Proximal Facies of the 1.38 (?)- 1.33 Ga Carrizo Mountain Group, Trans-Pecos Texas

Ongoing mapping of the ca. 1.33 Ga volcano-sedimentary rocks of the Carrizo Mountain Group (CMG)—some of the only exposed section of that age along the SE margin of Laurentia—shows that they are more proximal to volcanic sources in their NW exposures than previously-studied SE exposures. This forces a reappraisal of previously established stratigraphy.

Despite Grenville (ca. 1 Ga) deformation (mylonitization, 10- to 100-meter-scale isoclinal folding), and Bt-grade metamorphism in the study area, CMG primary structures are commonly preserved, and the stratigraphy is still relatively intact. Pyroclastic metarhyolites comprise the stratigraphically lowest units. These include local pyroclastic breccias, with pebble- to cobble-size clasts. A predominantly phyllitic unit occurs above the rhyolites, here provisionally named the Mineral Creek Formation (MCF). The phyllite is interbedded with metarhyolite, quartzite and coarser clastics (conglomerate and pyroclastic breccia), and rare calc-silicate schist. 1- to 30-cm graded beds (sandy to muddy or conglomeratic to sandy) are ubiquitous in the MCF; indeed, they are the primary way-up indicators in the unit. The stratigraphically highest unit in the study area comprises several hundred meters of cross-bedded quartzite and feldspathic quartzite.

The presence of interbedded pyroclastic breccias, local conglomerates, and ubiquitous grading indicates continuing active volcanism, relatively high relief (through faulting or volcanism), and episodic seismicity, mass wasting, or heavy rainfall (or some combination thereof). These stratigraphic elements are lacking from the SE CMG.

Previous stratigraphic division between the NW and SE exposures in the Carrizo Mountains (Grimes, 2005) was based on rhyolite ages of ca. 1.38 and 1.332 Ga, respectively. However, the ca. 1.38 Ga age is based on multigrain analyses with no detailed characterization of zircons. The stratigraphic sequence in the study area matches that in the upper part of the SE section, and may be the proximal equivalent—albeit with included xenocrystic zircon.