Triassic fossils found stratigraphically above 'Jurassic' eolianites necessitate the revision of lower Mesozoic stratigraphy in Picket Wire Canyonlands, south-central Colorado

Abstract

The recent discovery of Triassic tetrapod fossils in the Picket Wire Canyonlands of southeastern Colorado necessitates large-scale modification of the currently accepted stratigraphy of the area. The bone-bearing strata lie stratigraphically above a thick (∼80 meter [m]) eolianite historically identified as the Middle Jurassic Entrada Sandstone. The identifiable fossils include teeth and bone fragments of 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. Metoposaur bones consist of characteristically textured dermal bone fragments of the skull and pectoral elements, as well as a tooth. Phytosaur fossils consist of type C and B teeth, skull and jaw fragments, and some osteoderms. Aetosaurs are represented by several distinctive osteoderms, including some with evidence of prominent eminences and anterior bars. All identifiable tetrapods pertain to taxa known only from strata of Late Triassic age elsewhere, but none constrains the age of the fossil assemblage more precisely, although the assemblage is similar to lower Chinle Group assemblages of Carnian age (Otischalkian–Adamanian). The two most reasonable solutions to the discovery of Late Triassic index fossils stratigraphically above “Jurassic” beds are that the Triassic strata of this area have been mistakenly correlated with the Middle Jurassic Entrada Sandstone, or else the fossils are reworked into dramatically younger (Middle to Upper Jurassic) beds. The conglomerates are lithologically dissimilar from other Jurassic units regionally, but similar to Upper Triassic conglomerates of Wyoming (Gartra Formation) and New Mexico (Cobert Canyon Bed). Therefore, we consider the fossils to be in Upper Triassic strata. New lithostratigraphic data, including a composite measured section from the Picket Wire Canyonlands—as well as analysis and correlation of newly measured sections and others in the literature from south-central Wyoming, Colorado, Oklahoma, and New Mexico—suggest that the eolianite below the bone-bearing horizon and the finer clastic strata directly beneath the eolianite are best correlated to the Red Draw Member of the Jelm Formation. We correlate the bone-bearing conglomerates with 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 Picket Wire Canyon, are referred to the Middle Jurassic Ralston Creek (= Bell Ranch) Formation, a correlative of the Summerville Formation. These correlations 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 restudy of this unit.