POST-EOCENE.

Overlying the older rocks unconformably is a series of nearly horizontal basalt flows, which, with their intercalated pyroclastics, attain a thickness of 1,000 feet. They have an extensive distribution throughout the Talkeetna Mountains and compose many of the peaks and summits of the region. They weather in tints of red and break down in characteristic erosional forms which lend a picturesque castellated appearance to the mountain crest lines.

The basalt flows with their interbedded tuffs and breccias present a stratified appearance. The surface upon which these volcanics were accumulated appears to have been one of gentle relief, cut across the upturned edges of the older rocks. The discordance with which the nearly horizontal basalts rest upon the underlying strata varies from 10° to 90°. About the headwaters of Chickaloon Creek this surface slopes gently northward. East of Chickaloon Creek its altitude is about 5,000 feet. It is somewhat broken by small faults, the maximum displacement noted being 50 feet. At many places the basalts rest upon a conglomerate occupying broad channels in the Lower Cretaceous limestone. The basal portion of the volcanic series here consists of 100 feet of tuffs and amygdaloid breccias, which interleave with the sandstone beds in the upper portion of the conglomerate. The conglomerate consists of rather coarse wash embedded in a gravelly matrix. The pebbles are well rounded, though poorly sorted and shingled, and cobbles up to a foot or more in diameter are common. The material is largely of volcanic origin, consisting principally of greenstone amygdaloids and eutaxitic rhyolites extrusives of Jurassic age. Some irregular lenticular beds of sandstone are included in the conglomerate. It is 200 feet thick, and in the vicinity of Billy Creek fills a channel 1,000 feet wide, cut in the Cretaceous limestone and partly in the underlying sandstones. The arclike line of contact is finely exposed in the deep canyons tributary to Billy Creek. This conglomerate, whose fluviatile origin is thus apparent, has a very limited distribution and has been preserved only where buried under the covering of basaltic lavas. This conglomerate is later than Lower Cretaceous, but further than that its age is not known. Lithologic similarity suggests its correlation with the great mass of conglomerate occurring in the upper horizons of the Kenai in Matanuska Valley.

Locally the conglomerate is capped by an angular wash consisting of fragments of sandstone and cellular lavas, upon which rests the stratiform series of basalts and pyroclastics. At one point a breccia of unusual constitution was found near the base of the volcanic series. This breccia consists of large angular blocks of highly amygdaloidal

lava, blocks of sandstone containing Aucella, and a few blocks of the Cretaceous limestone characteristic of the region. A basalt block 8 by 10 feet was the largest noted. The matrix in which this angular material is embedded is composed of small, well-rounded gravel. These features are interpreted as indicating a volcanic mud flow occupying an ancient stream course.

The overlying basalts display a wide variation in habit and texture. Highly glassy types, amygdaloids, porphyries, and dolerites are variously represented. They belong to the acidic end of the basalt family and are in general non-olivinitic. The coarse-grained basalts consist of doleritic aggregates of labradorite (Ab,Ang) and augite. Plagioclase is the dominant constituent, and is in only a few specimens poikilitically inclosed in the augite. The basalt glass from the head of Chickaloon Creek, in spite of its holohyaline aspect, is found to contain sporadic phenocrysts of plagioclase (Ab.An,) and augite, and some small, irregularly shaped doleritic aggregates of labradorite and augite. The glassy base is of a deep, clear, brown color, and contains numerous augite and very minute feldspar microlites, the latter usually having forked terminations.

The basalts overlie a conglomerate of post-Lower Cretaceous origin and of probably late Kenai age. Their petrography suggests that they are the effusive equivalents of the diabase dikes and sills that are so common in the Kenai of the Matanuska basin. These volcanics of the Talkeetna Mountains can be correlated with the great series of Tertiary volcanics of the upper Nizina basin, described by Schrader and Spencer, and with an important volcanic series on Nabesna River, described by Brooks. Both these series are described as comprising andesites, rhyolites, and pyroclastics, and were deposited unconformably upon a general uniform surface. The volcanics of the Talkeetna region, however, show a greater petrographic uniformity and consist exclusively of basalts and their fragmental accumulations. In this respect they show a closer resemblance to the late Tertiary lavas of the Alaska Peninsula and the Aleutian Islands, which have been described by Spurra as andesitic basalts.

QUATERNARY DEPOSITS.

Extensive deposits of glacial and fluvioglacial gravels are prevalent. They extend entirely around the head of Cook Inlet, forming bluffs 50 to 100 feet high. Below Knik the gravel consists largely of subangular to rounded pebbles of hornblende-biotite granite and graywacke, with occasional muscovite-biotite granite, pyroxenite, and andesitic porphyries. Enormous angular blocks are frequently in

• Spurr, J. E., A reconnaissance in southwestern Alaska in 1898: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 7, 1900, p. 234.

cluded in this stream-worn material. Many of these have smooth, polished surfaces several feet square, much scratched and grooved. The gravel sheet at Tyonek, on the west side of Cook Inlet, with its extension up Susitna Valley as far as Chulitna River, has been recognized as divisible into an older and a younger formation. The older apparently consists of the ground moraine of an advancing ice stream, and the younger includes fluvioglacial gravels deposited upon the earlier sheet during the recession of the ice. These distinctions can not be drawn in the Matanuska region. The gravel sheet in Matanuska Valley is of considerable areal extent and mantles the old valley floor, especially in the region west of Chickaloon Creek. The gravels near the mouth of Kings Creek have a maximum thickness of about 300 feet. They are greatly waterworn, and the icerafted bowlders so conspicuous in the vicinity of Knik Arm were not observed. However, the fact that the bluff near this exposure in Matanuska Valley is composed of shales shows that gravels were deposited on a surface of irregular relief, or possibly in channels. This is also shown by the fact that the walls of the Matanuska gorge are in places composed of bed rock, and in other places of gravel.

In its upper course Matanuska River has cut a V-shaped gorge in the deposits underlying the southwest corner of the Copper River Plateau. The gravels here consist of angular blocks, scratched and striated, inclosed in a blue, clayey-sandy matrix. The river has cut down 300 to 400 feet in this glacial till, but does not yet reveal bed rock. Farther north, in the Copper River Plateau, stream bluffs on a tributary of the Nelchina show coarse gravels of subangular to well-rounded character. With this stream-worn material are associated numerous glacially scored bowlders and occasional angular blocks up to 10 feet in size. The gravel shows considerable variety in its petrographic charcter, including numerous fragments of granite, gabbro, gneiss, schist, slate, graywacke, and vein quartz. None of these rocks are found in the drainage area of this stream, so that the fragments appear to have come from the Alaska Range.

GEOLOGIC HISTORY.

PRE-QUATERNARY.

The oldest records of the geologic history of the region are preserved by the albite and garnetiferous mica schists. They appear to represent a series of calcareous argillites and plagioclase arkoses, recrystallized under conditions of deep burial. Subsequent to their metamorphism they have been uplifted, intruded by pyroxenite dikes, and subjected to a second period of dynamic activity. The dikes

a Spurr, J. E., op. cit., p. 102.

were thus sheared and the component minerals of the schists were crushed. The highly crystalline character of the schists and their isolated occurrence as a single narrow belt indicate the completeness of the geologic revolution which the region has undergone since their formation. On the basis of petrographic similarity these schists are correlated with those of the Yukon-Tanana region and assigned to a pre-Silurian age.

The time interval between the deposition of these oldest rocks and that of the Susitna slate is not known, but appears to have been of considerable duration, as is indicated by the far less advanced metamorphism of the latter. The Susitna comprises fissile slates, foliated graywackes, and microcrystalline hornblende and biotite schists. The possibility, however, that they may be contemporaneous in origin with the rocks of the Sunrise "series" is not excluded.

In late Paleozoic or early Mesozoic time sedimentary conditions prevailed over the region now occupied by the Chugach Mountains, and an extensive series of shales, arkose, and ferromagnesian sandstones derived from the destruction of a quartz-diorite land mass were laid down. Volcanic activity interrupted the normal sedimentary sequence and caused the accumulation of greenstone tuffs, associated with some massive basaltic greenstones. Rhyolitic lavas were also poured out, and the accompanying fragmental material was mingled with the sediments derived from the erosion of the granitic land area. Subsequently these deposits were closely folded and metamorphosed to graywackes, slates, and, locally, to phyllitic schists.

At the beginning of Middle Jurassic time marine conditions were inaugurated. Eruptive activity was then widespread throughout the region of the Talkeetna Mountains, and andesitic greenstone breccias. with intercalated amygdaloids were accumulated to a thickness of 1,000 feet. The relations of land and sea appear to have been unstable during this period, and a portion of the volcanics were doubtless extruded upon uplifted land areas. The exceedingly coarse volcanic conglomerates show the effectiveness of erosional agents, and the composition of the succeeding sandstones indicates their derivation from an andesitic land area.

Toward the close of the Middle Jurassic, quartz diorites forming the core of the Talkeetna Mountains were intruded and the Talkneeta area was uplifted. A vigorous denudation ensued, the sedimentary roof arching over the plutonic rocks was stripped off, and the granolites were uncovered.

By the beginning of the Upper Jurassic the granitic land mass thus laid bare furnished large amounts of material to the conglomerates then accumulating. Some andesitic eruptions occurred at this time. The relief of the land mass was soon subdued, and the succeeding sediments became normal sandstones and shales, interstratified with

occasional arkoses and crystal tuffs. Marine conditions persisted uninterruptedly until the close of Lower Cretaceous time. The final stages were marked by the deposition of a heavy limestone, now largely removed by erosion.

The revolution at the close of the Lower Cretaceous, as in adjacent regions, was of a gentle character. The strata involved were uplifted with but slight deformation and without folding. After this upheaval there came a long period of erosion and planation. In the Matanuska region, however, diastrophic movements produced a trough of fresh-water sedimentation in late Eocene time, and a thick series of arkoses, shales, and sandstones, with numerous coal beds were laid down. The arkoses, occurring probably at the base of the formation, indicate strong relief in the adjacent land mass at the beginning of Kenai time. During the progress of sedimentation erosion of the Talkeetna area continued and furnished the material for the Kenai deposits. At the close of the Eocene the strata were uplifted and sharply folded. Peneplanation, however, was far advanced in the Talkeetna area, especially on the flanks of the region. Broad rivers, whose beds were filled with well-rounded gravels, flowed over its surface.

This period of quiescence was finally closed, shortly after the folding of the Kenai, by a great outburst of volcanic activity. Drainage conditions were destroyed, lavas flooded the stream channels, and the fluviatile accumulations were buried to a depth of 1.000 feet under a succession of basalt flows and tuffs. While this extravasation of volcanic material occurred throughout the Talkeetna area an extensive series of diabase dikes and sills were injected into the Kenai strata of the Matanuska basin.

With the cessation of volcanic activity a new cycle of erosion was begun, initiating the modern history of the region.

GLACIATION.

PRESENT GLACIATION.

A considerable number of glaciers still remain in the higher portions of the Talkeetna Mountains (see Pl. I, pocket), but all appear to be rapidly receding. Chickaloon Creek and many of its tributaries are fed by glaciers at their sources. Of these Chickaloon Glacier, at the head of the stream of that name, is the largest, having a length of 9 miles and a width of 2,000 feet at its terminal end. In general, however, the ice streams do not descend far from the irregular névé fields occupying the higher recesses of the range. Several such masses lie on the Talkeetna side of the Chickaloon-Talkeetna divide and discharge their waters into that stream.

13070-Bull. 327-07-3