Marine Phreatic Zone Research Articles

Carbonate Diagenetic Textures from Nearsurface Diagenetic Environments

Understanding the processes and products of carbonate diagenesis is essential to exploration for, and optimum development of, hydrocarbon reservoirs in carbonate rocks. Much (and perhaps most) cementation and formation of secondary porosity (except fractures) in carbonates occurs at relatively shallow depths in one of four major diagenetic environments: the vadose zone, meteoric phreatic zone, mixing zone, and marine phreatic zone. Each of these zones may be divided into several parts on the basis of rate of water movement and saturation of the water with respect to calcium carbonate. Most carbonates are deposited in and begin their diagenetic history in the marine phreatic environment. This zone may be divided into two end members of a continuous spectrum: a zone of relatively little water circulation in which micritization and minor intragranular cementation occur, and a zone of good water circulation near the sediment/water interface of shelf margins or the upper shoreface in which extensive intergranular and cavity-filling cementation occur. Fibrous aragonite and micritic Mg-calcite are the dominant cements. With subaerial exposure, fresh water will replace sea water in the pores of shallow-water carbonates, and a zone of mixed fresh and marine waters may form. In long-lived mixing zones, dolomite may form if the water is of relatively low salinity, whereas bladed Mg-calcite may form if the water is relatively marine. Active water circulation in the mixing zone, which may be caused by seasonal rainfall, is necessary for dolomitization or cementation. Diagenesis in the freshwater phreatic environment may involve leaching in the zone of solution, neomorphism of grains accompanied by extensive intergranular calcite cementation in the active saturated zone, or neomorphism of grains without cementation in the stagnant saturated zone. Syntaxial overgrowths on echinoderm fragments and interlocking crystals of equant calcite that coarsen toward pore centers are typical of cementation in the active freshwater phreatic zone. The freshwater vadose environment is the zone with both air and meteoric water in the pores and may be divided into the zone of solution and the zone of precipitation. CO2 from the atmosphere and soil contributes to solution which generally occurs near the soil zone and forms vugs, molds, and etched grains. When the water becomes saturated with respect to calcite, evaporation or CO2 loss may cause precipitation of fine equant calcite in the form of pendant and meniscus cements. Grains may be altered to calcite, particularly in humid climates, and caliche crusts may be produced by evaporation and/or biologic (generally algal) factors. Climate plays an important role in early diagenesis if subaerial exposure occurs. In arid climates, cementation in freshwater environments may be limited and primary intergranular porosity may be preserved. In humid climates, little primary porosity is likely to escape cementation, but significant amounts of secondary moldic and vuggy porosity may form. Interpretation of diagenesis in carbonates is complicated by the fact that diagenetic environments may change many times in the history of a carbonate rock. By recognizing the processes leading to the formation or preservation of porosity, and the distribution of diagenetic End_Page 461------------------------------ environments in which those processes acted, the distribution of porosity in subsurface carbonates can often be predicted.

Interpretation of the origin and diagenesis of Pleistocene chalk, Eniwetok Atoll, Marshal Islands

ABSTRACT Three Pleistocene carbonate samples from a core from Eniwetok Atoll, Marshall Islands, were analyzed in order to describe and determine the origin of clay found extensively in the subsurface of the atoll. The samples represent a lagoonal depositional environment as evidenced by relatively high abundances of coral and Halimeda, and the lack of coralline algae. Sample depths of 51, 63, and 79 m below MSL occur below three distinct solution unconformities at 22, 60 and 70 m Consequently, each sample represents a zone with a different diagenetic history. Eight size fractions of the samples were grouped into three size intervals based upon the following compositional characteristics observed on scanning electron microscope micrographs: (1) generally biogenic carbonate grains (>5 µ), (2) significant amounts of calcite cement (2-5 µ), and (3) abundant aragonite needles (.1-2 µ), some of which are believed to be aragonitic cement. The mineralogy of the >5 µ fractions is determined largely by the mineralogic composition of the biogenic grains and their degree of neomorphism, whereas the mineralogy of the 2-5 and .1-2 µ fractions is determined largely by the addition of calcitic and aragonitic cements, respectively, superimposed upon neomorphosed remnants. Each sample has some calcite cement (most in the 79 m sample) which probably precipitated in the vadose and/or meteoric phreatic zones during periods of atoll emergence. On the other hand, differences in the strontium and magnesium concentration values for 100% aragonite suggest that: (1) the aragonite cement in the 51 m sample precipitated in a closed system, e.g., from hypersaline solutions during times of atoll emergence, and (2) the aragonite cement in the 63 and 79 m samples precipitated in an open system, e.g., from the marine phreatic zone during times of atoll submergence. Evidence for cycles of atoll emergence and submergence consists of growth relations between equant calcitic and aragonitic cement grains.

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

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.