AGES AND PROVENANCE OF DETRITAL MINERALS, TRIASSIC DOCKUM GROUP, TEXAS PANHANDLE

Redbeds of the Late Triassic (Carnian to Norian) Dockum Group are exposed around the perimeter of the High Plains of the Texas Panhandle, and in eastern New Mexico. The upper Dockum Trujillo Sandstone and overlying Cooper Canyon Formation contain detrital zircon, biotite, and deformed muscovite grains. Paleocurrent analysis indicates fluvial transport from a southeasterly source, candidates for which are (i) Grenville-age basement of the Llano Uplift, central Texas, (ii) metamorphic rocks of the Ouachita Interior Zone (today in the subsurface), and (iii) older grains recycled from Paleozoic sediments.

On a Rb-Sr isochron diagram, data from 26 muscovite samples (0.6 to 15 milligrams) resolve into two linear regressions corresponding to (i) t = 338 ± 6 Ma, R_i = 0.722 ± 003, and (ii) t = 371 ± 1.4 Ma, R_i = 0.7093 ± 0.0003. Population (i) muscovite, with a wide range of Rb/Sr and a radiogenic protolith, was deposited low in the stratigraphic sequence. Population (ii) muscovite, with uniformly low Rb/Sr and a less radiogenic protolith, was deposited higher in the sequence. A muscovite isochron age (n = 5) from the Llano Uplift is 1.16 ± 0.02 Ga. There is no hint of mixing between Llano and Dockum muscovite populations.

Detrital biotite is weathered, the freshest sample providing a model Rb-Sr age of 225 Ma, which is the age of Dockum deposition. Laser-ablation U-Pb ages of Cooper Canyon detrital zircons are clustered in the range of the muscovite Rb-Sr ages, but also with upward “streaking” to mid-Proterozoic values.

Ages of Dockum clastic detritus, deposited in a vertical sequence of 170 meters and over a lateral distance exceeding 460 kilometers, exhibit a consistent pattern of provenance. The Ouachita Interior Zone supplied abundant muscovite and zircon, as younger (~338 Ma) more radiogenic rocks were unroofed, followed by older (~371 Ma) less radiogenic metamorphic rocks. Yet older zircon suggests that Paleozoic sedimentary rocks in the Ouachita Frontal Zone were a subordinate source. Biotite is from contemporaneous volcanism. The Llano Uplift was not a significant source for Dockum sediments.