POSSIBLE SIGNATURES OF AN ACTIVE FORELAND BASIN SYSTEM IN THE EARLY CRETACEOUS DAKOTA SANDSTONE, NORTHERN SAN JUAN BASIN, SOUTHWESTERN COLORADO

Little attention has been given to the regional role of plate tectonics and its relationships to stratigraphic sequences and packages displayed in the Early Cretaceous Dakota Sandstone of the northern San Juan Basin of southwestern Colorado and northwestern New Mexico. Subduction and the formation of the Sevier fold-thrust belt in Nevada and western Utah led to the development of a foreland basin system that migrated eastward toward Colorado. Regional isopach data delineate possible foredeep, forebulge and backbulge depozones during fluvial and marine Dakota Sandstone deposition. Additionally, extrapolated forebulge positions based on rates of forebulge propagation from previous studies of Late Jurassic/Early Cretaceous strata in NE Utah/NW Colorado are consistent with a backbulge depression at the location of the San Juan Basin in the Early Cretaceous. Outcrop and subsurface data in this study reveal that aggradational and retrogradational stratal geometries and facies patterns within the Dakota Sandstone of the San Juan Basin are also consistent with deposition in a backbulge portion of this eastward propagating foreland basin system during the simultaneous southwesterly advance of the Cretaceous Western Interior Seaway. These sequences are interpreted as stalled shoreline/fluvial facies due to an increase in accommodation space induced by flexural subsidence of the back-bulge basin. These three lines of independent evidence suggest that the timing of subsidence and the resulting stratigraphic architecture of the Dakota Sandstone in the San Juan Basin may be better understood as a distal, backbulge portion of the Sevier foreland basin system.