Carbonate Diagenesis: Processes and Prediction

Whitaker, Fiona F.¹, Peter L. Smart¹, Richard J. Paterson² (1) University of Bristol, Bristol, England (2) Bristol University, Bristol, United Kingdom

Traditional approaches to carbonate diagenesis focus on the nature and distribution of diagenetic products. However interpretation of these data requires understanding of formative processes, which are controlled by the hydrology and geochemistry of diagenetic fluids, as well as the nature of precursor sediment. Here we focus on early diagenetic processes which can significantly modify porosity and permeability, and also influence later deep-burial diagenesis.

A formidable suite of complementary tools is now available to predict carbonate diagenesis. Laboratory experiments offer insight into the controls on diagenesis, although how far these can be spatially and temporally up-scaled is uncertain. Field studies of the hydrochemistry of carbonate groundwaters indicate the importance of organically-mediated processes, in addition to soil CO₂ and mixing. Increasingly sophisticated numerical models simulate geochemistry and hydrology individually, or via coupled reaction-transport models. Whilst sometimes cumbersome, these enable investigation of diagenetic scenarios poorly represented in the modern.

A particular challenge is understanding diagenesis under non-steady state conditions, such as with varying relative sea-level. Coupled sedimentological-diagenetic models provide a route forward, but predictions are limited by our understanding of diagenetic processes. During minor sea-level falls, both soil-derived dissolutional potential and dissolved carbonate are transmitted rapidly to the water table. During major falls, diagenetic potential appears to become exhausted within the much thicker vadose zone. However, recent hydrochemical data from uplifted carbonates indicates phreatic dissolution remains active due to bypass flow of recharge waters. Given the extended duration of sea-level low-stands, resolving this uncertainty is critical to understanding early diagenesis in Icehouse carbonates.