Dustin E. Sweet

Anomalous cold in the Pangaean tropics

The late Paleozoic archives the greatest glaciation of the Phanerozoic. Whereas high-latitude Gondwanan strata preserve widespread evidence for continental ice, the Permo-Carboniferous tropics have long been considered analogous to today9s: warm and shielded from the high-latitude cold. Here, we report on glacial and periglacial indicators that record episodes of freezing continental temperatures in western equatorial Pangaea. An exhumed glacial valley and associated deposits record direct evidence for glaciation that extended to low paleoelevations in the ancestral Rocky Mountains. Furthermore, the Permo-Carboniferous archives the only known occurrence of widespread tropical loess in Earth9s history; the volume, chemistry, and provenance of this loess(ite) is most consistent with glacial derivation. Together with emerging indicators for cold elsewhere in low-latitude Pangaea, these results suggest that tropical climate was not buffered from the high latitudes and may record glacial-interglacial climate shifts of very large magnitude. Coupled climate–ice sheet model simulations demonstrate that low atmospheric CO 2 and solar luminosity alone cannot account for such cold, and that other factors must be considered in attempting to explain this “best-known” analogue to our present Earth.

An Exhumed Late Paleozoic Canyon in the Rocky Mountains

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.

Late Paleozoic tectonics and paleogeography of the ancestral Front Range: Structural, stratigraphic, and sedimentologic evidence from the Fountain Formation (Manitou Springs, Colorado)

Sedimentologic, stratigraphic, and structural data indicate that the Fountain Formation at Manitou Springs consists of three tectonostratigraphic units, herein termed the lower, middle, and upper Fountain Formation. Both the lower and middle Fountain Formation were deposited in a fan-delta setting adjacent to the active ancestral Ute Pass fault. The lower is thin and predominantly characterized by depositional stability as recorded by abundant well-developed, deeply rooted paleosols. The middle Fountain Formation reflects fan progradation and highland rejuvenation as a result of increased activity of the ancestral Ute Pass fault. This activity was likely the result of a basinward splay of the ancestral Ute Pass fault with reverse dip-slip motion as indicated by (1) exhumation of the lower Paleozoic Sawatch Formation and (2) fault-parallel folding. The upper Fountain Formation reflects deposition in a NW-SE–trending, braided-river system and appears to postdate movement on the ancestral Ute Pass fault. Lithostratigraphic correlation allows for a Latest Pennsylvanian–Early Permian age for the upper Fountain Formation, thus constraining cessation of the ancestral Ute Pass fault to Middle to Late Pennsylvanian time. The Fountain Formation was deposited within a NW-SE–oriented structural trough (i.e., Woodland Park trough) that separated the ancestral Front Range into a northern block (i.e., ancestral Front Range block) and a southern block (i.e., Ute Pass uplift). The Woodland Park trough was bounded on its southern margin by the ancestral Ute Pass fault, which was active throughout deposition of the lower two Fountain tectonostratigraphic units, during which time sediments were shed northward across the ancestral Ute Pass fault into a marine environment. By the time of deposition of the upper Fountain Formation, movement on the ancestral Ute Pass fault had ceased or dramatically decreased. During this time, the Fountain Formation records predominantly axially oriented, braided-stream deposition. Comparison of the timing and kinematic history of the ancestral Ute Pass fault with other documented reverse and strike-slip faults of the ancestral Rocky Mountains suggests that: (1) cessation of faulting within the ancestral Rocky Mountains displays a slight east-to-west younging, but the crude age resolution of adjacent basin fill also allows for the possibility of relatively synchronous cessation of faulting, and (2) kinematics of all faults examined are consistent with a NE‑SW–oriented maximum horizontal compressional stress field. These inferences contrast with the proposed Laurentian–Gondwanan diachronous closure models for the ancestral Rocky Mountains, which require rotation of the maximum horizontal stress orientation through time and pronounced east-to-west younging of faulting. Rotation of the horizontal stress field may be recorded in multistage, kinematic histories of faults, but multiple faults require study to assess this model. Age constraints on post-tectonic units of the ancestral Rocky Mountains, such as the upper Fountain Formation tectonostratigraphic unit defined herein, are poor and further refinement would greatly aid understanding of the kinematic timing and tectonic driving force of these ancient mountains.

Upland Glaciation in Tropical Pangaea: Geologic Evidence and Implications for Late Paleozoic Climate Modeling

The late Paleozoic archives a prolonged icehouse, long recognized by means of Gondwanan continental glaciation. In contrast, the paleotropics have long been considered warm. Here we present the hypothesis of upland glaciation in the Ancestral Rocky Mountains (ARM) of western equatorial Pangaea, a region located within 11° of the paleoequator. The data to support this hypothesis include (a) a Permo-Pennsylvanian valley with glacial attributes and diamictite exhibiting rare striated clasts; (b) coarse-grained lacustrine strata onlapping the valley and preserving lonestones in Gilbert-type deltaic deposits proximally, along with (c) coarse-grained fluvial siliciclastic strata with microstriae and evidence for widespread flood deposition; (d) polygonally cracked paleosurfaces inferred to reflect frozen ground; and (e) voluminous paleoloess. Tropical glaciation occurs today at altitudes >4500 m and descended to 2100–3000 m at the last glacial maximum (LGM). However, ARM depositional systems terminating at sea level and emanating from inferred ice-contact facies indicate that ice-terminus elevations were lower (<1200–1600 m) than those of the LGM. If valid, tropical temperatures were ∼15°C cooler than today during intervals recording hypothesized tropical glacial conditions. This implies at least episodic cold within western tropical Pangaea, which conflicts with inferences from oxygen isotope paleothermometry. Furthermore, climate models for the late Paleozoic cannot reproduce tropical upland glaciation except under prohibitively low Pco2, implying the need to consider other forcings, such as cloud and aerosol behavior. Upland glaciation in the Permo-Pennsylvanian tropics was potentially widespread, given the global orogenesis accompanying Pangaean assembly. However, testing this hypothesis requires identification of pro- and periglacial indicators, owing to widespread erosion of upland (glaciated) regions. Midlatitude glaciation in both hemispheres also was likely, challenging climate models and paleogeographic consensus for this period.

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

We thank William Hood for his discussion of Soreghan et al. (2007). We recognize that our interpretation of the events leading to the formation of Unaweep Canyon, particularly our proposed Paleozoic age of the (ancestral) canyon, represents a significant departure from established models. Validation of our hypotheses regarding its age and origin would force revision of several longaccepted models, ranging from the Cenozoic tectonic and geomorphic evolution of this region to the climatic and perhaps tectonic framework of the Permo-Pennsylvanian tropics represented by this system. Accordingly, our work deserves close scrutiny, and Hood’s discussion provides such an opportunity. Hood (2009) begins his discussion by stating that Soreghan et al. (2007) presented the hypothesis that Unaweep Canyon is a Permian glacial valley that was filled by Paleozoic sediment and subsequently exhumed by Cenozoic rivers. To clarify, the focus of Soreghan et al. (2007) is the hypothesized Paleozoic age of the canyon, although we posed the question of a possible glacial origin in the final sentences of the article. A more complete analysis of the evidence for a glacial origin, however, appears in Soreghan et al. (2008), although only abstracts of this aspect (e.g., Soreghan et al. 2004) were published at the time that Hood submitted his discussion. Nevertheless, here we address all of the points raised by Hood; we treat each of his points using the subheadings he provides. Reexamination of the Field Evidence and Paleomagnetic Data

Anomalous cold in the pangaean tropics: Reply

We thank W. [Hood et al. (2009)][1] for their Comment on our Geology paper ([Soreghan et al., 2008][2]). We recognize that our hypothesis of episodic cold in the Pangaean tropics, which relies, in part, on our proposal of a late Paleozoic age and glacial origin for Unaweep Canyon, is radical. Hence