Miocene marine calcite concretions: a collaboration of microbial and inorganic processes?

ABSTRACT Late Miocene and early Pliocene mixed siliciclastic and carbonate deposits of the Cibao Basin, Dominican Republic, contain concretions cemented by low-magnesium calcite. These late Cenozoic concretion-bearing deposits have not been extensively altered or deeply buried and thus provide an opportunity to study a relatively young occurrence. Marine concretions are a feature of organic-rich siliciclastic shelves and basins, especially during certain times in Earth history. These pervasively cemented rocks can form low-porosity flow baffles, can be relatively fast acoustically, and are mechanically strong as they form near the seafloor and very early in their burial history. Preservation of soft grains, organic remains, and delicate fossils is exceptional in most concretions, due to an early, extensive pore-filling precipitation of small crystals (10–30 μm) of low-Mg calcite. This early cement allows concretions to preserve original grain structure in compaction-prone, mud-rich lithofacies. Stable carbon and oxygen isotope values of Miocene concretion calcite cement (δ13C = −10 to −35‰: δ18O = −2 to −5‰) are consistent with precipitation in pore water where alkalinity was generated by microbial processes. In addition to the long-recognized role of microbial sulfate reduction (δ13C values −5 to −25‰), our data suggest that anaerobic oxidation of methane (δ13C more negative than −25‰) may also contribute to an increase of alkalinity and may initiate precipitation. We favor a diffusion-based setting to supply sulfate and calcium ions. In order to facilitate precipitation of low-Mg calcite, a reduction of the Mg/Ca ratio of the pore fluids is needed to limit the inhibitory effect of Mg2+. One mechanism might be the absorption of Mg2+ by clay minerals such as chlorite, a clay common in the Cibao Basin and in many marine siliciclastic sediments. These concretions are thus a collaborative product of cementation from both microbial-induced and inorganic processes. We classify concretions as “stratigraphic” or “isolated” endmembers based on whether they form an extensive stratigraphic horizon, or occur individually, respectively. Most stratigraphic concretions mark the top boundary of their depositional cycle (meter scale), and are consistent with conditions needed for their formation (organic matter, low rate of deposition for sufficient diffusion and cement precipitation). Initial observations on the stratigraphic distribution of concretions from two depositional sequences (Cercado Formation, late Miocene, and Gurabo Formation, early Pliocene) indicate they cluster within the basal Cercado sequence and lower parasequences of the Gurabo sequence.