Stable carbon and oxygen isotope evidence of cretaceous sea-level fluctuations recorded in septarian concretions from Pueblo, Colorado, USA

Limestone concretions in the Blue Hill Member of the Carlile Formation (mid dle Middle Turonian) exposed in the Lake Pueblo State Park, Colorado, U.S.A ., range in size from 20-cm-diameter, approximately spherical forms to flat tened spheroids >2 m in width and >1 m in height. The earliest matrix and s eptarian calcite cement, with delta(13C) and delta(18)O values ranging most ly from -18 parts per thousand to -8 parts per thousand (PDB) and -3 parts per thousand to -8 parts per thousand (PDB), respectively, precipitated fro m diluted seawater during shallow burial where sulfate reduction largely co ntrolled porewater bicarbonate composition. In contrast, the latest septari an cements exhibit very depleted delta(18)O values (similar to -16 parts pe r thousand) with delta(13)C values approaching values of 1 parts per thousa nd to 3 parts per thousand, reflecting a strong meteoric-water component to pore fluids. Overall, delta(13)C and delta(18)O values are inversely covar iant. 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 t op of the Blue Hill Member Relative sea-level falls caused pore-water fresh ening as the coastal mining zone migrated seaward. Similarly, modified mari ne-derived pore waters replaced fresher pore fluids accompanying subsequent marine flooding. Carbonate cements precipitated during this active hydrolo gic history recorded the isotopic characteristics of their parent fluids. N egative delta(18)O values of the meteoric end-member calcite compositions s uggest deFivation from high-altitude precipitation in the Sevier highlands to the west. Septarian cements in concretions, despite their enclosure in r elatively impermeable host sediment, can act as sensitive indicators of rel ative sea-level fluctuations.