Model of Early Jurassic to Mid-Miocene GeologicalEvolution of the Back-Arc Sedimentary Basin of Northern

Chile and Northwestern Argentina

Terry Arcuri and George BrimhallDepartment of Earth and Planetary Science,

University of California, Berkeley, CA 94720-4767 tarcuri@uclink4.berkeley.edu

Abstract

A model has been constructed from stratagraphic analysis and magmaticemplacement data in which variations of the sedimentary environments of northern Chileand Argentina from the early Jurassic to the mid-Miocene were compiled. From this dataa series of maps at 2 million year intervals have been constructed and used to complete ananimation depicting the evolution of the regional sedimentary and magmatic systems.This animation was generated using computer facilities at the University of California,Berkeley, running Canvas 6.0 and Ulead GIF Animator software.

During the evolution of the region, numerous transgressive and regressive eventsoccurred causing variations in the lithologies and sediment thickness deposited is variousbasins. Also apparent from the magmatic emplacement ages is the eastward migration ofthe magmatic arc from the west coast in the early Jurassic to the present location on theborder of Chile and Argentina. Lithologic variations demonstrate the onset of Andeanuplift by depicting the rapid accumulation of conglomerates in the basins near the activeuplift areas of northeastern Chile in the late Tertiary.

Jurassic transgressions entering from the west caused the magmatic arc to becomeseparated from the coast of South America by a narrow, marine, back-arc basin.Regressions to the west tended to isolate sub-basins, which display distinct sedimentaryhistories recorded during the regressive intervals. The most extreme example of thisoccurred in the late-Jurassic (~154 Ma), when large sub-basins were isolated by a marineregression and underwent complete evaporation leading to the basin-wide precipitation ofthick deposits of evaporite minerals such as gypsum and halite.

The construction of sedimentary and magmatic depositional environment maps hasled to better understanding of the evolution of northern Chile and Argentina, and inparticular they allow for the examination of depositional environments present in specificlocations for particular time intervals. These observations can assist in the geologicalinterpretation of a region by showing such things as the sediments present in an areaduring magmatic emplacement.

Current applications of this visualization model include regional provenance studiesof chlorine and copper, both of which play essential roles in the varied ore deposits ofChile, Peru, Bolivia, and of Argentina. The model provides critical insight into thepositions of key sampling locations necessary to evaluate hypotheses relating themigration of elements via mobile magmatic and aqueous fluids. Modeling of this typeaffords unique opportunities to investigate the large crustal scale on which transportprocesses operate during certain periods of geological time.

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7071 69 68 67 66 65

Tupiza

Salta

TucumanCopiapo

Antofagosta

Olague

Calama

Iquique

Chile

Bolivia

Argentina

Location MapLongitude

Sou

thLa

titu

de

Figure 1. Location map of the region depicted in the evolution animation.

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Introduction

The evolution of the Andean magmatic arc has been the subject of numerousinvestigations regarding it’s emplacement and migration, (Ishihara et al, 1984; Petersen,1999; Sillitoe and KcKee, 1996) and the relationship between subduction and magmatism(Kay et al, 1999; Kay and Mpodozis, 2001) through geologic time. Similarly, studieswere conducted investigating the structural development of the Andes, (Dallmeyer et al,1996; Hartley et al, 2000, 1992; Marquillas and Salfity, 1988; Mon and Salfity, 1995;Reutter et al, 1988) and the sedimentology and stratigraphy of individual areas (Ardill etal., 1994, 1998; Gröschke et al., 1986, 1998; Harrington, 1961; Printz et al., 1994; VonHillebrandt et al, 1986). This study incorporates all of these styles of data to investigatethe geological evolution of the northern Chile, Argentina, and southern Bolivia as awhole from the early Jurassic through the mid-Miocene. The region investigated in thisstudy ranges from 20° to 28° South Latitude and from 65° to 71° West Longitude,(figure 1).

A model has been constructed from stratagraphic, sedimentologic, and magmaticemplacement data in which variations of the sedimentary environments of northern Chileand Argentina from the early Jurassic to the mid-Miocene were compiled. A series ofmaps have been constructed at 2 million year intervals from this data and used tocomplete an animation depicting the evolution of the regional sedimentary and magmaticsystems. Palinspastic paleogeographic regional maps by Pindell and Tabbutt, 1995 wereused to delineate the region to be investigated. These maps depict marine, continental,and magmatic environments from 245 Ma through 1.7 Ma at ~20 million year timeintervals. Figure 2 is a representative maps from Pindell and Tabbutt showing thesedelineation’s. These maps do not distinguish between the various sedimentary lithologiesdeposited nor indicate the location and timing of magmatic emplacements which areimportant for this study. The lack of temporal resolution to intervals smaller than 20million years was also an important factor in undertaking this project.

Sediment isopach maps like figure 3, (Printz et al, 1994) were used to constrain theboundaries between sedimentary environments and areas undergoing active erosion.Future sites of several porphyry copper deposits were plotted on this map to examine thesediment thickness through which the magmatic systems passed and potentiallyinteracted. From figure 3, it can be seen that during the Oxfordian age in the Jurassic,(159 to 154 Ma) a sedimentary basin was actively accumulating sediments. Individualdepocenters within the basin show variations in the sediment thickness during this 5million year interval. These variations are caused by structures which were active duringthe sediment accumulation. These structures continued to be active throughout theJurassic and helped to constrain sediment lithologies and their distributions in this area.

Maps depicting large scale events such as figure 4, taken from Marquillas and Salfity,1988, allowed for sedimentary systems to be correlated to regional and continental scales.In figure 4, Campanian age (~78 Ma) northern Chile and Argentina are shown at the endof a continental rifting stage. The sedimentary lithologies and their distributions were adirect response to large scale events such as the transgression depicted in figure 4.

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Futureemplacement

site of:Chuquicamata

Escondida

El Salvador

Figure 2. Middle to late Jurassic, (188 to 145 Ma) palinspastic paleogeographic map from Pindell andTabbutt, 1994. Dark blue region represents shallow marine environment separated from the pacificocean by a continuous volcanic arc (green). Marginal basin has good connection with ocean to theNorth. Colors and future emplacement sites of porphyry copper deposits added by the authors for thisstudy.

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Salar deA tacama

Chuquicamata

Es condida

El S alvador

Figure 12. Isopach map of northern Chile representing sediments of Oxfordian age, (159 to 154 Ma)deposited in a sallow marine, marginal basin (Prinz et al, 1994). The isopach contours indicate thebasin in connected with the ocean in the north and separated in the west by a volcanic arc. Futureemplacement sites of the El Salvador, La Escondida, and Chuquicamata porphyry copper depositswere added by the authors.

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Calam a

Chuqu icamata

Escond ida

E l Salvador

Exposed Areas

Sedim entary Basin s

Cretaceous-EoceneMagm atic Arc

Legend

Figure 4. Map of northern Chile and Argentina at the close of the Campanian age, (~78 Ma) depicting theend of a rifting stage from Marquillas and Salfity, 1988. Sub-basins are labeled: TC: Tres Cruzes, LO:Lomos de Olemedo, ER: El Rey, M: Metán, Al: Alemanía, S: Sey, A: Antofagasta. Colors and futureporphyry copper deposit locations were added by the authors.

Many world-class copper and gold deposits are located in the Andes of northernChile, Argentina, and southern Bolivia (Petersen, 1999; Sillitoe and McKee, 1996; Vila,1982). Magmatic activity since the early Jurassic is responsible for most of these mineraldeposits as well as for the grandeur of the Andean mountains. Some of the major Chileancopper deposits are plotted on figures 1, 2, and 3 to show the proximity of sedimentaryunits and depositional basins to these later magmatic emplacements. Recently, workershave indicated a potential link between specific sedimentary intervals containingevaporite minerals and economically important copper mineralization in several Chileanporphyry copper deposit (Arcuri and Brimhall, 2000). Occurrences of evaporitelithologies in late-Jurassic, early-Cretaceous, and mid-Tertiary sediments have beendocumented at various locations throughout the study region (Ardill et al, 1998; Bell,1989; Flint et al, 1986; Printz et al, 1994; Suarez and Bell, 1987; Suarez et al, 1985). It isthese sequences which were investigated for their depositional location, lateral extent,and potential interaction with later magmatic systems.

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Data Collection

Data was collected from a variety of sources including fieldwork, published articles,and corporate archives of Codelco, Chile. Vast quantities of data were collected in formssuch as stratagraphic columns, geological maps, lithologic facies maps, magmaticemplacement ages, volcanic ash ages, and mineralization ages. Field data was collectedover a two-month period in late 1999 while library and corporate archive investigationstook place throughout 1999, 2000, and early 2001. A complete bibliography of allcompiled data is presented at the end of the animation and is also available for downloadat the author’s web site at: http://www.ocf.berkeley.edu:80/~tarcuri/. Extrapolationbetween data points and interpolations through time were done using a facies transitionmodel developed for this project.

The facies transition model was based on several simple assumptions: 1) sediment istransported down-gradient, 2) grain size decreases with increasing distance from thesource, 3) erosion is a continuous process on exposed bedrock, and 4) carbonatedeposition is suppressed in environments with high clastic input. Using these assumptionsa continental lithologic sequence of conglomerate-sandstone-siltstone-shale and a marinesequence of carbonate-shale was developed. It was assumed that all lithologies with afiner grain size than the lithology described for a specific data point would beencountered with increasing distance from that point, down-gradient. For example, a datapoint of conglomerate transitions into sandstone, siltstone, and finally shale withincreasing distance. Multiple points of similar lithology were grouped into fields andlithologic homogeneity was assumed. This model allowed the authors to extrapolatebetween data points and through time to generate sedimentary environment maps forentire regions.

Geological Evolution

The geological evolution of the study area shows regional and continental scaleprocesses which influenced the type and distribution of the various deposited lithologies.The evolution shows the occurrence of numerous marine transgressions and regressionsaffecting the continent, with Jurassic events entering from the west and Cretaceous eventsentering from the east. These processes caused variations in the lithologies and sedimentthickness deposited in the various depositional environments. Also apparent from themagmatic emplacement ages is the eastward migration of the magmatic arc from the westcoast in the early Jurassic to it’s present location on the border between Chile andArgentina (Sillitoe and KcKee, 1996; Petersen, 1999). Lithologic variations demonstrateactive tectonic events, such as continental rifting and the onset of Andean uplift in theTertiary by depicting the rapid accumulation of conglomerates in basins near tectonicallyactive zones.

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The Jurassic Period

The Jurassic sediments of northern Chile have been the subject of numerous studiesexamining the environment in which they were deposited (Ardill et al, 1994, 1998;Chong and Pardo, 1976; Gröschke et al, 1988; Harrington, 1961; Printz, 1986; and VonHillebrandt et al. 1986). The tectonic history of the region has also been the focus ofworks investigating the connection between sediment thickness and forarc development,(Hartley et al, 2000) and sub-basin evolution (Printz et al, 1994). The conclusions of

Figure 5. Evolution maps from the Jurassic period representing 2 million year increments; A) 194-192 Ma,

B) 154-154 Ma, C) 146-144 Ma, D) Legend for all maps generated. Variations in lithologies and theirdistributions can be examined as well as magmatic emplacement ages. No distinction was madebetween volcanic and plutonic emplacements.

A B

C D

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these works are that the Jurassic system consisted of a marine back-arc basin whichexperienced numerous transgressive-regressive events prior to it’s disappearance in thelate Jurassic-early Cretaceous (Printz et al, 1994; Ardill et al, 1998). Jurassictransgressions entered from the west and caused the magmatic arc to become separatedfrom the coast of South America by a narrow, marine, back-arc basin. Subsequent marineregressions isolated sub-basins, which display distinct sedimentary histories recordedduring the specific regressive intervals.

Tracking the specific lithologic variations through these basins and over the continentas a whole shows how magmatic and tectonic processes act in concert to influencesedimentary environments. Figure 5 shows several time intervals from the Jurassic inwhich the focus is the marine back-arc basin. Figure 5A depicts the basin with strongconnections to the ocean and a large spatial distribution. As the basin is isolated from theocean by a marine regression and the development of the coastal magmatic arc, (figure5B) evaporite minerals such as gypsum and halite precipitate in various sub-basins.Evaporite minerals were deposited at various times throughout Jurassic in several typesof depositional environment, (i.e. deep water or sabkha). This process culminated inmassive, basin-wide deposition of gypsum in the late Oxfordian, approximately 154 Mawhen the basins underwent extreme evaporative concentration (Ardill et al, 1998; Bell,1989; Gröschke et al, 1986; Printz et al, 1994). In figure 5C the back-arc basin has nearlyvanished, with only small sub-basin remaining. These sub-basins continue to depositsediments in what was originally the deepest part of the original basin.

The Cretaceous Period

Continental scale processes continued to make major changes throughout the studyregions during the Cretaceous period. Of these processes, rifting coupled with atransgressive event from the east were the most notable. Marine basins developed innorthern Argentina and Bolivia which would later become the focus of major oil and gasexploration. It was this hydrocarbon exploration that motivated the sedimentologicalstudies forming one basis of this work. Compressional events associated with the activesubduction zone to the west and extensional tectonics throughout Argentina and Boliviareactivated Paleozoic faults and allowed for the thick accumulation of sediments in basinswhich would later be uplifted to for the Bolivian Altiplano (Welsink et al, 1995). Themigration of the magmatic arc eastward began in the early Cretaceous and by 90 Mareached the point where it would be located until the mid-Tertiary (Petersen, 1999).

Continental rifting began in the early Cretaceous which allowed a marinetransgression to penetrate far into the continental interior (Marquillas and Salfity, 1988;Pindell and Tabbutt, 1994; Reutter et al, 1988). Figure 6A shows the well developed riftsystem through Argentina and Bolivia at 90 Ma. Distinct, fault-bounded basins wereestablished at this time, and continued to be active into the Tertiary. This rift continued toexpand, allowing a marine transgression at 68 Ma to flood much of the continent (figure6B). The inland flood of marine waters was bounded on the west by the active volcanicfront of the Andes, which constituted a long, linear volcanic highland. West of the Andesvolcanic arc, sediment deposits indicate a gradual decrease of elevation down to sea

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level, with no evidence of transgression related flooding. A marine regression began soonafter 68 Ma, during which the marine waters drained from the continental interiors orwere stranded in isolated inland basins. Once again, these inland basins were subject toevaporation leading to the deposition of evaporite sequences as seen in figure 6C.

Figure 6. Evolution maps from the Cretaceous period representing 2 million year increments; A) 92-90 Ma,B) 70-68 Ma, C) 58-56 Ma. The legend displaying the lithologic identifications for these images ispresented in figure 5D.

A B

C

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The Tertiary Period

Tertiary regional evolution was dominated by compressional tectonics and continuedeastward migration of the magmatic arc. One of the major tectonic events which occurredduring the late Tertiary was the uplift of the Andes. The orogeney commenced in the lateEocene and continued well into the Miocene in a series of uplift phases, the Incaica (~38Ma), the Pehuenche (23-25 Ma), the Quechua (~10 Ma), and most recently the Diaguita(3-5 Ma), (Hartley et al, 2000). Figure 7A shows large fields of conglomerates being shedfrom the volcanic regions associated with the onset of uplift during the Pehuenche phase,at 24 Ma. The large non-depositing region in western Bolivia is the emerging Bolivianaltiplano, which began to form in the early Tertiary (Welsink et al, 1995).

Figure 7. Evolution maps from the Tertiary representing 2 million year increments; A) 26-24 Ma, B) 12-10Ma. The legend displaying the lithologic identifications for these images is presented in figure 5D.

The magmatic evolution is dominated by the rapid migration of the magmatic arcfrom it’s Cretaceous location through central Chile to the modern location at the borderwith Argentina. This migration can be divided into a series of four temporally distinct,magmatic belts which run in continuous parallel positions through most of northern Chile(Sillitoe and McKee, 1996). The magmatic belts have the following progression fromwest to east: an early Cretaceous belt located mainly in north-central Chile, a Paleocenebelt, a late Eocene to early Oligocene belt, and an early to middle Miocene belt onlypresent south of 26° S Latitude. The late Eocene to early Oligocene belt contains many ofthe world’s largest porphyry copper deposits and has generated much debate as to whythis one time interval saw the deposition of such a large quantity of base metal deposits.The active volcanic arc is currently the focus of much exploration related to preciousmetals such as gold and silver and base metals like copper.

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Conclusions

The construction of sedimentary and magmatic depositional environment maps andtheir compilation into an animation has led to better understanding of the geologicalevolution of northern Chile and Argentina. In particular, they allow for the examinationof depositional and magmatic environments present in specific locations for particulartime intervals. These type of maps allow observations to be made which can assist in thegeological interpretation of a region. For example, it is possible to investigate the type ofsediments present in an area prior to the emplacement of a magmatic system. The couldhave a profound affect on the way that data collected from the magmatic system was tobe interpreted.

Current applications of this visualization model include regional provenance studiesof chlorine and copper. These elements are among many which play essential roles in thevaried ore deposits of Chile, Peru, Bolivia, and of Argentina. The model provides criticalinsight into the positions of key sampling locations necessary to evaluate hypothesesrelating the migration of elements via mobile magmatic and aqueous fluids ascomplexing agents of metals. These fluids tend to be either completely or partiallyresponsible for the deposition of various metals forming the deposits of the region.Modeling of this type affords unique opportunities to investigate the large crustal scale onwhich certain transport processes operate during periods of geological time.

Animation

To view the complete animation, double click on the attached executable file labeledEvolution.exe. The program will take approximately 30 seconds to begin, depending onthe computer on which it is being viewed. The animation is presented as a continuousloop, allowing for repeated viewing. All references are presented at the end of theanimation. Press the escape key at any time to end the program. Approximate runningtime is 3 minutes. A complete version of the animation is presented here and is alsoavailable for download at the author’s web site: http://www.ocf.berkeley.edu:80/~tarcuri/.

Acknowledgments

The authors would like to express appreciation to the following people; IreneMontero S., Tina Takagi for discussions leading to improvements in the presentation ofthis material. This project was completed during doctoral research of Terry Arcuri,performed at the Earth Resources Center, University of California, Berkeley and madepossible by a grant from Codelco, Chile.

The animation was generated using computer facilities in the Earth Resources Center.Individual map images were generated in Canvas 6.0 by Deneba Systems. GIF Animator4.0 software from Ulead Systems was used to compile the images and generate the finalanimation.

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