Glaciation in equatorial Pangaea: Testing the hypothesis in the Pennsylvanian-Permian Fountain Formation (Colorado)

This study tests the hypotheses that glacial and proglacial conditions influenced sedimentation in the Late Paleozoic Fountain Formation where it crops out along the east flank of the Front Range of Colorado. In addition, data providing constraints on basin evolution in the Manitou Springs, Colorado, region and estimates of global sea level change are presented.;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 unit, 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.;Sand- and granule-filled polygonal fractures are present on bedding surfaces within the equatorial Fountain Formation (Pennsylvanian-Permian, Colorado). The polygonal fractures are inferred to have formed as frozen ground experienced thermal contraction induced by repeated cooling events. Owing to the equatorial location of the Fountain Formation, we suggest that diurnal, rather than seasonal, temperature variations provided the repeated cooling mechanism. Alternative causes of polygonal fracturing, such as desiccation of clay-rich sediments or thermal contraction of evaporite minerals, are untenable because the hosting strata contain minimal clay (<14%) and are framework-supported, indicating that there was insufficient space for either clay or evaporite minerals. A thermal contraction origin for these features implies that the equatorial Fountain Formation experienced at least two episodes of remarkably cold conditions.;Scanning Electron Microscopy (SEM) of first-cycle quartz grains from the equatorial, Pennsylvanian-early Permian Fountain Formation reveals microtextures that resulted from fracturing during depositional transport, even after diagenetic overprinting occurred under moderate burial conditions (up to 3.5 km depth and 100°C). Data from this study demonstrate that quartz grains from the Fountain Formation exhibit microtextures similar to both Quaternary till and glaciofluvial deposits, suggesting that periods of upland glaciation occurred in the Fountain Formation provenance region (Ute Pass uplift). Employing geologically reasonable stream gradients and estimated transport distance, elevation of the ice terminus is constrained to <1500 m. These data suggest that upland glaciers episodically existed within this equatorial setting and further use of this technique may reveal more evidence of ice in other proximal deposits of the ancestral Rocky Mountains, as well as other systems of various geologic ages.;At Manitou Springs, Colorado, the lower ∼240 meters of the Fountain Formation is composed of successive continental-marine cycles. Each cycle is characterized by alluvial-fan deposits overlain by a marine progradational package. A thin cobble conglomerate marks the base of marine strata and is inferred to represent a transgressive lag. Furthermore, each of these lags can be traced down depositional dip throughout the study area. The above field relationships allow for a minimum calculation of the magnitude of sea-level change because: (1) a minimum horizontal distance of transgression can be measured from the mapped transgressive lag deposits; and (2) the slope of the surface over which transgression progressed can be estimated through paleohydraulic analysis of the fluvial deposits. Magnitudes of sea level change over six successive cycles were calculated using an estimated depositional slope of 0.005; results suggest a minimum of 10-18 m of change. A large eustatic component is inferred for these six cycles because each marine package is separated throughout the study area by alluvial packages.;Glacio-eustasy is the result of ice growth and ice melt due to cyclical climate change. The data presented here indicate low levels of interglacial ice melt when compared to other times in the late Paleozoic. However, high- and low-latitude records indicate that cold conditions may have prevailed during the Morrowan-Atokan possibly suggesting that the estimates may reflect cooler interglacials with high-volume ice that experienced reduced melt. (Abstract shortened by UMI.)...