GEOLOGIC FORMATIONS IN THE LASSEN PEAK DISTRICT.
The portions of the country immediately observed by the writer while preparing the geologic map are indicated in Pl. XLVII by the routes of travel. About a thousand specimens of rocks were collected and most of them were examined microscopically. Ten formations, including stratified deposits of at least five geologic horizons and as many kinds of eruptive rocks, have been recognized in the region and delineated upon the geologic map (Pl. XLVII). Within the auriferous slate series of the district the only geologic horizon which has been determined by fossils is the Carboniferous limestone. Only the lower member (Chico beds) of the Chico-Tejon series was recognized within the district. The Tertiary consists chiefly of Miocene, but very probably embraces also the Pliocene. The eruptive rocks are rhyolite, quartz-andesite (dacite), hornblende andesite, hypersthene andesite, basalt, and quartz basalt; they will be described in another report. The sedimentary rocks will be considered in the order of age, beginning with the oldest, the auriferous slates.
AURIFEROUS SLATE SERIES.
The auriferous slate series is a very heterogeneous group of comparatively ancient sedimentary and eruptive rocks, most of which have been subjected to extensive modification, both in structure and position, since they were formed.
Distribution.They occupy two areas at opposite extremities of the Lassen Peak district, one in the vicinity of the North Fork of Feather River and the other near Pit River. The former is at the northern terminus of the Sierra Nevada Range, where this series of gold-bearing rocks is extensively developed, and the latter is at the end of the eastern extension of the Coast Range. These rocks do not cover as large an area in the Coast Range as in the Sierras; nevertheless they form a large portion of northwestern California and the adjacent portion of Oregon. All of the other rocks of the district are younger than the auriferous series, and there is every reason to believe that if all of these newer rocks were swept away from the region between the North Fork of Feather River and Pit River, it would be found that they occupy a large depression in the auriferous series. This depression, transverse to the general trend of the Sierras, is a very important feature in the geographic and geologic development of that country, and I shall have frequent occasion to refer to it as forming the boundary between the Sierras and the Coast Range. From the latter portion of the Cretaceous into the Pliocene, inclusive, the depression was occupied by water, to which, for convenience, the name Lassen Strait will be applied.
Carboniferous limestone.Among the auriferous series those formed of fine detrital matter appear to prevail, and their slaty character early suggested the name "auriferous slates" for the whole series. Interstratified with these are numerous lenticular masses of limestone containing fossils of Carboniferous age. Occasionally it is so metamorphosed as to obliterate all traces of organic remains; nevertheless their geographic distribution and relation to masses containing fossils are such as to indicate that all are essentially of the same age.1
The metamorphism of the series is general, but at different points it varies greatly in intensity. In some of the finer deposits the original sedimentary character is very well preserved, but in others it is entirely obliterated by the subsequent development of a completely crystalline structure. Although finer sediments appear to predominate, those of coarser texture are by no means wholly wanting, and some of them, as we shall see presently, contain interesting fragments of older terranes.
Serpentine.Mingled with the metamorphosed sedimentary rocks are numerous irregular masses of eruptive origin. They are chiefly of a basic character. Coarse granular plutonic rocks, such as diorites, gabbros, and peridotites, appear to be most abundant; but regular volcanic effusions are found also, together with strata composed of lapilli and volcanic sand usually in a highly altered condition.
Perhaps the most interesting rocks of the auriferous series within the Lassen Peak district are the serpentines. They appear to have a very different origin from those of the Coast Range described by Mr. G. F. Becker.1a The serpentine which forms the prominent "Red Hill" in the junction of Indian Creek (eastern branch) and the North Fork of Feather River may be taken as a typical example of these rocks, and its genesis is exactly analogous to that of many other masses of serpentine in the northern portion of the Sierras and Coast Range. Serpentine forms a large portion of Red Hill and frequently exhibits an interesting fissile structure, splitting up into more or less distinctly lenticular, platy fragments with slickensides, as if the serpentine had been subjected to great stress, producing motion within the mass. Intimately intermingled with the well developed serpentine are both large and small irregular compact masses of olivine, which are completely permeated by a fine network of serpentine resulting from its alteration. The reticulated structure of the serpentine and its intimate association with olivine throughout the whole mass demonstrate beyond question that all the serpentine is derived from the alteration of olivine, and it is possible that the increase of volume attending the change may have given rise to the stress necessary to produce the fragments with slickensides. The rock was originally a dunite, for it was composed almost wholly of olivine with small amounts of ilmenite and a few other accessory minerals. The dunite altered chiefly to serpentine, but frequently also to needle-shaped crystals and fibrous bunches of tremolite.1b In the latter case the weathered surface of the rock is usually colored yellowish-red by oxide of iron, and this suggested the name of the hill. Between Quincy and Spanish Ranch, as well as near Round Valley reservoir,2 west of Greenville, in Plumas County, and at Mt. Eddy, west of Berryvale, in Siskiyou County, are great masses of serpentine containing large remnants of olivine in such a way as to clearly indicate that the original rock was a peridotite.3
Age of the auriferous slate series.It is well known, from the investigations of Whitney, Becker, White, and others, that the auriferous series contains both Mesozoic and Paleozoic strata, and that a large part of it belongs to the latter system. Marcou has long contended that a large portion of the strata of the Sierra Nevada Range is older than the Mesozoic, and the evidence favorable to such a view appears to be cumulative. The writer was first led to regard a very considerable portion of the series as lying below the Carboniferous limestone by an examination of the section at the head of Soda Creek, south of Mt. Shasta. There it is evident that the large mass of black shale which has a wide distribution in that vicinity lies below the Carboniferous limestone in which fossils have been found. This view was strengthened by observations at numerous other points, among which may be mentioned Hough's Mountain, between Indian Valley and American Valley, where the apparently older portion of the series is exposed by a great fault.4 Upon the western slope of the mountain, however, the varied position of time strata renders this section less satisfactory.
A better exposure occurs in Shasta County, along Little Cow Creek, just below the point where it is crossed by the Oak Run road. The Carboniferous limestone, with traces of fossils, is distinctly underlaid by a conglomerate and black shale, both of which are evidently older than the limestone; for, as the conglomerate overlies the shale and contains its fragments, it is evident that there has been no overturning or the conglomerate would contain fragments of the limestone instead of the shale. The congolomerate contains much pyrite, amid is considerably altered. Some of its pebbles are so metamorphosed as to indicate that they were derived from an older metamorphic rock, and demonstrate that below the Carboniferous limestone there is a considerable series of older stratified rocks whose age is unknown. As it becomes more and more evident that a large part of the auriferous slates is Mesozoic, so also the evidence is accumulating in favor of the view that another large portion is older than the limestone and possibly pre-Carboniferous.
Last Updated: 28-Nov-2006