Abstract Thorough and accurate models of modern sand sources to the Santa Monica Basin, offshore southern California, are needed to facilitate the interpretation of sediment supply to the Hueneme–Mugu Fan, the largest submarine fan within the basin. Bathymetry and near-seafloor seismic-reflection isopach mapping of basin fill indicate that dominant sources of sand are canyons/channels that enter the basin from the northwest, which are likely fed via longshore drift by the Santa Clara River and Calleguas Creek. Sand within Calleguas Creek varies in composition across its drainage basin, ranging from more quartzofeldspathic in the northeast to more volcaniclastic in the southwest, ultimately producing sand with a compositional fingerprint distinct from that of the Santa Clara River and Santa Monica Mountains. The abundance of volcanic material and lack of metamorphic grains in downstream Calleguas Creek sand stand in stark contrast to the Santa Clara River’s relatively abundant metamorphic lithic fragments. In addition, Calleguas Creek sand can be further differentiated from sand derived from both the Santa Clara River and Santa Monica Mountains because both of these other sources have much higher proportions of plagioclase. The composition of late Pleistocene (<60 ka) sandy turbidites at Ocean Drilling Program Site 1015 on the distal Hueneme–Mugu submarine fan validates Calleguas Creek’s contribution of sandy sediment to this site in the Santa Monica Basin: out of 14 samples, four samples show compositions similar to the Santa Clara River sand, whereas five are similar to Calleguas Creek sand, and six exhibit mixed compositions. There is no indication of input to this distal environment from the southern Santa Monica Mountains. Trends in sand composition within the Santa Monica Basin can be related to alternating and/or mixing of sediment sources, possibly related to sea-level change, as well as frequency of floods/storms, earthquakes, and other destabilizing processes affecting offshore shelf-to-slope regions.

We describe the structure of the eastern Española Basin and use stratigraphic and stratal attitude data to interpret its tectonic development. This area consists of a west-dipping half graben in the northern Rio Grande rift that includes several intrabasinal grabens, faults, and folds. The Embudo–Santa Clara–Pajarito fault system, a collection of northeast- and north-striking faults in the center of the Española Basin, defines the western boundary of the half graben and was active throughout rifting. Throw rates near the middle of the fault system (i.e., the Santa Clara and north Pajarito faults) and associated hanging-wall tilt rates progressively increased during the middle Miocene. East of Española, hanging-wall tilt rates decreased after 10–12 Ma, coinciding with increased throw rates on the Cañada del Almagre fault. This fault may have temporarily shunted slip from the north Pajarito fault during ca. 8–11 Ma, resulting in lower strain rates on the Santa Clara fault. East of the Embudo–Santa Clara–Pajarito fault system, deformation of the southern Barrancos monocline and the Cañada Ancha graben peaked during the early–middle Miocene and effectively ceased by the late Pliocene. The north-striking Gabeldon faulted monocline lies at the base of the Sangre de Cristo Mountains, where stratal dip relations indicate late Oligocene and Miocene tilting. Shifting of strain toward the Embudo–Santa Clara–Pajarito fault system culminated during the late Pliocene–Quaternary. Collectively, our data suggest that extensional tectonism in the eastern Española Basin increased in the early Miocene and probably peaked between 14–15 Ma and 9–10 Ma, preceding and partly accompanying major volcanism, and decreased in the Plio-Pleistocene.

Abstract The slip rate on the Oak Ridge fault at South Mountain in the densely-populated lower Santa Clara Valley, California is estimated as 6.15 to 12.3 mm/yr since the end of Saugus deposition at 0.4 to 0.2 Ma. Slightly lower slip-rate estimates result from a different projection of the top of the Saugus in the hanging-wall block of the fault. An assumed displacement of 3 m per event gives an average recurrence interval of 250 to 500 yr, with the major uncertainty being the age of the top of the Saugus. Displacement of 2460 m of the top of the Saugus at South Mountain includes piercing-point displacement and distributed displacement from drag folds near the fault; both are the near-surface expression of faulting at potential mainshock depths beneath well control. Slip rates for the nearby San Cayetano and Red Mountain faults are not well constrained because post-Miocene strata are absent in their hanging-wall blocks, but available evidence suggests that their slip rates are about the same as that for the Oak Ridge fault. Earthquake return times measured in hundreds of years have been shown by others for the 1971 San Fernando fault and a small fault near the Red Mountain fault, and comparable return times are suggested for a normal fault in the hanging-wall block of the Oak Ridge fault. A trench on the Harmon alluvial fan near the Ventura County Government Center revealed evidence of cracks filled with sediments from below, suggesting liquefaction during an earthquake. The lower Santa Clara Valley has not had a large, damaging earthquake in 200 years of record-keeping, but one is expected there in the near future. The December 21, 1812 earthquake may have occurred on the offshore continuation of the Oak Ridge or Red Mountain fault.

We used tephrochronology for upper Neogene deposits in the Española Basin and the adjoining Jemez Mountains volcanic field in the Rio Grande rift, northern New Mexico, to correlate key tephra strata in the study area, identify the sources for many of these tephra, and refine the maximum age of an important stratigraphic unit. Electron-microprobe analyses on volcanic glass separated from 146 pumice-fall, ash-fall, and ash-flow tephra units and layers show that they are mainly rhyolites and dacites. Jemez Mountains tephra units range in age from Miocene to Quaternary. From oldest to youngest these are: (1) the Canovas Canyon Rhyolite and the Paliza Canyon Formation of the lower Keres Group (ca. <12.4–7.4 Ma); (2) the Peralta Tuff Member of the Bearhead Rhyolite of the upper Keres Group (ca. 6.96–6.76 Ma); (3) Puye Formation tephra layers (ca. 5.3–1.75 Ma); (4) the informal San Diego Canyon ignimbrites (ca. 1.87–1.84 Ma); (5) the Otowi Member of the Bandelier Tuff, including the basal Guaje Pumice Bed (both ca. 1.68–1.61 Ma); (6) the Cerro Toledo Rhyolite (ca. 1.59–1.22 Ma); (7) the Tshirege Member of the Bandelier Tuff, including the basal Tsankawi Pumice Bed (both ca. 1.25–1.21 Ma); and (8) the El Cajete Member of the Valles Rhyolite (ca. 60–50 ka). The Paliza Canyon volcaniclastic rocks are chemically variable; they range in composition from dacite to dacitic andesite and differ in chemical composition from the younger units. The Bearhead Rhyolite is highly evolved and can be readily distinguished from the younger units. Tuffs in the Puye Formation are dacitic rather than rhyolitic in composition, and their glasses contain significantly higher Fe, Ca, Mg, and Ti, and lower contents of Si, Na, and K. We conclude that the Puye is entirely younger than the Bearhead Rhyolite and that its minimum age is ca. 1.75 Ma. The San Diego Canyon ignimbrites can be distinguished from all members of the overlying Bandelier Tuff on the basis of Fe and Ca. The Cerro Toledo tephra layers are readily distinguishable from the overlying and underlying units of the Bandelier Tuff primarily by lower Fe and Ca contents. The Tshirege and Otowi Members of the Bandelier Tuff are difficult to distinguish from each other on the basis of electron-microprobe analysis of the volcanic glass; the Tshirege Member contains on average more Fe than the Otowi Member. Tephra layers in the Española Basin that correlate to the Lava Creek B ash bed (ca. 640 ka) and the Nomlaki Tuff (Member of the Tuscan and Tehama Formations, ca. 3.3 Ma) indicate how far tephra from these eruptions traveled (the Yellowstone caldera of northwestern Wyoming and the southern Cascade Range of northern California, respectively). Tephra layers of Miocene age (16–10 Ma) sampled from the Tesuque Formation of the Santa Fe Group in the Española Basin correlate to sources associated with the southern Nevada volcanic field (Timber Mountain, Black Mountain, and Oasis Valley calderas) and the Snake River Plain–Yellowstone hot spot track in Idaho and northwestern Wyoming. Correlations of these tephra layers across the Santa Clara fault provide timelines through various stratigraphic sections despite differences in stratigraphy and lithology. We use tephra correlations to constrain the age of the base of the Ojo Caliente Sandstone Member of the Tesuque Formation to 13.5–13.3 Ma.

The trip commences on Lavigia Hill where we gain an overview the physiography of the Transverse Ranges and to consider the framework and petroleum activity of the Santa Barbara Channel. The route proceeds westward from Santa Barbara to active tar seeps and exposures of fractured Monterey Formation at Carpinteria. Then we visit Conoco’s San Miguelito oil field to see the magnificent anticline in the core of the field and to examine turbidite sedimentation. We cross the axis of the fabled Ventura Avenue oil field, drive through Ventura, and turn northeast on to Hwy 126 and follow the Santa Clara River Valley up to Santa Paula. There we visit Texaco’s South Mountain oil field to observe depositional features of part of the Sespe Formation in the anticline. We continue to the Bardsdale oil field to consider the unique structure of the central Ventura basin and to reflect on the early history of its exploration and development. The trip concludes at the Torrey Canyon oilfield where the remarkable geologic structure of the Ventura basin is clearly displayed.