As in all these blog posts, any pictures below are all linked to full-sized versions.
I've wanted to transcribe Levi Noble's report on the Valyermo Quadrangle for many years now. It's printed on a large map, and that large map is folded into thirds both horizontally and vertically, so to read the report requires you to fold and unfold and refold the map many, many times. That being the case I've never had the patience to read the whole thing in a sitting. Rather, I've read it in bits and pieces over the years. I'm sure that's not what Noble envisioned for his report, however for almost 70 years now, that's how people have had to read it. The report below retains, to the best of my ability, Noble's spellings, grammar, punctuation, etc. Now that science and everything else has moved on, Noble's report is dated and in places is incorrect. However, the rocks are still in the same places they were when he surveyed the map and wrote the report. Only minor changes have occurred since there has not been a major movement on the San Andreas since the Fort Tejon earthquake of 1857. The hills, trees, creeks, and mountains look the same today as they did to Levi throughout the first half of the 20th century.
"What's Valyermo?", you might ask. I've loved the Valyermo area my entire life. The area lies on the northern slope of the San Gabriel Mountains, just above the Mojave Desert, my home, where I was born and where I long to return. Laying as it does at the base of the 10,000-foot San Gabriel range, Valyermo is made up of hills and valleys with creeks running through them under cottonwood and sycamore trees. Junipers dot the drier hillsides, and the San Gabriels rise above the hills, trees, and creeks, their snow-covered heights towering over Valyermo and the desert beyond.
Some history of the place, and how I came to care so much about Noble's map and report:
There's a group of Benedictine monks who were originally located in a monastery in China but were kicked out of that country by communist forces under Chairman Mao. In 1955 these monks purchased the Hidden Springs Ranch in Valyermo and the ranch became known as St. Andrew's Priory. Why "St. Andrew's"? Well, the priory, (now St. Andrew's Abbey), sits smack-dab on top of the San Andreas Fault, and "San Andreas", of course, is Spanish for Saint Andrew. I had a hand-painted wooden plaque for many years, painted by one of the monks, that read "Saint Andrew, protect us from your fault!", and displayed St. Andrew walking through a grassy field with a bird on his shoulder, halo burning bright, mountains rising in the background in a familiar silhouette.
Pallette Creek flows through the abbey's grounds, following the trace of the San Andreas, which has crushed and pulverized the surrounding rocks, creating an easy course for drainage to follow as it flows from the melting snows to pour out on the desert floor. In the 1980s, a geologist named Kerry Sieh obtained permission from the monks to bulldoze trenches across peat beds that had been deposited on the banks of Pallett Creek, and he was able to determine the average amount of time between large movements on the San Andreas fault. Those peat beds are mentioned in the report that follows.
Big Rock Creek flows directly out of the highest region of the San Gabriels. I spent countless days there training my dog Fremont, camping, hiking, and exploring the region. When my interest in geology sparked in my early 20's, I carried Levi Noble's map with me wherever I went around Big Rock. I even found some folds, small anticlines and synclines in the sandstone of Noble's Martinez formation, (later re-named the San Francisquito by Thomas Diblee), that weren't marked on Levi's map, and that at least my geology instructors at the time were completely unfamiliar with.
To say that the region mapped by Noble is near and dear to my heart would be a sinful understatement. I love the area with a passion usually reserved for a lover. Never have I found a place that speaks so directly to me as Valyermo. Levi Noble lived on the ranch that borders St. Andrew's to the east. He and his wife moved there in the early 1900s and lived there for the rest of their long lives. So this map and report are more than just science. They are a study of home; Levi's home as well as mine.
Geology of the Valyermo Quadrangle and Vicinity, California
By Levi F. Noble
The report/map can also be downloaded directly from the USGS here:
https://ngmdb.usgs.gov/Prodesc/proddesc_418.htm (Last accessed 10-11-2019)
The dominant geologic feature of the Valyermo quadrangle is a belt of intense disturbance in the rock formations that lie between the San Andreas and the San Jacinto faults. This tectonic belt, its relations to adjacent geologic and geographic features, and its geologic history form the principal theme of the present report.
The Valyermo quadrangle lies in northern Los Angeles County, about 40 miles northeast of Los Angeles. It covers a part of the northern San Gabriel Mountains designated Pleasant View Ridge on the map and a part of the southwestern Mohave Desert designated Antelope Valley.
In 1853 W. P. Blake accompanied, as geologist, an expedition sent out by Jefferson Davis, then Secretary of War, to explore routes for a railroad to the Pacific. In the course of the exploration, the party camped in the Valyermo quadrangle on lower Big Rock Creek, which they named Johnson River from the name of the soldier who discovered it. Blake’s observations are a fascinating and accurate account of the geology of a then uninhabited and almost inaccessible region (Blake, 1856, p. 62-66, 184-185, and 219-220). It is not strange that he failed to recognize the San Andreas fault, for in 1853 the significance of its surface features was unknown. Indeed, no geologist recognized it until more than 40 years later (Schuyler, 1896-1897, p. 711-713). If Blake had visited the Valyermo area 4 years later than he did, he would undoubtedly have recognized the great fault, for the Fort Tejon earthquake along the fault in 1857 is known to have disturbed surface features in the quadrangle. The late Charles Vincent told the author that these features were fresh and easily recognizable when he settled in the quadrangle in 1868. At that time the features were known to the few inhabitants as the “Earthquake crack.”
The trace of the San Andreas fault across the Valyermo quadrangle was mapped by Fairbanks for the California Earthquake Investigation Commission. Plate 26 B his report shows one of the recent fault features, a sink near the western border of the quadrangle (Lawson et al., 1908).
Dickerson, in 1912, assisted in part by the present author, mapped the Martinez formation in the quadrangle and separated it into two members – the lower predominantly brown sandstone and the upper predominantly dark shale. He assigned the formation to early Eocene age. He also described the Punchbowl formation and the angular unconformity between it and the Martinez, and assigned the formation tentatively to the Miocene (Dickerson, 1914).
The author has carried on a study of the 50-mile segment of the San Andreas fault zone between Soledad and Cajon Passes (fig. 3) intermittently since 1910. Preliminary results of the work have been published (Noble, 1926, 1932, and 1933); and a fuller report on the Tertiary structure and geology of this segment of the fault zone is in preparation. Before 1932, the geology was mapped on the 15-minute topographic sheets of the Geological Survey, and after that year, on the new 6-minute sheets as they became available. C. L. Gazin assisted the author with the work in 1932. The first map on a 6-minute sheet, showing the geology of the Pearland quadrangle, was published in 1953.
The fieldwork for the geologic map of the Valyermo quadrangle was begun in 1940 and completed in 1952. It is entirely the work of the author. Many geologists visited him in the field during the investigation of this part of the fault zone. Helpful suggestions were made by H. E. Gregory, Charles Schuchert, W. M. Davis, Bailey Willis, Andrew Lawson, J. P. Buwalda, F. L. Ransome, W. C. Mendenhall, George and Anna Stose, Philip King, and H. G. Ferguson. The author is also indebted to E. S. Larsen, Jr., and Charles Milton for study of thin sections of the different types of basement rock in the quadrangle.
The San Andreas fault is a major break within a zone of rupture, commonly half a mile to a mile wide, that extends more than 600 miles across California from north of San Francisco to south of the Salton Sea. This zone, the San Andreas fault zone or San Andreas Rift, has a curiously direct course across mountains and plains, with little regard for gross topographic features, yet it influences profoundly the local topographic and geologic features within it. It is a dominant part of the great fault system of southern California, which has broken the rocks of that part of the state into a mosaic of blocks (fig. 1).
In the Valyermo quadrangle another major fault, the San Jacinto, lies parallel to and about 3 miles to the southwest of the San Andreas. Southeast of the quadrangle it divers widely (fig. 1); a few miles northwest of the quadrangle it curves northward and merges into the San Andreas fault zone (Noble, 1953). Associated with the San Jacinto is the Fenner fault, which is exposed only the southeast corner of the area mapped.
The San Andreas fault has been intermittently active since at least early Tertiary time, and movements have continued to modern times. The northern part of the fault was the locus of the San Francisco earthquake of 1906. Similar disturbances caused the Fort Tejon earthquake of 1857, of equal or greater intensity. Movement on the San Jacinto fault south of the San Bernardino Valley (fig. 1) caused destructive earthquakes in 1899 and 1918, but there is no evidence in the Valyermo quadrangle of major movement on it during or after the Pleistocene, though movement on the Punchbowl fault parallels to and a short distance north of the San Jacinto fault has involved the older alluvium. The Fenner fault has not been active since middle Miocene time.
The tectonic belt (fig. 2) enclosing these faults is an area of extreme structural complexity, essentially a composite structural trough, renewed at intervals by synclinal folding and faulting concomitant with movements along the major faults.
In the northern part of the tectonic belt the San Andreas fault zone, a structurally depressed area of Tertiary and younger rocks encloses the San Andreas and subsidiary faults. The Tertiary and pre-Tertiary rocks and the Pleistocene Harold formation differ on opposite sides of the San Andreas fault, indicating a marked and persistent belt of horizontal movement. Within recent time, horizontal movements of about a mile have offset stream courses at the San Andreas fault, and these movements are still in progress. Earlier horizontal movements on the fault appear to have offset the Harold formation several miles. A combination of geologic features described below indicates that the offset of Tertiary and pre-Tertiary formations within the Valyermo quadrangle since early Tertiary time has been at least of the order of 25 to 35 miles.
In the southern part of the tectonic belt (fig. 2) rocks of early Tertiary age in the Pinyon block have been complexly folded and in part overthrust by pre-Tertiary granitic rocks. This area is bounded on the south by a reverse fault, the Punchbowl fault, which lies parallel to and a short distance north of the San Jacinto fault and is separated from it by the San Jacinto fault zone, a belt of crushed gneiss, granitic rock, and aplite. In the southeast corner of the area a similar belt of crushed rock accompanies the Fenner fault.
North of the tectonic belt a block of relatively undisturbed pre-Tertiary granitic rocks, uplifted in a low anti-clinal arch, underlies Holcomb Ridge and the pediment that slopes gently northward to Antelope Valley. South of the tectonic belt, Pleasant View Ridge, forming the northern flank of the San Gabriel Mountains, is underlain by pre-Tertiary granitic and gneissoid rocks. In contrast to Holcomb Ridge, this area has been relatively mobile and subject to repeated uplift.
In the following descriptions the different structural blocks indicated on figure 2 are described in order from north to south.
ANTELOPE VALLEY AND HOLCOMB RIDGE
In the northern part of the quadrangle Quaternary alluvial deposits cover the rock floor of the Antelope Valley and thicken northward to an unknown depth. Southward the rock floor rises, emerging in a gently sloping pediment that extends to the crest of Holcomb Ridge and thence descends gently southward toward the San Andreas fault zone. The pediment and the ridge are uplifted in a low anticlinal arch. The pediment is an exhumed feature and is part of a surface formed by erosion in pre-Harold time. These relations are shown clearly on the crest of the eastern part of the ridge, where Harold sediments are overlain by older alluvium lie upon the pediment, and on the south slope of the middle part of the ridge, where patches of Harold lie upon the pediment and dip south with it.
Three gaps occupied by antecedent stream courses, whose beds are graded from the San Andreas zone to the Mohave Desert, break the continuity of Holcomb Ridge. The westernmost is occupied by an unnamed wash; the middle by the combine courses of Pallett and Big Rock Creeks; and the easternmost by Bob’s Gap. The floor of Bob’s Gap is Holcomb quartz monzonite; the floors of the other gaps are younger alluvium. These gaps afford evidence of lateral displacement on the San Andreas fault.
The Holcomb quartz monzonite forming the rock floor of Holcomb Ridge and bordering pediment is a homogeneous body of granitic rock, part of a batholith of wide extent underlying much of the Mohave Desert. It is continuous with the granitic rock north of the San Andreas fault in the Pearland quadrangle (Noble, 1953) and thus borders the fault on the north for at least 25 miles. The quartz monzonite contains roof pendants of crystalline limestone and diorite-gabbro. The limestone forms long tabular masses that dip northward or are vertical; the largest, 4 miles in length, forms the crest of Holcomb Ridge east of Big Rock Creek. The diorite-gabbro masses are irregular but also dip to the north. All are aligned in trend with the ridge and form hills or ridges that rise above the general level of the pediment. The quartz monzonite is of Mesozoic age, probably Cretaceous (Woodford, 1939, p. 257). The age of the limestone is probably Paleozoic. The diorite-gabbro may be Jurassic.
The Holcomb quartz-monzonite is traversed by northeast-trending faults and by several sets of joints, of which the strongest set trends northeast and is nearly vertical. The faults offset the roof pendants and some of them appear to have a greater horizontal than vertical component. They are difficult to trace in the homogeneous quartz monzonite, but at some places they displace the surface of the pre-Harold pediment. They are certainly older than the older alluvium but probably post-Harold. These faults are apparently the result of compressive stresses accompanying lateral movement on the San Andreas fault.
In the eastern part of the quadrangle the Holcomb fault, a Recent east-trending thrust fault that dips north at angles of 25º to 50º, branches from the San Andreas fault zone near the middle of the quadrangle and continues to a point 2 miles east of the quadrangle, where it and the ridge die out. The fault is apparently structurally related to the anticlinal arching of the ridge. North of the fault the pediment, the Harold formation, and the older alluvium on the ridge stand 200 feet higher than they do south of the fault. A wide alluvium-covered embayment on the south side of the fault (sec. A-A’) extends east from the San Andreas fault zone to Antelope Valley and separates Holcomb Ridge from a similar ridge of basement rocks that borders the San Andreas fault zone to Antelope Valley and separates Holcomb Ridge from a similar ridge of basement rocks that borders the San Andreas fault zone eastward to Cajon Creek (fig. 3), 30 miles southeast of the Valyermo quadrangle.
The tectonic belt is a composite structural trough between the Holcomb Ridge and San Gabriel Mountain blocks. It consists of long parallel crustal blocks of exceedingly complex structure bounded for the most part by faults. One of the depressed fault blocks is the San Andreas fault zone, a topographic as well as a structural trough floored by moderately disturbed Quaternary deposits unconformably overlying a mosaic of intricately folded and faulted Tertiary and pre-Tertiary rocks. The Pinyon block next south includes Pinyon Ridge, an anticlinal arch of pre-Tertiary basement rocks and intensely disturbed Paleocene sedimentary rocks, and the Punchbowl trough, a faulted syncline of folded upper Miocene strata that lie unconformably across pre-Tertiary rocks and Paleocene. The Pinyon block is separated from the San Gabriel Mountain block of pre-Tertiary basement rocks by the San Jacinto fault zone, which forms the southwest boundary of the tectonic belt.
The tectonic belt is in striking topographic contrast to the areas north and south of it. North of the belt, Holcomb Ridge falls away in a broad even slope to Antelope Valley, a gently sloping alluvial plain above which rise occasional island mountains and broad low domes of granitic rock. South of the belt the San Gabriel Mountains rise abruptly to heights of nearly 8,000 feet just south of the Valyermo quadrangle. In Pleasant View Ridge, in a steep rugged face scored by deep canyons that drain northeast down the slope, peaks are of different heights, ridges are serrated and irregular, spurs are sharp edged and sprawling, and there are no flat-topped ridges.
Within the tectonic belt the ridges have even, gently sloping tops, and, although greatly dissected, they are essentially continuous. The topographic troughs are likewise continuous, although parts of their floors are warped and deeply dissected; they carry a subsequent drainage disrupted at places by faulting and warping.
The floors of the troughs and the long even tops of the ridges descend steadily northwestward across the quadrangle, but west of the quadrangle they flatten out and at places are buried under younger alluvium. Southeast of the quadrangle, they rise steadily to elevations between 6,500 and 8,500 feet, then descend continuously to Cajon Creek (fig. 3) and disappear under the alluvium of the San Bernardino Valley. This arch, 40 miles in width, is an upwarp of the northern part of the San Gabriel Mountains by doming in late Quaternary time (Noble, 1926, p. 421; 1932, p. 360) The Valyermo quadrangle lies far down on the northwest flank of this arch.
The slope to Antelope Valley at Holcomb Ridge, the crests of Holcomb and Pinyon Ridges, and the floors of the San Andreas fault zone and Punchbowl trough beneath the Quaternary deposits are parts of a widespread erosion surface that bevels all pre-Quaternary rocks without regard to character or structure. The surface was formed in late Pliocene or early Quaternary time prior to the deposition of the Pleistocene Harold formation. It has been dislocated by recent faulting and warping and given a northwestward tilt by the broad doming just described. It has been exhumed and is being dissected on the ridges, and has been obliterated by erosion in the high eastern part of the Punchbowl trough, but at most places elsewhere in the troughs it remains buried.
SAN ANDREAS FAULT ZONE
Within the Valyermo quadrangle, the San Andreas fault zone, half a mile to a mile wide, is a shallow depression that trends west-northwest and cuts across much of the local drainage. Its floor consists of broad alluvial surfaces that slope gently toward the northeast side of the depression and are diversified by occasional low ridges trending parallel with the depression. This trough belt, structurally as well as topographically a depressed area (secs. B-B’ and C-C’), is the feature for which the term “San Andreas Rift” was used in the report of the California Earthquake Commission (Lawson et al., 1908, p. 2-3).
In the central part of the quadrangle the zone is floored by coarse stream gravel that is being deposited by the shifting channels of Big Rock and Pallett Creeks, but in the eastern and western parts of the zone the older alluvium, Harold, and Shoemaker formations are being dissected. The dissected area bordering Pallett Creek, in the western part, affords the only good exposures of the pre-Quaternary structure. The Quaternary deposits unconformably overlie strongly folded and faulted late Tertiary rocks and a broad wedge of crushed pre-Tertiary crystalline basement rock that extends the length of the zone.
San Andreas Fault. – In this quadrangle the San Andreas fault lies within and near the southern border of the San Andreas fault zone, crossing the quadrangle in an almost straight course bearing N. 65º W. It is marked by an almost continuous chain of scarps, ridges, and trenchlike depressions, most of which involve Quaternary alluvial deposits and which afford clear testimony to many recent earth movements. These features are the result of irregular tearing of the poorly consolidated or unconsolidated alluvial deposits above a major break in the underlying rocks, the San Andreas fault. They roughly coincide with the position of the fault at depth.
The San Andreas fault is bordered on the south by a nearly continuous shallow alluvium-floored trench, commonly less than 500 feet wide, which is essentially a downwarp in the older Quaternary deposits modified by faulting. Shoemaker Canyon, on the Pearblossom Highway, is a segment of the trench. A low ridge of varying width, continuous except where cut by the larger stream valleys or where it is buried under younger alluvium, borders the fault trench on the north. The ridge is a composite feature, made up of a succession of low narrow ridges that trend eastward or westward from the San Andreas fault at a low angle, with en echelon arrangement. The component ridges are separated by small faults that branch from the San Andreas fault at low angles and dip toward it, a structure that appears to be the result of arching of the ridge by compression; some of the ridges actually are anticlines in the older Quaternary deposits.
Other faults of San Andreas zone. – There are numerous minor faults subsidiary to the San Andreas fault, many too small or too closely spaced to show on the map. These mostly branch from the San Andreas and from one another at very acute angles, diverging and curving in such a way that they enclose long sliver-like blocks – the typical pattern in the San Andreas fault zone. Most of the faults are marked by low scarps, low ridges, and shallow trenches. All are traceable on air photographs. Like the San Andreas fault, many exhibit apparent reversals of throw along the strike, indicating that they have a major horizontal component. Few have vertical displacements exceeding 50 feet. Some coincide with pre-Quaternary faults and area recurrent faults. Recent movement on some faults has irregularly warped even the youngest Quaternary deposits into low anticlinal domes and shallow undrained depressions.
The Hidden Springs fault crosses the quadrangle along the northern boundary of the San Andreas fault zone. It branches from the San Andreas at a very low angle just west of the quadrangle, diverges gradually from it as far as the middle of the quadrangle, and thence runs parallel with it to and beyond the east border of the quadrangle. Actually, it is not a single fault but a system of closely spaced parallel faults, arranged en echelon and exhibiting apparent reversals of throw along the strike. In the eastern part of the quadrangle, northeast of Mountain Brook Ranch, this composite fault forms a south-facing scarp. In the western part of the quadrangle, an apparent reversal of throw drops the Harold formation north of the fault against the Punchbowl formation south of it.
Crushed basement rocks. - Within the San Andreas fault zone lies a wedge of crushed basement rocks that at some places is overlain by the upper Miocene Punchbowl formation and at other places is faulted, dragged, or squeezed up through it. This wedge borders the San Andreas fault throughout the 50-mile segment of the fault zone between Cajon and Soledad Passes, although at most places it is buried under alluvium. It varies greatly in width, but in the area about Pallett Creek and near the center of the quadrangle it is as much as 1,500 feet wide. The crushing is the result of pre-Quaternary (pre-Harold) movements on the San Andreas fault zone. The wedge of crushed rock is believed to mark the major break upon which the greatest lateral displacement on the San Andreas fault zone has taken place.
In the area about Pallett Creek, pre-Quaternary faults, on which there has been some later movement, cut both the wedge of crushed basement rocks and the Punchbowl formation. The faults here dip toward the San Andreas, but the structure and distribution of these rocks under the alluvial cover in most of the quadrangle are matters of conjecture. The dip of the faults outward the San Andreas suggests that the wedge of crushed rock narrows downward, an inference supported by the fact that the outcrop of the wedge is very narrow southeast of the quadrangle in the higher part of the San Gabriel Mountain arch already described, where erosion has exposed a much deeper part of the fault zone than in the Valyermo quadrangle.
The area between the San Andreas and San Jacinto fault zones has had a complex geologic history. Periods of Tertiary deposition, at first marine, and later continental, have alternated with extreme compression and faulting, followed by recent uplift relative to the San Andreas fault zone.
The Martinez formation and the Pinyon Ridge granodiorite underlie Pinyon Ridge at the north side of the block. At the southeast only a narrow strip of the faulted Punchbowl syncline separates the Martinez from the crush zone along the San Jacinto fault; westward a wider are underlain by the Punchbowl syncline separates the Martinez from the crush zone along the San Jacinto fault; westward a wider area underlain by the Punchbowl formation occupies the southwest flank of the ridge. Three miles east of the quadrangle this structural block is crossed by the Fenner fault, but the block continues for 30 miles to the San Bernardino Valley (fig. 3). West of the quadrangle, close to the point where the San Jacinto fault joins the San Andreas, the block is broken into a mosaic of irregular fault blocks of crushed basement rock pushed up through upper Tertiary strata (Noble, 1953).
The surface of the western part of the Pinyon block is covered by older alluvium, dissected deeply enough to expose the Punchbowl formation along Pallett Creek and other streams that flow north from the San Gabriel Mountains. East of Pallett Creek the older alluvium rests directly on the erosion surface that bevels the upturned Punchbowl strata. West of Pallett Creek the alluvium overlies the Harold formation, which rests upon this erosion surface.
Martinez trough. - The older Tertiary formation, the Martinez formation of Paleocene age, is assumed to have been deposited in a marine embayment or estuary that occupied an elongate structural trough between and overlapping the San Andreas and San Jacinto faults. The lower member of the formation is in fault contact with the upper member, which was itself eroded before the deposition of the Punchbowl formation, so the total thickness is unknown, but it probably exceeds 6,00 feet.
Post-Martinez orogeny. - Prior to the deposition of the upper Miocene Punchbowl formation the Martinez was complexly folded and cut by a major thrust fault, the Pinyon fault, which was itself folded.
In the southeastern part of the quadrangle, an anticline consisting of the upper member of the Martinez formation trends west-northwest, approximately parallel to the San Jacinto and San Andreas faults. This anticline is truncated by the Pinyon thrust fault, which carries the pre-Tertiary Pinyon Ridge granodiorite an overlying lower member of the Martinez formation above the folded upper member. The Pinyon fault truncates the folds of the lower member of the Martinez but is itself folded along the same general trend. Beneath the southern segment of this thrust, small folds, in part overturned to the north, suggest relative northward movement of the upper plate. The stratigraphic displacement is unknown, as the upper and lower members of the Martinez are nowhere in normal contact, but outcrop of the thrust fault implies movement of at least 2 miles, suggesting the possibility of a fairly wide trough of original Martinez deposition. The Fenner fault, in the extreme southeast corner of the mapped area, brings Pelona schist the western end of a larger area of outcrop) against lower Martinez strata of the upper plate of the Pinyon thrust (fig. 3).
The Pinyon thrust fault, the Fenner fault, and the folded Martinez strata area overlain by the upper Miocene Punchbowl formation. The orogeny can, therefore, be dated as post-Paleocene and pre-upper Miocene and may have been contemporaneous with the break in the eastern part of the Ventura basin between the Vasquez formation (Oligocene to lower Miocene) and the overlying lower Miocene Tick Canyon formation of Jahns (Jahns, 1940; also, personal communication).
Punchbowl trough. - Like the Martinez, the formation was deposited in a trough parallel to the trend of the San Andreas and San Jacinto faults. The sediments, however, are all of continental origin, in large part derived from high ground to the south, and are largely alluvial fan deposits. The presence of fragments of Pelona schist implies that northwest-flowing streams also had access to the basin, and some increment from the northwest is shown by the presence of pebbles derived from the Vasquez formation. Where pre-existent Faultline scarps formed the northeast boundary of the basin the Martinez formation furnished a megabreccia of huge talus blocks to the basal part of the Punchbowl. Elsewhere the coarse material from the south lapped against the Martinez without substantial addition of material derived from the Martinez.
The northeast side of the Punchbowl trough is exposed in cross-section on Sandrock Creek, where the Punchbowl formation lies in depositional contact with Martinez rocks that form a steep slope toward the trough. The slope represents a pre-Punchbowl composite Faultline scarp trending subparallel with the trough. A red megabreccia several hundred feet thick lies at the base of the Punchbowl formation; it is a jumble of huge sandstone slabs of the Martinez formation, some as much as 200 feet long, lying in a matrix of red sand and smaller angular fragments of Martinez. Two east-trending faults in the Martinez strata forming the ancient Faultline scarp are overlapped by the megabreccia, from which it is inferred that at the time of Punchbowl deposition they formed scarps which, in wearing back, yielded the material of the megabreccia. The faults now stand nearly vertical, but prior to the folding of the Punchbowl formation, they may have dipped about 50º toward the Punchbowl trough.
Apparently, an ancient Faultline scarp also existed on the southwest side of the Punchbowl trough. At upper Punchbowl Canyon (sec. B-B’ and map) a narrow faulted wedge of basal conglomerate of the Punchbowl formation dipping steeply north lies in a belt of crushed basement rocks that borders the San Jacinto fault on the north. The conglomerate of the Punchbowl contains, along with rounded cobbles of other basement rocks, angular fragments of these crushed basement rocks, presumably derived from a scarp formed along the San Jacinto fault.
Post-Miocene orogeny. - The Punchbowl formation is folded into a broad syncline whose southern limb is truncated by the Punchbowl reverse fault, which has caused local overturning along the southern border of the quadrangle. The area of outcrop, which is 2 miles wide at the west border, is only a small strip in the eastern part, where it is very tightly compressed and the structure exceedingly complex. The Martinez of the upper plate of the Pinyon thrust is complexly folded, in general roughly parallel to the trend of the San Andreas fault, but the structure could not be worked out in detail as distinctive marker beds are lacking in the monotonous succession of sandstones, shales, and conglomerates. Near the contact with the Punchbowl formation the strata dip steeply to the south and southwest and the structure in this area appears to be in part the result of later folding, which also involved the Punchbowl formation (sec. B-B’).
Many small faults of pre-Harold age cut the Punchbowl formation. Displacement on these rarely exceeds 100 feet and is generally much less. In the trough these trend about N. 65º W., parallel to the axis of the syncline, but on the northern limb, the general trend is north of east, almost normal to the contact. These are not traceable far into the Martinez formation and are assumed to be associated with the folding of the syncline.
The small faults along the northern flank of Pinyon Ridge are clearly subsidiary to the San Andreas fault. Locally, however, there was faulting with a westerly trend prior to Punchbowl deposition.
The northwest corner of the Pinyon block is underlain by Pinyon Ridge granodiorite faulted against the Martinez formation along the west-trending Holmes fault. The latest movement on this fault may have been comparatively recent since its inferred extension west of Pallett Creek cuts off the older alluvium, but the major movement may be older. The dip of the western part of the fault is steep to the north, but east of the crest of the ridge the dip is to the south. Probably, it was originally a steeply dipping reverse fault, and the southerly dip of the eastern part is the result of distortion by movement on the San Andreas fault.
SAN JACINTO AND FENNER FAULT ZONES
The San Jacinto fault zone consists of a belt of crushed basement rocks bordered on the south by the San Jacinto fault and to the north by the Punchbowl fault. In the southeast corner of the area mapped, the Fenner fault is accompanied by a similar crush zone; both crush zones are overlapped by the Punchbowl formation, and the Fenner fault is believed to join the San Jacinto beneath the cover of Punchbowl rocks. Both faults are therefore pre-upper Miocene. The Punchbowl fault and some faults in the crush zone are post-upper Miocene.
Punchbowl fault. – The Punchbowl fault is a steep reverse fault upon which the pre-Tertiary basement rocks of the San Jacinto fault zone and of the San Gabriel Mountain block have overridden the Tertiary rocks in the Punchbowl syncline. The dip of the fault at the east border of the quadrangle is 50º S.; it steepens gradually northwestward across the quadrangle and becomes nearly vertical at the western border. The trace of the fault bears N. 65º W., parallel with the San Andreas fault. At most places, there is a layer of clay gouge a few inches to a few feet thick along the fault.
San Jacinto fault. – The San Jacinto fault is concealed everywhere by talus so that its attitude is uncertain; but at one place on the South Fork of Big Rock Creek, it appears to dip 55º SW. Southeast of the Valyermo quadrangle the San Jacinto fault with an accompanying narrow belt of crushed rock (Noble, 1932, p. 361) runs nearly parallel with the San Andreas fault to the San Bernardino Valley (figs. 1 and 3). Northwest of the quadrangle the San Jacinto fault zone curves gradually toward the San Andreas, and meets it in the confused area of granite blocks pushed up through Tertiary strata near the southeast corner of the Pearland quadrangle (Noble, 1953).
Fenner fault. - In the eastern part of the area mapped the Fenner fault brings the Pelona schist into contact with the lower member of the Martinez formation. Westward it is overlain by the Punchbowl formation and alluvium, but it is assumed to meet the San Jacinto fault beneath the Punchbowl formation between the forks of Big Rock Creek, near the point where the crush zone along the San Jacinto fault begins to widen. East of the area mapped (fig. 3) the Fenner fault trends eastward to the San Andreas. The dip is not directly determinable, but the trace of the fault plane in canyon spurs east of the area mapped indicates a steep southerly dip.
Crush zones. - Both the San Jacinto and the Fenner fault are bordered by zones of intensely crushed Pinyon ridge granodiorite and other rocks of the basement complex. These are similar to the wedge of crushed Holcomb quartz monzonite of the San Andreas fault zone but were formed prior to deposition of the Punchbowl formation. The crush zone bordering the San Jacinto fault is as much as 1,500 feet wide in the western and central parts of the quadrangle, narrowing to a few hundred feet in the eastern part beyond the projected junction with the Fenner fault. The belt may represent an injection zone active in Mesozoic times along an earlier fault zone in the basement rocks. Later fault movements have broken the injected igneous material along with the earlier rocks.
The rocks in the crush zone are more easily eroded than those of the mountain mass south of it, so that the crushed zone forms a gently sloping bench at the base of the steep mountain slope.
Later movements. - Post-Miocene faulting along the San Jacinto fault zone is apparent, not only from the Punchbowl fault but from the infaulted wedge of basal conglomerate of the Punchbowl formation at Punchbowl Canyon. Movement along the Punchbowl fault has also been renewed in recent time; between Holcomb Canyon and the west border of the quadrangle the older alluvium overlying the Punchbowl fault has been dislocated at several places, but the displacements are small, none of the exceeding 50 feet vertically.
No evidence of post-Miocene movement has been observed along the Fenner fault, but a later fault is assumed beneath the alluvium north of the outcrop of the Punchbowl formation north of the fault; for the conglomerate of the Punchbowl here consists of fragments of the underlying brecciated granitic rocks, Pelona schist, and Martinez formation without admixture of material derived from the Pinyon Ridge granodiorite.
SAN GABRIEL MOUNTAINS
The rocks of the San Gabriel Mountains, designated the Pleasant View complex, have not been studied in detail and are not subdivided. They include deep-seated igneous rocks intruded into older rocks of both igneous and sedimentary origin. In the Valyermo quadrangle the prevailing rocks are dark-gray hornblende-quartz diorite, locally gneissic, and metadiorite. The complex is cut by more faults than are shown on the map and the rocks are shattered along the faults, but nowhere are they as broken up as the rocks int eh crush zone along the San Jacinto fault.
LATERAL DISPLACEMENT ALONG THE SAN ANDREAS FAULT
In the following description, the writer offers quantitative evidence for post-Miocene lateral movement of at least 30 miles on the segment of the San Andreas fault extending from the western border of the Pearland quadrangle (long. 118º06’) to Cajon Creek (long. 117º26’), a distance of 42 miles (fig. 3). Discussion is confined to the San Andreas fault; possible additional lateral movement on subparallel faults is not taken into account.
Because the record of movements on the San Andreas fault is fairly plain in the youngest formation and becomes progressively more obscure in older formations, this account begins with the latest events and proceeds to the earliest, the reverse of the usual historical order.
RECENT DISPLACEMENT IN VALYERMO QUADRANGLE
Almost everywhere along the San Andreas fault, the younger alluvium is deformed into low ridges, small sinks, and discontinuous scarps. At many places in the San Andreas fault zone it is warped into low anticlinal domes and undermined shallow depressions. Elsewhere in the quadrangle it is undisturbed.
Just east of the Pearblossom Highway bridge over Big Rock Creek, a recent uplift of the creek gravel has produced a scarp facing upstream; this scarp probably was formed during the Fort Tejon earthquake of 1857. Before the earthquake, Big Rock Creek crossed the San Andreas fault 600 feet east of its present channel and flowed north through Mountain Brook Ranch; the old channel is still traceable. It had been shifted to that position by lateral movements on the San Andreas fault. Prior to the Fort Tejon earthquake, the land south of the fault had moved relatively 600 feet northwest. The 1857 uplift across the channel diverted the stream back to a course in line with its original channel on the southwest side of the fault.
Where the San Andreas fault crosses Pallet Creek a recent uplift of the younger alluvium has produced a scarp facing downstream. This scarp was probably also formed during the 1857 earthquake. As a result of the uplift, Pallett Creek is rapidly deepening its channel and dissecting a peat deposit that had accumulated in recent time in a depression south of the San Andreas fault ridge.
Although the San Andreas fault dislocates the younger alluvium in the San Andreas fault trench, the trench itself is an older feature, for the alluviu7m was deposited in the trench after the trench was formed. The San Andreas fault ridge is also an older feature, for parts of the ridge are buried under younger alluvium. Yet the ridge is younger than the older alluvium, because at places the older alluvium is involved in the faulting that formed the ridge. Clearly the trench and the ridge are the results of recurrent movements.
The older alluvium is cut by many faults in the San Andreas fault zone. All these faults are expressed topographically and are readily traceable on air photographs; but their scarps are degraded, and most of the fault trenches are floored with younger alluvium. On many faults the displacement of the surface is reversed abruptly from place to place along the trace, indicating that the faults have a dominant horizontal component.
The three gaps in Holcomb Ridge represent stream valleys whose upper courses have been offset by the San Andreas fault. The easternmost – Bob’s Gap – is the offset valley of Big Rock Creek; the middle gap, now occupied by the combined stream courses of Big Rock Creek and Pallett Creek, is the offset valley of Pallett Creek; and the westernmost gap, unnamed, is the offset valley of an unnamed large stream course on the west border of the Valyermo quadrangle. Each of these offsets indicates a horizontal, or strike-slip movement of more than a mile on the San Andreas fault, the land on the southwest side of the fault having moved relatively northwest. They are the same order of magnitude as the offsets of Littlerock Creek and other large stream courses in the Pearland quadrangle (Noble, 1953) and of Cajon Creek in the Hesperia quadrangle (Noble 1932, p. 357; 1933, p. 17).
The fact that Big Rock Creek now flows northwest in the San Andreas fault zone and passes through the middle gap in Holcomb Ridge is seemingly anomalous. Actually, the stream could not long have maintained its course through Bob’s Gap against the rising scarps of the recent Hidden Springs and Holcomb faults and against the strong northwesterly tilt of the San Andreas fault trough on the flank of the San Gabriel Mountain arch already described.
The area of older alluvium north of the San Andreas fault between Pallett Creek and the Valyermo Ranch has no counterpart directly opposite on the south side of the fault, but it is lithologically identical with the older alluvium south of the fault near the west border of the quadrangle. In both areas the gravel consists chiefly of cobbles from the Pleasant View complex, without admixture of material from the nearby Pinyon granodiorite. The existing relation indicates a horizontal offset of the older alluvium of 1 to 2 miles.
POST-HAROLD DISPLACEMENT IN VALYERMO QUADRANGLE
Toward the end of Harold deposition, uplift of the broad arch in the basement rocks of the northern San Gabriel Mountains caused the mountains to shed their debris northward in coarse alluvial-fan deposits (the Shoemaker gravel) all along the steepening northern flank of the mountains from the Valyermo quadrangle eastward to Cajon Pass, where they form the infacing bluffs at the summit of the pass (Noble 1926, p. 419; 1933, p. 18, 20). A check of precise levels run northward across Cajon Pass (fig. 3) from the San Andreas fault in 1906, 1924, and 1944 shows that this segment of the arch is still rising, apparently at the rate of 20 inches per century, (Gilluly, 1949, p. 562 - 565). Some erosion may have intervened before the deposition of the Shoemaker gravel, but no unconformity can be detected in the Valyermo quadrangle, where the boundary between the Harold formation and the Shoemaker gravel is essentially a change from the finer material of the Harold to the coarser material of the Shoemaker.
Movements after Harold time and before the older alluvium was deposited are indicated by the fact that the Harold formation and the Shoemaker gravel are more deformed than the older alluvium. Both lie flat in some places and dip gently in others, but locally they are considerably warped and folded and in the San Andreas fault ridge are violently disturbed. Both formations are cut by many faults parallel to the San Andreas or branching from it at low angles.
During this period the Harold formation may have been displaced as much as 5 miles by horizontal movements. The Harold rocks exposed north of the San Andreas fault in the western third of the Valyermo quadrangle are lithologically similar to Harold rocks exposed south of the fault 2 to 5 miles west of this quadrangle – a displacement similar to that of the same formation in the Pearland quadrangle (Noble, 1953).
Several lines of evidence, none in itself conclusive, but all pointing in the same direction, indicate that the total horizontal displacement on the San Andreas fault was of great magnitude. No accurate estimate is possible, but the amount of displacement since late Miocene time may have been of the order of 30 miles. (Current (2019) estimates of total displacement along the San Andreas are between 350 and 500 miles.) This estimate is considerably below that given by Hill and Dibblee (1953, p. 447 - 448) for movement on the northern part of the fault during this period. It should be noted, however, that the figure given here applies only to the San Andreas fault itself. If the speculation offered by these authors (p. 453), that the San Gabriel and San Jacinto faults may be ancestral portions of the San Andreas, can be proved to be correct, it is possible that the aggregate movement on these faults, could it ever be worked out, would bring the estimates more nearly in accord. Movement on the Fenner fault, a third possible “ancestor,” ceased before the Punchbowl formation was deposited.
Displacement indicated by Anaverde formation. – Within and northwest of the area shown in figure 3, all exposures of the lower to middle Pliocene Anaverde formation lie north of the San Andreas fault. In the Pearland quadrangle, at a point 3 miles west of Littlerock Creek, the formation lies unconformably upon the Holcomb quartz monzonite. The constituent materials of the formation are almost entirely quartz monzonite without admixture of material from the south side of the San Andreas fault, whereas those of the juxtaposed Punchbowl formation on the south side of the San Andreas fault are derived exclusively from rocks that crop out on that side of the fault. The Anaverde formation, which is younger than the Punchbowl, should contain material from the Punchbowl formation or from the rocks south of the San Andreas fault zone if it had been deposited in the position where it now lies, for cobbles weather out easily from the Punchbowl formation and are incorporated in great abundance in the Pleistocene Harold formation wherever it overlies the Punchbowl.
The evidence suggests that at the time the Anaverde formation was deposited, rocks other than those now adjacent were present on the south side of the fault. Thirty-five miles northwest of the Pearland quadrangle, Crowell’s Liegra quartz monzonite, probably the equivalent of the Holcomb quartz monzonite from which the Anaverde was derived, crops out south of the San Andreas fault (Crowell, 1952a, p. 11).
Displacement of Punchbowl formation. – In the Valyermo quadrangle the strata of the Punchbowl formation north of the San Andreas fault do not match lithologically those of the same formation exposed in the Punchbowl trough directly opposite them to the south; but some of them match beds in the facies of the Punchbowl formation exposed south of the fault from a point 2 miles west of the quadrangle to the Pearland quadrangle (fig. 3). As interpreted by the writer, these relations indicate that the two facies of Punchbowl rocks have reached their present juxtaposition by horizontal movements along the fault.
A combination of geologic features north of the San Andreas fault within an area several miles square bordering Cajon Creek, corresponds remarkably with a combination of geologic features south of the fault within and west of the Valyermo quadrangle (Noble, 1926, p. 420). On both sides of Cajon Creek, just north of the San Andreas fault, about 400 feet of marine beds of littoral origin lithologically similar to and correlated with the Paleocene Martinez formation of the Valyermo quadrangle overlie basement rock similar to the Pinyon Ridge granodiorite of the Valyermo quadrangle. The Martinez beds at Cajon Creek are complexly folded and faulted and are overlain unconformably by less complexly folded and faulted upper Miocene strata similar in lithology to the lower member of the upper Miocene Punchbowl formation of the Valyermo quadrangle. Both at Cajon Creek and in the Valyermo quadrangle these beds contain vertebrate remains which, according to Chester Stock (letter dated Sept. 18, 1950), indicate a stage of the upper Miocene near that of the Barstow formation, with a possibility that the fauna of the Valyermo quadrangle may be slightly younger than that of the beds of Cajon Creek. The beds in the Valyermo quadrangle and those at Cajon Creek both contain cobbles of volcanic rocks of the Vasquez formation, the only apparent source of which lies 4 to 10 miles west of the Valyermo quadrangle and south of the San Andreas fault. This offset of cobble-bearing beds from the source area of the cobbles is similar to that described by Crowell (1952b) as indicating a large lateral displacement on the San Gabriel fault.
Beds of limestone are rare in the Punchbowl formation. Three miles southwest of Cajon Pass a bed of algal limestone crops out on the north side of the San Andreas fault. A similar bed crops out south of the fault about 4 miles west of the Valyermo quadrangle. (fig. 3).
Two miles southwest of Cajon Pass, north of the San Andreas fault, a thin seam of lignitic material that has been prospected for coal is interbedded with yellowish and buff sandstone and dark shale in the upper part of the Punchbowl formation. A similar seam also interbedded with yellowish and buff sandstone and dark shale crops out in the upper part of the Punchbowl formation south of the San Andreas fault, just west of the Valyermo quadrangle; it has also been prospected for coal. (fig. 3).
The relations just described indicate that the upper Miocene rocks north of the San Andreas fault near Cajon Creek are displaced at least 30 miles relative to those on the south side of the fault (Noble, 1926, p. 420).
Displacement of Punchbowl fault. – It seems possible that the Punchbowl fault has also been offset 30 miles or more by the San Andreas fault. At a point 4 miles west of Cajon Creek the Cajon Valley fault (Noble, 1933, pl. 3) diverges northwestward from the San Andreas fault for several miles (fig. 3). The Cajon Valley fault closely resembles the Punchbowl fault: It is a reverse fault, dipping southwest and trending northwest; and, as on the Punchbowl fault, shattered basement rocks injected by quartz monzonite are faulted against the folded Punchbowl formation.
The faults and folds in the Martinez rocks record a major disturbance that took place before the Punchbowl formation was laid down. Although the relation of these movements to the San Andreas structure is a matter of conjecture, the alignment of the faults and folds of the Martinez and that of the structural trough of the Punchbowl formation indicate that these structures are closely related to movements on the San Andreas fault seems probably that the ancient marine estuary in which the Martinez rocks were deposited also coincides with this trough, implying that it too was structural in origin and related to movements on the fault. If this interpretation is correct, the San Andreas fault or its ancestral equivalent was in existence as a major structural feature at least as early as the beginning of Tertiary time.
The difference in the pre-Tertiary basement rocks on opposite sides of the San Andreas fault in the Valyermo quadrangle, and throughout the 50-mile segment studied by the writer (Noble, 1926, p. 420), suggests that horizontal movements greater than those already described may have taken place on the fault in the early Tertiary or even in pre-Tertiary time. It is even conceivable (Noble, 1932, p. 356 - 357) that horizontal movements on the San Andreas fault totaling more than 50 miles have pulled the similar rock masses of the San Gabriel and San Bernardino Mountains apart (fig. 1), but if evidence of this movement is to be forthcoming it must await a detailed study of the basement rocks that border the San Andreas fault.
(Transcriber’s note: Oct. 09, 2019: Noble’s summary in the last paragraph is correct. What he did not know was that the detailed study of the basement rocks that border the San Andreas would find that they were the Pacific and the North American tectonic plates. He was on to something but had to wait for knowledge to catch up with geologic history - a constant situation we find ourselves in today, and will tomorrow. The concept of plate tectonics was not grasped and accepted until the late-1960’s. Thomas Dibblee’s 1953 paper asserting that displacement along the San Andreas amounted to hundreds of miles, mentioned above under Pre-Harold Displacement, was a seminal finding contributing toward the acceptance of plate tectonics. KD)
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GEOLOGY OF THE VALYERMO QUADRANGLE
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