--> Abstract: Environment Of Aragonite Dissolution At Hardground Surfaces In Mid-Tertiary Cool-Water Carbonates, New Zealand And Southern Australia, by C. S. Nelson and N. P. James; #90928 (1999).

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NELSON, C. S.1 and N. P. JAMES2
1University of Waikato, Hamilton, New Zealand
2Queen's University, Kingston, Ontario, Canada

Abstract: Environment of Aragonite Dissolution at Hardground Surfaces in Mid-Tertiary Cool-Water Carbonates, New Zealand and Southern Australia

Cenozoic (Eocene-Miocene) cool-water, temperate shelf carbonate sediments throughout New Zealand and across southern Australia periodically contain grainy hardground units. These calcite-dominated, bryozoan-rich, abraded, wellsorted calcarenites are characterized by isopachous rinds of fibrous to bladed, nonferroan, low-Mg calcite crystals lining, but rarely filling, intra- and interparticle pores. The signature of these lithified layers, however, is geopetal interstitial micrite which overlies the calcite cement rind or occupies pore space in its absence. The fine material comprises homogeneous and micropeloidal micrite and/or microbioclastic micrite.

Such hardgrounds include molds of aragonite biofragments, particularly bryozoans and molluscs, an otherwise uncommon feature in the majority of associated limestones. Molds can be empty, be partially to complete y filled with the sediments and marine cements, or be infilled by burial or meteoric cement. The environmental setting of aragonite dissolution is controversial. In some cases aragonite loss resulted from meteoric exposure following hardground formation, at other times it occurred in cool marine waters accompanying prolonged seafloor exposure, and in still other situations it took place during shallow burial. Such a spectrum of settings for aragonite dissolution may be characteristic of cool-water diagenesis and so of utility when interpreting the older rock record.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas