Abstract

Limestone concretions in the Blue Hill Member of the Carlile Formation (middle Middle Turonian) exposed in the Lake Pueblo State Park, Colorado, U.S.A., range in size from 20-cm-diameter, approximately spherical forms to flattened spheroids >2 m in width and >1 m in height. The earliest matrix and septarian calcite cement, with δ13C and δ18O values ranging mostly from -18‰ to -8‰ (PDB) and -3‰ to -8‰ (PDB), respectively, precipitated from diluted seawater during shallow burial where sulfate reduction largely controlled pore-water bicarbonate composition. In contrast, the latest septarian cements exhibit very depleted δ18O values (∼-16‰) with δ13C values approaching values of -1‰ to -3‰, reflecting a strong meteoric-water component to pore fluids. Overall, δ13C and δ18O values are inversely covariant.

Concretion matrix calcite and septarian cements precipitated from mixtures of modified marine and meteoric fluids. Pore-water reversals resulted from sea-level fluctuations leading up to and associated with the formation of a sequence boundary, or possibly the development of parasequences near the top of the Blue Hill Member. Relative sea-level falls caused pore-water freshening as the coastal mixing zone migrated seaward. Similarly, modified marine-derived pore waters replaced fresher pore fluids accompanying subsequent marine flooding. Carbonate cements precipitated during this active hydrologic history recorded the isotopic characteristics of their parent fluids. Negative δ18O values of the meteoric end-member calcite compositions suggest derivation from high-altitude precipitation in the Sevier highlands to the west. Septarian cements in concretions, despite their enclosure in relatively impermeable host sediment, can act as sensitive indicators of relative sea-level fluctuations.

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