New Zealand Journal of Geology and Geophysics abstracts
Diagenesis of polymineralic temperate limestones in a cyclothemic
sedimentary succession, eastern North Island, New Zealand
Douglas W. Haywick
Department of Earth Sciences
University of South Alabama
Mobile, AL 36688, USA
Abstract Temperate carbonate petrofacies
(calcarenite and coquina) in the Pliocene–Pleistocene Petane Group of
eastern North Island, New Zealand, are dominated by aragonite faunas
consisting primarily of bivalves and gastropods. Unlike
calcite-dominated temperate limestones, these polymineralic carbonates
have undergone extensive early diagenetic alteration including
extensive calcite cementation induced by aragonite dissolution. Marine
cementation (type 1: pore-lining, bladed calcite) was isolated to
biogenic pores. It predated glauconite and may have been precipitated
as low-magnesium calcite, possibly in marine phreatic environments
during sea-level transgressions. Four phases of calcite cement with
varying but definitive degrees of meteoric influence occur in the
Petane Group. Type 2 (ferroan scalenohedral calcite) was the initial
pore-filling cement and precipitated from reduced pore fluids in a
phreatic environment, possibly during or soon after the transition from
marine to meteoric diagenesis. Type 3 (moderately ferroan drusy)
calcite and type 4 (non-ferroan drusy) calcite were sequentially
precipitated during meteoric conditions from pore waters that changed
from reducing to oxidising. Type 5 (sinter) cements comprise several
forms precipitated during vadose meteoric diagenesis, the final
meteoric phase of alteration in the Petane Group. Ferroan calcite
cementation of silt matrix in coquina limestones overlain by
terrigenous silt (type 6: matrix cement) probably occurred
simultaneously with type 2/3 pore-filling phases. A similar ferroan to
moderately ferroan to non-ferroan suite of drusy calcite cements also
lithified concretions in non-carbonate (siliciclastic sand) facies in
the Petane Group, but only after the onset of compaction. Extensive
skeletal diagenesis (stabilisation of magnesium calcite allochems,
dissolution/recrystallisation of aragonite) occurred during type 3 and
4 cementation phases. Diagenesis in the Petane Group was
stratigraphically influenced and ultimately controlled by uplift.
Alternating sequences of porous and non-porous formations developed a
stacked sequence of confined aquifers that forced carbonate diagenesis
to operate laterally.
Keywords aragonite; Pliocene–Pleistocene;
cementation; dissolution; meteoric diagenesis; Petane Group; Tangoio
block
G03042; Received 18 April 2003; accepted 27 January 2004; Online
publication date 1 December 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47:
839–855
0028–8306/04/4704–0839© The Royal Society of New Zealand 2004
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