Light–Dark Banded Rocks on Salt Diapirs: Not All Zebras are Born Equal

‘Zebra’ rocks display a characteristic fabric of alternating light and dark-colored bands that range in size from millimeters to centimeters. They often occur in association with carbonate-hosted lead-zinc mineralization (Mississippi Valley Type) and hydrothermal dolomite in hydrocarbon reservoirs, but their origin(s) remain controversial. Several different alternating light and dark banded rocks associated with a salt wall have been found in Gypsum Valley, Utah in the Paradox Basin. The first type is made entirely of alternating planar layers of light, finely-crystalline calcite and dark, coarsely-crystalline calcite. The second type is the same finely- and coarsely-crystalline layers, but with reversed coloring. The third type shows non-planar, wavy laminae that alternate between white calcite and black quartz, whereby the lamination sometimes displays a dome-like morphology.

Understanding the genesis of ‘zebra’ rocks may provide crucial information about processes that lead to the accumulation of associated resources. With its diversity of zebra rocks, the Gypsum Valley location could provide the key to deciphering their origin. This project aims to 1) establish the paragenetic sequences for the different zebra varieties, and 2) interpret underlying mechanisms, including the assessment if hydrocarbon-consuming microorganisms mediated mineral replacement in the creation of wavy zebra banding. Our findings indicate that despite the geographical proximity of the three different types of zebra rocks, they not only differ in the alteration processes they experienced, but also in their origin.