In general the sandstones and shales are barren of fossils, though belemnites are rather common. The following species, however, were found and were submitted to Mr. Stanton, who reports as follows: 6 A. K. 88A. Caribou Creek, tributary to Matanuska River. Inoceramus cf. lucifer Eichwald. Pleuromya. Pleurotomaria. Phylloceras. 6 A. K. 88B. Same locality as preceding, but found in float bowlder in stream bed. Inoceramus. Natica. Stephanoceras. Sonninia? (two species). The two lots are evidently from the same horizon, and nearly all the species occur in the lower part of the Enochkin at Snug Harbor. The age of these sandstones and shales is, therefore, substantially the same as that of the greenstones-lower Middle Jurassic. The sedimentaries from which the above fossils were collected do not differ much in appearance from the Tertiary sandstones of the region. The greenstones, however, wear a more ancient aspect, and are usually more internally disturbed than the sedimentaries, probably in consequence of the volume changes which they have undergone during their metasomatic alteration. QUARTZ DIORITE. DISTRIBUTION. Much of the rugged interior of the Talkeetna Mountains remains unexplored, but such evidence as is available indicates that the great central area consists wholly of granitic rocks. Extensive exposures of granular crystallines were encountered along the line of travel. The morainal material brought down by the glaciers from the remote recesses of the range and the gravels of the streams heading in the mountainous interior consist prevailingly of granitic material. A considerable variety of rock types is found, but quartz diorites appear to predominate. A quartz diorite north of Tsadaka Creek carries the dark minerals, hornblende and biotite, and is associated with a quartz-diorite gneiss rich in hornblende. This gneiss is found also on Granite Creek and is doubtless a cataclastic phase of the massive quartz diorite. Along the middle course of Chickaloon Creek quartz-augite diorite appears, but toward the headwaters it gives place to a muscovite granite. Along the upper Talkeetna gneisses again appear, but along the lower course more quartz diorites are exposed. Along the western flank of the Talkeetna Mountains granitic rocks are exposed in great abundance almost continuously from Talkeetna River to Willow Creek and again as a narrow band forming the southern boundary of the crystalline mica-schist area. Near Sheep Creek some uralitic gabbro is found. In the vicinity of Kashwitna River is a light-colored quartzose type, which proves to be a quartz monzonite. The quartz diorites on the south side of Knik Arm are probably to be correlated with those making up the main mass of the Talkeetna Mountains. Similarly a small isolated boss of epidotized quartz diorite a few miles east of the mouth of Caribou Creek appears to belong to the same general mass. AGE. Wherever the quartz diorites come in contact with the Susitna formation a recrystallization of the sediments has taken place and finely granular hornfels has been produced. Along the upper Chickaloon the contact of the granite with the andesitic greenstones is marked by a fine-grained marginal facies. Granite porphyries of nevadite habit, containing greenstone inclusions, appear near the contact and probably represent the chilled border of the invading granite magma. The quartz-diorite boss east of Caribou Creek, which appears to represent a satellitic intrusion attendant on the batholith of the Talkeetna Mountains, invades the andesitic greenstones of Sheep Mountain, which have been shown by their fossils to be of lower Middle Jurassic age. Accepting the correlation of this quartz-diorite mass with those of the central portion of the province, we have strong presumptive evidence that the batholithic core of the Talkeetna Mountains is later than the Middle Jurassic. From the abundance of granitic material in the coarse conglomerates of Upper Jurassic age it is inferred that the intrusion preceded the deposition of the late Jurassic strata. The granitic complex of the Talkeetna Mountains is thus contemporaneous in a general way with that great series of batholithic intrusions of late Mesozoic age which affected the entire cordilleran region from the Straits of Magellan to the Seward Peninsula of northwestern Alaska." UPPER JURASSIC ROCKS. DISTRIBUTION AND CHARACTER. The Upper Jurassic rocks consist largely of blue shales, locally carrying fossiliferous limestone nodules. With the shales are associated various sandstones, conglomerates, and some tuffs and arkoses. Strata of this age have a wide distribution east of Chickaloon Creek, but were not observed in the Talkeetna Mountains. a Lawson, A. C., The cordilleran Mesozoic revolution: Jour. Geol., vol. 1, 1893, p. 579. Brooks, A. H., The geography and geology of Alaska: Prof. Paper U. S. Geol. Survey No. 45, 1906, p. 250. The sandstones of this series are usually ordinary yellow sandstones, which in places are conspicuous on account of the multitudes of Aucella contained in them, this fossil making up as much as one-half of the rock. In some places the sandstones have light-gray color. This is due to the fact that such beds are highly feldspathic and contain much finely comminuted black mica with an abundance locally of small bits of carbonaceous material. On Billy Creek a number of interstratified tuff beds from 1 to 4 feet thick occur near the top of the series, a few hundred feet beneath the base of the Cretaceous. These tuffs differ greatly in appearance from the older Jurassic tuffs. They consist largely of fragments of glassy, striated feldspar, hornblende, and quartz with fine pyramidal terminations, set in an almost black matrix. Belemnites and lamellibranchs are embedded in these tuffs. Higher in the series arkoses, closely simulating granites in appearance, are interstratified with the sandstones. Coarser phases contain pieces of a fresh granitic rock-a hornblende-biotite-quartz monzonite. The arkose is composed largely of rounded and angular fragments of feldspar in a more or less advanced state of alteration. Quartz is notably absent. Deepbrown hornblende is somewhat common, and a few flakes of strongly pleochroic biotite are found. With these are associated fragments of hornblende andesite, highly altered, some of which were noted to contain large idiomorphic feldspar similar to those forming the bulk of the rock. The arkose thus appears to have been derived principally from the destruction of a volcanic land mass, but some of its material was possibly brought from a monzonite area, as indicated by the fragments of monzonite. Under the microscope the feldspars of these arkoses by their alteration contrast strongly with the clear, glassy feldspars of the somewhat older crystal tuffs already described. A notable accumulation of conglomerate, stretching over several square miles, occurs in the headwater region of Susitna River, near the Nelchina River, where extensive. glaciation has produced deep troughs that expose splendid sections of it. Perpendicular cirque walls reveal a thickness of not less than 1,100 feet. Occasional thin layers of sandstone show that the conglomerate is lying in horizontal attitude. It is almost exclusively composed of large well-rounded bowlders of augite andesite and quartz monzonite embedded in a tuffaceous matrix. In the upper horizons the bowlders of andesite preponderate. The bowlders of the conglomerate are ellipsoidal in shape, and many of them are as much as 2 feet in diameter. The conglomerate is lithified firmly enough to form large bowlders in the present stream wash. Sheets of lava are occasionally intercalated in the conglomerate. The andesite bowlders are composed of rather fresh-looking rock, and thus differ in appearance from the older greenstone andesite. The augite and feldspar are comparatively unaltered, while the hyalopilitic groundmass shows a rather strong pigmentation with iron. hydrate. The quartz monzonite, which carries abundant hornblende and biotite, is also of fresh, unaltered habit. It is the same type of granitic rock that is found in the arkoses on Billy Creek. Along Nelchina River, immediately below the junction of the north and south forks, the stream bluffs reveal an interdigitation of lenses of sandstone, shale, and coarse conglomerate. The sandstones show cross-bedding and contain remains of Aucella. The conglomerates are composed of well-rounded material, bowlders a foot through being common. The prevalence of granitic rocks is noteworthy. The conglomerate masses serve to emphasize the lenticular habit of the deposit. A lens 25 feet thick and 100 feet long was found completely inclosed by sandstone. Solitary waterworn bowlders have been embedded in the center of shale lenses. These various characteristics indicate a deposit formed under near-shore conditions, probably in the vicinity of the debouchure of a swift stream. In harmony with this interpretation is that fact that the Aucella characteristic of these rocks, though a true marine species, is often found in beds interstratified with sandstones inclosing abundant carbonaceous fragments. FOSSILS AND AGE. The age of these strata has been determined from the following invertebrates, identified by T. W. Stanton: Lot 6 A. K. 156. Headwaters of Nelchina River of Copper River drainage. mens. The horizon is that of the Cadoceras zone, which forms the upper third of the Enochkin formation. The horizon is probably in the upper part of the Enochkin formation. Lot 6 A. K. 136. Billy Creek. Aucella cf. bronni Rouiller. The same species occurs in similar rock in the Naknek formation of Kamishak Bay. The fossils indicate faunas of both late Middle Jurassic age and Upper Jurassic age. It is interesting to note that the Upper Jurassic rocks of this region, which are faunally allied to those of the Naknek formation on the west coast of Cook Inlet, show a certain lithologic similarity in the presence of interstratified tuffs and arkose." Further @ Stanton, T. W., and Martin. G. C., Mesozoic section on Cook Inlet: Bull. Geol. Soc. America, vol. 16, 1905, p. 402. more, an accumulation very similar to the remarkable andesitegranite conglomerate occurring on the headwaters of Susitna River is found at the base of the Naknek formation." STRATIGRAPHIC RELATIONS. The late Jurassic rocks overlie the lower Middle Jurassic rocks unconformably. On Sheep Mountain, at the head of Matanuska River, sandstones and shales with a thin basal conglomerate were found resting on the eroded edges of the lower Middle Jurassic greenstones. The following measurements indicate the amount of this discordance: The strike of the greenstone tuffs is N. 12° W., and the dip 50° E.; the strike of the superjacent sandstones is N. 85° W., and the dip 20° N. The thickness of the late Jurassic rocks (this term embracing the late Middle Jurassic and Upper Jurassic) is not definitely known. It certainly exceeds 2,000 feet, including a known thickness of 1,100 feet of conglomerate. Two conglomerates have been described as occurring in the late Jurassic rocks. Of these, the andesite-quartz-monzonite conglomerate overlies a portion of the late Jurassic strata, possibly of upper Middle Jurassic age, and overlaps the greenstones of lower Middle Jurassic age. The other conglomerate occurs interstratified with shales and sandstones, carrying Aucella cf. bronni, so that it is definitely known to be of Upper Jurassic age. The granite bowlders of this conglomerate do not, however, show the striking freshness so characteristic of the granite of the other conglomerate; hence it seems to have been derived from a granitic landmass which had been exposed for a longer period to subaerial decay. For this reason the andesite-granite conglomerate is regarded as occupying a lower position in the stratigraphic column, probably near the base of the Upper Jurassic, like its analogue on the west coast of Cook Inlet. The unconformity between the late Jurassic and the lower Middle Jurassic rocks records an important event in the dynamic history of the area. Below this break in the stratigraphic sequence the rocks are shattered and broken and in many places are rudely schistose, and the andesites have assumed a greenstone habit and in several places have been extensively pyritized and veined by quartz and calcite seams. Above the unconformity the strata lie in comparatively undisturbed attitudes, and the intercalated andesites are fairly fresh and unaltered, differing in this respect but slightly from the Tertiary volcanics of the region. Martin, G. C., The petroleum fields of the Pacific coast of Alaska: Bull. U. S. Geol. Survey No. 250, 1905, p. 44. |