Abstract
A sequence stratigraphic model has been developed for a seven core transect across a series of coastal limestone terraces exposed on the southern coast of the Dominican Republic. Combined, these cores provide a fairly continuous record of fringing-reef development and margin evolution over the past 1.6 myr. This period encompasses the transition from an obliquity dominated climate regime (40 kyrs cycles) to a high amplitude eccentricity dominated regime (100 kyrs) following the Mid-Pleistocene climate transition (MPT). Furthermore, the cores span two major ecological transitions in the predominant reef-building coral fauna which appear to be associated with the MIS-11 and MIS-31 super-interglacials; two of the longest and warmest interglacial periods over the past 2 myr. Combined, the DR cores provide an ideal setting to assess how the timing and amplitude of sea level change can influence facies distributions both laterally and vertically, as well as the influence of sea level fluctuations on diagenetic trends and porosity development across the margin. The sequence stratigraphic model for the cores is based on integrated chronostratigraphic tools including U-series dating, paloemagnetism, nannofossil, and lithostratigraphic and biofacies distributions. We have constrained ages from ~1.6 Ma to ~125 Ma (the last interglacial). One of my goals was to constrain the regional extinction and ecological decline of Stylophora (and other extinct taxa) and determine whether the transition from Stylophora to Acropora dominance was due to protracted climatic deterioration and cooling between 2.0 to 0.8 Ma, or the onset of high amplitude sea-level fluctuations at ~400 Ka. Chronological constraints suggest this ecological turnover was the result of the MIS 31 highstand event. Correlation of the sequences in the youngest and most seaward cores (Core 4 and Core 5) has provided grounds for interpreting reef morphology and architectural evolution from the shelf through the slope and basin for the last ~800 ka. The higher amplitude (100 kyr) cycles resulted in rapidly prograding, well developed, Acropora dominated reefal facies. Significant depositional changes are also associated with the extensive shelf flooding of MIS 11, a higher highstand. Reef development in the shallow shelf waters resulted in more carbonate production, steeper slopes and significant carbonate “highstand shedding” into the forereef. Geochemical characterization for this seven-core transect shows the evolution of diagenetic signatures as the deposits get progressively older. We have established the diagenetic history for each core by integrating stable isotopes of the carbonates (δ13C and δ18O), trace elements, total organic δ13C, Total Organic Carbon (TOC), and petrography. Over time, the fringing reefs margin hosts a diversity of geochemical regimes (marine phreatic, meteoric phreatic, and vadose) and different degrees of meteoric overprint. Cores 4 and 5 provide a complete record of diagenetic evolution from marine to meteoric diagenesis since the last interglacial (~125 kyr). Landward through the transect, the cores show the complexity that develops with meteoric stabilization and further diagenetic alteration associated with repeated sea-level changes. The complex depositional and diagenetic relationships that develop along a reefal margin make it difficult to predict associated porosity and permeability distributions. One of the challenges is to use the appropriate scale to properly address these highly porous vuggy carbonates. To address this problem, vertical profiles of hydraulic conductivity were calculated from short-interval straddle-packer injection tests in a three-well transect across the Pleistocene reefal limestones. Combined with whole-core porosity estimates and small diameter (2.54 cm) plug estimates of matrix porosity and permeability, these data provide a means of assessing the scale-dependent petrophysical variability within a complex carbonate pore system, as well as the primary factors that control flow within such a system. Permeability values (converted from hydraulic conductivity) based on in situ injection tests ranged between 5-25 Darcy (D), up to six orders-of-magnitude higher than associated plug permeability values (0.0001-19 D). Injection permeability strongly correlated to larger-scale vuggy porosity, quantified by subtracting the plug-based “matrix” porosity from the whole-core “total” porosity. Similarly, the underestimation in permeability that results from plug versus well injection tests for these vuggy carbonates becomes enhanced over time as cementation occludes matrix porosity and dissolution opens up larger molds and vugs especially corals and other large aragonitic grains. The in situ permeability values measured in the DR reefal carbonates provide realistic values useful in the assessment of flow potential in aquifers and reservoirs of other complex matrix-vug dual pore systems.