New Mexico Geology

Upper Cretaceous sedimentary rocks are exposed in central Sierra County, southern New Mexico, in the Fra Cristobal Mountains, Caballo Mountains and in the topographically low Cutter sag between the two ranges. The ~2.5 km thick Cretaceous section is assigned to the (ascending order) Dakota Formation (locally includes the Oak Canyon [?] and Paguate [?] members), lower interval of the Mancos Formation (Graneros, Greenhorn, and Carlile members), Tres Hermanos Formation (Atarque, Campana, and Fite Ranch members), D-Cross Member of the Mancos Formation, Gallup Formation, Flying Eagle Canyon Formation, Ash Canyon Formation, and the McRae Group, consisting of the José Creek, Hall Lake, and Double Canyon formations. The name Tokay Tongue of the Mancos Formation is abandoned as an unnecessary term that reduces lithostratigraphic precision. The new name Campana Member of the Tres Hermanos Formation is proposed to replace the preoccupied (duplicate) name, Carthage Member. The terms Mesaverde Formation (Group) and Crevasse Canyon Formation are no longer applied to part of the Cretaceous section in Sierra County. Instead, these strata are the Flying Eagle Canyon Formation (new name) and redefined Ash Canyon Formation. The very thick McRae Formation is raised in rank to the McRae Group, and its constituent members are raised to formations. The Double Canyon Formation is a new lithostratigraphic unit comprising the upper part of the McRae Group. Ammonite and inoceramid bivalve biostratigraphy indicates that the lower interval of the Mancos Formation is of middle Cenomanian-early Turonian age, the Atarque Member of the Tres Hermanos Formation is early Turonian, the D-Cross Member of the Mancos Formation is middle Turonian, and the Gallup Formation is late Turonian. Vertebrate biostratigraphy indicates that the lower part of the Hall Lake Formation is Lancian (late Maastrichtian) in age. Depositional environments of the Cretaceous strata in Sierra County are both marine and nonmarine. They range from offshore marine (lower interval and D-Cross Member of the Mancos Formation), to shoreline deposition of various types (part of the Dakota Formation, the Atarque and Fite Ranch members of the Tres Hermanos Formation, and the Gallup Formation), to non­marine fluvial channel and floodplain deposits (part of the Dakota Formation, Campana Member of the Tres Hermanos Formation, Flying Eagle Canyon Formation, Ash Canyon Formation, and the entire McRae Group). A comprehensive understanding of the Cretaceous strata in Sierra County allows a more detailed inter­pretation of local geologic events in the context of broad, transgressive-regressive (T-R) cycles of deposition in the Western Interior Seaway, and also in terms of Laramide orogenic history: (1) T1 transgression of the seaway during middle-late Cenomanian time resulting in deposition of the Dakota Formation, Graneros Member of the Mancos Formation and Greenhorn Member of the Mancos; (2) Turonian R1 regression with deposition of the Carlile Member of the Mancos Formation and the Atarque and Campana members of the Tres Hermanos Formation; (3) late Turonian T2 transgression marked by deposition of the Fite Ranch Member of the Tres Hermanos Formation and lower D-Cross Member of the Mancos Formation; (4) R2 regression during late Turonian-early Coniacian time, with deposition of the upper sandy part of the D-Cross Member, the Gallup Formation, and the lower part of the Flying Eagle Canyon Formation; (5) no clear record of the T3, R3 or T4 events in Sierra County, but the Flying Eagle Canyon Formation likely encompasses the time from the last phase of R2 through T4 (Coniacian–Santonian); (6) R4 regression of early Campanian age and the onset of the Laramide orogeny, when deposition of the Ash Canyon Formation took place; (7) the first significant influx of volcanic detritus at the base of the McRae Group derived from the late Campanian-early Maastrichtian (~70–75 Ma) Copper Flat igneous complex; (8) onset of Hall Lake Formation deposition during the late Maastrichtian (~66–68 Ma); (9) deposition of the bulk of the Hall Lake Formation and the Double Canyon Formation, possibly extending into the Paleocene; and (10), the Love Ranch Formation of likely Eocene age representing the final pulse of the Laramide orogeny in Sierra County.