Abstract

The UGS recently obtained nearly 30 ⁴⁰Ar/³⁹Ar ages and over 200 geochemical analyses on basaltic flows or groups of flows from 27 volcanic centers in southwestern Utah. These analyses were obtained in support of new geologic mapping in the St. George basin and Zion National Park areas, and additional sampling is underway. We intend to present these data as the core of a new geochemical database of basaltic flows in southwest Utah. When complete, major oxides, minor and trace elements, isotopic ages, sample numbers and locations, flow names, and other information will be tabulated. We will include published information from other reliable sources. The database is being developed with the intent of providing researchers with purely descriptive information on these flows based on new detailed geologic mapping.

Our preliminary interpretations of data from these flows: (1) show that regional downcutting rates are largely a function of relative uplift, implying that flows of markedly different ages can have similar erosional profiles depending on their location in relation to major faults in the area, (2) constrain the displacement history of the Hurricane fault zone over the past 1 million years, and (3) show that while commonly indistinguishable in hand sample, the flows are chemically distinct. The new geochemistry and ages, together with detailed mapping, has improved our understanding of the extent of and relationships among basaltic flows in the region.

We find that the most useful major oxides and minor and trace elements for correlating flows are, in addition to the total alkali versus silica diagram, TiO₂, P₂O₅, Ba, Cr, Sr, and Zr. These variables typically show small but significant variation between flows, but only minor variation within flows. We are currently assessing other variation diagrams for their suitability in distinguishing flows.