SCHOLLE, PETER A.¹ and D. W. LEWIS^2
1Southern Methodist Univ., Dallas, TX
²Canterbury Univ., Christchurch, New Zealand

Abstract: Formation and Diagenesis of a Cool-Water Hardground in the Oligocene Nile Group, Westland, New Zealand

A hiatal surface in the Nile Group provides an example of hardground formation on a narrow and steep, tectonically active, shelf-slope area in a cool-water setting. The sediment contains bivalves, bryozoans and brachiopods in a micritic matrix. Seafloor cementation, followed by extensive boring of the hardened sediment, provided pathways for shallow sub-seafloor dissolution of originally aragonitic bivalves without collapse of surrounding matrix. Bivalve molds were then filled with infiltrated or authigenic sediment--detrital terrigenous quartz and feldspar grains and peloidal glauconite (materials that are scarce in surrounding matrix), plus micritic and peloidal carbonate sediment. Carbonate mold-fills were later replaced by equant, medium-crystal line dolomite. Most of that dolomite, in turn, was altered to a "rusty" dedolomite (ferroan calcite after dolomite). This combination noted features leads to the following conclusions: 1) initial diagenesis had to be essentially penecontemporaneous; 2) a slow-down or cessation of carbonate production led to early lithification, presumably by Mg-calcite cement, and allowed time for formation of glauconite and accumulation of terrigenous sediment; 3) dolomitization of mold-filling sediment apparently also occurred in a near-seafloor setting, probably as a result of prolonged contact with seawater, as determined from stable isotope geochemistry; 4) subsequent burial and uplift of these strata led to formation of additional calcite cements and near-surface alteration of dolomite to calcite.

The Nile Group hardgrounds differ in many respects from those found in other, broader shelf settings of similar age in the Tertiary of New Zealand--they have far less phosphate and far more dolomite and are much less regionally extensive. Not enough is known about these sediments yet to fully explain such differences.

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