An Exhumed Late Paleozoic Canyon in the Rocky Mountains

Abstract

A B S T R A C TLandscapes 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.

A model-based evaluation of tropical climate in Pangaea during the late Palaeozoic icehouse

“…To the contrary, it has been argued that there was extensive glaciation across both the western and eastern Central Pangaean Mountains (e.g., Becq-Giraudon et al, 1996;Soreghan et al, 2007Soreghan et al, , 2008aSoreghan et al, , b, 2009Sweet and Soreghan, 2008;Hastenrath, 2009;. G.S.…”

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.…”

“…This is an interesting departure from Hood (2009), who attempted to refute a glacial origin for these strata, preferring an origin as a talus deposit. Regardless, Cenozoic glaciation here is inconsistent with its low elevation (Soreghan et al, 2007). 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.…”

“…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.…”

“…Oesleby's (1978) primary intent was to gauge fi ll thickness, and his assertion of a thick fi ll was, 25 years later, confi rmed by drilling (Soreghan et al, 2007). However, as Oesleby (1978) himself stated, and Cole and Young (1983) reiterated, the limitations of his data and methods, including the longitudinal orientations of his profi les, precluded an accurate assessment of the transverse basement profi le.…”