--> Concretions in the Mixed System: Microbial-Driven Early Marine Calcite Cementation

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Concretions in the Mixed System: Microbial-Driven Early Marine Calcite Cementation

Abstract

Marine concretions are a common feature of organic-rich, mixed-system shelves and ramps. These pervasively cemented rocks can form low-porosity flow baffles and mechanically stiff and brittle beds that may influence hydraulic fracturing in these mud-rich systems. Porous sediment is cemented by primary, low-magnesium calcite (LMC) marine cement (10-30 micron crystals). Stable carbon and oxygen isotope values of concretion calcite cement (-5 to -35‰ C: -2 to -5‰ O) are consistent with initial precipitation in pore water where alkalinity was generated by microbial processes: oxidation of methane (δ13C values < -25‰), and oxidation of organic matter in the zone of sulfate reduction (δ13C values -5 to -25‰). To facilitate marine LMC precipitation, a diffusion-based model invokes a mechanism for Mg2+ uptake to limit the inhibitory effect of Mg2+. One such mechanism would be the absorption of Mg2+ by clay minerals to decrease the Mg/Ca ratio. Concretions are thus a collaborative product of cementation from both microbial-induced and inorganic processes. Preservation of soft grains and delicate fossils is exceptional in most concretions, likely due to early, pore-filling precipitation of the small LMC crystals. This early cement allows concretions to preserve original grain structure in compaction-prone, mud-rich depositional systems. Most stratigraphic concretions mark the top boundary of the depositional cycle (meter scale) due to conditions needed for their formation (low rate of deposition or non-deposition, diffusion, organic degradation gradient, clay-rich filter). Concretions also form as isolated bodies within the depositional cycle, or rarely as vertical tube-like features between concretion beds. The stratigraphic distribution of concretions is poorly documented, and we make preliminary observations from two sections (Miocene, Gurabo Fm., Dominican Republic; and Jurassic, Vaca Muerta Fm, Argentina). Concretions in both examples were found to cluster within the basal part of the sequence or supersequence, implying formation during transgressive conditions.