REVISION OF LITHOSTRATIGRAPHIC NOMENCLATURE FOR THE LOWER CHINLE GROUP (LATE TRIASSIC: CARNIAN) BASED ON MAPPING IN THE CHAMA BASIN, NORTHERN NEW MEXICO

Extensive geologic field mapping in the Chama Basin of northern New Mexico has raised questions regarding the appropriateness of current nomenclature of the lower Chinle Group. Most recent revisions have been based on local stratigraphic section study and the utility of some stratigraphic subdivisions (especially formation rank) is suspect when mapping the units away from type areas. The Zuni Mountains Formation (= “mottled strata”) has been defined as a ~ 7 m thick, fine grained, extensively color mottled, bioturbated, pebbly quartz sandstone and shale that is present below and within the Shinarump Formation. The Shinarump Formation (= Agua Zarca) is a multi-storied, medium to very coarse, trough crossbedded quartz sandstone with extrabasinal clasts that varies considerably in thickness (~5 to > 40 m) and is locally color-mottled. The Salitral Formation consists of: the lower Piedra Lumbre Member, ~5 m of coarse grained, trough crossbedded quartz sandstones with extrabasinal clasts interbedded with green mudstones; the El Cerrito Bed, a 1-2 m thick, medium grained, crossbedded quartz sandstone; the upper Youngsville Member, ~30 m of brick red mudstone that is locally color mottled. Mapping of these formations leads to ambiguities that severely limit the utility of the current nomenclature. Issues include: 1) difficulties of confident distinction of the Zuni Mountains Formation from Shinarump strata and from deep paleoweathering profiles developed on underlying Permian strata and portions of Shinarump strata; 2) limited geographic distribution and outcrop size of mappable Zuni Mountains Formation; 3) confident identification of the base of the Salitral Formation within an interfingering contact with the underlying Shinarump Formation. A careful review of previously published stratigraphic sections and map relationships, and compilation of new sections indicate that revision of lower Chinle Group stratigraphic nomenclature is needed. We propose that the Zuni Mountains Formation and the lower member of the Salitral Formation be reassigned as members of the Shinarump Formation. Furthermore, the name “Zuni Mountains Formation” is not used in the Chama Basin because the weathered protoliths of the “mottled strata,” including Yeso Group, Cutler Formation and Shinarump Formation, can be readily identified.