HAYWICK, DOUGLAS W., University of South Alabama, Mobile, AL; and SHARON E. NESS, James Cook University, Townsville, Queensland, Australia

ABSTRACT: Meteoric Diagenesis of Temperate Aragonitic Faunas in Plio-Pleistocene Shallow Marine Limestones, Tangoio Block, Central Hawke's Bay, New Zealand

Cyclothemic strata in the Tangoio block of central Hawke's Bay, New Zealand are the product of inner- to mid-shelf sedimentation that occurred during glacio-eustatic sea level fluctuations in the Plio-Pleistocene. The shallow-dipping sedimentary rocks include three mudstone-temperate limestone couplets which constitute a stacked series of aquicludes (mudstones) and aquifers (limestones).

Limestone formations are primarily composed of aragonitic bivalves and gastropods. These fauna have experienced varying amounts of dissolution and replacement by calcite. Diagenesis began during regional uplift of the Tangoio block as meteoric groundwater initially infiltrated the aquifers and is presently continuing today. The intensity of diagenesis varies from negligible to pervasive, commonly over short distances (less than 1 m) within individual limestone formations.

The process of aragonite diagenesis within temperate limestone formations in the Petane Group is similar to that in tropical carbonate successions. Neomorphic replacement of aragonitic components proceeded by concomitant dissolution and precipitation. The transition was frequently preceded by partial dissolution in which pristine shells became increasingly more chalky. During dissolution, Sr{2+} was selectively lost from the aragonite and some isotopic exchange occurred between ambient pore water and aragonite.

We have also successfully applied high-resolution transmission electron microscopy to examine the crystallography of aragonitic bivalves in the Tangoio block. Lattice-fringe images suggest that the aragonite crystallites are composed of nm-scale "micro-domains" which might act as structural weaknesses and influence aragonite alteration. This may have major implications for skeletal diagenetic processes, particularly if such micro-domains prove to be ubiquitous within biogenic carbonate minerals.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.