JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B7, PAGES 14,031-14,050, JULY 10, 1994

Long river profiles, tectonism,and eustasy: A guide to interpreting fluvial terraces

Dorothy J. Merritts GeosciencesDepartment, Franklin and Marshall College, Lancaster, Pennsylvania

Kirk R. Vincent GeosciencesDepartment, University of Arizona,Tucson

Ellen E. Wohl Departmentof EarthResources, Colorado State University, Fort Collins

Abstract. Alongthree rivers at theMendocino triple junction, northern California, strath, cut, andfill terraceshave formed in responseto tectonicand eustatic processes. Detailed surveying andradiometric dating at multiplesites indicate that lower reaches of therivers are dominated by theeffects of oscillatingsea level, primarily aggradation and formation of fill terracesduring sea levelhigh stands, alternating with deep incision during low stands.A eustasy-drivendeposi- tionalwedge extends tens of kilometersupstream on all rivers(tapering to zerothickness). This distanceis greaterthan expected from studies of theeffects of checkdams on muchsmaller streamselsewhere, due in partto thelarge size of theserivers. However,the changein gradient is nearlyidentical to otherbase level rise studies: the depositional gradient is abouthalf thatof theoriginal channel. Middle to upperreaches of eachfiver aredominated by theeffects of long- termuplift, primarily lateral and vertical erosion and formation of steep,unpaired strath terraces exposedonly upstream of thedepositional wedge. Vertical incision at a ratesimilar to thatof uplifthas occurred even during the present sea level high stand along rivers with highest uplift rates. Strathterraces have steeper gradients than the modem channel bed and do notmerge with marineterraces at the river mouth;consequently, they cannotbe usedto determinealtitudes of sealevel high stands. Strath formation is a continuousprocess of responseto long-termuplift, andits occurrencevaries spatially along a river dependingon streampower, and hence position, upstream.Strath terraces are found only along certain parts of a coastalstream: upstream of the aggradationaleffects of oscillatingsea level, and far enoughdownstream that stream power is in excessof thatneeded to transportthe prevailing sediment load. For a givensize fiver, the greaterthe uplift rate, the greaterthe rate of verticalincision and, consequently, the less the like- lihood of strathterrace formation and preservation.

introduction Contemplationof the effectsof the latter,sea level change,began with Suess's[ 1888] postulatethat synchronousoscillations of sea Historical Prelude level were the causeof widespread,correlatable erosional sur- The generalobjective of this work is to achievegreater under- faces,and culminatedin the "greatunifying generalization"that standingof the interplayof long-termbase level changewith dominatedEuropean studies of denudationchronology [Chorley, fluvial processesof incisionand sediment deposition. Base level 1963]. Sincethen, the plate tectonicrevolution and recognition is the altitudinallower limit of the erosivecapacity of a river of the eustaticeffects of repeatedQuaternary glaciation have re- [Powell, 1961; Davis, 1902]; the ultimate base level for all rivers sultedin greaterunderstanding of the dynamic nature of both is a planar surface extendingfrom sea level beneaththe land land and sealevels and provide the impetusfor the followingex- [Mallott, 1928]. Regionalbase level changeis the resultof two aminationof fluvialresponse to simultaneoustect6nic and eu- processes:tectonic upheaval or subsidenceof the land surface, staticprocesses. andeustatic rise or fall of sealevel. Contemplationof the effects of the former, crustalinstability, led to cyclic schemesof ero- Base Level Change and Fluvial Terraces sionalhistory of landscapeswhich predominated early twentieth centurygeomorphic research [e.g., Davis, 1954; Johnson,1931]. River terraces are landforms that were at one time constructed and maintained as the active floor of a river but are now aban- Copyright1994 by the AmericanGeophysical Union. doned.As such,they can be usedto deducethe timing and ex- ternalcause of abandonment:base level change, climate change, Papernumber 94JB00857. or tectonicactivity. A river'sbed andits floodplaincan be used 0148-0227/94/94JB-00857 $05.00 to representthe longitudinalprofile (verticalsection) of the cur-

14,031 14,032 MERRITTS ET AL.: LONG RIVER PROFILES, TECTONISM, AND EUSTASY

rently active stream. The long profile of a river generally de- incisionof the bedrockfloor of the channelin responseto local creasesin gradient downstreamas a function of increasingdis- baselevel changeis concurrentwith upstreampropagation of a chargeof the fiver [Bagnold, 1977] and tangentiallyapproaches knickpointthat retreats in parallelfashion, leaving in its wakean sea level in a coastal fiver [Bloom, 1991]. In an alluvial fiver, abandonedchannel floor mantledby a thin veneerof sediments channelboundaries are composedof sedimentthat can be trans- [Seidland Dietrich, 1992]. As in thecase of the alluvialchannel, portedby the fiver. In a bedrockchannel, the boundariesare cut this surfaceis alsoa terracebut is referredto specificallyas a into rock. Both types, with exceptionof mountainousareas of strathterrace (Figure lb) [Bucher,1932; Leopold et al., 1964]. very steeprelief, form floodplainsin responseto continuously For both alluvial and bedrockchannels, if episodicvertical inci- variable discharge. sionoccurs, paired terraces are formed;if lateralerosion as well Alluvial channelsare very sensitiveto active tectonics[e.g., as continuousvertical incisionoccurs, unpaired terraces result Burnettand Schumm,1983; Ouchi, 1985], and adjustto vertical (Figures1 a and lb). deformation or base level change by channel modification, The utility of fluvial terracesto tectonic studiesis obvious. specifically by incising, aggrading, or altering sinuosity. Their occurrenceis ubiquitousworldwide [Fairbridge, 1968], Extensive valley aggradationburies evidenceof a stream'sprior andif they are tilted, faulted,or folded,they canbe usedto de- history,but cessationof aggradationand subsequentincision will ducehistory and ratesof tectonicdeformation [e.g., Keller and result in formation of one fill terraceand possiblylower cut ter- Rockwell,1984; Rockwellet al., 1984]. Their agescan be de- racescarved into the fill if incision is episodic(Figure l a). The terminedby radiometricdating of organicmaterial from within long profile of a bedrock channel, in contrast,responds more the veneer of depositson strathsurfaces or from within the allu- slowly to changesin gradientor baselevel, as its boundariesare viumof cut andfill terraces[e.g., Weldon, 1986]. Finally,the more rigid [Shepherd and Schumm, 1974]. Few studieshave existenceof a terracemight indicate that base level changeoc- been made of the long profiles of bedrockrivers, and even fewer curred, due either to vertical deformation [e.g., Bull and studieshave been made of their responseto baselevel change[cf. Knuepfer, 1987] or changingsea level [e.g., Pazzagliaand Summerfield,1991 ]. Field studyof a smalltributary to the South Gardner,this issue]. The difficultyof fully utilizingterraces in Fork of the Eel River, northern California, has documented that tectonicstudies, however, is that our knowledgeof fluvial re- sponseto baselevel processes(especially for bedrockrivers) is insufficientat presentto enableus to infer with confidencethe Fill Terraces historyof tectonicand eustatic processes from the terracerecord (comparediscussion by Seidl and Dietrich [ 1992]).

Project Design,Location, and Objectives The approachused here to providegreater understanding of the tectonic and eustatic controls on terrace formation is to exam- ine the geomorphicresponse of severalcoastal rivers to long- term baselevel change,as recordedby their fill, cut, and strath terraces.Marine terracesat the mouthof eachfiver provideesti- matesof ratesof late Quaternarybase level change.We surveyed andradiometrically dated fluvial landforms along three rivers (Bear, Mattole, and Ten Mile) in a tectonicallyactive area in northernCalifornia, near the Mendocinotriple junction. (MTJ; b) PairedStrath Figure2). The rivers were chosenspecifically because of their closesimilarities in bedrocktype (Franciscanargillaceous sand- stonesand mudstones)and climate (discussedbelow) and large dissimilarities in uplift rates [Merritts et al., 1989]. Late Pleistoceneuplift ratesincrease an orderof magnitudenorthward along the coastof California, from about 0.4 m/kyr at the Ten Mile River, to 2.5-3 m/kyr (locallyas high as4 m/kyr) alongthe Mattole River [Merritts and Bull, 1989]. The late Pleistocene airedSt• uplift rate at the mouth of the Bear River is intermediate,at about Terrace "t/l//% x• • 2-2.5 m/kyr [Merrittsand Bull, 1989]. In thispaper, we focuson analysisof the largestof theserivers, the Mattole (drainagebasin area655 km2; trunk stream length 97 km),and make comparisons with the other two in the discussionof results(Bear's drainage basinarea is 189 km2 andtrunk stream length is 44 km; Ten Mile'sdrainage basin area is 267 km2 and trunk stream length 36 Figure 1. Relationsamong the floodplain,valley floor, and ter- km). race formation. (a) Valley filling and subsequentepisodic inci- By working with rivers for which approximaterates of base sionresults in formationof one setof pairedfill terracesand one level changethrough time are known, our goal is to addressthe small, unpaired, cut terrace carved from the alluvium above the following questionsregarding the formationand significanceof floodplain. (b) Lateralvalley floor bevelingfollowed by sudden terraces: verticalincision that is episodicresults in formationof one setof When does a river carve the bedrock surface of a strath ter- pairedstrath terraces, one small,unpaired strath terrace, and one race? During sea level high standsor low stands?Or while sea activestrath, all carvedfrom the valley-wallbedrock. level is risingor falling? MERRITTS ET AL.' LONG RIVER PROFILES, TECTONISM, AND EUSTASY 14,033

124030 ' 124000 ' 123030 ' IJFP•J• m/ky z.s 40030 ' 40ø30 '

Mendocino Triple Junction 4.0 m/ky

1.2 m/ky 40000 ' Poinl Delgada

PACIFIC PLATE

0.4

39ø30 ' 39030 '

SurfaceUplift During April,1992, Earthquake I 124ø30 ' 124000 ' 123ø30 '

Figure 2. Platetectonic and locationmap of coastalnorthern California. Starmarks epicenter of April 25, 1992, Ms 7.1 thrustearthquake. Late Pleistocene rates of upliftare from Merritts and Bull [ 1989].Location of zoneof coseismicsurface uplift from surveysof die-off of marineorganisms and modeled surface displacement contours (150 mm contourinterval) are from Oppenheimeret al. [ 1993]. The triplejunction boundarybetween the North American(NAP), Pacific(PP), andJuan de Fucaplates (JFP) formedabout 29 m. y. ago, whenpart of the ances- tral spreadingcenter of the Farallon (present-dayJFP) plate was subductedbeneath the NAP [Atwater, 1970; Dickinsonand Snyder, 1979a, b]. Subductionof the JFP has continuednorth of the triple junction along the Cascadiasubduction zone (CSZ), while southof it right-lateralshear between the PP and NAP hasdeveloped into the San Andreas transform boundary. Right-lateral shear also occurs between the JFP and PP along the Mendocino fracture zone (MFZ).

Can terracesalong a large,meandering, bedrock river be corre- The Graded River, Base Level Change, latedin orderto reconstructpast longitudinal profiles? and Fluvial Terraces Do strathand fill terracesindicate that baselevel changehas occurred? ConceptualModels of the Graded River How far upstreamare effectsof baselevel changetransmitted? Mackin [ 1948, p. 464] describeda gradedstream as one with a Do adjacentrivers have similarnumbers and spacingof fluvial terracesthat are synchronous? ß.. slopethat will providethe velocityrequired for transportation Following discussionof a conceptualmodel of base level of all the load suppliedto it from above... [it] is a slopeof trans- change,a brief surveyof previousterrace studies, and presenta- portation;... influenceddirectly neither by the corrasivepower of tion of the resultsof analysisof the Matrole River, theseques- the stream nor bedrock resistance to corrasion. tions are addressed. To conclude, we summarize the relation be- tweenterrace formation and baselevel changein coastalnorthern Mackin viewedthe longprofile of a gradedriver as an indication California. Our goal is to providethe earthscientist considering of a systemin steadystate equilibrium, a systemadjusted to the the use of terracesas tectonicindicators with guidelinesto their prevailingsediment load. Le Chatelier,a chemist,had earlier de- analysisand interpretation. scribedthe reaction of a closedsystem in equilibriumto a change 14,034 MERRITTS ET AL.: LONG RIVER PROFILES, TECTONISM, AND EUSTASY

[cf. Prigogineand Defay, 1954], and Mackin [1948, p. 464] ap- regardingsea level oscillationsand the "eustaticapproach" of pliedthis principle to the caseof streamsas open systems: "If using terracescorrelated along the length of a fiver to estimate anystress is broughtto bearon a systemin equilibrium,a reac- the heightsof interglacialsea level high standsis relevantto this tion occurs,displacing the equilibriumin a directionwhich ab- study [Lamothe, 1918; Ramsay, 1931; Zeuner, 1945; Sibinga, sorbsthe effect of the stress." This reasoningcould be inter- 1953]. A special case of near-horizontalfill terraces,called pretedby geologiststo meanthat if the level of waterat the thalassostatic,refers to terracesformed along the lower, estuafine mouth of a streamis lowered (base level fall) and the stream's reachesof a coastalfiver in responseto sealevel high standsand gradientis increasedas a consequence,the channel will incisein drowning of the fiver mouth [Ramsay, 1931; Zeuner, 1945]. In orderto reduceits gradientto its previouslevel [e.g.,Leopold practice,few suchterraces have been identified with certainty, and Bull, 1979;Summerfield, 1985]. Conversely,if baselevel is and interpretationsof the relation between aggradationat the raisedand the stream'sgradient is decreased,the channelwill ag- mouth and fluvial processesupstream during high standshave gradein orderto increaseits gradient[e.g., Fisk, 1944;Zeuner, spanned the spectrum from extension of aggradation well 1945]. From thisbasic model, geologists developed a numberof upstream,to concurrentincision upstream [cf. Clayton,1968]. questionsregarding how base level change is transmittedalong a Many of the earliest ideas regardingeustatic terraces origi- stream,and how far upstreamthe effect of sucha changeis nated with the classic work of Lamothe [1918] on the Somme transmitted.A fundamentalquestion was simplyhow one deter- River, western Europe, a river nearly identical in size to the mines whetheror not a streamis graded. Leopoldand Bull Mattole. The Somme studies are relevant to the results of this [ 1979] respondedby statingthat a gradedstream is onethat is work, as the approachwas very similar,but the resultsare far dif- neitheraggrading or degrading,according to theoriginal ideas of ferent. The Sommeis outsidethe regionof Europeanglaciation Gilbert [ 1877]. and is fringed with strath surfacesmantled with a vertical se- In an activeorogenic belt, one might supposethat no streams quenceof depositsinterpreted as both glacial (older and lower shouldbe in equilibrium,as all are activelydegrading. Merritts deposits)and interglacial(younger and higher deposits)[Breuil, and Vincent [1989], however, working in coastal northern 1939] in age, and cappedwith loessinterpreted as synchronous California, showedthat a streamincising bedrock in responseto with glacial maxima. As discussedby Fairbridge [1968, pp. an effective base level fall can maintain a steady longitudinal 1129-1130], shapeby uniformincision along its length,with onecondition. Accordingto this model,the sizeand power of a streamare of ß.. it was... General Leon de Lamothe, who noticed the remark- greatimportance in determiningthe ability of thestream to incise able parallelismof terraces,both fluvial (in... easternFrance) and vertically. For a givenrate of uplift, smallerstreams will be marine (in Algeria); the marine terraceshe interpreted,with Suess steeperthan larger streams in orderto maintainthe power neces- [1888], as eustatic,which would necessarilybe high in inter- saryto incisevertically at a rate equalto that of the uplift rate. glacials,but he discovered... that they mergewith the high fluvial Thus, the streamis able to incise uniformly only if it is able to terraces,which... were identified by Penck [Penckand Bruckner, transmit all of the base level fall from its mouth upstreamalong 1909] as glacial [in age]. De Lamotheextended his surveysover its length.If a reachis encounteredin whichstream flow hasin- about half a century... [and over] other major rivers in western sufficientpower to incisevertically at an amountequal to the Europe. He claimed a general correlationwith the Algerian sea baselevel fall, the bed altitude and consequentlythe gradientof level data, with absolutealtimetric identity... No allowancewas that reach will increase relative to the next downstream reach. At made for... tectonics. anygiven point, the altitude of thebed will changeat a rateequal to the rateof uplift minusthe rateof incision. Figure 3 illustratesLamothe's estimation of interglacialsea level Thisconceptual model can be usedto considerthe occurrence high standaltitudes from the Sommestrath terraces. Figure 3 of fluvial terracesin the sameregion. Only the largeststreams will be comparedlater with the resultsfor the Mattole River, also and rivers in coastal northern California have strath terraces in a nonglacialsetting. alongsubstantial parts of theirlengths, and these are unpaired. The hypothesesdrawn from these observations are that (1) for a givenuplift rate, a streammust have sufficient drainage area, and Tectonic and Climatic Setting, hencepower, to erodelaterally as well asincise vertically, in or- Coastal Northern California der to leave a recordof unpairedterraces; and (2) the vertical componentof incisionwill be directlyproportional to theuplift The sinuousMattole River originatesless than 5 km from the rate. Theseideas are exploredfurther in thispaper. coastnear Point Delgada, flows inland and northwestward80 km along the region of greatestuplift rates in the MTJ, and exits north of the ruggedKing Range, the westernmostof California's The Graded River and Terrace Formation: Coast Ranges (Figure 2). Pronouncedrelief (up to 1246 m), Previous Terrace Studies rapid uplift rates (>2.5-3 m/kyr) [Merritts and Bull, 1989; Someworkers have consideredMackin's [1948] definitionof a Merritts et al., 1992a, b], and high seismicityare the result of gradedriver and Le Chatelier'smodel of systemresponse to complexinteractions among the threeplates joined alongthe mi- changeas an indicationthat river terraceformation (by aban- gratingMendocino triple junction (MTJ; Figure 2). Severaliso- donment)represents discrete events which acted to displacethe static, thermal, and mechanicalmodels have been proposedto systemfrom equilibrium.A fill terracemight represent aggrada- explain the mechanism(s)of crustaldeformation at the MTJ and tional responseto fluctuationsin sedimentloads due to climatic especiallythe rapid ratesof surfaceuplift betweenPoint Delgada change [e.g., Leopold et al., 1964; Weldon, 1986; Bull and andCape Mendocino [c. f. Jachensand Griscom,1983; Furlong, Knuepfer,1987], and abandonment of a strathsurface might rep- 1984; Merritts and Bull, 1989; Dumitru, 1991; Merritts and resentsudden movement along a fault or an increasedrate of up- Bonita, 1991; Merritts et al., 1992a,b]. The causeof rapiduplift lift [e.g., Bull, 1990]. The historicdevelopment of thought is not treatedhere. For the purposesof this paper,the ratesand MERRITTS ET AL.: LONG RIVER PROFILES, TECTONISM, AND EUSTASY 14,035

I EarlyHolocene toNeolithic peat F-/rj Peatand tufa ...... '• Marinedeposits :•..... Alluviumand coastalbeaches • Chalk Altitude, rn 8O - 8o

6o -•,• Deduced Trerace• 60

4o -_ SeaLevel _ Terrace- St.Acheul•••• ••"'•"• 4t) - TerraceMontie_•_•.• -••-'-• 2o •!.•...... t 20 -2o t ß i:::::•:, :,:,:, i,.,...... • • x280 It -20ø 0 10 20 30 40 50 60 70 Distance, km Figure 3. Longitudinalprofiles of the SommeRiver channelbed and strathterraces, with subsurfacedata (black dotsmark positionsof drill holes)and terraceprojections to presumedsea level high stands.Active channelbed shownas bold line. (Modified from Lamothe[1918] and Clayton [1968]). spatialpatterns of uplift are importantand are discussedbelow stant, and daily climatic data are almost identical for the Fort for the case of the Mattole. Bragg and Eureka coastal stations,near the Ten Mile and Bear Ongoinguplift of the coastat the mouthof the MattoleRiver Rivers, respectively(Figure 2). Mean monthly temperaturesvary is documentedby emergentPleistocene and Holocene marine ter- lessthan 6øC throughoutthe year; mean annualtemperatures are races,as well as by the recentcoseismic emergence of up to 1.4 12-14øC;and more than 90% of the mean annualprecipitation of m duringthe April 25, 1992,Cape Mendocino M• 7.1 earthquake 1.0 m occursas rain resultingfrom regional maritime air masses (Figure 2) [Merritts and Bull, 1989;Merritts et al., 1992a,b; during mild winters [National Oceanic and Atmospheric Oppenheimeret al., 1993]. The existenceof Holoceneand late Administration (NOAA ), 1985]. Pleistocenemarine platforms as far southas just north of Point Within the centerof the studyarea, however, local precipita- Delgada,however, indicates that the coastalong the entirelength tion and fog cover are modifiedby the orographiceffects of the of the Mattole River has been subjectto rapid surfaceuplift at King Range. Measuredmean annual precipitation at the coastat ratesof 2.5-3 m/kyr (andlocally up to 4 m/kyr) overtime periods Point Delgada is 1.65 m, and over the King Range crest at of tensof thousandsof years(Figure 2) [Merrittsand Bull, 1989; Honeydewit is 1.70-2.50 m. These differencesprobably affect Merritts and Bonita, 1991]. As the entire King Range terrane water dischargeand sedimentyield within the King Range and block, which extendsinland to the Mattole trunk stream,has been consequentlyin westerntributaries to the Mattole River. Despite uplifted during the Quaternary[McLaughlin et al., 1982; evidence of local climatic variations; however, the dominant Dumitru, 1991] andis at very high altitudeswithin 4-5 km of the controlon fluvial processes(as indicatedby substantialseasonal coast, at least the western part of the Mattole Valley (which variationsin discharge(U.S. GeologicalSurvey gauging station drainsthe King Range) and the trunk streamare presumedto recordsfor the Mattole and Ten Mile Rivers)) is regional,heavy havebeen subject to rapidrates of uplift duringmost of thistime. winter rainfall, which probablyaffects all coastaldrainages in a Apatite fission track studiesin the King Range and adjacent similar manner. Coastalterranes indicate that Quaternaryuplift and unroofingde- The late Pleistocene climate of northern coastal California was creasesin magnitudenortheastward, from a local high in the characterizedby a seriesof cool-moist(glacial) and warm-dry King Range[Dumitru, 1991]. (interglacial) alternations[Heusser, 1960], but alternationswere Modern and glacial climatesof northernCalifornia are de- of a much lessermagnitude than thosethat producedthe major scribedhere to supportthe assumptionthat modernclimate is climatic changesof glaciatedand continentalinterior regions similar for the Ten Mile, Mattole, and Bear Rivers, and because [Johnson,1977]. Climatic variationsbetween glacial and inter- pastclimates have some bearing on thepossibility that environ- glacial periodswere stabilizedby maritimeinfluences on intensi- mentalchange might have invoked terrace formation. The north- ties of temperatureand precipitationgradients, and paleontologi- em Californiacoastal region has a cool, temperate,highly sea- cal evidence indicates that the northern California coastal zone sonalMediterranean climate, characterizedby mild, wet winters acted as an "ice age refugium" for cold-sensitivespecies dis- anda prolongedsummer dry season.During the winter, cold, dry placedelsewhere [Johnson, 1977]. During the most recentfull- air from the Asiaticcontinental anticyclone moves into the trough glacial climatic conditions at 18 ka, estimatedsea surface sum- of the Aleutian low-pressure system, growing warmer and mer (August) temperatures(13øC) were similar to modern sea moisteras it movesuntil, as the polarPacific air mass,it encoun- surface summer temperatures(-13øC;

11øC [CLIMAP Project Members, 1981]. Although the region and closeeach surveyloop. After surveying,tidal chartsand was not glaciated and is known to have acted as an ice-age benchmarks were usedto correlatesurveyed altitudes to absolute refugium, the decreasedwinter temperaturesmight have resulted altitudes. Similar surveyswere done for the Bear and Ten Mile in sufficientenvironmental change to have an impact on onshore Rivers in 1990 and 1989, respectively [Merritts et al., 1989, sedimentyields and consequentlyterrace formation. 1991]. The coordinatesof surveypoints were plottedwith a computer Methods of Surveying,Mapping, and Sampling as a large-scale (-1:7000) plan view map and combined with field mapsand notes to createa geomorphicand topographic base Prominentterrace surfaces are ubiquitousand nearly continu- map (schematicexample in Figure 4). This map (3 m in length ousalong the lower40 km of theMattole River downstream from for the Mattole River) was then usedto constructa midvalley ax- Honeydew(Figure 2). In the summerof 1991, we surveyed ial line drawn along the midpoint between the bedrock valley 1300 pointsand mappedin detailthe activechannel width and walls (dashedline in Figure 4). This line is lesssinuous than the bottom,gravel bars, floodplain, and terraces (alluvial and strath modemriver channel. All pointsfor long profileswere projected surfaces;Plate 1) along the lowermost40 km of the Mattole perpendicularlyto this midvalley line, as is illustratedin the top River with a Lietz totalgeodetic station (angular accuracy +_3 arc of Figure 4. The distanceto each point from the Pacific Ocean at minutes at one standard deviation; linear accuracy +_(5mm + the river mouth was measuredwith an Alteck digitizer, with a 3ppm)at one standarddeviation). Surveying began at the mouth resolutionof _+0.2m at the scaleof our map. Althoughthe in- of the river, where marineterraces previously had been surveyed stmmenthas great precision,as a resultof repeatedstopping and [Merritts and Bull, 1989; Merritts and Bonita, 1991; Merritts et startingto get cumulative distancesto each point, the total error al., 1992a, b]. The surveytransect ended at about40 km, near for distance along the river is about +_100m out of 40,000 m. the townof Honeydew,just upstreamof whichseveral large trib- Distancesand altitudes of each point were then plotted to con- utariesconverge and the remaining57 km of the trunk stream stmct long profiles for each terrace surface,the active channel flows in a narrow bedrock gorge with few to no terraces. bed, gravel bars,and the water surface(Figure 5b). Becausethe surveytraversed a 40-km-longstrip of raggedtopog- Samples of charcoal and wood were collected from strata raphy,it wasnot possible within the severalmonth survey period within fill terracesor above strathsurfaces for radiocarbonage to returnto the originalstarting position to closethe entiresur- dating at eight siteson the Mattole, 12 siteson the Bear, and two vey. To minimizerandom survey errors, at leastthree temporary siteson the Ten Mile Rivers. Age estimatesand their uncertain- benchmarks were surveyedand resurveyedprior to and after ties were corrected for carbon reservoir effects and calibrated to a each time the total station was moved in order to check accuracy dendrochronologically-tunedrecord of varying carbon 14 in the

Plate1. Twostrath te•aces on the right ba• atkm 34 on the Matrole River. Note solid line added to highlight strathsurhces and riser between them. Thickness of alluvialdeposits above lower strath surface is about2.5 m, andabove upper surface about 3 m. Depositsfine upward, from channel bed gravel immediately above strath to fine-grained(clay, silt, and fine sand) overbank deposits. Upper surhce isthe same as that dated at •9.9 ka on left ba•, 0.5 km upstream. MERRITTS ET AL.: LONG RIVER PROFILES, TECTONISM, AND EUSTASY 14,037

EXPLANATION OF SYMBOLS

• Activefloodplain Active channel and survey points • Lowby100-yrterrace; eventflooded

Midvalley axial "...•ß'"... Terrace of historicaboveflooding level line

[•,•:'•'•••d•! High terrace

Bedrock; valley Higher terrace wall

Figure 4. Schematicmap illustratingmethod of constructinga geomorphicmap of a river system,including its activechannel (thalweg and banks), floodplain, terraces, and valley floor walls. Dashedaxial line is constructed from midpointsalong valley. Arrows in top part of diagramshow how surveypoints are projectedperpendicu- larly to midvalleyaxial line. Distanceto eachpoint along the midvalleyline is thendetermined with a digitizer to constructlongitudinal profiles. atmosphere,according to establishedmethods (Table 1). In some Results of Terrace Investigations cases,calibration of carbon-14ages with the dendrochronological General Observations record results in more than one likely possibility for the age range,and agesare presentedas multipleranges. In the text, ages The longitudinalprofile of the Mattole River obtainedfrom are presentedas the calibratedage rangefor +_onestandard devi- 7.5 arc minutetopographic maps (Figure 5a) is segmented.The ation, in years B.P., accordingto protocol(e.g., 2000-2575 cal reach within longitudinaldistances of zero and 60 km is com- yearsB.P.); for brevity, they are referredto by the centroidof this posedof two linearsegments that join at about30 km. Upstream rangeon somefigures. Radiometricage estimatesmentioned in of 30 km, the river has a gradientof 0.0032 (metersper meter), the text below include the sample number for identification in but downstreamof 30 km the gradientis 0.0014, a decreasein Table 1. gradientof-55%. Only-8 km of thesteeper segment is depicted 14,038 MERRiTTSET AL.' LONG RIVER PROFILES,TECTONISM, AND EUSTASY

• o

o

• o o o o • • 00 • • o • o• o

-H -H -H -H -H -H -H -H -H u-• o o • o • • o o

o

d o

x

o

o o o o u• o ou• o ou-• o 0 0 ':' -._...... "" • 0,1 N e.- ,,.,- u• 0

i i i i i 14,040 MERRITTS ET AL.: LONG RIVER PROFILES, TECTONISM, AND EUSTASY on the detailed surveyprofile of Figure 5b, but, for the segment follows the river upstream.Although strath surfaces are subpar- below 30 km, the profiles of the channelbed (thalweg), water allel to one another,they are steeperthan the modemchannel bed surface,and gravelbars are paralleland remarkably straight. For downstreamof 32 km (0.0032 gradientversus 0.0014), have the example,linear regressionresults (in meters)for the channelbed samegradient as reachC-D (Figure 5a) of the Mattole, and do andwater surface (from surveypoints) yield y = -1.30 + 0.0014x not mergewith marineterraces at the river mouth(Figure 5b). (R2 -- 0.99) and y = -0.81 + 0.0014x (R2 = 1.00),respectively. The upstreambreaks in slopeof prominent,near-horizontal fill The variable vertical difference between the channel bed and terracesalong the river at 3 and 12 km, coincidewith the inner water surfaceprofiles illustratesalternating pool and riffle se- edgealtitudes of the two youngestmarine terraces, 6.5 and30 ka quences[Wohl et al., 1993]. in age, respectively.The positionof the 30 ka interstadialsea A secondimportant observation is that two differenttypes of level high standmarine terrace (inferred from altitudinalspacing terracesexist along the Mattole River, and the type has a general analysis[see Merritts and Bull 1989]) is obtainedfrom surveys associationwith valley floor width and distanceupstream. From alongthe coast1 km southof the mouthof theMattole. The po- 0 to 18 km, the wide (1-2 km) valley floor is dominatedby allu- sitionof the inneredge of the mid-Holocenemarine platform (the vial terraces.From 18 to 40 km, the valley floor is narrowerthan preemergenceterrace), dated via a musselshell (NC-1-87) at- that downstream(0.3-0.5 km), and is fringed with a steppedse- tachedto the platform(6163-6863 cal yearsB.P.), is from a sur- ries of unpairedstrath terraces with cover sediments.Upstream vey 1 km northof the mouth[Merritts et al., 1992a,b]. of 40 km, the valley becomesa narrow gorge,with no or occa- sional strath terraces and a bedrock floor. Interpretationof DepositsOverlying Strath Surfaces Terracesand the active channelfloor alongthe lower -18 km The coarse,bouldery, subrounded to roundedgravels overly- of the river are compossedof alluvium (cut and fill terraces; ing strathsurfaces (Plate 1) aresimilar in size,sorting, and shape shownas fine lines in Figure 5b). Occasionalbedrock knobs are to those in the active channel bed, and we assumethat they were surroundedby alluvium,as at km 10 (altitude= 40 m). An esti- left as depositsupon the strathsafter abandonmentby the chan- mate of the minimum thicknessof the alluvium comprisingthe nel. Ages of thesechannel bed gravel depositsprobably are elevated fill terrace at km 10 is available from multiple water- roughlysynchronous with thetime of formationof thestrath sur- well logs which indicate that sand,gravel, and clay occurto a face. In contrast, cumulic, stratified, very fine-grained sands, depthof at least 12 m. In an upstreamdirection from km 18, the silts,and clayswhich cap the gravelspostdate the time of strath occurrence of strath terraces along the valley walls and of formation and are the result of flood events that were large bedrockin the channelfloor increasesin frequency(e.g., Plate 1' enoughto overtopthe terrace gravels. These are referred to here strathsshown as heavy lines in Figure 5b). These surfacesare as overbankdeposits. distinct, can be followed for up to severalkilometers, and in Radiometricdating of overbankdeposits at multiplesites on someplaces merge together upstream, as at km 26, 32, and 37. both the Bear and Mattole Rivers indicates that they can be at At all locations, straths are overlain by up to 1-2 m of coarse leastas muchas 3000 yearsyounger than the underlyinggravels gravels,which in turn are overlainby up to 1-3 m of fine-grained at a given locality. For example,at km 34.8 on the Mattole sandsand silts (Plate 1). Alluvial fans at tributary mouthscause (Figures5 and 6), a log in channelbed gravels<1 m abovea thesedeposits to be locally muchthicker (up to 12 m). The tops strathyielded an age of 9951-10545or 10766-10784cal years of thesesediments overlying the strathsurfaces are shownalso as B.P. (MT-A-2). Two samplesof charcoalfrom thin bedsof very fine lines in Figure 5. These terracetreads, unlike thosedown- fine-grainedsand and interbedded silt directlyabove the gravel,2 stream,do not representthe surfacesof fill or cut terracesbut m above the strath, yielded ages of 5896-5929 cal years B.P. ratherthe cappingof sedimentson a strathsurface. (MT-A-1) and 6191-6289 cal years B.P. (MT-A-3). The older A third observation is that the active channel floor of the lower date from the gravelsis a minimumage for the strath(strath >9.9 part of the Mattole (0 to 30 km) is the top of a gravelwedge that ka), but is much closer to the strath age than a date from the tapersto zero upstream.The surfaceof this wedge (Figure 5a, overlying sand and silt (strath>>5.8-6.3 ka). Terrace gravels reachA-C) hasa gentlergradient (0.0014) thanthe bedrockreach provide an upper limit on the altitudinalposition of the channel upstream(Figure 5a, reachC-D, gradient0.0032). An estimate floor associated with the strath and a minimum limit on the time of the total thicknessof the gravel veneernear the mouth,at km of strath formation (Figure 6). The overbankdeposits are also five, is obtained from recent U.S. Geological Survey (USGS) useful,in that they providean approximateage and an upperlimit drilling alongthe right bankof the channel(Figures 5a and5b). for the altitudinal positionof the channelbed that floodedto that Bedrockwas not encountereduntil a depthof least37 m, when a level to depositthe fine-grainedmaterial (Figure 6). large mass of blue, argillaceousrock, identical to the local bedrock,was hit (T. Dunklin, drill logger,personal communica- Interpretation of DepositsWithin Cut and Fill Terraces tion, 1992, USGS Cape Mendocino Seismic Reflection Experiment;E. Criley projectdirector). At km 35, deepbedrock Near the Mattole River mouth, several of the alluvial terrace poolsalternate with gravelriffles on the activechannel floor, and surfacesare nearly horizontal,while othershave steepergradients the gravelveneer is lessthan severalmeters thick. The longpro- and convergeupstream with them, as at 10-12 km (Figure 5b). file of the Mattole obtainedfrom topographicmaps (1:24,000) We concludethat these representboth fill and cut terraces,re- reveals additional information to the surveyand subsurfacedata spectively, with the latter cut from the alluvium of the former. (Figure 5a). The river's long profile in bedrockreach C-D is Few exposuresof the stratathat comprisethe fill and cut terraces nearlystraight at this scaleand, if projecteddownstream beneath occur, except in occasional gullies. Even these provide little the gravelwedge (Figure 5a), wouldoccur just beneaththe 37-m stratigraphicinformation because of slumpingand the ubiquitous depthto estimatedbedrock. This upperreach and its gradient cover of poisonoak, vines, and nettles. Where exposuresdo ex- mightindicate a zoneof adjustmentto long-termuplift. ist, however, no bedrock is found, with exceptionof occasional The final generalobservation is that strathsproject into the isolated knobs. Data from numerous water wells indicate that all modem channelbed between 18 and 38 km, forming a seriesof terracesdownstream of about 14-18 km are composedof thick benchesthat emergefrom the channel,one after the other,as one sequencesof cobbles,sand, silt, and blue clay (discussionswith MERRITTS ET AL.' LONG RIVER PROFILES, TECTONISM, AND EUSTASY 14,041

841o-855o)yrs BP Active channel floor >>9.9 ka, < 1 2 ka? ALTITUDE, rn channelatthis• • . height~8.5Waterflowkai> •••/ lOO Ve channel - incisionr.ti.c•a ' Activefloor >9.9 ka? 9o

6214-62745912yrs yrsBP; BP • Waterflowat this• height ~$.8-6.3 ka 8o 10157-10278 yrs BP Active channel floor >9.9 ka i 7o Active channel floor <<9.9 ka , VE -120 x , • I , , , , , , , I , , , ' ' I I I I I I I I I I 6o 34 36 38 40

DISTANCE UPSTREAM, KM Bedrock exposure and

waterLogin surfacegravel (•.•o-•sso)•r.., onCalibratedfine-grained radiocarbondeposits date

Strath (with survey points) Calibrated radiocarbon overlain by channel bed 10157),rs BP date on channelbed gravels gravels and flood deposits

Figure 6. Detailedstratigraphy and terrace correlation from 34 to 39 km, indicatingmeans of estimatingrates of vertical incision. local drillers and landownersin Petrolia). As local groundwater and the positionsof meandersmigrate with time, we do not know levelsare high, drillers rarely go belowdepths of 12 m, andac- the exact location of all points along the river's channel for a tual well log datawere only obtained for wellsat km 6 and 10 giventime. For this reason,and because age control is not avail- (Figure5b). Thesewell logs indicate that the upper 12 m of al- able for all terraces or for all reaches of the river, the recon- luviumare probably dominantly fluvial and estuarine rather than marine(no shellsnoted) and composedof sedimentssimilar to Altitude, m thatfound at thepresent mouth. The blue clay is describedby lo- Sea Level Fluctuation Curve 50 calsas very common and likely to be foundanywhere one drills alongthis part of the river. Blueclay is a commonweathering productfrom argillaceousrocks of the CaliforniaCoastal and KingRange terranes. However, blue clay also commonly indi- catesreducing conditions and might be similarto the estuarine -5O claysnoted by Ota et al. [ 1991]in the strataexamined in the mouths of rivers in New Zealand.

Results -150 I ' ' ' ' i , , , Terrace Surfaces and Reconstructionof Paleo-Long Profiles 200 101• 0 Justas it is possibleto constructa long profile of the active Time Before Present, ka channeland floodplain for the modernMattole River, it is also Figure 7a. Late Pleistocenesea level curve from Chappelland possiblefrom the data presentedin Figure5b to reconstructap- Shackleton[ 1986]. Note that sea level has been low for most of proximatepaleo-long profiles. Becausethe channelis sinuous the past125 kyr, andreached its loweststand about 18 ka. 14,042 MERRITTS ET AL.' LONG RIVER PROFILES, TECTONISM, AND EUSTASY

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N MERRITrS ET AL.: LONG RIVER PROFILES, TECTONISM, AND EUSTASY 14,043 structedpaleoprofiles are referredto as approximate.They are least well-constrained,but still provides useful information for anchoredby the surveyand age data available,as illustratedin later analysis. the followingexample for reconstructinga 6.5 ka paleo-longpro- file. Why Straths Are Steeper Than the Modern Stream Bed A maximumheight for the altitudinalposition of the Mattole River channelbed during the -6.5 ka sealevel high standis re- One of our major observations(Figure 5b) is that the Mattole constructedfrom four "anchor"points along the river's length River strath terraces are roughly parallel to each other but are (starson Figure 7b). At the downstreamend, we anchorthe pro- steeperthan the active streambed by 0.0018 rn/m. In a down- file at the terrace break in slope which is coincident with the streamdirection, the strathsconverge with and disappearinto the horizontalprojection of the inner edgealtitude of the marineplat- channelbed. One explanationfor this differencein gradientis form datedat 6163-6863 cal yearsB.P. (NC-1-87), whereit coin- that the terraceshave been tilted by tectonicprocesses. This hy- cideswith the breakin slopein the alluvialfill terrace. The -6.5 pothesiscan be evaluated by examining the hypothesizedse- ka marine terraceoccurs 7-10 m abovesea level today (due to 17 quenceof eventsrequired to causesuch tilt. First, older straths m of uplift since its formation at an altitude of about -10 m are nearly parallelto the >9.9 ka strathsurface downstream of 36 [Fairbanks, 1989]). At the upstreamend, at km 35, we anchor km. Therefore, no tectonic tilt occurredduring the period of the profile just below the fine-graineddeposits that date at 5896- about 20-30 ka to 9.9 ka, but since9.9 ka, significanttilt has oc- 5929 cal years B.P. (MT-A-1) and 6191-6289 cal years B.P. curred. As a consequence,ten thousandyears is the maximum (MT-A-3). To deposit sand and silt in the channelor over the tilt duration. If we assumethat the streamgradient at the time of strath formation was the same as that of the active channel bed bank, the channel bed itself had to be at or below, the level of the dated deposits,but within the range of maximum possibleover- (0.0014), the tilt magnitude is 0.0018 rn/m. This convertsto a bank flooding (Figure 6). At present,this range is about 7 m, minimumtilt rateof 0.18 m km4 kyr-• (equivalentto 0.18 grad basedon the maximum stageof the largestflood in historictime, yr-•;this is a veryrapid rate of tilt relativeto otherknown rates in the 1955 event (Figure 5, km 24). Betweenthe downstreamand the world). upstreamendpoints, we project the 6.5 ka profile below fine- Uplift of the King Rangeis not uniform. For example,along graineddeposits at two sites(32 and24 km) that yield datesfrom the coastbetween Cape Mendocinoand Big Flat, differentialup- nearthe topsof the depositsof -4650-4972 cal yearsB.P. (M5-3- lift hasresulted in tectonictilt of 0.04 gradyr -• overthe past70 91), and -4536-5035 cal yearsB.P. (M4-5-91). We assumethat kyr or so [Merritts and Vincent, 1989]. This stretchof coastis depositsthat are 6.5 ka in age exist below the youngeroverbank parallel to but outboard of the middle reach of the Mattole. deposits. The final approximatepaleoprofile constructed for 6.5 Although the differential uplift along this middle reach (where ka is shownin Figure 7b. strathsoccur) is unknown,the required tilt rate(0.18 gradyr 4) is We use similar reasoningto reconstructlong profiles for the almost five times greater than that observed along the coast. 9.9 and 12 ka profiles of the Mattole River (Figure 7b). Several Furthermore,evidence suggests that the uplift rates(and therefore details of thesecorrelations are as follows. The paleoprofileon tilt) probablydecrease inland from the coastrather than increase. the age-constrained>9.9 ka surface(from the dateon a basallog) Recent analysisof Holocene surfaceuplift rates obtainedfrom is obtainedmerely by following the well-exposedsurface. Note marineplatforms at the coastdetermined that ratesare high from that this surface has a nearly constantgradient (0.003) that is Cape Mendocinoto just north of Point Delgada, parallel to the similar to that of all other straths and to that of bedrock reach C- length of the Mattole River [Merritts et al., 1992a, b]. However, D (Figure 5a) of the Mattole River. Paleoprofiles for older duringthe 1992 Cape Mendocinoearthquake, the magnitudeof strathsare not as well-constrained.A higherstrath at km 39 has a surfaceuplift decreasedto zero inland from the coast(Figure 2) minimum radiometric age of 8410-8550 cal years B.P. (M6-1- [Oppenheimeret al., 1993]. Furthermore,apatite fission track 91), basedon charcoalfrom the baseof the fine-graineddeposits [Dumitru, 1991] and vitrinite reflectance [Underwood et al., overlying the strath. This date indicatesthat the strathformed 1988; Laughland et al., 1990] studies indicate that rapid more than -8.5 kyr ago, and perhapsmore than 11 or 12 kyr ago, Quaternary rock uplift and unroofing decreasenortheastward if depositionof silts and sandspostdates strath formation by as from the King Range, perpendicularto the lengthof the Mattole muchas 3.5 kyr (the maximumage difference from datedsites on (seeabove discussion in sectionon tectonicand climatic setting). the Mattole and Bear Rivers). It also indicatesthat a channelex- In conclusion,tectonic tilt as the causefor strathterraces being so istedbelow that altitudeat -8.5 ka, with floodwatersrising to that much steeperthan the modem channelbed seemsunlikely. level at that time (Figure 6). We assumean approximateage of A more reasonableexplanation for why strathsare steeperis >12 ka for the time of formationof this strathsurface, by adding that repeated,rapid sea level rise during the QuaternaryPeriod 3.5 kyr to -8.5 ka. The profile for this paleobedrocksurface is has causeda wedge of sedimentbackfilling (Figure 5a) that has then determinedby noting that all continuousbedrock straths migrated upstream,with decreasedgradient, and buffed the ex- have about the same gradient (Figure 5b), and using this tensionof the strathsshown on Figure 5b. Study of the effect of observationas an assumptionto determinethe gradientof the -12 base level rise on streamshas centeredlargely on the effect of ka surface. For the 30 ka profile, we only have anchorpoints at small check damsbuilt acrossvery small streams[e.g., Leopold the mouth,where a prominent,near-horizontal fill terraceis at the and Bull, 1979; LeopoM, 1992]. The conclusionof thesestudies same altitude as the inferred 30 ka marine terrace [Merritts and is thatbase level rise will causea wedgeof sedimentdeposition Bull, 1989]. However, we assumethat the channelbed upstream to migrateupstream a relatively shortdistance in a shorttime, af- was higherthan all youngerstraths, as the channellater incisedto ter whichit will remainstable. When deposition is complete,the those levels. These strathsprovide a minimum height for the gradientof depositionis abouthalf the gradientof the original profile. Furthermore,if we assumean averagelong-term incision channel(LeopoM, 1992). The depositionalwedge on the Mattole ratefor the river (seediscussion below) of 0.8-1.5 m/kyr,the 30 (Figure 5a) extendsupstream about 30 km (31% of the total ka paleoprofilewould be at least 24-45 m above the present stream length) and to an altitude of about 60 m above sea level channelupstream of km 32 (Figure5b). This paleoprofileis the (in effect the lip of the checkdam). Thesevalues of distanceand 14,044 MERRITTS ET AL.: LONG RIVER PROFILES, TECTONISM, AND EUSTASY height are large comparedto thoseof smallerstreams and might rate of eustatic rise (up to 24 m/kyr at 12 ka in Barbados be the result of the much greater size of the Mattole River. The [Fairbanks,1989]) exceededthat of tectonicuplift in northern one parameter directly comparableto previous work on small California, sea level was still low relative to its earlier interstadial streamsis the changein gradientto aboutone half during aggra- high standat 30 ka, and it was not until between9 and 6 ka that dation. As mentioned,the gradient of the lower or depositional strathsat the mouthof the river were submergedand buried dur- reach of the Mattole is 45% of the gradientof the channelup- ing backfilling,at an altitudeof---10 to -20 m relativeto modem streamof the wedge. sea level. We tentatively project, in a downstreamdirection, strathsexposed upstream of 18 km beneaththe channelbed, be- History of Terrace Formation low the minimum depthto bedrockof -37 m near the mouth,and to the coast(Figure 7b), where we indicatethe estimatedrelative From constraintsregarding the positionof the mouth of the positionof sealevel about10 ka (usingthe sealevel historydata Mattole River during sea level high stands[Merritts and Bull, of [Fairbanks, 1989] from Barbados,and an uplift rate of 2.6 1989] and its channelbed altitudinal positionat different times rn/kyr). between 30 ka and the present,we can examineits responseto Sealevel hasbeen so low sincethe last major interglacial high sea level fall from 30 ka to 18 ka, and to rapid sealevel rise from standat 125 ka, that even duringtimes of sealevel rise prior to then until about 6.5 ka (Figure 7a). Since 6.5 ka, sea level has eachinterstadial high stand, it waslow relativeto thebedrock lip risen very slowly relative to the preceding12 kyr. As no local presumedto be buried beneath the gravel at the mouth of the sea level curve is available, data from New Guinea [Bloom et al., Mattole gorge (Figure 7a). Becauseof this local baselevel con- 1974; Chappelland Shackleton,1986] andBarbados [Fairbanks, trol, it is possiblethat the channel was flowing on bedrock 1989] are used(Figure 7a). Their use is consideredvalid for this downstreamof 40 km duringtimes of risingas well asfalling sea analysis,as it dependsonly on general and relative trendswith level. Supportingevidence for this interpretationis the radiocar- time. Note that we assumethe simplest case of base level fall bon date of -9.9-10.8 ka for the log sampledfrom channelbed due to uplift, i.e., that an averageuplift rate of 2.6 m/kyr is uni- gravelson the strathat 35 km (Table 1; Figures5 and 6). This form alongthe lengthof the Mattole, and tilting or folding along dateindicates that gravel was beingtransported across the strath, its lengthare insignificant. andhence it was probablyactive, at thattime, while sealevel was rising at the mouth. As this strathis now 5-7 m abovethe active Terraces and Sea Level Fall From 30 to 18 ka channelbed (Figure 6), net vertical incisionhas occurredsince then,during a time of deceleratingsea level rise,but net tectonic The most recentsea level high standbefore the presentwas 30 uplift of 7-10 m. kyr ago, at -44 m. This high standcaused inundation and back- filling of what is now the lowermost6-12 km of the mouthof the Terraces and Relative Base Level Fall From Mattole (Figures6b and 7b), althoughthe upstreamextent of the 6.5 ka to the Present depositional wedge is unknown. The level of the northern California coastalland masshas been uplifted about76 m since During and sincethe time of stabilizingsea level at -6.5 ka, that time, and the relative positionof the 30 ka high standis now wave-cutbedrock marine platforms have formed along the coast •-32 m above modem sea level (2.6 m/kyr * 30 kyr + (-44 m) = immediatelynorth and southof the mouthof the Mattole River, 32 m). Between 30 and 6.5 ka, global eustaticsea level reached but inundationof the incisedvalley mouthgorge, from which its loweststand in perhapsthe past125 kyr, about-125 m at--17- muchbedrock and alluvium probably had been removed already 18 ka. During this period of exceptionallylow global sea level, during earlier times of relatively low sea level, was associated including the 12 kyr of sea level fall just prior to it, we propose with aggradationand formationof a fill terrace. Near the mouth, that alluvium and bedrock in the river mouth were removed by thisdeposit is a mixtureof interbeddedgravel, sand, silt, andblue deep incision, and strath surfaceswere formed that dip more clay. Charcoalfrom a 1.5-mbed of blueclay 2 m belowthe top steeplythan the modem, backfilled channelbed. of the fill terraceat 5 km yieldsa radiometricage of 2143-2469 The USGS drilling data providescorroborative evidence that cal yearsB.P. (M2-1-91). The resultingdeposit decreases rapidly the bedrockfloor of the valley is below many 1Os of metersof in thicknessupstream but perhapswas associatedwith burial of gravel 5 km upstreamof the channelmouth (Figure 5 and 7b). older strath surfacesas far upstreamas km 24-35, where mid- We have no other data regardingthe natureof the bedrockvalley Holoceneage overbankdeposits overlie strathsurfaces and early floor at the mouth,except from offshorebathymetry, which indi- Holocenegravels. The river has incisedabout 5-7 m at km 35 catesa narrow (<3 km wide) and very steepcontinental shelf and since -6.5-10 ka, and consequentlyit has exposedolder strath a deep submarinecanyon (Mattole Canyon)just northof the pre- terraces that were partially buried during the early- to mid- sent river mouth. The offshore profile of the Mattole River Holocene sea level rise. shown in Figures 5b and 7b follows the Mattole Canyon. The buried bedrock valley floor might be a prominentknickpoint Conclusions Regarding Terraces, Eustasy, which actsmuch like a bedrockwaterfall when exposed,or a se- and Tectonism ries of knickpointsbetween the mouth and the first clear evidence of exposedbedrock valley floor (at 34-38 km on Figure 5a). Tectonicand Eustatic Significance Regardless,the steepnessof the shelf and offshore canyon are of Strath and Fill Terraces such that the slope of the bedrock valley floor must steepen The responseof the Mattole River to high-frequencyfluctua- rapidly somewherenear the presentmouth. tionsof sealevel and long-termtectonic uplift supportsthe con- clusionsof Leopoldand Bull [1979] andLeopold [1992] regard- Terraces and Sea Level Rise From 18 to 6.5 ka ing the impact of base level rise on a river's long profile. Between18 ka and6.5 ka, sealevel roserapidly in responseto Furthermore,the resultsdiscussed here add importantscale rela- globalmelting of ice sheetsand valley glaciers. Althoughthe tionsfor a river muchlarger than those which they studied.Sea MERRITTS ET AL.' LONG RIVER PROFILES, TECTONISM, AND EUSTASY 14,045 level high standshave resultedin episodicbackfilling at the river LARGE SMALL mouth when the water surface rises relative to the land mass. Fill (High Stream (Low Stream terraceswith gentle gradientsalso occur at the mouths of the Power) Power) Bear andTen Mile Rivers. Aggradationhas been restricted to the lower reaches of the rivers, however, and even as sea level con- a) b) tinuedto rise at the coast,vertical bedrock incision occurred up- stream at km 35 on the Mattole since -6 kyr ago. Although LOW aggradationprobably also occursin responseto climate change UPLIFT [e.g., Weldon, 1986; Bull, 1990], the evidencethat full-glacial RATE climateswere not substantiallydifferent than at presentin this re- gion, the locationof fill terracesalong only the lower reachesof the rivers, and the overwhelmingevidence of tectonic and eu- staticcontrols on terraceformation, suggest that climateplays a relatively minor role. The nature of the straths shown in Figure 5b (subparallel, closely spaced,upstream-merging surfaces with about the same RAPID gradientas bedrockreach C-D of the Mattole, Figure 5a) indi- UPLIFT cates that despiterepeated sea level high stands,the dominant RATE long-termtrend of the Mattole upstreamof •-18 km has been one of nearly continuousvertical incision, as well as lateral erosion, due to long-termtectonic uplift. The terracesformed by incision are unpairedand do not indicateperiods of baselevel stability and lateralvalley floor bevelingpunctuated by episodicincision. As notedby Davis [ 1902, 1954], a meanderingchannel swinging e) f) sidewayswhile also incising downwardswill remove parts of LOW older terracesduring its sweepsback and forth acrossthe valley lo 5 UPLIFT width, and leave unpairedremnants that are partsof what can be imaginedas a continuouslydownward-moving, switch-backing RATE surface(dashed line in Figure 8), of which only marginalrem- nantsnow exist. The time of formationof a strathsurface is syn- chronouswith the river's sweepingalong a bedrock surfacein- clined toward the valley bottom, but the time of formationof the g) h) riser (and hencethe terraceitself) is synchronouswith the side- ways sweepof the river that removedthe rest of the inclined sur- RAPID lo face. UPLIFT •2 It is curiousthat a river that is nearly continuouslyincising RATE would form a series of terracesinstead of a V-shaped gorge. Many suchvalleys with few or no terracesdo existin the region, includingthe uppermost50-60 km of the Mattole and numerous adjacent rivers in the Coast Ranges (compare discussionof nearbyinner gorges by Kelsey[ 1988]). The criticalfactor for ter- Figure 8 Model of strathsurface development along a meander- race formation is upstreamdrainage area, as it relates to dis- ing bedrock river for two uplift rates (relative units of 1 and 2) chargeand hencestream power. The sizeand valley-ward slope and two sizesof river. Sketchesin Figures8a, 8b, 8e, and 8f are of the terraceremnants are controlledby the ratio of the rate of for low uplift rate, while sketchesin Figures 8c, 8d, 8g, and 8h lateral migration to the rate of vertical incision. The former is a are for a rate two times greater. Sketchesin Figures 8a, 8c, 8e, functionof dischargeand streampower (and thereforemainly and 8g are for a large river with high stream power, while drainagearea), while the latteris a functionof the long-termrate sketchesin Figures8b, 8d, 8f, and 8h are for a river with a valley of uplift. If the ratio is very large,the valley-wardslope will be width half that size and small streampower. These proportions gentle, and broad strathsurfaces will be carved(Figures 8e and indicate relative lateral erosion (units of 10 and 5) versusvertical 8g); if it is small,steep, narrow surfaces will be carved(Figure 8f incision (units of 1 and 2). Units are arbitrary. Dashedline indi- and 8h). If it is very small, a V-shapedgorge with no terraces cates switch-backingpath of incision along which terracerem- will form. A functionalpower/incision rate ratio can be defined nants occur. for eachreach of a river, for which the greaterthe value of this ratio, the greaterthe likelihood of formationof broad strathsur- faces. 8a and 8c with 8b and 8d). For a given size river, the greaterthe The fact that few to no terracesoccur in the bedrockgorge uplift rate, the greater the rate of vertical incision, and conse- farther upstreamin the Mattole valley, and none in the smaller quently the less the likelihood of formation and preservationof rivers immediately to its south, indicatesthat erosion there is broadstrath terraces (compare Figures 8a and 8b with 8c and 8d). dominantlyvertical, and the lateral component,or meandermi- These interpretationslead us to concludethat strathterraces gration,is insufficientto form and preservestrath terraces (i.e., will form only along certain parts of a coastal stream during the power/incisionrate ratio is small). Riverswith largedrainage times of fluctuating sea level. This location is upstreamof the areasare more likely than small onesto have the power to erode aggradational effects of a bobbing sea level surface, and far laterally at ratesgreater than vertical incision(compare Figures enoughdownstream from the headwatersthat streampower is in 14,046 MERRITTSET AL.: LONGRIVER PROFILES, TECTONISM, AND EUSTASY excessof that neededto overcomeresistance, to transportthe 4. Correlationof terracesamong reaches based on isolated prevailingsediment load, and to executevertical incision. Uplift surveyswithin each reach, and matchingby relative heights rate is the maximum rate of incisionpossible, and maximumin- abovethe channelbed (the most commonlyused method [cf. cision occurswhere a streamtransmits all base level fall up- Thornbury,1969]), is not likely to yieldenough information to stream. Along the lengthof the stream,if incisionis lessthan the reveal the potential complexities of terrace sequences. rate of uplift, the channel gradient steepens[Merritts and Continuous,detailed surveying from reachto reach,and correla- Vincent,1989]. Downstreamof this point,however, straths will tion basedon followingsurfaces rather than matching relative form if the streamhas excess power for meandermigration and positions,is criticalto avoidsubjectivity and error. lateral erosion. This lastproblem, terrace correlation, is perhapsthe greatest. For the Mattole, we can comparerates of uplift at the coast Until recently,regional terrace studies have been limited by the (2.5-3 m/kyr) with average rates of net vertical incision into practical necessityof obtaining most altitudinal control from bedrock. Two vertical bedrock incision rates can be estimated at small-scaletopographic maps and of correlatingterraces based km 34-36 on the Mattole (Figure 6). The >9.9 ka strathis 7 m primarily on local studiesof altitudinalposition within each above the channel bed, which is thinly mantled with gravel. reach.With recentlydeveloped equipment such as total geodetic Assumingthat about 7 m of net vertical incisionhave occurred stations,however, one can surveya largeriver and its terracesin sinceat least 9.9 ka resultsin a maximum,average incision rate severalmonths and producehighly detailed,large-scale maps of 0.7 m/kyr. The next higher strath,which is older than 8.5 ka from whichto generatelong profiles. and perhapsat least as old as 12 ka (and must be older than the In the case of the Mattole, both approacheswere used: the lower 9.9 ka gravel), is 18 m above the presentchannel bed. former in the late 1980s (preliminary mappingand local hand Assumingan age of at least 12 ka yields a net vertical incision level surveying; terrace profiles published by Merritts and rate of 1.5 m/kyr for the past-12 kyr. This rate is uncertainbe- Vincent [1989]), and the latter in this work. The resultsare dif- causeof the poor constraintson the strathage; however, a maxi- ferent, and the two main reasonsfor this have importantramifi- mum incision rate of 1.8 m/kyr is certain, becausea minimum cations for terrace studies worldwide. First, in the earlier work we age for this strathis 9.9 ka. Therefore,possible incision rates did not clearly recognizethe fact that the gradientsof strathter- range from 0.7 to 1.8 m/kyr and are 23-72% of the long-term racesmight be significantlysteeper than that of the modernchan- averageuplift rate of 2.5-3 m/kyr at the river mouth(Figure 6) nel bed (more than 2 times). Second, we correlated terraces [Merrittsand Bull, 1989]. This similaritysuggests that the long- among reachesbased on altitudinal position(i.e., height above term uplift rate controls the long-term rate of vertical incision the river). into bedrock,as recentlynoted also by Bull [ 1990]. A comparisonof the two approachesis shownin Figure9, in a The Ten Mile River (uplift rate -0.4 m/kyr) has formedin a schematicbut accurateportrayal of the Mattole studies.Consider manner similar to the Mattole and Bear Rivers. However, the Figure 9a, in which data pointsrepresenting the heightsof ter- reducedamount of baselevel fall over a giventime perioddue to races above the channel bed were obtained from local hand level uplift ratesan order of magnitudelower resultsin a "vertically surveys at several reaches, and the long profile of the active compressed"series of strath surfacespinched close together. channel was obtained from a topographicmap. With no other Furthermore,the estuaryis much largerthan that of the Mattole, data, one might correlatethe terracesas in Figure 9b, the obvious and the tidal influenceoccurs several kilometers upstream. For and most simple solution (as done by Merritts and Vincent thesereasons, backfilling during sea level high standshas buried [1989]). The subsequentsequence of landscapeevolution is mostbedrock surfaces for many kilometersupstream, and the in- dominatedby long-term incision, with the river maintaininga termediatezone of strathexposure before the upstreamcanyon is nearly constantform throughtime. Strath terracesmerge with only severalkilometers in length. marine platforms at the mouth, as in the case of the Somme River, westernEurope (Figure 3). This solutionis appealingbe- causethe correlationsmatch an alreadypresent pattern, the shape Problems in Correlating Terraces in Order to of the modern channel bed during the presentsea level high ReconstructPast Longitudinal Profiles stand. It is inevitablefor many geomorphologiststo assumethat A numberof problemsmake it very difficult to correlateun- riverstend to maintaina fairly constantprofile shapeover time pairedterraces along a meanderingcoastal river that is nearly (the graded river) and furthermore to assumethat the modern continuouslyincising. The four most obvious are related to profileis a goodrepresentation of thispreferred state. stratigraphy,burial history,the natureof unpairedterraces, and Now considerFigure 9c with the samedatabase as in Figure methodsof surveyingand correlation: 9a, but with the modernchannel profile removed to preventpat- 1. Much caution must be used to correlate terraces based on tern recognitionand bias, and with additionalaltitudinal control radiometricages. Datable materialin the sedimentsabove strath providedfrom continuousgeodetic surveying. From thesedata, surfacesprovides minimum agesfor the time of strathformation terracesare correlatedas in Figure 9d and are steeperthan the but can spana long time period if the strathremains within the modern channel bed. Correlation of fluvial terraces based on to- zone of overbankdeposition by extremefloods. Baselevel rise pographicmaps for altitudinalcontrol, relative terraceposition, after strathformation can result in overbankdeposition up to and an array of radiometricdates (see problem 1 above) would thousandsof yearsafter the originalstrath formed. probablynot have led to suchan interpretation.However, based 2. The treadsof strathterraces may be buriedby subsequent on our recentdetailed survey results from the Mattole, Bear, and fill terraces,especially in lower reachesnear the coast,as at km Ten Mile Rivers, this solutionis correct,with exceptionof the 18-26 alongthe Mattole River. correlationsin reacha-b. In this reach,it might be reasonableto 3. Strathsurfaces are incisedvertically as well as laterally concludethat the fluvial terracesmerge with the marineterraces duringtheir abandonment, resulting in upstreamconvergence that (as in Figure9b) and hencefurther to deducethat lateralvalley producesa very complexrecord of time-transgressivesurfaces floor planationoccurs during times of sealevel high stands[cf. [cf. Seidl and Dietrich, 1992]. Bull, 1990]. However,in Figure9e, the totalstation survey data MERRITTS ET AL.' LONG RIVER PROFILES, TECTONISM, AND EUSTASY 14,047

a continuousTerrace b Surfaces MarineTerrace / Correlation of Terraces /Platform / • SurveyPo•nt on , Terrace •Modern Channel Bed

d

Additional Survey Points on Terraces

ß%• 5 3

e f Continuous Strath Strath Surface ContinuousFillTerrace / Surfaces / _ ß TreadofF•llTerrace •• m b

D_TSTANCE UPSTREAH Figure9. Idealizedterrace survey data (Figures 9a, 9c, and 9e) with three alternative correlations and interpreta- tionsof theoriginal continuity of thefluvial surfaces (Figures 9b, 9d, and9f). In Figures9a and9b, dataare from localsurveys of heightof terraceabove channel. In Figures9c-9f, data are from continuous geodetic surveying. The interpretationspresented in Figures9e and9f are discussedin the conclusionsand are similarto the resultsof theanalysis of theMattole, Bear, and Ten Mile Rivers.Note that only four terraces occur in Figure9b whereas fiveoccur in Figure9d, and seven occur in Figure9f. In Figure9f, theterraces 6 and7 areunrelated in originto terraces 1-5. indicate that terrace treadsin reach a-b are not steep,and farther givento illustratethe historical development of thoughtregarding upstreamin reach b-c they partially bury strathswith gradients sea level changeand formationof fluvial terraces(Figure 3). muchsteeper than the modemchannel slope. The final interpre- Larnothe[1918] andothers [e.g., Breuil, 1939]proposed that fill tation (Figure9f) makesit clear that two typesof terraceswith (thalassostatic)terraces at the mouths of coastal rivers formed different slopesoccur: fill terracesnear the mouththat formed duringtimes of sea level high stands. Lamothealso correlated duringtimes of sealevel high stands, and strath terraces farther strathterraces along the Sommewith fill terracesat its mouth and upstreamthat form most of the time as a resultof ongoingnet concludedthat he could use the profilesof terracesalong the vertical incision. length of a river to deduce the altitudes of late Pleistocenesea level high stands. In the case of the Mattole River, also in a Comparison of the Mattole, Somme, nonglaciatedsetting but on an activerather than a passiveplate and SusquehannaRivers margin, we concludethat strath surfaceshave much steeper Earlier, the classicexample of the nonglaciatedSomme River, slopesthan both the modem channel bed and the fill terracesat along the western European passive continental margin, was its mouth. Consequently,they cannotbe projecteddownstream 14,048 MERRITTS ET AL.: LONG RIVER PROFILES, TECTONISM, AND EUSTASY

to where they merge with high standfills, nor can they be usedto McLaughlin, D. Simpson,J. McAbery, and JereMelo. We also deducethe altitudesof sea level high stands. Their formationis thank S. Schumm,F. Ethridge,and N. Humphreyfor helpful not limited to, or necessarilyassociated with, times of sea level suggestions.We thank the following for financial support: high stands. National Science Foundation (Doctoral Dissertation A U.S. river that is perhapsmore similar to the Somme, the Improvement grant EAR-8405360 to Merritts; Research at Susquehannaalong the westernAtlantic passivemargin, also has UndergraduateInstitutions grants EAR-89-04785 and 89-17116; a seriesof late Cenozoic, strath terracesalong its length and fill supplementalgrant EAR-91-49176), and the U.S. Geological terracesat its mouth. Like the Somme, both are interpretedto Survey(NEHRP grant 14-08-0001-GN2334). Supportfor re- have formed during times of rising sea level prior to and during productionof the colorplate has been provided by NASA grant major interglacials [Pazzaglia and Gardner, this issue]. NAGW-3338 to Michael A. Ellis. Finally, we give a special However, Lamothe did not consider the effects of tectonics on thankyou to the geologystudents inaugurated to long field sea- the Somme River. Pazzaglia and Gardner, in contrast,modeled sons in this area: S. Polaski, R. Ticknor, L. Hammack, G. fluvial responseto both sealevel changeand flexure of the North Dudkiewiscz,R. Bruant,J. Buchman,I. Rose,and many others. American crust (due to denudationand offshore loading), and Thoroughreviews by W. Dietrich, R. Anderson,M. Ellis, and an concludedthat strathsform in areas of long-term, slow crustal anonymousreviewer greatly improved the manuscript. uplift and tilt resultingfrom isostasyand flexure and are at peak formation during times of rising sea level. Accordingto their in- terpretation,peak strath formation is the result of maximum lat- References eral incision at times of high baselevel. In contrast,in this study of rivers in a region of rapid uplift, we concludethat strathfor- Atwater, T., Implicationsof plate tectonicsfor the Cenozoictectonics of mation along a meanderingriver is not isolatedto times of high westernNorth America, Geol. Soc.Am. Bull., 81, 3513-3536, 1970. Bagnold,R. A., Bed-loadtransport by naturalrivers, Water Resour. Res., sea level or stable base level but rather occurscontinuously in 13, 303-312, 1977. reachesthat have both excessstream power and are upstreamof Bloom,A. L., Geomorphology:A SystematicAnalysis of Late Cenozoic the reach affected by sealevel rise. Landforms,2nd ed., 532 pp., Prentice-Hall,Englewood Cliffs, N.J., The results of the study described here are similar to those 1991. from investigationsof smaller rivers and baselevel change. As Bloom,A. L., W. W. Broecker,J. M. A. Chappell,R. K. Matthews,and observedby Leopold and Bull [ 1979, p. 195], a streamreacts to K. J. Mesolella,Quaternary sea level fluctuationson a tectoniccoast: processesof aggradationor degradationin "closely adjoining New230Th/234U dates from the Huon Peninsula, New Guinea, Quat. reaches,and not to a baselevel far removedin space". Baselevel Res., 4, 185-205, 1974. rise will result in a depositionalwedge that migratesupstream, Breuil,H., The Pleistocenesuccession in the SommeValley, Geogr.J., but it will only migrate upstreamuntil a gradientof depositionis 130, Part. 1, 87-105, 1939. established[Leopold, 1992]. Upstreamof that point, the base Bucher,W. H., Strathas a geomorphicterm, Science, 75, 130-131,1932. level rise has no influence. Rather, the streamresponds to up- Bull, W. B., Stream-terracegenesis: Implications for soildevelopment, Geornorphology,3, 351-367, 1990. streamcontrols (climate-influenced supply of sedimentand wa- Bull, W. B., andP. L. K. Knuepfer,Adjustments by the CharwellRiver, ter). If, however, this upstreamreach is in an area of long-term New Zealand,to uplift andclimatic changes, Geornorphology, 1, 15- baselevel fall inducedby uplift, as is the casefor the Mattole and 32, 1987. Bear Rivers in northern California, it may continueto downcut Burnett,A. W., andS.A. Schumm,Active tectonics and fiver response in even during a sea level highstand, becauseof the presenceof Louisianaand Mississippi,Science, 222, 49-50, 1983. nearby knickpointsmigrating upstreamthat have their origin in Chappell,J.M., andN.J. Shackleton,Oxygen isotopes and sea level, baselevel fall from long ago. Nature, 324, 137-140, 1986. In sum, interpretationof the terracesalong any coastalriver, Chorley,R. J.,Diastrophic background totwentieth century geomorpho- be it on an active or a passivecontinental margin, requiresexam- logicalthought, Geol. Soc. Am., Bull., 74, 953-970,1963. ination of both eustatic and tectonic processesof base level Clayton,K. M., ThalassostaticTerraces, in, The Encyclopediaof change. The ratio of lateral erosion to vertical incision varies Geornorphology,edited by R. W. Fairbridge, pp. 1142-1143, due to uplift rate, sealevel position,and upstreamdrainage area, Reinhold, New York, 1968. CLIMAP ProjectMembers, A. Mcintyre,Leader LGM Project,Seasonal resulting in complex flights of terraces that differ in number, reconstructionsof the Earth'ssurface at the last glacialmaximum, spacing, and age distribution even along a single coastline. Geol. Soc.Am.. Map Chart Ser.,36, 1981. Adjacent rivers with the same uplift rate do not have similar Davis, W. M., Baselevel, grade,and peneplain,J. Geol., 10, 77-111, numbersand ages of strath terracesbecause these landformsare 1902. not the resultof instantaneous,external events but ratherare part Davis,W., M., 1909, GeographicEssays, 777 pp.,Dover Reprint, New of a time-continuum that representsconstant adjustment to York, 1954. changingbase level. The degreeof adjustment,or fine tuning, Dawson,A. G., Ice Age Earth.' Late QuaternaryGeology and Climate, dependsupon the size and power of the river relative to the rate 293 pp., Routledge,New York, 1992. of tectonicuplift. Dickinson,W. R., andW. S. Snyder,Geometry of subductedslabs related to SanAndreas transform, J. Geophys.Res., 84, 609-627, 1979a. Acknowledgments. For permissionto work alongthe Mattole, Dickinson,W. R., andW. S. Snyder,Geometry of triplejunctions related Bear, and Ten Mile Rivers, we are grateful to J. Zanone, J. to SanAndreas transform, J. Geophys.Res., 84, 561-572,1979b. Dumitru, T., Major Quaternaryuplift along the northernmostSan Cooke, B. McBride, M. Etter, J. Wilson, J. Short,S. Lowry, M. Andreasfault, King Range,northwestern CA, Geology,19, 526-529, and G. Smith; the Roscoefamilies; and many more. We are in- 1991. debted to many at the Bureau of Land Management,the U.S. Fairbanks,R. G., A 17,000-year glacio-eustaticsea level record: GeologicalSurvey, the Georgia-PacificLumber Company,and Influenceof glacialmelting rates on the YoungerDryas event and the Mattole RestorationCouncil, for technicaland logisticalsup- deep-oceancirculation, Nature, 342, 637-642, 1989. port. We are especially grateful to J. Tinsley, T. Dunklin, R. Fairbridge,R. W., Fluvial terraces,environmental controls, in The MERRITTS ET AL.: LONG RIVER PROFILES,TECTONISM, AND EUSTASY 14,049

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D. J. Merritts, GeosciencesDepartment, Franklin and Marshall College,Lancaster, PA 17604-3003. (e-mail'[email protected]. (Received April 12, 1993; revisedFebruary 16, 1994;accepted March edu) 31, 1994.)