Stratigraphy of the Bisbee Group (Jurassic-Cretaceous), Little Hatchet Mountains, New Mexico
— Spencer G. Lucas and Timothy F. Lawton
Upper Jurassic–Lower Cretaceous strata of the Bisbee Group exposed in the Little Hatchet Mountains of Grant and Hidalgo Counties, New Mexico, are assigned to four formations: Broken Jug, Hell-to- Finish, U-Bar, and Mojado. The Broken Jug Formation is 1228 m thick and consists of marine strata and volcanic rocks assigned to the (ascending) dolostone, lower conglomerate, fine-grained upper conglomerate, and basalt members. The Hell-to-Finish Formation is 525 m thick and consists of two members named here: (1) Stone Cabin Gulch Member, a lower, 200-m-thick interval of interbedded bioturbated siltstone and marine limestone; (2) Winkler Ranch Member, an upper, 325-m-thick interval of conglomerate, enterolithic dolostone and red siltstone with pedogenic calcite nodules deposited in fan-delta and fluvial channels, sabkha and subaerial coast-plain environments, respectively. The U-Bar Formation is 237 m thick in the central part of the range at Howells Ridge, where it consists of the Carbonate Hill, Old Hachita, and Howells Ridge members. In the southern part of the range at Winkler Ranch, it is 972 m thick and consists of the Carbonate Hill, Old Hachita(?) and Still Ridge members. The Still Ridge Member is mostly skeletal sandstones deposited in shallow-water carbonate- ramp settings coeval with rudistid reefs that make up the Howells Ridge Member. The U-Bar Formation thus differs substantially in thickness and facies from north to south. The Mojado Formation is 1245 m thick and consists of the Fryingpan Spring, Sarten, and Rattlesnake Ridge members. Biostratigraphic data support the following age assignments: Late Jurassic upper conglomerate member of Broken Jug Formation; late Aptian- Carbonate Hill Member of U-Bar Formation; early Albian-Old Hachita Member of U-Bar Formation; middle Albian-Howells Ridge Member of U-Bar Formation; late Albian middle? Cenomanian-Mojado Formation. There are substantial unconformities at the Stone Cabin-Winkler Ranch, Carbonate Hill-Old Hachita, and the U Bar- Mojado contacts. The unusually thick, complete Bisbee Group section in the southern part of the Little Hatchet Mountains was deposited near the thick keel of a rift basin that formed in Late Jurassic time. This section provides an important link between marine Jurassic rocks in the Chiricahua Mountains of southeastern Arizona and the Chihuahua trough in northern Mexico. Thickness changes in the U-Bar Formation record differentialsubsidence in the Bisbee rift basin.
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- Lucas, Spencer G.; Lawton, Timothy F., 2000, Stratigraphy of the Bisbee Group (Jurassic-Cretaceous), Little Hatchet Mountains, New Mexico, in: Southwest passage. A trip through the Phanerozoic, Lawton, Timothy F.; McMillan, Nancy J.; McLemore, Virginia T., New Mexico Geological Society, Guidebook, 51st Field Conference, pp. 175-194.