DEPOSITIONAL CONTROLS ON COAL DISTRIBUTION AND QUALITY IN THE EOCENE BRUNNER COAL MEASURES, BULLER COALFIELD, SOUTH ISLAND, NEW-ZEALAND

The Buller Coalfield on the West Coast of the South Island, New Zealan d, contains the Eocene Brunner Coal Measures. The coal measures unconf ormably overlie Paleozoic-Cretaceous basement rocks and are conformabl y overlain by, and laterally interfinger with, the Eocene marine Kaiat a Formation. This study examines the lithofacies frameworks of the coa l measures in order to interpret their depositional environments. The lower part of the coal measures is dominated by conglomeratic lithofac ies that rest on a basal erosional surface and thicken in paleovalleys incised into an undulating peneplain surface. These lithofacies are o verlain by sandstone, mudstone and organic-rich lithofacies of the upp er part of the coal measures. The main coal seam of the organic-rich l ithofacies is thick (10-20 m), extensive, locally split, and locally a bsent. This seam and associated coal seams in the Buller Coalfield are of low- to high-volatile bituminous rank (vitrinite reflectance betwe en 0.65% and 1.75%). The main seam contains a variable percentage of a sh and sulphur, These values are related to the thickening and areal d istribution of the seam, which in turn, were controlled by the nature of elastic deposition and peat-forming mire systems, marine transgress ion and local tidal incursion. The conglomeratic lithofacies represent deposits of trunk and tributary braided streams that rapidly aggraded incised paleovalleys during sea-level stillstands. The main seam repr esents a deposit of raised mires that initially developed as topogenou s mires on abandoned margins of inactive braidbelts. Feat accumulated in mires as a response to a rise in the water table, probably initiall y due to gradual sea-level rise and climate, and the resulting raised topography served as protection from floods.The upper part of the coal measures consists of sandstone lithofacies of fluvial origin and biot urbated sandstone, mudstone and organic-rich lithofacies, which repres ent deposits of paralic (deltaic, barrier shoreface, tidal and mire) a nd marine environments. The fluvial sandstone lithofacies accumulated in channels during a sea-level stillstand. The channels were infilled by coeval braided and meandering streams prior to transgression. Conti nued transgression, ranging from tidal channel-estuarine incursions to widespread but uneven paleoshoreline encroachment, accompanied by mod erate basin subsidence, is marked by a stacked, back-stepping geometry of bioturbated sandstone and marine mudstone lithofacies. Final retro gradation (sea-level highstand) is marked by backfilling of estuaries and by rapid landward deposition of the marine Kaiata Formation in the late Eocene.