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Coal in the Coos Bay field, southwest Oregon, was an important source of fuel along the western seaboard of the United States from about 1855 to the early 1920's when the largest operating mine in the field was closed. In an effort to stimulate production of coal to meet the increased war-induced needs of 1943-45 for the Pacific Northwest a core drilling program and geologic investigation of one of the more promising areas, about 21 square miles, occupying the central part of the Coos Bay coal field, was undertaken by field units of the Geological Survey and Bureau of Mines, cooperating agencies of the U. S. Department of the Interior. Sedimentary rocks of the Coos Bay Coal field include a series of sandstones and shales of Tertiary age mostly concealed by a mantle of Quaternary terrace deposits, alluvium, or deep soil. The coal beds penetrated by drilling in the central part of the coal field are confined to a sequence of beds about 1,100 feet thick in a sandstone member of the Coaledo formation, of late Eocene age. Attempts at correlating the drilled sequence, about 2,200 feet thick in the report area, with a stratigraphic section of the Coaledo formation, about 5,600 feet thick, exposed about 10 miles to the northwest along the sea cliffs at the mouth of Coos Bay, indicate that the main coal-bearing member of the drilled area may be correlated with either the upper (sandstone) member of the Coaledo or the lower (sandstone) member of the Coaledo in the coast section. These correlations suggest that the upper and lower members of the Coaledo formation in the coast section may be approximately the same stratigraphic unit, which has been duplicated by faulting in the middle (shale) member of the Coaledo. The Coos Bay Coal field occupies a structural basin about 15 miles wide and 30 miles long, elongate north-south. Tertiary sedimentary rocks in the basin have been deformed into several steep, and almost parallel folds which trend northward. One of these folds, the Beaver Slough syncline, underlies most of the map area of this report and the coal-bearing sequence is exposed mainly along the flanks of the syncline. Northward-trending faults in the northern part of the map area are interpreted to be high-angle thrust faults. Several smaller west-, northwest-, and northeast-trending faults with displacements of less than 100 feet offset the coal beds in the mined areas. These faults and adjacent sheared zones have limited the size of several of the smaller mines and prospects in the area. The main coal-bearing member of the Coaledo formation in the drilled area is about 500 feet thick. It consists mainly of alternating sandstone, shaly sandstone, and minor shale and contains 7 or 8 coals in most sections. The coal beds range from about 1 foot to 20 feet in thickness. Coal of the area is of sub-bituminous B rank. Heat values of 65 coal samples range from about 5,300 Btu per pound for the impure coals to about 10,000 Btu per pound for the better coals, 53 54 A CONTRIBUTION TO ECONOMIC GEOLOGY on as-received basis. The Beaver Hill coal bed at the base of the main coalbearing member is the most persistent bed of minable thickness and good quality in the drilled area. The bed ranges from 3.6 to 8.2 feet in thickness and its average heat value on as-received .basis ranges from 9,000 to 10,000 Btu per pound. Coal reserves were, estimated for beds 30 inches or more thick, yielding 8,000 Btu or more per pound, and extending to depths as much as 1,500 feet below the surface. Based on spacing and completeness of sample data and reliability of other observations, the reserves are divided into measured, indicated, and inferred categories. Estimated remaining reserves of coal in the report area, including all categories, aggregate about 63 million tons as of July 1945. Remaining reserves in the Beaver Hill coal bed, underlying 5,355 acres of the map area, total about 56.5 million tons of coal, including 17.2 million tons of measured reserves, 26.6 million tons of indicated reserves, and about 12.6 million tons of inferred reserves. Other coal beds in the area are too thin or of too low grade to be of value except at a few localities. These coal beds contain an estimated 6.2 million tons of measured reserves. Recoverable coal reserves from the report area are believed to be onefourth to one-half of the estimated remaining reserves. INTRODUCTION Coal has been mined from the Coos Bay field, southwest Oregon, at intervals since about 1855, and it was an important source of fuel for railways, coastal shipping, and industrial and domestic heat in Oregon until the early 1920's when the largest operating mine, the Beaver Hill mine, was closed. At about the same time, less expensive fuel oil from California became available, resulting in a curtailed market for coal along the west coast. With a reduced demand for coal, no large-scale coal-mining operations were carried on in the Coos Bay area until 1944, when mining was resumed to meet the war-induced needs for fuel in the Pacific Northwest. An acute shortage of fuel developed in Washington and Oregon during the war period from 1942 to 1945 when many coal mines in western Washington curtailed production or ceased operations for lack of men, and shipments of fuel oil from California were reduced. As a means of stimulating fuel production in the Pacific Northwest a program of mapping and prospecting was begun in. 1942 by the Geological Survey and the Bureau of Mines, U. S. Department of the Interior. This program included the investigation of the area in the Coos Bay field herein described. The Bureau of Mines and the Geological Survey cooperated in the work in the Coos Bay field, examining mines and prospects, and core-drilling to obtain samples and learn the extent and depth of the coal beds. This report describes the geologic results of the investigation, and a report by the U. S. Bureau of Mines (Toenges, A. L., and others, 1948) supplies complementary data on the quality and mining possibilities of the coal. Keports on the geology and coal resources of the Coos Bay quadrangle by Diller (1899, 1901) and Diller and Pishel (1911) supplied the first comprehensive data on the coal resources of the area. More recent reports by the Oregon State Department of Geology and GEOLOGY AND COAL DEPOSITS, COOS BAT COAL FIELD, OREG. 55 Mineral Industries and by the U. S. Bureau of Mines have added to knowledge of the coal in the field. The most comprehensive of these is a report by Alien and Baldwin (1944), which supplies a full account of the geology and coal resources of the Coos Bay quadrangle. The examination carried on by the Oregon State Department of Geology and Mineral Industries during 1943 and 1944 was planned to locate and test near-surface coal deposits and to reappraise the entire coal field. The Oregon State investigation preceded and overlapped that of the Geological Survey and Bureau of Mines, and much helpful information was supplied the Federal agencies by Messrs. Alien and Baldwin prior to publication of their results. Thanks are due also to several local men who supplied information and contributed their efforts as guides to some of the exposures and coal prospects in the area. H. E. Vokes, of the U. S. Geological Survey, identified fossils obtained from the drill cores and supplied opinion on correlation of the faunas. The investigation by the Geological Survey and the Bureau of Mines was confined largely to an area of 20 square miles near the center of the Coos Bay field between the towns of Coos Bay and Coquille, Oregon (pi. 4). D. A. Andrews, assisted by R. C. Robeck, started the geologic examination in November, 1943. In May, 1944, Andrews and Robeck received other assignments and Duncan was assigned to the Coos Bay project. The field investigation, including the drilling, was completed in June, 1945. A map, originally on a scale of 1 : 31,680, was compiled from aerial photographs. Many of the section corners in the area were located on the photos, and a land net, compiled from Bureau of Land Management and Coos County engineers' records, served as the base. Spot locating of prospects and mine entries was made with reference to the photos; some prospects, especially those in heavily wooded sectors, were difficult to locate accurately; but points in or near clearings, roads, or other identifiable places on the photos were located with greater accuracy. A few prospects, particularly those in the wooded areas, were located by planetable traverse. Many coal prospects were found. Of these, some were opened and a face of coal exposed for examination, measurement, and sampling. Others, particularly those in the thinner and relatively impure beds, were, not opened, as the work necessary to prepare adequate exposures of the coal, or to dewater a mine or prospect, would have been more costly than was justified by the value of the information to be obtained. Drilling began at about the same time that geologic mapping started. Each drill site was selected jointly by the geologist and the engineer. Cores from the drilling were logged in detail by careful inspection at the drill site. Samples of all coal beds and most carbonaceous shale beds were then packaged and sent to the coal-constitution laboratories of the U. S. Bureau of Mines for further exami56 A CONTRIBUTION TO ECONOMIC GEOLOGY nation and analysis. Fossils and rock samples were saved for later study and comparison. Holes 1-23 and 2-23 were put down to test the continuity of coal at depth in the Southport mine (pi. 4). When it was found that these initial holes were located on the upthrown side of a fault, and that they penetrated rocks below the main coal-bearing member of the Coaledo formation, drilling was moved south to the vicinity of the Delmar, Overland, and Martin mines. Here holes 3-3, 4-9, 5-9, 6-3, and 8-3 were drilled deep enough to penetrate the principal coal bed, the Beaver Hill coal bed, at the base of the main coa

Geology and coal deposits in part of the Coos Bay coal field, Oregon