Geology and Coal Resources of the Coos Bay Quadrangle, Oregon: ABSTRACT

Abstract

The Coos Bay coal field is situated on the coast of southwest Oregon, readily accessible to railroad and to the harbor of Coos Bay. It lies within a roughly elliptical structural basin measuring 35 miles north and south by 11 miles east and west. Mesozoic sediments, schists, and volcanics, tentatively correlated with the Franciscan-Knoxville group of California, are exposed in the southern part of the quadrangle and are overlain unconformably by the middle Eocene Umpqua formation, consisting of more than 1,800 feet of tuffaceous sandstone and shale with thick lenticular basalts and pyroclastics. The Tyee massive feldspathic sandstone, about 2,000 feet thick, overlies the Umpqua and occupies the northeastern corner of the quadrangle. About 6,000 feet of upper Eocene Coaledo sediments are confined to a complex structural basin occupying the central portion of the quadrangle. The lower and upper Coaledo members consist of medium-bedded tuffaceous sandstones made up largely of basaltic glass, separated by the middle Coaledo member consisting of 400 to 2,300 feet of dark tuffaceous shale of more acidic composition. The principal coal beds occur in the upper and lower sandstone members of the Coaledo formation. The Bastendorf shale and Tunnel Point sandstone represent the transitional and Oligocene strata, and their outcrops appear mainly on the western edge of the basin, although the Bastendorf is also found in remnants farther eastward. The Bastendorf is 2,900 feet thick, composed predominantly of basaltic glass, and the Tunnel Point with a minimum thickness of 850 feet is composed of basaltic with less amounts of andesitic glass. The Coaledo and the later Oligocene formations in the major basin were compressed during the Miocene into north-trending folds, and faulted by major north-trending faults and by more numerous transverse faults. The Pliocene Empire formation, comprising at End_Page 1779------------------------------ least 2,000 feet of poorly bedded sandstone, unconformably overlies the Oligocene and Eocene strata in the South Slough syncline and has been folded along the same axis to a lesser degree than older formations. Pleistocene terrace and estuarine deposits cover the coastal plains and major valley bottoms. Coal was first mined in 1854, and production reached 100,000 tons a year during the early part of the century, but since the increased use of fuel oil during the twenties coal has been mined only for local needs. The total production for the field is probably of the order of 3 million tons. The Beaver Hill bed, lowest coal of the upper group, has been mined more extensively than any other bed; with a few exceptions other beds of the upper and lower groups have not yielded great tonnages; these beds are ordinarily higher in ash and contain more numerous partings. Detailed mapping and drilling on four properties have resulted in developing 541,000 tons of measured coal; an additional 800,000 tons was indicated and 3,200,000 tons was inferred. More than 160 mines, prospects, and outcrops were examined and are described; 60 of them were sectioned and sampled. Coos Bay coal is subbituminous in rank, with a heating value of 9,000 to 10,000 B.t.u. per pound as received, with a low sulphur content, moderate percentage of ash, and a relatively high moisture content. The coals of the lower group have a higher heating value and a higher ash content, but mining conditions are relatively unfavorable. End_of_Article - Last_Page 1780------------