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 1 to 3/Ma for ¹³C_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 ¹⁸O_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°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.