Tectonic Implications of Late Cenozoic Sedimentation from the Coastal Cordillera of Northern Chile (22-24"S)

Home , Atacama Fault, La Herradura Formation, La Negra Formation, Mejillones Peninsula, Volcanism of Chile

Journal ofrhe Geological Sociery, London, Vol. 152, 1995, pp. 51-63, 11 figs, 1 table. Printed in Northern Ireland

Tectonic implications of Late Cenozoic sedimentation from the Coastal Cordillera of northern Chile (22-24"s)

A. J. HARTLEY' & E. J. JOLLEY2 'Production Geoscience Research Unit, Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB9 2UE, UK 'BP Exploration, Farburn Industrial Estate, Dyce, Aberdeen AB2 OPB, UK

Abstract: Late Cenozoic sediments from northern Chile (22-24"s) are exposed on the coastal plain between the Coastal Cordillera to the east and tilted fault blocks of the Mejillones Peninsula to the west. During the mid-Miocene to Pliocene (?up to mid-Pleistocene) a shallow marine basin developed unconformably over basement. Sedimentation was initially restricted to a small half-graben on the western margin of the Mejillones Peninsula during the Miocene. Expansion of the basin during the early Pliocene resulted in widespread shallow marine sedimentation across the study area. Alluvial, aeolian and beach sediments were restricted to the basin margins, where sediment was supplied from theCoastal Scarp (a major cliff-line bounding the western margin of the CoastalCordillera) and isolated(islands) fault blocks. Areas of restrictedclastic input were characterized by carbonate deposition. Marine planation surfaces or terraces (0.5-600m elevation) and associated palaeo-sea cliffs cut intoMiocene-Pliocene sediments and basement rocks, developed along the coastline of northern Chile in the ?late Pliocene to late Pleistocene. A similar age for a number of late Pleistocene terraces (125 OOO years) now at different elevations, suggests that they were cut during interglacial highstands (oxygen isotope stage 5). Variations in terrace elevation are attributed to faulting. Regionalscale uplift (over hundreds of kilometres) of shallowmarine sediments beneath an extensive pre-mid Pleistocene shoreline is considered to be related to variations in subduction zone geodynamics. The favoured hypothesis is that of aseismic ridge subduction.

Keywords: Chile, Upper Cenozoic, tetonics, sedimentation, marine terraces.

Thelate Miocene toRecent uplift of the centralAndean of fault-relateduplift resulting from active N-S-trending Pacific margin of South Americais recorded by the faults which cross-cut thepeninsula and glacio-eustatic development of a number of marine terraces and exhumed sea-levelfluctuations. Descriptions of these uplift related shallowmarine and continental sediments of Miocene to phenomena are restricted to the area around the Mejillones Recent age. However, whilst terrace development has been Peninsula and little attention has been paid to uplift related recognizedfor some time (e.g. Fuenzalida et al. 196S), features north and south of the peninsula. Here we present correlationalong the Pacific marginhas proved extremely data on Miocene to Recent shallow marine sedimentation, difficult. Correlation difficulties havearisen because of terrace, beach ridge and alluvial fan development along the variations in theages, numbers and heights of terraces. north Chilean coast between 22" and 24"s. These data allow These variations are thought to be related to a combination constraints to be placed on uplift mechanisms at both local of fluctuationsin sea-level resulting from the Quaternary and more regional scales. glaciation (e.g. Radtke 1987) superimposed on areas of the Pacific marginsubjected to differential uplift. This Geological setting differentialuplift is likely to berelated to changes in the The Coastal Cordillera forms a prominent topographic high geodynamic configuration of the subduction zone (e.g. Flint throughout northern Chile with an average height of 1km et al. 1991; Machare & Ortleib 1991). Here we illustrate how (locally up to 2 km). The Cordillera is bounded to the east a detailed study of Miocene to Recent sediments from the by the Atacama Fault Zone and to the west by a major cliff north Chilean coastal margin can help to constrain the scales line known as the Coastal Scarp (Fig. 1). The Coastal Scarp at which different uplift mechanismshave affected the extends for over700 km and has previously been interpreted leading edge of the continental margin. The understanding as either an extensional fault (Paskoff 1980) or palaeo-cliff of these uplift mechanisms hasimportant implications for line (Mortimer & Saric 1972). The study area comprises the the tectonicevolution of theCoastal Cordillera and the coastal plain between 22" and 24"s (Fig. 2). In this area the relationship between forearc deformation and subduction. plain shows a gradual decrease in gradient from alluvial fans at the Coastal Scarp to the present day shoreline. At the shoreline, the fans terminate in one or a series of terraces Previous work which variably exposealluvial, beach or shallow marine Previouswork on coastal uplift innorthern Chile has sediments. Between 22" and 23"s the plain increases in width focused onmarine terrace and beach ridge development from SO0 m to 4500 m (Fig. 2). Between 23" and 23'30' the acrossthe Mejillones Peninsula (Herm 1969; Okada 1971; alluvial fans pass westwards from the Coastal Scarp onto the Armijo & Thiele 1990: Fig. 1). These authors suggested that easternmargin of the Mejillones Peninsula (Fig. 2). The terracedevelopment could be attributed to a combination peninsulacomprises a series of fault-bounded blocks 51

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Fig. 1. Geological map of the Coastal Cordillera of northern Chile between 22 and 24% (modified from Boric er al. 1990), illustrating the main fault patterns and extent of Miocene to Recent shallow marine and associated sediments. Area east of the Atacama Fault Zone left blank for simplicity.

Fig. 2. Geomorphology of the Coastal Cordillera between 22 and separatedfrom the Coastal Scarp by alow-lying plain of 243, illustrating the distribution of marine terraces, beach ridges uplifted marinesediments of Miocene to Pleistoceneage and main faults. The coastal plain is defined as being below the (Fig. 1). South of the peninsula (south of 23'30') the coastal 200 m contour. Boxes indicate position of aerial photographs and plain is relatively narrow and similar in morphology to the detailed maps (Figs 5, 8 & 9). CH, Calita Herradura. northern part of the study area. The Coastal Cordillera itself largely comprises andesite- dominated volcanicrocks of the 10km thick Jurassic La Negra Formation(Garcia 1967) andassociated Jurassic Miocene-Recent stratigraphy granodioriticintrusions (Fig. 1). As well asthe igneous FiveMiocene Recentto stratigraphic unitscanbe rocks,fault blocks onthe MejillonesPeninsula andthe recognized onthe coastal margin between 22" and 24"s southernpart of thestudy area include Palaeozoic unconformably overlying older rocks (Fig. 3). (1) Miocene metamorphic rocks and Cretaceous red beds and limestones shallowmarine sediments of the 160 mthick Caleta of the Coloso and El Way Formations (see Ferraris & Di HerraduraFormation (Martinez-Pardo 1980; Krebs et al. Biase 1978; Hartley et al. 1992 forfurther details). Clast 1993). The formation crops out at Caleta Herradura (on the typeswithin thePleistocene and Holocene alluvial fans northwesterncoast of the MejillonesPeninsula) and is indicate an igneous and metamorphic source area. unconformablyoverlain by the La PortadaFormation. (2)

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Thickness aspre-mid-Pleistocene. The unit is considered to be Strat.Lithology equivalent in age to the La PortadaFormation. (4) (m) Pleistocene Holoceneto shallowmarine and beach sediments of the Mejillones Formation (Ferraris & Di Biase 1978). The formation is up to 80 m thick and unconformably overlies the La PortadaFormation. The Mejillones Z Formation is exposed over much of the northern part of the 0 MejillonesPeninsula. (5) Pleistocene to Holocene alluvial i=* fan, aeolian and beach sediments (up to 50m thick) which flank the Coastal Cordillera and are equivalentin age to the Mejillones Formation. 0.1 Sedimentology L> *3 Caleta Herradura Formation 0 The formation was defined by Krebs et al. (1993) who also ~n described the sedimentology of the 160 m thick section. The JZ section comprises three main facies associations: (1) a lower unit of well rounded, fossiliferouspebbly conglomerates W withcoarse-graineda matrix interpreted asnearshore reworking of alluvial fandeposits (Krebs et al. 1993), interbedded withshell-rich, cross-stratified coarsegrained sandstones interpreted as wave-dominated upper shoreface 1.64 deposits, (2) fossiliferousmudstones interbedded with cross-stratified sandstonesand pebbly fossiliferous con- glomeratesorganised into coarsening upwards units and interpreted as pulsed offshore bar construction (Krebs et al. 1993)and (3) fine-grainedburrowed and cross-stratified sandstonesinterpreted as middle upperto shoreface ... deposits. Thetop of theformation is capped by a d.. . .'.l . diatomaceousmarine mudstone unit up to3m thick 5.2 deposited in an offshore pelagic environment. La Portada Formation The formation was defined by Ferraris & Di Biase (1978) v::... and recentlymodified by Krebs etal. (1993)(Fig. 3). It is ...... o:. exposed in sea cliffs at La Portadato the north of l Antofagasta where it can be seen to unconformably overlie the La Negra Formation and at Caleta Herradura where it Jurassic volcanics, granitoids + unconformablyoverlies theCaleta Herradura Formation. Cretaceous sediments The LaNegra Formation unconformity surface is strongly weatheredparticularly along old cooling joints within the lavas. The unconformity itself is colonized by barnacles indicatingarocky shoreline (Fig. 4) and is overlain by Alluvium m Shallowmarine low-angleplanar cross-stratified (withseaward to coast- Fig. 3. Generalized stratigraphy of the Coastal Cordillera between paralleldips) and horizontally stratified coarseto very 22 and 24"s based on Ferraris & Di Biase (1978) and Krebs er al. coarse grained shelly sandstones (beds up to 1.5 m thick). (1993). Sandstonebeds commonly have scoured contacts with a maximum relief of 65 cm. Scours generally have large width to depth ratios and are marked at the baseby pebble lags. In Shallowmarine sediments of the 40 mthick La Portada places,trough to planar cross-stratified medium to coarse Formation which contain fauna of Pliocene age (Herm 1969; grainedshelly sandstones are present. Large arthropod Ferraris & Di Biase 1978; Krebs et al. 1993). This formation burrows are present throughout the section. The sandstones crops out over the southern half of the Mejillones Peninsula areinterpreted as wave-dominated upper shoreface and andextends further southwards tothe northern edge of transitionalupper shoreface toforeshore deposits (cf. the Antofagasta. It is unconformably overlain by the Mejillones inner rough facies of Clifton et al. 1971). Formation.(3) Shallow marine and fan delta deposits Atthe top of the section oneto twohorizons of exposed in terraces to the north of the Mejillones Peninsula conglomeratesare present. Well-cemented conglomerates (Herm 1969). From macrofaunal studies, Herm (1969) dated are exposedwithin thebackshore terraces interbedded these sediments as Pliocene (sensu lato) and as such, they withfan apronconglomerates, marine-reworked alluvial areequivalent in age tothe La PortadaFormation. The conglomeratesand shallow marine sandstones. The con- upper age limit is constrained by dated terraces (see later) glomeratescomprise well rounded clasts rangingfrom

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reworked alluvial deposits and is equivalent in age to the La Portada Formation.

Beach deposits. They comprise well rounded clasts ranging frompebble to boulder grade andesitic and granodioritic materialwith a coarse sandstone matrix often dominated (upto 100%in places) by shell fragments. These conglomeratesare identical to those described fromthe La PortadaFormation and are interpreted as beachdeposits. Beach deposits are present in all terraces south of Gatico (22'30's) along the Coastal Cordillera.

Shallowmarine sandstones. Excellentsections of shallow marinesandstones are exposedwithin the backshore terraces along the Coastal Cordillera between Gatico and the Mejillones Peninsula. Three main facies can be recognized: (1) very low angle (<4') planar cross-stratified and parallel laminated fine- to medium-grained black sandstones (beds up to 30cm thick)largely comprising heavy minerals and interpreted as foreshore deposits;(2) low-angle planar cross- stratified (coastparallel to oblique,occasionally seaward dipping) and horizontally stratified coarse- to very coarse- grainedshelly sandstones (beds up to 1.5 mthick) with pebbles on the base of small scours and occasional current andwave ripples, interpretedas wave-dominated transitional upper shoreface to foreshore deposits; (3) large-scale (up to 3 m high) trough to planar cross-stratified medium to coarsegrained shelly sandstones (Fig. 4) with arthropod burrows interpreted as upper shoreface deposits.

Marinereworked alluvial deposits. Interbedded within the beach and shallow marine deposits are a series of 0.3-3 m thickangular framework-supported conglomerates. They have a matrix of coarse-grained sandstone. The conglomerates can be mapped out as lobeswhich become progressively (h) morereworked away from a main alluvial feederchannel 4. (a) Unconformity surface colonized by Miocene barnacles Fig. system towards the present-day shoreline. This is illustrated between the La Portada Formation and La Negra Formation (La by increasedrounding and sorting of clasts together with Portada, Fig. 2). (b) Large-scale trough cross-stratified coarsegrained sandstones interpreted as upper shoreface deposits, note the incorporation of shell material into the matrix. The location development of beach deposits (boulders) overlain by alluvial fan of conglomerate lobes coincides with the present day posi- deposits above the sandstones (Gatico, upper terrace, ruler is 1 m tion of incised gullies suggesting that incision and gully high). developmenttookplacepre-mid-Pleistocene (?late Pliocene).

pebble to boulder grade andesitic and granodioritic material Mejillones Formation witha coarsesandstone matrix often dominated (up to The formationcomprises seriesa of shallowmarine 100% in places) by shell fragments.Bed thicknesses vary sandstonesand limestones previously described by Herm from single clast lags to up to 3 m. Byanalogy with the (1969) andFerraris & Di Biase(1978). The sandstones present-daybeach where large, well-roundedboulders of comprisetrough and low angle planar cross-stratified granodioriteandandesite areseparated by coarse (seaward-dipping) and horizontally stratified coarse to very grained-sands with a high proportion of shell material, the coarse grained shelly sandstones (beds up to 1 m thick) with conglomerates are interpreted as Pleistocenebeach deposits. currentand wave ripples in places.Shallow scours are Different facies are occasionally developed in other parts of particularlycommon (upto 50 cm relief) withoccasional the basin. In particular calcareous mudstones and bioclastic pebblespreserved above scours. These sandstones are limestonesare present on the western margin of the interpretedas transitional upper shoreface to foreshore MejillonesPeninsula. The mudstonesand limestones are deposits.Beds of coarse-grainedbioclastic limestone and thought to reflect periods of limited clastic input. fine-grained diatomite are interbedded with the sandstones and have irregular tops which are interpreted as the product Pliocene to ?early Pleistocene shallow marine of erosion prior to deposition of overlying sandstone units. sediments north of the Mejillones Peninsula The limestone and diatomite deposits appearto be restricted This stratigraphic unit is exposed in a series of backshore to the central partsof the low-lying plain around the town of marine terraces north of the Mejillones Peninsula to Gatico. Mejillones suggesting that in these areas, more distal from It comprises interbedded beach, shallow marine and marine the palaeo-coastline, clastic sedimentsupply was limited.

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Thelimestones are interpreted as shoreface deposits and (lee) side of coastal spurs. Carbonate cemented Quaternary diatomites as lower energy shelf deposits. aeoliandunes commonly occur at fan apron toes inter- Inland, southwards from the present day coastline Herm bedded with fan streamflow deposits between Antofagasta (1969) interpreted a series of ridges, which can be mapped and the southern edge of the Mejillones Peninsula (Fig. 5). fromaerial photographs over much of the Mejillones Trough cross-strata in these older dunes dip at up to 26". Formation outcrop, as palaeo-beach ridges. The ridges have The dunes are texturally mature but compositionally imma- an arcuate pattern mimicking the present day coastline and turecontaining shell fragments,glauconite and occasional comprise well cementedpebbly coarse grained sandstones heavy mineral grains derived potentially from either one, or rich in shell material. a combination of (1) the underlying La Portada Formation andequivalents, (2) deflated dune surfaces or (3) aeolian Pleistocene-Holocene sediments of the Coastal reworking of beach sands. Cordillera Thisstratigraphic unit comprises alluvial fan,aeolian and Marine reworked alluvial deposits. Thesesediments rep- marine reworked alluvial sediments. resent the coastal terminationof the alluvial fans and record theinteraction between marine and alluvial processes. At Alluvial fan deposits. Over 30 coastal alluvial fans and fan the present day this is represented by the development of deltas exposed between the Coastal Scarp and the shoreline conglomerate lobes at the mouths of incised gullies where havebeen studied (a preliminarystudy was published by the lobes are actively prograding across the backshore (see Flint e6 al. 1991). Typically the fans comprise a steep (8-28" also Flint et al. 1991). Conglomerate lobes are progressively slope) cone of sediment adjacent to the Coastal Scarp cen- reworked seawards as indicated by increased rounding and tered around the main fan feeder channel. The cone covers sorting of clasts together with incorporation of shell material approximately 10-15% of the fan area and largely comprises into the matrix. very poorly sorted, ungraded beds of angular cobble, pebble and locally boulder grade material in a matrix ranging from mudstone to very coarse-grained sandstone. No evidence of Summary of Miocene-Recent sedimentology imbricationwas found. These sediments are thought to represent cohesive debris flow, grain flow and high density Throughout mid-Miocene to Holocenetimes virtually flood deposits. The rest of the fan forms an apron with a continuousshallow marine sedimentation dominated over gentle slope (between 2 and 8') from the foot of the cone. much of the study area withsynchronous fault movement Thefan apron comprises poorly sorted, subangular, un- indicated by (1) interruption of deposition in theearly graded cobble and pebble grade material in a coarse grained Pleistoceneindicated by thedevelopment of anangular sandstone matrix. Clasts display a crude b-axis imbrication unconformitybetween the La Portadaand Mejillones and bedding-parallel alignment. These sediments are inter- Formations and (2) localized fault activity during the early preted as poorly confined streamflow deposits with variable Pliocene at CaletaHerradura indicated by unconformity sediment load concentrations. Fan apron widths are typically development between the Caleta Herradura and La Portada a few kilometresacross and may comprise a number of Formations restricted to the Caleta Herradura half-graben. coalesced fans. Many of the fans display spectacular incision Pliocene to ?early Pleistocene shallow marine sediments of with gullies upto 20m deep developed where the fan the La PortadaFormation and equivalent age sediments terminatesat the terrace on the beach backshore. Incised were developed throughout the study area with some input fans commonly display a deflated, weathered surface where from alluvial fans at the basin margins (Fig. 6). Pleistocene oxidationhas resulted in anochre colouring over inactive alluvial andaeolian sedimentation was restricted to the parts of the fan surface. Along the coast discrete fan aprons margins of the basin where sediment was supplied from the are separated by areas with a thin veneer of sediment a few CoastalCordillera and isolated (islands) fault blocks (Fig. crns thick or small?fault-bounded headlands of Jurassic 6). Areas of restricted clastic input during the Pleistocene andesites and granitoids. arecharacterized by thedevelopment of bioclastic Important observations include: (1) the bipartite division limestoneswith calcareous mudstones and diatomites of the alluvial fans into debris cones and braidplain aprons suggesting deposition below fair-weather wave base. reflecting thevariation in sedimentaryprocesses active on the fansurface: (2) Theamount of incision onthe fan surface in response to base-level fall andby-passing of Palaeo-cliff lines and marine planation sediment from the fan surface. surfaceslterraces

Aeoliandeposits. Recentunconsolidated aeolian sediments comprisingtrough cross-stratified, sinuous-crested dunes Description and distribution rangingfrom a few crns to a few metres in height, are present in interfan areas or interbedded with distal alluvium Twotypes can be recognized: (1) palaeo-cliff lines with at fan toes. Aeolian sediments are particularly common on associated marine planation surfaces or terraces developed andaround the MejillonesPeninsula where there is no in Miocene to Holocene sediments and (2) marine planation direct fan connection to the sea and aeolian processes have surfaces on basement rocks (Fig. 7). reworked alluvial sediments.Dunes are common at the (1) Palaeo-cliff lines andassociated terraces cut in edges of inactivefans and in placescan be seen to be Miocene to Holocenestrata are present throughout the actively migrating over the fan apron. Aeolian sediments are study area. Cliff lines comprise a vertical to sub-vertical face particularlycommon onfan aprons located on the south with a horizontal trace which lies parallel or sub-parallel to

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the present day shoreline. Palaeo-cliff heights vary from a lines. However,where incision on individualfans is few centimetres to over 70m. A thin veneer of shell-rich widespread (e.g. near Hornitos, Fig. 8) sediment transport marine sediments (generally a few tens of centimetres thick has been concentrated within incised gullies thus by-passing althoughlocally up to acouple of metresthick) may be the main fan surface and preserving earlier cliff-lines. One presenton terraces developed at the top of cliffs. These or two palaeo-cliff lines developed close to the shoreline can sedimentsoverlie Plio-Pleistocene alluvial fan,beach and be recognized throughout the study area. However, in some shallow marine sediments and cannot be traced more than a areasup to 6 palaeo-cliff lines are presentover 1.5 km few metres inland from the terrace top. In many cases only inland. Comparison of numbers of cliff lines is difficult as, theyoungest cliff-lines canbe recognizedadjacent tothe depending on rates of coastal erosion (in part dependent on shoreline(within a few tens of metres of the present-day the nature of the substrate), cliff lines may well amalgamate beach) as, alluvial fan sedimentation has covered older cliff such that any cliff line is likely to have a composite recordof

Fig. 5. (a) Aerial photograph illustrating distribution of alluvial fans, beach ridges and aeolian sediments at the southern end of the Mejillones Peninsula. (b) Interpretation of photograph. Note the roll-over of the Pleistocene planation surface on top of the terrace towards the fault east of Cerro Moreno (see Fig. 2 for location).

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W E

.. *.l.. ~~ri~~terracewith Palaeo-sea cliff (oldea to east) ...... marine deposits marine ..+.* associated with *...,...... *...I.**...... f...... 1 . 4 urn -. . . * ...... f ...... I '1~''1 ,

a La NegraFormation m Alluvial Fan Pliocene-Recent [7 Plio-Pleistoceneshallow marine deposits Pleistocene-Recent aeolian deposils

N Basementplanation S Marine terrace incised valley / cut to sea-level

Fig. 7. Schematic E-W and N-S cross-sections illustrating the relationships between Pliocene to Recent sedimentation, palaeo-sea cliff formation, marine terrace development and faulting in the study area. Fig. 5(b).

development of beachridges (Fig. 9) which representa a number of drops in relative base-level, rather than each more gradational fall in sea-level. South of the Mejillones cliff line simply representing a single base-level fall. Peninsula,up to 4 palaeo-cliff lines aredeveloped with a Onand adjacent to the MejillonesPeninsula up to3 height of over 40 m recorded in a single cliff at La Portada. terraces are developed in late Cenozoic sediments. Relative Ingeneral, cliff-line development is closelysimilar to that sea-level falls in this areaare alsoreflected in the north of the peninsula. (2) Planation surfaces can be recognized throughout the study areadeveloped on pre-Miocene basement including Pianatian surlaces the La Negra Formation, and to a lesser extent Cretaceous on basement sediments, Jurassic granodiorites and Palaeozoic metamor- Shallow marine sands phic rocks (Figs 2, 9). Characteristically planation surfaces dip gently (generally less than a few degrees) seawards and comprisestrongly fractured and weathered basement with areas of shell material locally preserved within fractures. North and south of the Mejillones Peninsula, planation surfacesare present at the top of smallheadlands which separate alluvial fans and fan deltas (Fig. 2). It is generally notpossible to tracethe surfaces from the present day shoreline to the foot of the Coastal Cordillera due to their partialcoverage by younger alluvial fandeposits and localised weathering of the Coastal Scarp. Only one general planation surface can be recognized on the coastline north of Mejillones (Fig. 2). This surface generally, but not always, hasa slightly higherelevation (30-60m) than terraces developed in the intervening sediments. Threeplanation surfaces were recognized by Okada (1971) on the tilted fault blocks of the Mejillones Peninsula (Fig. 9). Okada noted that these surfaces range in elevation from 600-330 m (highest), 500-250 m (middle) to 280-30 m (lowest)with the highestbeing 8 kminland from the shoreline.He alsonoted a general tilting of thesurfaces from north to south with an increasing amount of tilt from the youngest(lowest) 0.07" tothe oldest(highest) 0.44'. Detailedexamination of aerialphotographs indicates that whilst the observations of Okada are correct, a number of intervening minor planation surfaces (in some places up to 5) are recognizable between the present day shoreline and Fig. 6. Schematic palaeogeography of the study area during the highestplanation surface (Fig. 9). The highestand Plio-Pleistocene times. M, Mejillones. middle planation surfaces have only been recognized on the

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northernpart of thepeninsula, whilstonly thelower (Radtke 1985,1987) techniques. Based on this dataset, planationsurface can be recognized around Morro Mor- terrace ages can be roughly assigned into two groups. eno in thesouth (Fig. 2).

Mid- to late Pleistocene terraces. In areas where a numberof Age terraces have been developed (3 or more) the first or lowest Published data on the age of marine terraces in northern terrace ranges from 50 cm to 36 m in height, but generally Chile are summarized in Table 1 and Fig. 10. Age dates are averages between 3 and 12m. The lower terrace has been derived from marine molluscs foundon terraces using U-Th dated between 80300 and 168000 years. (Table 1). Hsu et disequilibrium (Radtke 1985 1987), amino acid racemization al. (1989) based on amino acid racemization data suggest a (Hsu et al. 1989) andelectron spin resonance (ESR) mean age of 125 000 years for lower terrace development

Fig. 8. (a) Aerial photograph of the Coastal Cordillera around Hornitos (see Fig. 1 for location) and (b) interpretation (for key see Fig. 5). Note the variations in the amount of incision related to individual terraces. The amount of incision into individual terraces is considered proportional to the height of the terrace. This relationship allows terrace lines to be identified from variations in the amount of incision of present day alluvial fan channels (see Fig. 2 for location).

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In areas where a number of terraces have been dated the lower terrace always has the youngest age. This could result fromeither (1) arelative sea-level fall fromthe highest terracewith lower terraces cut and associated marine deposits formed during regression or (2) formation of terraces through periodic transgression and uplift such that the oldest terraces have the highest elevation. The lower terrace date of 125 000 yearscorresponds to oxygenisotope stage 5 (Imbrie et al. 1984), and is considered to haveformed through transgression during an interglacial with a warmer climate and sea-level highstand. Where a number of terraces aredeveloped (i.e. terraceshave not amalgamated over time)the lower terrace is thought to representthe last interglacial highstand. The highest terrace at El Rincon (Fig. 2) has been dated at 342400 years using ESR and 320000 years using U-Th disequilibria (Radtke 1987; Table 1). Thisterrace can be traced on to the lowest basement planation surface on and aroundthe MejillonesPeninsula (Okada 1971; Armijo & Thiele 1990). As this lowest planation surface is likely to be equivalent to thebasement planation surfaces along the Coastal Cordillera, this planation surface can be regarded as at least mid-latePleistocene in age.Consequently, all terracescut in lateCenozoic sediments andthe lowest basement planation surface are considered to be of mid to late Pleistocene age.

?Pliocene to earlyPleistocene terraces. The middleand Fig. 8(b). higher planation surfaces developed on the fault blocks at the north and central parts of the Mejillones Peninsula are older than the lowest planation surface. The height of the in the north of the study area (Table 1). Whilst, there is a Mejillonesterraces suggested to Paskoff(1977) and Clap- wide spread of agesfor the lower terrace a mean age of perton (1983) alate Pliocene age although it shouldbe 125,000 years is consideredrepresentative of thetime of noted that terrace height is not age diagnostic, particularly lower terracedevelopment in mostareas. Higher terrace as the highest terraces are associated with active faults (Fig. ages are problematical ranging from 154000 to 342 000 years 2and see below). Consequently, the two oldest planation (Table 1). Hsu et al. (1989) suggest a date of 280000 years surfaces are likely to range in age between the early Pleis- for the next lowest terrace in the northern part of the study tocene to late Pliocene. Pliocene marine deposits have been area (Table 1). However, because of the large variation in found on terraces with over 100 m elevation to the east of terrace dates and heights it is impossible to further constrain the city of Antofagasta and to the south of Antofagasta near the age of older terraces. Coloso (Martinez & Niemeyer 1982) (Fig. 2).

Table 1. Summary of ages for marine terraces in the study area

LocationLatitude Terrace U-Th age (years) ESR(years) age Aminostrat. Reference elevation

Coloso 23'45' 8-10 m 100 148000 82(+21400, 900) -17 Radtke 1987 8-10m -8000)(+loOO, 110000 111 000 Radtke 1987

El Rincon, 23"05' 6-10m 106 000 (+6000, -4000) 167 800 Radtke 1987 Mejillones 14-24 m 154000 (+10000, -6000) >l93 000 Radtke 1987 31-36m 320000 (+infinity, -70000) >342 000 Radtke 1987

Playa de 10s Hornos 22"52' 30 m l05 000 140 OOO Radtke 1985 36-45 m 230 000 Radtke 1985

Hornitos 36 22O.54' m 134 000 ( + 2000,80 - 8000) 300 Radtke 1987

*Michilla, 22"41' lowest Hsu 000 125 et al. 1989 *Los Hornos, 22"52' next lowest 000 280 (+SO 000) Hsu et al. 1989 *Bahia Tames 22"32'

* Localities grouped together. Error limits in parentheses, where given.

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Interpretation developed throughout the study area and therefore likely to A combination of interglacial sea-level highstands during the represent interglacialhighstands. Differences in terrace Pleistoceneand tectonically-related uplift throughfault height and some of the scatter in age dates are likely to be movement are considered responsible for terrace formation. due to differential fault block uplift. This is demonstrated in The evidencefor glacio-eustaticsea-level highs is the the northern and central parts of the Mejillones Peninsula development of common terrace ages along some parts of wherethe middle and higher planation surfaces and the margin (Radtke 1985,1987; Hsu et al. 1989; Table 1). sub-surfaces are developed. In this area, planation surfaces The common age of some terraces (coincidentalwith oxygen progressivelyincrease in dip tothe south withage, isotopestage 5), eventhough presently differentat suggestingfault controlled uplift and tilting alongblocks elevations,suggests that they were cut by the same event which are currently bounded by active faults.

..- 'Y

Fig. 9. (a) Aerial photograph of the northern end of the Mejillones Peninsula illustrating the planation surfaces developed on the tilted fault blocks, Pleistocene beach ridges, fault scarps and alluvial fans. (b) Interpretation (for key see Fig. 5). Note the faulted dykes on the NW part of the peninsula (see Fig. 2 for location).

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A ESR .urn m Amino 400$ r T TI

l I I a 2730' 2T 23'30 24" Latitude "S

f 04 I b Fig. 9(b). 22930 23" 2330' 24' Latitude "S

Regional uplift

On a more regional scale, subtle variations in uplift within the study area, canalso be determined by examining the present day height above sea-level of marine and marginal 20 marinesediments beneath theyoungest La Portada 301 Formationand equivalent-aged shoreline deposits exposed within terraces (as distinct from the shell lags formed on the terraces). The shoreline deposits are older than the oldest terrace dated at 344000 years and macrofauna beneath the beachdeposits are considered by Herm (1969) to be of Plioceneage (sensu lato). The beachdeposit is therefore likely to be latePliocene to earlyPleistocene in age. The Fig. 10. (a) Dated terrace ages plotted along the coastline with deposits can be traced along strike over much of the study error limits where given (see Table 1); note the breakdown into two area and are considered to represent a single chronostrati- main groupings. (b) Dated terrace heights plotted along the graphicevent. Figure 10illustrates the uplift of the coastline (see Table 1); note height variations of similar aged Plio-Pleistocenebeach deposit across thearea. The terraces suggesting possible fault-control. (c) Plot of metres above shorelinehas undergone relatively uniform uplift with the sea-level of Plio-Pleistocene beach deposit which can be traced exception of the area adjacent to the Mejillones Peninsula throughout the study area. Note the relatively uniform uplift since the mid Pleistocene across the study area with the exception of the wherethe large amount of uplift is considered to be area around the Mejillones Peninsula. fault-related. Faulting may locally haveaffected the height of theshoreline deposits, however the general trend depicted in Fig. 10 shows that whilst faulting was active on a small-scale,Plio-Pleistocene shoreline height across the study area was controlled by a larger scale mechanism. buoyant, topographically positive oceanic crust is thought to Pleistocene to Recent marine terraces have been uplifted induce local uplift of the margin. Machare & Ortlieb (1992) alongmuch of the PacificMargin of SouthAmerica (e.g. have documented the effects of aseismic ridge subduction on Hsu et al. 1989) suggesting an uplift mechanism developed the coastal morphology of Peru where the Nazca Ridge is on the scale of the continental margin. The scale and aerial currently being subducted. Bathymetric data for the ocean extent of uplift in the study area precludes a fault-related floor west of thestudy area revealsthe presence of an origin. Giventhe proximity to thesubduction zone it is aseismic ridge which forms a spur to the Iquique Ridge,west likely that any potential uplift mechanism will be related to of the Mejillones Peninsula (Fig. 11). It is possible therefore heterogeneitiesatthe subduction zone. One potential that regional uplift in the study area is related to aseismic regional uplift mechanism is that of aseismicridge ridge subduction centered around the Mejillones Peninsula. subduction,where subduction of anomalously thick, The trace of the Atacama Fault Zone (Fig. 11) parallels the

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northernChile also noted that no direct seismic activity couldbe attributed to the Coastal Scarp.Consequently, it seemsunlikely that the Coastal Scarp represents an active extensional fault system linked to the subduction zone. Rutland(1971) and Mortimer et a/. (1974) recognized thatthe Coastal Cordillera formed a topographic barrier confining sedimentation to the Central Depression (Figs 1 & 2) during the Miocene. Mortimer & Saric (1972) and Paskoff 0 100 -400 U OOfl (1973, 1977, 1980)suggested that theCoastal Scarp KM I\ represents a weathered Miocene extensional fault, possibly formedduring uplift of the coastalmargin, and that post-Mioceneweathering has resulted in fault scarp degradation.Alternatively, given the absence of tectonic activity along thescarp it couldrepresent aweathered palaeo-cliff line whichoriginated through uplift of the coastalmargin during the Miocene and is nottectonic in origin.

Conclusions

Thelate Cenozoic to Recent stratigraphy of theCoastal Cordillera of northern Chile records the following series of events. (1) Subsidenceand sedimentation in fault-bounded extensional(half-graben style) shallowmarine sub-basins (theCaleta Herradura and La PortadaFormations) took place in lateMiocene to Pliocenetimes, thepresence of localised unconformities indicates sporadic fault movement. Thedevelopment of the oldestplanation surfaces on northernand central parts of theMejillones Peninsula, aroundthe Cerro Moreno Fault and eastand south of Fig. 11. Location of the aseismic ridge currently being subducted Antofagasta took place at this time. beneath the Andean forearc between Iquique and Taltal, together (2) Local tilting resulted in the development of a minor with the outcrop trace of the Atacama Fault Zone. Note that the unconformity and continued subsidence and sedimentation landwardly convex trace of the Atacama Fault corresponds to the in a fault-bounded shallow marine basin during Pleistocene position of the aseismic ridge possibly suggesting a causal times (Mejillones Formation) with marginal marine, alluvial relationship between the two. In addition note that the Atacama and aeolian sedimentation along basin margins. Fault Zone is offset by a transform fault just to the north of Taltal (3) Sporadic uplift and development of marine terraces such that transform fault development may oe caused by aseismic in the form of palaeo-cliff lines (up to a maximum of 5) and ridge subduction. Oceanic area based on American Association of basementplanation surfaces occurred during the ?late Petroleum Geologists (1981). Forearc area based on Servicio Pliocene and Pleistocene. Uplift resulted in the exposure of Nacional de Geologia y Minera, Chile (1982). Miocene-Pleistocenemarine sediments within terraces, prolonged weathering of inactive fan surfaces and large scale zone of uplift andcorresponds tothe position of the incisionwithin alluvial fanchannels, some of which have aseismic ridge possibly suggesting a relationship between the followed the same channel course for over 340 000 years. two(?strain partitioning related to subduction of the (4) Marine terraces with the same age (125 000 years) are aseismic ridge). relatedto interglacialsea-level highstands. They are presently at different heights due to differential uplift caused by fault movement. Origin of the Coastal Scarp (5) Uplift of Plio-Pleistocenea shoreline reveals a regional scale uplift phenomenon centered on theMejillones TheCoastal Scarp (Fig. 1) hasbeen interpreted as an area which is of too large a scale to be fault-related. Due to extensional fault linked to the subduction zone (Armijo & the proximity of the subduction zone (16-30 km below the Thiele 1990 after Paskoff 1980). Detailed observations from coastline) uplift is considered to berelated to local aerialphotographs and fieldwork have notidentified any variations in subductionzone geodynamics. The favoured exposed fault breaks along the scarp with the exception of hypothesis is that of aseismicridge subduction within the theCerro Moreno Fault (Figs 1 and2) whichhas study area. demonstrableleft-lateral kinematic indicators and extends (6) The upliftedarea forms a broad, easterly convex forapproximately 35 km south of the mainscarp. Active zonebounded by theAtacama Fault Zone, possibly faultswithin theCoastal Cordillera generally trend NNE indicating that development of the Atacama Fault Zone in cross-cutting the Coastal Scarp and with the exception of the this area may result fromstrain partitioning between the AtacamaFault Zone rarelyextend for more than a few CoastalCordillera and the Central Depression related to kms. Comte et a/. (1992), in a study of shallow seismicity in aseismic ridge subduction.

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(7) The CoastalScarp is alate Miocene to Pliocene HERM,D. 1969. Marines Pliozan und Pleistozan in Nord und mittel Chile palaeo-cliffline and notmajora extensional fault, as unter besonderer Berucksichtigung der Entwicklung derMollusken- Faunen. Zitteliana. indicated by the absence of active faulting along the scarp Hsu, J.T., LEONARD,E.M. & WEHMILLER,J.F. 1989. Aminostratigraphy of and lack of any linear features which could be interpreted as Peruvianand Chilean Quaternary marine terraces. Quaternary Science possible palaeo-fault traces. Reviews, 8,255-262. IMBRIE,J., HAYS, J.D., MARTINSON, D.G..ET AL. 1984. The orbital theory of Pleistoceneclimate: support from a revised chronology of thedellxO of Funding from the Nuffield Foundation, the University Aberdeen, record. In: BERGER,A.L., IMBRIE,J., HAYS,J., KUKLA,G. & SALTZMAN, MobilNorth Sea Ltd. (A.J.H.) and NERC fellowship B. (eds) Milankowirch and Climate Part 1, G. Reidel,Dordrecht, GTS.F/87/GSS(E.J.J.) is gratefullyacknowledged. G. Chong 269-305. (UniversidadCatolica del Norte, Antofagasta) is thanked for KREBS,W.N., ALEMAN,A.M., PADILLA,H., ROSENFELD,J.H. & NIEMEYER. logisticalhelp, S. Kape,P.M. Scanlon and T. Buddin for field 1993. Ageand paleoceanographic significance of theCaleta Herradura assistance and P. Turner and S. Flint for discussion in the field. The diatomite,Peninsula de Mejillones, Antofagasta, Chile. Revista Geologica de Chile, 19, 75-81. authors aregrateful to Hans Niemeyer for preprintsof papers. C. MACHARE,J. & ORTLIEB,L. 1992. Plio-Quaternary vertical motions and the Dart and an anonymous reviewer are thanked for their comments. subduction of the Nazca ridge, central coast of Peru. Tectonophysics, 205, 97-108. MARTINEZ,E. & NIEMEYER,H. 1982. Depositosmarinos aterrazados del References Pliocene Superior en la ciudad de Antofagasta, su relacion con la Falla AMERICANASSOCIATION OF PETROLEUM GEOLOGISTS1981. Plate Tectonic Map de Atacama. 111 Congresso Geologico Chileno, Concepcion, Chile. Actus, of the Circum-Pacific Region, Southeast Quadrant. American Association A176-AI88, of Petroleum Geologists, Tulsa, Oklahoma, USA MARTINEZ-PARDO,R. 1980. Hallazgo de Mioceno marino en la peninsula de ARMIJO,R & THIELE,R. 1990. Active faulting in northern Chile: ramp Mejillones, Antofagasta, Chile. In: Congreso Argentino de Paleontologia stackingand lateral decoupling along a subduction plate boundary?. y Biuestratagrafia, Nu. 2 y Cungresu Latinamericano de Paleontologia, Earth and Planetary Science Letters, 98, 40-61. No. 1 (1978), Acras, 3, 57-66. BORIC,R., DIAZ, F. AND MAKSAEV,V. 1990. GeologiaYacimientosy MORTIMER, &C. SARIC,N. 1972. Landform evolution in the coastal region of Metaliferos de la Region de Antofagasta. Servicio Nacional de Geologia y Tarapaca Province. Revue de Geomorphologie Dynamique, 21,162-170. Minera, Chile, Bolletin 40. -, FARRAR,E. & SARIC,N. 1974. K-Arages from Tertiary lavas of the CLAPPERTON,C.M. 1983. The glaciation of theAndes. 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Received 25 October 1993; revised typescript accepted 19 April 1994.

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