GENERAL RECONNAISSANCE WORK
The earliest report on the geology of the region is by James Blake.1 In the early seventies he examined Pueblo Mountain, which lies directly to the south of Steens and is, in part, defined on both sides by continuations of the same faults. Here are exposed both the southern continuation of the Steens Mountain volcanic series and the underlying crystalline rocks on which it rests. Blake gave a brief description of both the metamorphic and the volcanic rocks and suggested that the latter might form the sequence for a large part of southern Oregon, perhaps even extending northward beyond the Columbia River.
In 1881 and 1882, I. C. Russell made a general survey of the geology of the northern end of the Great Basin. His subsequent report2 contained a remarkably good reconnaissance map showing the major faults. He considered the structure to be due to normal faulting. About twenty years later he made a superficial examination3 of Steens Mountain. He noted the basal tuffs, but considered the lavas to consist entirely of basalt.
In 1903, a week's study of the southern part of Steens Mountain enabled W. M. Davis to make some general observations4 on the structure and physiography of the mountain. In this report he suggested that the southern and middle parts of Steens Mountain are defined by faults on the western side as well as on the east, and therefore, that that portion of the mountain is a true horst. Time did not permit Davis to visit the western scarp, but observations by the writer have subsequently confirmed his impression.
About five years later, Gerald A. Waring spent two years in preparing a very able reconnaissance report and map on the geology and water resources of most of the volcanic region of southern central Oregon. This was published in two papers.5 Waring did not attempt to do detailed work on the stratigraphic succession, but merely subdivided the lavas into the Earlier Effusives and the Main Lava Flows. R. J. Russell has since called attention to the fact that the acidic lava forming one of the "Earlier Effusives is apparently a later intrusive.6
In 1910, Merriam published an article on the "Tertiary Mammal Beds at Virgin Valley and Thousand Creek in Northwestern Nevada."7 Owing to their proximity to the southern extension of the Steens Mountain volcanic series exposed on Pueblo Mountain, both the age and the relationship of these beds are of considerable importance.
South of Pueblo Mountain, and immediately west of the tilted spur formed by the southern extension of the volcanic series as it plunges southward, lies the Thousand Creek Basin. This basin, which is but superficially eroded, contains stratified tuffs of unknown thickness. These deposits directly overlie the Pueblo Mountain series which the writer correlates with the upper flows exposed on Steens Mountain. From their vertebrate fauna, these beds were considered by Merriam to be of Lower Pliocene age, but subsequent studies by Dr. Chester Stock8 indicate them to be "definitely Pliocene and fairly late in that period." These light colored tuffs are locally capped by a thin flow known as the Railroad Ridge Basalt. This lava was thought to have originally filled a river bed.
Immediately adjacent to the back of the Pueblo Mountain block no satisfactory exposures are apparent. Merriam observed that the sediments as they near Pueblo Mountain develop a slight dip to the southwest,9 but, although the contact was not exposed, he considered the tuffs to bear a strongly non-conformable relationship10 to the underlying lavas. This interpretation would place the period of faulting and its accompanying deformation relatively early in the Pliocene. This age would be contradictory to the prevailing evidence found elsewhere in the northern portion of the Great Basin, for Louderback's work11 clearly indicated a late Pliocene or a post-Pliocene time for the beginning of faulting. Viewed at a distance from the south, the tuffs appear to the writer to curve upwards to the east, conformably overlying the flows. These indications, however, are based only on indefinite criteria dependent upon the physiographic expression, soil color and vegetation.
The Thousand Creek Basin is bounded on the south by a steep fault scarp exposing a thickness of rhyolite of at least 400 feet. To the south a series of tuffaceous sediments about 1,500 feet in thickness lies on the irregular surface of this acidic lava, which is referred to as the Canyon Rhyolite. These sediments are overlain by a thin cap of basalt called the Mesa Basalt in reference to its physiographic form. This basalt was thought by Merriam possibly to correlate with the similar Railroad Ridge Basalt in the down-faulted basin to the north. Immediately beneath the Mesa Basalt the exposures are either very steep or covered with talus and unsuitable for the collecting of fossils.
The fauna of the lower beds was considered by Merriam to belong to the Upper Miocene, while the upper part was considered possibly to correlate with the very similar beds in the Thousand Creek region. Subsequent work by Stock,12 however, indicates the beds in Virgin Valley to be of Middle Miocene age and, therefore, unrelated to those exposed in the basin immediately to the north. Traced northward the great volcanic series of Pueblo Mountain, which is but 10 miles away, also overlies sediments of supposed Middle Miocene age. To prevent the Virgin Valley Beds from being flooded with these relatively fluid northern lavas it is necessary to postulate a barrier, which may have been formed by the great mass of Canyon Rhyolite.
Recently, Richard J. Russell published an excellent report13 on the geology of the Warner Range of California, which forms the western scarp of the southern continuation of the Warner Valley lying about 70 miles west of the southern spur of Pueblo Mountain. Russell was primarily interested in the structure and physiography of the region, but for the purpose of mapping, he divided the volcanic series into stratigraphic units. Although most of this series is probably of a relatively local distribution, a broader correlation was suggested for two of the upper members.
Near the top of the volcanic series in the Warner Range, basaltic flows form a sheet 30 to 600 feet in thickness. This lava, which Russell named the Warner Basalt, was indicated by its stratigraphic relationship to be Lower Pliocene. Russell considered that this basaltic series thickened to the north to form the great exposures in the northern part of Warner Valley and at Abert Rim. He also correlated it not only with the Mesa Basalt and the Railroad Ridge Basalt, but considers "that it is very likely that this same sheet is, at least, in part, his (Merriam's) Pueblo Mountain series.14
In northeastern California, a variable thickness of rhyolitic lava caps the Warner Basalt and forms the uppermost member in the local stratigraphic sequence. In a similar manner the extrusion of the acidic lava in this broad volcanic field was thought by Russell to have been confined chiefly to a prolonged rhyolitic period subsequent to the basalt.15 In order that the sequence in Virgin Valley and Thousand Creek basin might coincide with the type locality he considered that Merriam failed to appreciate the full significance of faulting and, in consequence, misinterpreted the geologic history. Although his evidence is not very coherent, Russell apparently considered that the great exposures of rhyolite adjacent to Virgin Valley and Thousand Creek are due to post-Mesa Basalt extrusion, followed by the down faulting of these capping volcanics.
The present writer, however, is in full accord with Merriam in considering the Canyon Rhyolite to be below the sediments. The actual contact was locally excavated by the writer, and fragments of the glassy rhyolite were found to be included in the stratified tuffs. In addition, the acidic flow capping the Pueblo Mountain series is definitely older than the Thousand Creek Beds. The basaltic series which it caps has no apparent analogy to either the Mesa or the Railroad Ridge Basalt.
Although the base of the Canyon Rhyolite is not exposed, it shows a thickness of at least 300 feet within a horizontal distance of a few hundred yards from the flat persistent cap of Mesa Basalt. If the rhyolite was subsequent to the Mesa Basalt, a considerable thickness must have extended over this level cap. It seems impossible to the writer to have erosion completely strip this resistant acidic lava and still leave the undissected surface that characterizes the thin basaltic cap. Russell, however, gives evidence of local acidic volcanic activity subsequent to the Mesa Basalt. Scattered pebbles of obsidian resting on the surface of this lava offer additional testimony. These may have been deposited in tuffs, which have since been eroded.
Russell's correlation of the Warner Basalt with the Steens Mountain series will be considered in later pages.
Last Updated: 28-Mar-2006