Shallow, subsurface growth and burial alteration of Middle Devonian calcite concretions

Organic-rich, shale-hosted septarian and nonseptarian calcite concretions, in the Middle Devonian (Givetian) Hamilton Group of central New York State, formed early in the shallow subsurface as indicated by detailed petrography, SEM, X-radiographic, lithologic, geochemical, and isotopic analyses. delta ¹³ C _carb values suggest that carbon was derived from microbial breakdown of organic matter in the sulfate reduction zone at subsurface depths of less than 10 m. However, strongly depleted delta ¹⁸ O (-7 to -10 per thousand PDB) suggests much greater burial depths. The delta ¹⁸ O data appear to represent an altered isotopic signature, as indicated by their narrow, depleted range and petrographic evidence for recrystallization of concretion carbonate. Evidence of primary porosity in concretions during growth indicates that early stage diagenetic alteration occurred in an open-marine, phreatic environment that homogenized isotopic and geochemical signatures of metastable carbonate minerals. The association of septarian calcite and bitumen, along with the depleted delta ¹⁸ O values, suggests that late-stage diagenesis was a result of migration of warm, basin-derived, connate waters through the open system. Preburial condensation of sediments in younger stratigraphic units within the Hamilton Group, possibly caused by increased winnowing, bioturbation, and erosional events during shallowing of the Hamilton sea, has generated stratigraphic variations in concretion lithology and shape. Within any given unit, however, variation in characteristics of concretions remains a primary function of their timing of growth versus burial compaction. Unlike Cenozoic carbonate concretions in organic-rich marine sediments that possess a dolomitic matrix, no dolomite concretions were discovered in the Hamilton Group. Preliminary data suggest that restriction of calcite as the host mineral for Early to Middle Paleozoic concretions may be controlled by elevated sulfate concentrations that inhibited dolomite precipitation and/or primary secular variations in carbonate mineralogy.