1989/03/01 by Geoffrey D. Thyne, James R. Boles · 4 citations
Earth and Planetary Sciences · #Building materials and conservation #Geological and Geophysical Studies Worldwide #earthquake and tectonic studies #Geology #Geochemistry #Paleontology
paper · doi:10.1306/212f8f6c-2b24-11d7-8648000102c1865d
openalex publication_date 1989/03/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
Abstract Oxygen isotopic analyses show that the Moeraki septarian concretions formed in marine porewaters at shallow depths. Carbon isotopic analyses indicate that porewaters were influenced by sulfate reduction and methanogenic bacterial action. Carbon becomes progressively lighter toward the concretion margins at a calculated rate of depletion of nearly equal 1 to 3/Ma for delta 13 C PDB in a two meter concretion. Trace element and isotopic data demonstrate that the initial septaria form contemporaneously with or soon after the rim of the concretion. Septarian vein-fill of yellow spar calcite and rare, very late dolomite are both interpreted as forming in mixed meteoric-marine porewaters, probably related to the regressive sea level cycles during the Tertiary after deposition of the Moeraki formation. The calculated delta 18 O SMOW values for the mixed porewater from which the late calcites and dolomite could have precipitated are -5 and -8, respectively, assuming a maximum crystallization temperature of 40 degrees C. The observed decrease in the Mn/Fe ratio and change in oxygen isotopic values with growth of the yellow spar stage is consistent with a change in the redox conditions and the gradual alteration of the mixed meteoric-marine water chemistry.