Date of Award

1992

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Clyde H. Moore

Second Advisor

Harry H. Roberts

Abstract

Wave driven pressure variations in reef porewater were used to investigate the physical processes that affect early marine carbonate cementation in a bank-barrier reef. Cores taken along a transect, and porewater geochemistry sampled from wells were used to characterize sedimentary and geochemical environments. The cores show an Acropora palmata framework with a patchily cemented sandy matrix composes the forereef and reef crest, with dominantly loose sediments in the backreef. Hydraulic conductivities are approximately 1 $\times$ 10$\sp{-4}$ m/s. The wave climate is moderate, with up to 75 % wave height attenuation across the reef crest. Porewater pressure variations correspond to surface waveforms, and are attenuated with increasing depth. Hydraulic head gradients are approximately 0.01. The resulting instantaneous porewater velocities are about 1 $\times$ 10$\sp{-6}$ m/s. Time series of pressure variations suggest flow is dominantly oscillatory, with no apparent net motion. Shallow reef porewater is well oxygenated and supersaturated for aragonite or Mg-calcite. Deeper porewater becomes nearly anoxic, with some sulfate reduction, both due to biological respiration. A corresponding decrease in pH causes carbonate saturation values also to decrease, limiting the potential for cementation to the shallow reef surface. Oxide and sulfide coatings on sediments are thought to reflect porewater redox conditions. The distribution of coatings suggests deeper porewater exchange in the forereef and reef crest than in the backreef. Modeling of symmetrical and asymmetrical linear wave motion suggests net advective transport is negligible. Instead, mechanical dispersion is believed to dominate porewater exchange processes. Gradients from wave generated setup, tides and infragravity waves also drive porewater circulation, and may contribute to porewater exchange. Typical early marine carbonate cements are present throughout the reef, although more abundant in the forereef and reef crest, especially intergranular, sediment binding Mg-calcite cements. The abundance of cements in the shallow reef apparently reflects a greater intensity of wave driven porewater exchange with supersaturated seawater. Minor occurrences of cathodoluminescent cements were observed, apparently precipitated in the anoxic zone. The resulting pattern of early marine carbonate cementation in the reef directly reflects oceanographic conditions as a fundamental control on cement distribution, with biological activity and sedimentation as additional influences.

Pages

228

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