Evaluating the effects of carbonate saturation state on carbonate platform geometry and porosity evolution - a case study from the Great Bank of Guizhou of south China
Keywords: Geological and Environmental Sciences.

Carbonate saturation state (Ω) plays a key role at regulation of oceanic chemistry through geologic time. Ω not only controls precipitation and growth of sediments at the grain scale; it also influences the growth and geometry of carbonate platforms through bulk accumulation rates and timing of lithification. At different scales, how Ω influences growth of depositional grains and carbonate platform geometry in the ancient is less constrained because properties of ancient seawater, such as Ω, have varied across time and are often difficult to constrain even through chemical proxy data. Quantifying the influence of Ω on carbonate depositional processes from the grain to platform scale requires studies on carbonate platforms that were formed in oceanic chemical conditions distinct from the modern and on depositional grains that allow to quantitatively estimate coeval Ω through their growth mode or process. The Great Bank of Guizhou (GBG), a Permian-Triassic isolated carbonate platform sitting in the Nanpanjiang Basin of south China, offers a great opportunity to provide insights into the question, because (1) it grew through the Early Triassic during which oceanic chemistry is considered to be unstable and lead to delayed life recovery until the Middle Triassic; (2) its stratigraphic architecture from initiation to demise is exceptionally well-exposed, allowing to observe how carbonate platform geometry changed through time; and (3) occurrences of giant ooids (> 2 mm in diameter) at the platform margin in the Early Triassic allow to link their growth to coeval Ω. Firstly, giant ooids are developed at the platform margin of the GBG in the Early Triassic, co-occurring with microbialite and carbonate radial fans in shallow marine environment. The abrupt change in the types of depositional fabric after the end-Permian extinction might simply represent a result of the loss of skeletal carbonate production or they also reflect a change in the extent of Ω. Despite this attention, significant questions remain about the way to quantify how saturated the Early Triassic tropical shallow seawater was relative to modern tropical ocean, since it does not allow us to directly measure Ω in ancient seawater and other proxies remain beyond available. I applied a physio-chemical model of ooid growth to the Lower Triassic giant ooids collected from the margins of the GBG, Yangtze platform, and Chongzuo-Pingguo carbonate platform in the Nanpanjiang Basin of south China in order to quantify the Ω of shallow water required for the formation of the giant ooids. The Lower Triassic giant ooids are composed of aragonite and calcite, so I separately simulated the required Ω for the two end members of minerals to bracket the possible conditions. Simulation results of the physio-chemical model manifest that the Lower Triassic ooids require a higher Ω than typifies modern sites of ooids formation. Early Triassic oceans were at least eight to ten times supersaturated with respect to aragonite and eight to fifteen times supersaturated with respect to calcite. When combined with independent constraints on Early Triassic atmospheric pCO2 and oceanic calcium ion concentration, the findings require that Early Triassic oceans had more than twice the modern levels of dissolved inorganic carbon and alkalinity and a pH near 7.6. Secondly, while the Lower Triassic giant ooids are developed at the platform margin of the GBG, the platform shows distinct changes in its morphology. It was a low-relief, gently dipping carbonate ramp in the earliest Triassic. It formed an aggraded, high-relief, steep-sloped carbonate platform by the end of the Early Triassic. However, there was no microbial or metazoan reef framework and synsedimentary tectonic movement at the margin and upper slope that usually lead to steepening of carbonate platforms, implying there was another mechanism responsible for the change in morphology. The steepening process of the GBG in the Early Triassic can be explained by two hypotheses related to Ω. One is that Ω was high enough to promote enough carbonate sediment production that was able to catch up with coeval subsidence rate and form a steep-slope platform. The other one is that high saturation state in pore water within sediments helped enhance precipitation of early marine cements that restricted shallow-water sediments from transport to the adjacent basin and can maintain a high-relief steep-sloped platform. To test the hypotheses, I employed stratigraphic forward modeling (DIONISOS) to evaluate the sensitivity of Early Triassic platform geometry to sediment production and transport that is controlled by Ω. The simulation results reveal that the change of the Early Triassic platform morphology is more sensitive to sediment transport than sediment production. In addition, simulation results from mimicking following Middle Triassic platform geometry of the GBG, in comparison to the Early Triassic geometry, suggest that the growth of steep-sloped carbonate platform lacking slope microbial factory may often be limited by transport of sediment from the platform-top factory to accommodation on the slope rather than by the intrinsic production capacity of the platform-top factory. Thirdly, primary interparticle pores in the Lower Triassic oolite from the GBG are largely filled by early marine cements. However, following dolomitization helps create secondary porosity. The stratigraphic architecture of the GBG is well exposed through an interior-to-basin transect providing an exceptional opportunity to trace, observe and compare features of dolostone to its neighboring host limestone across different environments, which very few studies can rely on for general investigation of dolomitization mechanisms. An integrated approach including field relationships, petrography, cathodoluminescence, microthermometry of fluid inclusions, trace element, carbon, oxygen, and strontium isotopes is applied to decipher the formation mechanisms of the Lower Triassic dolostone of the GBG. The results suggest that an early-formed reflux dolomitization was overprinted by a burial brine dolomitization of a later stage. Albeit primary interparticle porosity is largely filled by early marine cements, precursor limestone with a grain-supported texture (esp. oolitic grainstone at the platform margin) is more prone to have the highest secondary porosity than precursor limestone with other textures. Results from this study could provide insights on dolomitization process and prediction of reservoir quality for age- and facies-equivalent analogs including the Feixianguan Formation in the Sichuan Basin of southwest China and the Khuff Formation in the Middle East