PERIGLACIAL FEATURES IN THE FOUNTAIN FORMATION (PERMO-PENNSYLVANIAN, COLORADO)?

The Fountain Formation (Permo-Pennsylvanian) near Manitou Springs, Colorado, comprises coarse clastics shed as an alluvial wedge off the Ancestral Front range during the Ancestral Rocky Mountains orogeny. Large bedding-plane exposures within this unit locally exhibit a distinct geometric pattern composed of white, erosionally resistant ridges 2-5 cm wide and up to 50 cm deep rimming red polygons 15-50 cm across. Both the white and the red lithologies comprise extremely poorly sorted and very angular granular arkose. However, in thin section, the white lithology tends to exhibit a silicified micro-brecciated texture whereas the red lithology exhibits a diamictite texture with clasts suspended in a muddy matrix comprising ~25% of the rock. Mineralogical analysis (FTIR) of the <62 micron fraction (matrix) indicates that, relative to one another, the white lithology contains substantially more quartz (<60%), whereas the red lithology contains substantially more clay minerals (<45%). In addition to the polygonal features, wedges filled with white granular arkose cut across a nearby stratigraphic interval. The wedges are up to 20 cm in width, penetrate to a depth of at least 2.5 m, and exhibit a faint vertical foliation. Furthermore, the host strata are bent upward immediately adjacent to the wedges.

On the basis of our preliminary analyses, we postulate that the polygonal features represent fossil patterned ground whereas the granule wedges record ice-wedge casts. The color contrast is a diagenetic enhancement, the result of bleaching of the original polygonal surface by reducing fluids channeled by permeability contrasts between the muddy arkose (red) and the granule fills (now white). Polygonal patterns result from contraction related to either desiccation (mud-cracking) in an arid environment or freeze-thaw in a cold (periglacial) environment. Owing to a lack of sufficient clay (~10%) overall, in addition to other considerations, we prefer a periglacial origin in lieu of a desiccation origin. Modern unsorted polygons form in mean annual temperatures of -2° to -10° C and slopes of <5°, within surfaces subjected to seasonal variations in moisture content. Thus, if our interpretation is correct, the Fountain Formation records extraordinarily low temperatures and seasonality in low-latitude western Pangea.