SANDSTONE DIAGENESIS RELATED TO VARYING BURIAL DEPTH AND TEMPERATURE IN GREYMOUTH COALFIELD, SOUTH-ISLAND, NEW-ZEALAND

Substantial variation of vitrinite reflectance within the Greymouth Coalfield permits correlation of vitrinite reflectance trends with diagenetic modifications, documented from a suite of 34 sandstones taken from eight drillholes. Vitrinite reflectance and apatite fission track data provide a framework for estimating downhole paleoburial depth and paleotemperature trends; the paragenetic sequence of sandstone diagenesis is fitted to this framework. Results indicate a potential utilisation of paragenetic suites in detrital sandstones for estimating paleoburial conditions. Primarily granite-derived Eastern Compositional Suite (ECS) and primarily metasedimentary (Greenland Group)derived Western Compositional Suite (WCS) in the Rewanui Member of the Paparoa Coal Measures (Upper Cretaceous) show different paragenetic suites of diagenetic minerals. In ECS, macrokaolinite began forming very early (pedogenesis?); interstitial microcrystalline siderite spherulites formed later but still early in diagenesis, followed by macrocrystalline (av. c. 0.5 mm) siderite, quartz overgrowths, illitisation of original clayey components, and authigenic muscovite growth. This study suggests that kaolinitisation and siderite development are complete before 2.5 km burial depth; illite begins to form early in diagensis, possibly above 1.0 km, but mainly forms below 3.0 km, at which depth authigenic mica begins to grow. In WCS, macrokaolinite is absent, probably because of differences in the original detritus and pedogenetic conditions, and paragenetic stages later than microcrystalline siderite spherulites are absent because of the elimination of original porosity resulting from deformation of metasedimentary rock fragments to pseudomatrix between 2 and 3 km paleodepth.