John C. Crowell

Probable Large Lateral Displacement on San Gabriel Fault, Southern California

The San Gabriel fault zone, which trends northwesterly subparallel with the San Andreas fault for about 90 miles, appears to have a post-late Miocene right-lateral displacement of 15-25 miles. Southwest of this nearly vertical fault, 6 miles northwest of Castaic, Los Angeles County, upper Miocene coarse conglomerates grade southwest into finer sediments from their source, now disappeared, across the fault on the northeast. The conglomerates, which contain boulders of anorthosite and norite, were derived from a near-by basement terrane containing these rock types. A buried source is eliminated because at present, so far as known, all basement rock across the San Gabriel fault from the conglomerates for several miles northeast is covered by sedimentary rocks older than the onglomerates. Basement types exposed still farther northeast do not include anorthosite and norite. In fact, the only in situ occurrence of these rock types known in the entire region is in the San Gabriel Mountains 23 miles southeast of the conglomerates and on the other (northeast) side of the San Gabriel fault zone. It thus seems clear that this was their source area and that the conglomerates have been displaced 15-25 miles relatively northwest. Large right-lateral displacement is also suggested by the presence of upper Miocene sedimentary breccia now found northeast of the fault about a mile northwest of Castaic. This breccia, containing blocks of gneiss almost exclusively, has been offset from its source now at least 6 miles but probably 15 miles or more northwest. Again, sediments older than the breccia across the fault from its present exposure preclude noteworthy dip-slip displacement.

Geology of Hungry Valley Area, Southern California

During late Tertiary time a thick section of coarse and fine continental clastics accumulated in Ridge Basin. Deposition was concurrent with movement on the San Gabriel fault zone which delimited Ridge Basin on the southwest. Movement on this fault ceased in late Pliocene time and the younger sediments overlapped southwestward across the fault and onto a pediment cut into the crystalline basement. The low-angle Frazier Mountain thrust moved in the Pleistocene relatively southeastward across the veneer of sediments on the pediment and in part onto the thick Ridge Basin deposits. Subsequently the thrust, veneer of sediments, and pediment were folded and faulted along an east northeast trend whereas the thick sediments in Ridge Basin were folded along trends nearly at right ngles to this and subparallel with the basin margins. At this time the San Gabriel fault zone was reactivated in the southern part of the area.

Problems of Fault Nomenclature

The distinction between separation and slip is fundamental to the proper geometric understanding of fault displacements. Ideally geologists strive to find slip: the relative displacement of formerly adjacent points on opposite sides of a fault. In practice we recognize these points where lines formed by geological elements meet the fault plane at piercing points. Such lines are primarily those formed: (1) by intersecting planes, such as dikes transecting strata; (2) by the trace of one plane against another, as where a bed meets an unconformity; (3) by linear geological features, such as attenuated sand lenses, ore shoots, and stream courses; (4) by stratigraphic lines, such as pinch-out lines, lines of facies changes, and fossil shorelines; (5) by constructed lines, such as isopachs, lithofacies lines, and traces of axial surfaces with bedding. Not always are such data in three-dimensional space available to determine slip. Usually we have only information on displaced planes, such as bedding planes, unconformities, dikes, sills, contacts, etc., and lack recognizable lines lying within these planes. Furthermore, data are commonly given in two dimensions only. More widespread recognition that we must describe the geometry of fault displacements in terms of separation is therefore necessary. Moreover, since separation measures the displacement of traces of displaced planes as shown on a cross section or map, it is as essential to define the orientation of this view as to describe and give the location, amount, and sense of the separation. Where we employ the term apparent in fault definitions, in general we refer to separation Geologists must therefore distinguish habitually between geological situations with displaced lines and those with displaced planes. Our fault practice and terminology fail to draw this distinction sharply, so that faults are inadvertently described incorrectly when slip terms are applied carelessly to separation. A qualified committee of geologists should now examine our fault nomenclature and make recommendations that will dispel ambiguities. Precise definitions would also stimulate the search of geological data for clues to slip that alone can reveal significant facts on deformation kinematics.

Origin of Late Cenozoic Basins in Southern California: ABSTRACT

Several sedimentary basins in southern California, within and south of the Transverse Ranges, display a history suggestive of a rhombochasmic origin. Beginning in the early Miocene, segments of the continental margin at the soft and splintered border between the Pacific and Americas plates were apparently fragmented so that basins originated as irregular pull-aparts. Basin walls were formed by both transform faults and by crustal stretching and dip-slip faulting. Deep basin floors grew as a complex of volcanic rocks and sediments. As basins enlarged, high-standing blocks are pictured as separating laterally from terranes that were originally adjacent. Older rocks exposed around margins therefore cannot be extrapolated to depth within the basins. Support for such a speculative model comes from accumulating understanding of the Salton trough. This narrow graben is being pulled apart obliquely, with faults of the San Andreas system serving as transforms. With widening, the walls sag and stretch, and margins are inundated by sedimentation that occurs simultaneously with deformation and volcanism in the basin floor. The Los Angeles basin apparently started to form as a rhombic hole in the middle Miocene, with basin-floor volcanism accompanied and followed by voluminous sedimentation. The Miocene Topanga basin in the western Santa Monica Mountains contains vast thicknesses of volcanic and sedimentary rocks that were laid down adjacent to high ground, from which sediments and huge detachment slabs were carried into a spreading hol . Other basins that perhaps reveal stages in the history of crustal stretching, culminating in pull-aparts and rhombochasms, are parts of Ventura basin, Ridge basin, and several offshore depressions, including the Santa Barbara Channel. End_of_Article - Last_Page 774------------

Theme: Stratigraphy Guides Structure: (A) Interrelation Between Stratigraphy and Structure (Crowell); (B) Ventura Basin, Example of Theme (Paschall): ABSTRACT

Earth deformation initially delineates basins and, together with climate and provenance, guides the distribution of sediments. In geosynclines and mobile belts the rise and fall of welts and troughs influence the facies sharply. Even in cratonic regions, tectonic control of sedimentation is clear. Crustal deformation also occurs after deposition, and the positionings and geometric details of structures are controlled by the mechanical properties and inhomogeneities of the strata. In such cases stratigraphy clearly has guided structure. In many regions, however, deformation and deposition have occurred together, and an interplay continues intermittently for long periods of time. As a result, deformation guides deposition which in turn guides deformation et cetera. Such an nterrelated continuum regulates the movements of fluids, including oil and gas, within the strata. Modern analysis of basin history requires a careful reconstruction of the interplay between deformation and deposition. The analysis is most effective if one begins with the present and works backward in time, sorting out the geological events and their effects one by one. Knowledge gained recently of modern depositional environments and the geometry and distribution of sedimentary facies within them provides the geologist with reference models of the appearance of his study area in the past. It is not sufficient to visualize static strata as having been deformed suddenly after deposition and lithification. Instead the geologist must find techniques which permit him to reconstruct the panorama of continuous changes not only of the stratigraphy through time, but also the folding, fault ng, and movements of fluids within the strata. (Crowell) The sediments of the Ventura basin are more severely deformed than those of most oil-producing provinces. This circumstance, in combination with the narrow linear aspect of the basin and the abundance of outcrops, yields more conspicuous examples of structural-stratigraphic relations than usually are encountered. The basins early history reveals a characteristic common to all depositional areas, i.e., the manner in which basin and basin-margin structure affected sedimentation. A second feature of basin history that is not so conspicuous elsewhere is the manner in which stratigraphy affected later deformation of the basinal sediments, as well as oil accumulation in them. Major high-angle reverse faults now exist locally along the north and south boundaries of the main Pliocene basin. The very thick (world-record) Pliocene section not only thins toward these faults, but also typically has a notable decrease in permeable sandstone percentage. The fault zones contain fine-grained terrigenous clastic rocks and siliceous shale, which served as a lubricant for fault movements. The fault zones probably End_Page 460------------------------------ also served as intermittent avenues for migration of sub-fault oil into producing structures above the fault. (Paschall) End_of_Article - Last_Page 461------------

Deep-Water Sedimentary Structures, Pliocene Pico Formation, Santa Paula Creek, Ventura Basin, California: ABSTRACT

The Pliocene Pico Formation, according to paleogeographic and paleoecologic interpretations, was deposited in marine waters at least 300 m. deep. Sedimentation of mud, sand, and some gravel was largely the result of bottom-following underflows generally travelling west. Resulting sedimentary structures--some viewed in stilled-stages of development--are: stratification with eroded and deformed contacts, internal stratification, graded bedding, small-scale cross-stratification, disturbed bedding, fossils with preferred orientation, imbricated clasts and shells, ripple marks, flame structures, pull-aparts, load pockets, load waves, and many others. Ideal graded bedding is generally rare, but most sandstones display grading superposed on other structures such as internal lamination. Thin but persistent strata with signature sedimentary structures imply infilling on a nearly horizontal sea floor by bottom-contact currents tending to level the accretional surface. Eroded and deformed contacts at the base of beds imply vigorous current impact and drag. Larger disruptions such as deformed or disturbed zones, several beds thick, may result from current drag rather than from gravity-induced downslope slumping. Accordingly, some penecontemporaneous folds are less reliable indicators End_Page 522------------------------------ of paleoslopes than current-induced structures such as cross-stratification. Underflows, acquiring energy through flow down the trough margin, probably debouched from submarine canyons; many flows were ephemeral but others deposited relatively continuously for longer periods. Most underflows possessed sufficient energy to move a tractional load, and the stronger ones vigorously eroded and disturbed the sea floor. Complexity in form and genesis of the sedimentary structures dictates a comparable complexity in the depositing current. End_of_Article - Last_Page 523------------

Geology of Orocopia Mountains, Southeastern California: ABSTRACT

The Orocopia Mountains border the Salton Sea northeast of the San Andreas fault in Riverside County, California. The range core, composed of Orocopia schist, is separated on the southwest from End_Page 218------------------------------ deformed late Cenozoic non-marine strata by the Hidden Spring fault zone, a branch of the nearby San Andreas. Northeast of the Orocopia schist a mile-wide wedge of gabbro, diorite, anorthosite, gneiss, and alaskite lies between a northeast-dipping fault, at places folded, and a high-angle major fault marked by great crushing. These rocks, intruded by volcanics and highly deformed, resemble rocks in the western San Gabriel Mountains, about 150 miles northwest. Northeast of the high-angle fault, augen gneiss with migmatite on the southeast and granite on the north underlies unconformably about 4,800 feet of newly discovered fossiliferous marine Eocene strata which are probably correlative with Coast Range middle Eocene. Unconformably overlying this sequence is a 5,000-foot thick variable series of undated non-marine conglomerate, sandstone, shale, and tuff, with volcanic flows and intrusions. In this series, lenses of granitic breccia characterize the northwest, and platy tuffaceous sandstone with gypsum-bearing interbeds the southeast. Major faults separate the area into tectonic blocks of different geologic history, and local correlation across the faults is not possible. Understanding of the significance of these faults and others in the vicinity, like the San Andreas, awaits regional study such as that now underway. Strike separations dominate over dip separations on minor faults associated with complex folds. End_of_Article - Last_Page 219------------

Eocene Stratigraphy and Paleontology of Orocopia Mountains, Southeastern California: ABSTRACT

Marine Eocene strata underlie about 26 square miles in the northeastern Orocopia Mountains, Riverside County. The newly discovered section, which totals about 4,800 feet in thickness, lies in a structural trough within basement rocks and is overlain unconformably by about 5,000 feet of undated non-marine clastic and volcanic rocks. The Eocene beds consist of interbedded siltstone, sandstone, and breccia with some sandy limestone and conglomerate. On the east, at the base of the section, large granitic boulders up to 30 feet in diameter lie along the unconformity with granite. These give way upward to thick lenses of coarse granitic breccia with interbeds of buff siltstone and arkosic sandstone. The upper part of the section on the east consists of massive buff siltstone with sandstone and boulder beds. On the west the section consists largely of interbedded siltstone and sandstone with conspicuous isolated boulders of granite. Mollusks and Foraminifera, including orbitoids, occur at many localities throughout the section. Some of the characteristic forms are: Turritella andersoni cf. lawsoni Dickerson, Turritella uvasana cf. applini Hanna, Clavilithes sp., Marginulina mexicana (Cushman) var., Pseudophragmina (Proporocyclina) psila (Woodring) and Pseudophragmina (Proporocyclina) clarki (Cushman). This fauna indicates middle Eocene age, and the strata are possibly correlative with similar rocks of the Coast Ranges. End_of_Article - Last_Page 219------------

Geology of Hungry Valley Area, South of Gorman: ABSTRACT

During late Tertiary time a thick section of coarse and fine continental clastics accumulated in the northwestern part of the Ridge basin. Deposition was concurrent with movement on the San Gabriel fault which bounded Ridge basin on the southwest. Movement on this fault ceased in early Pleistocene (?) time and the younger sediments overlapped southwestward across the fault and onto a pediment cut into the basement. Later in the Pleistocene the low-angle Frazier Mountain thrust moved relatively southeastward across the veneer of sediments on the pediment and in part onto the thick Ridge basin deposits. Subsequently the thrust, veneer of sediments, and pediment were folded and faulted along an east-northeast and west-southwest trend. End_of_Article - Last_Page 2319------------