Field excursion: Petroleum trapsand structures along the SanAndreas convergent strike-slipplate boundary, CaliforniaThomas L. Davis and Jay S. Namson

SUMMARY

Fault-related fold models that illustrate the geometry and kine-matic development of petroleum traps and structures are fre-quently used to assist basin exploration and development ofstructurally complex oil fields.Worldwide, several petroleum-richprovinces are situated in convergent strike-slip settings with ad-jacent convergent structures that are commonly petroleum traps.Strain studies andmodeling of these settings are dominated by thewrench fault model, and examples from the San Andreas faultplate boundary and its trapping influence on adjacent large oilfields in California abound (Wilcox et al., 1973). Use of thismodel in petroleum exploration and geologic education is prob-lematic and can lead to poor choices and wasted drilling dollars.Here, we show at three field trip stops that the wrench model andits associated flower structures (Harding, 1976) and palm treestructures (Sylvester and Smith, 1976; Sylvester, 1988) fail toexplain the oil trapping style and structure of the uppermostcrust near the San Andreas fault (Figure 1).

The San Andreas transform fault through much of southernand central California is oblique to the direction of motion be-tweenNorth America and the Pacific plates, and twomodels havebeen used to explain the strain response to the stress field: (1) thewrench model that results from a high shear strength on the SanAndreas fault and (2) strain partitioning along a weak SanAndreasfault that is characterized by pure strike-slip and an adjacent beltof convergent structures that are parallel to subparallel to the SanAndreas fault (Mount and Suppe, 1987; Zoback et al., 1987;Townend and Zoback, 2004). During the field trip, we presentdata and interpretations that support the strain-partitionedmodel that is characterized by a strike-slip San Andreas faultwith no vertical offset and development of an adjacent and

AUTHORS

Thomas L. Davis ~ Thomas L. DavisConsulting Geologist, 212 Lincoln Drive,Ventura, California 93001; tdavis@thomasldavisgeologist.com

Thomas L. Davis is a State of Californiaprofessional geologist and founderanddirectorof thenonprofitGeologicMapsFoundation, Inc.

Jay S. Namson ~ Namson Consulting Inc.,222 Trafalgar Lane, San Clemente,California, 92672; jay@namsonconsulting.com

Jay S. Namson is a structural geologist withexpertise in balanced cross sections and thegeology of tectonically complex areas.

Copyright ©2017. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received January 16, 2017; final acceptance January 18, 2017.DOI:10.1306/011817DIG17040

AAPG Bulletin, v. 101, no. 4 (April 2017), pp. 607–615 607

coeval fold-and-thrust belt with convergent struc-tures that have little or no strike-slip component(Namson and Davis, 1988a, b). Further, we showthat application of geometric and kinematic modelscommonly used in fold-and-thrust belts, for example,fault-bend and fault-propagation folds (Suppe, 1983;Suppe and Medwedeff, 1990), provides a realistic,testable, and economically successful methodologyfor basin exploration and oil-field development in the

convergent petroleum traps of southern and centralCalifornia (Figure 1). A more optimistic view of thisarea’s oil and gas exploration potential is provided bythe fold-and-thrust model, because the larger thrustsheets conceal footwalls with untested subbasinsand structures with known oil source and reser-voir rocks (Davis et al., 1988; Davis, 2015).

Other implications of using the strain-partitionedmodel combined with restorable cross sections

Figure 1. Map of southern and southern-central California showing the three field trip stops, petroleum basins, oil fields, the San Andreasfault (SAF), and many of the regional cross section lines constructed by Namson and Davis since 1988. Oil fields are dominantly trapped byyoung, convergent structures that are the result of late Cenozoic transpression along the SAF plate boundary, which will be demonstrated at thefield trip stops. Stop 1 is at theWheeler Ridge oil field and the convergent San Emigdio Mountains, Stop 2 is along the western big bend segmentof the SAF, and Stop 3 is at the Russell Ranch oil field and the convergent Caliente Range. The Neogene basins of southern California are very oilprolific (with a cumulative production of nearly 20 billion bbl of oil and daily production now at 560,000 BOPD). Much of the oil is sourced fromthe Miocene Monterey Formation, and basin modeling shows that only the deepest parts have recently reached sufficient depths for oilgeneration (Davis et al., 1996). Integration of basin modeling and cross section work shows a very young and active petroleum system withdiscrete generation pods within the deepest parts of the basins and trapping structures formed just before and during oil generation. Fuis et al.(2012) proposed that the SAF is dipping southwest from 55° to 75° through its western big bend segment based on deep geophysical data, andthe southward dip supports the cross section interpretation of Namson and Davis (1988b) that the SAF is dipping southward as shown in Figure2A. However, it is unclear to us at this time if the SAF dips southwest under the Cuyama basin and Carrizo Plain as mapped by Fuis et al. (2012).Cross sections 1–15 are available at www.thomasldavisgeologist.com (Nameson and Davis, 1996), and many are cataloged as NationalEarthquake Hazards Reduction Program–US Geological Survey (USGS) Final Technical Reports, USGS Open File Reports.

608 Traps and Structures Along the San Andreas Plate Boundary, California

Figure 2. (A) Structural transect across the western Transverse Ranges (modified from Namson and Davis, 1988b). Note southward dipof San Andreas fault (SAF) that is required by restoration of the Pleito thrust system. (B) Line-length restoration of late Pliocene throughQuaternary compressive structures along cross section (modified from Namson and Davis, 1988b). Restoration shows late Eocene andOligocene convergence (Ynezian orogeny), Miocene and Pliocene normal faults, and SAF strike-slip offset. The SAF restores to a verticalfault, separating terrain now offset horizontally approximately 100 km (62 mi) since late Pliocene. (C) and (D) Schematic cross sectionsshowing how shortening above the brittle-ductile transition is caused by subduction of the lower crust and lithosphere of the Pacific plateand the shallow part of the plate boundary is translated over the leading edge of the North American plate (modified from Namson andDavis, 1988b). (C) Shows the edge of the North American plate as a vertical buttress to deformation. (D) Shows the leading edge of theNorth American plate as a crustal-scale wedge driven into the Pacific plate. Circled A (away) and T (toward) indicate strike-slip motion of theSAF in and out of plane of section. CCF = Caballo Canyon fault; Fm = Formation; LF = Lion Fault; LMA = Lion Mountain anticline; MTN =Mountain; NFMT = North Frazier Mountain thrust; NT = North Tejon oil field; ORF = Oak Ridge fault; PMT = Pine Mountain thrust; PTS =Pleito thrust system; SCT = San Cayetano thrust (SCT1 and SCT2 are splays); SFMT = South Frazier Mountain thrust; SGF = San Guillermofault; SL = sea level; SMT = South Mountain thrust; SYF = Santa Ynez fault; TT = Tejon thrust; VA = Ventura Avenue anticline; WRA =Wheeler Ridge anticline; WRT = Wheeler Ridge thrust; WWF = White Wolf fault.

DAVIS AND NAMSON 609

610 Traps and Structures Along the San Andreas Plate Boundary, California

Figu

re3.

(A)Crosssection

acrosstheSanEm

igdioMountain

s(Mtns)andsouthernSanJoaquinbasin

show

ingafold-and-thrustinterpretationoflatePliocene

andQuaternary,north-directed

convergence.Thecrosssectionintegratessurfacegeologyandwelldataandshow

sthatthe

thrustfaultsofthePleitothrustsystem(PTS)flattenwith

depthandrampsinthethrustfaultsurfaces

makefault-bendandfault-propagationfolds.Thelong,hangin

g-wallthrustflatsatthe

baseoftheTemblorFormation(Fm)(Tom)and

withintheEocene

formations(Teo)indicatethatthePTS

intersectstheSan

Andreasfault(SAF)atah

ighangle

andshow

thattheSAF

andPTSm

ergeintoasouth-dipping

strike-slipfault(NamsonandDavis,1988b;Fuis,etal.,2012).The

WhiteWolffault

(WWF)isinterpretedtobe

anoldernormalfaultthatformed

theedge

oftheTejondepocenter(deepportion

ofsouthernSanJoaquinbasin)duringtheMiocene

andPliocene.A

normalfault

interpretationispreferred,becauseb

othsidesofWWFw

eresubsidingw

ithrespecttosealevel(SL),with

then

orthernsidesubsidingatafasterrate,asshow

nby

coevalgrow

thstrata.Theshallow

partoftheWWFw

assubsequentlyfolded

intoareversefaultgeometryby

grow

thofafault-propagationfoldassociatedwith

latePliocene

andQuaternarym

ovem

entonthedeeperTejonthrust

(TT).Tv=volcanicandshallow

-levelin

trusiverockso

fthe

Tecuya

andTemblorFormations.(B)

Amigrated

two-dimensionalseismiclineimageacrosstheCuyamabasin,CalienteRange,and

CarrizoPlain

andadjacenttotheSAF.The

wellshow

nanditsupdipredrillarethe

ARCO

1DrakeFederalthatconfirmed

thep

ositio

noftheM

oralesthrust(MT)andthestratigraphyo

ntheseismic

line,andthewell

encounteredthetopofcrystallinebasementneartotaldepth(TD;17,193

ft[5240m]m

easureddepth).The

imageshow

sthatreflectorsfromtheMTflattentowardtheSAF

(located1.2mi[1.9km

]northeastoftheendoftheline),thatthe

CalienteMtnanticline(CMA)andWellRanchsyncline(W

RS)areunderlainby

thethrust,andthatbeneaththeanticline/syncline

pairandthrustisathick

sequence

ofreflectorssuggestiveofaconcealed

basin

whoseexistence

wasconfirmed

bysubsequentdrilling.Tv=Vaqueros

Formation.(C)A

USGeologicalSurvey

(USGS)crosssectionadjacenttoandroughlyp

aralleltotheseismiclinein(B)showsa

steepening-with-depthMoralesfaultthatdipstow

ardtheSAF

(Vedder,1970;Vedderand

Repenning,1975).

Thewell

andseism

icdatadevelopedby

ARCO

show

theadditionalexplorationpotential

oftheCuyamabasin

thatisabsentintheUSGS

interpretation.Wellsshownon

crosssectionare1=

Richfield

OilC

o.1Schaefer,TD=8630

ft(2630m);2=Richfield

OilC

o.C-5Russell,TD=9808

ft(2989m);3=Richfield

OilC

o.C-2Russell,TD=12,981

ft(3957m);4=Richfield

OilC

o.C-1

Russell,TD=2748

ft(838

m);5=C.W.Colgrove5

3-4Sawyer,TD

=1399

ft(426

m);6=Westates2

Graham

etal.,TD=2612

ft(796

m);7=InternationalExplorationCo.1Evelyn,TD

=2452

ft(747

m);8=C.G.LewisOilD

evelo

pmentAssoc.1

Lewis,TD

=2320

ft(707

m);9=TexasC

o.KCLTraver,TD=5999

ft(1828m).Ho

rizontalscaleandverticalscaleareequal.C

S=Cuyama

shelf;CPA=Carrizo

Plainanticline;Kgr=

graniticrocks;KJf=FranciscanAssemblage;KJop

=CoastRange

Ophiolite;Kto-gn

=tonaliteandmafic-richgneiss;M

zgr=

graniticrocks(inclu

des

oldergneissicrocks);M

zms=

metasedimentaryrocks;Mzrs=Rand

Schist;

Qa=Activestreamdeposits;Qf=

Deform

edalluvialdeposits

ofPadrones

Canyon;Q

n=Deform

edalluvial

deposits;Qoa

=Olderalluviu

m;Q

tp=Paso

RoblesForm

ation;Qtr=Oldertalusand

rockslide

deposits;QTu

=Paso

RoblesForm

ation?;Q

Tu+Tes=

Tulare,San

Joaquin,andEtchegoin

(Chanac)Formations;Q

ya=Youngeralluviu

m;RNF

=Rinconada-Nacim

ientofault;Tb

=basaltflow

sand

dikes;Tb1/Tb2/Tb3/Tb4/Tb5=BasaltFlo

ws;Tbc=

Branch

Canyon

Sandsto

ne;Tbs=

Branch

Canyon

Sandsto

neandSantaMargaritaFormation,undifferentiated;Tbw

/Tbw

s=Bitterwater

CreekShale;Tc/Tcu/Tcc/Tc1/Tc2/Tc3/Tc4/Tc5=CalienteFormation;Tcb=Caliente

FormationandBranchCanyon

Sandsto

ne,undifferentiated;Tcy=

TecuyaFormation;Ti=Intrusiveign

eousrock;TKu

=Up

perCretaceousand

lowerTertiarystrata;Tm=MontereyFormation;

Tmo=MoralesFormation;Tm

u=MontereyShale;Tp=Pattiw

ayFormation;Tpc=

QuailC

anyonSandsto

neMem

berofthe

VaquerosFormation;Tpr/Tpr1=PaintedRockSandsto

neMem

berof

theVaqueros

Formation;Tq

=QuatalFormation;Ts/Tsu

=Sim

mler

Formation;Tsa=SaltosShaleMem

berofthe

MontereyFormation;Tsh/Tsc1/Tsc2/Tss=

Soda

Lake

Shale

Mem

berofthe

Vaqueros

Formation;Tsi =

Simmler

Formation;Tsm

=SantaMargaritaFormation;WRT

=WheelerRidgethrust.

DAVIS AND NAMSON 611

include the enhanced understanding of the deeperstructure of the plate boundary and of seismic riskevaluation (Namson and Davis, 1988b). As shownat Stops 1 and 3, large thrust fault systems do notsteepen with depth into the San Andreas fault buthave listric shapes and must intersect the San Andreasfault at a high angle (Figure 2A). In our interpretation,the San Andreas fault and the thrust systems sharea common displacement surface that dips at lowangles and offsets the shallow plate boundary from itsdeeper continuation. In the western big bend segmentof the San Andreas fault, this displacement surface isa south-dipping fault that has accommodated bothright lateral strike-slip on the San Andreas fault andnorth-directed convergence along the Pleito thrustsystem during the late Cenozoic (Namson and Davis,1988b). Fuis et al. (2012), unaware of the work ofNamson and Davis (1988b), proposed that theSan Andreas fault is dipping southwest from 55° to75° along the western big bend segment based onstudies of potential field data, active-source imag-ing, and seismicity. Large strike-slip faults withlow-angle fault surfaces are not prevalent in thegeologic literature but have been observed (Umhoeferet al., 2007).

The western big bend segment of the San Andreasfault is not the cause of north–south shortening, be-cause shortening continues well to the west of thebend. In our interpretation, the north–south shorten-ing in the western Transverse Ranges is driven bydeeper processes in the lithosphere and asthenosphere(Figure 2C, D) and not caused by constraining bendgeometry as proposed by many. Instead, the bendresults from the north–south shortening and de-formation of the fault’s trace, and the shallowest partof the San Andreas fault plate boundary is offset andtranslated north- and northeastward over the leadingedge of the North American plate. A line-lengthrestoration of late Pliocene through Quaternaryconvergent structures across the Transverse Rangesyields 53 km (33 mi) of shortening (Figure 2B) andrequires 53 km (33 mi) of crustal thickening or in-cipient subduction to balance shortening above thedetachment (Figure 2C, D). Incipient subductionseems favored, because a 60-km-thick (37 mi) slab,shown by seismic tomography, extends to approxi-mately 120 km (75 mi) depth and is coincident witha high-velocity anomaly in the western TransverseRanges (Humphreys et al., 1984).

STOPS

Stop 1 (35.005061°N, 118.986575°W) is at theWheeler Ridge oil field (~59 million bbl of oil cu-mulative as of 2009) that is an actively growing an-ticline located along the deformational front of theSan Emigdio Mountains and the southern end of theoil-prolific San Joaquin basin (Figure 1). To the southare the San Emigdio Mountains, a fold-and-thrustbelt that is bounded on the south by the western bigbend segment of the SanAndreas fault. The proximityof the bend to the San Emigdio Mountains and theWheeler Ridge oil-field trend (that includes northTejon, Pleito, and White Wolf fields) provides anexcellent area to evaluate the structural geometry,kinematics, and associated oil-field trapping mecha-nisms adjacent to a large strike-slip fault with asso-ciated convergent structures.

Pliocene and Quaternary uplift of the east–west-trending San Emigdio Mountains across thenorthwest-trending depositional strike of the SanJoaquin basin provides a unique, natural cross sectionof a basin fromnonmarine deposits in the east to deep-marine deposits in the west. Abundant surface ex-posures and well data provide mapping control of thestratigraphic trends and reveal fault-piercing points inthe hanging wall and footwall of the Pleito thrust(“sensu stricto”) that indicate little or no lateral dis-placement despite its nearby proximity to the SanAndreas fault. The lack of lateral offset shows thatthe positive flower-structure model with oblique-slip convergent faults cannot account for the fold-ing and thrusting and uplift of the San EmigdioMountains (Davis et al., 1996). The east–west trendof the fold axes and thrust faults indicates north–south-directed shortening, and the absence oflateral slip allows for north–south cross sections(Figure 3A) to be retrodeformed in their two-dimensional plane (Figure 2B) and dismisses thecriticism that cross sections adjacent to the San Andreasfault cannot be retrodeformed because of strike-slipin and out of the cross section plane. Since the late1980s, construction of oil-field cross sections usingfault-bend and fault-propagation fold models con-strained by matching hanging wall and footwallcut offs and flats has provided a better under-standing of the movement history and geometryof the oil-trapping White Wolf fault and WheelerRidge thrust. The interpretation that the upper

612 Traps and Structures Along the San Andreas Plate Boundary, California

segment of the older White Wolf normal fault hasbeen rotated by deeper and younger folding andoffset by the younger Wheeler Ridge thrust (Daviset al., 1996; Gordon and Gerke, 2009) has assisted inmaking deeper pool discoveries and provided keyinformation about untested traps, including theirpotential to have cross-fault seals as well as the thick-ness of oil column.

Stop 2 (34.864257°N, 119.248282°W) is atApache Saddle, which is located at the head ofSantiago Canyon and within the western big bendsegment of the SanAndreas fault (Figure 1).With theexception of the pressure ridges, a tectonic geo-morphic feature abundant within the fault zone,there is no evidence of vertical displacement alongthis segment of the fault. One of deepest exposures ofthe San Andreas fault zone is at Apache Saddle, anddetailed mapping shows that the internal structureof the larger composite pressure ridges has a geom-etry similar to a positive flower structure (Davis andDuebendorfer, 1987). The southern side of the SanAndreas fault zone is dominated by the ApacheSaddle fault and its upper plate, which steepens withdepth into SantiagoCanyon andmerges with the SanAndreas fault. The steepening of the Apache Saddlefault into the San Andreas fault and the incohesivefault material within the upper plate of the ApacheSaddle fault are the result of material faulted upwardand outward from the fault zone. Along the fault zoneare numerous pressure ridges that have an en echelonmap pattern, and the ridges probably result fromsimple shear during fault offset. These ridges areconfined to the fault zone, consist mostly of in-cohesive fault rock, and have no relationship to themany map-scale anticlines and convergent faults ex-terior to the fault zone, some of which are the largepetroleum traps found along the southwesternmarginof the San Joaquin basin.

Stop 3 (35.000699°N, 119.824675°W) is inthe Cuyama oil basin (Figure 1), near the base ofthe Caliente Range, and along the southwestern edgeof the Russell Ranch oil field (~70 million bbl of oilcumulative produced as of 2009). The range consistsof two large, southwest-vergent anticlines bounded onthe south and southwest by the surface traces of theMorales and Whiterock thrusts, which belong to thenortheast-dipping Morales thrust system. To the north-east and on the other side of the range, out of view butnearby, are theCarrizo Plain and theSanAndreas fault.

The thickest and most stratigraphically complete partof the Cuyama basin is within the Caliente Range andwas thrust southward over the thinner and stratigraphi-cally equivalent units of the “Cuyama shelf,” result-ing in “basin inversion.” A dip seismic line (Figure 3B)shows the position and listric shape of the Moralesfault that is consistent with the drilling results of theAtlantic-Richfield Company (ARCO) 1 Drake Federalwell. The seismic image shows the various splays ofthe Morales thrust system root into a horizontaldetachment at approximately 4 km (2 mi) depthbeneath the surface expression of the Wells Ranchsyncline; the detachment is a footwall flat, and itsgeometry, depth, and line length are required tomatch the hanging wall flat observed beneath theCaliente Range. Overthrusting has concealed a pre-viously unknown and extensive part of the Cuyamaoil basin. The presence of this subthrust basin, withknown source and reservoir units, was suggested byseismic lines and balanced cross sections and provenby subsequent drilling of the ARCO 1 Stone well(Davis et al., 1988). A US Geological Survey crosssection (Figure 3C) adjacent to and subparallel tothe seismic image shown in Figure 3B interprets theMorales thrust to dip steeply under the Caliente Rangeand toward the San Andreas fault, and the structuralstyle is typical of many southern and central Cal-ifornia cross sections that adopt the flower-structuremodel when interpreting convergent fault geometry atdepth. The additional well and seismic data developedbyARCO and combinedwith fault-related foldmodelsand restorable cross sections show the additional ex-ploration potential of the Cuyama basin that is absentin a flower-structure interpretation.

CONCLUSIONS

The North American/Pacific plate boundary hasbeen strain partitioned during the late Cenozoic, withstrike-slip motion taken up along the San Andreasfault and convergence taken up by adjacent fold-and-thrust belts with little or no strike-slip. Fault-related fold models and cross section constraintscommonly used in fold-and-thrust belts providea testable method for exploration and oil-field de-velopment in southern and central California. Thismethod yields a more commercially successful ap-proach to understanding known oil-field traps, and

DAVIS AND NAMSON 613

offer a more optimistic view of southern and centralCalifornia’s oil and gas exploration potential, becausethe larger thrust sheets conceal extensive footwallareas with untested structures. As shown in the oil-prolific southern San Joaquin (Stop 1) and theCuyamabasins (Stop 3), and contrary to the conclusions ofWilcox et al. (1973), Harding (1976), and Sylvester(1988), positive flower structures fail to explain thestructural style and evolution of the basins and thegeometry and kinematics of the numerous petro-leum traps. Pressure ridges that probably result fromsimple shear and resemble positive flower structuresare only found within the narrow San Andreas faultzone, but such ridges are not petroleum traps nor dothey develop into the numerous petroleum trapsadjacent to the San Andreas fault. Our regional in-terpretation of the San Andreas fault plate boundaryalong its western big bend segment requires thata horizontal detachment or a low-angle fault accom-modates both strike-slip and convergent movements(Namson and Davis, 1988b), that the bend is a resultof shortening and is not causing shortening, and thatthe shallow plate boundary is being offset and trans-lated over the leading edge of the North Americanplate (Figure 2C, D).

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DAVIS AND NAMSON 615