BRIDGING THE GEOSCIENCE GAP: SOUTH TEXAS UNDERGRADUATE STUDENTS UNCOVER THE GREAT UNCONFORMITY IN THE LLANO UPLIFT BY ZIRCON LASER ABLATION (U-TH)/HE

The North American Great Unconformity was first defined in the Grand Canyon as Paleozoic sedimentary rocks overlying crystalline basement. It is today recognized as a global phenomenon marking a substantial stratigraphic gap of up to 1.5 billion years in the geologic record between the Proterozoic and early Phanerozoic. The nature and cause of the Great Unconformity remains hotly debated and generally falls into two competing hypotheses: (1) Extensive erosion associated with the diachronous breakup of the Rodinia supercontinent (~850-600 Ma), or erosion and subsequent non-deposition linked to multiple global glaciation events referred to as the Cryogenian “Snowball Earth” period (~720 - 630 Ma). In the Llano Uplift of Central Texas, the Great Unconformity is expressed as an unconformity between late Mesoproterozoic Grenville basement, incl. the late-orogenic Town Mountain granites (~1070 Ma), and the Upper Cambrian Hickory Sandstone (~535 Ma). While hypotheses regarding the Great Unconformity's origins have been extensively studied on a number of localities in North America, the Llano Uplift in Central Texas remains underexplored.

This study leverages laser-ablation zircon (U-Th)/He (ZHe) thermochronometry to explore thermal histories from several Llano Great Unconformity locations. This novel technique allows for the rapid analysis of large number of zircons, compared to conventional methods, and does not require HF dissolution. Numerical modeling, conducted using HeFTy, constrains the thermal history of the samples, allowing us to reconstruct a detailed exhumation and burial history. These new ZHe results reveal a significant period of rapid exhumation of the Llano basement at ~850 Ma which appears to be linked to Rodinia rifting and to predate “Snowball Earth” glaciations. Hence, our data provide strong evidence for a tectonic cause of the Great Unconformity in central Texas. Our results also identified three major phases of Phanerozoic burial and exhumation, including burial depths of up to 3 km during the Mesozoic. Our findings offer a refined understanding of the Great Unconformity's complex history in the Llano Uplift, highlighting the interactions between tectonic activity, supercontinent cycles, glaciation events, and the region's long-term thermal evolution.