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Spatial Variability in the Character and Extent of Early Marine Cementation


In many marine systems, diagenesis of carbonates can be essentially syndepositional and rapid, and markedly modify depositional porosity and permeability. Although it is generally appreciated that reef margin carbonates tend to be better cemented than platform interior sediments, quantitative measures of change in abundance or type of cement, and the influence of grain size and climate (such as rainfall), are rare. The purpose of this study is to explore the nature and controls on spatial variability of early cementation. Well-exposed Holocene strata on three atolls (Tarawa, Aranuka, and Beru) in Kiribati (equatorial Pacific) provide a transect across a rainfall gradient: ∼2300 mm/yr (Tarawa) to 1500 mm/yr (Beru). Sampling on atolls captured the range of geomorphic variability, including transects (across strike) from multiple locations across one margin, and among different margins around each atoll (e.g. waveward vs. leeward margin). Petrographic analysis of Holocene outcrops provides means to evaluate and quantify grain type and size, mineralogy, texture, and abundance of cements, and pore types. Results show trends in the nature of cements within and between atolls. Multiple generations of cement, alternating between clear, fibrous cement and darker, micritic cement, occur in most samples. Fibrous and micritic cement generally form isopachous rims in intergranular pores. Intergranular cements tend to be fibrous and decrease in average thickness (e.g. on Aranuka from 170 to 100 μm) from platform margin to platform interior. Conversely, intergranular micritic cements increase in average thickness towards the interior (e.g. on Aranuka from 27 to 92 μm). Dissolution and thickness of fibrous cement appear to be positively correlated with rainfall, whereas micritic, isopachous cements may be negatively correlated. The processes of cementation and dissolution vary spatially, and systematic trends in the products allow for first-order predictions of porosity and permeability in reservoir analogs.