Contrasting Depositional Styles in Tertiary Fluvial Deposits of Nenana Coal Field, Central Alaska: ABSTRACT

Abstract

Oligocene and Miocene fluvial deposits contain about 1.4 billion tons of minable subbituminous coal in the Nenana coal field and are prospective for petroleum in the nearby Middle Tanana basin. These deposits, in ascending stratigraphic order, are in the Healy Creek, Suntrana, and Lignite Creek Formations of the Usibelli Group. To better understand the depositional setting of these units, the authors studied their facies and microarchitecture in outcrops along Suntrana and Healy Creeks, about 120 km southwest of Fairbanks. The lower Healy Creek Formation consists mainly of amalgamated, basally scoured, lenticular conglomerates and sandstones. The conglomerates are normally graded and crudely imbricated, and the sandstones exhibit planar, trough, and ripple cross-stratification. These were likely deposited by migrating longitudinal and transverse bars in braided streams. Minor channel-form lenses of mudstone also occur and probably represent quiet-water deposition in abandoned channels. In contrast, the Suntrana Formation includes several fining-upward sequences in which normally graded pebble conglomerates and cross-stratified sandstones are overlain by mudstones and finally by coals as much as 6 m thick. The conglomerates and sandstones are interpreted as stacked high-energy fluvial channels that were filled by longitudinal gravel bars, sandy midchannel bars, and point bars. The overlying mudstones occupy amore » series of crosscutting abandoned channels that suggest a complex history of channel abandonment and reoccupation by high-sinuosity streams. Fining-upward sequences also occur in the Lignite Creek Formation, but they differ from those in the underlying Suntrana Formation by having fewer mud-filled abandoned channels, thick intervals of flood-plain mudstone with well-developed crevasse-splay sandstones, and thinner coals (generally < 1 m thick).« less