Triassic-Jurassic stratigraphy in southwestern Colorado
In southwestern Colorado (south of the Uncompahgre Plateau), Triassic and Jurassic strata are locally exposed and most have been assigned parochial lithostratigraphic names that should be abandoned and replaced with the names of regional lithostratigraphic units. The Lower-Middle Triassic Moenkopi Group is divided into the (in ascending order) Tenderfoot, Ali Baba and Sewemup formations. These are mostly red-bed siliciclastic rocks typically about 107-137 m thick, but locally they are up to 400 m thick or more. Upper Triassic strata of the Chinle Group rest unconformably on the Moenkopi strata and belong to three formations (in ascending order): (1) Moss Back Formation, up to 49 m of sandstone and conglomerate; (2) Petrified Forest Formation, up to 90 m thick, mostly mudrock and some sandstone and conglomerate; and (3) Rock Point Formation, as much as 213 m of repetitively bedded siltstone and fine-grained sandstone. Sparse age data from southwestern Colorado and correlation to outcrops and age data in adjacent states indicate the Moenkopi Group in southwestern Colorado is Dienerian-Anisian in age, and the Chinle Group is Norian-Rhaetian in age. In southwestern Colorado, Middle-Upper Jurassic strata of the San Rafael Group (Entrada, Todilto, Summerville and Bluff formations) are ~ 300 m thick and consist of eolian sandstone, salina limestone and siltstone/sandstone deposited on an arid coastal plain. The Upper Jurassic Morrison Formation (sandstone-dominated Salt Wash and overlying, mudstone-dominated Brushy Basin members) is as much as 244 m thick and consists of sandstone and mudstone deposited in fluvial environments. Important regional unconformities in the Triassic-Jurassic section of southwestern Colorado are Tr-1 (base of Moenkopi Group), Tr-3/Tr-4 (base of Chinle Group), J-2 (base of Entrada Sandstone), J-3 (base of Todilto Formation), J-5 (base of Salt Wash Member of Morrison Formation) and K (base of Cretaceous strata above Morrison Formation).
- Lucas, S.G., 2017, Triassic-Jurassic stratigraphy in southwestern Colorado, in: The Geology of the Ouray-Silverton Area, Karlstrom, Karl E.; Gonzales, David A.; Zimmerer, Matthew J.; Heizler, Matthew; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 68th Field Conference, pp. 149-158.