Dustin E. Sweet

Climatic interpretations of the upper Paleozoic (Permo-Pennsylvanian) Fountain Formation, a coarse-grained fan-delta system that formed in western equatorial Pangea, are difficult to constrain owing to a general lack of climatic indicators so typical of coarse clastic systems. We applied scanning electron microscopy (SEM) to analyze quartz grains in this system in an attempt to test the hypothesis of a glacial influence on these strata. SEM observations of first-cycle quartz grains from these strata reveal microtextures formed from fracturing during grain transport, even after diagenetic overprinting occurred under moderate burial conditions (up to 3.5 km depth and 100°C). Transport-induced microtextures can be grouped based on inferred fracture process into: (1) high-stress fractures, consisting of fractures created through sustained high shear stress, such as grooves, deep troughs, and gouges, and are inferred to occur predominantly during glacial transport; (2) percussion fractures, consisting of fractures created by grain-to-grain contact during saltation or traction flow, such as randomly oriented v-shaped cracks and edge rounding; and (3) polygenetic fractures, such as conchoidal fractures, arc-shaped steps, linear steps, and linear fractures, that occur under a wide range of transport processes and thus possess no environmental significance. Delineation of high stress, percussion, and polygenetic fracture types demonstrate that the Fountain Formation quartz grains exhibit microtextures similar to both till and glaciofluvial deposits, suggesting that periods of upland glaciation occurred in the source region of the Fountain Formation (Ute Pass uplift). The abundance of high-stress fractures peaks at two stratigraphic intervals. These intervals are inferred to record the presence of ice in the Ute Pass uplift and are correlative with polygonally fractured paleosurfaces in the Fountain Formation that are interpreted to reflect cold-temperature weathering. Moreover, the peak intervals are approximately coeval with inferred episodes of ice maxima from high-latitude localities, as well as other low-latitude localities. Geologically reasonable stream gradients and estimated transport distance suggest a best-estimate elevation of the ice terminus of ~ 1500 m, but possibly ranging to 3000 m. These data suggest that upland glaciers episodically existed within this equatorial setting and that 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.

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.

The Permo-Pennsylvanian Cutler Formation in the proximal Paradox basin (western equatorial Pangea) has long been interpreted as low-latitude alluvial-fan deposits shed from the Uncompahgre uplift during the Ancestral Rocky Mountains orogeny. Here, we employ sedimentologic analysis of the Cutler strata within 10 km of its exposed source to hypothesize the alternative interpretation of deposition in a tropical proglacial–periglacial system. At this proximal location, the Cutler Formation onlaps Precambrian basement and dips gently southwestward, such that the section youngs distally. Along the onlap contact, crudely bedded and subhorizontal granule to cobble conglomerate of traction-flow origin is angularly juxtaposed atop inclined (15–30°) boulder diamictite and granule conglomerate of mass-flow origin (inferred subaqueous mass flows). The steeper dips form a semi-radial pattern that, together with facies evidence, suggest a subaqueous fan near this location. Immediately basinward (1–3 km from the onlap) is a gently dipping (≤ 10°) assemblage of cobble diamictite, granule conglomerate, sandstone, and mudstone of inferred subaqueous mass-flow origin. We propose that these associations collectively record topset, foreset and proximal bottomset deposits, respectively, of a lacustrine Gilbert-type delta. The slightly younger Cutler Formation exposed from 3 to 10 km from the onlap contains subhorizontal (≤ 4–7°) granule conglomerate and subordinate sandstone deposited by hyperconcentrated flood flows and traction flow in a fluvial system characterized by frequent high-discharge flow. Scattered siltstone units form a distinct assemblage reflecting silt genesis and eolian sorting (loess deposits), but with common overprinting by lacustrine and fluvial processes. We propose that these facies document the presence of generally abundant water. However, chemical and mineralogical immaturity record relatively minimal chemical weathering most compatible with a cold and wet system. Lateral to the inferred topset exposure, the Cutler strata fill a paleotrough within the Precambrian basement and contain abundant lonestones interpreted to record ice rafting, suggesting that the lacustrine system was, at times, ice-contact. Abundant faceted clasts and loess are also consistent with a glacial influence. Ultimately, the onlap of the Cutler strata onto the Precambrian basement of ancestral Unaweep Canyon, which has elsewhere been hypothesized to be a late Paleozoic valley of glacial origin, establishes a potential link between the two. We propose that, as an alternative to the long-held view of an alluvial fan in a hot (tropical) climate, the facies and climate indicators of the proximal Cutler system record a proglacial to periglacial, largely lacustrine–fluvial system related to tropical glaciation of the Uncompahgre uplift during the late Paleozoic. The proximity of marine and paralic facies to the Cutler “fan” system further implies that such glaciation resulted from cold temperatures, rather than high elevations, thus indicating a globally cool planet during the time recorded by the study section. If valid, this hypothesis raises the possibility that upper Paleozoic strata in other, uplift-proximal equatorial systems record anomalously cold conditions, and perhaps merit reassessment.