An Exhumed Late Paleozoic Canyon in the Rocky Mountains: A Reply

Abstract

Landscapes are thought to be youthful, particularly those of active orogenic belts. Unaweep Canyon in the Colorado Rocky Mountains, a large gorge drained by two opposite-flowing creeks, is an exception. Its origin has long been enigmatic, but new data indicate that it is an exhumed late Paleozoic landform. Its survival within a region of profound late Paleozoic orogenesis demands a reassessment of tectonic models for the Ancestral Rocky Mountains, and its form and genesis have significant implications for understanding late Paleozoic equatorial climate. This discovery highlights the utility of paleogeomorphology as a tectonic and climatic indicator.

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“…7). Furthermore, the Cutler Formation here projects into Unaweep Canyon, a hypothesized exhumed landscape with remnant Pennsylvanian-Permian fi ll (Soreghan et al, 2007(Soreghan et al, , 2009b. The age of the canyon fi ll is inferred from the combined evidence for its on July 11, 2012 geosphere.gsapubs.org Downloaded from exclusively Precambrian provenance, Pennsylvanian-Permian palynomorph content, and shallow (late Paleozoic) paleomagnetic inclinations (detailed in Soreghan et al, 2007Soreghan et al, , 2009b.…”

“…Furthermore, the Cutler Formation here projects into Unaweep Canyon, a hypothesized exhumed landscape with remnant Pennsylvanian-Permian fi ll (Soreghan et al, 2007(Soreghan et al, , 2009b. The age of the canyon fi ll is inferred from the combined evidence for its on July 11, 2012 geosphere.gsapubs.org Downloaded from exclusively Precambrian provenance, Pennsylvanian-Permian palynomorph content, and shallow (late Paleozoic) paleomagnetic inclinations (detailed in Soreghan et al, 2007Soreghan et al, , 2009b. A pre-Mesozoic age for the Precambrian (inner) gorge of Unaweep Canyon is also inferred from geomorphologic relationships wherein Mesozoic strata crosscut tributary valleys carved in Precambrian basement (Soreghan et al, 2007), analogous to the crosscutting relationships noted for the Wichita Mountains.…”

Anomalous cold in the Pangaean tropics: REPLY

“…Willard, 2008, personal commun.). Those four samples all occur in the basal 31 m of the (320 m) core (Soreghan et al, 2007(Soreghan et al, , 2009. Moreover, the appearance of late Paleozoic forms here overlaps with a shift to an exclusive Precambrian provenance, and the appearance of low paleomagnetic inclinations, which contrast with the mixed provenance and steep (modern) inclinations of overlying sediment.…”

“…Hence, scrutiny of our data and reasoning is justifi ed. Note that we addressed several of Hood's arguments about the canyon age in a separate Reply to a Discussion of Soreghan et al (2007) (Soreghan et al, 2009;Hood, 2009). Hood et al (2009) question the age of diamictite found in an outcrop and core within Unaweep Canyon.…”

“…Hood et al (2009) conclude by stressing their view of a Cenozoic age for the canyon, citing as evidence the infl uence of Laramide-age faults, and the "bowl-shaped" longitudinal profi le of the (bedrock) canyon. We cannot test the validity of these suppositions; no one knows the earliest ages of movement on any faults that may have infl uenced the canyon, and the longitudinal shape of the bedrock canyon is unknown because the basement location and relief is obscured by a fi ll now known to (locally) exceed 320 m depth and 2 km width (Soreghan et al, 2009).…”