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Controls on Early Diagenetic Overprinting in Icehouse Carbonates: Insights from Modeling Hydrological Zone Residence Times Using CARB3D⁺

¹ Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, U.K.; present address: ExxonMobil International Limited, ExxonMobil House, Leatherhead, KT22 8UX, U.K.; Richard.Paterson{at}ExxonMobil.com
² Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, U.K.
³ School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, U.K.
⁴ ExxonMobil Development Company, 800 Bell Street, Houston, Texas 77002, U.S.A.; present address: Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, California 94583-2324, U.S.A.
⁵ School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, U.K.

Icehouse carbonate diagenesis is complex, with carbonate sequences being exposed to multiple meteoric hydrozones (vadose-zone, freshwater-lens, mixing-zone) on multiple occasions. A forward modeling approach using CARB3D⁺ has enabled quantitative exploration of the nature of this diagenetic overprinting, by explicit simulation of meteoric hydrozone residence times during evolution of an isolated carbonate platform. A sensitivity analysis was performed, encompassing the fundamental controls on hydrozone residence time; frequency and magnitude of sea-level variation, subsidence rate, and climate, the latter via its effect on freshwater-lens thickness and rate of dissolutional lowering of the land surface.

Residence time within all meteoric hydrozones is characterized by a high degree of lateral continuity. Vertical continuity is much more limited, with significant variations on a fourth order sequence scale across the platform. Below sequence boundaries prolonged exposure to the vadose zone results from multiple missed beats (when the platform remains exposed at relative sea-level maxima). Overprinting gives total meteoric residence times significantly in excess of the duration of exposure of the overlying sequence boundary. There is a hierarchy of eustatically driven overprint cycles, with higher-frequency cyclicity modifying the primary signal generated by lower-frequency cycles. Fifth-order cycles thus have only a minor impact on hydrozone residence times, whilst third order (seismic scale) cycles are a critical control, and could be used as a first-order approximation of the amount of diagenetic overprinting.

Subsidence has a strong control on residence time, with greater overprinting at lower subsidence rates, but in high-subsidence regimes sediment packages move below the zone of meteoric-water influence sufficiently rapidly that residence time can be minimal. High land-surface dissolution rates, which result from a wetter climate, remove a large proportion of the subaerially exposed sediment and reduce vadose-zone residence time. Higher effective recharge can increase the thickness of freshwater lens and mixing zones which increases residence times significantly. Residence times in the vadose zone can be directly linked to a sequence stratigraphic framework, but for the freshwater lens and the mixing zone this is possible only with a well constrained relative sea-level history.