Tectonosedimentary Relations of Pennsylvanian to Jurassic Strata on the Colorado Plateau
William R. Dickinson, 2018. "Tectonosedimentary Relations of Pennsylvanian to Jurassic Strata on the Colorado Plateau", Tectonosedimentary Relations of Pennsylvanian to Jurassic Strata on the Colorado Plateau, William R. Dickinson
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The Colorado Plateau in the southwestern United States is within the Paleozoic transcontinental arch, an area of thin, cratonic strata. The plateau was broken by latest Mississippian to early Permian Ancestral Rocky Mountain orogenesis, which produced bedrock uplifts that influenced lower Mesozoic sedimentation before Jurassic burial. Clastic sediments shed from uplifts interfinger with eolian Permian strata ultimately derived from eastern Laurentia. Triassic and Jurassic strata of the Colorado Plateau are here divided into five depositional systems, each representing a different sedimentary and tectonic setting and forming stratal associations referred to as “deposystems.” The five deposystems, which largely but not entirely correspond to formation or group names, were deposited during northward continental drift from tropical latitudes (fluvial, tidal, and nearshore marine Moenkopi and fluvial Chinle) through desert latitudes (the erg-dominated Glen Canyon and San Rafael) to temperate latitudes (fluvial Morrison). Paleomagnetically determined paleolatitudes, corrected for inclination shallowing due to postdepositional sediment compaction, place the Glen Canyon and San Rafael eolianites firmly within expected latitudes for desert environmental conditions. Lower Triassic strata of the Moenkopi deposystem form a westward-thickening wedge of fluvial and shallow marine strata and are overlain by entirely fluvial strata of the Chinle deposystem. Both contain 240–280 Ma detrital zircon populations derived from the east Mexico magmatic arc, but more northern Chinle fluvial deposits contain a higher fraction of zircons derived from Paleozoic, Neoproterozoic, and Grenville provinces in eastern Laurentia. Westward thickening of Moenkopi strata is attributed to subsidence in the proforeland basin of the east-vergent Sonoma orogeny in central Nevada, whereas accommodation space for Chinle sedimentation was provided by dynamic subsidence above the upper Triassic subduction zone behind the newly established Cordilleran magmatic arc to the southwest. Overlying, largely Jurassic Glen Canyon and San Rafael deposystems are dominantly eolian. Detrital-zircon geochronologic analysis indicates that eolian sands were derived largely from eastern Laurentia. Interbedded marginal marine, lacustrine-sabkha, and fluvial strata have been associated with regional unconformities, but evidence for such unconformities is here regarded as indicating facies transgressions without development of plateau-wide unconformities or disconformities. Upper Jurassic northward continental drift carried the plateau out of the desert belt and into the zone of prevailing westerly winds. This coincided with a flare up of magmatism in the Cordilleran magmatic arc, leading to transgression of Morrison fluvial sediments over erg deposits of the San Rafael deposystem. Eastward dispersal of Morrison sediments marked the initiation of the Cordilleran orogen as the dominant topographic feature of the plateau region.