GENERAL STRATIGRAPHY OF THE USIBELLI COAL MINE
Formation and Grubstake Formation. The group is overlain unconformably by the Nenana Gravel Formation.
As many as 30 coal beds are recognized in the Usibelli Group, which are mainly 2.5 ft (0.7 m) thick but can be as much as 30 ft (9.1 m) thick. UCM has encountered and mined seams as thick as 40-60 feet. The depositional environments of the Usibelli Group have been interpreted as fluvial and lacustrine deposits.
Healy Creek Formation
The Healy Creek Formation is the oldest rock unit in the Usibelli Group. The formation is as thick as 445 ft (136 m) and consists of interbedded sandstones, conglomerates, siltstones, and mudstones, including carbonaceous shale and coal beds. Sandstone is the most common rock type and coal is the least common. The formation unconformably overlies the pelitic and quartzose schist sequence with erosional relief of as much as a few hundred feet. In most of the synclinal coalfields, the Healy Creek Formation is early to middle Miocene.
The Healy Creek Formation consists mainly of fining- upward sequences of conglomerates, sandstones, and silty sandstones. The Healy Creek Formation was interpreted to originate as braided to high-sinuosity stream.
Sanctuary Formation
The Sanctuary Formation is composed mainly of 130 ft (40 m) of gray, thinly laminated, varved mudstone and shale that weathers to chocolate brown in color. The formation also contains minor sandstone, siltstone, and limestone. This for- mation conformably overlies the Healy Creek Formation and is assigned to the middle Miocene. The Sanctuary Formation is interpreted to have accumulated in a large, shallow lake.
Suntrana Formation
The Suntrana Formation unconformably overlies the Sanctuary Formation and is as thick as 1,310 ft (400 m). The formation as a whole, thickens gradually southeastward and pinches out in the northwestern part of the coal province. It consists of interbedded sandstones, siltstones, mudstones, carbonaceous shales, and coal. Coal beds are interbedded with carbonaceous shales and have a combined thickness ranging from 1.6 to 65 ft (0.5 to 20 m). Most of the coal beds can be traced laterally over distances of as much as 15 mi (25 km). Three of the thicker beds (Nos. 3, 4 and 6) are currently mined at UCM’s Two Bull Ridge and Jumbo Dome mine sites. The Suntrana Formation was assigned to middle Miocene.
The fining-upward, erosional-based sandstones of the Suntrana Formation probably were deposited in braided streams by migrating longitudinal bars and transverse side channel bars. A thick coal bed commonly overlies the fining- upward sandstones, which reflect accumulation of peat on raised mires.
Lignite Creek Formation
The Lignite Creek Formation, ranging from 490 to 790 ft (150 to 240 m) thick, overlies and is conformably gradational with the Suntrana Formation. The Lignite Creek consists of interbedded sandstones, siltstones, mudstones, carbonaceous shales, and coals. The sandstones are fining-upward pebble to coarse grained in the lower part and fine grained in the upper part. The dominant mudstones and sandstones in the Lignite
Photo of the “drag line” in action at the Usibelli mine. The massive machine removes overburden with a capacity rate of 30 to 40 cubic yards. For scale, a pick- up truck is visible at the base of the drag line that was named by local students as “Ace in The Hole.” Photo by Chris Arend.
Creek Formation reflect its deposition in a high-sinuosity or meandering stream setting.
Grubstake Formation
The stratigraphically highest formation assigned to the Usibelli Group is the Grubstake Formation. This formation consists of dark gray laminated shale and mudstone that is 590–980 ft (180–300 m) thick in the northeastern part of the Nenana coal province but only 2–6 ft (0.6–1.9 m) thick in the southwestern part. The Grubstake Formation probably accu- mulated in a lake closely similar to that of the Sanctuary lake.
Nenana Gravel
The Nenana Gravel consists of poorly consolidated, buff to red, pebble- to boulder-size conglomerates overlying the Usibelli Group. It ranges in thickness from 3,940 ft (1,200 m) at the south edge of the Nenana coal province to 980–1,310 ft (300–400 m) along the north edge of the Alaska Range foothills. It is comprised of gravel detritus that was shed northward from the rising Alaska Range. Its age is bracketed between 8.3 and 2.75 Ma. The Nenana Gravel is much more widely dis- tributed than the Usibelli Group, which is primarily confined to synclinal basins deformed early in the orogeny that later deposited the Nenana Gravel.
A typical train loaded with Usibelli coal to be delivered along Alaska’s scenic railways. Photo by Chris Arend.
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