Phreatic vs. Vadose Diagenesis: Stratigraphy and Mineralogy of a Cored Borehole on Barbados, W. I.

Abstract

ABSTRACT Meteoric water diagenesis may take place in the vadose-subaerial environment or in the fresh water phreatic environment. This paper presents data to suggest meteoric water diagenesis may occur more rapidly in the phreatic diagenetic environment. Continuous drill core was recovered from a borehole in the radiometrically dated 105,000 years Before Present (B.P.) reef tract on the south coast of Barbados, W. I. Diagenetic environments have passed through the sediment of this borehole in response to Pleistocene glacio-eustatic fluctuations of sea level and gradual tectonic uplift of the island. Carbonate sediments from the upper 1.5 m of the borehole have been in the vadose diagenetic zone since initial emergence from the depositional environment. These carbonate sediments are for the most part mineralogically unstable; they have not been affected by extensive dissolution. Fresh water phreatic lenses associated with high stands of sea level have occupied the pore systems of the sediments between 1.5 m and 15.3 m depth at least twice and possibly three times. Those zones occupied at least once by a fresh phreatic lens show complete mineral stabilization of the inter-coral matrix to low-Mg calcite, and extensive cementation and dissolution. Sediment from the lowest part of the core still retains most of its depositional mineralogy and shows only minor dissolution or cementation. This portion of the borehole has alternately been subjected to the vadose zone (during glacial low stands of the sea) and the marine phreatic zone (during interglacial high stands). Although interpretations of sea level fluctuations indicates the sediment from this borehole spent more time in the vadose diagenetic environment than all other environments combined, it is significant that only those portions of the borehole section once occupied by fresh phreatic pore fluids show extensive diagenetic modification. Indeed, the lower portion of the borehole section, despite prolonged exposure to the vadose diagenetic environment, shows only minor diagenetic modification. Thus on the south coast of Barbados, early carbonate diagenesis takes place more rapidly in the fresh water phreatic zone than in the other diagenetic environments combined.