DEPOSITION DOWNWIND OF THE COLORADO PLATEAU ERGS: UPPERMOST TRIASSIC?-MIDDLE JURASSIC STRATA OF THE PLOMOSAS UPLIFT, EAST-CENTRAL CHIHUAHUA, MEXICO

The Plomosas Formation in the Plomosas uplift, located 90 km NE of the city of Chihuahua, contains diverse sedimentary facies that record deposition in an extensional back-arc setting. Conglomerate, channel-form pebbly sandstone complexes, and heterolithic sandstone-siltstone were deposited in alluvial fan, ephemeral fluvial, and tidal to marginal-marine settings, respectively. Abundant red to gray siltstone, locally exceeding an aggregate thickness of 1000 m, was deposited in tidal-flat and overbank environments. Marginal-marine sandstones locally contain abraded spherical grains that suggest prior eolian transport.

New detrital and igneous U-Pb ages indicate that the Plomosas Formation, formerly considered Permian, likely spans latest Triassic(?)-Middle Jurassic time, equal to the age of the Glen Canyon and lower San Rafael groups, including the Wingate, Navajo and Page erg deposits of the Colorado Plateau. A distinctive ignimbrite assemblage, ~184-179 Ma, separates informal lower and upper members of the Plomosas Formation. Prominent Triassic (~252-201 Ma) and Grenville (~1250-900 Ma) detrital zircon (DZ) age groups dominate the lower member and broadly resemble DZ ages of the Springdale Sandstone Member of the Moenave formation in southern Utah (Dickinson & Gehrels, 2009, GSA Bull). The upper Plomosas member contains the Triassic age group and a diverse suite of Archean, Paleoproterozoic, Grenville, Neoproterozoic and early Paleozoic DZ grain ages. The pre-Triassic age distribution is strikingly similar to DZ age distributions in the Navajo, Aztec and Nugget sandstones and attributed to eastern Laurentian sources (ibid.).

The Plomosas Formation provides a basis for extending Jurassic eolian sediment well southeast of the erosional edge of the Jurassic erg deposits in northern Arizona and New Mexico. Similar DZ age populations in the lower member and Springdale Sandstone suggest a common provenance in Triassic igneous rocks locally exposed in northern Mexico. Upper Plomosas deposits contain a component of reworked distal eolian sand from the Navajo and Page ergs, indicating that trail’s end for some Appalachian grains lay in northern Mexico. Thick Plomosas siltstone intervals represent a preserved remnant of a desert loess fringe that formerly lay downwind of the Jurassic sand seas.