Cementation scenarios for New Zealand Cenozoic nontropical limestones

Abstract

Cenozoic limestones are widely distributed in New Zealand, especially in the Oligocene-earliest Miocene in both islands, and the Pliocene-Pleistocene in North Island. A spectrum of limestone types exists, but all are skeletal-dominated (>70%), with usually <20% interparticle cement-matrix and <10% siliciclasts, and they have facies attributes typical of nontropical carbonates. The range of diagenetic features identified within the limestones is the basis for assigning them to a small number of “end-member” cementation classes that are inferred to be associated with four, broad, diagenetic settings. Class I limestones have very open fabrics dominated by abraded bryomol (bryozoan + bivalve mollusc) facies skeletons coated with isopachous rinds of typically nonferroan, dull to blotchy luminescent, fibrous to bladed spar, often with porosity occlusion by detrital and/or precipitated micrite. The limestones are usually thin and rare, of Oligocene — early Miocene age, and are interpreted to have formed by marine cementation at the seafloor under special environmental conditions associated with high energy levels and reduced sedimentation rates. Class II limestones have open fabrics with meagre amounts of dull to bright luminescent, ferroan to nonferroan, often substrate-specific, bladed to dog-toothed scalenohedral fringe or syntaxial rim cement. These porous, bryomol, echinofor (echinoderm + benthic foraminifera), and less commonly barnamol (barnacle + bivalve mollusc) facies 1 imestones are relatively common, especially in Oligocene— Miocene eastern South Island and in some Pliocene eastern North Island occurrences, where diagenesis occurred mainly under conditions of only shallow burial (<500 m) in the presence of essentially marine pore fluids. Class III limestones have open to moderately open fabrics with typically nonferroan to ferroan, variably luminescent, fine to coarse drusy equant spar, and often include a range of neomorphosed, formerly aragonitic, bivalve skeletal fabrics. Composed predominantly of barnamol and bimol (bivalve mollusc) facies, these usually rather porous Pliocene—Pleistocene limestones are especially common in eastern North Island, and were characterised by diagenesis initially in the shallow burial realm and subsequently under variable influence from meteoric fluids, both in the subsurface and following uplift. Class IV limestones are characterised by tight and highly pressure-dissolved fabrics, with scant pore spaces filled by usually ferroan, dull luminescent, clear drusy equant spar. These hard, flaggy, bryomol-dominated limestones are common, of Oligocene-Miocene (and also Paleocene-Eocene) age, and their diagenesis proceeded mainly under conditions of moderate to deep burial (500–1500 m) in the presence of marine-modified connate fluids. The spectrum of marine, shallow burial, meteoric, and deep burial spar cements recognised in the New Zealand limestones^ together with special diagenetic features such as neomorphosed aragonitic fabrics and polygenetic micrites, remains to be fully appreciated and incorporated into existing models of nontropical shallow-marine carbonate evolution. © 1996 Taylor & Francis Group, LLC.