Unravelling Reflux Dolomitization: Why Size Matters?

Abstract

Refluxing brines have been invoked to explain extensive dolomitization of numerous platform carbonates, including the Permian Basin of West Texas, the Mississippian Madison of the Western US, and the Jurassic Arab Formation of the Middle East. Though orders of magnitude smaller in scale, Bonaire Island in the Netherland Antilles is an often cited, early example of recent reflux dolomitization. Comparisons were drawn between the salt-ponds of the modern Pekelmeer and the fluids forming dolomite bodies in Miocene slope deposits, and the impact of reflux on rock fabric and porosity prior to burial characterized. Using data from a number of new field sites, we re-examine this model for dolomitization of the Mio-Pliocene limestones of Bonaire.

At our type section of Seru Grandi, in the Washington Slagbaai National Park, tongues of replacement dolomite extend down from an erosional unconformity which marks the transition to overlying undolomitized limestone. Dolomite geobodies develop along clinoforms within shallow-marine coral-algal deposits, with preferential alteration of high-Mg calcite red algae. The dolomite is largely 20 to 100 µm sucrosic crystals, with cloudy centers and patchy zonation, and is non-stoichiometric and calcium-rich (45 Mol% MgCO3). This, together with the absence of restricted facies or associated evaporites, supports dolomitization by reflux of mesohaline fluids, rather than dense brines. Stable isotope measurements show significant enrichment relative to precursor limestones, with δ13C values +1 to +4 ‰ VPDB and δ18O values of +1.5 to +5 ‰ VPDB. Assuming Miocene oceans were δ18O enriched (+1 to +2 δ18O VSMOW) relative to modern oceans, this suggests dolomitizing fluids with salinities of 40-44 ‰.

Several studies have used reactive transport models to better understand dolomitization driven by reflux of brines up to and above gypsum saturation over distances of 10s to 100s of km. Our simulations, constrained by field data from Seru Grandi, indicate that at much smaller scales waters of no more than 44 ‰ can reflux through these permeable bioclastic deposits at 3 to 8 m/yr. These flow rates are comparable with those modeled for high salinity brines suggested to cause dolomitization of larger scale systems. Although the geochemical potential of these mesosaline fluids is lower, our models suggest that at 40^oC, dolomite geobodies of comparable scale to those at outcrop could form from only marginally evaporated seawater within 200 kyr.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018