Oak Ridge Fault, Ventura Fold Belt, and the Sisar Decollement, Ventura Basin, California

Oak Ridge fault, Ventura fold belt, and the Sisar decollement, Ventura basin, California

Robert S. Yeats, Gary J. Huftile, F. Bryan Grigsby* Department of Geology, Oregon State University Corvallis, Oregon 97331-5506

ABSTRACT (Yeats, 1977). The Fernando Formation of Pliocene and early Pleistocene The rootless Ventura Avenue, San Miguelito, and Rincon anti- age consists of abyssal-plain turbidite sandstone alternating with and over- clines (Ventura fold belt) in Pliocene-Pleistocene turbidites are fault- lain by hemipelagic mudstone; this is overlain by shallow-marine to non- propagation folds related to south-dipping reverse faults rising from a marine conglomerate and sandstone of the Saugus Formation. On the decollement in Miocene shale. To the east, the Sulphur Mountain anti- basis of amino-acid racemization age estimates near Ventura (Yerkes et al., clinorium overlies and is cut by the Sisar, Big Canyon, and Lion 1987; K. R. Lajoie, 1987, personal commun.), the youngest Saugus is south-dipping thrusts that merge downward into the Sisar decolle- 0.2-0.4 Ma. The Fernando is underlain by the Sisquoc, Monterey, and ment in lower Miocene shale. Shortening of the Miocene and younger Rincon Formations, all dominated by mudstone and shale of much higher sequence is ~3 km greater than that of underlying competent Paleo- ductility than overlying strata or the underlying Paleogene sequence. The gene strata in the Ventura fold belt and ~7 km greater farther east at Paleogene is dominated by sandstone; i.e., the marine Vaqueros Forma- Sulphur Mountain. Cross-section balancing requires that this differ- tion, the underlying nonmarine Sespe Formation, and various marine ence be taken up by the Paleogene sequence at the Oak Ridge fault to formations of Eocene and Cretaceous age. These formations are competent the south. Convergence is northeast to north-northeast on the basis of enough that they are not folded within the central part of the basin, unlike earthquake focal mechanisms, borehole breakouts, and piercing-point the overlying Miocene and younger formations. offset of the South Mountain seaknoll by the Oak Ridge fault. A The Ventura basin (Fig. 1) is flanked on the north by the Red northeast-trending line connecting the west end of Oak Ridge and the Mountain and San Cayetano reverse faults on which microearthquakes east end of the Sisar fault separates an eastern domain where late have been recorded (Lee et al., 1979; Yeats et al., 1987) and on the south Quaternary displacement is taken up entirely on the Oak Ridge fault by the Oak Ridge reverse fault, which has no instrumental or historical and a western domain where displacement is transferred to the Sisar seismicity onshore, but is probably seismogenic (Yeats, in prep.). All three decollement and its overlying rootless folds. This implies that (1) the faults cut through the brittle crust and may flatten downward at the Oak Ridge fault near the coast presents as much seismic risk as it does brittle-ductile transition (Yeats, 1983; Webb and Kanamori, 1985). To farther east, despite negligible near-surface late Quaternary move- evaluate displacement on the decollement in Miocene shale, we pin the ment; (2) ground-rupture hazard is high for the Sisar fault set in the north end of balanced cross sections against the Red Mountain and San upper Ojai Valley; and (3) the decollement itself could produce an Cayetano faults, and we consider separately the displacement on the Oak earthquake analogous to the 1987 Whittier Narrows event in Los Ridge fault. Angeles. The data and structural interpretation are presented in four cross sections extending from the Oak Ridge fault across the fold and thrust belt to the northern end of the basin (Fig. 2); displacements are given in INTRODUCTION Table 1. Much of the late Quaternary convergence across the western Trans- Cross section A-A' (modified from Grigsby, 1986) is constrained by verse Ranges is taken up in the Ventura basin and is characterized by well data north of the Pitas Point fault, but is speculative in the Pitas Point thin-skinned thrusting and decollements at two levels. The deeper level is anticline, where well data are proprietary (except for the report of Way- the brittle-ductile transition in the middle crust, as described by Yeats land et al., 1978). The San Miguelito and Rincon anticlines are related to (1983) and Webb and Kanamori (1985), and the shallower level is near the Padre Juan and C-3 faults, respectively, and the Padre Juan fault loses the base of a ductile sequence of Miocene shale and mudstone separating a separation upward toward its fault tip as it crosses the anticlinal axis. A 1.2 folded, competent sequence of Pliocene-Pleistocene turbidites (Ventura Ma ash bed near horizon 5 is folded in the Rincon anticline, but a sheared fold belt) from unfolded competent Paleogene strata (Yeats, 1983). We unconformity with some of the post-horizon 5 strata missing on the north analyzed these thin-skinned structures by using retrodeformable (bal- flank of the anticline (Grigsby, 1986) suggests that some deformation took anced) cross sections, whereby rock layers are restored to their unde- place during upper Fernando deposition. Well data show that the Padre formed state without loss of bed length or bed thickness (Dahlstrom, 1969; Juan fault and the south flank of the Rincon anticline flatten southward Price, 1981; Namson, 1987). Restoration of displacement on thrusts ramp- (Grigsby, 1986). On the basis of relations in the Venetura Avenue anticline ing upward from the brittle-ductile transition and on structures above the to the east, we suggest that the Vaqueros is not involved in the Rincon fold decollement in ductile Miocene beds permits an estimation of total crustal and is not cut by the Padre Juan and C-3 faults. Comparison of thicknesses shortening across the Ventura basin. For the upper decollement and fold north and south of the Oak Ridge fault shows that all displacement on that belt to be fully retrodeformed, the shortening of the Miocene and younger fault occurred prior to the end of Saugus deposition. Similarly, much of the sequence must be taken up by a ramp thrust cutting up from the brittle- movement on the Red Mountain fault occurred during Fernando deposi- ductile transition through the Paleogene and older, sequence. tion (Yeats et al., 1987), bringing indurated Paleogene strata against ductile Miocene rocks so that the Red Mountain fault acted as a backstop and GEOLOGIC SETTING caused the Padre Juan and C-3 faults to ramp up toward the surface. The The central Ventura basin contains an enormously thick succession of Padre Juan fault arched across the crest of the Rincon anticline, and the post-Miocene strata, including the world's thickest Pleistocene sequence younger Javon Canyon fault (JCF in Fig. 2) broke through to the surface (Grigsby, 1986) and displaced Holocene sediments (Sarna-Wojcicki et al., 1987).

•Present address: 706-M Eagle Heights, Madison, Wisconsin 53705. Cross section B-B' extends from the Oak Ridge fault across the

1112 GEOLOGY, v. 16, p. 1112-1116, December 1988

Figure 1. Tectonic map of central Ventura basin with lines of four cross sections shown in Figure 2. Arrows are slip vectors based on earthquake focal mechanisms (Red Mountain fault, San Cayetano fault, 1973 Point Mugu earthquake in western Santa Monica Mountains) and on piercing-point offset of facies boundary (1 Ma) by Oak Ridge fault. Heavy dashed line is domain boundary, west of which there is virtually no late Quaternary near-surface movement on Oak Ridge fault, and east of which there are no interbasin folds and thrusts. U.O.V. = Upper Ojai Valley. T653 locates Ventura Avenue anticline deep test well, projected onto section B-B'.

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Figure 2. Cross sections through central Ventura basin; locations in Figure 1. Heavy lines = faults; light lines = depo- sitional contacts. Biostrati- graphic horizons 5 and 1 in Pico Formation are estimated as 1 and 2 Ma, respectively. No vertical exaggeration. Inset E shows how displacement at brittle-ductile transition (X) is transferred to Oak Ridge fault as ramp, and partitioned at shallow depths to shallow Oak Ridge fault (Y) and Sisar dé- collement (Z).

Qs Saugus Formation (Pleistocene) QTp Fernando Fm. (Pico)(Pliocene-Pleistocene) Trp Fernando Fm. (RepettoKLower Pliocene) Sisquoc Fm. (Upper Miocene-Pliocene) Monterey or Modelo Formation (Miocene) Rincon Shale (Lower Miocene) Vaqueros Sandstone (Oligocene) Sespe Formation (Eocene-Oligocene) Coldwater Sandstone (Eocene) Cozy Dell Shale (Eocene) Tma Matilija Sandstone (Eocene) Tj Juncal Formation (Eocene) Te Marine Eocene undifferentiated

° BRITTLE-DUCTILE. TRANSITION ,.¿11 Partitioning of displacement on Oak Ridge fault.

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Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/16/12/1112/3510492/i0091-7613-16-12-1112.pdf?casa_token=_kBljaooCGMAAAAA:0hZt65X7FXyCj17oDC8XqJ86uhFT-CzV1e8XwWxHNxHzi9byZTKJnh5pdwmJ2RUp91jXNows by California Geological Survey, 19774 on 09 November 2019 same fades as Repetto of the Santa Clara syncline, and it shows no Eocene Cozy Dell Shale that is a frontal thrust to the San Cayetano fault evidence of local derivation; therefore, the anticlinorium formed after the (Huftile, 1988). Repetto was deposited. The folds are truncated by the Sisar, Big Canyon, East of Santa Paula Creek, the Sisar thrust turns sharply into the San and Lion thrusts, and the Lion thrust cuts Saugus, which rests with angular Cayetano fault in Timber Canyon oil field (Schlueter, 1976), and shorten- unconformity on strata as young as Repetto. There is no evidence that the ing of Pliocene turbidites with respect to the Vaqueros is near zero. Where Pliocene-Pleistocene sequence of the Santa Clara Valley becomes thinner is this shortening taken up? How is the shortening in Pico turbidites toward Sulphur Mountain, but the angular unconformity at the base of the transferred to the competent Paleogene section beneath? Saugus indicates that some of the folding occurred prior to Saugus deposi- The first fault south of the fold and thrust belt that cuts competent tion and after the deposition of the Repetto. Wells in the western part of Paleogene and older rocks is the Oak Ridge fault. At South Mountain, the the upper Ojai Valley show that the Sisar, Big Canyon, and Lion thrusts do post-Saugus slip on the Oak Ridge fault, including associated drag folding, not cut the competent sequence beneath, but instead merge into a decolle- is 2.5 km (Yeats, in prep.). Post-Saugus slip on the Oak Ridge fault ment in Rincon mudstone and shale called the Sisar decollement by Huf- decreases westward to zero at the town of Saticoy, and displacement on tile (1988). We suggest that the Sisar decollement extends westward and this fault from Saticoy west into the Santa Barbara Channel occurred also underlies the Ventura fold belt. during or prior to Saugus deposition. A northeast-trending line parallel to Cross section D-D' extends from Oak Ridge to Timber Canyon oil the slip vector between the town of Saticoy and the east end of Timber field beneath the San Cayetano fault. In contrast to sections farther west, Canyon oil field separates an eastern domain, where all post-Saugus dis- there is no evidence of intrabasin folds and thrusts, and the Oak Ridge placement is on the Oak Ridge fault, and a western domain, where all fault underwent post-Saugus displacement (Yeats, in prep.). The Rincon post-Saugus displacement is on the Sisar thrust set and the rootless Ventura and Monterey Formations extend south to the Oak Ridge fault, because fold belt above it (heavy dashed line, Fig. 1). We explain this by suggesting horses of these formations are found in the fault zone (Rieser, 1976). that post-Saugus displacement was transferred from the Oak Ridge fault to the Sisar decollement. CALCULATION OF DISPLACEMENT Inset E of Figure 2 illustrates how horizontal displacement at the The direction of tectonic transport is not north-south, normal to the brittle-ductile transition (X) is transferred to the ramp where the Oak axes of the Ventura folds: the average slip vector on thrust-fault earth- Ridge fault cuts the competent sequence and is partitioned above the top quakes on the Red Mountain fault beneath Lake Casitas trends N22°E of the competent sequence into displacement on the shallow part of the (Yeats et al., 1987), and a thrust-fault earthquake located on the San Oak Ridge fault (Y) and displacement on the decollement in Miocene Cayetano fault has a slip vector that trends N34°E (Lee et al., 1979; shale (Z). For post-Saugus displacements, Y = 2.5 km in cross section Yerkes and Lee, 1979). Northeast to east-northeast slip vectors were calcu- D-D', and Z = 2.7 km in cross section B-B' and 3.3 km in cross section lated for three deep low-angle thrust events associated with the 1973 Point A-A', assuming that the Ventura fold belt formed in post-Saugus time. Mugu earthquake (Stierman and Ellsworth, 1976; Webb and Kanamori, Shortening across the upper Ojai Valley cannot be used in this calculation 1985). Offset of the northwest shoulder of the South Mountain seaknoll of because the 3.3 km displacement on the Sisar thrust set may have begun Yeats (1965) results in a northeast-trending slip vector for the Oak Ridge during Saugus deposition. These three estimates of displacement are sim- fault near Santa Paula (Yeats, 1976). Maximum principal compressive ilar enough to lead to the conclusion that deep displacement on the Oak stress based on borehole breakouts are oriented northeast-southwest at 80° Ridge fault was largely and perhaps entirely transferred from the Oak to 90° to the San Andreas fault (Zoback et al., 1987; Mount and Suppe, Ridge fault east of the domain boundary to the Sisar decollement west of 1988). the boundary in late Quaternary time. On the San Miguelito and Rincon anticlines and associated faults, the Pre-Saugus displacements are more difficult to evaluate. In cross top of the Repetto, a representative horizon in the deformed turbidite section C-C', post-Repetto displacements include 6.7 km on the Sisar sequence, has been shortened 3.3 km more than the competent Paleogene thrusts and related folds and an additional 4 km on the Oak Ridge fault, sequence (Table 1; Grigsby, 1986, and in prep.). To the east, this horizon is a total of 10.7 km. In cross section B-B', post-Repetto displacement is at shortened at least 2.7 km more than the Paleogene on the Ventura Avenue least 2.7 km on the fold belt and 2.5 km on the Oak Ridge fault, far less anticline and associated faults in the past 0.2—0.4 m.y. There is probably than the displacement farther east. This difference could be accounted for additional shortening on the north limb of the Canada Larga syncline, but by early bedding slip on the Sisar decollement or by an undetected buried without the Red Mountain or San Cayetano fault as a backstop, this western extension of the Sulphur Mountain anticline. Similarly, post- cannot be documented. Repetto displacement on the Oak Ridge fault in cross section D-D' is only Farther east, shortening of 3.4 km on the Sisar imbricate thrusts 6.9 km, suggesting that the Sulphur Mountain anticline mapped farther restores the Sulphur Mountain anticline and adjacent syncline, both of west by Schlueter (1976) in the Timber Canyon field may extend into this which predate thrusting. An additional 3.3 km shortening is calculated by section. unfolding the anticline and syncline because they, too, are rootless with Ikeda (1983) noted that active reverse faults in Japan as well as the respect to the competent Paleogene (Huftile, 1988). Folds underlying the reverse faults at the northern end of the San Fernando Valley ruptured first Saugus of the upper Ojai Valley are not included; these south-verging folds at the range-basin boundary along a fault he called the master boundary involve the Paleogene and may be formed above a decollement in the fault. Younger movement was transferred outward into the basin along a frontal active thrust. In the San Fernando Valley, movement was transferred from the Santa Susana and Hospital faults at the range front to the San Fernando fault in the basin during the 1971 earthquake. Flattening of thrusting is facilitated by the change from high-strength rocks below to TABLE 1. DISPLACEMENT ON OAK RIDGE FAULT AND IN FOLD-THRUST BELT low-strength valley fill above, based on elastic dislocation, photoelastic, Rincon Ventura Upper Ojai Timber Canyon and clay models by Rodgers and Rizer (1981). Near the coast, the Oak A-A' B-B' C-C' D-D' Oak Ridge fault displacement: 0 0 0 2500 Ridge fault would be Ikeda's master boundary fault, and the Sisar de- post-Saugus 1100 2500 3960 6860 collement would be his frontal active thrust. post-Repetto 3300 2700+ 6700 0 Fold-thrust belt shortening* The reason that the Sisar decollement is not found east of Saticoy and Note: Values are in metres. Cross-section locations shown in Figure 2. Timber Canyon may be the increase in strength of the Miocene decolle- ^Relative to Paleogene competent sequence. ment zone because of facies change to interbedded sandstone and shale. In

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Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/16/12/1112/3510492/i0091-7613-16-12-1112.pdf?casa_token=_kBljaooCGMAAAAA:0hZt65X7FXyCj17oDC8XqJ86uhFT-CzV1e8XwWxHNxHzi9byZTKJnh5pdwmJ2RUp91jXNows by California Geological Survey, 19774 on 09 November 2019 the outcrop section north of the San Cayetano fault, the Sisquoc and Price, R.A., 1981, The Cordilleran foreland thrust and fold belt in the southern Monterey Formations change facies to the Towsley and Modelo Forma- Canadian Rocky Mountains, in McClay, K.R., and Price, N.J., eds., Thrust and nappe tectonics: Geological Society of London Special Publication 9, tions, respectively, both of which contain interbedded turbidite sandstone. p. 427-448. The Rincon Formation also acquires sandstone interbeds eastward. Rieser, R.B., 1976, Structural study of the Oak Ridge fault between South Moun- Another factor may be the eastward narrowing of the Santa Clara syncline tain and Wiley Canyon, Ventura County, California [M.S. thesis]: Athens, as the San Cayetano and Oak Ridge faults approach each other. Ohio University, 93 p. Rockwell, T.K., Keller, E.A., Clark, M.N., and Johnson, D.L., 1984, Chronology and rates of faulting of Ventura River terraces, California: Geological Society IMPLICATIONS FOR SEISMIC RISK of America Bulletin, v. 95, p. 1466-1474. Yeats (in prep.) has calculated a slip rate of 6 to 12 mm/yr for the Rodgers, D., and Rizer, W.D., 1981, Deformation and secondary faulting near the Oak Ridge fault at South Mountain for the past 0.2-0.4 m.y. Because the leading edge of a thrust fault, in McClay, K.R., and Price, N.J., eds., Thrust fault is locked, this slip must take place by earthquakes. If each earthquake and nappe tectonics: Geological Society of London Special Publication 9, p. 65-77. produces 3 m slip at seismogenic depths, the recurrence interval on this Sarna-Wojcicki, A.M., Lajoie, K.R., and Yerkes, R.F., 1987, Recurrent Holocene fault is 250-500 yr. Transfer of displacement to the Sisar decollement displacement of the Javon Canyon fault—A comparison of fault-movement would mean that slip on the Oak Ridge fault at seismogenic depths would history with calculated average recurrence intervals: U.S. Geological Survey occur at about the same high rate near the coast and offshore, even though Professional Paper 1339, p. 125-135. the post-Saugus displacement on this fault is near zero above the decolle- Schlueter, J.C., 1976, Geology of the upper Ojai-Timber Canyon area, Ventura ment. The near-surface trace of the fault has become inactive as displace- County, California [M.S. thesis]: Athens, Ohio University, 76 p. Sibson, R.H., 1985, Stopping of earthquake ruptures at dilatational fault jogs: ment is transferred northward onto the Sisar decollement. Ground-rupture Nature, v. 316, p. 248-251. hazard should be as high along the surface trace of the Sisar fault system on Stierman, D.J., and Ellsworth, W.L., 1976, Aftershocks of the February 21, 1973 the southern edge of the upper Ojai Valley as it is on the Oak Ridge fault Point Mugu, California earthquake: Seismological Society of America Bul- from South Mountain eastward. letin, v. 66, p. 1931-1952. Suppe, J., 1985, Principles of structural geology: Englewood Cliffs, New Jersey, We may speculate on the nature of earthquakes to be formed west of Prentice-Hall, 537 p. the displacement-transfer zone. Displacement on an earthquake generated Wayland, R.G., Acuff, A.D., McCulloh, T.M., Raleigh, C.B., Vedder, J.G., and near the base of the seismogenic zone would propagate upward to the Yenne, K.A., 1978, Facts relating to Well No. 5, Lease OCS-P 0234, Pitas decollement, but not farther, because at the time of rupture the decolle- Point unit area, and the earthquake of August 13, 1978, Santa Barbara Chan- nel, California: U.S. Geological Survey Open-File Report 78-906, 59 p. ment would be in a position analogous to a dilatational jog on a strike-slip Webb, T.H., and Kanamori, H., 1985, Earthquake focal mechanisms in the eastern fault (Sibson, 1985). Dilatational jogs have been proposed to stop the Transverse Ranges and San Emigdio Mountains, southern California and evi- propagation of a fault rupture, because in dilating, fluid pressure is reduced dence for a regional decollement: Seismological Society of America Bulletin, and effective stress is increased, moving the stress field away from the v. 75, p. 737-757. Coulomb-Mohr failure envelope. Yeats, R.S., 1965, Pliocene seaknoll at South Mountain, Ventura basin, California: American Association of Petroleum Geologists Bulletin,v. 49, p. 526-546. Later, as fluid pressure reequilibrated, a second earthquake might 1976, Neogene tectonics of the central Ventura basin, California, in Fritsche, propagate on the decollement itself, relieving the stress concentration at the A.E., Ter Best, H., Jr., and Wornardt, W.W., eds., The Neogene symposium: edge of the earlier fault rupture. Even though rock strength is lower at Society of Economic Paleontologists and Mineralogists, Pacific Section, shallower depths, the moment magnitude of such an event might be large p. 19-32. because of the large surface area of the decollement. This earthquake may 1977, High rates of vertical crustal movement near Ventura, California: Science, v. 196, p. 295-298. be analogous to the M 5.9 earthquake that occurred in the Whittier 1981, Deformation of a 1 Ma datum, Ventura basin, California: Menlo Park, Narrows area of the Los Angeles basin on October 1,1987. The preferred California, U.S. Geological Survey, Final report, Contract 14-08-0001-18283, rupture plane of this event is a low-angle, north-dipping thrust that could Modification 3, 17 p. be a frontal decollement thrust related to the Sierra Madre fault system at 1983, Large-scale Quaternary detachments in Ventura basin, southern Califor- the southern edge of the San Gabriel Mountains. nia: Journal of Geophysical Research, v. 88, p. 569-583. Yeats, R.S., Lee, W.H.K., and Yerkes, R.F., 1987, Geology and seismicity of the eastern Red Mountain fault, Ventura County, California: U.S. Geological REFERENCES CITED Survey Professional Paper 1339, p. 161-167. Dahlstrom, C.D.A., 1969, Balanced cross sections: Canadian Journal of Earth Yerkes, R.F., and Lee, W.H.K., 1979, Maps showing faults and fault activity, and Sciences, v. 6, p. 743-757. epicenters, focal depths and focal mechanisms for 1970-1975 earthquakes, Grigsby, F.B., 1986, Quaternary tectonics of the Rincon and San Miguelito oil fields western Transverse Ranges, California: U.S. Geological Survey Miscellaneous area, western Ventura basin, California [M.S. thesis]: Corvallis, Oregon State Field Studies Map MF-1032, 2 sheets, scale 1:250,000. University, 110 p. Yerkes, R.F., Sarna-Wojcicki, A.M., and Lajoie, K.R., 1987, Geology and Quater- Huftile, G.J., 1988, Geology of the Upper Ojai Valley and Chaffee Canyon areas, nary deformation of the Ventura area: U.S. Geological Survey Professional Ventura County, California [M.S. thesis]: Corvallis, Oregon State University, Paper 1339, p. 169-178. 103 p. Zoback, M.D., and 12 others, 1987, New evidence on the state of stress of the San Ikeda, Y., 1983, Thrust-front migration and its mechanism—Evolution ofintraplate Andreas fault system: Science, v. 238, p. 1105-1 111. thrust fault systems: University of Tokyo, Department of Geography Bulletin 15, p. 125-154. ACKNOWLEDGMENTS Lee, W.H.K., Yerkes, R.F., and Simirenko, M., 1979, Recent earthquake activity Funded by U.S. Geological Survey Earthquake Hazard Reduction Program and focal mechanisms in the western Transverse Ranges, California: U.S. Grants 14-08-0001-G1194 and 14-08-0001-G1372 and based on work funded by Geological Survey Circular 799-A, p. 1 26. two earlier grants, 14-08-17730 and 14-08-0001-18283. Huftile was supported by a Mount, V.S., and Suppe, J., 1988, Present-day stress directions in California deter- scholarship from Chevron, Inc., and Grigsby received summer support from Con- mined through borehole breakout analysis [abs.]: American Association of oco, Inc. Jim Schlueter did much of the preliminary subsurface study of the upper Petroleum Geologists Bulletin, v. 72, p. 390. Ojai Valley and the eastern end of the Red Mountain fault. Discussions with Ken Nagle, H.E., and Parker, E.S., 1971, Future oil and gas potential of onshore Ventura Lajoie, Carl Wentworth, Andrei Sarna-Wojcicki, Bob Yerkes, Ed Hall, and the late basin, California: American Association of Petroleum Geologists Memoir 15, Jim Taylor were very useful in working out the geology. I thank Mark Cloos and v. 2, p. 254-296. Thomas Davis for reviewing the manuscript. Namson, J., 1987, Structural transect through the Ventura basin and western Transverse Ranges, in Davis, T., and Namson, J., eds., Geologic transect across the western Transverse Ranges, southern California: Society of Economic Manuscript received February 5, 1988 Paleontologists and Mineralogists, Pacific Section, Volume and Guidebook 48, Revised manuscript received June 20, 1988 p. 29-41. Manuscript accepted August 15, 1988

1116 Printed in U.S.A. GEOLOGY, December 1988