From the Reef to Slope: Architectural, Facies and Diagenetic Evolution During High-Frequency Sea Level Cycles

Abstract

Integration of lithology, diagenesis, and sequence stratigraphy constitutes a powerful tool for predicting the spatial and temporal distribution as well as the evolution of carbonate sequences and reservoir properties. The prograding Plio-Pleistocene reefs of the southern Dominican Republic provide an opportunity to characterize the diagenetic overprint, and assess lateral, vertical, and temporal trends along a reefal transect that has been exposed to repeated episodes of meteoric and marine diagenesis during high frequency sea-level cycles. A depositional model has been developed for a 7-core transect drilled perpendicular to a series of uplifted terraces from the oldest deposits 50 m above sea level (ASL) to the youngest deposits 6 m ASL. Chronological constraints include paleomagnetic, U-Th, and U-Pb dates. Diagenetic evolution for the cores is resolved with stable isotopes, trace elements (Sr, Mn, Fe), mineralogy, sedimentary organic carbon (δ13Corg) and total organic carbon (TOC). Age control for the 2 youngest cores has allowed for correlation of 7 sequences associated with sea level fluctuations during marine isotope stage (MIS) 5 (∼125 ka) to MIS 17 (∼700 ka). A U-Th age of 0.128±0.01 Ma (2σ) at the top of the 15 m terrace correlates with the youngest sequence of the 6 m terrace and evidences deposition during MIS 5e. Meteoric diagenesis along the upper 40–50 m of the 2 cores is evidenced by subaerial exposures, stabilization to low-magnesium calcite and negative δ13C (-6.60‰ to -0.03‰) and δ18O (-5.46‰ to -0.98‰). A U-Pb age of 0.646±0.013 Ma (2σ) for the shallow water reef deposits at 110 m below sea level evidences deposition during sea level lowstand MIS 16. These lowstand deposits are characterized by an increased amount of siliciclastics and positive marine δ18O up to 1.60‰ and δ13C up to 2.94‰, with preservation of original mineralogy. The flooding of MIS 11 marks a sedimentological transition. Reef development in the shallow shelf resulted in more carbonate production, steeper slopes and significant carbonate shedding into the forereef. The sequences of MIS 11 to MIS 5 show an overall shallowing-upward trend, where transgressive cycles are less evident as the margin progrades seaward. The combined depositional model from all 7 cores provides a record of reef response to the onset of high-amplitude cyclicity including changes in facies and architectural heterogeneities, as well as the resulting diagenetic overprint.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015