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Origin of Meter-Scale Submarine Cavities and Herringbone Calcite Cement in a Cambrian Microbial Reef, Ledger Formation (U.S.A.)

¹ Department of Earth and Environment, Franklin & Marshall College, Lancaster, Pennsylvania 17604-3003, U.S.A.; carol.dewet{at}fandm.edu
² Department of Geology, University of Michigan, Ann Arbor, Michigan 48109-2143, U.S.A.
³ Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, U.S.A.
⁴ Department of Geology, University of Tennessee, Knoxville, Tennessee 37996-1410, U.S.A.
⁵ Department of Earth and Environment, Franklin & Marshall College, Lancaster, Pennsylvania 17604-3003, U.S.A.
⁶ Soil Sciences Department, Oregon State University, Corvalis, Oregon 97331-5503, U.S.A.

Meter-scale submarine cavities in Middle Cambrian shelf-margin microbial reef strata indicate large-scale dewatering processes, in conjunction with substrate instability related to interreef channeling and shelf-edge downslope creep and slip. Syndepositional cement precipitation within the cavities preserved delicate microbial fabrics and stabilized the reef system. Radiaxial fibrous calcite and herringbone calcite cements line the cavity interiors isopachously. The two phases cannot be discriminated on the basis of Fe, Mn, or Sr contents, but do have different isotopic signatures. Slightly more negative ¹³C values in herringbone calcite suggest that abrupt transitions between radiaxial fibrous and herringbone calcite cement are the result of rapid and repeated changes in pore-fluid oxygen levels. Storm-driven pore-water circulation renewed oxygenated seawater flow into the cavities, resulting in precipitation of radiaxial fibrous calcite. A threshold level of oxygen reduction resulted in the change to herringbone calcite precipitation. The pore fluids associated with herringbone calcite did not have elevated Mn or Fe concentrations, as suggested in previous studies. Herringbone calcite appears to be more susceptible to diagenetic alteration than radiaxial fibrous cement however, as indicated by greater resetting of oxygen isotope values.