New Mexico Geology

The Tokay Tongue of the Mancos Shale (new name) is that portion of the Mancos Shale lying between the undifferentiated or main body of the Dakota Sandstone and the Tres Hermanos Formation (or offshore equivalent). Its type locality is in the Carthage coal field, Socorro County, New Mexico. At its type section the Tokay Tongue is 575 ft (175 m) thick and consists of the following five bed–rank units (in ascending order): 1) the shale and sandstone unit, 182 ft (55 m) thick; 2) the calcareous shale and bentonite unit, 120 ft (36 m) thick; 3) the Bridge Creek Limestone Beds, 72 ft (22 m) thick; 4) the calcareous shale unit, 113 ft (34 m) thick; and 5) the noncalcareous shale unit, 88 ft (27 m) thick. The Tokay Tongue was deposited during the initial depositional cycle of the Late Cretaceous Seaway in New Mexico: units #1 and #2 as the western shoreline transgressed southwestward (T-1); unit #3 at maximum transgression (end T-1); and units #4 and #5 as the shoreline regressed northeastward (R-1). The Tokay Tongue at Carthage ranges in age from middle Cenomanian to middle Turonian; both its base and top are diachronous across its outcrop belt in southern New Mexico. There are more than 70 discrete bentonite beds in the tongue at its type locality, ranging in thickness from a fraction of an inch to 14 inches (36 cm); a 12 inch- (30 cm-) thick bentonite near the base of the tongue that lies just above strata containing the middle Cenomanian ammonite Acanthoceras amphibolum is correlated with the “x” or marker bentonite that occurs throughout the Western Interior. A thin, white limestone near the middle of the Bridge Creek Limestone Beds contains the bivalve Mytiloides puebloensis, which marks the base of the Turonian Stage at the Global Boundary Stratotype section and point for the Turonian Stage at Pueblo, Colorado.