Abstract

The synthesis of mudstone petrology and interbedded sandstone detrital zircon geochronology provides insights into the provenance, weathering regime, hydrodynamic effects, and diagenesis of the Mesoproterozoic Unkar Group. Unkar mudstones were derived largely from the Grenville Orogen (GO) of southwest Texas and the adjacent Yavapai–Mazatzal (YM) and Southern Granite Rhyolite (SGR) terranes. Detrital zircon data indicate distinct pulses of GO-derived sediment during Hakatai and Dox Formation deposition whereas Shinumo sediment contains a larger component of YM and SGR material. An increase in plagioclase and biotite abundances in the Dox further suggest an orogenic pulse. Bulk chemistry, including elevated Cr and Ni abundances and REE systematics, implicate a heterogeneous provenance for Unkar sediment best approximated by a mix of granite or granodiorite with some basalt.

Weathering of Unkar sediment, as determined from mudstone and crystalline-source bulk-chemical trends, is characterized as moderate (CIA values between 55 and 70) indicative of a temperate climate in an orogenic setting. Small (less than 10 wt%) amounts of detrital kaolinite and chlorite are consistent with a moderate (temperate) weathering regime. Illite, the dominant clay mineral in the Unkar Group, is largely the 2M₁ polytype and detrital in origin. However, it is estimated that ~ 7% of the K in the Escalante Creek Member results from K-metasomatism and may form up to ~ 15 wt% authigenic 1M or 1M_d illite.

The clay-size fraction and zircon are the chief contributors to the REE budget in Unkar mudstones. Lithostratigraphic variations in the HREE distributions can be attributed partially to variable zircon abundance in the silt-size fraction, a likely function of hydrodynamic sorting. Age constraints on the duration of Unkar Group sedimentation range from 30 to 100 My for the ~ 1100 m conformable Shinumo–Dox succession and result in compacted sedimentation rates of between 1.0 and 3.6 cm/10³ yr. These rates are comparable to Mesozoic foreland-basin settings.