ARMO REACTIVATION OF CAMBRIAN OR OLDER STRUCTURES: DETRITAL ZIRCON EVIDENCE FROM "STRUCTURELESS" SANDSTONES OF THE SOUTHERN FRONT RANGE IN COLORADO SPRINGS

In the Colorado Front Range, the sedimentary–tectonic association of Pennsylvanian Fountain Formation and Ute Pass fault (UPF) offers clear evidence for an active UPF in Frontrangia during the Ancestral Rocky Mountains orogeny (ARMO). Bordering the UPF, Fountain Formation thickness exceeds 1300m and the unit contains large-boulder clasts derived from proximal sources. Here we present new U-Pb detrital zircon data for tabular sandstones, dikes and sills that are hosted by crystalline basement along the UPF. The data provide evidence that 1) the quartz sandstones did not form during the ARMO, and 2) the formation of the UPF predates the ARMO.

Informally designated as Tava sandstone (Siddoway et al. 2013 GSA Field Trip guide), the UPF associated sandstones form massive tabular bodies bounded by faults, dikes and sills that all consist of fine or very fine quartz arenite matrix supporting isolated rounded granules and pebbles (0.5 to 5 mm). The ‘floating clasts’ and a near-absence of stratification and grading distinguish the Tava sandstone from Cambrian Sawatch Formation and younger strata, and indicate formation as hyperconcentrated flows and injectites associated with the UPF.

Detrital zircon U-Pb geochronology offers an avenue to determine Tava sandstone provenance and the time period of formation of dikes and sills as injectites, through comparison with detrital zircon reference distributions for North America (e.g Gehrels, this vol.; Gehrels et al. 2011, Lithosphere). Thereby Tava sandstone can constrain the age of the UPF. Here we report zircon data for 3 Tava and 3 Paleozoic quartz arenites analyzed at the Univ. Arizona Laserchron Facility. Tava zircons yield a broad 1.0 – 1.2 Ga age spread indicative of derivation from the Grenville orogen; notable is the lack of a Rocky Mountain source for the abundant 1.10 to 1.20 Ga detrital zircon. Elsewhere in the US West, sandstones with similar distributions are Cambrian or older. The Tava results contrast with those of the Glen Eyrie member of the Fountain Formation, which displays isolated narrow peaks at circa 1.72 and 1.48 Ga, and minimal presence of 1.08 Ga Pikes Peak Granite-derived zircon. We interpret the data to mean that Tava sandstone is Cambrian or older, a hypothesis being tested through comparison with the remaining two Paleozoic sandstones (in progress).