mass doubtless stood at a lower level relative to the sea. Some time after the flooding of the region with basaltic lavas, a domal uplift of the area was inaugurated, probably accompanied by faulting along the north side of Matanuska Valley. North of Talkeetna River the old surface, buried beneath the basalts, can be seen dipping gently northward. Eastward, from an altitude of about 5,200 feet, it appears to merge gradually into the flats of the Copper River basin at 3,000 feet. This arching demanded a stretching of the earth's crust, and as a result the numerous small dislocations of the basement upon which the volcanics rest were produced. The domal character of the uplift is expressed in the general radial drainage, the main streams flowing from the center of the region. The shortness of the streams tributary to the Matanuska, in comparison with those flowing to the Susitna, appears to indicate an obsequent origin for this portion of the drainage. Along the western flank of the Talkeetna Mountains a broad bench several miles wide parallels Susitna River at an altitude of about 2,500 feet. The surface of this bench has been considerably modified by glacial erosion, and a mammillated topography has been impressed upon it but does not obscure its terrace character. Tributaries of the Susitna have cut normal stream valleys across it, many displaying marked evidence of a position at base-level in their recent history by the intricate and beautiful systems of oxbows developed at an elevation of about 500 feet. Below these stretches of oxbows the streams enter sharply incised canyons, whose small amount of recession indicates the comparative recency of the latest uplift of the region. It appears probable that the stage of the oxbows represents a halt in the greater uplift which has given the broad bench its present elevated position. COPPER RIVER BASIN. The Copper River basin has been discussed in some detail by Mendenhall, who advanced the hypothesis, as one most consistent with the facts, that it is of tectonic origin, representing a depressed area between the Alaska Range on the north and the Chugach Range on the south." The relation of the Talkeetna Mountains to the basin was not discussed. Certain facts in the region under consideration appear to favor Mendenhall's hypothesis. On the western edge of the basin, near the head of Matanuska River, the hills rise abruptly from the plateau. Their summits form a gently rolling surface at an average altitude of 5,000 feet and evidently constitute a remnant of the ancient peneplain upon which, a Prof. Paper U. S. Geol. Survey No. 41, 1905, p. 88. farther west, at an altitude of 5,200 feet, were found the river channels buried beneath lava. These features strongly suggest faulting along the base of the hills, the old plain being broken off along this line and its eastward extension buried beneath the glacial débris of the Copper River basin. Farther northwest, however, the Talkeetna Mountains gradually merge into the lowlands of the Copper River basin-a fact indicating, perhaps, that faulting gives way in this direction to warping of the crust. MINERAL RESOURCES. COAL. AREAL DISTRIBUTION. Coal outcrops have been examined on Tsadaka, Eska, Kings and its tributaries, Chickaloon, Coal, Hicks, and Billy creeks, and the north fork of Nelchina River; along the southern flank of Anthracite Ridge; and in the banks of Matanuska River, about 3 miles above the mouth of Chickaloon Creek. In addition, coal float has been found close to its source on Glacier Creek, and as far north as the south fork of the Tyonek. Coal has also been reported from Little Susitna River, Willow Creek, the Kashwitna, and the Chunilna, and from a creek on the south side of the Matanuska, 9 miles above Coal Creek. The field work of 1906 extended the known area of Tertiary coalbearing rocks approximately 18 miles up Chickaloon Creek, though no outcrops of coal were observed. The discovery of an area of Mesozoic rocks in Matanuska Valley extends the field also in that direction, as is indicated on the map, though all the coal that outcrops north of Anthracite Ridge between Hicks and Boulder creeks is lignitic. So far as known the Tertiary coal-bearing rocks in the Matanuska cover an area of about 380 square miles. The known coalbearing rocks of Mesozoic age in the upper Matanuska basin cover approximately 500 square miles. The areal extent of the divisions is shown on the map (Pl. II, pocket), and the character of the beds has been described on page 25. The mapping of the coal-bearing rocks must in no sense be taken to mean that areas so mapped are underlain by workable coal. So far as known the actual area underlain by coal from Moose Creek to Hicks Creek, inclusive, approximates 70 square miles. Localities where coal of commercial importance has been observed will be described. The area of coal-bearing rocks can be subdivided from an economic standpoint into three fields: (1) The Matanuska field, comprising the valley floor of Matanuska River west of the mouth of Hicks Creek. This field is underlain by a series of Tertiary strata containing bituminous coals of various grades. (2) The Anthracite Ridge field, forming a limited area along the southern flank of the Talkeetna Mountains between Boulder and Hicks creeks. The coal here is an anthracite, the age of which has not yet been determined with certainty. (3) The northeastern field, comprising the area drained by the headwaters of Matanuska River and the adjacent areas in the Copper and Susitna drainages. The coal, which is of Jurassic age, is generally lignitic, but locally is semianthracitic. MATANUSKA FIELD. EXTENT. The area of Tertiary coal-bearing rocks found in the Matanuska basin is approximately 380 square miles, of which 70 square miles are actually underlain by coal, according to a most conservative estimate. STRUCTURE. The coal-bearing rocks have a strike in general parallel to the easterly trend of Matanuska Valley. The dips are variable, with a tendency toward steep angles. East of Tsadaka Creek, where the gorge of Matanuska River exposes a voluminous succession of sandstones and shales, the strata stand on edge. The folding is open, although locally, as in the anticline on Chickaloon Creek, the deformation may be quite sharp. The strata are affected by a large number of faults of small throw. One extensive fault involving a probable displacement of several thousand feet is indicated, mainly on physiographic evidence. The range of hills rising within Matanuska Valley and extending in an east-west direction from Kings to Tsadaka Creek represents an upheaved fault block. Between Tsadaka and Eska creeks this range consists of a great mass of conglomerate, 1,000 feet thick, with a gentle dip slope toward Mata 0 2 3miles FIG. 1.-Section across Matanuska River at Conglomerate Mountain. 1, Sandstones, shales, arkoses; 2, conglomerate with sandstone lenses; 3, graywackes and slates; 4, quartz diorite. nuska River and a steep scarp toward the north (see fig. 1). During the uplifting of this block, differential movement has taken place within it, producing a series of nearly parallel faults, and the southern slope of the range descends to the level of the valley by a succession of steplike scarps. On the very brink of the northward-facing scarp of the range the differential movement has produced small fault troughs, within which lie small inclosed lakelets. (See Pl. IV, 4.) These features are shown with great clearness on the topographic map. The recognition of this fault is of some economic importance, in that it indicates the limit of the available coal. DESCRIPTION OF THE COAL. The bituminous coal field of lower Matanuska Valley may be divided into two districts, the eastern and the western. Under the first may be included the coals of Kings and Chickaloon creeks and those on both sides of Matanuska River in the vicinity of Chickaloon Creek; under the second, the coals of Tsadaka and Eska creeks. EASTERN DISTRICT. a Martin reports in regard to the eastern district: The coal in this area all possesses about the same physical characteristics, and, as will be seen by the analyses, the variation in chemical composition is not great and supports this grouping. It has the ordinary properties of most bituminous coal. It is soft and fragile, but often without any well-defined planes of fracture. It burns with a short flame and a small amount of smoke and possesses distinct coking properties. The seams generally contain a large amount of impurities, both in the form of thick partings of shale and as thin bands of shale and bone. Many of these can be separated in mining. The coal is soft and friable, and much of it will not stand severe handling without crushing. Pyrite is present both as balls and as scales, but not abundant. The friable character of the coal is not a great detriment when it is considered that much of it will probably have to be crushed and washed (especially for coke making), and that the coal when used for steam or heating will cake as soon as put in the furnace, so that there will consequently be little or no loss through the grates. "Op. cit., p. 19. |