| Summary: | Chert and dolomite in sediments of coastal Marlborough occur in the detrital units of the Upper Iwitahi Group (Wool shed Formation (WF) and Claverley Sandstone - Late Cretaceous), and in Late Cretaceous – Early Tertiary micritic sediments (foraminiferal nannofossil oozes) of the Amuri Limestone Group (ALG). All sediments were deposited in a NW-SE trending trough. Chert and dolomite are generally restricted to what was the deepest part of the basin. There is a greater volume of chert than dolomite.
Two chronologically and stratigraphically separated types of dolomite occur: concretions of aphanitic dolomite in the WF, and phaneritic, rhombic dolomite in beds, lenses, or disseminated in chert or micrite in the ALG. Both dolomite types are moderately well ordered but Ca-rich. Isotopic and elemental analyses indicate all dolomites formed in wholly marine sediments at temperatures <60°C, in association with sulphate reduction, in the upper sediment column. Concretions formed from the
dolomitization of early diagenetic calcite whereas in the ALG primary depositional carbonate was dolomitized. Mg2+ was supplied by seawater, with dolomite formation restricted by the presence of dissolved SO42-.
Chert formation in the WF is limited. Most chertification occurs in the lower ALG. Chert consists predominantly of quartz with rare opal-CT. SiO2 concentrations are usually ≥90 weight% with all other elements forming trace components, except in detrital-rich cherts where SiO2 concentrations may be <90 weight percent. The chert chemistry is consistent with replacement of primary carbonate and the expulsion of carbonate-bound elements from the site of chert formation, effectively diluting noncarbonate-bound insoluble residue. Formation was by precipitation of either opal-CT or quartz depending on SiO2 saturation conditions. Isotopic analyses indicate formation temperatures similar to
those of dolomite. The SiO2 was initially derived from biogenic sources but large amounts are inferred to have been derived from the underlying WF. Clay mineral transformations in the WF produced SiO2-rich pore waters through which ALG sediments are thought to have compacted. Initiation of silica deposition resulted from localized oxidation of the H2S produced by SO42- reduction. Initially deposited silica provided sites for further chertification.
Evidence for early chertification (such as differential compaction) have lead to development of a combined model for dolomite and chert formation in association with SO42- reduction. Although chert and dolomite both commenced nucleation early, the initial rate of dolomite crystallization exceeded that of silica. The extent of dolomitization was governed by the SO42- concentration in pore waters and by the availability of Mg2+. Where SO42- concentrations were too high no dolomite formed, only calcite recrystallized. Chertification, although slower than dolomitization, continued after the cessation of dolomite formation, certifying undolomitized beds or parts of beds, and the matrix between dolomite
crystals. The extent of chertification was governed by the availability of SiO2 This model of contemporaneous chert and dolomite formation explains the alternation of chert and dolomite beds, the dissemination of dolomite in chert, and the distribution of chert and dolomite.
|
|---|
