INCISED VALLEY DEPOSITIONAL SEQUENCE AND ROOT TRACES OF THE LATE DEVONIAN (FRASNIAN) TEMPLE BUTTE FORMATION IN MARBLE CANYON, EASTERN GRAND CANYON

The Devonian Temple Butte Formation is exposed as incised valley fills (IVF) in Marble Canyon. A widespread stream network incised into Cambrian Muav Limestone that filled with nonmarine to valley-mouth coastal beds. Valleys are steep-walled, flat-topped, with transverse to longitudinal sediment profiles as much as 120 m wide by 30 m thick. One exceptional ~20 m thick exposure offers an ideal IVF facies model, comprising three stacked units: basal and upper are discrete end-members; medial is a transition zone between them. The most distinctive deposits are concentrated in the upper unit, crowded with calcified progymnosperm root traces from rich forest growth.

The basal unit is reddish-purple carbonate-cemented siltstones with gray rhizohaloes, interpreted as paleosols, interbedded with <1 m thick gravelly lenses of locally-derived carbonate with lateral accretion beds of channels. Rhizohaloes are 5-31 cm diameter, range up to 1.3 m, and some have central vacuities. This unit is interpreted as lower valley mouth to floodplain with marine influence during transgression. The medial unit is more thinly bedded with characteristics transitional between base and top. The upper unit has pale tan, resistant, very thick carbonate beds of densely packed and cross-cut root networks, rarely with dark gray matrix and caliche nodules, and interbedded with broad, thin channel deposits. The well-cemented rhizoliths are 6-24 cm in diameter, with median lengths of 1.2 m, ranging up to 2.7 m. Root traces are vertical to oblique, branching, and taper downward. Based on analyses of field images and measurements, the rhizoliths are inferred to be roots of Archaeopteris, a major constituent of Frasnian landscapes.

Within each unit, 1-2-m-thick cycles commenced with an inherited hardground surface from transgressive erosion. The subsequent interval supported soil profile development, in which progymnosperm trees established a forest with large-scale root networks. Next, saltwater inundation during transgression led to the death and decay of trees, as well as further cementation. Hardground development and erosion paved the way for the next cycle. The preservation of Temple Butte root traces and paleosols provides a key window into a unique non-marine interval of deposition in the Paleozoic succession of Grand Canyon.