Hydrological Influence on Carbonate Diagenesis in Modern Barrier Reef, St. Croix, United States Virgin Islands

Brian L. Carter, C. H. Moore, M. Simms, H. H. Roberts

The flow of water through reefs has long been held to be important in the resulting patterns of early reef diagenesis. Pore-water flow driven by waves in a bank-barrier reef in a microtidal setting on the island of St. Croix has been measured in order to quantitatively assess the importance of this process. Cores were also taken of reef sediments and framework in order to correlate observations of diagenetic products with the patterns of pore-water flow.

Wave-generated pressure fluctuations were measured inside the reef with electronic pressure transducers. The hydraulic conductivity of the reef was measured with standard hydrological pump and slug tests on piezometers installed in the reef. Darcy's law was used to calculate instantaneous flow velocities, and a numerical modeling technique was used to describe the motion of a single pore-water particle. Due to the oscillatory nature of wave motion and the strong permeability contrast between the open-water column and the reef framework, the resulting pore-water flow pattern is a strongly vertically oriented ellipse. The dominant solute transport direction is therefore going to be vertically oriented from the reef surface, decreasing in intensity with depth and largely a function of ve tical dispersive mixing as water moves in and out of the reef. The effect of wave pumping will also be strongly influenced by reef morphology as waves are attenuated by breaking across the reef crest, significantly reducing exchange in the back reef.

This inferred transport mechanisms correlates well with the distribution of the oxic to anoxic pore-water transition inside the reef as indicated by the presence of oxide and sulfide coatings. Ferrous-sulfide coatings appear dispersed within reef sediments and cements and delimit the extent of anoxic waters deeper inside the reef. Iron and manganese oxides, probably produced from recycled reduced metals from the underlying anoxic zone, coat the outside of reef materials and are present in the near-surface oxic or perhaps suboxic environment. The zone of oxide coatings is significantly thicker and deeper in the forereef and reef crest and nearly absent in the back reef, supporting the inferred wave-induced pore-water flow pattern.

Carbonate coatings and cemented sediments occur dominantly in the fore-reef and reef-crest zones. Most void-fill cements and intergranular cements are largely pelloidal in nature, although rare, coarse aragonite void-fill cements are present also. Circumgranular cements are rare and weakly developed, if present. The cement distribution strongly correlates to the zones of greatest wave-driven water/solute flux. The pelloidal nature of much of these cements, however, may indicate the influence of biogeochemical processes in some cement formation, perhaps down in the zone of sulfate reduction.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.