TRIASSIC TETRAPOD FOSSILS FOUND ABOVE "JURASSIC" STRATA IN PICKETWIRE CANYON, SOUTHEASTERN COLORADO, REQUIRE MAJOR REVISION OF LOWER MESOZOIC STRATIGRAPHY ACROSS THE SOUTHERN HIGH PLAINS

Recent discovery of Triassic tetrapod fossils in the Picketwire Canyonlands of SE Colorado necessitates large-scale modification of the currently accepted stratigraphy of the area. The bone-bearing strata lie atop a thick eolianite historically identified as the Middle Jurassic Entrada Sandstone. Fossils include teeth and bone fragments from typical Late Triassic tetrapods, including metoposaurs, phytosaurs, and aetosaurs, recovered from thin (m-scale) discontinuous channels of limestone-pebble conglomerate deposited in a high energy fluvial environment. The assemblage is reminiscent of lower Chinle Group faunas of Carnian age (Otischalkian-Adamanian). The two most reasonable solutions to this conundrum are that the Triassic strata of this area have been mistakenly correlated with the Entrada Sandstone, or else the fossils are reworked into dramatically younger (Middle to Upper Jurassic) sediments. The conglomerates are lithologically dissimilar from other Jurassic units regionally, but similar to those in the Upper Triassic of Wyoming, Texas and New Mexico. Therefore, we consider the fossils to be in Upper Triassic deposits. New lithostratigraphic data, including a composite measured section from the Picketwire Canyonlands, analysis and correlation of new measured sections and others in the literature from south-central Wyoming, Colorado, Kansas, Oklahoma, and New Mexico, suggest that the eolianite below the bone-bearing horizon is best correlated to the Sips Creek Member of the Middle Triassic Jelm Formation, and the finer clastics directly beneath the eolianite to the Red Draw Member of the Jelm Formation. The bone-bearing conglomerates are correlated to the Cobert Canyon Bed at the base of the Chinle Group, described by previous authors as limestone and lithic pebble conglomerate underlying the Travesser Formation in northern New Mexico. The gypsiferous and clastic strata overlying the conglomerates and below the Morrison Formation, ~30 m higher in Picketwire Canyon, are referred to the Middle Jurassic Ralston Creek Formation. These correlations dramatically extend the known distribution of Jelm Formation strata southeastward from north-central Colorado and south-central Wyoming and highlight the need for a major, modern study of this unit.