The Precambrian bedrock of northern Wisconsin hosts the 1.8-1.9Ga billion-year-old Penokean Orogeny. The Penokean Orogeny involves the collision of volcanic arcs and smaller continental crustal blocks. These blocks are known as the Pembine-Wausau Terrane and the Marshfield Terrane. The Eau Pleine Shear Zone marks the boundary between these two terranes. Recent studies within the Marshfield Terrane indicate a more complex geological structure than previously understood, the highly variable ages in this area highlight the need to re-evaluate the significance of this structural boundary between the Pembine-Wausau and Marshfield terranes. Clusters of magmatic rocks found near the Eau Pleine Shear Zone could help determine the nature of this crustal boundary. Magmas that are generated before, during, or after the collision inherit the geochemical and isotopic characteristics of the crustal blocks they interact with. For this research project, samples were collected from both intrusive and volcanic rocks that are found on both sides of the Eau Pleine Shear Zone, these samples were then prepared for geochemical and petrographic analysis. The results of this work will allow us to evaluate historical tectonic models of the Lake Superior region and determine whether the Eau Pleine Shear Zone is indeed an appropriate terrane boundary.
Climate change is impacting water resources globally. In the US Mountain West, warming is pushing watersheds beyond historical hydroclimate conditions and altering snowpack, groundwater recharge, and stream baseflow. This project investigates intermittent groundwater discharge, i.e., flow that pulses following snowmelt and ceases during late summer low-flow conditions. Here we address three research questions to better understand shifting ecohydrological baselines: (1) What groundwater flow path distributions support intermittent flow? (2) What is the chemical signature of intermittent groundwater discharge? And (3) How do groundwater age and flow duration relate to landscape geomorphological characteristics? The study will be conducted in the Sagehen Creek Basin, CA, a well-instrumented Sierra Nevada watershed with long-term climate, streamflow, and groundwater records. We will sample five intermittent groundwater sites for age-dating using CFCs and SF6, analyze major ion chemistry and field water quality parameters, deploy in-situ loggers to record flow persistence or absence, and compute high-resolution terrain metrics to evaluate landscape controls. Pending results and analysis will provide insight into how climate-driven changes in recharge and snowpack influence groundwater flow paths, water quality, and the resilience of mountain groundwater systems.
The volcanic rocks of the Penokean orogen of northern Wisconsin are well known for hosting 1875 Ma Cu-Zn-Pb-Au-Ag volcanogenic massive sulfide (VMS) deposits that formed in deep marine extensional settings. New U/Pb ages obtained from VMS-hosting volcanic strata suggest the extensional tectonic setting was active at 1835 Ma when previous tectonic models state the Penokean Orogen is thought to have closed the ocean. These inconsistencies in dates of samples from the Penokean Orogen may suggest a different geologic timeline than previously recognized. However, younger tectonic or hydrothermal may have altered the zircon U/Pb systematics. This study compares U/Pb ages taken from zircon cores or rims to test if there is a difference in the data. If there is a correlation between core/rim analyses and ages, then it is possible that the 1835 Ma age is an artifact of tectonic or hydrothermal events and not related to timing of VMS mineralization. Preliminary core vs. rim comparisons indicates there is no distinct difference in the ages. This suggests that the younger submarine extensional VMS-forming environment existed and that the timing of major events in the Penokean orogen need to be revisited.
