SUMMARY 283 Tion Between the Cordilleran Revolution of Eastern
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SUMMARY 283 tion between the Cordilleran revolution of eastern California and the post-Franciscan revolution of the Coast Ranges. It is clear, how ever, that at some time not far from that of the Cordilleran revolu tion Coast Range rocks were intruded and extensively metamor phosed by basic volcanic rocks. Perhaps the two events were con temporaneous ; perhaps the latter antedated the former. By the beginning of the Cretaceous or a little later, at any rate, all of Cali fornia had undergone a series of profound diastrophic changes ac companied by vulcanism of several different types ; in both Sierra Nevada and Coast Ranges, moreover, and to a great extent in eastern California as well, the structure of the rocks had become intricately folded and faulted. The structure was mountainous, whatever the general elevation may have been. If we may judge from the distribution and lithology of later sediments, coastal Cali fornia was of mountainous height only locally and temporarily if at all, while eastern California became an upland underlain by so rigid a basement that it did not again yield to orogenic stresses until middle Tertiary, and has never since, except locally (Death Valley and Colorado Desert), sunk below sea-level. The Cretaceous subsidence was very widespread in western California, and was locally extreme-30,000 feet or more. It seems to have been distinctly variable in amount, however, since there is much change in thickness from place to place. These changes have not yet been mapped in satisfactory detail, but the available evidence suggests strongly that the variations in subsidence were related to the growth of folds which, in many cases, have exercised a dominating influence over the entire later structural history of the Coast Ranges. The conditions are particularly favorable for a study of these relations on the eastern flanks of the northern Temblor and southern Diablo ranges.l At the close of Cretaceous time there may have been a mild period of folding and uplift; there was at any rate a partial displacement of the loci of most rapid subsidence during the succeeding Eocene epoch. The Oligocene epoch as a whole seems to have been a period of emer gence and of continental deposition, but some of the subsiding areas of the southern and western San Joaquin Valley were more or less continuously under the sea. During the Miocene the sea spread widely over coastal California 1 Chapter VI, Fig. 14 A and B, and R. D. Reed, "Structural History of the Coalinga District," abstract, Bull. Geol. Soc. A mer., Vol. 41 (1930), p. 150-51. Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3840644/9781629812526_backmatter.pdf by guest on 28 September 2021 284 GEOLOG Y OF CA LIFORNIA and advanced inland in a few places farther even than the Eocene sea had spread. There are great differences in thickness of deposits from place to place, however, and some of the upfolded and upfaulted areas were above water during much or all of the epoch. About the middle of the Miocene came the culmination of a very important volcanic and diastrophic period. If batholiths were intruded into the crust of California, they have so far failed to be revealed by later erosion. Vulcanism was widespread, however, in the southern Coast Ranges, the Transverse Ranges, in the Sierra Nevada, and over the eastern desert region. The exact dating of the flows, intrusions, and erup tions is not always clear, but there is a great accumulation of evidence tending to prove that they became prominent during the Temblor and died out during the Monterey-Santa Margarita stages, with a culmi nation at about the close of the Temblor. In the eastern California region the volcanic processes gave rise to great lava flows which interrupted the drainage, heretofore continuously westward-flowing since the Jurassic. They were accompanied also by faulting which has been far less studied than it should be. Perhaps the faulting was sufficiently widespread and intensive to account for the interruption of drainage without much aid from the lava flows. Both phenomena are demonstrated, however, and the only problem remaining is to determine their relative importance in creating the basins that became the sites of deposition of the widely distributed and variable Rosa mond series. Some of the faulting was normal, and much or all of it may have been so. Recent studies in Nevada suggest, however, that part of it may have been of the reverse type, or perhaps that the dominant normal faulting of this stage may have been preceded or followed by a period of reverse faulting. This is one of the many fascinating problems still awaiting solution in the Mohave Desert region. So far as the Coast Ranges themselves are concerned, there is good evidence to prove that some of the geologists of an earlier day exaggerated the amount of diastrophism during the later Miocene. Local folds developed in the Santa Monica Mountains, in the Devils Den region, and elsewhere, but they are far from constituting the post-Mont;e�ey revolution of the text books. They belong rather to the subsidiary- phases of the great diastrophic period which cul minated before Monterey time, and which had as the scene of its most important and far-reaching episodes not the Coast Ranges but the old land areas of Mohavia and Salinia. The northwest end of the Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3840644/9781629812526_backmatter.pdf by guest on 28 September 2021 SUMMARY 285 latter sank beneath the sea at this time and has never been resurrected since. Part or all of Catalinia, which had been high and rugged dur ing the Temblor, sank also during the Upper Miocene, but some of it rose again during the next succeeding epoch. On the whole, the middle Miocene diastrophism embraced local folding, widespread faultingand basin development, and the extrusion of a great variety of lavas at many different centers. The lavas of the Columbia River region belong to the same great epoch of vul camsm. One of the most striking features of the Cenozoic history of Cali fornia is the rapidity with which the approximate conditions of pre-Miocene time were restored over southern California after the middle Miocene diastrophism. The Pliocene sea spread somewhat more widely than that of the Oligocene, and was supplied with detritus similar in texture and in general lithology to Oligocene or Eocene detritus. This fact seems to mean that the basins of Mohavia were filled and that the interrupted drainage lines were beginning to be restored by the close of the Miocene or soon afterward. Since a high proportion of the coarse Pliocene detritus of many districts seems to have come from the Coast Ranges rather than from the old eastern land, there were probably higher land areas in the Coast Ranges at this epoch than there had been in the Oligocene. The rate of subsidence of different areas was again very variable during the Pliocene, the total amount ranging from 15,000 feet in the middle of the Ventura Basin to a fraction of that amount a few miles away. As a matter of fact, some Coast Range areas seem to have been rising intermittently throughout the epoch. Possibly "primary faults" were already in operation here and there, but the favorable evidence available at present is not very cogent. Toward the end of the Pliocene most of the basins ceased to subside so rapidly as before, and became filled to or above sea-level. In the San Joaquin Valley, for example, the dominantly marine Etchegoin beds are succeeded by the nonmarine Tulare formation, 2,000-3,000 feet thick. In the Paso Robles Basin of the Salinas Valley district the Paso Robles beds are an approximate equivalent of the Tulare ; in the Sacramento Valley the Tehama formation is another; in southern California the Saugus formation is a third. In the more important basins marine or nonmarine deposition continued until some time within what is now considered Pleistocene. Then the folds that had been growing gradually since the early Tertiary, or even since the Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3840644/9781629812526_backmatter.pdf by guest on 28 September 2021 286 GEOLOGY OF CALIFORNIA Cretaceous, suddenly began growing at a vastly increased rate. Many of them presently became overturned and even overthrust. Old faults in some cases probably suffered so much movement as to mask the evidence of their earlier existence. New faults were cer tainly formed all over the State, however, and some of the known old ones did not move at all. The uplift of the Sierra Nevada be came so strongly accelerated that half of the present height of the range dates from this time. Probably even more of the present height of the Coast Ranges dates from the same time. Strong folding and reverse faulting were the order of the day, with normal faulting as a casual accompaniment. In contrast to the conditions of the late Jurassic or middle Miocene diastrophism, that of the Pleistocene was not accompanied by much vulcanism except in the Cascade Mountains and locally in eastern California or elsewhere (Table XXV). FOSSILS Coast Range paleontologists are faced with two serious difficulties: one is the occurrence of great thicknesses of barren strata in many formations ; the other is the poor preservation that has resulted from the widespread folding and faulting that have affected all formations except those of the uppermost Pleistocene and later. The barren strata are largely siltstone or sandy shale, though partly sandstone and conglomerate. In recent years careful search has shown that many of the formations classed as barren are really well supplied with small fossils.