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* School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom
** Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom
*** United States Geological Survey, MS 430, Reston, Virginia 20192, USA
Present address: ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77027, USA; email: gareth.d.jones{at}exxonmobil.com
Density-driven seawater circulation may occur in carbonate platforms due to geothermal heating and / or reflux of water of elevated salinity. In geothermal circulation lateral contrasts in temperature between seawater and platform groundwaters warmed by the geothermal heat flux result in upward convective flow, with colder seawater drawn into the platform at depth. With reflux circulation, platform-top waters concentrated by evaporation flow downward, displacing less dense underlying groundwaters. We have used a variable density groundwater flow model to examine the pattern, magnitude and interaction of these two different circulation mechanisms, for mesosaline platform-top waters (50
) and brines concentrated up to saturation with respect to gypsum (150 ) and halite (246 ). Geothermal circulation, most active around the platform margin, becomes restricted and eventually shut-off by reflux of brines from the platform interior towards the margin. The persistence of geothermal circulation is dependent on the rate of brine reflux, which is proportional to the concentration of platform-top brines and also critically dependent on the magnitude and distribution of permeability. Low permeability evaporites can severely restrict reflux whereas high permeability units in hydraulic continuity enhance brine transport. Reduction in permeability with depth and anisotropy of permeability (kv << kh) focuses flow laterally in the shallow subsurface (<1 km), resulting in a horizontally elongated brine plume. Aquifer porosity and dispersivity are relatively minor controls on reflux. Platform brines can entrain surficial seawater when brine generating conditions cease but the platform-top remains submerged, a variant of reflux we term "latent reflux". Brines concentrated up to gypsum saturation have relatively long residence times of at least 100 times the duration of the reflux event. They thus represent a long-term control on post-reflux groundwater circulation, and consequently on the rates and spatial patterns of shallow burial diagenesis, such as dolomitization.
This article has been cited by other articles:
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  G. D. Jones and Y. Xiao Geothermal convection in the Tengiz carbonate platform, Kazakhstan: Reactive transport models of diagenesis and reservoir quality AAPG Bulletin, August 1, 2006; 90(8): 1251 - 1272. [Abstract] [Full Text] [PDF]
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G. D. Jones and Y. Xiao Dolomitization, anhydrite cementation, and porosity evolution in a reflux system: Insights from reactive transport models AAPG Bulletin, May 1, 2005; 89(5): 577 - 601. [Abstract] [Full Text] [PDF]
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 H. G. Machel Concepts and models of dolomitization: a critical reappraisal Geological Society, London, Special Publications, January 1, 2004; 235(1): 7 - 63. [Abstract] [PDF]
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F. F. Whitaker, P. L. Smart, and G. D. Jones Dolomitization: from conceptual to numerical models Geological Society, London, Special Publications, January 1, 2004; 235(1): 99 - 139. [Abstract] [PDF]
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