Abstract

Linkages among surface sediments, depositional processes, geomorphic forms, stratigraphic architecture, and pore distribution in Bahamian sand bodies and oolite-capped depositional successions have been characterized in recent studies. An understanding of how these parameters, especially porosity, are preserved or modified in the eogenetic environment is essential in bridging the gap between modern and ancient analogs. Two complementary 3D (200 MHz and 400 MHz) ground-penetrating-radar surveys and three shallow cores reveal that the uppermost part of the upper Pleistocene (Marine Isotope Stage 5e) Lucayan Formation at Red Bays, Andros Island, Great Bahama Bank, comprises a single, 6-m-thick upward-shallowing parasequence. Six radar packages (P1–P6) bounded by radar surfaces (S1–S6) have been identified in the two 3D radar volumes. The stratigraphic succession indicates development of a burrowed backshoal lagoon during early flooding of the bank top followed by deposition of flood-tide-driven, low-angle sheet sands and sigmoidal subaqueous dune deposits as the shelf-margin sand shoal moved bankward as a function of continued sea-level rise. Constructional accommodation on the front of the prograding subaqueous dune in the northwest part of the survey area and erosional accommodation created by downcutting of a shallow tidal channel into the upper part of the subaqueous dune depositional unit in the northeast part of the study area were filled with peloid- and ooid-dominated carbonate sands as sea level stabilized. The uppermost radar package is a thin (0.15 to 0.55 m), partially calichified carbonate sand sheet that mantles the entire survey site and that reflects late highstand and fall of sea level.

Exposure of the studied interval to meteoric vadose diagenesis over the past 100,000+ years has resulted in partial stabilization of the largely aragonitic precursor sediment through selective partial dissolution of aragonitic grains (mainly ooid cortical layers, peloids, and mollusk shells) accompanied by precipitation of patchily distributed low-magnesium calcite cement. Core data demonstrate a relationship between stratigraphically controlled sedimentological attributes, diagenesis, and pore distribution. Three distinctive zones of porosity were present upon deposition: a lower zone of relatively high interparticle porosity (21.3% to 33.2%) in the medial part of P1, a low-porosity zone (< 10%) in muddy peloidal sediments in the upper part of P1, and a zone of high interparticle porosity (19 to 43%) in grain-dominated radar packages P2 through P6. Dissolution and cementation resulted in a net depositional porosity loss of approximately 50%. The existing porosity (average = 15.5%), which is a combination of cement-reduced primary interparticle, secondary laminamoldic, and secondary pelmoldic porosity is more evenly distributed throughout the stratigraphic succession than was precursor depositional porosity. X-ray diffraction analysis reveals that aragonite persists as a volumetrically important mineral constituent (38% to 56%) and as an important driver of continued diagenesis in rocks that constitute the uppermost Lucayan depositional succession.

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