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Abstract

The diagenetic evolution of thick, cool-water Pliocene limestones that formed within a forearc basin to accretionary wedge setting in eastern North Island New Zealand can be usefully tracked by applying at the thin-section scale the concepts of stratal patterns (onlap, offlap, discontinuity surfaces) in sedimentary sequences. The petrographic approach, supported by geochemical data, involves recognizing genetically related packages of zoned cements under cathodoluminescent (CL) light, named cement suites, which are bounded in thin section by (correlative) diagenetic discontinuities, including dissolution surfaces, renucleation surfaces and/or fractures. Based initially on detailed petrographic study of the early Pliocene Kairakau Limestone formation, a bryozoan–epifaunal bivalve–barnacle grainstone to rudstone, this procedure identifies five distinctive cement suites labelled K1–K5 separated respectively, by discontinuity surfaces d1 to d4, and referred to collectively as the Kairakau diagenetic motif. Suites K1 and K2 have a pre-compaction origin and are inferred to have formed in a sedimentary system paced by high-frequency glacio-eustasy cycles, and reflecting deposition in transgressive (TST), highstand (HST) and regressive (RST) systems tracts, followed by initial shallow burial. Cement suite K1 is developed only locally, typically immediately above (early TST) and sometimes below (late RST) sequence boundaries. It consists of neomorphosed marine turbid cements growing upon the abraded surface of skeletons, and is bounded above by a dissolution surface (d1). Pre-compaction cement suite K2 has this dissolution surface at its base and a fracture surface (d2) at its top. K2 cements formed from oxidizing waters, either under shallow marine burial or, more likely, mixed marine–meteoric influences; they are inferred to relate mainly to the HST–RST portion of a depositional cycle. Post-compaction cement suites K3–K5 comprise pore-filling and fracture-hosted cements that formed during the burial of depositional sequences by overlying sequences, and during subsequent uplift. Suite K3 comprises ferroan calcite cements precipitated from compaction-driven reduced fluids that are terminated against fracture event d3, whereas suites K4 and K5 are interpreted as telogenetic cement phases that formed from meteoric, dominantly oxidizing, waters during uplift and exhumation of the whole succession and are separated by fracture and dissolution surface d4. Significantly, the same Kairakau diagenetic motif is developed in all the other Pliocene limestone occurrences in the study area. In an attempt to explain the emplacement of the successive cementing aquifers within limestones of different ages and separated by thick siliciclastic deposits, a cement stratigraphic model for the Pliocene succession concludes the paper, utilizing the concept of onlap and downlap cementational trends within the pre- and post-compaction cement suites of the eastern North Island carbonates. Ideal pre-compaction onlap–downlap diagenetic suites K1 and K2 mimic the evolution of the depositional environment from marine to subaerial forced mainly by short-term high-frequency (104–105 a) relative sea-level changes, whereas their post-compaction counterparts (suites K3–K5) record burial followed by exhumation of the sediment pile forced by subsidence and tectonic mechanisms of longer duration (105–107 a).

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