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Quaternary Intertzatiottal.Vol. 15/16, p. 99-112,1992.Printed in Great Britain. All rights reserved. 104&6182/92 15.001992 NQUNPergamonPressLtd

QUATERNARY SHORELINES IN SOUT HERN PERU: A RECORD OF GLOBALSEA-LEVEL FLUCTUATIONS AND TECTONIC UPLIFT IN CHALA BAY

Jos Luis Goy,* Jos Machar,? Luc Ortliebt' and Cari Zazo*Dep artm ento de Geologa, Universidad de Salamanca/37008, Salamanca, S paintlnstituto Geofisico del Per, Aparta do 3747 L i m a 100, PeruMission OR ST O M au Prou, Apartad o 18-1209, Lima 18 PeruMu seo Nacional de Ciencias Naturales, CS IC, JosGutierrez Abas cal 2, 28006 Mad rid, SpainTh e coastal area bordering the bay of Chala (14 5O'S., 4 15'W.), southern Peru, contains one of the most complete sequencesof Quaternary shorelines in South America. Remnants of about 27 high seastands have been preserved between present meansea-level and +275 m. Most remnan ts consist of staircased marine te rrace s and associated dep osits which are partly covered byalluvial fan and colluvial units deposited during intervening periods of lower sea-levels.No geochronological data are yet available; a tentative chronostratigraphy of the terr ace sequence is based on the geometricand stratigraphic relationships between successive landforms, and deposits. We group most marine terraces into 15 .majormorphostratigraphic units' (MMUs). Some of these major units seem to correlate with interglaciations, for example IsotopicStage 5, or with interstadials (Stage 3). Higher in the Chala sequenc e, ma jor morpho stratigraphic units mapped during field andair-photo studies may represent only parts of Middle Pleistocene interglaciations. We infer that shorelines located at +68, +121,+168, and +184 (or $200) m correlate with the highest seastands of Isotopic Stages 5, 7 9, and 11, respectively.The proposed chronostratigraphy suggests an average uplift rate of ca. 460 m d k y r fo r the Chala Bay area during the last 500ka. This rate is slightly higher than in the surrounding coastal areas (ca. 270-350 m d k y r ) , but significantly lower than rate s inthe area where the Nazca Ridge is being subducted below the South American P late (maximum rate ca. 740 m d ky r) . Deform edshorelines evidence several fault displacements within the Chala basin, but such deformation does not seriously distort therecord of former sea-levels in the basin. The Chala terrace sequence is the first reliable record of Middle and Late Quaternarysea-level fluctuations described from Peru.

INTRODUCTIONStudy of emerged marine terraces provides one of

the best ways to assess vertical motions experienced bycoastal regions during the Quaternary. This approachrequires (1) assumptions about the vertical position ofthe geoid during each high seastand throughout theQuaternary, and (2) ages for marine terraces along theuplifted coasts. These conditions are commonly metwith reasonable accuracy for Late Quaternary time(last 125 ka). The identification of remnants of highsea-level during the last interglacial, and the elabora-tion of models of sea-level variations in the last climaticcycle (Chappell, 1974a, b; Bloom et al., 1974; Chappelland Shackleton, 1986; Shackleton, 1987) were largelybased upon radiometric methods (U-series) used onfossil nearshore carbonates (mainly corals) from tropi-cal areas. But for the Middle Pleistocene (740-125 ka)and Early Pleistocene (1600-740 ka) periods, there aremany uncertainties about the position reached byglobal sea-level during the warmest interstadials andinterglacials and about the age of these episodes. Aframework for Quaternary chronostratigraphy, as wellas the best proxy for sea-level variations, are providedby deep-sea core isotopic data (Shackleton andOpdyke, 1973, 1976; Williams et al., 1981, 1988;Shackleton, 1987). However, isotopic data is insuffi-cient to assess paleo-positions of global sea-level.

'Presently at: ORSTOM-University de Antofagasta, Facultad deRecursos del Mar, Casilla 170, Antofagasta, Chile.

Reconstructions of relative sea-level variations need tobe based on direct measurement of former shorelines,and the best areas to study shorelines are in coastalareas with constant and relatively low rates of uplift.Such coasts should have been uplifted at a high enoughvelocity to permit paleo-shorelines to be distinguishedalong vertical profiles, but low enough so that theshoreline record goes as far back in time as possible(Ortlieb, 1987, 1988).

Coastal areas raised by constant uplift during theQuaternary are not numerous. In South America, alikely area of long-term constant uplift is along thePeruvian coast, where the Nazca Plate has beensubducting beneath the South American Plate at aconstant rate for the last few million years (80 k d my r ,Pilger, 1983; Pardo and Molnar, 1987), and where Plio-Quaternary uplift is documented (Broggi, 1946; Sbrieret al., 1982; Machar, 1987; Hsu, 1988; Ortlieb andMachar, 1990b). Thus, long sequences of staircasedQuaternary shorelines should be preserved along thispart of the Peruvian coast.Marine Terraces o n the Peruvian Coast

North and south of a relatively stable, 1,000 km-long,segment of the central coast of Peru (6 30'-14 S.), thecoast shows much evidence of Quaternary uplift(Sbrier et al., 1982; CERESIS, 1985; Ortlieb andMachar, 1990a). In northern Peru, Pleistocene marineterraces, known as 'tablazos', are rare and only locallyextensive (Bosworth, 1922; Devries, 1984,,1986,1988).

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99Fonds DocumentaireORSTOMCote: 6jtRS03-4 Ex O10015031


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100 J.L. Goy et al.In southern Peru, relatively narrow but numerousmarine terraces commonly rise up to +loo-200 m, asshown on regional maps south of 14 s. (e.g. Bellidoand Narvez, 1960;Mendvil and Castillo, 1960; aenand Ortiz, 1963;Narvez, 1964;Garca, 1968;Caldas,1978; Olchausky, 1980). These emerged terracesdemonstrate uplift (Steinmann, 1929; Broggi, 1946;Sbrier t al.,1982;Teves, 1977;Machar, 1987), butonly recently have they begun to be dated (Hsu, 1988;Ortlieb and Machar, 1990b;Ortlieb t al.,1990, 991).

The most studied region of Pleistocene shorelines inPeru is located near San Juan-Marcona (15 20'S.)(Broggi, 1946;Hsu, 1988;Hsu t al.,1989;Machar,1987; Ortlieb and Machar, 1990b; Machar andOrtlieb, in press .There, staircased Quaternary marineplatforms are particularly extensive and numerous; theoldest (earliest Pleistocene) terrace, the highest knownQuaternary marine unit in South America, reaches amaximum elevation of +780 m. Because the aseismicNazca Ridge is being subducted below the SouthAmerican continent in the San Juan-Marcona region, itwas assumed that this tectonic feature was responsiblefor the rapid uplift of the area (Machar, 1987;Hsu,1988;Machar and Ortlieb, 1990, in press .

Farther south, Pleistocene marine terraces arecommon, but are seldom found in long, continuous,sequences that would permit a reconstruction of sea-level variations. Some tens of kilometers south of SanJuan-Marcona, for instance, several sets of Quaternarybeach ridges range in elevation from sea level to ca.+200 m. But beach-ridges are lithologically similar andare poor indicators of paleo-sea levels because theyformed during regressions (instead of transgressivemaxima). In many sectors of the southern Peru coastwhere marine terraces are preserved, they are oftenpartly hidden by sand dunes or alluvial deposits.However, one sequence of marine terraces at Chala(1430's.;Fig. l , first described by Laharie (1970),offers a long record of well-preserved shoreline fea-tures and deserves closer study.

The Chala Sequenceof Marine TerracesThe area bordering the bay of Chala (15'45'-15 55 s. , 74'10'-74'2O'W.) displays a staircase-likesequence of 27marine terraces that climbs to +275 m.More terraces are found here than in any other sectorof the southern Peruvian coast, except for the SanJuan-Marcona area. The marine terraces are separatedfrom one another by steep erosional escarpments(risers). The resistant igneous bedrock of the Chalaarea has helped preserve a record of successive marineencroachments. The scarcity of beach sands along theembayment and the unusual W N W - E S E orientation ofthe coastline in this sector (at angle with the dominantwind direction) explain the lack of covering eoliansands, like those in coastal regions north and south ofChala (Gay, 1962).

But the Chala area is unique in its preservation ofcolluvial and alluvial units deposited during or soonafter each episode of, marine terrace formation. Thin

alluvial deposits partly overlie most of the terraces, ormay locally be interstratified with marine beds formedduring transgressions. Small fans were deposited aboveshoreline remnants at the foot of ancient sea cliffs.These continental sediments have been little erodedbecause of the low rainfall that affects the Peruviancoast, and because continuous uplift quickly raisesthem above the level of wave erosion.

As a result of these favorable conditions, the Chalaarea recorded successive marine transgressions andintervening pulses of alluvial deposition. It thus offers aunique opportunity for a detailed morphostratigraphicstudy of a particularly long series of marine andcontinental Quaternary sediments.Methodological Approach

Our reconnaissance study of Quaternary coastaldeposits and landforms in the Chala area aimed to use alocal morphostratigraphy based on geomorphologicalmapping to support a chronologic interpretation. Themaps of Quaternary marine and continental depositsand major recent landforms are based on an interpreta-tion of air photos at two different scales (approximately1/70,000: ig. 1, and 1/6,000: Fig. 2), as well as onsedimentological, paleontological and altimeter sur-veys. Figure 1 shows the major morphostratigraphicunits distinguished in Chala Bay and the main faultsactive in Plio-Pleistocene time in the area. The secondmap (Fig. 2) focuses on the central zone of the bay,along Chala River, and depicts with more detail thegeometrical relationships between successive coastalcliffs, marine deposits, alluvial fans and other continen-tal sediments. A common legend is attached to the twomaps.

Combined geomorphological, sedimentological andpaleontological studies helped to identify the deposi-tional environment of each sedimentary unit, to estab-lish lateral correlation of ancient coastal features, andto reconstruct for each marine terrace the position ofthe highest paleo-shoreline. The fossil content of themarine emerged sediments (Ortlieb and Diaz, 1991;Ortlieb, unpublished dutu proved to be of limitedchronostratigraphic utility since no significant variationwas observed among the distinct terrace deposits.

Mapped continental sediments include alluvial ter-races, alluvial fans and cones, colluvium units and slopedeposits (Fig. 1 . Most alluvium is carried by severalrivers that have progressively entrenched the volcanicbedrock of Chala basin. The back-edge of some marineterraces is covered by a relatively large amount ofalluvium that forms coalescent fans and cones while theback-edges of others are practically devoid of alluvialcover. In the former case, the thick alluvial unitssuggest relatively long periods of deposition.The morphostratigraphic study focusedon the recordof high seastands and on the reconstruction of thehistory of sea-level variations. The altimeter measure-ments were performed with a high-precision aneroidaltimeter (readings with a 1 m accuracy) and involvedrepeated calibrations at sea level to take into account


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102 J.L. Goy et al.LEGEND -- Part 1

HEIGHT MARINE LANDFORMS CONTINENTAL LANDFORMSm) AND DEPOSITS AND DEPOSITS

Alluvial conesFluvial terraceFluvial terrace

+3 lc c31 Beachesandlittoral ridges5 Erosionalsurface

++18 Terrace at +16m+31+45+5468 Terrace at +68m

+90 Terrace at +90m99 Terrace at +99m

+111 Terrace at + I l l m+121

Erosional surface at +11mscarpAlluvial cones

Alluvial conesAlluvial conesAlluvial cones

Terrace at +25 31mTerrace at +42 +45mTerrace at 50 +54m

scarpAlluvial cones,

colluviumscarpColluvium

scarpTerrace at +118- +121m I Alluvial cones.+145 errace at t145m colluvium+154 errace at +151 +154m I+162 Terrace at t162m Alluvial cones,colluvium

Alluvial cones,colluvium+I77 Terrace at t177m

+IV Terrace at t184m+184 (w)- 170 (E)Terrace at +184= t170m

+I80 Terrace at t180m+ I 9 0 Terraceat +190m+195 errace at t195m

t205 errace at t202 - t205msc rpColluviumsc rpAlluvial fan

+218 Terrace at +216- +218mAlluviaifan

scarpAlluvial fan,colluvium+228 Terrace at t228m

+235 Terrace a +235mt254 (w)37235 (E)Terrace at t235 = t254mAlluvial ia nsw

Alluvial fanErosion or scarp

266 Terrace at t266m274 Terrace at t274m

I 1 Marine Pliocenere-Pliocene substratum

SECTION I MMUIn F1g.l

FIG. 1. continued


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Quaternary Shorelines in Southern Peru 103LEGEND -- Part

GEOMORPHOLOGIC SYMBOLS

Alluvial conesand fansCoalescing alluvialfansColluvial deposits

Alluvial deposits

Alluvial terrace

DunesOverlapping deposits

Uplifting area

Landslide trackLandslide

Hanging valley

Morphological scarpRiver escarpment

Drainage gridSummit level

Uplifting area (minor)

Transgressive maximum

Scarp of m arine terraceMarine erosionalsurfacesFaultInferred fault

TiltingTriangular fa cets

Stratigraphic sections

FIG. 1 Geomorphological map of the Chala Bay area, Peru, showing major faults and the distribution of the majormorphostratigraphic units (MMUs) and Qua ternary deposits described in the text. Symbols are explained in the Legend (in twoparts), which is common to both maps of Figs 1 and 2. First part of Legend: The first column indicates our chronologicinterpretation. Th e second column lists marine terraces and their elevations surveyed in the C hala basin, along cross-sections 1-5(column 4) located in Fig. 1. Column 3 shows the types of continental landforms and deposits closely associated with eachepisode of marine terrace formation. T he last column lists the ma jor morphostratigraphic units (MMU), that were identified inthe Chala sequence (numbered with roman numerals, see text). In he second column, nu mbe rs to the left indicate the highestmeasured elevation of the corresponding shoreline (this elevation is used to refer to th e terraces in the text and in Fig. 3), whilenumbers to the right indicate one or two measured elevation(s) of the shorelines. Breaks in the horizontal lines in this columnshow differences in elevations resulting from faulting. T he second part of the Legend explains geomorphologic symbols used onthe two maps.

pressure and temperature variations. Uncertainty inthe determination of the shoreline positions combinedwith the instrumental accuracy resulted in an estimatedoverall precision of the measurements of -t3m. In mostcases, the shoreline elevations (relative to mean sealevel, MSL; Fig. 1 refer to the highest position reachedby sea level during each highstand, as carefullyestimated from well-preserved marine deposits anderosional features (paleo-seacliffs, shoreline angles,terrace back-edges) We use measured shoreline eleva-tions to refer to each marine (highstand) episode and tothe corresponding marine terrace deposits and land-forms (Figs 1-3).

The staircased arrangement of the marine terraces andthe favorable conditions for terrace preservation in thearea made lateral correlation of the landforms anddepositional units relatively easy. The few river valleyscutting across the Chala terrace sequence also exposedthe inner edges of the terraces. Two easily recognizablesand units that have a high reflectance on air-photosand crop out throughout the Chala basin were alsohelpful in the lateral correlation of the terraces.

We grouped successive marine terraces in what weterm 'major morphostratigraphic units' (Figs 1-3).Major morphostratigraphic units (MMUs) are assemb-lages of sedimentary deposits and landforms thatformed in relatively short spans of time during highseastands (interglacials). These units are separated by

features which suggest periods of low seastands(glacials), for example, thick alluvium overlying amarine terrace or high sea-cliffs, which indicate rela-tively long time periods between high seastands(assuming constant uplift). Our air-photo interpreta-tion suggests that the Chala sequence may be dividedinto about fifteen major units. We discuss below therelationships between the individual marine terracesand major morphostratigraphic units of the Chalasequence (Figs 1-3), and the high seastands andinterglacials of the Pleistocene.Geoclironological Liniitatioris

Studies of vertical motions in coastal regions rely onaccurate ages for marine terraces. Late Pleistocene(and latest Middle Pleistocene) shorelines may often bedated by radiometric (U-series, electron spinresonance) or bio-geochemical (amino-acid racemiza-tion) methods. Commonly, the identification of theshoreline of the last interglacial maximum (IsotopicSubstage 5e) by one of these dating methods then leadsto extrapolated age estimates for earlier shorelines.

At Chala, a few preliminary U-series analyses wereconducted (at GEOTOP Laboratory, Montral) onmollusk shells from the lowest-lying terraces. Theseanalyses did not produce reliable ages because theshells behaved as open geochemical systems. Otherauthors have encountered the same problem in the San


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104 J.L. Goy et al

FIG. 2. Deta iled geomorphological map of a central sector of the C hala Bay are a, along the, Chala River valley (see location inFig. 1). Explanation of symbols in Legend for both Figs 1 and 2 (see Fig. 1 .


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A _____ -_ J105

_ _ _ _ _ _ _ _ _ _ _ _5U T ---Jm -.

O O0 i O00Distance (m) 15 mFIG. 3. Schematic composite cross-section of the lower part of the Chala sequence of marine terraces, based on the 5 cross-sections located in Fig. 1 (circled numbers). M aximum elevation (in meters above present M SL) reached by sea level during theformation of each ma rine terrace is indicated on the vertical axis. Horizontal distances give an idea of the width of each m arineterrace. The roman numerals refer to major morphostratigraphic units (MMUs of Figs 1 and 2) defined in text. Black dotsindicate coarse marine terrace sediments; small points correspond to relatively thick marine sand units; open circles representthe largest alluvial fans and cones which generally mark the boundary between successive MMUs (except for MMUs III andVIII)

Juan-Marcona area (Osmond, 1987; Hsu, 1988; Hsu etal., 1989). Amino-acid racemization analyses on mol-lusks and barnacles from several terrace deposits are inprogress (at GEOTOP). Interpretation of initial analy-ses is a problem because of uncertainties in calibratingamino-acid age estimates using regional data fromprevious studies (Hsu, 1988; Hsu et al., 1989). Thebasis for calibration of the regional aminostratigraphyproposed by Hsu (1988) and Hsu etal. (1989) in the SanJuan-Marcona area is debatable both on morphostrati-graphic (identification of the last interglacial shoreline)and geochronological (questionable ESR age esti-mates) grounds (Ortlieb and Machar, 1990b; Ortliebet al., 1991). A thorough discussion of the amino-acidracemization data from the Chala area will be pre-sented elsewhere.Calibration of relative dating methods, includingESR and amino-acid racemization methods requiressound numerical ages. In the regions where U-seriesages are unavailable (for lack of fossil corals forinstance), detailed morphostratigraphic studies ofmarine terrace sequences, may sometimes provide avalid basis for amino-acid and ESR data calibration(Ortlieb, 1987; Ortlieb and Machar, 1990b).

GEOLOGICAL AND STRUCTURAL SETTINGThe Chala area is located in the emerged part of a

forearc massif called the 'Cordillera de la Costa', which

was a structural ridge during most of the Tertiary(Machar et al., 1986). Near Chala the Cordillera de laCosta basement is composed of a Jurassic volcanicsedimentary sequence (Chocolate Formation) that wasintruded by Cretaceous monzonites and monzo-dioritescorrelated with the Coastal Batholite (Olchauski,1980). The Tertiary is represented by marine siltstones,sandstones, and conglomerates that crop out in fault-bounded blocks in the northern and central parts ofChala Bay, and by subaerial pyroclastic rocks in thenortheastern corner of Fig. 1.Although not radiometri-cally dated, these volcanoclastic and sedimentary rockshave been correlated with the Pliocene La PlanchadaFormation of the Caman basin, 100 km south of Chala(Beaudet et al., 1976; Huamn, 1985). Remnants of a(Late ?) Pliocene marine transgression are preserved inriver valleys west and east of the study area, but were'not observed near Chala. During the Late Cenozoic,the paleogeographic evolution of the Chala basin waspartly controlled by tectonic activity, notably bydisplacements on several major faults (Huamn, 1985).

Three major fault systems bound the Chala basin:the first trends roughly N . 4 . along the Honda River,the second is oriented N120 E., almost parallel with thecoastline about 4 km inland, and the third has an E.-W.strike and extends east of Chala (Fig. 1). Within thezone delimited by these major faults, several secondaryfaults can be detected, especially along the course ofthe Chala and Huanca Rivers (N30 E., Fig. 1). Pre-vious studies (Sbrier et al., 1984; Huamn, 1985;


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106 J.L. Goy et alMachar et al., 1986) indicate that this part of thesouthern Peruvian forearc has been recurrently sub-jected to episodes of stretching and shortening duringthe Late Cenozoic. Since the last compressional phaseof Andean tectonics, which probably occurred in theEarly Pleistocene, the region has experienced a pre-dominant N.-S. trending extensional state of stress.

MORPHOSTRATIGRAPHY OF THECHALA SEQUENCEOur morphostratigraphic study of the Chala

sequence is based on a detailed analysis of thedistribution of the landforms and major marine andcontinental deposits associated with each of the succes-sive emerged shorelines.Marine Terraces and Major Morphostratigraphic Units>

At Chala, 27 distinct marine terrace remnants ofQuaternary high seastands were distinguished: theyounger two are Holocene and all older terraces arePleistocene. The typical uplifted marine terrace isseveral tens of meters wide (rarely several hundreds ofmeters) and slopes slightly (1-5) toward the sea. Theabrasion platforms of the terraces are eroded intovolcanic bedrock (or, locally, into Tertiary sedimentaryunits) and are covered by marine sediments (0.5-2 mthick), which in turn are often overlain by a thin bed ofalluvium. The marine deposits consist of unsorted sandand gravel containing pebbles and boulders, and aregenerally poorly cemented. The well-rounded, largerclasts, reworked from older marine and/or fluvialdeposits, are locally abundant and may form conglo-merates several meters thick. In a few localities,sublittoral sequences of sand are preserved with an insitu fauna (paired valves of pelecypods), but most oftenshells in the marine deposits are fragmented and beach-worn.

The back (inland) edges of marine terraces aregenerally marked by the topographic scarp of a paleo-seacliff; their external (seaward) edge is normallyformed by the top of the cliff, hat was eroded duringthe subsequent highstand. The cliffs that separate themarine terraces may be divided into two categories (atleast in the lower part of the sequence): larger cliffsabout 20-25 m high, and smaller scarps only a fewmeters high. This difference in scarp heights is one ofthe criteria that we use to distinguish major morpho-stratigraphic units in the Chala sequence.

Major morphostratigraphic units (MMUs) areassemblages of landforms (one or more marine ter-races) and associated deposits (both continental andmarine sediments) that seem to have formed in arelatively brief episode. These large scale groups ofmorphologic features can be observed in the Chalasequence, either in the field (from elevated viewpointsalong the Pan-American road) or on air-photos. Thegroup of marine terraces that lies between +25 and$70 m (MMU III), in particular, appears from somedistance as a wide, multi-phased feature (Fig. 1 .

Fifteen MMUs were recognized and numbered fromthe youngest to the oldest, I to XV (Figs 1 and 2). Thenine youngest MMUs include between one and fourindividual marine platforms , while the oldest (MMUsX to XV) are represented by single marine terraces.Identification of the oldest MMUs is provisionalbecause erosion of the earliest marine landforms anddeposits, or burial by alluvial deposits in some sectors,obscures their chronologic relationships.

The legend of Figs 1 and 2 presents a compositesequence of the main features identified in the Chalabasin, as well as a tentative reconstruction of the mainprocesses that produced the Quaternary sequence.Most detailed information used for this reconstruction,including shoreline elevations, was collected along the5 transects shown on Fig. 1.The Nine Younges t MM Us

The youngest major morphostratigraphic unit(MMU I) is represented by landforms and sediment ofHolocene age: the present beach, an isolated remnantof an emerged shore platform, a high seacliff cut inlithologically varied bedrock, and alluvial sedimentalong river valleys. The beach is mostly sandy in theeastern part of the Chala embayment (near a source ofTertiary sedimentary strata) and gravelly (roundedclasts up to boulder size transported by the rivers) inthe rest of the bay. The clasts accumulate.in temporarystorm berms as much as 3 meters high (Fig. 2). A small,emerged, abraded platform is observed at a maximum+5 m elevation, near the farmer Hotel de Turistas ofChala. The limited width of the platform and itslocation between two faults (Fig. 1) suggest it is anuplifted remnant of a Mid-Holocene terrace (Ortlieband Machar, 1989).

Terrace remnants corresponding to MMU II weremapped only along the eastern part of Chala Bay (Fig.1). They mainly consist of a small abraded platform at+11m on which the Hotel de Turistas was built) anda marine terrace deposit with a maximum observedelevation of +18 m (Fig. 2). Because these featureshave no lateral equivalents in the rest of the embay-ment we hesitated to recognize them as a separateMMU. But since they are cut into the youngest units ofMMU III, and because the marine deposit is alteredenough to be pre-Holocene, we have grouped theminto a morphostratigraphic unit distinct from MMUs Iand III. The present elevation of the platform andterrace deposit is probably due to recent uplift on anearby N45E. trending fault (Fig. 1).

MMU III is the most widely exposed unit of theChala sequence. It is the only major unit that includesfour marine terraces. The highest shorelines of each ofth e terraces are found at +31, +45, +54, and +68-64m respectively (Fig. 3). The youngest marine terrace ofMMU III (+31m) is almost continuous along the Chalaembayment (Fig. 1). Roadcuts along the Pan-Americanroad, NW of Chala, show that the sedimentarysequence overlying the abrasion surface includes poorlysorted and fossiliferous sediment, conglomeratic


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Quaternary Shorelines in Southern Peru 107marine beds, and lenses of lagoonal fine sand and silt.Locally, this sequence is partly cut into similar sedi-ments deposited during the previous transgression(terrace at $45 m), or onlaps a 3 m thick alluvialconglomerate, which overlies deposits of the earlier+45 m terrace. The span of time between the twoperiods of terrace formation was brief (a single alluvialphase) and apparently corresponds with an episode ofsea-level lowering between two interstadial highstands.The small variations observed in the elevation of theshoreline (+31-25 m) of the +31 m terrace documentsmall, recent, vertical displacements on faults duringtilting of tectonic blocks.

The second youngest marine deposit ($45 m) ofMMU III is well preserved in the central part of theembayment (Fig. 1). North and south of the ChalaRiver valley, this deposit cuts deeply into the sedimentsof the previously deposited marine unit (+54 m).

The third youngest marine terrace of MMU III ($54m) is laterally very extensive (Figs 1 and 2). It mainlyconsists of a thick sequence of coarse regressive marinesand several meters thick and more than a hundredmeters wide. The chronologic relationships betweenthis terrace deposit and the next older terrace (+68-64m) are uncertain. The +54 m terrace undoubtedly post-dates the +68 m shoreline, but it is unclear if it is muchyounger than the shoreline (distinct interstadial), or if itconstitutes a regressive feature of the $68 m highstandepisode. The +54 m terrace does not seem to cut intothe +68 m terrace, although alluvial units weredeposited between the marine sediments of the twoterraces.The earliest shoreline of MMU III is found at thefoot (+68-64 m) of a 25 m-high paleo-seacliff. Themorphological similarity between the paleo-seacliff andthe modern one eroded by the sea during the Holoceneargues for a correlation between the +68 m shorelineand the earliest highstand of the last interglacial(Oxygen Isotopic Substage.5e). Extensive alluvial fansand colluvium (Figs 2 and 3) cover the foot of theseacliff and most of the marine sediment coeval withthe transgressive 5e maximum. In spite of the lack ofoutcrops of marine sediment, this paleo-shoreline isone of the most conspicuous in the Chala sequence.

MMU IV is comprised of two narrow marine terraces+90 and +lo0 m) around most of the bay. In thecontemporaneous with terrace formation has split thesetwo features into 3 or 4 distinct terraces (Figs 2 and 3).

Landforms of MMU V are similar morphologicallywith those of MMU IV, and include two narrowterraces (+111 and +121 m) (Figs 2 and 3). The 20 mhigh paleo-seacliff backing this MMU is steeper andmore conspicuous around the bay than the cliff separat-ing the landforms of MMUs IV and V. It is similar tothe Holocene seacliff and the cliff backing the terracesof MMU III (Fig. 3).

The main landforms of MMU VI are a narrow, well-formed, marine abrasion platform (shoreline at +135m, locally at +145 m) and a much wider terrace (+154

I

southeastern part of the embayment, surface faulting

m) characterized by an irregular surface. The sea-cliffthat marks the inner edge of this MMU is particularlylow; near the southeastern bank of Chala River it isreplaced by a swale-like depression (due to runofferosion or possibly to displacement on a small fault).

In MMU VI1 we grouped two marine terraces (+162and $168 m) that are often difficult to distinguish onair-photos. The scarp that divides these two terraces issmall (3 m), but the paleo-seacliff bounding the inneredge of this MMU is steep and high (10 m). Abundantalluvial fans overlie the marine deposits of this unit.

The landforms and deposits in MMU VI11 arelaterally extensive and somewhat similar to those ofMMU III. This MMU includes two marine terraces($177 and +184 m). High reflectance from the surfaceof these terraces on air-photos is due to an extensive,light-colored, gypsiferous sand, locally called yapato.The sand is the result of a pedogenetic alteration of themarine terrace deposits. Below the yapato surface,the sequence of sediment underlying the lower of thetwo terraces on the western bank of Chala Riverconsists of, from top to bottom:fluvial (?) conglomerate (1.5 m thick) withreworked marine shingle-like cobbles and a gypsiferousmatrix (secondary gypsum migrates from underlyingbeds and/or evaporates from marine overflow waterbehind beach-ridges);

wo superposed lagoonal beds (each ca. 0.5 mthick), the uppermost with fine marine sand andgypsum, and the lower one with coarse, greyish sandand gravel;n alluvial gravelly conglomerate (1 m) with abun-dant sandy matrix;lternating beds (ca. 2.5 m) of marine sand andgravel with small boulders that overlie a marineabrasion platform. The fauna of this unit includes thepelecypods (predominantly Mesodesma doizacium andEurhonzalea lenticularis), gastropods and barnaclescommonly found in the nearshore zone in the area.

The whole sequence described, which is probablythicker than in most terrace deposits of the Chala area,indicates that the sediments underlying the marineterraces are not restricted to a single transgressivemarine deposit. The yapato soil on the MMU VI11terrace documents surface faulting in the center ofChala Bay. The highest outcrops of yapato surface areat different elevations on both sides of Chala River:+184 m on the southeastern bank and +170 m on thenorthwestern bank. Thus, displacement of the ChalaRiver fault (Figs 1 and 2) may have occurred betweenthe deposition of MMU VI11 and the formation ofMMU VI1 landforms, which do not show a 14 mvertical displacement across the river.

Three prominent marine terraces of MMU IX on thenorthwestern bank of Chala River were identified at ca.+BO, +185 and +190 m. In the eastern part of theembayment, the landforms related to this MMU (andolder ones) are extensively covered by the distal edgesof alluvial fans that head in the foothills of the Andes(Figs 1 and 2).


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108 J.L. Goy et al.The Oldest MM Us

The deposits and landforms of the oldest MMUs (i.e.X, XI, XII, XIII, XIV) are found mostly in thenorthwestern part of the embayment, particularly alongthe eastern side of the Honda River (Fig. 1). TheseMMUs include the highest marine terraces of the Chalaembayment at +205, f218, +228, +235, +266 andf274 (?) m, respectively (Figs 1 and 2). Elevationswere measured at only a few localities, so the listedelevations may not be accurate paleo-sea-levels for theoldest high sea-stands in the Chala area.

The two oldest MMUs reported in the Chala areaabove an elevation of +240 m may be of Late Plioceneage. Outside the study area, on the western bank of theHonda River, a Quaternary marine limit is mapped at+240/+260 m; there, the highest Pleistocene marinesediments in the area onlap Pliocene sandstones at+220-230 m. Thus, unless recent movement on theHonda River fault (Fig. 1 has raised the marine limit,the oldest MMUs may be Pliocene.

CHRONOSTRATIGRAPHIC INTERPRETATIONGl oba l Sea-level Variations

Since the work of Shackleton and Opdyke (1973,1976) on the deep-sea cores V28-238 and V28-239, thechronostratigraphy of the Pleistocene has been closelylinked to the oxygen-isotope stratigraphy (Bowen,1978; Imbrie and Imbrie, 1980; Pisias et al., 1984;Ruddiman et al., 1986; Williams et al., 1988). Thisisotopic chronostratigraphy provides only approximateages for the interglacials and glacials of the Pleistocene,although several time scales based on different mathe-matical treatments of deep-sea core data have beenproposed (Shackleton and Opdyke, 1976; Innbrieet al. ,1984; Williams et al., 1988; Ruddiman et al. ,1989). Forthe Early Pleistocene, there is not even a consensus onthe identification and numbering of the isotopic stages.Therefore, the absolute chronology of Middle andEarly Pleistocene high seastands remains largelyapproximate. Isotopic data, however, do provide arough chronology, and indicate the magnitude of globalsea-level variations, both of which are of great help ininterpreting sequences of marine terraces.

Radiometric dating of Late Pleistocene coral reefsallowed calibration of the youngest part of the deepsea-core isotopic chronology (Shackleton and Opdyke,1973, 1976) and provided estimates of paleo-sea-levelheight for each high seastand of the last interglacial:Isotopic Substage 5e (125 ka), 5c (105 ka) and 5a (80ka) (e.g. Chappell, 1974a, b; Bloom et al., 1974). Mostresearchers agree that sea level reached a maximumelevation of about +6 m above present MSL at thebeginning of the last interglacial (Substage 5e), and thatduring the ensuing Substages 5c and 5a sea levelprobably was close to present MSL, or at most a fewmeters below MSL (Chappell and Shackleton, 1986).

During the interglacial maxima of the Middle Pleisto-cene, sea level was also probably close to MSL. Thedeep-sea core isotopic data suggests that the Isotopic

Stage 7 lasted some 70 kyr (Imbrie et al., 1984;Martinson et al.,1987; Williams et al., 1988) and that itincluded at least two major and one minor highstand.During the Stage 7 highstands, sea level was probablybelow the present datum (Shackleton, 1987). ForIsotopic Stages 9 and 11, some core data show singlemajor peaks that are high enough to suggest that sealevel attained a position similar to present MSL. Thelast four interglacials (Stages 5, 7, 9, and 11) wereprobably longer and warmer than earlier Pleistoceneinterglaciations because the older isotopic stages arecharacterized by lower peaks that suggest high sea-stands lower than present. Stages 13, 17, 19 and 21, inparticular, were apparently characterized by relativelylow seastand maxima (Shackleton, 1987).

Early Pleistocene sea-level fluctuations also hadsmaller amplitudes and frequencies than later Pleisto-cene fluctuations (Shackleton and Opdyke, 1976; ,Williams et al., 1981, 1988; Ruddiman et al., 1986).Records of Early Pleistocene highstands are extremelyscarce, possibly because the shorelines of this periodwere almost entirely eroded during the subsequenttransgressions of the Middle Pleistocene.Tentative Clironostratigrphy of the Chala Sequenc e

Following our description of the major morpho-stratigraphic units found in Chala Bay, and the briefoverview of the present knowledge about globalQuaternary sea-level variations, let us examine how theChala sequence may be interpreted chronostratigraphi-callyM M U s I and

MMU I consists of Holocene coastal features and is,therefore, correlated with the Isotopic Stage 1.

The MMU II terrace remnants, preserved in thetown of Chala only, are latest Pleistocene. Theseterraces formed after the youngest terrace of MMU III(80 ka ?) and before the Mid-Holocene sea-levelmaximum (5-7 ka), probably during Isotopic Stage 3.These terraces may correlate with either (or both ?) ofthe Stage 3 high seastands observed elsewhere at about60 or 40 ka (Bloom et al., 1974; Chappell, 1974a, b).Their maximum elevation (+18 m) may indicaterelatively rapid uplift of a small fault-bounded block,because paleo-sea level at 60 and 40 ka rose only to ca.-30 m (Bloom et al. , 1974; Chappell and Shackleton,1986).M M U I I I

The well-developed features of MMU III includefour marine terraces (+31, -1-45, i-54, +68 m) andintervening continental deposits. The stratigraphy ofMMU III shows that alluvial episodes, coeval with lowseastands, alternated with episodes of marine terraceformation (highstands). The thickness and sedimentol-ogy of the continental deposits suggest that the alluvialphases were brief and did not correspond to entireglacial periods. So, we group the landforms anddeposits preserved between about i-25 m and $68 m


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Quaternary Shorelines in Southern Peru 109into a single MMU, which we infer encompasses alimited period of time, most probably a full interglacialstage.

The physical boundaries of MMU III are two majorseacliffs of comparable shape and height: the Holocenesea-cliff and the scarp between $70 and $90 m. Themorphological and geometrical similarity of the twoscarps may indicate that the duration of cliff formationwas equivalent for both features. Because the seacliffbacking the modern shore was eroded during thepresent interglacial (Holocene), the $70-90 m seacliffmay have been eroded during an early phase of the lastinterglacial (Substage 5e, 125 ka). In both cases, theseacliff height may reflect the amount of uplift experi-enced by the coastline during entire glacial periods (ca.80-10 ka and ca. 180-130 ka).

Thus, if the +68 m shoreline corresponds to the sea-level maximum of Substage 5e (125 ka), the highseastands responsible for two of the younger marineterraces (+54, +45 and +31 m) may have occurredduring Substages 5c (105 ka) and 5a (80 ka). Becausethe landforms and associated deposits related to thepaleo-shorelines at 1-31 and $45 m are typical ofmarine terraces formed during such highstands, theseterraces may be coeval with Substages 5a and 5c. Analluvial unit between the deposits associated with the+45 and +31 m terraces confirms that a sea-levellowering of interstadial magnitude occurred betweenthe two high seastands marked by the terraces. Thewide and thick sand at +54 m may have been depositedduring a second highstand within Substage 5e or duringa late Substage 5e regression.

*

Older M M U sMMUs IV to IX in the Chala sequence are foundbetween ca. +90 and +200 m. All these older unitsinclude a pair of marine terraces (except for the oldestone which has 3 marine terraces). Our definition ofmajor morphostratigraphic units implies that a seriesof depositionaVerosiona1 events occurred in a limitedspan of time during interglacial periods of high sea-level. If so, most MMUs probably include groups oflandforms and deposits related to the successive inter-glacials recorded by the isotopic stages of Pleistocenedeep-sea cores. On this basis, MMUs IV, V, VI, VII,VIII, and IX, might be coeval with Stages 7, 9, 11, 13,15 and 17, respectively. However, there is anotherinterpretation supported by several lines of evidence. Itshould be noted that we correlated MMU II withIsotopic Stage 3, which is an interstadial, not aninterglacial period. Furthermore, the oxygen isotopecurves from many deep-sea cores strongly suggest thatduring Stages 15 and 17, sea level did not reach aposition similar to the present one (Shackleton andOpdyke, 1973, 1976; Shackleton, 1987); consequently,it is unlikely that these minor interglacial highstandscorrelate with the extensive morphologic features anddeposits of MMUs VI11 and IX (Figs 1-3). In fact, acloser examination of morphostratigraphic features andsome assumptions about Middle Pleistocene sea-level

fluctuations suggest alternative chronologic interpreta-tions.

Beginning with Isotopic Stage 7, this interglacial (thepenultimate) may be represented by landforms of bothMMU IV and V. As Isotopic Stage 7 lasted at least aslong as Stage 5 and included (at least ?) 3 highseastands, it should be represented by more than thetwo small terraces (at +90 and $99 m) of MMU IV.MMU IV features are preserved between $90 and+ l o 0 m (10 m elevational spread), whereas those ofMMU III are vertically spread between $25 and $70 m(45 m); if the landforms and deposits of MMUs IV andV are combined, their total vertical spacing is ca. 30m ($90-121 m). Another argument in favor of acorrelation of both MMU IV and V with Stage 7 is thesteep, 20 m-high scarp between +120 and $140 m,which is similar to the scarps at +4-20 m and $70-90m . Thus, we favor a tentative correlation between bothMMU IV and V and the Stage 7 interglacial.For the same reasons (similar time span for Stages 5,7 and 9 and the small height of the scarp between MMUVI and VI1 landforms), Isotopic Stage 9 may berepresented by the landforms and deposits of bothMMU VI and VI1 (however, the paleo-seacliff backingthe landforms of MMU VI1 is less than 10 m high).

Following this chronologic interpretation, the thicksequence of deposits in MMU VI11 at ca. $180 m andcapped by the yapato soil, would correlate withIsotopic Stage 11. MMU IX features may also berelated to Stage 11. Oxygen-isotope data from manydeep-sea cores (Shackleton, 1987) suggest that Stage 11was a long-lasting interglacial that was as warm as Stage5 and during which sea level reached higher thanpresent MSL. In northwestern Mexico, marine terracescorrelated with this stage appear to be more extensivethan any other terraces of Middle Pleistocene age(Ortlieb, 1987, 1991). So, it is plausible that all theterraces observed between $170 and $200 m in Chalaarea (MMUs VI11 and IX), which have a similar widthand extent to those correlated with Stage 5, wereformed during Stage 11.

Obviously, our tentative chronology needs to beconfirmed by independent dating control. Such controlmay be provided by future amino-acid analyses (oncethe regional aminostratigraphy is well-defined), orpossibly by paleomagnetic studies. Meanwhile, thistentative chronostratigraphy is a working hypothesisthat is consistent with the morphostratigraphic subdivi-sion of the Chala sequence.

UPLIFT AND FAULTINGRegional UpliftRates

In Chala Bay, the mean uplift rate calculated fromthe elevation of the +64-68 m shoreline that we assumeto be of Substage 5e age (last interglacial maximum) isca. 460 mdkyr. Both north and south of Chala,remnants of the shoreline that seem to correlate withthe +68 m feature are generally found at +40-50 m, a nelevation indicating lower regional uplift rates of ca.


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110 J.L. Goy et al.270-350 mdkyr. Thus, the faulted block of the Chalabasin has been uplifted slightly more rapidly thanadjacent parts of the coast during the Late Quaternary.

Our chronostratigraphic interpretation of the lateMiddle Pleistocene units suggests that the highestseastands of the Isotopic Stages 7, 9 and 11 arerecorded at +121, +168, and +184 (or +200) m,respectively. Accordingly, the mean uplift rate duringthe period 430-200 ka was about the same order ofmagnitude (500-430 mdkyr) as the rate in the LateQuaternary (460 md kyr). The relative vertical spacingof the marine terraces above -I-200 m suggests thatearlier in the Pleistocene (490-1600 ka), the meanuplift rate may have been significantly lower (ca. 250m d k y r ?). But, of course, we still know very littleabout the paleo-sea level heights, and ages of the EarlyPleistocene highstands.Surface FaultingThe three major faults bounding the Chala basin(Fig. 1) and the secondary faults that partly control thedrainage system of the area, vertically deformed theterraces during the Quaternary. Slip on the N120 E.trending fault system that limits the Chala basin to thenortheast, for instance, is indicated by the thick alluvialsequences deposited along the mountain fronts duringmost of the Quaternary. In our mapping of marineterraces, we found several vertically offset shorelines,which provide estimates of the amount of displacementand the age of deformation. For example, a 14 mvertical offset of the MMU VI11 terraces was producedby displacement on the Chala River fault. This dis-placement occurred shortly after the formation of the+180 m marine terrace, probably during Isotopic Stage10 (approximately 360-390 ka), and before the forma-tion of the MMU VI1 terraces (Stage 9). We also foundsome displacements of a few meters on faults boundingsmall blocks within the Chala basin.

Other displacements between major fault blocksfound during our general survey of shorelines include:Late Quaternary westward tilting of a block limited bythe (N35 E. trending) Chala River fault and a faultalong the unnamed river that ends north of Chala town(Fig. 1);Late Quaternary westward tilting of the blockbounded by the (N35 E. trending) Huanca and ChalaRiver faults; and eastward tilting of the block limited bythe Honda River ( N . S . trending fault ?) and HuancaRiver (N35 E. trending fault). As a consequence ofthese displacements, the Pleistocene shorelines areslightly more elevated on the western and eastern basinmargins than in the central part of the Chala basin.

'

CONCLUSIONSStudies of the vertical tectonic deformation of marineterraces and of Pleistocene sea-level variations are

closely interdependent. One cannot reccmstruct paleo-sea-level heights during Pleistocene highstands withoutassumptions about uplift rates. Conversely, neotectonicstudies of coastal areas need ages for emerged shore-

lines as well as estimates of former sea-level positions(relative to present MSL) when marine terraces wereformed. But where ages for terraces are lacking, studiesmust rely on morphostratigraphic mapping and chrono-stratigraphic interpretation.

Morphostratigraphic analysis is most likely to besuccessful on extensive sequences of emerged shore-lines. In Peru, where there is an urgent need toestablish a chronostratigraphic framework for themarine Pleistocene, the small basin of Chala, with itslong, well-preserved, sequence of terraces, wasselected as the most promising area for a reconnais-sance study of the Quaternary landforms and deposits,even though no radiometrical ages were available.

Field observations and air-photo interpretation ofthe Chala marine terrace sequence led us to groupmarine and continental landforms and deposits intomajor morphostratigraphic units (MMUs). These unitswere initially interpreted as groups of features relatedto single interglacial episodes. But interpretation of thedeep-sea core isotopic record, as well as the morph-ology and heights of marine terrace scarps and thethickness of associated alluvial deposits suggest that therecord of high seastands in the Chala area is morecomplex than suggested by our initial interpretation:successive interglacial episodes may be represented bypairs of MMUs. The two MMUs that probably arecorrelative with single interglacial episodes are those ofMMU I (Holocene) and MMU III (last interglacial,Stage 5). However, neither of these MMUs is typical;the Holocene MMU is not yet completed, and theassemblage of features included in MMU III is equiva-lent to assemblages in groups of at least two MMUshigher in the sequence.

Our chronology for pre-last interglacial of the Chalasequence is based on correlation of MMU III with thelast interglacial and then on extrapolation of olderMMUs back in time (upwards in the sequence); the lastfour interglacials (Stages 5 , 7 , 9and 11) are representedby the groups of marine terraces between +20 and+200 m. The earliest part of the Pleistocene is less wellrepresented by terrace remnants between +200 and+240-274 m.According to our chronostratigraphic interpretation,long-term uplift of the Chala basin may have beenconstant in the last half million years, and more rapid(460 mm/kyr) than the regional rate of uplift inferredfor the southern Peru coast. Due to this higher rate ofuplift, the Chala marine terrace sequence registers Lateand Middle Pleistocene sea-level fluctuations withgreater detail than areas to the north and south.

Surface displacements on faults within the Chalabasin appear of limited magnitude, and can generallybe measured accurately. This is not the case in the SanJuan-Marcona area where surface faulting has signifi-cantly displaced many marine terraces (Ortlieb andMachar, 1990b; Machar and Ortlieb, in press). Forthis reason, in spite of excellent preservation of manyPleistocene shore platforms at San Juan-Marcona, it isdifficult to determine the age and amplitude of the fault

,

,


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Quaternary Shorelines in Southern Peru 111displacements and to correlate marine shorelines (Hsu,1988). Because the rates of surface faulting near Chalaare much lower, the sequence of Chala yields a moredetailed record of Pleistocene sea-level variations thanthe only previously studied area on the Peruvian coast.

ACKNOWLEDGEMENTSThis work results from the com bined efforts of a Franco-Peruvianscientific agreement between ORSTOM, L'Institut Franais deRecherche Scientifique pour le Dveloppement en Coopration (UR1E) and IGP, Instituto Geofsico del Per (L.O. and J.M.), on onehand, and of Spanish research supported by the projects PBSS-0125(J.G.) and PB89-0049 (C.Z.) on the other hand. The authors areindebted to Brad Pillans and Eric Leona rd who reviewed an earlierversion of the pape r, and to Alan Nelson who provided many helpfulcomments to improve the text. This is a Contribution to IGCPProject 274, done in cooperation with the activities of the Neotec-tonics and Shorelines Commissions of INQUA.

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