Morphostratigraphic evolution of the northwestern margin of the basin (230 S-68°W)

José Antonio Naranjo Servicio Nacional de Geologla y Mlnerla, Casilla 10465, ,

Carlos Felipe Ramírez Cambior Chile S.A., la Concepción 1n, piso 4, Santiago, Chile Roland Paskoff Unlvershé lumillre de lyon, France

ABSTRACT

The 2,500 km 2 Salarde Atacama lorms the centerol a large Cenozoic depositional basin in the 01 northem Chile. The stratigraphy and geomorphology 01 the westem uplifted margin 01 this basin correlates well witt- the stages 01 the piedmont evolution resulting Irom the main Andean uplift' These uplifting/erosional processes produced the accumulation 01 sediments which infill the basin. Major NNE oriented morphostructural leatures largely controlled the distribulion 01 the sedimentary units in this part 01 the basin. The sediments inelude red sandstones, siltstones, conglomerates and evaporites. Evaporites were mainly deposited to the centerol the basin, whereas the piedmont deposits accumulated closerto the basin margino The basin was originated during the Oligocene and deposition is still active. A pediment-type planation which produced a gravel veneer matches well with the Atacama pediplain. It is suggested that the Oligo-Miocene morphostratigraphic evolution 01 the Salar de Atacama basin, which is correlated with the Andean piedmont, is closely related to the rates 01 convergence and geodynamic effects on the southeastem Pacific region.

Key worris: Morphos/ra/igraphic evo/u/ion. Cenozoico Andean piedmon/. Pedimento K-Ar dating. Salar de A/acama, r.orthem Chile.

RESUMEN

Evolución morfoestratigráfica del margen occidental de la cuenca del Salar de Atacama. El Salar de Atacama, de 2.500 km 2, constituye el centro de una cuenca deposicional cenozoica en los Andes del norte de Chile. La estratigrafía y geomorfología del margen occidental solevantado de esta cuenca, muestra una buena correlación en las diversas etapas de evolución del piedemonte formado como consecuencia del alzamiento de los Andes. La erosión de los terrenos alzados produjo la acumulación de los sedimentos que rellenan la cuenca del salar, cuya distrbución en el borde occidental está controlada por rasgos morfoestructurales mayores de orientación NNE. Los sedimentos incluyen arenas y limos rojos bien consolidados, así como gravas y evaporitas. Estas últimas fueron depositadas principalmente hacia el centro de la cuenca, mientras que los depósitos de piedemonte se acumularon hacia el margen occidental. La cuenca se originó durante el Oligoceno y los procesos de acumulación aún son activos. Un allanamiento del relie'le por procesos de pedimentación que produjeron una cubierta de gravas, coincide bien con el pediplano de Atacama. Se sugiere que la evolución morfológica oligo-miocena de la cuenca del Salar de Atacama que se correlaciona con el piedemonte andino, se vincula con la velocidad de convergencia de placas y efectos geodinámicos en la región del Pacífico suroriental.

Palabras e/aves: Evo/ución morfoestra/igráfica, Cenozoico, Piedemon/e andino, Pedimento, Dalaciones K-Ar, Salar de A/acama, norte de Chile.

Revista Geológica de Chile, Vol. 21. No. 1. p. 91-103. 8 Figs .• 2 tables. July 1994. 92 M ORP HOSTRATIGRAPHIC EVOLUTION OF THE NORTHWESTERN MARGIN OF THE SALAR DE ATACAMA BASIN ...

INTRODUCTION

Most Cenozoic sediments in northern Chile are ranjo and Paskoff, 1985). Between the Cordillera de the erosional pro:lucts ofthe Andean uplift and include Domeyko and the Andean Cordillera there are some ignimbrite deposits. The main Andean physiographic closed basins where debris Ilows and evaporites elements inclucia the Coastal Cordillera, a central have been deposited. At the western margin 01 the depression, the Cordillera de Domeyko and the An­ most complex 01 these endorheic basins, the Salar dean Cordillera (Fig.1). Generally, Cenozoic sed­ de Atacama basin (Fig.1), a sedimentary series iments have been trapped in the central depression reveals the main leatures 01 the erosion and between the Coastal Cordillera to the west and the accumulation processes. A 40 km northwest-south­ Cordillera de Dcmeyko to the east. east lield transect across this margin is analysed The evolution 01 the central depression was (Fig. 2), fromthe Altos de Purilactis Range to the east discussed by Naranjo and Paskoff (1985) who pro­ across the Llano de la Paciencia, the Cordillera de la posed that sediMentation there started in Oligocene Sal down to the village 01 . An time, as a consequence 01 the uplift 01 the adjacent ignimbritic plateau borders the basin to the east, ranges. The continental sediments are apparently extending Irom the present surface 01 the salar at nonlossililerous but numerous K-Ar dates on inter­ 2,500 to 4,000 m a.s.l, obliterating any sedimentary bedded pyroclastic Ilow deposits have provided succession. chronostratigra¡:hic inlormation (Lahsen, 1982; Na- This paper provides a review 01 the stratigraphy in

2~'r-__~7+IO ______~r- ______~6~~~ __~MT· ______~6~70

80· TARA PACA REGION

lOO· \ \ \ \ \ \

\ \ \ \ \ \--+-.~\ \ -o: \ t­ \ Z \ o W u U

ANTOFAGASTA 1> t----,f-- REGION

... --+-1

FIG.1. Locatlon ma::> 01 the Salar de Atacama in northern Chile. J.A. Naranjo, C.F. Ramlrez and R. Paskoff 93

the area and discusses the relation of sedimentation logicframeworkgiven by Abele (1988), are suggested to the stages of morphologic evolution of the sedi­ in the light of new data. Maeellari et al. (1991) have mentary cover of the western marginal range of the pointed out that the sediments of the Paciencia basin (Cordillera de Domeyko). New K-Ar dates also Group were accumulated at the toe of a thrust front provide a time framework for landform chronology. whieh produced the uplift of their souree and a Local stratigraphie nomenclature defined by Brüggen shortening of 4.2 km. More recentl~', Flint el al. (1993) (1934), Dingman (1963,1965), Moraga etal. (1974) interpreted the evolution of the Salar de Atacama and Marinovic and Lahsen (1984) has be en preserved. from a non-arc-related rift, through back-are and in­ Modifications to the stratigraphie framework defined ter-are stages, to a Neogene fore-are basin, sinee by Flint (1985, 1987) and Hooper and Flint (1987) as Permian times. the Llano de la Paciencia Group, and of the morpho-

MAJOR MORPHOSTRUCTURAL FEATURES

The northern extension of the Cordillera de westernmost part of this basin is the plain ealled Domeyko is locally termed Altos de Purilactis in the Llano de la Paciencia (2,500 m), interrupted to the study area and is oriented NNE (Fig. 2). Its western east by the Cordillera de la Sal (3,000 m). The present slope is gently inclined towards the Loa River basin Salar de Atacama (2,300 m) forms the central part of (Marinovic and Lahsen, 1984). Morphological the basin. Farther east, the western slope of the elements extending parallel to the Purilactis Range Altiplano dips westwards from approximately 4,000 (3,500-4,000 m) formthe Salar de Atacama basin.The m to the border of the salt crust forming the salar.

STRATIGRAPHY

The arid conditions prevailing in the area have give a revised and extended stratigraphic framework created a unique morphology, where the stra­ (Table 1) which includes the inferrad morphogenetic tigraphical relationships of the sedimentary units are events. well exposed. This corresponds to one of the best areas to identify morpho-sedimentary events that occurred in conneetion with the main Andean uplift. The succession exposed alongthe western margin TABLE 1. STRATlGRAPHY ALONG THE NORTIIWEST-SOUTHEAST TRANSECT, WESTERN MARGIN OF THE SALAR DE of the Salar de Atacama basin (Fig. 2) consists ATACAMA BASIN. essentially of sandstones, siltstones, evaporites and debris-flow deposits, with some tuffaceous inter­ Ma Altos de Purilacti. Cordillera de L. Sal calations. Radiometrie dates on these tuffs indieate Quatemary Uano de La Pacienda Gravels Oligocene to Pliocene ages. The stratigraphy of this Pliocene-Qualernary 2· Vilama Formation area has been described by several authors. Brüggen Upper Miooeno-Pliocene Ignimbrées (1934, 1942) defined the main units as San Pedro lO. and Tambores formations and assigned these conti­ HoIlingworth Gr.... els (pedimentation) nental clastics to the Eocene. More recently, Flint (1985, 1987) proposed an extension of the Tertiary stratigraphy based on field observations in the San Tambores Formation ( .....in Andean upiift) Lower Mioceno Bartolo area, with newly delineated lithostratigraphic 2> units grouped in the Llano de la Paciencia Group. In (paraHel uncon'ormity) this paper, observations through a northwest-south­ Oligoceno San Pedro Formation east transect, normal to the main structural trend, Cretaceous Purilactis Formation 94 MORPHOSTRATIGRAPHIC EVOLUTlON OF THE NORTHWESTERN MARGIN OF THE SALAR DE ATACAMA BASIN ...

. ...' ...... 11 ' ..

LEGENO

o AlluvllJn

~ Lklno d~ lo Pa cie ncia Groll els ~ B vHomo ~o Jma tion rrrr=l l...!:....!:.J Artolo,Slfon ond Puripicof' undIUeren1tat@d iQnlmbrites r r-

Hollinq;.orth Grove l$

~ ombol"ll!'s Formo iOIl ~ !2IIJ Son PE'CfO formo ton fZ2J Bosemenl r o(~s Alti tudu 'o ff'..ete'15 above no level

~ Aoroll1TlO1e profl~ IlOe of fic;¡ure 3

o 5 10 km ~I =====-....~I ====~.... "I

FIG.2. Geological map 01 Ihe northweslern margln 01 Ihe Salar de Atacama basin (modilied Irom Marinovic and Lahsen 1984). Clrcled numbers reler "o K-Ar dates in table 2 (see p. 99). J.A. Naranjo. C.F. Ramfrez and R. Paskoff 95

SAN PEDRO FORMATION Paleocurrent directions indicate that fine clastic debris was shed southeastward frcmthe Cretaceous The San Pedro Formation (Brüggen, 1934, 1942) Purilactis Formation (Flint, 1985; Wilkes and Garler, is formed of continental siltstones, sandstones, con­ 1988)atAltos de Purilactis,intotheearly Atacamabasin. glomerates and evaporites, with scarce tuffaceous intercalatic-ns. To the north ofthe Atacama basin, this TAMBORES FORMATION unit overlies unconformably Cretaceous volcanic and sedimentary rocks of the Lomas Negras Formation The Tambores Formation is a succession of al­ (Marinovic and Lahsen, 1984) and in the western most unconsolidated gravels and sands distributed margin ofthe basin (Quebrada Tambores) it is overlain, alongthe Quebrada Tambores onthewestern margin with parallel unconformity, by continental conglomer­ of the Salar de Atacama basin, which were also de­ ates of the Tambores Formation (Figs. 3, 4). The thick­ rived from the western flank and Mrthern part of the ness ofthe unit is variable. Dingman (1963) measured Altos de Purilactis. The source of these sediments 2,100 m of fine-to coarse-grained red sandstones was the unconformably underlying Purilactis For­ and siltstones, containing intercalations of thick beds mation of Late Cretaceous age. Downstream, to the of salt, minor conglomerate intercalations and rare east of Quebrada Tambores, this succession is also cemented :uffaceous beds, in the northern part of the unconformably underlain bythe San Pedro Formation Cordillera de la Sal. Siltstones predominate in the (Fig.4). ln bothflanks of Altos de Purilactis, Tambores lower part ·::>f this unit in the San Bartolo area (Fig. 2), Formation gravels are partly eroded and are uncon­ where Ranírez (1979) reported a thickness of 1,000 formably overlain by the Hollingworth Gravels (Fig. 6) m. These red sediments are well stratified, in beds of (see p. 80). The Tambores Formation is dominated 0.2-1.4 m :hick and cut by abundant gypsum veins. by crudely stratified matri~ supparted gravels (and Detailed descriptions of the San Pedro Formation baulders), containing intercalations of sandy beds, lithologies are given in Marinovic and Lahsen (1984). up to 20 cm thick, partially cerrented by calcite. The parallel unconformity, exposed at Quebrada Gravel beds are 0.1-2 m thick being pale gray­ Tambores between the red sediments of the San brownish to pale reddish-brown in colour. The clasts Pedro and the overlying gravels of the Tambores For­ are angular, 1-15 cm in diameter and correspand to mation (Fig. 4) conspicuously indicates a deposi­ , sandstones, and granitoid gravels derived tional regime change at the western proximal facies from the Purilactis Formation. of the basin. The sedimentary structures of these deposits The San Pedro Formation was deformed into indicate that mast of them were deposited as matrix large scale folds about NNE axis (Fig. 5) during the supported debris-flows. Cross-bedding in sands and early-midcle Miocene (Hooper and Flint, 1987). A imbrication of rare clast-supported gravels indicate a series of salt domes and doubly plunging synclines transport direction from the Altos de Purilactis in the and anticlhes are present in the northern Cordillera west (Marinovic and Lahsen, 1984; Flint, 1985). de la Sal (Dingman, 1962). This deformation is likely The Tambores Formation is overlain by 8.3-10 related to 3 major basement fault dipping steeply to Ma ignimbrites constraining its upper-age /l mit to the the west underthe Cordillera de Domeyko (Forsythe, Miocene (Table 2). Since the gral/els of the Tambo­ 1988; written communication). res Formation overlie the fine grained sediments of Tuff intercalations have yielded K-Ar ages of 28 ±6 the San Pedro Formation it is sug~ested that there is Ma and 24 9 ±1 Ma at Río Salado and Cordillera de la no interfingering between these two successions Sal, respectively (Table2)(Travisany, 1978; Marinovic alongthe studied profile area, as in"erred by Dingman and Lahsen, 1984). Thus,the San Pedro Formation was (1963, 1967) and Marinovic and Lahsen (1984). deposited duringthe Oligocene. Furthermore, Wilkes and Garler(1988)found lacustrinefossils (charophytes, HOLLlNGWORTH GRAVELS· gastropodes and ostracodes) around Río Grande and San B3rtolo, which suggested an oligohaline and These are alluvial gravels, up to 2 m thick, that mixohaline environment and indicated distinctive cover unconformably the Tambo'es Formation, on freshwate- influx at the northern margin of the basin. both the eastern and western f anks of Altos de

This new name is propased here lo honour Professor S.E. Hollingworlh who worked wnh a group of graduale sludenls from Ihe Lniversily College, Lendon. in Ihe Alacama Desert during Ihe 60's. According lo his will. his ashes were spread in Ihis area. ~

¡¡: NW sSOzs' 68°15' SE o J) I I "U I ALTOS DE PURILACTlS (Cordill.ra d. Domeyko) CORDILLERA DE LA SAL o ~ Qu.brado Tambores ~ I 3,500 15 »J).., I ñ m 3,000 < ~ e -< \~ (5 z O 2,500 o o 000000°0 "-< • km I m z O ~ ~W~~~~IIIIIIIII~EI I :;: m en m-< J) z ¡:; J) 1- Purilaclis Formalion 4- Hollingworlh gravels 7- Puripicar ignimbrile Gl 2- San Pedro Formalion 5- Arlola ignimbrile 8- Vilama Formalion Z O 3~ Tambores Formation 6- Sifón ignimbrile 9- Llano de la Paciencia gravels. Age dala (Ma) as in table 2. "-< I m FIG.3. Sketch prolile between El Hotel and Cordillera de la Sal, northwestern margin 01 the Salar de Atacama basin (22°50'S). en ~ » J) O m ~ ()» ¡:; »'" :zen J.A. Naranjo, C.F. Ramfrez and R. Paskoff 97

FIG. 4. ConlollTlable Iransilion Irom Ihe red sinslones and sandslones 01 Ihe San Pedro Formation lo coarserdebris-flc·w deposns ofthe Tambc.res Formalion al Quebrada Tambores. looking lo lhe soulh. Bushes are aboul 0.5 m lall.

FIG.5. Aerial view 01 large-scale (5-10 km wide) anticlinal pericline in Ihe San Pedro Formalion al the southe-n end 01 Cordillera de la Sal.

Purilactis. They are coarser than the Tambores gra­ interval of pedimentation which was probably due to veis, with sl..brounded clasts up to 10-20 cm in an attenuation orcessation ofthe continuous tectonic diameter, within a subordinate grey-sandy matrix. uplift, in addition tofavourable arid climate conditions. Gradingwas not observed, and stratification is absent. During that interval, a pediment-type surface de­ These deposits form conspicuous veneers (rock­ veloped on the Tambores and Purilactis formations; floor) on the Tambores gravels (Figs. 6, 7). the surface is capped by two ignimbrite flows, with They are hterpreted as deposits resulting from an ages of 10 and 8.3 Ma (Table 2, see p. 99). 98 MORPHOSTRATIGRAPHIC EVOLUTION OF THE NORTHWESTERN MARGIN OF THE SALAR DE ATACAMA BASIN ..•

FIG. 6. In Ihe background, Ilghl colour debrls-llow deposHs 01 lhe Tambores Formatlon capped by darker Ihln deposlls 01 Ihe Holllngwor1h gravels in lhe westem flank 01 Altos de Purilactis. To lhe left, ignimbrile sheels al El HolellocalHy cover bolh unns.

FIG. 8. Unconlormily belween Ihe Oligocene San Pedro Formatlon and Ihe Pliocene Vilama Formalion besides Ihe road lo San Pedro de Alacama.

FIG . 7. Thin dep·:>sils 01 Hollingwor1h gravels «50 cm Ihick) mation. The succession is welllaminated with layers conlormably sealed by Ihe snón ignimbrHe and uncon­ commonly less than 2 cm thick. Intercalations of lormably overlying bareiy-slratnied and weslward dipping stromatolitic limestones and white diatomites are gravels 01 the Tambores Formation; easlem llank 01 Anos de Purilactis. subordinate, as well as minor ash beds less than 1 m thick. Biotites in these ash beds have yielded a K-Ar age of 2.0±0.9 Ma (Table 2). The Vilama Formation VILAMA FORMATION contains lacustrine and piedmont deposits. In some places, coarse debris deposits record small rivers The Cordihera de la Sal, in the northwestern part flowing into the basin. of the Salar de Atacama basi n, is partially covered by Local overturned folds of 50 cm wavelength disturb the 60 mthickVilama Formation (Moraga et al., 1974). these strata with axes trending northeast, parallel to This unit overlies unconformably the San Pedro the San Pedro Formation folds (Marinovic and Lahsen, Formation (Figs. 3, 8). 1984). Normal faults with a similar strike produce P ale reddish-brown silts and fine to coarse-grained displacements of up to 25 m in the Vilama Formation, sands (calcite and halite cemented) are the dominant as well as in the overlying deposits (Llano de la Pa­ and characteristic components of the Vilama For- ciencia gravels). J.A. Naranjo. C.F. Ram/rez and R. Paskoff 99

LL4NO DE LA PACIENCIA GRAVELS basin towards the Llano de la Paciencia depression. The lans are inlerred 01 Quaternary age, and spread This uni- consists primarily 01 coarse gravels with out Irom the gorges on the eastern Ilank 01 the Altos polymictic clasts up to 15 cm in diameter with a grey­ de Purilactis and the western Ilank 01 the Cordillera sandy matrix, and thick planar cross-bedding. These de la Sal. Reverse laults (Forsythe, 1988: written deposits constitute coalescent alluvial lans draining communication) andcontinued erosion have disrupted southwest ..... ard and lorming a wide, open, lunnel-like these deposits along the core 01 this cordillera.

K-AR GEOCHRONOLOGY

Some radiometric dates have been previously the stratigraphy and relative chronology observed lor reported (Table 2) Irom sites within the study area. the units. Five new K-Ar ages (this paper) combined Unlortunately, it was not possible to lind the exact with previously published dates provide an absolute location 01 lour samples dated by Dingman (1965), in chronology lor the events that occurred during the the immediate area 01 the transect. evolution 01 the basin. They al so constitute a basis lor K-Ar ages displayed in table 2 are consistent with the stratigraphic correlations.

TABLE 2. K-AF ANALYTlCAL DATA FROM DATED SAMPLES OF THE NORTHWESTERN MARGIN OF SALAR DE ATACAMA f!ASIN.

Age(') Anallzed Vol.

1 NP-90 22°53.9' 68°12.9' Ashtuff Biotite 6.082 0.477 96.1 2.0±0.9 This paper \'ilama Formation 2 NP-76 22°48.9' 68°18.5' Ignimbrite Biotite 6.957 1.150 90.0 4.2±O.8 This paper Puripicar Ignimbrite 3 NP-69.A 22°40.8' 68°18.5' Ignimbrite Biotite 6.762 2.189 62.0 8.3±O.4 This paper Sifón Ignimbrite 4 NP-91 22°47.0' 68°20.5' Ignimbrite Biotite 6.039 1.974 61 .0 8.4±O.4 This paper Sifón Ignimbrite 5 NP-69E 22°40.0' 68°28.0' Ignimbrite Biotite 7.371 2.915 80.7 10.1±O.9 Thispaper Artola Ignirrbrite 6 MC-77, 22°54.2' 68°13.3' Ashtufl Biotite 24.9±1.0 Marinovic and San Pedro Formation Lahsen, 1984 7 SB-T 22°47.7' 68°13.0' Ashtufl Biotite 28.0±6.0 Travisany, 1978 San Pedro Formation

, Constants used: 1..,= 0.581x10·Hlyr·'; A.~=4.96x10·"'Yr'; "'KlK=0.01167

LANDFORM CHRONOLOGY AND STRATIGRAPHIC CORRELATIONS

The Andean uplilt, which gave rise to the present Domeyko or Sierra de Moreno and the upper Loa mountain range, is the consequence 01 tectonism river, about 150 km to the north 01 th~ Atacama basin probably initiated during early Oligocene times (Na­ (Maksaev, 1978); an age 0134.1 ±1 'v1a was reported ranjo and Paskoff, 1985). This uplilt also conditioned lor an ignimbrite intercalated in the lower-middle part thelormati01 01 continental sedimentary basins where 01 the Sichal Formation. Lithologic features indicate Oligocene-Miocene sequences such as the San Pedro that the lower part 01 the Sichal an::J the San Pedro and Tamberes Formations were deposited. These lormations were deposited in an environment which basins wer~oriented aproximately NNE and received is similar to that 01 the present salars. the sediments eroded Irom orogenic highlands. An aridclimate, with high evaporation rate, explain Remarkable similarities with the San Pedro For­ the abundance 01 evaporites (salt and gypsum) inter­ mation, are lound in the sandy-conglomerate and bedded with fine to medium-grained red sediments in evaporitic li:holacies associations 01 the Sichal For­ the Cordillera de la Sal and in the mouth 01 the mation, distributed between the Cordillera de Quebrada Tambores. Some volccnic activity also 100 MORPHOSTRATIGRAPHIC EVOLUTION OF THE NORTHWESTERN MARGIN OF THE SALAR OE ATA':.AMA BASIN . ••

occurred at the e1d 01 the deposition 01 these sed­ die Miocene pedimentation-type surfaces are 01 re­ iments in the San Pedro and San Bartolo area (Fig. markable wide distribution and are synch ronous along 2), represented !Ji thin beds of volcanic ash. the Atacama deserto They have been recognized The uplift 01 th3 Altos de Purilactis blocktriggered between 18°S and about 28° S (Naranjo and Paskoff, a period of erosion which was strongly active and 1985). In most places the surface is sealed by lavas debris-flow accunulations were spread towards both and ignimbrites of about 9-10 Ma (Mortimer, 1973; sides of the Altos de Purilactis block. Probably due to Naranjo and Paskoff, 1980). the instability of the basin, the relief of this block was Strong subsequent incision affected the area, gradually increased, andonlycoarserdebris (gravels) resulting in gorges that cut the former formations as was deposited in the Tambores Formation. deeply as the folded Cretaceous Purilactics Formation. A similar situ3tion could be interpreted for the The erosional detritus was spread on to the Llano de uppermost gravels of the Sichal Formation in the la Paciencia basin. Ignimbritic volcanism occurred in western flank 01 the Sierra de Moreno (Skarmeta and this part of the Andes at 4.2 Ma and the Puripicar Marinovic, 1981). n both areas such deposits clearly ignimbrite (Guest, 1969) filled some gorges and represent piedmcnt gravels as those that are widely covered parts of the area. The thicker outcrops are distributed in the , better known as close to the basins, while the ignimbrite thins towards the Atacama Gravels to the south (Mortimer, 1973; the highlands. Naranjo and Pas~:of1, 1980), Calama Formation in the A new quiescence stage was reached and the Cal ama area (Na-anjo and Paskof1, 1981; Marinovic lowest topographic areas were covered by shallow and Lahsen, 19E4) and Altos de Pica Formation in lakes where so me alluvial and chemical deposits northern Chile (Galli and Dingman, 1962; Naranjo and (fresh water limestones) formed the Vilama Formation. Paskoff, 1985), o- Oligocene to Miocene in age. The A thin ash layer dated at 2 Ma denotes some coeval accumulation of tnese gravels probably ceased after explosive volcanism, probably derived from the east. the climatic desiccation during the middle Miocene Younger fine grained sediments corresponding to alongthe Atacarra Desert (Alpers and Brimhall, 1988). this formation could be correlated with other localized After this episode, a tectonic quiescence stage lake deposits of Plio-Pleistocene age (Chiuchiu For­ apparently began probably associated with climatic mation) on a paleo-karst surface developed on the conditions propitious for sheetflood generation, which calcareous Formation (Naranjo and Paskoff, might have been responsible for the areal planation 1981 ). phenomena of pediment type, producing the high­ Deformation affecting the Vilama Formation, the land retreat. This ::Jedimentation process left the thin, Puripicar ignimbrite and, consequently, the San Pe­ but conspicuoU!: deposits defined here, as the dro Formation, is displayed at the mouth 01 Quebrada Hollingworth Gravels, which unconformably cover Tambores (Dingman, 1962), changing the base level the Tambores Formation . of the basins (Llano de la Paciencia and Salar de Highly explos ve volcanism became more active Atacama). Erosion began subsequently to supply to the east and S011e ignimbrites were spread into the coarse materials to these basins originating the sands basin, passing oV3rthe Altos de Purilactis, and sealing and gravels grouped here as Llano de la Paciencia thecontemporaneous pedimentation surfaces. These Gravels, which are currently filling the basins, as pyroclastic flows are the Artola (10.1±0.9 Ma) and debris-flow alluvial sediments derivedfromthe uplifted Sifón (8.3 Ma) igrimbrites (Ramírez, 1979). The mid- terrains.

DISCUSSION

The stratigra:Jhic and morphologic evolution of that have led to confusion arises because The San the sedimentary cover at the western margin of the Pedro and The Tambores Formations have been Salar de Atacam~ basin as presented here, appears considered as a single chronostratigraphic unit (the to be more cOrTlllex than that described by Flint Paciencia Group), without any significant age-break (1985, 1987). In 1act, one of the main controversies between them (Flint et al., 1993). These authors J.A. Naranjo, C.F. Ramfrez and R. Paskoff 101 described the Group as a 2 km thick succession of Atacama (Fig. 8), the Vilama Formation isconsiderably continental sedimentary rocks, whose minimum age younger than the important deformation that affected would be restricted by a lava that 'unconformably the San Pedro unit. Because of its lithology, structure overlies the Paciencia Group', dated in 17±2 Ma and stratigraphic position, the V lama Formation (Ramírez, 1979). However the date was obtained in conspicuously represents a different and younger the Jorquencal-Machuca Volcanic Group, which un­ stage in the basin evolution. conformably overlies the San Pedro Formation (Ra­ It is noted here that the sediment accumulation mírez, 1979), but not the Tambores Formation, the and morphogenic evolution of the Salar de Atacama conglomerate succession that crops out to the south­ basin were linked to the past changes in the plate­ west of The San Bartolo area. tectonic setting along the western South American In addition to the above mentioned formations, margin (Cross and Pilger, 1982; Pi ger, 1981, 1983, the Atacama basin-fill sequence includes other 1984). These changes have been recorded as va­ sedimentary units which involve a wider and younger riations in the sedimentary regime among the des­ age-range, and represent varied stages in the mor­ cribed units. In fact, thetransition fromthe San Pedro phologic evolution of this margino Contraction pro­ to the Tambores Formation indicates an uplift that cesses which took place during the early-middle might be induced by a rapid rotation of the oceanic Miocene (Flint et al., 1993), possibly associated with plate from an oblique southeast subduction to a more local subsidence of the basin, are indicated by litho­ orthogonal direction to the South American margin at logic and structural breaks among the units. ca. 26 Ma (Cande and Leslie, 1986). Due to the rapid The parallel-unconformable transition from the (ca. 120 km'Ma) relative normalconvergence, through fine-grained sediments of the San Pedro Formation a low-angle subduction zone, an efficient transmis­ to the coarse clastics of the Tambores Formation sion of compressional stress was induced (Cross and evidences a clear change in the sedimentation style, Pilger, 1982). Thus, thiscompressionalphasecaused as a result of an increasing rate of relief generation or deformation and high rate of relief formation, which uplifting. The gravels ofthe Tambores Formation were are recordedasthefolding ofthe Sar Pedro sediments probablythe product of radical changes in the tectonic andthe intense and widespread debris-flowdeposition processes occurring by that time. Thus, the debris­ (Tambores Formation) originated frcmthe denudation flow deposit record corresponds to an outstanding of the uplifted areas. stage of the Andean uplift, which occurred between At ca. 10 Ma, the relative convergence rate bet­ 25 and 10 Ma in the area. In addition, climatic condi­ ween the Nazca and South American plates was tions less arid than at present probably occurred dramatically reduced to 93 km/Ma (Cande and Leslie, during the early-middle Miocene along the Atacama 1986) . lnterestingly, in contrast, uplift might have stop­ Desert (Alpers and Brimhall, 1988), which contributed ped and subsequently, mountain masses (e.g. Altos to generation of debris-flows. de Purilactis) were partially consuned by pediment­ Although Flint (1 985) did notdistinguishthe Vilama type denudation (Hollingworth Gravels). Additional Formation as a separate unit in the Paciencia Group, evidence for non-compressional deformation in about Hooper and Flint (1987), after Moraga et al. (1974), middle-Iate Miocene has been given by Hervé (1987) recognized it as a Pleistocene poorly-consolidated and Naranjo (1987). These authors concluded that alluvial deposit or as a 'braided fluvial and lacustrine final high angle normal movements along the 1,000 sequence' deposited during the Pleistocene, due to km long Atacama fault zone, in the Coastal Range of climatic cyclicity induced by glaciation (Flint et al., northern Chile, occurred at approximately 1 O Ma B.P. 1993). As shown besides the road to San Pedro de

CONCLUSIONS

Regarding its location, the Salar de Atacama ba­ history of the western par! of the Salar de Atacama sin has been geographically classified as a fore-arc basin agree well with those of the Andean piedmont basin (Jordan and Alonso, 1987). The main stages of evolution in the Atacama Desert (Naranjo and Paskoff, the Oligo-Miocene geomorphic and sedimentary 1980, 1985), which include: 102 MORPHOSTRATIGRAPHIC EVOLUTION OF THE NORTHWESTERN MARGIN OF THE SALAR DE ATACAMA BASIN •••

• Basin formation and their consequent infilling (e.g. the first time as the counterpart of the Andean uplift, San Pedro Formation sediments). and hence formed under a non-compressional pro­ • Relief formation, which triggered erosion and cess. Therefore, it is suggested that at 10 Ma there intense debris-110w emplacement as a piedmont was either a quiescent tectonic stage or extensional constituent (Tambores Formation). deformation, at least along the western flank of the • Pediment-type denudation (Atacama pediplain) north Chilean Andes. locally represented by veener deposits of the Locally, the northwestern margin of the Salar de Hollingworth Gravels. The pediplain surface is Atacama is still active and deformation affecting the sealed by ignimbrites of about 10 Ma. youngermost Quaternary units, is al so taking place at The 'Atacama pediplain' surface is regarded for present.

ACKNOWLEDGEMENTS

Criticism and comments from T. Jordan, M. Bell, Miranda for typing the manuscript. This work was R. Forsythe, D. School, J. Corvalán and Y. Ota are supported by the Servicio Nacional de Geología y gratefully acknowledged. The authors al so thank E. Minería (Chile).

REFERENCES

Abele, G. 1988. Geomorphological west-eastsection through of the San Pedro area, Chile. U.S. GeologiealSurvey, the north Chilesn Andes near : In Lecture Professional Paper, No. 525C, p. C63-C67. Notes in Earth Sciences (Bahlburg, H.; Breitkreuz,C.; Oingman, RJ. 1967. Geology and groundwater resources Giese, P.; editors). The Southem Central Andes. Springer ofthe northem partofthe Salar de Atacama, Antofagasta Ver/ag, Vol. 17, p. 153-168. , Chile, U.S. GeologiealSurvey, Bulletin, No. Alpers, C.N .; Brimhall, G.H. 1988. Middle Miocene climatic 121-9,49 p. change in the At3.cama Oesert, northern Chile: Evidence Flint, S. 1985. Alluvial fan and playa sedimentation in an from supergene mineralization at La Escondida. Andean arid closed basin: the Paciencia Group, GeologiealSoeietyofAmeriea , Bulletin, Vol. 100, p. 1640- Antofagasta, Province, Chile. GeologieaJ Soeiety of 1556. London, Joumal, Vol. 142, p. 533-546. BlÜggen, J. 1934. Las formaciones de sal y petróleo en la Flint, S. 1987. Oiagenesis of Tertiary playa sandstones of Puna de Atacama. Boletín de Minas y Petróleo, Vol. 4, Northern Chile; implications for Andean uplift and No. 32, p. 5. S~ntiago . metallogeny. Sedimentology, Vol. 34, p. 11-29. BlÜggen, J. 1942. Geología de la Puna de San Pedro de Flint, S. ; Tumer, P.; Jolley, E.J.; Hartley, A.J. 1993. Ex­ Atacama y sus formaciones de areniscas y arcillas tensional tectonics in convergent margin basins: An rojas. In Congreso Panamericano de Ingeniería de example from the Salar de Atacama, Chilean Andes. Minas y Geología, No. 1, Anales, Vol. 2, p. 345. GeologiealSocietyofAmeriea, Bulletin, Vol. 105, p. 603- Cande, S.C.; Leslie, RB. 1986. Late Cenozoic tectonics of 617. the southem Chile trench . Joumal of Geophysieal Galli, C.D.; Oingman, RJ. 1962. Cuadrángulos Alca, Pica, Research, Vol. 91, No. B1, p. 471-496. Matilla y Chacarilla. Carta Geológica de Chile, Vol. 3, Cross, T.A.; Pilger, RH. 1982. Controls of subduction No. 2, 3, 4 Y 5, 125 p. geometry, location of magmatic arcs, and back-arc Guest, J.E. 1969. UpperTertiary ignimbrites in the Andean regions. Geologieal Soeietyof America, Bulletin, Vol. 93, cordillera of part of the Antofagasta Province, northem p. 545-562. Chile. Geological Soeiety of Ameriea, Bulletin, Vol. 80, Oingman, RJ. 1962.. Tertiary saltdomesnearSan Pedro de p.337-362. Atacama, Chile. U.S. Geologieal Survey Professional Hervé, M. 1987. Movimiento normal de la falla Paposo, Paper, No. 450-0, p. 092-094. zona de Falla Atacama, en el Mioceno, Chile. Revista Oingman, R.J. 1963. Cuadrángulo Tulor. Instituto de In­ Geológica de Chile, No. 31 , p. 31-36. vestigaeionesGeológieas, Carta Geológieade Chile, No. Hooper, B.; Flint, S. 1987. Miocene-Recent tectonic evolu­ 11,37 p. tion of the San Bartolo area, northem Chile. Zentralblatt Oingman, RJ. 1965. Pliocene age of the ash-flow deposits fürGeologie und Palaontologie, Vol. 1, No. 7, 8, p. 967- J.A. Naranjo, C.F. Ramírez and R. Paskoff 103

981. sierto de Atacama. Revista Geológica de Chile, No. 13- Jordan, T.E.; Alonso, A.N. 1987. Cenozoic stratigraphyand 14, p. 79-85. basic tedonics of the Andes Mountains, 2°_28° South Naranjo, J.A.; Paskoff, A. 1985. Evolución cenozoiea del Latitude. American Association ofPetroleum Geologists, piedemonte andino en la Pampa del Tamarugal, norte Bulletin, vol. 71, No. 1, p. 49-64. de Chile (18-21 OS). In Congreso Geológico Chileno, No. Lahsen, A 1982. UpperCenozoic Volcanism andtectonism 4, Actas, Vol. 4, p. 5/149-5/165. in the Andes of Northem Chile. Earth-Science Reviews, Pilger, A.H. 1981. Plate reconstructions, aseismic ridges, Vol. 18, p. 285-302. and low-angle subduction beneath the Andes. Geological Macellari, C.E.; Su, M.J.; Townsend, F. 1991. Structureand Society of Amerlea, Bulletin, Vol. 92, p. 488-456. seismic stratigraphy of the Atacama basin, northem Pilger, A.H. 1983. Kinematics of the South America Chile. In Congreso Geológico Chileno, No. 6, Actas, Vol. subduction zone from global plate reconstructions. In 1, p.133-137. Geodynamics of the eastern Pacific region, Caribbean Maksaev, V. 1978. Cuadrángulo Chitigua y sectorocciden­ and Scotia Ares, Geodynamics Series, Vol. 9 (Cabre, tal del cuadrángulo Palpana. Instituto de Investigacio­ A., S.J.; editor). American Geodynemics, Geophysical nes Geo/:Sgicas, Carta Geológica de Chile, No. 31 ,55 p. Union, p. 113-125. Washington. Marinovic, N_; Lahsen, A. 1984. Hoja Calama. Servicio Pilger, A.H. 1984. Cenozoic plate kinematics, subduction Nacional de Geología y Minería, Carta Geológica de and magmatism: South American Andes. Geological Chile, Ne. 58, 140 p. Society of London, Journal, Vol. 141, p. 793-802. Moraga, A; ' ~hong, G.; Fortt, M.A.; Henríquez, H. 1974. Ramírez, C.F. 1979. Edades Potasio-Argón de rocas volcá­ Estudio geológico del Salar de Atacama, provincia de nicas cenozoicas en la zona San Pedro de Ataeama-EI Antofagasta. Instituto de Investigaciones Geológicas, Tatio, Región de Antofagasta. In Congreso Geológico Boletín, No. 29, 56 p. Chileno, No. 2, Actas, Vol. 1, p. F3"-F41. Mortimer, C. 1973. The Cenozoic history of the Southem Skarmeta, J.; Marinovic, N. 1981. Hoja Cluillagua. Instituto Atacama desert, Chile. Journal ofthe Geological Society de Investigaciones Geológicas, Carta Geológica de of Lond01, Vol. 129, p. 505-526. Chile, No. 51,63 p. Naranjo, J.A. 1987. Interpretación de la actividad cenozoica Travisany, V. 1978. Mineralización cuprífera en areniscas superior a lo largo de la zona de falla Atacama, Norte de de la Formación San Pedro en el d strito San Bartolo. Chile. Revista Geológica de Chile, No. 31, p. 43-55. Memoria de Título (Inédito), Universidad de Chile, De­ Naranjo, J.A.: Paskoff, A. 1980. Evolución geomorfológica partamento de Geología, 70 p. del Desierto de Atacama entre los 26" y 33° latitud sur: Wilkes, E.; Geirler, K, 1988. Sedimentary and structural revisión cronológica. Revista Geológica de Chile, No. evolution of the Cordillera de la Sal, 11 Región, Chile. In 10, p. 85·89. Congreso Geológico Chileno, No. 5, Actas, Vol. 1, p. Naranjo, J.A. Paskoff, A. 1981. Estratigrafía de los depósi• A173-A188. tos cenozoicos de la región de Chiuchiu-Calama. De-

Manuscript receil.ed: June 11,1992; approved, June 6, 1994