Marine Calcite Concretions as Base Level Markers: Examples From the Transgressive and Lowstand Systems Tract in the Mixed System

Abstract

Outcrops (Miocene-Pliocene) in the Cibao Basin (Dominican Republic) that contain subaerial exposure boundaries are used to test the hypothesis that marine (concretion) cementation provides a reliable marker of marine versus non-marine deposition. The first test case is the flooding of the fluvial Bulla Member with the marine facies of the Cercado Formation. The second test case is where a base-level fall exposed the marine Gurabo Formation to incision, deltaic deposition, and subsequent fluvial deposition. Marine microbial-related cementation (MRC) involves the bacterial sulfate reduction (BSR) of organic material through diffusion of seawater into the seafloor, sufficient to raise alkalinity, and reduce the Mg/Ca ratio (Mg²⁺ adsorption) of the diffused fluid. Cementation forms small (<15μm) low-Mg calcite crystals. We use calcite crystal texture and δ¹³C value of the calcite to confirm cementation in a marine setting. Carbon isotopic values are diagnostic of calcite formed through BSR with values from -10‰ to -25‰ PDB. Oxygen isotopic values are -2‰ to -5‰ PDB and are mostly non-diagnostic. Base Level Rise: the late Miocene marine flooding of fluvial gravel and sand. Marine deposits of the early transgression contain reworked gravel and lack diagnostic marine indicators. The first occurrence of concretions typical of MRC marks the nonmarine to marine boundary. Marine origin of these boundary concretions is confirmed with stable isotope values (negative δ¹³C) and calcite cement texture (subhedral 4-15μm calcite matrix). Base Level Fall: the upper surface of the Gurabo Formation is eroded, incised, and subaerially exposed to form a sequence boundary. The most prominent feature is the irregular, eroded surface of the mud-rich Gurabo lithofacies overlain by 25m of gravel and coarse sand. At ~5m above the basal erosional surface, distinct cemented lenses occur among the sand and gravel. These cemented features contain typical concretion calcite cements and relatively light carbon isotopic values, suggesting that they formed from MRC. These examples demonstrate the utilization of concretions and concretion geochemistry to refine the marine-nonmarine boundary associated with base level changes. In core or cuttings, difference in cement texture can distinguish marine concretions from meteoric cementation. Concretion cements are mainly dense microspar-size low-Mg calcite, whereas meteoric cements (either early or late stage) are predominantly larger blocky spar.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018