OOID CORTICAL STRATIGRAPHY: THE TOPOLOGY OF CONCENTRIC LAMINATIONS IN CARBONATE SAND GRAINS RECORD GRAIN-SCALE ENVIRONMENTAL HISTORIES (Invited Presentation)

Ooids are a type of carbonate sand grain composed of a concentrically laminated cortex precipitated around a central nucleus that occur commonly in carbonate successions of all ages. They are unique among sedimentary grains in that they form through the combination of constructive and destructive mechanisms: growth via precipitation and diminution via physical abrasion. Each 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. Previous authors have recognized that the stratigraphic information recorded in ooid cortices could be used to discern paleoenvironmental conditions, specifically in the case of bimineralic ooids whose cortices contained laminae composed of two different CaCO₃ polymorphs—calcite and aragonite. However, applications of the concept of ooid cortical stratigraphy have remained limited to qualitative assessments, in large part because there is no framework with which to quantify and compare these stratigraphic relationships within ooid cortices. We developed an approach to study cortical stratigraphy using grain shape—e.g. roundness, sphericity, and aspect ratio—that quantitatively constrains an individual grain’s history of growth and abrasion. Recent work has demonstrated that surface-normal growth, collisional abrasion, and frictional abrasion produce predictable and distinguishable trajectories in particle shape. Because each cortical layer provides a record of the former grain shape and size, the radial differences between adjacent boundaries can be used to estimate the volumes of carbonate added via growth and subsequently removed via abrasion during the deposition of each lamina. This data can be inverted to constrain the length of time associated with each cortical layer using recent experimental constraints on ooid growth and abrasion rates, which are both functions of grain size. We applied this approach to thin sections of modern lacustrine ooids from several locations in the Great Salt Lake, UT to demonstrate how cortical stratigraphy via ooid shape can be used to assess the spatial and temporal variability of environmental conditions—from the perspective of individual grains within a single deposit.