Deposition and Diagenesis of the Bonneville Salt Flats

Abstract

The Bonneville Salt Flats (BSF) is an ephemeral saline pan consisting of a lens-shaped surface layer of interbedded halite and gypsum. This evaporite layer changes seasonally in response to flooding, evaporation, desiccation, and human uses. BSF is a remnant of the desiccation of Lake Bonneville, as an evaporite deposit in a fault bound basin it is an excellent analog for continental margin rift lake systems where halite serves as the cap/seal rock, such as those of South America and West Africa. This research explores the depositional and diagenetic history of the Bonneville Salt Flats, with a specific aim of understanding timescales of deposition and the development and alteration of porosity/permeability within the salt crust.

This research draws upon surface observations of the salt crust, thin sections, fluid inclusions, geochemical (mineralogical and elemental), and micro-CT analyses. It entails a stratigraphic assessment of ~70 shallow cores (~ 0.3 to 2 m) distributed across BSF. The crust overlies a carbonate mud which is the focus of micropaleontological study (ostracods, brine shrimp fecal pellets/ooids).

Preliminary results indicate that carbonate muds were deposited following the desiccation of Lake Bonneville ~13k ybp. Micropaleontological assessment will reveal trends in freshening/desiccation of this system. A season of deposition begins with precipitated chevron halite covered by cumulate rafts during the evaporation stage. These precipitates are cemented by efflorescence from evapotranspiration of subsurface brine during the desiccation stage (depth to brine is variable from ~5 cm to over 1 m). Micro-CT indicates that at the end of the desiccation period the halite crust has a porosity of 15 to 26% and a permeability of 2 x10^-6 cm² to 5x10^-6 cm². Human use for racing also compacts portions of crust during this stage. During the flooding stage efflorescence dissolves, dissolution pipes develop, and distinct layers of gypsum sand accumulate. Both vertical and horizontal porosity are present within the crust and diagenesis increases with burial.

This research extends our understanding of the development of halite crusts and the dynamics of changing porosity/permeability over time. It examines the conditions of deposition for halite with sealing potential. In addition, it explores the development of porosity and permeability within a heterogenous material with multiple stages of diagenesis.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018