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

GEOLOGIC FORMATIONS IN THE LASSEN PEAK DISTRICT.
(continued)

PLIOCENE.

Within the Lassen Peak district deposits of Pliocene age have not been definitely distinguished from those of the Miocene, but their presence is rendered altogether probable by several considerations. The auriferous gravels were regarded by Professor Whitney as accumulating upon the western slope of the Sierras throughout the whole of the Tertiary, reaching their culmination in the Pliocene. This view is very probable, indeed; for, although the mass of the gravels of the gold belt are Pliocene, it appears to be evident that other portions, especially that at Cherokee as well as about Mountain Meadows and near the head of Light Cañon, belong to the Miocene.

Overlying the Miocene of the Piedmont region is a very extensive development of the volcanic tufas. As already remarked, in the upper portion of the Miocene conglomerate, especially in the region of Bear Creek, Diamond Peak, and Monntain Meadows, pebbles of lava become abundant; in fact, they rapidly increase upwards, so as to completely predominate, and the formation passes into tufa without any apparent stratigraphic interruption. It has been completely demonstrated by Whitney, Le Conte, and others that in the Sierras the great volcanic outbreak took place in the Pliocene, and there is reason to believe that the early eruptions in the Lassen Peak district, which gave birth to the immense mass of tufa in the Piedmont region, occurred about the same time. The tufas were unquestionably deposited later than strata known to be Miocene, and earlier than the Quaternary, so that we feel constrained to regard them as belonging to the Pliocene.

This formation has its greatest development in the southwestern quarter of the Piedmont region and determines its peculiar features. All of the mountain streams between Battle and Butte Creeks on their way to the Sacramento have cut deep cañons across the Piedmont, and thus furnish excellent exposures of the tufa. Its genesis is a very perplexing problem.

The material of which it is composed is chiefly hypersthene andesite, but fragments of hornblende andesite and basalt are also common. Much of the detritus has been subjected to considerable water attrition, so as to appear in the form of rounded pebbles or sand; but the larger portion, ranging in size from fine dust to blocks of lava several feet in diameter, is rough and angular without distinct traces of abrasion. In the cañons where the tufa is well exposed it may be observed that much of the material is assorted and distinctly stratified. The accompanying illustration (Pl. XL VIII), which represents its general features very well, was taken near the western limit of the Piedmont, northeast of Tehama. It is true that in many places a stratum of material apparently thrown together pell-mell may be so large as to create the impression that the whole mass is without arrangement, but this is not the case. Much of the sediment has been picked out and laid down in such a way as to clearly indicate that its deposition took place in water. In the cañon of Mill Creek and also in that of Deer Creek, near the mountains, where the tufa has a thickness of nearly one thousand feet, it is roughly divisible into three parts, as indicated in Fig. 17. The upper and lower portions are agglomerate and between them the stratified arrangement of the tufa is clearly discernible. In all parts the sediments are essentially the same, being composed largely and perhaps chiefly of fragmental material ejected from volcanoes.

PLATE XLVIII.—CAÑON OF LITTLE DRY CREEK, SHOWING THE STRATIFIED TUFAS OF THE PIEDMONT REGION.

FIG. 17. Mill Creek Cañon, 20 miles east of Tehama: Depth 1000 feet. 1. Basalt. 2. Agglomerated tufa. 3. Stratified tufa. 4. Yellowish sandstone.

The peculiar manner in which the tufa was formed is an enigma whose complete solution has not yet been attained; nevertheless there are some conditions attending its formation which we may understand. The aqueous stratification of a large part of the tufa appears to me to be evident, and the body of water in which it was laid down filled the northern end of the Sacramento Valley. Piute Lake, with modifications, may have continued into the Pliocene. At any rate, it is apparent that the water body about the northern terminus of the Sierras during the Pliocene was shallower and more extensive than the Miocene Lake, and differed also in the character of its deposits. The distinctly assorted material is of two varieties: (1) That which has been considerably abraded and was produced by normal shore agents, and (2) that which was ejected during violent volcanic action and after being wafted for a considerable distance by the winds fell into the water.

The agglomerated portion of the tufa is complicated in its genesis, and probably received additions in various ways. Most important of these along its eastern margin was the tumultuous contribution from the active volcanoes near shore.

Intercalary lavas such as are shown in the section (Fig. 17) are not uncommon, and by irregular decomposition and disintegration they frequently give rise to pseudo-agglomerates which may closely resemble real agglomerate in general aspect. They may be readily distinguished from the hatter, however, by the uniformity of their composition throughout both matrix and fragments.

A large part of the pell-mell beds are heterogeneous in composition, and contain large angular fragments of compact lava, which bear no marks of having ever been ejected from a volcano, but appear rather to have been driven from their parent bed in the process of secular decay.

The manner in which superficial lava flows may be broken up by atmospheric influences and the angular fragments strewn over the plain is well illustrated by several examples in the Piedmont region, especially in the vicinity of Paine's Creek. This process is evidently subaerial and is doubtless accompanied to some extent by the sort of action which gives rise to alluvial cones. The cause which produces alluvial cones is the main one to which Captain Dutton appealed to explain the accumulation of the enormous detrital masses of volcanic rocks in the high plateaus of Utah,¹ and it has probably contributed to the upbuilding of the Piedmont region; but the greater influence in the development of its peculiar features must be ascribed to the Pliocene lake which occupied the northern part of the Sacramento Valley.

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¹Geology of the high plateaus of Utah (Capt. C. E. Dutton), 1880, p. 77.

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The topographic features which always accompany the degradation of heavy beds of agglomerate are often prominent, and enable the observer to recognize the formation at a considerable distance. They usually present either cliffs or irregular columns scattered promiscuously and abundantly over the slopes.

The plain of the Piedmont region is often extremely stony. The rough, angular fragments strewn over the surface in great numbers are in part the surviving pieces of a once continuous lava sheet, and in part the enduring portions of the disintegrating agglomerate, the finer material having been removed. A typic example of this sort of stony waste may be seen eight to twelve miles east of Red Bluff, south of Paine's Creek, on the Susanville road, The dreary plain is often for wide stretches without arboreal vegetation; but the stunted oak (Quercus Garryana), taking hold below, and the bull pine (Pinus Sabiniana) with the manzanita and other shrubbery above, mitigate its otherwise complete barrenness. The traveler over the desolate Piedmont region is not likely to forget its peculiarities, especially if he crosses it in an open conveyance under the blaze of a summer sun. It fits him to enjoy the salubrity of the mountain clime beyond, and it makes him feel that he has earned an abode for a time in the garden of the Hesperides by having successfully passed through Pandemonium.

UPHEAVAL OF THE PIEDMONT REGION.

That the tufa of the Piedmont region was deposited near the shore of a large body of water which occupied the northern portion of the Sacramento Valley is apparently certain, and that the water was fresh is inferred from the fact that it appears to have been a continuation of the fresh-water Miocene lake. It is evident that the champaign character of the formation was determined by the water in which the deposition took place. Some time after the deposits were formed they were raised above the lacustrine waters in such a way that the mountain streams run directly across them and each stream excavated for itself a deep cañon. The uplifting was differential, being greatest to the eastward, so as to give the plain a westward declivity of about four degrees. At the same time the western limit of the region was defined by the development of an interesting monoclinal fold, which is exposed in the gates of all the cañons where the streams debouch into the Sacramento Valley proper. It gave rise to the prominent bluff which is well illustrated in the following view (Pl. XLIX), taken from the Sacramento Valley opposite the gate of Mill Creek. The bluff is here over a hundred feet high. Across the Piedmont region, at a distance of about twenty miles, the volcanic ridge of Lassen Peak is conspicuous. A section of the monoclinal fold south of Antelope Creek is given in Fig. 18, and shows the relation of the tufa of the Piedmont region to the Quaternary gravels of the Sacramento Valley. This monoclinal fold extends from the neighborhood of Battle Creek on the north beyond Deer Creek, gradually dying out at both ends, so that the Piedmont region beyond the limits of the fold continues its gentle slope without interruption to the level of the Sacramento Valley. The displacement along the monoclinal fold is on an average somewhat more than one hundred feet and was formed before the deposition of the Quaternary. At that time the cañons were already cut across the Piedmont, and the streams entered the Sacramento Valley at the level of its alluvial plain, to which each creek contributed a low, deltoid, gravel cone.

PLATE XLIX.—VIEW OF THE PIEDMONT BLUFF FROM THE SACRAMENTO VALLEY. Lassen Peak volcanic ridge in the distance.

FIG. 18. section of the Piedmont monocline near gate of Antelope Creek. 1. Quaternary gravels 2. Volcanic tufa (Pliocene?).

Within the cañon of Deer Creek, as also in that of Mill Creek already illustrated (Fig. 17), there is a small cañon containing the present stream. It is best developed near the gate of Mill Creek, where the annexed view (Pl. L) was taken. The depth is frequently over seven times its width, and it extends out into the valley gravels for more than a mile. It is evident that this seoondary cañon is due to the differential uplifting of the Piedmont region, by which the declivity of the stream was so increased as to augment the corrasion upon its restricted bed and in the same proportion to diminish the wearing away of its banks. The relation of the valley gravels to the Piedmont bluff, and the fact that the younger cañon extends some distance out into the Quaternary deposits indicate that the latest uplifting in that region, which gave birth to the little cañon, was post-Quaternary, although the formation of the monocline took place at the close of the Pliocene.

PLATE L.—VIEW SHOWING THE RELATION BETWEEN OLDER AND YOUNGER CAÑONS OF MILL CREEK. Eight miles east of Tehama.