Geology of the Las Vegas area
We document a rich Upper Triassic bonebed in Borden County, Texas in either the uppermost Tecovas Formation or, more likely, the Trujillo Formation of the Chinle Group, that represents one of the highest stratigraphic occurrences of the aberrant archosauromorph reptile Trilophosaurus. The associated fauna includes indeterminate coprolites, unionid bivalves, osteichthyans, indeterminate reptiles, a possible ornithischian dinosaur, and the putative theropod dinosaur Spinosuchus caseanus Huene.
Discovery of Major Basement‐Cored Uplifts in the Northern Galiuro Mountains, Southeastern Arizona: Implications for Regional Laramide Deformation Style and Structural Evolution
The Laramide orogeny is poorly understood in southeastern Arizona, largely because of complex structural overprinting by mid‐Cenozoic extension that occurred over large areas. This study integrates new geological mapping with previous work, combined with structural reconstructions and forward modeling, to determine the primary structural style, timing, evolution, and kinematics of Laramide shortening in the northern Galiuro Mountains. Cenozoic normal faulting in the study area is minor and has only resulted in up to 13° of eastward tilting, as indicated by the gentle dips of synextensional strata. Detailed mapping has revealed newly identified reverse fault systems measuring at least 50 km in combined strike length. Each major fault strikes north‐northwest, dips moderately to the west, places older rocks on younger, and has related fault‐propagation folds. Once restored to their original orientation, reverse faults range in dip from 38° to 47°. These moderate dips of faults combined with related folds, the significant degree of basement involvement, and cover sequence lacking obvious penetrative deformation indicate that these faults are thick‐skinned, basement‐cored uplifts. Forward modeling and Cenozoic erosion surfaces suggest regionally extensive Laramide‐age tilting to the west‐southwest and gentle folding, possibly caused by a regional‐scale reverse fault underlying the study area. These results are consistent with the interpretation that Laramide shortening in southeastern Arizona was primarily characterized by thick‐skinned tectonics. Kinematic indicators, folded basement rocks, north‐northwest strikes of reverse faults, and lack of evidence for basin inversion suggest that preexisting basement faults and fabrics had little or no effect on the subsequent structural evolution.
Thrusting in Huerfano Park, Colorado, and related problems of orogeny in the Sangre de Cristo Mountains
INTRODUCTION Huerfano Park, in south-central Colorado, is bounded on the west by the Sangre de Cristo Mountains and on the east by the southern end of the Wet Mountains (Pl. 1). At its south end, the park opens onto the plains near the termination of the bordering Wet Mountains uplift, but, at the north, it merges across a low divide into the Wet Mountain Valley, with which it is, in part, structurally continuous. Huerfano Park itself is of particular geologic interest, because there are Eocene sedimentary deposits of Green River and lower Bridger age, equivalents of which are not found elsewhere along the eastern front of the southern Rocky Mountains. Thrusts of the Sangre de Cristo Mountains intersect these remnants of Eocene sedimentary rocks along the western border of the park, and thus record some later stages of mountain-building that might, otherwise, be scarcely distinguishable from much earlier stages of . . .