Massive dolomitization of a Messinian reef in the Great Bahama Bank: a numerical modelling evaluation of Kohout geothermal convection

Abstract

The hypothesis that Kohout thermal convection may have induced the massive dolomitization of the 60 m thick lowest more reefal unit in well Unda [top of Great Bahama Bank (GBB)] is evaluated through numerical modelling. A twodimensional (2-D) section, including lithological and petrophysical data, together with datings for the sediments of the GBB, was used in the basin model TEMISPACK to reconstruct the history of the whole platform, with a focus on the reef unit.Simulations showed that during high sea-level periods, Kohout convection is a valid mechanism in the settings of the GBB, although the convection cell remains¯at in most cases because of high permeability anisotropy. This mechanism induces rapid¯uid¯ow in the super®cial as well as in the deeper parts of the platform, with velocities of at least two orders of magnitude higher than with compaction alone. Lithology appears as a strong control of¯uid circulations at the margin scale through the permeability anisotropy, for which a critical value lies between values of 10 and 100. The reefal unit in Unda is part of a larger area determined by the lithologic distribution, in which¯ow velocities are signi®cantly higher than in the rest of the platform. These velocities are high enough to bring the magnesium necessary to precipitate the observed amounts of dolomite, within durations in agreement with the available time of post-reef deposition high sea level(s). However, neither¯uid¯ow pattern nor¯ow velocities are able to explain the preferential massive dolomitization of the lower reef unit and the complete absence of dolomite in the upper one.

Three-dimensional flow in the Florida platform: Theoretical analysis of Kohout convection at its type locality

“…Following Caspard et al (2004), we calculate the time for complete dolomitization by assuming that (1) dolomitization occurs as a replacement process, and (2) dolomitization is limited by seawater infl ow rates and seawater Mg molarity (0.045 mol/L). For the geometry of our case, we further assume that (3) the axis of the predevelop- on August 20, 2015 geology.gsapubs.org Downloaded from ment Upper Floridan aquifer potentiometric surface (Fig.…”

“…This type of convection has been invoked as a conceptual model for dolomitization in numerous modern carbonate platforms, including the Bahamas (Whitaker et al, 1994;Caspard et al, 2004), Enewetak Atoll (Saller, 1984), Mururoa (Buigues, 1997), French Polynesia (Rougerie et al, 1997), and other atolls (Leclerc et al, 1999). Numerical simulation of two-dimensional (2-D) fl ow in generalized carbonate platforms has suggested that geothermal heat fl ow can cause convective circulation to depths of several kilometers (e.g., Kohout et al, 1977;Sanford et al, 1998;Wilson et al, 2001).…”

Whiting-related sediment export along the Middle Miocene carbonate ramp of Great Bahama Bank

“…The absence of aragonite needles at the proximal part of the slope implies granulometric sorting. Previous authors, however, postulated that pore fluid circulation inside the bank is preferentially active in permeable sediments in the slope domain (Eberli et al 1997;Caspard et al 2004). For this reason, the absence of needles in the proximal sites is certainly enhanced by aragonite dissolution processes resulting in precipitation of diagenetically more stable authigenic carbonates.…”

Bed‐scale spatial patterns in dolomite abundance: Part II. Effect of varied fluid chemistry, flow rate, precursor mineralogy, temperature, textural heterogeneity, nucleation density and bed geometry

“…Yet, horizontal flow is appropriate because it prevails over large portions of nearly all larger‐scale dolomite RTMs. Streamlines are dominantly horizontal for tens of kilometres with geothermal circulation in stratified carbonates (Caspard et al ., ; Whitaker & Xiao, ). Models of compaction‐driven flow or hot fluids injected from faults into layered limestones all exhibit horizontal flow due to permeability differences between layers (Consonni et al ., ; Xiao et al ., ; Corbella et al ., ; Lu & Cantrell, ).…”