Stratigraphy, petrography, and depositional history of the Ignacio Quartzite and McCracken Sandstone Member of the Elbert Formation, southwestern Colorado, U.S.A.

The Ignacio Quartzite—exposed in the San Juan Mountains of southwestern Colorado—is composed of red and brown arkose and subarkose sandstones and minor interbedded shales. The formation is newly divided here into the Tamarron Member (0–24 m) and the overlying Spud Hill Member (0–21 m). The Spud Hill Member has a greater abundance of sandstones with shale clasts, weakly fissile shale beds, and trace fossils than the Tamarron Member. The McCracken Sandstone Member of the Elbert Formation, which overlies the Ignacio, is chiefly white and off-white quartz-cemented quartzarenites. The McCracken is divided for the first time into the Mill Creek facies (0–12 m) to the south of Coal Bank Pass and the Sultan Creek facies (0–36 m) to the north of the pass. The Sultan Creek facies contains dolostone-sandstone parasequence tidal-flat cycles up to 70 cm thick with a composite thickness of 14 m. Eastward transgression across the western edge of the Transcontinental Arch permitted the accumulation of fluvial deposits of the lower Tamarron Member in the deepest channels incised into the craton. As sea level continued to rise, fluvial channels evolved into estuaries dominated by sandy tidal flats (upper Tamarron Member, Mill Creek facies, and Sultan Creek facies) and mixed sand and mud tidal flats (Spud Hill Member). Sandstone composition and ages of detrital zircons indicate that sand grains were derived from a complex terrain that included granitoid plutonic rocks (∼0.46 to >2.4 Ga), metamorphic rocks, and well-rounded quartz sand from eolian ergs. The area between the present Coal Bank and Molas passes was a boundary between a northern fluvial source with an abundance of superbly rounded quartz grains of eolian erg origin from a southern fluvial source with few such grains, but large amounts of K-feldspar. The Ignacio and McCracken units are, at least in part, coeval and of Late Devonian age as shown by the stratigraphic distribution of rocks resting on the basement, the presence of an Ordovician zircon in the Ignacio, and the presence in the same Ignacio sample of oboloid brachiopods of questionably late Cambrian age with well-dated Late Devonian placoderm fish plates.