Polygonal cracking in coarse clastics records cold temperatures in the equatorial Fountain Formation (Pennsylvanian–Permian, Colorado)

Abstract

Sand- and granule-filled polygonal fractures are present on bedding surfaces within the equatorial Fountain Formation (Pennsylvanian–Permian, Colorado). The surfaces are areally extensive (> 120,000 m 2) and occur within inferred braided braided-river deposits. The fractures penetrate downward into coarse sandstone to granule conglomerate and range from 3–55 cm to 13–> 220 cm in width and depth, respectively. At one locality (Manitou Springs), both fracture fill and polygon interiors display a grain-supported texture and contain < 14% clay; additionally, the fracture fill is microbrecciated. At the other locality (Loveland), both the fracture fill and polygon interiors display a grain-supported texture and contain < 3.5% clay. The polygonal fractures 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. Furthermore, using maximum reasonable stream gradients (~ 0.02) between the polygonally fractured surfaces and the shoreline (gauged from shallow-marine deposits of the Denver basin), the fractures formed at relatively low elevation (≤ 1800 m).