--> Abstract: Sedimentology and Lithofacies of the Eocene Skookumchuck Formation in the Centralia Coal Mine, Southwest Washington, by R. M. Flores and S. Y. Johnson; #90958 (1995).

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Abstract: Sedimentology and Lithofacies of the Eocene Skookumchuck Formation in the Centralia Coal Mine, Southwest Washington

Romeo M. Flores, Samuel Y. Johnson

The late middle to late Eocene Skookumchuck Formation is well exposed in highwalls of the Centralia mine, southwest Washington. Three coal zones and intervening sandstone-rich successions occur in a 220-m-thick interval that extends from below the Smith to above the Tono No. 1 coal beds.

The Smith-Big Dirty, Lower-Upper Thompson, and Tono Nos. 1-2 coal zones contain coal beds that range from 0.5 to 15 m thick. The coal beds are interbedded with coarsening-upward units of mudstone, siltstone, and sandstone that are burrowed, flaser- and lenticular-bedded, mud-draped in their lower part, and rippled, wavy bedded, and tabular crossbedded (with reactivation surfaces) in their upper part. These coarsening-upward units are commonly overlain erosionally by fining-upward beds of trough-crossbedded, rippled, burrowed, and rooted sandstones. Facies characteristics of these units indicate deposition in a tidally influenced coastal plain that included intertidal flats, tidal channels and bars, and supratidal environments. Coal formed in low-lying peat mires above mean high tide levels.

Two discrete facies occur in the sandstone-rich successions between the coal zones. The first facies consists of very fine to coarse grained sandstones that have sharply defined bases and tops and are heavily bioturbated, horizontally bedded, trough crossbedded, hummocky bedded, and rippled. Trace fossils include Ophiomorpha and Thalassinoides; fossil lags include oyster shells, thin- to thick-shelled bivalves, and Turritella gastropods. The sandstones are interpreted as shoreface deposits. The second facies of the sandstone-rich successions consists of sandstone, siltstone, and mudstone beds with common ripple lamination, lenticular and flaser bedding, trough crossbedding, bioturbation, bivalve fossils, and root marks. These rocks were deposited in tidal, i tertidal, and supratidal environments.

The Skookumchuck interval includes a parasequence that extends from major flooding surfaces between the Smith-Big Dirty coal zone in the lower part and Lower-Upper Thompson coal zone in the middle part. Upward thinning of coal beds is consistent with deposition during a major transgression. These coals formed in mires of the tidally influenced coastal plain in the Centralia mine area, in contrast to coals elsewhere in Washington that accumulated in mires of the fluvial- and distributary-channel-influenced coastal plain. The Northcraft volcanic center to the east probably deflected fluvial drainages of the coastal plain to the north and south.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California