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Long River Profiles, Tectonism, and Eustasy: a Guide to Interpreting

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Long River Profiles, Tectonism, and Eustasy: a Guide to Interpreting 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 the caseof 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 set of 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
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