REEVALUATING THE DISTRIBUTION OF THE LAVA CREEK TUFF, YELLOWSTONE

Recent work on the Huckleberry Ridge Tuff (HRT, 2.08 Ma, 2500 km³), the oldest and largest caldera-forming eruption of the Yellowstone Plateau Volcanic Field (YPVF), reveals that the opening phase of the eruption tapped eight separate magma bodies. Upon eruption, these once discrete bodies were chaotically mixed and distributed in all directions around the collapsed caldera (Swallow et al. 2019). While detailed field and geochemical investigations have deepened our understanding of the storage configuration and eruption dynamics of HRT magmas, a similarly in-depth study of the younger and spatially proximal Lava Creek Tuff (LCT, 0.631 Ma, 1000 km³) eruption is lacking. Currently, the LCT is divided into two members, A and B, defined by reverse welding profiles and phenocrystic amphibole. We use outcrop, hand sample, and thin section observations to begin the process of reconstructing the storage configuration and emplacement history of LCT magmas in comparison to the HRT, and to reevaluate the current classification and distribution of LCT-A and -B.

To begin the process of understanding the LCT eruption and its distribution, ignimbrite bulk samples were collected at six locations around the Yellowstone Caldera, with data from 22 preliminary samples presented here. Using physical descriptions and thin sections, we are able to visually distinguish four rock types based on glassy vs. devitrified matrix and crystal content. These varying characteristics of the LCT exhibit overlaps between members A and B as currently defined by Christiansen (2001). Additionally, we have not found amphibole in any of the samples examined thus far. Further, use of single-sanidine chemistry has found compositional dependence based on location, which has the potential to help restore eruption dynamics. These preliminary data suggest that LCT-A and -B are not as simple as previously thought and require reevaluation. Additional samples from seven localities along with single grain and melt chemistry will aid in our understanding of how the LCT is distributed around the park and if redefining the members is required.