Landform Evolution in the Coastal Region of Tarapaca Province, Chile

LANDFORM EVOLUTION IN THE COASTAL REGION OF TARAPACA PROVINCE, CHILE

par C. MORTIMER * et N. SARlt **

RESUMEN many enclosed basins of the coastal mountains is thought to be by sub-surface seepage. The Coastal Se considera .que la plataforma continental y Cordillera was uplifted at the close of the Paleo- el alto acantilado costero del Norte Chileno han gene, and, because of the low erosion rates, the sido formados durante el retroceso Pliocénico. amount of sediment deposited in the Perú-Chile Subsecuentemente la tectonica local produjo una Trench area during the N eogene has been restricted emergencia del litoral, permitiendo asi la forma- to that derived {rom coastalrecessión. ción de terrazas marinas al pié del cliff. El fallamiento en bloques de la Cordillera de la Costa, se produjo con anterioridad a la consti- 1. INTRODUCTION tucion de las formas litorales y a la depositación del aluvio más joven de la Pampa del Tamarugal. La reconstrucción de la topografta pre-fallamiento, Throughout the greater part of northern Chile mediante diagramas, revela que correspondta a exist well defined physiographic and tectonic un pediplano. provinces which are, from east! to west; the Andean block, the Pampa del Tamarugal La red de .drenaje a traves la Cordillera Costera, (central tectonic depression), the Coastal Cordil- se implantó cuando el alu vio, que empezo a rebasar lera and, offshore of a locally terraced littoral; desde la Pampa, alcanzó el acantilado retrocedente. the Perft-Chile Trench. Virtualmente no existe desarrollo de cauces fuera de los cañones y se piensa que, el drenaje hacia los This paper is based on a detailed study of that numerosas cuencas cerradas de las montañas part of the Coastal Cordillera east of the town costeras, es por percolación subterranea. of Iquique lying between 19°30t and 20030t lato S. (SARlé, 1971). The area can be regarded as typicál La Cordillera de la Costa se levanto al fin del of the coastal region ot the Province of Tarapacá Paleogeno; a causa de la baja tasa de erosión, which has been studied in less detail between la cantidad de sedimentos depositados en el área Arica (180 30' lato S.) and the Río Loa (210 de la fosa Chileno-Peruana, durante el Neogeno, se ha restringido a los derivados del retroceso 30' lato S.) (Figs. 1 and 2). costero. This is one of the most raid regions in the world, Iquique has an annual average rainfall ABSTRACT of 1.9 mm. (Almeyda, 1949) and vegetation is accordingly completely absent from most of The continental shelf and high cliff of northern- the area. It only occurs in the deep river valleys most Chile are considered to have been formed which carry exotic water from the Andes, and during Pliocene coastal recession. Subsequentlocal on high summits near the coast where conden- sation from the coastal fog allows a few shrubs tectonically induced emergenceof the littoral per- to survive. mitted marine terracing at the cliff foot. Block-faulting of the Coastal Cordillera occurred Geological mapping (THOMAS1970 a, b, SALAS before the formation of the littoral landforms and and others, 1966; SILVA, in prep,) has revealed before the deposition of the younger alluvium of that J urassic and Cretaceous sediments and the Pampa del Tamarugal. The pre-faulting volcanic rock's which have been intruded by topography can be. diagramatically reconstructed various elements of the Andean Batholith cons- to reveal a pediplain. titlite the predominating bedrock of the region. Drainage acrossthe Coastal Cordillera was esta- Orogenic movements occured in the Cretaceous, and since then the area has been continuously blished when overspilling alluvium from the Pampa met the receeding cliff. above sea leve!. A sequence of Tertiary continen-

Channel development is virtually non-existant (*) Bickerton, Wetherby, Yorkshire, England. away from the canyons, and drainage towards the (**) Compañia Minera Exotica, Calama, Chile.

------163 T.XXI C. MORTIMBR ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION

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Area 01 Fig. 2 2,0 lO O ,lO 20,.30 'f! SC I KILOM ETERS -' FIG. 1. - Physiog1'aphy and d1'ainageof n01'lhe1'moslChile. FIG. 1. - Physiog1'aphie el d1'ainage de l'eXI1'~me n01'd du Chili. 164 C. MORTIMER ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION T.XXI ral sediments mantles the topography, whereas on the coastal terraces there are thin deposits of Plio-Pleistocene marine sediments.

2. LITI'ORAL LANDFORMS

A clifI which in places is over 1000 m. high extends along the greater part of the coast of northern Chile. In Tarapacá Province it is only broken where infrequent gorges cut through the coastal mountains. South from Iquique there is a continuous flight of terraces at the clifI-foot which are covered by marine sediments. To the north between Arica and Iquique, terraces are only locally developed. BRÜGGEN(1950) proposed that the clifI was (THOMAS,op. cit.; SILVA,op. cit.). We consider an actual fault scarp, and that the terraced littoral was a downthrow splinter. Since geological mapping has shown that the fault do es not exist (Thomas op. dt.; Silva op. cit.). We consider that the clifI originated by wave action during the Pliocene transgressive episode, evidence of which has been noticed all along the north Chi- lean coast (Herm, 1969). Coastal subsidence is thought to have occurred, which permitted the waves to attack continuously across the deve- loping wave"':cut platform. Evidence of subsidence occurs in the Lluta Valley near Arica, where continental Miocene sediments and subaerially deposited ash-flows now lie at least 300 m. below sea level (KARZULOVIé,1968). Near to the mouth of the Rio Loa, warped lacustrine terraces in the coastal mountains probably indicate downward tilting to the west of the irnmediate coastal region. The world-wide tectono-eustatic preglacial sea level rise proposed by GALLOWAY(1970) may have aided the transgression. Prolilt __'o\llt If coastal retreat has occurred in the manner -IKmConyon .~ COClstolEs(ormenI ,,-l.' 1::' Principó)l Summits suggested, then one would expect to find evidence .~~ , .. KI1..0HEJUS of an extensive submarine abrasion platform representing the distance of the clifI recession. Data on the submarine topography irnmediately FIG. 2. - Actual fault scarps in the Iquique region. west of Tarapacá are not available, but recent FIG. 2. - Escarpements de faille actuels dans la région work further south has revealed a continental d' 1quique. shelf of about 10 km. width (SCHOLLel al., 1969). This is known to extend, with local variations, well to the south of Chile, and there is no reason south from here. At Patillos some 60 km. to the to believe that the feature dres not extend to south it lies at about 100 m.a.s.!., and at Pisagua the north. It is here suggested that this continental it is only 45 m.a.s.!. Between Pisagua and Arica shelf is at least in part a complementary feature there are apparently no terrace remnants and to the high clifI, and formed during the coastal recession. the high clifI is still receeding under wave attack. The relative fall in sea level of unequal efIect The flight of terraces at the clifI-foot indicates along the coast was probably therefore caused that there has been a local regression which was by local tectonic movements. Mapping of the most probably-non uniform and may have invol- coastal terraces to the south of Iquique has ved reingressions. The back of the highest terrace revealed a great many terraces of difIering alti- occurs just south of Iquique at 150 m.a.s.!., tudes and of short longitudinal extent, and it but the highest terrace level falls both north and has not been possible to distinguish any evidence

- T.XXI C. MORTIMER ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION 165

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FIG. 3. - PrQfile E-E' showing local topographic effect of FIG. 3. - Profil E-E' montrant les effets des failles E-W E- W faults. sur la topographie locale.

which can be said to be theeffect of glacio-eustatic show east-west and north-south predominating sea level changes on the littoral. trends and a11 faults are apparentIy high-angle Where terraces are not present the cliff reaches and normal. It is interesting to note that although an inclination of up to 800, elsewhere terraces the north-south striking suite of faults do es not have protected the cliff and degradation since have any apparent preferred direction of throw, the sea retreated has reduced the inclination to those faults which strike east-west nearly always nearer 45°. downthrow to the north (Fig. 3). Field observation led to the conclusion that the east-west fault 3. THE TOPOGRAPHY system is the younger of the two suites because OF THE COASTAL CORDILLERA of the fresher appearance of the fault scarps. In the detailed study area (Fig. 2Ythe coastal There is no evidence that the faulting, which mountain - belt is approximately 40 km. wide, has had such a marked effect on the landscape narrowingto the north. Though the approximate of the Coastal Cordillera, has in any way affected average height is about 1,000 m.a.s.l., the maxi- the terraced littoral or its superficial layer of mum height of 1761 m. is on Cerro Constancia marine sediments, although fault lines have been (Fig. 2). At the coast the range is terminated 10ca11y etched-out by marine abrasion. It is. by the steep cliff, but the eastern border of the therefore apparent that the cutting of the terraces Coastal Cordillera is irregular where the moun- took place after the topographica11y expressed tain slopes finger into flat-Iying alluvium in the fault movements. Pampa del Tamarugal. This latter tract is a depo- The alluvium of the Pampa del Tamarugal has sitional surface of a11uvium" which slopes gentIy partia11y submerged the eastern flank of the down from the Andean foothills to meet the barrier Coastal Cordillera, so that this latter unit conti- " of the "C6astal Cordillera~ .." nues as a structural block for an unknown dis- Fault movements in the Coastal Cordillera tance beneath the Pampa del Tamarugal physio- have produced a topography of isolated internal graphic province. There is only one fault trace drainage formed from a- complex of uplifted and which can be seen to displace the Pampa surface downsunken blocks (Fig. 2). The margins of the (Fig. 2). This indicates that by for the greater blocks have a clear topographic expression and part of the alluvium forming the Pampa surface have inclinations of up to 45°. The fault pattern was deposited after the faulting.

168 C. MORTIMER ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION T.XXI

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0 co Sq!'l Anl~nio 12"'. CO C~orodo 1 PompQ "el TomOn:filOI K>09 ...... ( "ISO. i ...... 50.. i i A' '00 _ Preservedold londscope oj s.1.. _ YounQsurfaee CId&ondscopehidden by alluvium Km _Foult

FIG. ~. - The Tarapaca Coastal Pediplain (1) reconstructed FrG. ~. - La pédiplaine cotiere de Tarapaca reconsittuée a from the present topography (2) in section A-A'. partir de la topographie actuelle. Légende dans l'ordre : paysage ancien conserpé. surface récente. paysages anciens marqués par des allupions. faille.

The tectonic basins within the Coastal Cordil- 4. DRAINAGE lera ha ve been partially infilled with alluvium. Sections through the alluvium occuring in the In the region between 19° 30' and 20° 30' lato cliff-face reveal a completely continental sequence S., the only break in the Coastal Cordillera is of interbedded sandstones and conglomerates the deep Tiliviche gorge, which cuts across the with occasional evaporites and ashf1ows. The Pampa del Tamarugal and the coastal mountains continental sediments reach an observed maxi- from a source in the high Andes. Further north mum thickness of over 200 m. at Pisagua. At where the Coastal Cordillera is narrower, several this same locallity some 50 m. of the sequence more canyons cut the range, thouth to the south is seen to be composed of ash-flow tuff which there is no other similar drainage until the Río can only have had its origin near to the volcanic Loa some 130 km. distant (Fig. 1). Andean crest far to the east. The alluvium is CHRISTENSENand others (in press) envisage a considered to have been locally derived, hav- form of fan development from the east crossing ing its origin in scarp retreat following the block .the Pampa. Rivers are considered to have cut faulting. In the central and eastern parts of the down across the apex of the fan when the level Coastal Cordillera the top few metres of the allu- of the alluvium within the fan was sufficient to vium contain nitrates and other salts. overtop the range and achieve a marine base- The landscape which existed prior to the block level, thus establishing such canyons as Tiliviche. faulting and alluviation can be locally recons- We were unable to find evidence of such fans, tructed by diagramatically neutralising the effects and consider that after the uplift of the Coastal of the faulting and other, later, landform deve- Cordillera relative to the other physiographic lopment (Fig. 4). The manner of reconstruction provinces, the Pampa del Tamarugal began to is clear aiíd 'thé' form' of the oldlandscape is infill 'with debris from the Andes. The level of well brought out. The exhibited lack of relief deposition of the alluvium slowly rose submerging with no evidence of drainage is typical of the old the eastern flank of the Coastal Cordillera by landscape throughout the investigated area, alluvium. As the alluvial fill increased in depth and the process responsible for the modelling it worked progressively westwards, !!pilling over of at least the latest forms of the old landscape into successive tectonic basins and infilling is considered to have been pedimentation. Accor- them at low points of the Coastal Cordillera. At dingly the old landscape has been named the the same time the coast was retreating towards Tarapaca Coastal Pediplain. the west, so that a marine baselevel was establis-

-- T.XXI C. MORTlMER ET N. SARIC : LANDFORM EVOLUTlON IN THE COASTAL REGION 169

hed when these two processes meto Once the only through the interpretation of landforms alluvium from the Pampa overtopped the cliff and their associated alluvial deposits can any there would have been a rapid down-cutting by history of the Cenozoic be worked out. There the exotic water. are no ancient river valleys in the studied region It is interesting to note that the bed profile which could have transmitted relative changes of the lower Tiliviche demonstrates three major of a marine base-level inland, so a hierarchy breaks of slope (Fig. 5) which are neither struc- of landscapes cannot be seen. That which can turally nor lithologically controlled, whereas be seen is the tectonically dislocated end result the river profile above the highest change of of a single continuous phase of erosion. Never- slope is in virtual coincidence with' the level of theless, by examining and relating the histories the Pampa. The upper profile is considered to of the Pampa del Tamarugal and coastal area be a section of the river whose erosive state is to that of the Coastal Cordillera an idea of the still in equilibrium with baselevel on the Pampa Cenozoic evolution can be obtained. del Tamarugal. The lower changes in the profile The coastal mountains have acted as a dam of the river may have formed subsequent to to easterly derived sediment until late in the relative falls in sea level during the regression Cenozoic, and still act as a partial barrier (TRICART, after the formation of the cliff. 1966). Consequently the greater part of the ero- The other canyons to the north of Tiliviche sion products derived from the western flanks are considered to have evolved in a similar manner, of the Andes during Neogene times was deposited but since the Coastal Cordillera narrows north- in the longitudinal depression east of the Coastal ward, the role of coastal recession in facilitating Cordillera. Volcanic horizons intercalated in the establisment of a marine base-level to the drai- sediments on the eastern flanks of the longitudinal nage may have been greater there. depression have yielded K/Ar ages not older The tilted lacustrine sediments near to the than Miocene (MORTIMER,FARRARand SARIé, in Río Loa mouth suggest that the saline Lake prep; CHRISTENSENand others. op. cit.) and it Soledad (BRÜGGEN,op. cii.) was tectonically can be reasonably concluded that the coastal tipped over the cliff, in this manner establishing mountain belt did not act as a barrier before this the lower course of the river. Some of the water time. It is therefore suggested that the present from the lake was displaced westward during Coastal Cordillera in Tarapacá Province was warping into a tectonic depression to the north uplifted in relation to the rest of the Andean of the Loa, which, after evaporation, gave rise mass in the late Paleogene-Lower Miocene periodo to the salt deposits of Salar Grande. GALLI-OLlVIER (1967) working on the flanks Stream development away from the vicinity of the Andes immediately to the east of our of the major canyons is barely evident except area, recognised in the Juan de Morales regio n immediately adjacent to the coast, and perhaps a faulted Tertiary landscape which he called the developed during a marginally more pluvial Choja Pediplain. It is very probable that this climate at some time in the pasto None of the landscape can be correlated with the similar valleys reach sea level or anywhere near it, landscape we have recognised in the coastal and are hanging in the cliff topo The rest of the mountains. There exists a strong possibility drainage in the Coastal Cordillera is entirely that these surfaces can be further correlated internal towards the many basins, and the lack with the Atacama Pediplain which lies between of channel development within these prompts 260 and 290 lato S. This later pediplain was shown the conclusion that any rainfall is removed by to have ceased to develop at the end of the seepage through the alluvium towards the centre Miocene (MORTIMER,1969, CLARK el al., 1967; of the depressions. SILLlT, MORTIMER and CLARK, 1968). . Evaporation is undoubtedly more than adquate Faulting dislocated the Tarapacá Pediplain to prevent standing water accumulation, but and there after the landscape took on its modern the numerous faults which cross the range appearance after limited scarp retreat had occurred adjacent to the fault blocks, resulting in must provide a potentially rapid transportation basin alluviation. route for water to pass through to the coast. Several seepages of brackish ground water at The arid climate which has prevailed through or near sea level along the foot of the cliff could much of the Caenozoic (GALLI-OLlVIER,1969) and be explained this way. resulting lack of dissection are concieved as the reasons for the remarkable preservation of both 5. OVERVIEW OF THE LANDSCAPE HISTORY fault scarps and the coastal cliff. ONSHORE OFFSHORE The north of Chile has had a history of subae- Profiles across the Coastal Cordillera reveal rial erosion since the Cretaceous, consequently that thelli sumit altitudes show no tendency to 170 C. MORTIMER ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION T.XXI

fall towards the coast. In the concept of the - 1969, Climate : A primary control of sedimentation in the Perú-Chile Trench: Geol. Soco coastal subsidence during cliff formation the America. Bull., v. 80, p. 1849-1852. landscape should therefore have had an earlier GALLOWAY(R.W.), 1970, Coastal and shelf geomor- easterly inclination. This suggests that the cliff phology and late Cenozoic sea levels: lour. in the early part of its evolution, when the coas- Geology, v. 78, p. 603-610. tline was further west, was probably even higher HERM(D.), 1969, Marines Pliozan und Pleistozan than it is today. in Nord-und Mittel - Chile, unter besonderer Sedimentation in the northern part of the Beriicksichtigung der Entwicklung der Mollu- sken - Faunen : Zitteliana, v. 2, 159 p. Perú-Chile Trench has been slight, this is to KARZULOVIc(J.), 1968, Estudio hidrogeológico de be expected if the erosion products of the hin- la región de Arica, Provincia de Tarapacá : terland were trapped in the longitudinal depres- Santiago, Univ. de Chile, 129 p. . sion east of the Coastal Cordillera. Sedimentation MORTIMER(C.), 1969, The geomorphological evo- in the Perú-Chil~ Trench area in Neogene times lution of the southern Atacama Desert, Chile : would be virtually restricted to material intro- Ph. D. thesisj Univ. of .London, 283 p. duced by the formation of the continental shelf MORTIMER(C.), FARRAR(E.), SARIC(N.), in prep., and coastal cliff. This quantity, for the whole Cenozoic K/ Ar ages from the Quebrada Tili- of Tarapacá Province, is estimated as approxi- viche Camina transect, north Chilean Andes. mately 3.4 X 103 k3 of removed rock. In addi- SALAS(R.), KAsT (R.), MONTECINOS(F.), and SALAS (1.), 1966, Geologia y recursos minerales del tion, a very little debris has been derived through departamento de Arica: Inst. Tnvest. Geol., the canyons. paca, Chile: Memoria Universidad de Chile, SARIC(N.), 1971, Evolution Cenozoica de la Cot- REFERENCES dillera de la Costa, en la Provincia de Tara- paca, Chile : Memoria Universidad de Chile, ALMEYDA(E.A.), 1949 : Pluviometría de las zonas 176 p. del' desierto y las estepas calidas de Chile: SILLITOE(R.H.), MORTIMER(C.), and CLARK(A.H.), Santiago, Edit. Univ., 162 p. 1968, A chronology of landform evolution and supergene mineral ¡ilteration, southern Ata- BRUGGEN(J.M.) 1950 : Fundamentos de la geología cama Desert, Chile: Inst. Mining and Metal- de Chile: Santiago, Instituto Geografico Mili- lurgy Trans., v. 77, Seco B., p. 165-169. tar, 374 p. SILVA(L.1.), in prep., Geología de las Hojas Pisa- CHRISTENSEN(M.N.), PÉREZ (G.), MONTECINOS(F.), gua y Zapiga, Provincia de Tarapaca, Carta and CURTIS (G.H.), in press, Late Cenozoic Geologica de Chile: Santiago, Inst. Invest. vo1canism, deformation, and denudation in Geologicas. northern Chile: Geol. Soco America Bull. THOMAS(A.), 1970 : Cuadrangulos Iquique y Caleta CLARK(A.H.), MAYER(A.E.S.), MORTIMER(C.), SILLI- Molle, Provincia de Tarapaca. Santiago (Insti- . TOE (R.H.), COOKE(R.U.), and SNELLING(N.J.), tuto de Investigaciones 'Geologicas) Carta Geol. 1967: Implications of isotopic age of ignimbrite Chile, 21 y 22. flows, southern Atacama Desert, Chile: Nature, THOMAS(A.), 1970: Beitrag zur Tektonik Nord- v. 215, p. 723-724. chiles. Geol. Rundschau, 59, p. 1013-1027. GALLI- OLIVIER(C.), 1967, Pediplain in northern TRICART(J.), 1966 : Un chott dans le désert chilien ; Chile and the Andean uplift : Science, v. 158, la pampa del Tamarugal : Rev. de Géom. Dyn., p. 653-655. v. 16, p. 12-22.

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