GEOLOGY OF THE LOST CREEK COAL FIELD,

MORGAN COUNTY, UTAH.

By FRANK R. CLARK.

INTRODUCTION.

The Lost Creek coal field lies in Morgan County, Utah, about 10 or 12 miles northeast of Devils Slide station, on the main line of the Union Pacific Railroad. (See fig. 44.) The field takes its name from Lost Creek, a tributary of Weber River, and comprises about 8 or 10 square miles in T. 5 N., R. 5 E., Salt Lake base and meridian. It lies between the Echo Canyon drainage basin on the east and the Ogden Canyon drainage basin on the west. The area examined includes parts of Tps. 4, 5, and 6 N., Rs. 4 and 5 E., but no coal of economic importance was found outside T. 5 N., R. 5 E., where it has been mined in a small way at several places to supply demands of ranchers. A considerable portion of the coal mined was hauled by team to the town of Croydon, 8 or 9 miles from the mines. The wagon road from the railroad to the mines is good for most of the distance except during the winter. The average grade of the road up Lost Creek is about 50 feet to the mile, but the grades of branch roads up the side canyons to the mines are from 200 to 300 feet to the mile.

The coal bed, which is lenticular and varies greatly in thickness even in small areas, is confined to one coal-bearing zone. It is subbituminous and contains much moisture and many impurities in the form of small lenses or partings of bone and shale.

The field was examined in 1914 for the purpose of verifying or disproving the many reports regarding the possibility of a commercial field in this area and to obtain data from which a proper classification and valuation of the land might be made. The mapping was done with the Gale telescopic alidade and plane table. Locations were made by the three-point intersection method from a system of triangulation expanded from a carefully measured base line. Secondary locations were made by stadia readings or short stadia traverses in deep canyons where points of the primary triangulation were invisible. These traverses were tied to Government land corners and to the primary system of triangulation.

The area outside the coal-bearing portion was mapped in a reconnaissance way. Approximate altitudes were carried into the coal field from the railroad at Devils Slide by stadia traverse and vertical angles.

The writer was assisted in the field by Gordon W. Clark. Ranchers in the area furnished information relating to the coal and its previous

FIGURE 44.-Index map showing location of Lost Creek coal field, Utah.

development, and in this connection especial credit should be given to Mr. J. S. Turner for his hospitality and desire to aid the work in every way.

LAND SURVEYS.

The townships covered by this report were surveyed many years ago Tps. 5 and 6 N., R. 5 E., Salt Lake base and meridian in 1891 and the remaining townships in 1899. Most of the corners found were well marked and check closely with the positions indicated by

the General Land Office plats, but several of the monuments appeared to have been moved from the positions in which they were originally set by the authorized surveyor. Those for the northwest corner of sec. 25, T. 5 N., R. 5 E., and the quarter corner between secs. 25 and 26 now appear to be about 100 feet north and 150 feet east of their true positions according to the Land Office plats, and that for the southwest corner of sec. 34, T. 5 N., R. 5 E., is about 200 feet north of its original position. All other corner monuments found appear to be in their original positions and to check with the official plats.

GEOLOGY.

STRATIGRAPHY.

The rocks exposed in this field comprise two formations which differ widely in character as well as in age; the older formation is of Jurassic age and the younger of Tertiary age (Wasatch formation). These formations are separated by a great unconformity representing a long interval of time during which the older rocks were minutely folded and the folds were later truncated by erosion.

JURASSIC ROCKS.

The rocks that are here assigned to the Jurassic consist of limestone, shale, and well-indurated sandstone. Some of the limestone beds are relatively pure, some contain many veinlets of quartz, and others are shaly and grade into calcareous shale. The shale, which in places is calcareous, is usually thin bedded and weathers silver-gray but is brown to slate-colored on freshly broken surfaces. In a casual search these beds yielded but few fossils-Pentacrinus, which is indicative of the Jurassic, being the only form identified.

TERTIARY ROCKS (WASATCH FORMATION).

The rocks overlying the Jurassic unconformably are correlated with the Wasatch formation of Echo Canyon, to the east, largely on the basis of their position and general character. No mapping has been done between the two areas, but there is little doubt that the correlation is justified. The Wasatch in the Lost Creek field may be divided for convenience of description into three parts-an upper and a lower conglomerate and intervening sandstone and shale. It is composed largely of coarse materials, the greater part being coarse conglomerate; a minor portion near the middle consists of conglomeratic sandstone, coarse sandstone, shale, and in places thin, lenticular, and valueless coal beds.

The Wasatch beds rest unconformably upon the upturned and eroded edges of rocks of Jurassic age. The thick, massive beds of

21544°-19-Bull. 691-21

coarse conglomerate lie mainly in small areas east of Lost Creek, namely, in Coal Hollow, Joe's Hollow, Paradise Canyon, Toone Canyon, and the head of Cedar Canyon; a small outcrop occurs at Smith's mine, on the west side of Lost Creek. The Wasatch in surrounding areas is composed of thin beds of shale and of coarse sandstone which at many places is conglomeratic.

The maximum thickness of the Wasatch in this area is about 1,800 feet. At the head of Toone Canyon it is coarse conglomerate except for 300 or 400 feet of sandstone and shale near the middle. In 'Toone and Paradise canyons the lower conglomerate is from 700 to 900 feet thick and is composed largely of quartzitic pebbles and boulders which range from a fraction of an inch to 11 feet in diameter. The lower conglomerate is usually gray to white, but locally it may be distinctly red. The upper surface of this conglomerate is exceedingly uneven and in many places varies as much as 12° from the bedding of the overlying sandstone, shale, and coal. The coal beds rest almost directly on this lower conglomerate and are overlain by the sandstone and shale member above mentioned. In most places where exposed the coal rests on the conglomerate or is separated from it by only a few feet of shale or sandstone.

Apparently the lower conglomerate was laid down in an erosional basin, for in Toone and Paradise canyons and Joe's Hollow this heavy conglomerate abuts squarely against the upturned, eroded edges of the Jurassic limestone, and the westward extension of the conglomerate is abruptly cut off by the limestone. It would seem from this that a fault may have cut the beds at this place, but no evidence of faulting was seen to the north or the south. The abrupt ending of this conglomerate suggests that either the Jurassic limestone in Toone and Paradise canyons, and especially in Joe's Hollow, extended above the general level of the surrounding country at the time the conglomerate was deposited, or during subsequent uplift the Jurassic limestone was faulted up against the lower conglomerate. Toward the east the contact between the Jurassic and Wasatch in secs. 17 and 16 rise about 1,000 feet in a mile. The base of the lower conglomerate in the southeast corner of sec. 16 is about 400 feet lower than the top of the Jurassic limestone.

Above the middle sandstone and shale member lies the upper conglomerate, which is distinctly red owing to the red sand matrix in which the pebbles are embedded. The pebbles and boulders are similar in character and size to those of the lower conglomerate and were laid down under similar conditions. The pebbles are usually well rounded though not greatly streamworn. They consist chiefly of quartzite with minor amounts of sandstone fragments (very irregular), limestone, and small masses of conglomerate of which the component pebbles are largely quartzite.