonstrate that these fossils are in fact Cambrian obolid brachiopod shells. However, we provide multiple lines of evidence, including carbonate δ13C chemostratigraphic data, distinctive lithofacies, and stratigraphic relationships to argue that a thin interval of shale and dolostone, as well as a previously unrecognized paleokarst and regolith interval at the top of what was mapped as Lodore Formation, are in fact part of the Upper Devonian Parting Formation. The thick sub-Mississippian paleoregolith is consistent with elevated rates of weathering during the Late Devonian–Early Mississippian. δ13C chemostratigraphic data for the Madison Limestone include a large double-peaked Kinderhookian positive isotopic excursion (up to ~6.5 ‰), providing a precise estimate of the timing of Mississippian transgression in this part of the Rocky Mountain region.Detrital zircon geochronologic data for Cambrian to Pennsylvanian strata show remarkably consistent patterns dominated by ~1100 Ma, ~1400 Ma, and ~1700 Ma peaks, reflecting relative igneous quiescence over much of the Paleozoic for this region, a span of ~200 m.y., despite some orogenic activity (e.g., Antler Orogeny, Ancestral Rockies) during that time interval.