EARLY LARAMIDE DEFORMATION IN NORTHERN NEW MEXICO AND SOUTHERN COLORADO

In the Late Cretaceous, prior to about 80 Ma, broadly uniform patterns of subsidence and sedimentation prevailed in the northern New Mexico and southern Colorado parts of the Western Interior Basin. Stratal accumulation rates (not decompacted) were ~15-60 m/m.y., as shown by 40Ar/39Ar-calibrated biostratigraphic zonation (Obradovich, 1993). Laramide intraforeland deformation first occurred ~80-75 Ma as shown by: (1) a doubling or tripling of stratal accumulation rates in the present axial parts of the San Juan Basin (Lewis Shale) and Raton Basin (upper Pierre Shale); (2) initial apatite fission-track (AFT) cooling in areas of Laramide uplift (Kelley et al., 1992) between these basins (~81 Ma, Nacimiento uplift; >74 Ma, Santa Fe Range); (3) a two-fold northeastward (seaward) thinning of the Lewis Shale between the axial part of the San Juan Basin and the San Juan sag indicates differential subsidence between these two areas began ~78-75 Ma; (4) syndepositional slumping within the Menefee Formation (~78 Ma) and stratigraphic thinning or omission of the Pictured Cliffs Sandstone (~75 Ma) along the western margin of the rising Nacimiento uplift; and (5) beginning of magmatism (~74 Ma; Semken and McIntosh, 1997) and related doming in the southwestern part of the Colorado Mineral Belt. Rapid subsidence and sedimentation in the San Juan and Raton Basins continued until the late Campanian or early Maastrichtian (~72-70 Ma). From ~72-70 Ma to ~67-65 Ma, stratal accumulation rates decreased in both basins, and periods of no accommodation resulted in development of widespread unconformities in both basins. Many workers have interpreted these unconformities and their superjacent, coarse-grained deposits (Ojo Alamo Sandstone, basal Raton Formation) to herald the beginning of the Laramide orogeny. Instead, these features appear to mark an episode of decreased subsidence and accommodation that intervened between the early and middle phases of the Laramide orogeny.