Retrogradational Fluvio-Paralic Coal-Forming Environments, South Island, New Zealand: ABSTRACT

Abstract

The West Coast Region of South Island, New Zealand, records Paleogene history of peneplanation, fluvio-paralic aggradation, and marine transgression. At Buller coalfield, Eocene Brunner coal measures rest unconformably on Paleozoic to Cretaceous basement rocks and interfinger upward with Eocene, marine Kaiata Formation. During Eocene, rising sea level controlled the architecture of fluvio-paralic deposits and strongly influenced the origin and properties of Brunner coals. The Brunner coal measures contain as much as 40-ft thick, high to low volatile bituminous coal beds with 0.5-16% ash and 1-9% sulfur that is greatest in the upper coal beds. The Brunner can be divided informally into lower and upper coal measures. The lower coal measures are dominated by pebble conglomerates and multierosional, trough-crossbedded conglomeratic sandstones; the upper coal measures by stacked, multilateral, trough crossbedded, granular sandstones with subordinate siltstones and mudstones. Bioturbated sandstones with brackish-marine Ophiomorpha-like trace fossils are common in the upper coal measures. The lower coal measures were deposited in exhumed paleovalleys of the peneplain surface. These valleys, as deep as 50 ft, were initially aggraded by gravelly braided streams. Raised bogs formed in abandoned braid valleys and extended onto surrounding basement highs. The upper coal measures were deposited in sandy bedload, meanderingmore » streams with paralic wave-reworked deltas, and attached barriers. Peat bogs in the paralic environments accumulated thin coals, whereas bogs that developed on abandoned meander ridges formed thick, lenticular coals. Sea-level rise resulted in inundation introducing sulfur into the peat and resulted in high sinuosity fluvial architecture. Gradual incursion of brackish-marine waters favored bioturbation of paralic sediments along retrograding coastlines.« less