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Ooids as Archives of Past Conditions

Abstract

Ooids are concentrically coated, sand-size carbonate grains that are a key environmental indicator of high-energy depositional conditions and are significant to the evolution of carbonate platforms, shelves and reservoirs from around the world.

How do ooids form?- Latest studies support the stance that ooid cortex formation is driven mainly by a microbially mediated organomineralization process: (a) a biologically induced mechanism, whereby by-products of metabolic activities can change the physicochemical conditions of the microenvironments, inducing an increase in alkalinity and deposition of mineral particles, and (b) a biologically influenced mechanism, whereby an organic material, such as microbial extracellular polymeric substance (EPS) exudates, serve as a template for carbonate mineralization through the adsorption of ions and mineral nucleation.

Where do ooids form?- Ooid sand bodies generally develop in platform-margin sites with water depths between 0 and 8 m and where tidal velocities are intensified.Hydrodynamic modeling reveals a predictive relationship in that doubling the peak current velocity increases the area of a sand body by a factor of three, and there are distinct sedimentological trends observed in the field that parallel the decrease in current speed across a sand body in a platform-ward direction. Positive feedback among flow velocity, ooid grain size, tidal bar height, channel depth, bar shape and orientation, and even the width of the sand body is a general pattern that supports the notion that ooid dispersal and accumulation are controlled by hydrodynamic processes.

Ooids as archives of past conditions- Besides the microbial involvement in precipitation that leads to ooid formation, microbes can induce textural and geochemical transformations through a cascade of microscale biological processes in tandem with geochemical conditions. This provides cautionary implications for the use of ooids as archives for paleo-environmental reconstructions. However, grain size, sand body size and morphology seemingly have a direct relationship to the current strength, suggesting that ooid sand bodies can be exploited to discern hydrodynamics that existed in the past and aspects of sand body morphology are predictable. Thus an important consequence of the dual influence of ooid formation and accumulation is extracting the paleophysical energy record from oolitic deposits is potentially more viable than extracting the paleochemical record.