GEOLOGY OF THE BREAD SPRINGS 7.5-MINUTE QUADRANGLE, MCKINLEY COUNTY, WEST-CENTRAL NEW MEXICO

Not previously mapped larger than scale 1:125,000, the Bread Springs quadrangle near Gallup, New Mexico, showcases the importance of updated fine resolution geologic mapping to address societal and scientific issues. For the first time in the Gallup area at scale 1:24,000, the Late Cretaceous Crevasse Canyon and Menefee formations are being mapped at the member level. Members are traceable to surrounding quadrangles, and help reveal subtle intra-basin Laramide folds related to regional structure. Both aspects are important for subsurface hydrology in an area where water levels have dropped 60 m since the year 2008. Three previously unidentified normal fault populations are mapped: (1) NE-striking up to 10 m throw normal faults are perpendicular to fold axes and do not cut overlying Late Cretaceous units, suggesting ca. 85 Ma motion; (2) A SSW-striking normal fault (or faults) of unknown throw and age close to the Zuni arch front; and (3) A NW-striking 10–20 m throw normal fault zone of unknown age near Gallup that cuts Late Cretaceous strata. Obvious hill slope changes are coincident along this latter fault, but Quaternary alluvium is not discernibly displaced. Lastly, perched gravel deposits in the southeast part of the map composed dominantly of Permian Glorieta Sandstone and a white-purple rhyolite define an upslope surface from ~2230–2300 m elevation eastward toward the Zuni Mountains. The age of these perched gravels is currently unknown, but their elevation and composition suggests (1) a possible connection to Miocene Bidahochi Formation deposits located in the southwest part of the map, their basal contact at ~2210 m elevation, and (2) that this paleo-drainage eroded a Laramide thrust sheet in the Zuni Mountains, with possible connection to the Mogollon-Datil volcanic field 100 km further southeast.