Petrology of tertiary and cretaceous coals of southern Alaska

Abstract

Petrologic analysis of more than 400 coal samples from four general areas of southern Alaska show that the coals possess overall similar compositions. The areas sampled include the Nenana field, Susitna lowland (Beluga and Yentna fields), the Matanuska field, and the Alaska Peninsula (Chignik, Herendeen Bay, and Unga Island fields). The coals are all of Tertiary age ranging from Paleocene to Miocene except for those of the Chignik Formation (Chignik and Herendeen Bay fields) which are of Late Cretaceous age. The Tertiary coals are from the Healy Creek, Suntrana, and Lignite Creek Formations (Nenana field), Kenai Group -Tyonek and Beluga Formations (Beluga and Yentna fields), Chickaloon Formation (Matanuska field), and Bear Lake Formation (Unga Island field). The southern Alaska coals share broad compositional affinities with coals from other areas of the world including British Columbia and Australia. They are consistently high in vitrinitic or huminitic components, and have variable but typically low amounts of inertinitic and liptinitic components. The coals with relatively high liptinite contents, the hydrogen-rich components, may be well-suited for use in synthetic fuel generation. Variations in the maceral compositions generally reflect changes in the primary vegetation cover and in the environment of coal formation. However, post-depositional thermal effects have progressively more severely altered coal-maceral assemblages in the easternmost part of the Matanuska field, and also have more strongly influenced the compositions of bituminous coals of the Chignik Formation on the Alaska Peninsula. Petrology of the southern Alaska coals indicates that they formed predominantly in forest-moor backswamp (paludal) environments on valley flats or flood plains of nonmarine, continental-fluvial systems. They typically originated as tree-vegetation peats in the telmatic zone.