LITHOTYPE SEQUENCES IN THE EARLY MIOCENE MARYVILLE COAL MEASURES, NEW-ZEALAND

Coals of the Early Miocene Maryville Coal Measures show well developed dulling-up trends, defined by a decrease in the proportion of bright coal, that culminate in sapropelic horizons. Field determination of li thotype is difficult but the use of polished surfaces enables classifi cation by either the brightness or Stopes-Heerlen systems. During upwa rds dulling, gross petrographic composition remains essentially consta nt, at 80-90% huminite (vitrinite), 10-15% liptinite and 1-4% inertini te, except in sapropelic types, which are liptinite dominated. Dull ba nds are characterized petrographically by fragmentary macerals, especi ally detrogelinite, whereas bright bands are composed of intact plant fragments. Dulling up is related to the development of raised (ombroge nous) mires where growth and subsequent preservation of large, intact plant fragments is favoured during early stages of mire development an d around the mire margins.