Surface and subsurface stratigraphy of the Burro Canyon Formation, Dakota Sandstone, and intertongued Mancos Shale of the Chama Basin, New Mexico

Abstract

—The Burro Canyon Formation (Lower Cretaceous) and the Dakota Sandstone (Upper Cretaceous: Cenomanian), including two tongues of the Mancos Shale, are present throughout the Chama Basin. The Dakota-Mancos section is divided into 7 marine members in the Chama Basin. The entire stratigraphic section thins southward across the Chama Basin, most notably the Clay Mesa Shale Tongue of the Mancos and the Paguate Sandstone of the Dakota. Prominent sequence-bounding unconformities, K1 and K2, bound the Burro Canyon Formation. A correlative conformity equivalent to the K3 sequencebounding unconformity occurs at the base of the Cubero Sandstone Tongue of the Dakota. A possible sequence boundary occurs at the base of the marine La Jollas bed, a local sandstone lens at the stratigraphic position of the upper Twowells Sandstone Tongue of the Dakota in the southern Chama Basin. FIGURE 1. Index map of the Chama Basin with location of cross-sections (Figs. 3, 4, and 5). The Tierra Amarilla Grant is not surveyed into townships and ranges. 219 STRATIGRAPHY OF THE BURRO CANYON FORMATION miscorrelation was corrected in Owen (2001, p. 32). Ridgley (1987) correlated the Burro Canyon Formation and units within the Dakota Sandstone in a series of uranium drill-hole logs to two measured sections near Heron Dam. She recognized the unconformities at the base and top of the Burro Canyon and divided the Dakota into informal units A (=Encinal Canyon Member), B (=Oak Canyon Member and Cubero Sandstone Tongue), and C (=Clay Mesa Shale Tongue of Mancos and Paguate Sandstone Tongue of Dakota). Varney (2001) proposed three sequences in the Dakota Sandstone at the Heron Dam section. Details are included in Varney (2005, this guidebook).