Clarence A. Hall

Stratigraphy and Structure of Mesozoic and Cenozoic Rocks, Nipomo Quadrangle, Southern Coast Ranges, California

A composite section of more than 30,000 feet of Mesozoic and Cenozoic rocks is present in the Nipomo quadrangle, San Luis Obispo County, California, ranging from the characteristic but heterogeneous mixture of Jurassic Franciscan rock types to a few remnant patches of upper Pliocene Careaga Formation. Mudstone and conglomerate of the Lower Cretaceous Jollo Formation with Buchia pacifica Jeletzky and Jurassic Franciscan group locally containing B. piochii (?)(Gabb) are unconformably overlain by Upper Cretaceous Carrie Creek Formation (new name). Acila demessa from this Upper Cretaceous formation dates the sequence of sandstone, siltstone, and conglomerate west of the Nacimiento fault as middle Coniacian to uppermost Campanian. The early Tertiary (i.e., Zemorrian to Relizian) marine Vaqueros, Rincon, and Point Sal Formations are thin and discontinuous in most places and display changes in facies from clastic sedimentary rocks to basaltic and andesitic lavas or to tuffs of the Obispo Formation. Middle and upper Miocene rocks of the Monterey (Luisian to upper Mohnian) and Santa Margarita (upper Mohnian to upper Delmontian or “Repettian”?) Formations are characterized by sandy or in places oytser- and echinoid-rich eastern facies and siltstone or silty claystone western facies largely devoid of macro-invertebrate fossils. The Huasna syncline which lies between the East and West Huasna faults and 35 miles west of the San Andreas fault is the dominant structural feature of the area. Many smaller folds are near the bordering faults. Although the major Huasna faults and the Nacimiento fault which occurs in the eastern part of the quadrangle are conspicuous features, evidence is not conclusive as to the history of their movement. Axial traces of folds along the West Huasna fault, different rock types or facies relationships and thicknesses of the Monterey Formation exposed on opposite sides of the fault, and offset contacts within the fault zone suggest strike-slip and probably left slip. The East Huasna fault also has a probable strike-slip component because of the marked differences in stratigraphy and facies on its opposite sides, and drag features suggest right slip. Movement along the Nacimiento fault zone may have begun near the end of or after Upper Cretaceous sedimentation. Owing to its apparently long and complex history, several periods or types of movement may have occurred. The last record of movement in this area is post-late Miocene reverse slip.

Geology and Petrology of the Cambria Felsite, a New Oligocene Formation, West-Central California Coast Ranges

Geologic mapping in the Cambria and Black Mountain areas, western San Luis Obispo County, California, has led to recognition of the two stratigraphically and petrologically distinct extrusive felsic igneous units: (1) the Obispo Formation; and (2) a new unit, named here the Cambria Felsite. The Obispo Formation was deposited above the Rincon Shale and below the Monterey Formation during middle Miocene time. The Cambria Felsite rests with angular unconformity on Franciscan rocks of Jurassic and (or) Cretaceous age, and occurs as clasts in the nonmarine Lospe Formation of Oligocene age. Textural and mineralogic intergradations exist between the Cambria Felsite and the nearby upper Oligocene hypabyssal volcanic necks, plugs, lava domes, and dikes of the Morro Rock–Islay Hill complex; this suggests that the Cambria Felsite represents an effusive equivalent of the Morro Rock–Islay Hill. Cambria tuffs are enriched in alkalis plus silica and are depleted in ferromagnesian constituents relative to samples of the Morro Rock type. Chemical contrasts in the two units probably arose either because of a winnowing of airborne crystals from the glassy shards or because of magma fractionation in the conduits followed by pyroclastic eruption of the upper portions of the melt columns. No obvious correlation exists between the two-stage silicic volcanism, separated by a 10-m.y quiescent interval, and middle Tertiary plate tectonics. However, generation of these igneous rocks was probably a thermal response of the western margin of the continental crust-capped Americas plate to underflow of the Farallon plate, resulting in Morro Rock and Cambria units, followed by a complex encounter with the East Pacific Rise, resulting in the Obispo Formation.

The structural and sedimentary evolution of the Cretaceous North Pyrenean Basin, southern France

During the Cretaceous, the North Pyrenees of southern France suffered complex deformation. The relatively deep marine North Pyrenean Basin formed, a narrow band of rock adjacent to the North Pyrenean fault was metamorphosed, Iherzolite was emplaced, and the Bay of Biscay opened. Compression during the Late Cretaceous and Tertiary Pyrenean orogeny overprinted and partially masked this earlier history. In order to better understand the regional tectonics off the Cretaceous northern Pyrenees and, specifically, the early history of the North Pyrenean Basin, we compiled and then palinspastically restored a geologic map of the western North Pyrenees. Lithofacies, paleotransport, and isopach maps for the early deposits of the basin, and a pre-Albian palinspastic subcrop map show that the basin was an east-west-elongate, fault-bounded trough at least 300 km long and from 40 to 60 km wide, containing a siliciclastic composite sedimentary package as much as 4,700 m thick. Detailed structural and stratigraphic studies of the Massif d9Igountze-Mendibelza, which is located along the southern limit of the North Pyrenean Basin in the western Pyrenees, suggest that its Cretaceous cover was deposited against an active, relatively low-angle listric normal fault. This fault is on strike with the North Pyrenean fault zone, a major long-lived zone of deformation in the central and eastern North Pyrenees.

Salinia revisited: a crystalline nappe sequence lyingabove the Nacimiento fault and dispersed along theSan Andreas fault system, central California

Salinia, as originally defined, is a fault-bounded terrane in westcentral California. As defined, Salinia lies between the Nacimiento fault on the west, and the Northern San Andreas fault (NSAF) and the main trace of the dextral SAF system on the east. This allochthonous terrane was translated from the southern part of the Sierra Nevada batholith and adjacent western Mojave Desert region by Neogene-Quaternary displacement along the SAF system. The Salina crystalline basement formed a westward promontory in the SW Cordilleran Cretaceous batholithic belt, relative to the Sierra Nevada batholith to the north and the Peninsular Ranges batholith to the south, making Salinia batholithic rocks susceptible to capture by the Pacific plate when the San Andreas transform system developed. Proper restoration of offsets on all branches of the San Andreas system is a critical factor in understanding the Salinia problem. When cumulative dextral slip of 171 km (106 mi) along the Hosgri–San Simeon–San Gregorio–Pilarcitos fault zone (S–N), or dextral slip of 200 km (124 mi) along the Hosgri–San Simeon–San Gregorio–Pilarcitos–northern San Andreas fault system, is added to the cumulative dextral slip of 315–322 km (196–200 mi) along the main trace of the SAF north of the San Emigdio–Tehachapi mountains, central California, there is a minimum amount of cumulative dextral slip of 486 km (302 mi) or a maximum amount of cumulative dextral slip of 522 km (324 mi) along the entire SAF system north of the Tehachapi Mountains. When these sums are compared with the offset distance (610–675 km or 379–420 mi) between the batholithic rocks associated with the Navarro structural discontinuity (NSD) in northern California, and those in the ‘tail’ of the southern Sierra Nevada granitic rocks in the San Emigdio–Tehachapi mountains, central California, a minimum deficit of from ∼100 km (∼62 mi) to a maximum deficit of ∼189 km (∼118 mi) is needed to restore the crystalline rocks associated with the NSD with the crystalline terranes within the San Emigdio and Tehachapi mountains – the enigma of Salinia. Two principal geologic models compete to explain the enigma (i.e. the discrepancy between measured dextral slip along traces of the SAF system and the amount of separation between the Sierra Nevada batholithic rocks near Point Arena in northern California and the Mesozoic and older crystalline rocks in the San Emigdio and Tehachapi mountains in southern California). (i) One model proposes pre-Neogene (>23 Ma), Late Cretaceous or Maastrichtian (<ca. 71 Ma) to early Palaeocene or Danian (ca. 66 Ma) sinistral slip of 500–600 km (311–373 mi) along the Nacimiento fault and of the western flank of Salinia from the eastern flank of the Peninsular Ranges (sinistral slip but in the opposite sense to later Neogene (<23 Ma) dextral slip along and within the SAF system. (ii) A second model proposes that the crystalline rocks of Salinia comprise a series of 100 km- (60 mi-) scale allochthonous (extensional) nappes that rode southwestward above the Rand schist–Sierra de Salinas (SdS) shear zone subduction extrusion channels. The allochthonous nappes are from NW–SE: (i) Farallon Islands–Santa Cruz Mountains–Montara Mountain, and adjacent batholithic fragments that appear to have been derived from the top of the deep-level Sierra Nevada batholith of the western San Emigdio–Tehachapi mountains; (ii) the Logan Quarry–Loma Prieta Peak fragments that appear to have been derived from the top of a buried detachment fault that forms the basement surface beneath the Maricopa sub-basin of the southernmost Great Valley; (iii) The Pastoria plate–Gabilan Range massif that appears to have been derived from the top of the deep-level SE Sierra Nevada batholith; and (iv) the Santa Lucia–SdS massif, which appears to be lower batholithic crust and underlying extruded schist that were breached westwards from the central to western Mojave Desert region. In this model, lower crustal batholithic blocks underwent ductile stretching above the extrusion channel schists, while mid- to upper-crustal level rocks rode southwestwards and westwards along trenchward dipping detachment faults. Salinian basement rocks of the Santa Lucia Range and the Big Sur area record the most complete geologic history of the displaced terrane. The oldest rocks consist of screens of Palaeozoic marine metasedimentary rocks (the Sur Series), including biotite gneiss and schist, quartzite, granulite gneiss, granofels, and marble. The Sur Series was intruded during Cretaceous high-flux batholithic magmatism by granodiorite, diorite, quartz diorite, and at deepest levels, charnockitic tonalite. Local nonconformable remnants of Campanian–Maastrichtian marine strata lie on the deep-level Salinia basement, and record deposition in an extensional setting. These Cretaceous strata are correlated with the middle to upper Campanian Pigeon Point (PiP) Formation south of San Francisco. The Upper Cretaceous strata, belonging to the Great Valley Sequence, include clasts of the basement rocks and felsic volcanic clasts that in Late Cretaceous time were brought to a coastal region by streams and rivers from Mesozoic felsic volcanic rocks in the Mojave Desert. The Rand and SdS schists of southern California were underplated beneath the southern Sierra Nevada batholith and the adjacent Salinia-Mojave region along a shallow segment of the subducting Farallon plate during Late Cretaceous time. The subduction trajectory of these schists concluded with an abrupt extrusion phase. During extrusion, the schists were transported to the SW from deep- to shallow-crustal levels as the low-angle subduction megathrust surface was transformed into a mylonitic low-angle normal fault system (i.e. Rand fault and Salinas shear zone). The upper batholithic plate(s) was(ere) partially coupled to the extrusion flow pattern, which resulted in 100 km-scale westward displacements of the upper plate(s). Structural stacking, temporal and metamorphic facies relations suggest that the Nacimiento (subduction megathrust) fault formed beneath the Rand-SdS extrusion channel. Metamorphic and structural relations in lower plate Franciscan rocks beneath the Nacimiento fault suggest a terminal phase of extrusion as well, during which the overlying Salinia underwent extension and subsidence to marine conditions. Westward extrusion of the subduction-underplated rocks and their upper batholithic plates rendered these Salinia rocks susceptible to subsequent capture by the SAF system. Evidence supporting the conclusion that the Nacimiento fault is principally a megathrust includes: (i) shear planes of the Nacimiento fault zone in the westcentral Coast Ranges locally dip NE at low angles. (ii) Klippen and/or faulted klippen are locally present along the trace of the Nacimiento fault zone from the Big Creek–Vicente Creek region south of Point Sur near Monterey, to east of San Simeon near San Luis Obispo in central California. Allochthonous detachment sheets and windows into their underplated schists comprise a composite Salinia terrane. The nappe complex forming the allochthon of Salinia was translated westward and northwestward ∼100 km (∼62 mi) above the Nacimiento megathrust or Franciscan subduction megathrust from SE California between ca. 66 and ca. 61 Ma (i.e. latest Cretaceous–earliest Palaeocene time). Much, or all, of the westward breaching of the Salinia batholithic rocks likely occurred above the extrusion channels of the Rand-SdS schists; following this event, the Franciscan Sur-Obispo terrane was thrust beneath the schists, perhaps during the final stages of extrusion in the upper channel. Later, the Sur-Obispo terrane was partially extruded from beneath the Salinia nappe terrane, during which time the upper plate(s) underwent extension and subsidence to marine conditions. Attenuation of the Salinia nappe sequence during the extrusion of the Franciscan Complex thinned the upper crust, making the upper plates susceptible to erosion from the top of the Franciscan Complex near San Simeon, where it is now exposed. In the San Emigdio Mountains, the relatively thin structural thickness of the upper batholithic plates made them susceptible to late Cenozoic flexural folding and disruption by high-angle dip–slip faults. The ∼100 km (∼62 mi) of westward and northwestward breaching of the Salinia batholithic rocks above the Rand-SdS channels, and the underlying Nacimiento fault followed by ∼510 km (∼320 mi) of dextral slip from ∼23 Ma to Holocene time along the SAF system, allow for the palinspastic restoration of Salinia with the crystalline rocks of the San Emigdio–Tehachapi mountains and the Mojave terrane, resolving the enigma of Salinia.

POTASSIUM-ARGON AGE OF THE OBISPO FORMATION WITH PECTEN LOMPOCENSIS ARNOLD, SOUTHERN COAST RANGES, CALIFORNIA

Fossil pelecypods, Pecten (Amusium) lompocensis Arnold and Pecten (Lyropecten) crassicardo Conrad, were discovered in rhyohtic tuff and breccia of the Obispo Formation, San Luis Obispo County, California. The potassium-argon age of plagioclase obtained from the middle of the tuff is 20.9 ± 1.5 × 10 6 years. The age of plagioclase in the Morro Rock andesite-dacite volcanic neck is 23.5 ± 1.8 × 10 6 years.

Significance of lherzolite at the Etang de Lherz, central Pyrenees, southern France

Abstract Geologic relationships in the central Pyrenees of southern France demonstrate that lherzolite has been emplaced, as a plastic solid, into middle or upper Cretaceous calcareous rocks; that it has been eroded and clastic peridotite debris deposited in rocks of approximately the same age as those it intruded; and that it has also been juxtaposed against Cretaceous limestone or marble along or within the North Pyrenean fault zone. There are at least three types of late Cretaceous lherzolite breccias known from this region. Metamorphism of the country rock and penetrative deformation of the lherzolite and marble took place during shearing. Shearing was accompanied by an important period of motion (perhaps 85–100 m.y. ago) along the North Pyrenean fault and an associated thermal event which involved temperatures of 400 ± 100°C. Cretaceous metamorphism along the North Pyrenean fault zone was not due to forcible primary hot intrusion of lherzolite.