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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California