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 ¹³C and ¹⁸O values ranging mostly from -18o/oo to -8o/oo (PDB) and -3o/oo to -8o/oo (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 ¹⁸O values (-16o/oo) with ¹³C values approaching values of -1o/oo to -3o/oo, reflecting a strong meteoric-water component to pore fluids. Overall, ¹³C and ¹⁸O 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 ¹⁸O 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.