Teresa E. Jordan, Naomi E. Kirk-Lawlor, Nicolás Blanco P., Jason A. Rech, ..... Miocene consolidated sandstone and conglomerate of the Altos de Pica.
Teresa&E.&Jordan,&Naomi&E.&Kirk3Lawlor,&Nicolás&Blanco&P.&,&Jason&A.&Rech,& Nicolás&Cosentino& in&press,&March&2014& Landscape&modification&in&response&to&repeated&onset&of&hyperarid& paleoclimate&states&since&ca.&14&Ma,&Atacama&Desert,&Chile& Geological&Society&of&America&Bulletin& &
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& Pre3print&from&the&authors&
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Published as: Jordan, T. E., Kirk-Lawlor, N. E., Blanco, N., Rech, J. A., and Cosentino, N. J., 2014, Landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 Ma, Atacama Desert, Chile: Geological Society of America bulletin, v. 126, no. 7/8, p. 1016-1046, doi: 10.1130/B30978.1
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Landscape&modification&in&response&to&repeated&onset&of&hyperarid& paleoclimate&states&since&ca.&14&Ma,&Atacama&Desert,&Chile& & Teresa&E.&Jordan1,&Naomi&E.&Kirk3Lawlor1,&Nicolás&Blanco&P.&2,&Jason&A.&Rech3,&Nicolás& Cosentino1& 1Department&of&Earth&&&Atmospheric&Sciences,&Cornell&University,&Ithaca,&NY&1485331504& 2Servicio&Nacional&de&Geología&y&Minería,&Avenida&Santa&María&0104,&Santiago,&Chile& 3Department&of&Geology,&Miami&University,&Oxford,&Ohio&4505631846&
& ABSTRACT' The&landscape&of&the&hyperarid&Atacama&Desert&in&northern&Chile&records&extremely& slow&change&on&Earth’s&surface.&Disputed&ages&for&the&onset&of&hyperaridity&range&from&the& late&Paleogene&through&Pleistocene.&A&long3term&paleoclimate&record&is&recorded&in&a& nonmarine&basin&whose&fill&is&primarily&alluvial&strata.&For&this&setting,&the&primary&proxies& for&climate&state&are&the&mineralogical&and&chemical&composition&of&soil,&which&varies& across&a&precipitation&gradient,&and&the&landforms&and&deposits&of&alluvial&fans.&During&the& most&recent&~15&million&years,&five&climate3related&landscape&stages&are&resolved&for&the& Pampa&del&Tamarugal&sedimentary&basin,&with&each&successively&younger&stage&inset&lower& in&the&local&topography&than&its&predecessor.&The&earliest&landscape&stage&is&expressed&as&a& set&of&alluvial&strata&inherited&from&a&time&of&arid&or&semi3arid&climate,&~14–12&Ma.&The& younger&four&landscape&stages&generated&a&composite&long3lasting&exposure&surface.& Predominantly&hyperarid&conditions&have&persisted&since&~12&±&1&Ma,&during&which&four& intervals&of&arid&to&semi3arid&climate&occurred.&Each&wet&interval&was&short&lived,&a&million& 2
years&or&less,&whereas&some&of&the&hyperarid&periods&were&lengthy,&1–5&million&years.&The& hyperarid&intervals&were&roughly&11–5.5&Ma,&4.5–4&Ma,&3.6–2.6&Ma,&2.2–1&Ma,&and&repeated& intervals&during&the&last&1&Ma.&The&onset&of&hyperaridity&~12&Ma&likely&reflects&the&growth& of&the&Andes&Mountains&above&a&climate&threshold.&In&contrast,&sea&surface&temperature& variability&likely&controlled&Atacama&paleoclimate&changes&since&the&Late&Miocene.& INTRODUCTION' The&rates&of&weathering&and&erosion&in&hyperarid&environments&are&orders&of& magnitude&lower&than&in&more&humid®ions&(Dunai&et&al.,&2005;&Hoke,&2006;&Kober&et&al.,& 2007;&Matmon&et&al.,&2009)&and&the&processes&of&soil&development&differ&markedly& (Ericksen,&1981;&Ewing&et&al.,&2006)&from&those&of&the&vast&majority&of&Earth’s&surface.&Here& we&document&the&major&consequences&to&the&landscape&of&a&series&of&transitions&between& hyperarid&and&moderately&wetter&climate&states.&The&study®ion&is&the&Atacama&Desert&of& northern&Chile&(Figures&1,&2),&the&landscape&record&spans&approximately&15&million&years,& and&the&focus&is&on&the&landforms&and&associated&deposits&within&a&sedimentary&basin.& The&landscape&of&the&Atacama&Desert&is&extraordinarily&uncommon,&last&subject&to& significant&erosion&millions&of&years&ago&even&though&certain&parts&of&the&Atacama&Desert& exist&across&tectonically&tilted&and&uplifted&terrain&(Mortimer,&1980;&Paskoff,&1980;&Dunai&et& al.,&2005;&Kober&et&al.,&2006;&Hoke&et&al.,&2007;&Evenstar&et&al.,&2009;&Jordan&et&al.,&2010;& Schlunegger&et&al.,&2010).&Studies&using&independent&methods&demonstrate&that&hyperarid& conditions&persisted&over&long&time&intervals&(≥2&million&years)&throughout&the&Atacama& Desert&(Alpers&and&Brimhall,&1988;&Sillitoe&and&McKee,&1996;&Hartley&and&Rice,&2005;& Arancibia&et&al.,&2006;&Rech&et&al.,&2006;&Reich&et&al.,&2009;&Bissig&and&Riquelme,&2010;& Amundson&et&al.,&2012).&Extending&geomorphological&reasoning&to&sedimentary&detritus,&
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Nester&and&Jordan&(2012)&proposed&that&the&nonmarine&sedimentary&record&of&hyperarid& climate&must&be&facies,&stratal&architecture,&and/or&stratigraphic&sequences&that&are& markedly&different&than&those&formed&under&more&humid&climate&states.&They&suggested& that&extremely&slow&erosion&would&lead&to&the&lack&of&transport&of&siliciclastic&detritus&on& hillslopes&and&in&stream&systems,&with&the&result&that&there&would&be&little&detrital& sedimentary&accumulation&in&sedimentary&basins.&Instead,&hyperarid&states&would&tend&to& produce®ionally&extensive&unconformities.&Nevertheless,&because&climate&varies&through& time&at&both&global&and®ional&scales&(Zachos&et&al.,&2001;&Placzek&et&al.,&2006),& hyperaridity&in&the&Atacama&Desert&was¬&likely&a&constant&condition&over&millions&of& years.&Indeed&specific&proxy&studies&for&the&late&Quaternary&demonstrate&that&millennial3 duration&intervals&of&greater&precipitation&and&hydrologic&activity&interrupted&the&mean& hyperarid&state&(Rech&et&al.,&2002;&Lowenstein&et&al.,&2003;&Latorre&et&al.,&2006;&Placzek&et& al.,&2006;&Nester&et&al.,&2007;&Quade&et&al.,&2008;&Reich&et&al.,&2009;&Placzek&et&al.,&2010;&Gayó& et&al.,&2012).&& Unlike&the&late&Quaternary,&there&are&sharply&differing&interpretations&of&the°ree& and&tempo&of&climate&variability&in&the&Atacama®ion&during&the&Miocene&and&Pliocene.& The&controversy&results&in&part&from&two&inherent&limitations&of&study&of&ancient& continental&surfaces:&i)&the&lower&precision&of&dating&methods&for&the&Neogene&as&compared& to&the&Quaternary;&ii)&the&less&complete&preservation&of&landforms&in&the&older& geomorphological&and&stratigraphic&records.&In&addition,&a&third&cause&for&confusion& regarding&paleoclimate&history&arises&from&mixing&a&variety&of&proxies&irrespective&of&which& particular&part&of&the&climatic,&geographic&and&hydrologic&system&each&proxy&records.&In&
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combination,&the&result&has&been&the&publication&of&seemingly&conflicting&interpretations&of& the&long3term&climate&history&of&the&Atacama&Desert.&& For&example,&within&and&near&this&report’s&study&area,&the&time&when&hyperaridity& began&remains&contentious.&Some&studies,&using&cosmogenic&nuclide&dating&of&the&exposure& history&of&individual&clasts&found&on&geomorphological&surfaces,&reached&the&interpretation& that&hyperaridity&began&near&26&Ma&(Dunai&et&al.,&2005)&or&near&15&Ma&(Evenstar&et&al.,& 2009).&More&recent&studies&reveal&that&cosmogenic&nuclide&inheritance&of&clasts&on&alluvial& surfaces&in&the&Atacama&Desert&must&be&taken&into&consideration&(e.g.&Jungers&et&al.,&2013),& with&the&result&that&the&exposure&ages&reported&by&Dunai&et.&al.&(2005)&and&Evanstar&et&al.&& (2009)&warrant&revision.&Nevertheless,&Dunai&et.&al.&(2005)&and&Evanstar&et&al.&(2009)& demonstrated&extraordinary&longevity&of&materials¤tly&at&the&Earth’s&surface.&Other& studies,&based&on&the&chronology&of&supergene&enrichment&of&ores,&suggested&an&age&of& hyperarid&onset&~14&Ma&(Figures&1)&(Sillitoe&and&McKee,&1996;&Arancibia&et&al.,&2006),&yet& Hartley&and&Rice&(2005)&point&out&that&a®ional&paleoclimate&interpretation&is¬& justified&until&the&supergene&data&have&been&parsed&by&latitude,&local&geography,&and&state& of&exploration&of&the&deposit.&Some&studies,&based&on&the&facies&history&of&the&basin&fill,&have& suggested&the&onset&of&hyperaridity&~8–7&Ma&(Schlunegger&et&al.,&2010;&Sáez&et&al.,&2012)&or& ~3&Ma&(Hartley&and&Chong,&2002).&A&final&set&of&studies,&using&the&nature&of&ancient&soils,& interpreted&the&onset&of&hyperaridity&prior&to&10&Ma&(Rech&et&al.,&2006)&or&more&recently& than&2&Ma&(Amundson&et&al.,&2012).&It&seems&plausible&that&part&of&the&divergence&of& opinions&about&the&time&of&onset&for&hyperaridity&results&from&a&multiplicity&of×&of& onset&as&a&consequence&of&the&variability&of&climate&over&millions&of&years&(Le&Roux,&2012).&
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Yet&little&attention&has&been&given&to&documenting&a&series&of&Neogene&climate&states,&which& is&the&theme&of&this&paper.& Climate&scientists&and&geologists&interested&in&the&paleoclimate&of¢ral&western& South&America&have&focused&on&the&relative&roles&played&by&ocean¤ts&and&sea&surface& temperature,&global&climate&change,&topographic&uplift&of&the&Andes,&and&southern& hemisphere&glaciation&(e.g.,&Takahashi&and&Battisti,&2007;&Insel&et&al.,&2009;&Garreaud&et&al.,& 2010;&Le&Roux,&2012;&Le&Roux,&2012).&The&analysis&of&causal&relations&is&greatly&hampered& by&uncertainty&whether&hyperaridity&began&during&the&Oligocene,&the&Middle&Miocene,&the& Late&Miocene,&the&Early&Pliocene,&or&the&Late&Pliocene.&Likewise,&the&multiplicity&of& paleoclimate&interpretations&exacerbates&the&challenge&of&analyzing&the&causes&of&uplift&of& the&Andes&Mountains,&because&some&of&the&plausible&causations,&such&as&lubrication&of&the& subducting&plate&boundary&by&water3rich&sediments,&are&tied&to&climate&(Lamb&and&Davis,& 2003;&Garreaud&et&al.,&2010).& A&primary&objective&of&this&paper&is&to&document&changes&in&landscape&form&and& process&driven&by&climate&variability&for&a&sedimentary&basin&within&the&hyperarid&core&of& the¢ral&Atacama&Desert&of&northern&Chile.&A&second&major&objective&is&to&reconstruct&the& paleoclimate&history&over&the&last&15&Ma.&New&results&presented&here&include&maps&of&the& spatial&distribution&of&geomorphic&surfaces,&descriptions&of&properties&of&soils&found&on& those&surfaces&and&interbedded&within&strata,&and&descriptions&of&cross3cutting& topographical&relationships&between&geomorphic&surfaces.&To&achieve&these&primary& objectives,&our&results&are&combined&with&published&information&about&the&ages&and&facies& of&strata&that&fill&the&sedimentary&basin.&Although&features&of&the&landscape&will&seem& analogous&to&the&Quaternary&geology&of&many&arid®ions&and&our&approach&is&similar&to&
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that&used&by&some&Quaternary&geologists&\e.g.,&\Monger&2009,&the&results&show&that&~30%& of&otherwise&familiar&forms&are&relicts&from&the&Neogene,&created&and&fossilized&~12–2.5& Ma.&This&time&scale&and&the&unique&processes&of&the&hyperarid&environment&stretch& terminology&and&concepts&widely&used&in&Quaternary&geology&far&beyond&familiar&bounds.& In&addition,¬ions&of&the&genetic&relationships&between&climate&states&and&soil& development&suited&to&other&arid&lands&\i.e.,&soils&mature&fastest&during&the&wetter&phases& of&climate&cycles;&\Monger&2009&do¬&fit&the&saline&soils&found&in&the&core&of&the&Atacama& Desert.& Our&method&is&a&source3to3sink&landscape&analysis&in&a&stratigraphic&context&(Figure& 3).&Amundson&et&al.&(2012)&recently&illustrated&the&strengths&of&a&holistic&geomorphic& approach&in&the&reconstruction&of&the&paleoclimate&history&of&the&Atacama&Desert&between& ~23–25°S,&south&of&our&area&of&interest.&They&considered&the&geomorphological&record&of& fluvial&activity&in&a®ion&that&lacks&sedimentary&basins.&Nevertheless,&because& sedimentary&basins&retain&products&of&Earth&surface&conditions&over&much&longer&periods& of&time&than&do&erosional&landscapes,&we&include&strata&in&conjunction&with&landforms&to& develop&a&history&encompassing&many&million&years.& The&study&area&is&the&elongate&lowland&(~900–1100&m&altitude)&Central&Depression& and&the&adjacent&Precordillera&foothills&of&the&Andes&(~20°20’&–&21°50’S;&~69°&–&69°30’W)& (Figure&1,&2).&Over&the&time&range&of&the&Neogene,&the&Central&Depression&and&lower&slopes& of&the&Precordillera&acted&as&a&forearc&sedimentary&basin&(Figure&2B,C).&The&sedimentary& basin&is&referred&to&broadly&as&the&Pampa&del&Tamarugal&basin,&and&in&its&southwest§or& as&the&Quillagua&basin&(Sáez&et&al.,&1999;&Nester&and&Jordan,&2012).&The&climate&setting&of& this&sedimentary&basin&and&its&hydrologic&catchments&in&the&foothills&is&hyperarid.&For&a&
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>9000&km2&area&at&elevations&between&1000–2500&m&we&analyzed&landforms,&soils& including&modern,&relict,&and&buried&examples,&and&nonmarine&strata.&Within&the&southern& half&of&the&study&area,&where&alluvial&processes&have&dominated,&we&illustrate&the&end3 product&spatial&distribution&of&a&sequence&of&low3relief&geomorphic&surfaces.&To&determine& the&paleoclimate&history&of&the®ion&(Figure&1)&we&rely&on&paleoclimate&proxies&that& reveal&information&about&the&precipitation&history&at&the&specific&location&where&the&proxy& is&now&found,&such&as&soil,&and&within&the&immediately&adjacent&Andean&foothills& catchments,&such&as&alluvial&fan&sediments.&We&purposefully&avoid&proxies&related&to&major& rivers,&whose&records&integrate&the&climate&conditions&of&distant&highlands&in&the&Andes.& CLIMATE Today’s&climate&of&the&western&margin&of&South&America&between&18°S–27°S&is& hyperarid. There is a strong&increase&in&precipitation&with&increasing&altitude.&At&the& latitudes&of&the&study&area,&the&annual&precipitation&at&500&m&altitude&measures&100&mm/yr&(Houston&and&Hartley,&2003).& The&aridity&is&controlled&by&three&main&factors&(Houston&and&Hartley,&2003;&Garreaud&et&al.,& 2010).&The&first&two&factors&work&together:&a&cold&water&coastal¤t&that&chills&the& Pacific&Ocean&surface&water&and&the&SE&Pacific&anticyclone&that&causes&upper&levels&of&the& atmosphere&to&descend&and&create&a&temperature&inversion.&Together&these&factors& markedly&reduce&the&advection&of&Pacific&water&vapor&inland.&The&third&factor&is&the&Andes& Mountains,&which&serve&as&an&orographic&barrier&that&prevents&advection&of&water&vapor& from&sources&in&the&continental&interior.&Despite&the&proximity&of&the&western&part&of&the& study&area&to&the&Pacific&Ocean,&rainfall&is&greatest&in&the&eastern&mountains&and&least&in&the& lowland&sedimentary&basin&(Figures&1,&2).&Because&the&controls&on&aridity&are&large3scale&
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features&of&atmospheric&circulation,&the&altitudinal&increase&in&precipitation&is&likely&to&be&a& persistent&feature&of&the&climate,&even&if&the&magnitude&of&precipitation&varies&over&long& time&periods.& During&the&Late&Pleistocene&there&were&multiple&periods&of&wetter&conditions.&Two& major&pluvial&stages&affected&the&Central&Andes&and&Atacama&Desert&widely&between&18–14& ka&and&13–10&ka.&Those&wet&periods&generated&a&major&lake&in&the&southern&Altiplano& plateau&(Figure&1)&(Placzek&et&al.,&2006;&Placzek&et&al.,&2009),&high&lake&levels&in&small&basins& of&the&Chilean&highlands&(Geyh&et&al.,&1999;&Grosjean&et&al.,&2001),&and&expanded&wetlands& and&riparian&habitats&in&canyons&and&closed&basins&(Rech&et&al.,&2002;&Quade&et&al.,&2008)& under&conditions&corresponding&to&twice&the&precipitation&that&occurs&today&(Latorre&et&al.,& 2006).&During&an&intervening&dry&interval,&~14.2–~13&ka&Placzek&2009,&the®ional& climate&returned&to&a&hyperarid&state.&& The&late&Quaternary&record&reveals&several&environmental&proxies&that&reveal&the& relationships&between&precipitation,&vegetation&cover,&erosion&rates,&and&the&nature&and& rate&of&sediment&deposition&at&both&low&and&high&elevations.&Within&the&Pampa&del& Tamarugal&lowlands&(Figure&1),&the&latest&Pleistocene&pluvial&interval&is&recorded&in&fossil& leaf&litter&and&wood&of&riparian&and&ground3water&supported&basin3floor&ecosystems&Nester& et&al.&2007;&Gayó&2012.&A&lack&of&evidence&of&hillslope&taxa&led&Gayó&et&al.&%Gayó&2012&to& conclude&that&the&Pampa&del&Tamarugal&lowland&remained&hyperarid&while&precipitation& increased&at&elevations&greater&than&their&study&sites&(i.e.,&&>1900&m).&The&wet&climate& excursions&also&are&represented&in&a&series&of&aggradational&fluvial&terraces&(Nester&et&al.,& 2007).&In&contrast,&none&of&the&canyons&or&fans&of&the&Pampa&del&Tamarugal&have&yielded&
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fossil&leaf3litter&deposits&dated&14.2–12.1&ka,&when&the&perennial&rivers&and&associated& riparian&ecosystems&retreated&to&higher&elevations&in&the&Andes&(Gayó&et&al.,&2012).&& STUDY'AREA'GEOMORPHOLOGIC'AND'STRATIGRAPHIC'FRAMEWORK' The&study&area&(Figure&2)&is&~150&km&long,¶llel&to&the&Andean&topography,&and& ~60&km&wide.&Nearly&all&the&study&area&lies&at&elevations&between&900&m&and&2500&m,&in&a& region&where&hillslopes&below&2300&m&lack&vegetation.&To&the&east&of&the&Central& Depression,&the&general&crestline&of&catchments&in&the&Precordillera&extends&above&4000&m& and&local&peaks&exceed&5000&m&(Figure&1).&To&the&west,&the&Coastal&Cordillera&separates&the& lowland&Central&Depression&from&the&Pacific&coast.&North&of&~21°S&the&study&area&is& endorheic.&Farther&south&the&surface&drainage&is&technically&exorheic,&although&under& modern&climate&conditions&all&parts&of&the&surface&drainage&system,&except&for&the&Loa&River& (Figure&2),&are&ephemeral,&and&drainage&into&the&Loa&is&almost&entirely&fed&by&groundwater& discharge.& The&structural&relief&between&the&basement&of&the&Central&Depression&and&the& basement&of&the&Andes&has&increased&by&monoclinal&folding&throughout&the&Neogene&(e.g.,& Victor&et&al.,&2004;&Farías&et&al.,&2005;&Jordan&et&al.,&2010;&Nester&and&Jordan,&2012).&During& the&time&interval&of&hyperarid&climate&there&has&been&little&erosion,&allowing&the&structural& relief&to&be&largely&expressed&in&the&landscape&relief&(Jordan&et&al.,&2010).&The&topographic& separation&between&the&headwater&areas&in&the&Precordillera&and&the&Central&Depression& floor&increased&by&more&than&1200&m&during&the&15&million&year&time&interval&considered&in& this&paper&(Jordan&et&al.,&2010).& Modern&and&relict&alluvial&fans&are&the&primary&landforms&that&express&sedimentary& deposition&and&accumulation.&The&sediment&and&surface&water&are&derived&from&mountain&
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catchments&in&the&Precordillera&(Figure&2).&The&catchments&north&of&21°10’S&that&drain&into& the&study&area&contain&extensive&tracts&above&3400&m&altitude,&whereas&catchments&farther& south&contain&little&area&above&3400&m.&& Wind&is&an&important&secondary&agent&that&modifies&the&small3scale&landscape&and& contributes&to&pedogenesis.&Throughout&the®ion&wind3borne&transport&of&dust&and,& locally,&wind&transport&of&fog&aerosols&distribute&salts&from&which&gypsic&soils&form.&North& of&21°S,&wind3driven&transport&and&deposition&are&expressed&in&eolian&dune&fields&located&in& both&the&lowland&valley&and&on&the&lower&slopes&of&the&Precordillera.&South&of&21°S,&the& wind&climate&is&influenced&by&strong&diurnal&winds&funneled&through&the&canyon&of&the&Loa& River,¢ered&at&21°42’S.&For&tens&of&kilometers&north&and&south&of&the&intersection&of&the& canyon&with&the&Central&Depression,&the&distal&basin&(west&of&~69°40W)&displays&broad& wind3deflated&tracts,&with&some&zones&deflated&to&at&least&a&depth&of&2&m.&Sand&sheets&occur& farther&east&and&in&the&lee&of&topographic&barriers.&In&the&wind3deflated&tracts,&resistant& layers&such&as&indurated&anhydrite3nodule3rich&horizons&of&soils&and&gravel3rich&alluvial& deposits&preferentially&constitute&the&landscape&surface.&& The&landforms&transition,&from&west&to&east,&from&active&depositional&forms&(salt& pans,&alluvial&fans),&to&an&extensive&piedmont&slope&that&is&dissected&by&widely&spaced&east3 trending&canyons&(“quebradas”),&to&a&dissected&mountainous&terrain&at&the&eastern&limit&of& the&study&area&(Figure&2).&The&Quillagua&paleolake,&which&occupied&the&western&fringe&of& the&lowland&basin&during&part&the&latest&Miocene&and&Pliocene,&was&hydrologically& connected&to&the&Loa&River&and&Calama&basin&(Bao&et&al.,&1999;&Sáez&et&al.,&1999;&Sáez&et&al.,& 2012),&whose&water&was&at&least&partly&sourced&in&the&Western&Cordilllera.&The&lake&and& subsequent&salt&pans&are&expressed&in&remnant&landforms&and&in&playa&deposits.&This&paper&
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focuses&on&the&alluvial&strata&of&the&eastern&and¢ral&parts&of&the&Pampa&del&Tamarugal& basin.& Oligocene&through&Middle&Miocene&alluvial,&eolian,&and&lacustrine&strata&of&the& Pampa&del&Tamarugal&forearc&basin&are&widespread&in&the&Central&Depression&and&below& ~2500&m&in&the&Precordillera&(Figures&2B,C,&3)&(Galli&Olivier&et&al.,&1962;&Hartley&and& Evenstar,&2009;&Jordan&et&al.,&2010;&Nester&and&Jordan,&2012).&But&during&the&last&~12& million&years,&accumulation&has&been&short3lived&(Kiefer&et&al.,&1997;&Nester&et&al.,&2007;& Sáez&et&al.,&2012)&and&focused&in&the&western&(distal)§or&of&the&sedimentary&basin& (Figures&2B,C,&3)&(Sáez&et&al.,&2012).&& Upper&Cenozoic&alluvial&strata&north&of&21°15’S&are&well&characterized,&with&formal& stratigraphic&divisions&systematically&applied&during&mapping&at&1:100,000&scale&(Figure&3)& (Blanco&P.&and&Tomlinson,&2013).&The&lithologies&and&facies&of&the&southern&continuation&of& those&strata&have&been&documented&through&surveys&of&individual&canyons&(Nester,&2008;& Jordan&et&al.,&2010;&García&et&al.,&2012);&the&formal&stratigraphic&divisions&of&Blanco&and& Tomlinson&(2013)&are&tentatively&extended&into&the&southern&area&(Figure&3).&& Strata&south&of&20°30’S&consist®ionally&of&several&hundred&meters&of&Upper& Oligocene3Middle&Miocene&consolidated&sandstone&and&conglomerate&of&the&Altos&de&Pica& Formation,&unconformably&overlain&by&consolidated&conglomerate&and&sandstone&of&the&El& Diablo&Formation&(Figure&3).&The&ages&of&volcanic&interbeds&in&the&El&Diablo&Formation&are& typically&~13–14&Ma&(Table&1)&(Blanco&P.&and&Tomlinson,&2013).&Between&20°–21°S&in&the& mountainous&eastern®ion,&overlying&those&strata&across&an&erosional&unconformity&are& poorly&consolidated&gravels&and&sands,&locally&over&100&m&thick,&mapped&as&two&informal& units&(“upland&piedmont&deposits”&and&“old&eolian&deposits”)&whose&numerous&volcanic&
12
interbeds&range&in&age&between&10.7–5.6&Ma&(Blanco&P.&and&Tomlinson,&2013)&(Table&1).&All& of&these&units&are&rich&in&well3sorted&sandstones&(Figure&3)&whose&facies&reveal&important& eolian&sediment&delivery&(Dingman&and&Olivier&Galli,&1965;&Blanco&P.&and&Tomlinson,&2013).& In&the&Pampa&del&Tamarugal&lowlands,&an&extensive&unit&of&gravels&(“lowland&piedmont& deposits”),&now&poorly&consolidated,&began&somewhat&later&to&accumulate&above&the&El& Diablo&Formation,&with&volcanic&interbeds&ranging&between&6.5&±&0.06&Ma&and&5.&38&±&0.10& Ma&(Table&1).& Between&21°–22°S&(Figure&3),&consolidated&strata&of&the&Altos&de&Pica&and&El&Diablo& formations&are&dominated&by&conglomerates,&ranging&from&clast3supported&to&matrix3 supported,&from&tabular&to&lenticular,&and&from&pebble&to&cobble&maximum&grain&size.& Poorly&sorted&sandstones&are&interbedded&with&the&conglomerates.&The&debris&flow,& sheetflood&and&channelized&flow&facies&are&all&interpreted&to&have&been&deposited&on& alluvial&fans&(Jordan&et&al.,&2010;&Nester&and&Jordan,&2012).&These&units&are&dated&to& between&~20&Ma&and&12&±&1&Ma&(Table&1).&An&overlying&unconsolidated&gravel&unit,&tens&of& meters&thick,&is&laterally&extensive&along&the&western&margin&of&the&outcrop&belt&and& contains&a®ionally&extensive&ignimbrite,&whose&age&is&~5.3&Ma&(Hoke&et&al.,&2007;&Jordan& et&al.,&2010).&Based&on&similarity&in&landscape&position&and&composition&to&the&area&north&of& 21°S,&we&describe&this&unit&as&the&“lowland&piedmont&deposits”&(a&revision&from&Jordan&et& al.&(2010)&who&referred&to&the&same&unit&as&the&informal&Arcas&unit).&& In&the&western&third&of&the&study&area,&deposits&of&relatively&fresh3water&lakes&and& salt&pans&are&interspersed&with&thin&mudstone&and&sandstone&units&that&were&derived&from& the&east&(Figure&3).&The&age&and&the&subsurface&spatial&distribution&of&Upper&Miocene&salt3 pan&anhydrite&deposits,&the&Hilaricos&Formation,&are¬&well&documented&(Sáez&et&al.,&
13
1999;&Nester,&2008),&but&locally&include&tephras&dated&8.76&±&0.05&Ma&(Sáez&et&al.,&2012)&and& 5.6–6.0&Ma&(A.&Jensen,&personal&communication,&2013).&Lacustrine&diatomite&and&sandstone& of&the&overlying&Quillagua&Formation&are&restricted&to&distances&within&25&km&from&the& canyon&of&the&modern&Loa&River,&which&suggests&that&the&lake&water&was&supplied&by&the& paleo3Loa&River.&The&lacustrine&deposits&are&constrained&by&tephrachronology&and& magnetic&polarity&stratigraphy&to&the&span&of&5.5–4.5&Ma&(Sáez&et&al.,&2012).&An&episode&of& widespread&salt3pan3facies&gypsum&and&localized&halite&formed&the&Soledad&Formation& during&the&Early&Pliocene,&with&interbedded&tephra&dated&as&4.3&±&0.2&and&3.73&±&0.2&Ma& (Table&1)&Quezada&2013a.&The&Soledad&Formation&overlies&the&Quillagua&paleo3lake& deposits&near&the&Loa&River&canyon&and&also&extends&more&than&70&km&north&of&the&canyon,& which&suggests&that&much&of&its&parent&water&may&have&been&sourced&from&the& Precordillera&catchments.&& Kiefer&et&al.&(1997)&used&landscape,&geophysical,&and&stratigraphic&analysis&to&study& the&partially&relict&Arcas&alluvial&fan&(Figure&2),&which&drains&a&comparatively&large& catchment&(700&km2).&Keifer&et&al.&(1997)&mapped&two&age&units&on&the&fan.&They& considered&the&most&widespread&map&unit&to&be&relict&from&the&time&of&initial&deposition&of& the&alluvial&strata&between&7.2&and&6.8&Ma.&Their&second&fan&surface&unit&was&the&active&fan,& limited&to&an&east3west&elongate&belt&in&the&northern§or&of&the&fan.& METHODS' This&integrated&landscape&analysis&is&based&on&geomorphological,&stratigraphic,&and& pedogenic&data.&Geomorphic&surfaces&with&their&constituent&allostratigraphic&units&and& soils&constitute&the&organizational&core&of&the&work&Monger&2009.&Landforms&were&mapped& by&a&combination&of&field&observation&and&satellite&image&analysis&(Figure&4).&Satellite&image&
14
analysis&included&spectral&and&morphological&mapping&based&on&CNES/SPOT&imagery&(e.g.,& cloud3free&acquisitions&dated&27&February&2006&and&28&April&2007,&displayed&by&Google& Earth),&Digital&Globe&images,&and&Landsat&TM&data.&Pixel&sizes&vary,&but&are&commonly&2.5– 15&m&ground&resolution.&In&an&attempt&to&identify&variations&in&mineral&composition&of&the& landscape&surface,&and&hence&either&variations&in&rock&weathering&or&soil&development,&we& investigated&ASTER&imagery&band&ratios.&We&found&that&the&'quartz&index'&of&(Ninomiya&et& al.,&2005)&inversely&correlates&with&gypsum3rich&surficial&deposits,&leading&to&use&of&an& ASTER&band&ratio&(10x12&/&11x11)&as&an&indicator&of&gypsum3rich&surfaces&(Data& Repository,&Figure&DR31).&For&those&Aster&bands,&the&90&m&pixel&resolution&has&a&coarse& resolution&compared&to&the&other&types&of&imagery.&The&landform&map&was&compiled&using& Google&Earth&mapping&tools,&from&which&shape&files&were&transferred&to&Canvas©&for& further&analysis.& Field&observations&were&conducted&by&the&authors&in&multiple&week3long&campaigns& in&2003–2008,&during&1.5&months&in&2009,&and&during&a&week&in&2011&and&another&in&2012.& Part&of&the&field&study&(20°–21°S)&was&conducted&in&conjunction&with&the&2008–2012& geological&mapping&program&of&the&Chilean&Geological&and&Mining&Service& (SERNAGEOMIN),&which&generated&1:100,000&scale&map&products&and&many&of&the& chronological&constraints&(Table&1).&Field&observations&for&this&study&focused&on&four& different&tasks:&1)&identification&of&surface&materials&that&correspond&to&the&major&units&that& display&consistent&spectral&characteristics&visible&in&the&satellite&imagery,&2)&identification,& characterization,&and&mapping&of&modern&and&relict&saline&soils&and&paleosols;&3)& description&of&sedimentary&lithologies,&textures&and&structures&in&Neogene&stratigraphic& sections;&4)&identification&and&sampling&of&volcanic&tephra&interbedded&in&Neogene&strata.&&
15
We&organized&field&observation&campaigns&based&on&the&geomorphic&and&spectral& units&identified&in&the&satellite&imagery.&In&the&field&we&recorded&the&cross3cutting& topographic&relations&between&adjacent&categories&of&surface.&We&also&recorded&the&small3 scale&topographic&relief&of&geomorphic&surfaces&and&the&nature&of&the&materials&that&cover& those&surfaces,&inclusive&of&extent&of&coverage&by&eolian&sands&and&granules,&gravel3size& clasts,&sizes&of&the&largest&clasts,°ree&of&desert&varnish,°ree&of&shattering&of&clasts,& degree&of&cementation&of&materials&in&the&upper&few¢imeters,&and&mineralogy&of& cement,&if&present.&Experience&in&the&field&revealed&that&a&thin&cover&of&wind3blown&sand&or& grit&substantially&impacted&the&Aster&“gypsum&index”&and&reduced&its&utility.&Soils&were& characterized&in&natural&exposures,&which&enable&lateral&tracing&of&major&soil&features&over& horizontal&extents&of&tens&of&meters&to&several&kilometers&(Figure&4C).&Soil&descriptions& focused&on&soil&morphological&features,&determination&of&the&composition&and&abundance&of& salts&by&visual&inspection,&hardness&and&taste&tests,&and&documentation&of&the&nature&and& condition&of&parent&material.&In&an&initial&survey,&samples&were&collected&at&two&depths&in& six&widely&separated&soil&profiles&for&chemical&analysis&of&salts.& Approximately&25&g&of&bulk&soil&material&from&saline3cemented&soils&was&crushed& and&homogenized&in&a&ball&mill,&and&then&soluble&salts&from&a&50&mg&subsample&were& dissolved&overnight&in&ultrapure&water&by&repeated&agitation&in&an&ultrasonic&bath.&The& dissolution&procedure&was&repeated&three×&at&room&temperature.&The&decanted& solution&was&analyzed&on&a&Dionex&DX3500&HPLC&ion&chromatograph.&For&anions,&an&IonPac& AS14&analytical&column&with&a&3.5&mM&sodium&carbonate/1.0mM&sodium&bicarbonate& eluent&was&used.&For&cations,&an&IonPac&CS16&analytical&column&with&a&25&mM&methane& sulfonic&acid&eluent&was&used.&Measurements&were&by&suppressed&conductivity.&
16
We&mapped&all&the&landform&surfaces&for&4830&km2&in&the&southern&part&of&our&study& area&(Figure&4C).&Unit&boundaries&of&the&units&were&generalized&in&Canvas©&relative&to&the& primary&mapping&with&Google&Earth.&Using&Canvas©,&we&quantified&the&spatial&extent&of& each&landform&stage.&The&generalization&of&boundaries&between&distinctive&landscape& surfaces&that&are&in&some&cases&intricately&interwoven&introduces&errors.&The&criteria&with& which&to&distinguish&landscape&stages&from&satellite&images&are¬&sufficient&to&recognize& and&map&all&thin&veneers&or&small3area&patches&of&younger&materials.&The&errors&in&the&map& increase&toward&the&north,&where&eolian&sand&more&widely&covers&surfaces.& To&establish&the&ages&of&the&landscape&components,&the&primary&criteria&used&for& relative&age&relations&between&neighboring&surfaces&is&topographic&cross3cutting&relations.& Whereas&McFadden&et&al.&(1989)&demonstrate&the&utility&of&rock&weathering&of&alluvial& materials&as&well&as&the&maturity&of&soils&as&indicators&of&relative&ages&of&exposure&(Dan&et& al.,&1982;&McFadden&et&al.,&1989),&we&do¬&use&this&chronosequence&approach&(i.e.,&the& thickness&of&gypsic&soils&and&the&salt&concentrations)&as&a&major&criteria&for&the&relative&age& of&a&landscape&stage.&Numerical&ages&of&landscape&surfaces&are&constrained&by&volcanic& tephra&interbedded&in&the&alluvium&on&which&the&fan&surfaces&were&constructed,&but&in&the& case&of&late&Quaternary&and&Holocene&fans,&constraints&come&also&from&14C&ages.&The&ages&of& volcanic&deposits&were&determined&by&40Ar/39Ar&and&K3Ar&methods&in&SERNAGEOMIN’s& laboratory&in&Santiago,&Chile&(http://www.sernageomin.cl/laboratorio.php)&and&are& reported&by&Jordan&et&al.&(2010),&Blanco&and&Tomlinson&(2013),&and&others&(Table&1).& Radiocarbon&ages&used&in&this&study&have&been&published&elsewhere&(Supplemental&file&2)& (Nester&et&al.,&2007;&Gayó&et&al.,&2012;&Blanco&P.&and&Tomlinson,&2013).&Our&primary& criterion&for&numerical&ages&for&landscape&surfaces&is&that&a&maximum&age&for&a&given&
17
surface&is&younger&than&the&minimum&reliable&age&of&a&stratigraphic&unit&that&occurs& beneath&that&surface.&The&minimum&age&of&initiation&of&a&surface&is&defined&by&the&maximum& age&of&surfaces&inset&against&it,&yet&all&surfaces&continue&to&evolve&until&present.&The&low& degree&of&temporal&resolution&provided&by&this&method&leads&to&large&uncertainties,& especially&for&Quaternary&age&surfaces.&Indirect&numerical&age&constraints&come&from& cosmogenic&nuclide&surface&ages&within&for&the&Coastal&Cordillera&near&the&study&area& (Carrizo&et&al.,&2008).&& PROXY'SELECTION:'SOILS'AND'SEDIMENTS'OF'THE'HYPERARID'STATE'AND'THE' LATE'PLEISTOCENE'WET'PHASES' Under&the&modern&hyperarid&climate&state,&“absolute&desert”&exists&in&the&Central& Depression&and&the&slopes&of&the&Andean&foothills&below&2300&m,&where&it&is&so&dry&that& macroscopic&plant&life&is&absent.&Without&plants,&abiological&proxies&for&past&climate&states& are&needed&that&are&readily&preserved.&In&the&following§ion,&the&differences&between& modern&soils&and&sediments&and&those&that&formed&during&the&Late&Pleistocene&wetter& intervals&are&characterized.&The&features&that&contrast&strongly&between&those&two&climate& states&are&then&used&as&proxies&to&investigate&pre3Late&Pleistocene&climate&history.& First=Order'Soil'Properties:'Gypsic'and'Calcic'Horizons' (2007; 2013)Soil&formation&during&the&Late&Pleistocene&pluvial&events&was& characterized&by&the&precipitation&of&soil&carbonate.&Nester&et&al.&&and&Blanco&and& Tomlinson&reported&calcic&rhizoconcretions&associated&with&in&situ&latest&Pleistocene& vegetation&on&fluvial&terraces&(Supplemental&File&2).&Calcic&soils&&have¬&been&found&on& surfaces&covered&by&relict&gypsic&soils&that&must&have&been&the&interfluve&surfaces&during& the&latest&Pleistocene&wet&intervals.&
18
In&contrast,&today’s&hyperarid&conditions&and&lack&of&plants&result&in&weathering,& soils&and&erosion&in&the&absolute&desert&that&differ&drastically&from&those&of&a&typical&Earth& landscape.&Where&precipitation&is&less&than&20&mm/yr,&normal&soil3forming&activities&do¬& occur&(Ewing&et&al.,&2006).&Instead,&there&is&a&net&volume&expansion&of&a&soil&horizon&due&to& addition&of&salts&and&dust&(e.g.,&Figures&5,&6d).&Salts&are&added&due&to&delivery&of&both& particulate&and&dissolved&salts&from&the&atmosphere&to&the&land&surface&(Rech&et&al.,&2003;& Ewing&et&al.,&2006).&The&most&abundant&pedogenic&salts&in&the&Atacama&Desert&are&sulfate& salts,&occurring&mostly&as&gypsum&and&anhydrite,&which&produce&soil&gypsic&(By)&or& petrogypsic&(Bym)&horizons.&Rare&precipitation&events&play&a&key&role&by&wetting&the&soil& surface,&which&leads&to&differentiation&of&soil&horizons&(e.g.,&Amundson&et&al.,&2012).&Fossil& gypsic&soils&(paleosols)&are&commonly&referred&to&as&Gypsisol&if&the&gypsic&horizon&is&the& dominant&feature&of&the&paleosol&(Mack&et&al.,&1993;&Rech&et&al.,&2006).&For&the&Atacama,&a& mean&annual&precipitation&of&20&mm/yr&is&a&useful&threshold&for&a&geological&definition&of& “hyperarid”,&and&ancient&examples&of&Gypsisols&derived&from&atmospheric&salts&are&a&proxy& for&hyperarid&paleoclimate.&& However,&the°ree&of&gypsic&soil&development&must&be&spatially&variable,&even&on& surfaces&of&equal&age,&because&of&spatial&variability&of&i)&air3borne&supplies&of&salts,&ii)&wind& removal&of&dust&as&well&as&sand,&and&iii)&overland&flow&of&water.&Salt&supply&variability&is& likely&controlled&in&part&by&the&location&of&a&surface&of&interest&relative&to&salt&pans,&which& are&sources&of&eolian&sulfates&(McFadden&et&al.,&1986),&and&relative&to&zones&prone&to&fog& incursions&(Farías&Salvador&et&al.,&2005),&which&bring&marine3sourced&sulfates&(Rech&et&al.,& 2003;&Sträter&et&al.,&2010).&Wind&patterns&also&cause&salt&removal&that&is&variable&over&large& and&small&spatial&scales.&Remote&sensing&and&field&observation&demonstrate&that&large&
19
regions&tend&to&be&deflated,&whereas&other®ions&display&dust&and&sand&accumulation.& Surface&water&and&variations&in&infiltration&also&promote&spatially&variable&pedogenesis.& Locations&prone&to&water&erosion&or&water3borne&deposition&are&less&likely&to&accumulate& gypsic&soil.&In&addition,&the°ree&of&salt&segregation&and&gypsum&accumulation&in&a&soil& seems&to&be&controlled&by&the&effectiveness&of&the&local&landscape&surface&to&concentrate& soil&waters&in&isolated&areas&that&evaporate&and&precipitate&soil&sulfates.&Within&the&study& area&no&systematic&study&of&the&spatial&variations&among&modern&gypsic&soils&has&been& conducted.Multiple&horizons&in&a&typical&mature&gypsic&soil&in&the&Atacama&Desert& (Ericksen,&1981;&Searl&and&Rankin,&1993;&Rech&et&al.,&2003;&Ewing&et&al.,&2006)&can&be& simplified&to&two&major&horizons&(Figures&5,&6d).&The&uppermost&layer&is&an&unconsolidated& or&weakly&consolidated&A&horizon,¢imeters&thick,&composed&of&a&mixture&of&eolian& siliciclastic&dust&rich&in&gypsum&and&anhydrite,&and/or&anhydrite&dust,&and&pebble&to&cobble& detrital&clasts.&Several&layers&with&differing°rees&of&induration&comprise&the&second& horizon,&B,&which&is&rich&in&salts&and&commonly&more&than&1&meter&thick.&The&base&of&the& gypsic&soil&transitions&to&the&underlying&parent&material&through&a&gradational&basal& horizon&of&salt3cemented®olith&and&loose®olith.&Within&the&B&horizon,&an&upper§or,& decimeters&thick&and&vesicular,&is&composed&of&weakly&indurated&dust&of&gypsum,& anhydrite,&and&siliciclastic&detritus,&with&prismatic&peds&and&vertical&fractures&that&contain& an&infill&of&sand&and&fine&gravel&cemented&with&sulfate&(Figure&5).&In&some&cases&this&layer& includes&cushion3shaped&nodules&of&anhydrite&or&pendants&(Buck&and&Van&Hoesen,&2002).& Water&that&occasionally&infiltrates&this&upper&sulfate3rich&horizon&causes&leaching&and& downward&movement&of&salts,&to&produce&a&more&firmly&indurated&lower&interval&with& halite,&gypsum,&carbonate,&and&often&nitrate&minerals.&The&B&horizons&contain&parent&
20
material&(e.g.,&conglomerate&clasts)&surrounded&by&a&displacive&salt&cement&(Ericksen,&1981;& Ewing&et&al.,&2006;&Prellwitz,&2007).&Polygons&on&the&landscape&surface&are&the&upper& surface&expression&of&B&horizon&vertical&fractures.&The&vertical&fractures&are&a&few& centimeters&to&decimeters&in&width&and¢imeters&to&~4&meters&in&vertical&extent,&and& commonly&their&vertical3laminated&salt3cemented&detrital&fill&is&the&most&easily& recognizable&diagnostic&feature&of&a&mature&gypsic&soil&(Figure&5)&(Ericksen,&1981;&Rech&et& al.,&2003).&The&polygons&represent&expansion&and&contraction&fractures&caused&by&the& addition&of&salts&to&the&upper&dust&layer,&followed&by&wetting,&dissolution,&and&downward& transportation&of&solutes&along&the&polygon&boundaries,&in&turn&followed&by&crystallization& of&evaporite&minerals&along&the&fracture&walls&(Ericksen,&1981).&The&salt&crystallization& process&also&causes&fracturing&of&parent&material&clasts&at&shallow&depths&in&a&soil&subjected& to&rapid,&extreme&changes&in&temperature&and&moisture&content&(Figure&5)&(Amit&et&al.,& 1993;&McFadden&et&al.,&2005),&leading&to&a&progressive&decrease&of&particle&size&within&the&B& horizon&compared&to&the®olith.&& Gypsic&soils&deteriorate&when&the&climate&state&shifts&beyond&the&suitable&climate& threshold,&and&in&an&extreme&case&only&vestiges&remain.&During×&that&the&climate&is& wetter,&enhanced&dissolution&of&the&more&soluble&salts&(i.e.,&halite&and&nitrate)&from&shallow& horizons&augments&the&salt3differentiation&among&horizons,&but&if&a&wet&time&interval& persists&the&soluble&salts&may&be&completely&removed,&and&only&the&less&soluble&calcium& sulfates&are&likely&to&remain&in&the&soil.&Under&wetter&conditions,&the&physical&structure&of& the&gypsic&soil&also&changes.&A&loss&of&salt&terminates&the&expansion&and&contraction&process& that&previously&maintained&partially&open&polygon&boundaries.&At&the&extreme,&these& processes&transform&the&horizons&that&once&constituted&a&soil&into&a&residual&mass&that&
21
resembles&an&unsorted&gypsum3cemented&alluvial&deposit.&Between&the&extremes&of&single3 climate3state&gypsic&soil&and&unrecognizably&deteriorated&gypsic&soil&exists&a&set&of&gypsic& soils&that&formed&under&hyperarid&conditions,&were&altered&during&prolonged&states&of&less& aridity,&and&again&were&active&in&subsequent×&of&hyperaridity.&Ericksen&(1981),&Ewing& et&al.&(2006),&Rech&et&al.&(2006),&and&Prellwitz&(2007)&documented&Atacama&gypsic&soils&that& are&the&final&products&of&a&long&period&of&time&during&which&climate&variations&included& times&that&were&wetter&than&the&hyperarid&mean&state.& These&typical&Atacama&Desert&soils&are&more&extreme&examples&of&salt&accumulation& than&the&gypsic&soils&in&other&arid®ions.&Soil&chronosequences&in&the&Negev&Desert&and& New&Mexico&display&tendencies&of&gypsum&accumulation&through&stages&of&increasing&soil& maturity&(Dan&et&al.,&1982;&Buck&and&Van&Hoesen,&2002),&yet&the&B&horizons&of&most&non3 Atacama&gypsic&soils&contain&significant&concentrations&of&soil&carbonate&(e.g.,&examples&in& Iran&contain&2–70%&gypsum&accompanied&by&1–18%&carbonate;&Moghiseh&and&Heidari,& 2012;&examples&in&the&Negev&Desert&contain&horizons&where&1–29%&gypsum&is& accompanied&by&1–63%&carbonate;&).&In&contrast,&most&gypsic&soils&of&the¢ral&Atacama& Desert&at&altitudes&comparable&to&our&study&area&contain&3300&m.&Given&a&strong&altitudinal&control&on&precipitation& today&(Figure&13),&one&should¬&assume&the&“onset&of&hyperaridity”&at&various& paleoaltitudes&to&be&equal.&& The&results&presented&here&imply&that&the°ree&of&moisture&transport&to&the&west& coast&of¢ral&South&America&was&more&variable&during&the&latest&Late&Miocene&and& Pliocene&than&during&most&of&the&Late&Miocene&(Figure&14A).&The&global&climate&had&cooled& sufficiently&during&the&Miocene&(Figure&14B)&(Zachos&et&al.,&2001)&that&the&West&Antarctic& ice&sheet&initiated&near&14&Ma,&and&thereafter&this&ice&sheet&varied&in&extent&until&it&reached&
52
a&near3modern&volume&at&some&point&during&the&Late&Miocene.&During&the&Late&Miocene&the& Antarctic&Circumpolar&Current&was&vigorous,&and&by&6&Ma&the&modern&system&of&global&heat& distribution&and&deep&ocean&water&circulation&had&been&established&(Haywood&et&al.,&2008).& The&~16–14&Ma&hiatus&in&the&Pampa&del&Tamarugal&basin&correlates&to&the&global&Middle& Miocene&Climatic&Maximum&(Zachos&et&al.,&2001).&However,&the&onset&of&hyperaridity&~12&±& 1&Ma&shows&no&correlation&to&significant&changes&of&global&ocean&temperatures&(Figure& 14B).&During&the&Late&Miocene,&the&Atacama&Desert&appears&to&have&been&persistently& hyperarid,&but&variability&increased&during&the&Pliocene.&A&million3year3long&positive& excursion&from&the&long3term&ocean&temperature&decline&curve¢ered&on&5&Ma&(Figure& 14B)&(Zachos&et&al.,&2001)&appears&to&correlate&with&the&Stage&NIIa&wet&interval.&Whitehead& and&Bohaty&(2003)&and&Escutia&et&al.&(2007)&identified&intervals&of&warmer&surface&water&in& the&oceans&adjacent&to&Antarctica&at&approximately&4.6–4.8&Ma,&4.2–4.4&Ma,&and&3.6–3.8&Ma& (Figure&14B).&Likewise,&since&4&Ma&during&four&intervals&whose&durations&were&10 Ma 8-14 Ma; 1000 years
Table DR-1. Locations within study area of Late Pleistocene alluvial and fluvial sediments related to latest Pleiostocene pluvial periods# Reference Latitude Longitude Degrees S Degrees W A. Locations with latest Pleistocene Carbon-14 dates of organic matter Matilla Chintagua 20.51716 69.34688 Blanco and Tomlinson, 2013 Matilla Chintagua 20.51716 69.34688 Blanco and Tomlinson, 2013 north of Challacolla Chipana 20.89841 69.33467 Blanco and Tomlinson, 2013 north of Challacolla Chipana 20.89846 69.33949 Blanco and Tomlinson, 2013 north of Challacolla Chipana 20.89849 69.34034 Blanco and Tomlinson, 2013 north of Challacolla Chipana Nester et al. (2007) 20.90365 69.34179 north of Challacolla Chipana Nester et al. (2007) 20.90365 69.34179 Guatacondo medial fan Guatacondo 20.99517 69.30124 Blanco and Tomlinson, 2013 Guatacondo medial fan Guatacondo 20.99542 69.30444 Blanco and Tomlinson, 2013 69.32 Nester et al. (2007) Maní medial fan Maní 21.09 69.3 Gayó et al. (2012) Maní medial fan Maní 21.09 69.19 Nester et al. (2007) Sipuca medial fan Sipuca 21.23 69.2 Nester et al. (2007) Sipuca medial fan Sipuca 21.23 69.42 Gayó et al. (2012) Lomas de Sal Tambillo 21.39 69.44 Gayó et al. (2012) west of Lomas de Sal Tambillo 21.4 69.42 Nester et al. (2007) Lomas de Sal Tambillo 21.4 69.43 Nester et al. (2007) Lomas de Sal Tambillo 21.4 69.25 Nester et al. (2007) Tambillo medial fan Tambillo 21.43 69.46 Gayó et al. (2012) west of Lomas de Sal Tambillo 21.43 69.46 Nester et al. (2007) west of Lomas de Sal Tambillo 21.43 69.26 Nester et al. (2007) Tambillo medial fan Tambillo 21.44 69.31 Nester et al. (2007) Tambillo distal fan Tambillo 21.44 69.25 Nester et al. (2007) Tambillo medial fan Tambillo 21.44 #Blanco and Tomlinson (2013) also report latest Pleistocene wetlands and salt-pan facies deposits Location name
Drainage Basin
B. Locations reported to contain carbonate rhyzoconcretions or other carbonate accumulations in soil lowest stream terrace Guataconda 20.97 69.19 Blanco and Tomlinson, 2013 lowest stream terrace Chipana 20.86 69.19 Blanco and Tomlinson, 2013 69.2 Nester et al. (2007) Sipuca medial fan Sipuca 21.23 69.25 Nester et al. (2007) Tambillo medial fan Tambillo 21.43
Figure'DR*1:'Satellite'remote'sensing'highlights'varying'amounts'of'gypsum'in' surface'materials'of'the'Pampa'del'Tamarugal,'northern'Chile.'These'data'are'from' the'Advanced'Spaceborne'Thermal'Emission'and'Reflection'Radiometer,'ASTER,' using'bands'10,'11,'and'12.'Whereas'the'specific'band'ratio,'(10x12'/'11x11),'was' developed'by'Ninomiya'et'al.'(2005)'for'recognition'of'quartz*rich'surface'materials,' the'inverse'(the'white'to'pale'gray'sectors'of'this'image)'correlate'with'gypsum*rich' surficial'deposits'in'northern'Chile.' ' Ninomiya,'Y.'B.,'Fu,'B.,'and'Cudahy,'T.,'2005,'Detecting'lithology'with'Advanced' Spaceborne'Thermal'Emission'and'Reflection'Radiometer'(ASTER)'multispectral' thermal'infrared'“radiance*at*sensor”'data":'Remote'Sensing'of'Environment,'p.' 127*139.' ' '
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70° W
69° W
21° S
21° S
22° S
22° S
. 70° W
20
10
0
20 Km
69° W
Figure'DR*2:'Photographs'of'Stage'NIV'geomorphic'surface'and'materials.'A)'view' SE'of'relict'gravel'deposits,'incised'~1'm'by'younger'channels,'within'the'Arcas'fan.' The'landscape'is'somewhat'more'rounded'than'the'Stage'NV'landscape'of'Figure'6A.' B)'Weakly'developed'gypsum'soil'on'Stage'NIV'surface'of'medial'Maní'fan.'Rounded' clasts'of'the'parent'moderately'well'sorted'alluvium'contrast'with'the'more'angular' clasts'of'the'soil,'in'which'the'angularity'increases'due'to'fracturing.'Fractured'clasts' include'gypsum'and'anhydrite'within'fractures,'and'sulfates'also'occur'between'the' original'clasts.'Locations:'A'near'21.67°S,'6933°W;'B'near'21.08°S,'69.37°W.' ' '
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Supplemental Files Figure S2 Supplemental*Files*Figure*S2*Stage*NIV* A.
B.
landscape surface
soil
parent material
Figure'DR*3:'Photographs'of'a'Stage'NIII'surface'(A)'and'soil'(B)'in'the'southern' Pampa'del'Tamarugal'basin.'A)'is'a'view'east'across'a'very'smooth'surface.'The'light' toned'patches'have'a'millimeter*thick'crust'of'gypsum'(or'anhydrite)'cemented' medium*grained'sand,'and'dark'patches'are'desert*varnished'pebbles'and'cobbles.' B)'is'a'cliff*exposure'of'the'soil'drawn'in'Figure'10B.'The'parent'for'this'soil'is' alluvium'of'the'“lowland'piedmont”'unit.'Locations:'A'near'21.436°S,'69.208°S;'B'at' 21.435857°S,'69.208310°W.' '
'
Jordan et al. Supplemental Figure S3 created as a .cvx file; submitted as Figure S3.pdf
Supplemental*File*Figure*S3:*Stage*NIII* A.
B.
Figure'DR*4:'Photographs'of'a'Stage'NII'surface'(A)'and'soil'(B)'in'the'southern' Pampa'del'Tamarugal'basin.'A)'shows'this'landscape'surface'which'covers'small' patches'of'the'“piedmont'deposit.”'There'is'a'concentration'of'pebbles'on'the' surface,'but'the'underlying'sulfate*rich'dust'is'sufficiently'cohesive'that'little'dust' rises'into'the'air'when'the'surface'is'disturbed.'B)'is'a'gypsic'soil'found'on'the'Stage' NII'surface.'Locations'for'both'photos:'21.426°S,'69.205°W.' '
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Supplemental*File*Figure*S4:*Stage*NII* Jordan et al. Supplemental Figure S5: created as a .cvx file; submitted as Figure S5.pdf A.
B.
By
Bym1
Bym2
10 cm
Figure'DR*5:'Photographs'of'a'Stage'NI'landscape'features'in'the'southern'Pampa' del'Tamarugal'basin.'A)'shows'obliquely,'and'B)'is'a'close*up'of,'the'surface'of'the' sparse'remnants'of'Stage'NIb.'C)'is'a'close'up'of'the'ledge*forming'unit'within'photo' A,'and'illustrates'that'this'indurated'horizon'is'the'poorly'structured'relict'of'a' deeply'leached'gypsic'soil,'formed'during'Stage'NIb.'This'soil'profile'is'shown'in' Figure'10D.'Locations:'A'and'C'near'21.4827°S,'69.1376°'W;'B'near'21.477°S,' 69.134°W.' '
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Supplemental*File*Figure*S5:*Stage*NI*
Jordan et al. Supplemental Figure S5 created as a .cvx file; submitted as Figure S5.pdf
A.
B.
C.
Figure'DR*6:'Paleosols'in'the'catchment'basin'of'the'Chipana'fan.'A)'At'the'contact' between'the'underlying'Middle'Miocene'El'Diablo'Formation'and'overlying'Upper' Miocene'“old'eolian'deposits”'(Blanco'P.'and'Tomlinson,'2013),'gypsum'columns' (white)'occur'in'a'25*cm*thick'bed'of'gravelly'sandstone'that'lacks'primary' sedimentary'structures'(Location:'20.8548°S,'69.1171°W).'The'columns'are' dominantly'gypsum'with'traces'of'halite'and'nitrate'(Table'4,'sample'09TJ*13).'The' columns'are'interpreted'to'be'the'remnants'of'salt'pendants'that'would'have' originally'been'found'under'gravel'clasts'in'a'mature'gypsic'soil.'Other'patches'of' gypsum'of'similar'thickness'but'only'a'few'tens'of'meters'of'lateral'extent'occur' sparsely'along'the'stratigraphic'contact.'Scale'on'notebook'is'10'cm.'A'nearby' (~260'm'horizontal'distance)'volcanic'tuff'located'~50'm'higher'in'stratigraphic' position'was'dated'9.7±0.4'Ma'(Table'1)'(Blanco'P.'and'Tomlinson,'2013).'B)'weakly' consolidated,'~50'cm'thick'Gypsisol'(arrow'indicates'stratigraphic'horizon'of' Gypsisol,'which'is'more'resistant'than'surrounding'strata),'interbedded'about'10'm' below'the'top'of'the'Upper'Miocene'“upland'piedmont'deposits.”'Circles'highlight' people,'who'serve'as'scale.'Left'set'of'two'people'is'near'location'20.8512°S,' 69.1265°W.'The'soil'contains'~25%'gypsum'with'trace'amounts'of'halite'and' nitrate'(Table'4,'Chip'1'and'Chip'2).'The'ridgeline'above'the'Gypsisol'(note'two' people'on'ridge)'exposes'an'unconsolidated'ash'flow'deposit.'A'nearby'welded'ash' flow'tuff'in'a'similar'landscape'position'and'1.6'km'distant'was'dated'8.1±0.4'Ma' (Table'1)'(Blanco'P.'and'Tomlinson,'2013).' ' '
Jordan et al., created in Canvas X, Supplemental Figure S6.cvx; File: Supplemental Figure S6.pdf
Supplemental* File*Figure*S6:* Stage*NIII*
A.
B.
10 m Gypsisol
REFERENCE'CITED' Blanco'P.,'N.,'and'Tomlinson,'A.'J.,'2013,'Carta'Guatacondo,'Región'de'Tarapacá:' Santiago,'Chile,'Servicio'Nacional'de'Geología'y'Minería,'Chile,'Subdirección' Nacional'de'Geología'Carta'Geológica'de'Chile,'Serie'Geología'Basica'15,'109'p.,'1' map'1:100,000'.' ' '
