Geological Survey 8th Annual Report (Part I) Geology of the Lassen Peak District

The primary relief features of the district are the Great Basin platform upon the east, the Piedmont region upon the west, and the Lassen Peak volcanic ridge in the middle. To the southeastward this ridge appears to merge into the Sierras, and in the opposite direction to unite with the Coast Range, but in reality it is simply an accumulation of lavas occupying a depression between these ranges and belongs geologically to the Cascade Range.

The stratified deposits, excepting the most recent lake beds, lying for the most part beneath the lavas belong to three groups, the auriferous slates, the Cretaceous member of the Chico-Tejon series, and the Tertiary, including both Miocene and Pliocene.

The auriferous slate series, which embraces both massive and water-laid rocks, occupies two areas at opposite extremes of the 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 the gold-bearing rocks are extensively developed, and the latter is at the end of the eastern extension of the Coast Range. Between these two exposures there is a great depression in the auriferous series transverse to the general trend of the Sierras. This depression marks the boundary between the northern end of the Sierras and the Coast Range, and from the latter portion of the Cretaceous to the Pliocene, inclusive, was occupied by Lassen Strait, which filled it with sediments.

Among the auriferous slates there are large masses of serpentine which have resulted from the alteration of peridotites, in some cases so rich in olivine that they may be classed with the dunites.

Concerning the age of the auriferous series it may be said that, 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 of them is older than the limestone and possibly pre-Carboniferous.

The Chico beds laid down as marine sediments are the most ancient, unaltered, stratified rocks known in the district and repose discordantly upon the plicated auriferous slates. They occupy the great central valley of California and, extending through Lassen Strait, connect with a large mass covering a wide expanse in eastern Oregon. During the Chico epoch the Lassen Peak district lay almost wholly within Lassen Strait, which separated the large Cretaceous island of northwestern California from the continental land to which the Sierra country belonged.

Some time between the close of the Chico epoch and the beginning of the Miocene there was an upheaval west of the Sacramento Valley, and also apparently in the vicinity of the Cascade Range. The oceanic waters were thus excluded from the Sacramento Valley, Lassen Strait, and a large area in eastern Oregon, for we find that the whole region of Upper Cretaceous marine sediments was covered during the Miocene by a great body of fresh water, to which the name Piute Lake has been given. Whether the transition from marine to fresh water conditions was direct or, as at other places, through a series of intermediate brackish water stages, is as yet only conjectural. Positive evidence concerning the history of the district during the latter portion (Eocene) of the Chico-Tejon period has not been clearly made out, but it may be surmised that if the moderate upheaval which gave birth to Piute Lake occurred at the close of the Chico epoch the equivalent of the marine Tejon deposits may be looked for in the lower portion of the Piute Lake beds.

Before the Miocene there was a change in the relative elevation of the Sacramento Valley and the Sierra region, because the shore deposits of Piute Lake are found farther up on the slopes of the Sierras than those of the Chico group. This change was doubtless a part of the result brought about by the movement which raised the Sacramento Valley above the sea level and prepared the slightly unconformable surface for the Piute lake deposits.

The relations of the Miocene lacustrine deposits appear to fully justify the statement that at the time they were laid down part of the Great Basin country was higher than the region now occupied by the Sierras, at least in the latitude of Diamond Peak. During the Miocene that country was a broad platform with the gentle relief that indicates a near approach to its base level of erosion. The evidence drawn by paleobotanists from the plant fossils found in the Miocene strata strongly corroborates this view. They have compared the climatic conditions of that country during the Miocene to those of a country like Florida.

The Miocene conditions appear to have continued into the Pliocene, at least as far as Piute Lake is concerned, and to have finally terminated in a great revolution which gave to the country its present aspect.

The extravasation of massive rocks occurred at various intervals in the geologic history of northern California, but the final great volcanic outburst reached its maximum near the close of the Pliocene and continued with gradually declining vigor almost to the present day.

The upbuilding of the Lassen Peak volcanic ridge was effected by these outbursts, and at the same time birth was given to the material from which the Piedmont region was developed at the close of the Tertiary. This material was deposited chiefly under the influence of lacustrine waters, and upheaved bodily in such a way as to produce a monoclinal fold and bluff which marks the western limit of the Piedmont region.

Since the deposition of the Quaternary gravels of the Sacramento Valley there has been a differential uplifting of the Lassen Peak district, by which the streams crossing the Piedmont have been enabled to cut small cañons within the larger ones of pre-Quaternary corrasion.

Gentle slopes prevailed during the deposition of the Pliocene gravels. The uplifting of the Sierras took place for the most part at the close of the Pliocene, and was accompanied by the manifestation of great volcanic activity

The profile of a cross-section of the Sierras and the recurrence of the same fossiliferous strata in analogous topographic and stratigraphic positions demonstrates that the northern end of the range is made up of three orographic blocks separated from one another and from those of the Great Basin by profound dislocations. These displacements occurred chiefly if not wholly after the elevation of the Sierras, when the volcanic energy had almost spent itself. The Sierras were made to appear as a distinct range by the subsidence of the Great Basin region.