Cortical Stratigraphy of Ooids from Great Salt Lake, UT

Ooids are a type of carbonate sand composed of a concentrically laminated cortex precipitated around a central nucleus that occur commonly in carbonate successions of all ages. They form through the combination of constructive and destructive mechanisms: growth via precipitation and diminution via physical abrasion. We posit that each cortical lamination represents a "deposit"—an interval of net precipitation on the ooid surface; each contact between laminations reflects an unconformity—an interval of net abrasion juxtaposed between growth episodes. Because growth and abrasion obey distinct morphometric rules, we developed an approach to quantitatively invert the history of growth and abrasion of individual ooid grains using the record of evolving particle shape preserved by its cortical laminae. We designed a model that fits particle shape data by simulating >10⁶ possible growth-abrasion histories for pairs of cortical laminae in an individual ooid, using recent experimental constraints on ooid growth and abrasion shape parameters and rate data. This approach provides best estimates for the durations of growth and abrasion of each cortical layer by inverting the simulated history that best fit the observed particle shape. We applied this approach to thin sections of 'modern' lacustrine ooids collected from several locations in the Great Salt Lake, UT, to assess the spatial and temporal variability of environmental conditions—from the perspective of individual grains within a single deposit. Results show that Great Salt Lake ooids do not all share the same histories: clustering ooid histories by Fréchet distance reveals distinct subpopulations, some specific to an individual locality and others shared among localities. These findings are consistent with recent radiocarbon constraints on ooid ages and the expectation of varying frequency and mode of transport in different parts of the lake. This general approach to invert ooid cortical stratigraphy could be applied to characterize environmental variability over short timescales—i.e., the lifetimes of ooids, of order 1000 years—in both marine and lacustrine ooid grainstones of any geologic age.