Aaron Clement Waters

Protoclastic border of the Colville batholith

In the Okanogan Highlands of Washington a complex plutonic mass called the Colville batholith intrudes folded and dynamometamorphosed sedimentary and volcanic rocks of late Paleozoic and Triassic age. Along the sharply discordant contact the wall rocks are much fractured and granulated, but contact metamorphism is slight or absent. The batholith is remarkably heterogeneous both structurally and petrographically. A central mass of structureless granodiorite grades outward into a belt of foliated igneous rock which commonly shows intricate swirling of the foliation. These swirled rocks grade into a peripheral belt of variable but well-foliated migmatitic gneisses characterized by severe granulation of the constituent minerals. Over broad zones this rock is a mylonite; locally recrystallization has produced types resembling metamorphic granulites. That the crushing was protoclastic and not due to dynamic metamorphism following batholithic solidification, is proved by the relations with the wall rocks and by the widespread cementation of the broken materials by films and stringers of undeformed quartz and microcline. Along the contact between the approximately contemporaneous Osoyoos and Colville batholiths occurs a narrow belt of heterogeneous syenite with highly complicated internal structure. This is believed to be a hybrid rock formed by the action of magmas and emanations from both batholiths upon a thin wall rock septum. Deep erosion revealed the Colville rocks over wide areas. Flows of spilite and keratophyre were poured out, and after another erosion interval were covered by Tertiary sediments and volcanics. These were then folded and deeply eroded. Ice sheets covered the area during the Pleistocene.

GEOMORPHOLOGY OF SOUTH-CENTRAL WASHINGTON, ILLUSTRATED BY THE YAKIMA EAST QUADRANGLE

Geologic mapping of the Yakima district has been neglected since publication of the Ellensburg and Mount Stuart folios (G. O. Smith, 1903a; 1904; see also Calkins, 1905). Reports on other parts of the Columbia River plateau, however, pose new problems and call for revision of some interpretations. This paper presents the results of extending Smith9s mapping eastward into the adjacent Yakima East quadrangle, and of considerable reconnaissance work in adjoining areas. The new mapping shows that to the east and south the lower part of the Ellensburg formation interfingers with the Yakima basalt. It also indicates that local diastrophism and volcanism continuously modified and interrupted deposition of the Ellensburg formation. Smith9s conclusions that the great topographic ridges are growing anticlines, not fault blocks, and that the major rivers are antecedent to these folds, are confirmed. The anticlines appear to have grown in a single epoch of deformation instead of in two orogenic episodes separated by a period of peneplanation. The “Cascade lowland” is a local pediment formed on the flanks of a growing anticline, not a remnant of a widespread peneplain. The new data do not support Warren9s hypothesis of defeat and diversion of Columbia River by rise of the Horse Heaven uplift, nor Flint9s hypothesis of the cutting of Wallula Gap by a river thus diverted.

Terraces and Coulees Along the Columbia River Near Lake Chelan, Washington

Introduction Near the close of the last century the region around the mouth of Lake Chelan was visited by several of the early reconnaissance parties.[2][1] Somewhat diverse interpretations of the glacial features, particularly of the broad terrace that hangs 600 feet above the Columbia, were proposed and the region became the subject of considerable controversy.[3][2] It was the writer’s good fortune to spend most of the summer of 1931 in a portion of the Columbia Valley where the physiographic features associated with the advance and recession of the last ice sheet are well displayed. Although the investigation was primarily of petrologic character, it is believed that sufficient data regarding the glaciation of the district were gathered to be worthy of record, and that the observations here presented will be of assistance in evaluating some of the controversial interpretations. Eliot Blackwelder has kindly read and offered suggestions regarding the manuscript, and . . . [1]: #fn-1 [2]: #fn-2

A Petrologic and Structural Study of the Swakane Gneiss, Entiat Mountains, Washington

The Entiat Mountains, a spur of the Cascade Range in central Washington, are composed dominantly of a metamorphic complex of pre-Ordovician age. Although originally made up of a diversified series of rocks partly of sedimentary and partly of igneous origin, metamorphism has been so complete that the original heterogeneity of the group, save that inherent in the chemical composition, has been almost completely obliterated, and the entire metamorphic mass now participates in a common foliation and forms a single definite structural unit. Subsequent to the development of the foliation, mechanical movements superimposed cataclastic structures upon the metamorphic rocks and locally developed flat thrusts of some magnitude within them. Along the thrust surfaces mylonites are developed. Three distinct periods of batholithic intrusion of pre-Tertiary age are recorded within the area, and these, together with the Miocene batholiths to the west, establish the presence of four periods of plutonic invasion in the Northern Cascades. A striking fact brought out by the study is that the late Tertiary folding which trends northwest-southeast and east-west transverse to the main axis of the Cascade Range was preceded by several episodes of pre-Tertiary deformation that occurred along the same axes.

The Latest Eruptions from Mount Rainier Volcano

Brief notices in books and old newspapers report eruptions from Mount Rainier on at least fourteen dates between 1820 and 1894. Descriptions of the supposed eruptive activity are vague, however, and in no case is there a clear record of the emission of new lava or ash. Recent mapping of the geology of Mount Rainier National Park brought forth no geologic evidence to substantiate such recent activity. The youngest blanket of pumice and ash from Mount Rainier consists of two or more distinct ash falls. Both tree-ring and radiocarbon methods of dating indicate that it was deposited about 550-600 years ago. Mount Saint Helens, on the other hand, was in eruption more recently. Historic accounts of eruptions from this volcano in 1831, 1842, and 1843 appear to be reliably documented, and there is also geologic evidence of very recent activity.

The Nature and Origin of the Horst and Graben Structure of Southern Oregon

In southern Oregon east of the Cascade Mountains a varied series of Tertiary lavas is cut by the northern continuation of the Basin Range faults, giving rise principally to relatively undissected horst and graben structure rather than to tilted fault-block mountains. The tensional origin of these faults is indicated by definite normal faulting, curving, and zigzag faults, step faults, circular fault basins, volcanic activity parallel to the faults, absence of folds, thrust faults, or other pressure effects, and by the even distribution of faults rather than their local concentration in zones of initial failure.

Resurrected Erosion Surface in Central Washington

Results obtained in mapping the geology of the Wenatchee-Chelan and of the Okanogan district suggest modifications of the earlier interpretations regarding the erosional history: 1) The existence of a “Methow Plain” or “Cascade Peneplain” is questioned. Most of the areas formerly considered to be remnants of this peneplain are surfaces controlled by the structure of the Columbia River basalt flows without regard to a widespread base level of erosion. One remnant, which bevels the Columbia River basalt and the Ellensburg formation on the flanks of an anticline in the Yakima district, is believed to be a pediment of very local extent. 2) The mature surface of the “Entiat stage” is well developed only on resistant granodiorite and gneiss and is exceedingly fragmentary or absent in areas underlain by greenstone, phyllite, schist, and Tertiary sedimentary rocks. It has not been recognized in areas underlain by the Columbia River basalt. The writer interprets nearly all the residuals of the “Entiat surface” described in the literature as resurrected fragments of a widespread late mature erosion surface buried beneath the Miocene Columbia River lavas and associated sediments and then partially exhumed in post-Miocene time. Certain fragmentary remnants of this surface on massive crystalline rocks beyond the limit reached by the basalt flows are believed to have been similarly preserved by burial under andesitic pyroclastics erupted from volcanoes in the Cascade Range.

A Structural and Petrographic Study of the Glass Buttes, Lake County, Oregon

The Glass Buttes, a small mountain range composed entirely of volcanic rocks, have as their dominant structural feature an anticline. This anticline has been very greatly modified by a multitude of normal faults. The lavas of the district represent three periods of extrusion. The older flows of basalt were followed by a series of acidic lavas which were, in turn, partially covered by a later series of basalts.

Petrology of the contact breccias of the Chelan batholith

At Chelan Butte in central Washington the marginal granodiorite facies of a large batholith is irruptive into hornblende schist. Along the margin a broad zone of contact breccias and striped migmatites has been formed which occupies an area of many square miles. Field study, petrographic examination, and chemical analyses, all converge to show that the inclusions of schist caught up by the batholith were first thoroughly recrystallized, feldspathized, and silicified until they became fine-grained granoblastic rocks which megascopically resemble diorites, and then were differentially assimilated as a result of the liquefaction and running together of the felsic constituents allowing the remaining still-solid material which consisted chiefly of hornblende to pack together into clots of amphibolite. During this process the hornblende crystals of the zenoliths grew in size by precipitation of material upon them from the adjacent magma. Such amphibolite clots are indistinguishable from the “basic schlieren” which occur throughout the batholith. Although definitive proof is lacking, it is suggested that the basic schlieren of this and other areas may represent similarly reconstituted foreign inclusions. A modification of Beger’s hypothesis of the origin of lamprophyres is suggested in view of this conception of the origin of schlieren.

Basaltic rocks in the Umpqua formation

INTRODUCTION In 1930, during a study of the geology of an area in west-central Oregon within which occur the quicksilver deposits of Blackbutte, Elk-head, Bonanza, and Nonpareil, a series of extrusive and intrusive basaltic igneous rocks were encountered. Part of this area lies within the Roseburg quadrangle, mapped by Diller in 1898. Diller called all these rocks “diabase.” 1 It is the purpose of this paper to show that the basaltic rocks include amygdaloidal and ellipsoidal basalt flows, olivine basalt dikes, norite sills and necks, and hypersthene-augite basalt dikes and that they represent more than one period of igneous activity. SUMMARY OF GEOLOGY A full discussion of the geology of this area is found in a recent bulletin of the United States Geological Survey,2 it can be briefly summarized as follows: The southwestern part of the area (Fig. 1) is underlain by the Umpqua formation, a series of interbedded sandstones . . .