PALEOGEOLOGIC ANALYSIS OF ANCESTRAL ROCKY MOUNTAINS SUGGESTS REGIONALLY VARIED AND DIACHRONOUS UPLIFT HISTORY

Analysis and comparison of diverse sedimentary information suggest that the Colorado-Utah Ancestral Rocky Mountains (ARM) were formed as complex, long-lived geomorphic elements. Data were summarized on isopach, facies, and subcrop maps to examine patterns and trends from Late Mississippian to Early Cretaceous. Proterozoic data suggest strong inheritance for ARM patterns. Both proximal and distal areas were analyzed to fingerprint regional patterns. Results were used to generate detailed paleogeographic maps that portray the dynamic history of ARM. Complex patterns of Pennsylvanian-Permian rapid uplift and erosion not only produced coeval narrow bodies of arkosic conglomerate and sandstone that abruptly fined into sandstone and mudstone away from uplifts, but also extensive sheets of sandstone and conglomerate. Height and/or extent of the mountains were rapidly diminished following uplift, but onlap patterns of Mesozoic strata suggest prolonged local topographic expression, possibly indicating subtle post-orogenic pulses of uplift; stratigraphic patterns in areas surrounding ARM also support changing patterns of uplift, erosion, and deposition over long periods of time. The mountains may not have been towering edifices because estimated volume of conglomeritic arkose and related deposits suggests that only two kms of uplifted basement is necessary across the area of ARM footprints to supply arkosic sediment. On the other hand, amounts of quartz sand in Colorado Plateau and Oquirrh basin are too voluminous to have come from ARM, so much was derived from Pangaean uplifts to south and east. The traditional model of elongate, unbroken, symmetrical, sausage-like uplifts mantled by numerous, short alluvial fans may not be an accurate portrayal of ARM topography; rather, a topography composed of distinct areas of local, asymmetrical, and diachronous uplift and long, incised, integrated stream systems and broad pediments better match facies patterns.