Covering the Great Unconformity in southern Laurentia during Rodinia breakup: Detrital zircon studies of provenance evolution during Cambrian marine transgression (Llano Uplift, Texas)

Abstract

The Llano Uplift in central Texas, USA, exposes the southernmost expanse of Laurentian crystalline basement in North America and the overlying lower Paleozoic strata deposited on the Great Unconformity. Systematic detrital zircon (DZ) U-Pb provenance analysis of the Hickory Sandstone—the basal unit of the Cambrian Riley Formation that onlaps the Mesoproterozoic core of the Llano Uplift—yielded locally variable DZ U-Pb signatures with ages ranging from 1800 Ma to 485 Ma (n >1700). The Hickory Sandstone zircons are dominated by 1550−1300 Ma (50%) and 1300−1000 Ma (46%) ages. These two dominant Mesoproterozoic DZ age components likely were sourced from local Grenville metamorphic and igneous basement of the Llano Province (1300−1000 Ma) and from the extra-regional Granite-Rhyolite Province basement to the north and northwest of the Grenville Front, which suggests both local sourcing and a regional drainage system supplying sediment to the southern Laurentian margin during the Cambrian. This interpretation is supported by a minor component of 1800−1600 Ma zircons, sourced from the Yavapai-Mazatzal Province, and zircons with Early Cambrian ages that were likely sourced from the Wichita Igneous Province, which is located to the northwest and to the north of the Llano Uplift, respectively. Documented NNW-trending topographic ridges in the Mesoproterozoic basement surface of the Llano Uplift could have funneled aeolian and fluvial sand supply to the southern Laurentian coast that was reworked in the marginal marine environment. Samples from the western Llano Uplift are dominated by regionally sourced, early Mesoproterozoic Granite-Rhyolite Province DZ ages, whereas samples from the eastern Llano Uplift exhibit a dominant locally sourced Grenville signature. Hickory Sandstone samples also contain a small number of Neoproterozoic (850−600 Ma) and Cambrian (541−487 Ma) zircons. Sources for Neoproterozoic zircons likely were located along the Laurentian continental margins and derived from extension-related magmatism associated with the breakup of Rodinia. Cambrian zircons are most common in Hickory Sandstone samples on the southern and western flanks of the Llano Uplift. Some are reasonably sourced from the Wichita Igneous Province to the north, but the younger Cambrian zircons suggest sources to the west. Upper Hickory strata are dominated (>60%) by 1550−1300 Ma grains with up to 10% >1600 Ma Paleoproterozoic grains. Upper Cambrian sandstones of the Wilberns Formation also contain a significant contribution of >1600 Ma grains, which suggests a possible enlargement of the fluvial headwaters over time extending into older Laurentian provinces to the north and northwest. Differences among DZ populations over relatively limited distances may have been related to NW-oriented ridge and corridor landforms that developed on the Great Unconformity surface, which were products of the Proterozoic tectonic and lithologic architecture affected by aeolian and fluvial processes at this unusual stage in Earth history. Paleocurrent data and atypical detrital grain characteristics for a Hickory Sandstone locality in the southeastern Llano Uplift and its DZ age similarities with Cambrian sandstones of the Argentine Precordillera suggest a common provenance prior to or during Rodinia breakup.