LOWER CRETACEOUS ROCKS.

Rocks of Lower Cretaceous age are represented mainly by a single formation—a massively bedded limestone, 300 feet thick, which overlies the Upper Jurassic strata conformably. The limestone has a limited areal distribution and is known only from the headwaters of Billy Creek and Nelchina River, where it weathers in prominent gray cliffs encircling the hilltops. (See Pl. IV, B, p. 42.) The limestone is white and of very finely saccharoidal texture. On fresh fracture it emits a strong fetid odor. It is readily soluble in cold hydrochloric acid, and evolves an abundance of unpleasant hydrocarbon gases. Qualitative tests show the presence of not more than a trace of magnesia. Under the microscope the detrital organic origin of this limestone is readily apparent. It is largely composed of irregular granules of dirty-looking calcite, but with these are associated numerous prismatic sections referable to organic fragments. Rhombohedral cleavage is here and there finely developed, but the characteristic calcite twinning is notably absent. Some of the carbonate is still in the aragonite form, as is indicated by negative biaxial interference figures. Accessory constituents are limpid plagioclase, quartz with hairlike rutile, hornblende, and, more rarely, augite and apatite. The incomplete conversion of the carbonate to the stable modification, and the absence of twin lamellation in the calcite indicate the essentially static conditions in which the limestone has remained since its formation.

a

The age determination of this Cretaceous horizon is based on the presence of Aucella crassicollis Keyserling, obtained by Mendenhall from a conglomeratic sandstone at the base of the limestone. This conglomerate may indicate a time break between the Lower Cretaceous rocks and the Upper Jurassic strata in conformable sequence beneath them.

TERTIARY.

EOCENE.

DISTRIBUTION.

Strata of Kenai age were found only in the basin of the Matanuska west of Hicks Creek, underlying an area of about 380 square miles. This figure represents a minimum area, as the lower Matanuska Valley is not included in the above estimate on account of the heavy gravel sheet which effectually conceals all bed rock in that part of the region. Strata of the same age are found at Tyonek, on the west side of Cook Inlet, and at various points in Susitna Valley, but they differ considerably from those of the Matanuska region in their lith

a Mendenhall, W. C., op. cit., p. 309.

ology and structure, in the absence of associated intrusives, and in the character of the interstratified coal seams. Rocks of Kenai age are widely distributed throughout Alaska, but usually in areas of restricted extent.

CHARACTER.

The Kenai of the Matanuska basin comprises a series of sandstones, shales, arkoses, numerous coal seams, and a large volume of conglomerate. The rocks are well indurated, and, as first noted by Mendenhall, resemble the Paleozoic coal measures of the Appalachian region. The sandstones are gray, hard, and tough, and are characterized by the presence of silvery shreds of white mica. At some localities, as in the leaf-bearing strata in the Kings Creek coal measures, the sandstones contain shale fragments. The shales show no unusual features; they are dark-bluish, fissile, and interstratified with the sandstones in very regular beds. The arkoses were noted only on the ridge north of Tsadaka Creek, abutting against the quartz-diorite mass. They are composed of feldspar, quartz, and chlorite, and locally contain granite pebbles. Certain strata are fine examples of regenerated granites.

Conglomerate is found at Castle Mountain, between Kings and Chickaloon creeks, and at Conglomerate Mountain, between Tsadaka and Eska creeks. (See Pl. III, A, p. 16.) At both places the deposit is at least 1,000 feet thick. Thin sandstone beds, not exceeding a few feet in thickness, are intercalated with the conglomerate and pinch out along the dip. The pebbles of the conglomerate are well worn and possess a well-defined shingling. Their diameter averages 4 inches and ranges up to 8 inches. Greenstone porphyries form the principal material of the conglomerate, with some quartz porphyry and very rarely some granite and vein quartz. The matrix of the pebbles is a grit.

a

Mendenhall describes the conglomerate of Castle Mountain as follows:

This bed is probably not less than 1.000 feet thick, 600 feet of it having been measured, and the estimated height of the cliff above me at the time of the measurement being 300 to 400 feet. The heavy conglomerate plates, which together make up the mountain, are separated by thin sandy shale sheets, which constitute but an insignificant quantity of the mass. The pebbles of the conglomerate contain a great variety of rocks, among them many igneous types. They are rounded and vary in size up to a foot in diameter. Overlying the Castle Mountain bed is a body of soft shales with a cap of red sandstone.

Heavy beds of conglomerate dipping to the north can be seen on the east side of Chickaloon Creek. They are part of a great series of

• Mendenhall, W. C., A reconnaissance from Resurrection Bay to the Tanana River, Alaska, in 1898: Twentieth Ann. Rept. U. S. Geol. Survey, 1898-99, pt. 7, p. 308.

sandstones, shales, and conglomerates, probably not less than 3,000 feet thick, which are overlain by basaltic porphyries at an elevation of about 4,700 feet. Canyons cut through this series of rocks have nearly vertical walls and give to the mountains a massive castellated form. (See Pl. III, C', p. 16.)

The coal of the Kenai of the Matanuska basin is bituminous except where modified by igneous rocks. The seams vary in thickness up to a maximum of 17 feet.

FOSSILS AND AGE.

The age of these beds has been determined from their fossil plants. These have been submitted to F. H. Knowlton, who reports as follows:

This material, consisting of six small lots, is in the main very poorly preserved and insufficient in quantity to make the report of fullest value. The various localities and the forms detected at each are as follows:

Tsadaka Creek, 25 feet above 11-foot coal seam. Age, Kenai.

Sequoia langsdorfii (Brgt.) Heer.

Ficus? grönlandica? Heer.

Magnolia inglefeldi Heer.
Fragments of other dicotyledons.

Kings Creek coal. Age, apparently Kenai.

Ficus? grönlandica Heer.

Populus arctica? Heer.

Rhamnus eridani Heer.

Viburnum sp., cf. V. nordenskiöldi
Heer.

Arkose Ridge, north of Tsadaka Creek. Age, apparently Kenai.

Taxodium distichum miocenum Heer.

Taxodium tinajorum Heer.

Populus sp.?

Ficus? grönlandica? Heer.
Paliurus colombi Heer.

Fruits, cf. Leguminosites sp.

Chickaloon coal strata, Watson's camp. The age indicated is Kenai.

In addition to the plant remains, shells have been found by Martin in the thin ironstone (No. 17) in the section on page 12 of his report. Stanton says:

This lot (No. 3316) consists entirely of fresh-water gasteropods, of which all but one specimen belong to a single species of Viviparus of a type that occurs in both the upper Cretaceous and the Tertiary. The other specimen is a more slender form, too imperfect for generic determination. These fossils are apparently an undescribed species-at least they are new to Alaska-and they do not fix the horizon more closely than is above indicated.

a Op. cit., p. 12.

The evidence from the flora of these beds shows, therefore, that they belong to the Kenai, which is regarded as of upper Eocene age. The evidence from the fossil gasteropods is valuable in confirming the fresh-water origin of this thick series of strata and, in a general way, their late Cretaceous or Tertiary age.

STRATIGRAPHY.

The thickness of the Kenai in this field is not definitely known, but is over 3,000 feet, of which 1,000 feet, near the top of the series, consists of the conglomerate.

The stratigraphic relations of these Tertiary strata to the older rocks are complex. North of Tsadaka Creek, where arkoses come in contact with granitic rocks, simple erosional unconformity is suggested. West of the Little Susitna there is evidence that the position of the sandstone beds against the crystalline rocks is due to faulting. At one locality the beds overlie at a considerable angle crystalline mica shists, while a mile or two farther west a granite similar in type to the central granite mass has taken the place of the sandstone beds against the schists, and the sandstones lie farther south, against the granite. (See Pl. II, pocket.) Along the south flank of Anthracite Ridge the Kenai has been faulted against Jurassic beds.

The Kenai beds have been folded and faulted since they were laid down. The folding is at some places of a sharp, asymmetric character. On account of the presence of coal beds in the Kenai, it is thought best to present the main discussion of its structure in the description of the coal fields.

IGNEOUS PHENOMENA.

Dikes and sills of diabase are numerous in the Kenai rocks of the Matanuska region, especially in the region east of Chickaloon Creek, where they form prominent "trap ridges." The thickness of the intrusives ranges from several feet up to several hundred. Equally noticeable variations in grain, ranging from extremely fine-grained to coarse ophitic textures, are easily recognized by even the naked eye. At Anthracite Ridge one of these dikes, 75 feet thick, composed of augite and labradorite, has exerted strong contact metamorphism on the adjoining shales, baking them into a porcelanite zone 8 inches thick.

The igneous sheet overlying the shales on Kings Creek at the coal prospects differs petrographically from the prevailing diabase intrusives of the region. It carries porphyritic feldspars in a lightcolored granular groundmass and has been determined microscopically to be a monzonite porphyry.

The effect of these dikes and sills on the coal is of particular interest. Wherever they cut the coal or come close to it they have converted it

to a hard, dense coke. Besides this local effect the presence of thick sills in the coal-bearing rocks appears to have raised the grade or fuel value of the coal. As will be shown under the discussion of the mineral resources, the Matanuska coal field can be divided into two districts, of which one, the western, contains low-grade bituminous coal, approaching lignites in composition, while the other, the eastern, contains high-grade bituminous coal. The difference in character of the coals can not be explained as due to difference in intensity of folding in the two districts. Igneous intrusives are absent from the western district, but are prevalent in the eastern. This association of high-grade coal with abundant intrusives is at least suggestive. Furthermore, it is probable that the anthracitic character of the coal of Anthracite Ridge is due to the presence of unusually thick and abundant dikes in rocks of Kenai age.

COMPARISON WITH ADJOINING REGIONS.

The Kenai rocks of Matanuska Valley differ markedly from those of the type section of the Kenai as exposed on Kachemak Bay, on Cook Inlet, where the sandstones are soft and incoherent, the shales are plastic when wet, and the lignite seams form the resistant members. The beds on Kachemak Bay lie at low angles in undisturbed attitudes. The lignite is in general hard and glossy, but in the higher seams becomes dull and shows a more woody fiber. The lower seams have been buried under a depth of nearly 10,000 feet of strata. If the Kenai of Matanuska Valley be compared with the Kenai exposed at Tyonek, the contrast is even greater, though Tyonek is nearer than Kachemak Bay. The Kenai at Tyonek consists" of alternating beds of clay, sand which has hardened to sandstone in huge nodules, and lignite." According to Eldridge, the lignite in places hardly appears to be more than a compressed mass of carbonized wood. The strata are tilted at angles ranging from 35° to 60°. Though more sharply deformed, the coal is of lower grade than that of Kachemak Bay.

From the foregoing comparisons it appears that the Kenai of Matanuska Valley differs considerably from that of the adjacent regions in its higher degree of induration and in the superior quality of its coal seams. The conclusion can be drawn that the higher character of the Matanuska coal is due not to conditions of deeper burial or sharper folding, but to the prevalence of intrusives in the coal-bearing strata.

a Stone, R. W., Coal fields of the Kachemak Bay region: Bull. U. S. Geol. Survey No. 277, 1906, p. 58.

Spurr, J. E., Reconnaissance in southwestern Alaska in 1898: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 7, 1898-99, p. 102.