Mortimer, Estelle

Pratt, Jonathan R.

Schoenbohm, Lindsay

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coarsening-up alluvial shallowbraided alluvial plainReworkedbasalt clastsPalaeosols braided alluvial plainphyllitebivalvesin green-greysiltLake and ephemeral streamsdistal alluvial plainreworked basalt clastsPalaeosols interbedded withgravel sheets, rare channelsred-brown alluvial plain sedimentsbasaltcoarsening-upalluvial massive conglomeratedebris fanChannellisedalluvial plain7.82 ± 0.32 Ma6.93 ± 0.15 Ma13.6 ± 0.5 Ma12.2 ± 0.5 Ma12.0 ± 0.5 MaQTt: Quaternary-Tertiary Tortoral Fm.Tpz: Tertiary Playa del Zorro Fm.Tl: Tertiary lacustrineTpa: Tertiary Penas Azules Fm.Tb: Tertiary basaltpC: Precambrian (?) basementTgm: Tertiary green mudstone markerPenas Azules Formation~Sections ARG1 - 6 Section ARG8 Section ARG 9Playa del ZorroFormationTotoralFormation300 mBasementSequence 1Sequence 2Sequence 3Sequence 430003000350040003500300030003500250030003000??QalQalQalQalQalQalQalQalQalQalQalQlQlQlQtQtQtQtQtQtQtQtQtQtQtQtQtQtQtQlsQlsQlsQlsQlsQlsQlsQlsQlsQlsTTTTpaTpaTpaTpaTpaTpaTpaTpaTpaTpaTpaTpaTfTpzTpzTpzTpzTpzTpzTpzTpzTpzTpzTpzTpzQTtQTtQTtQTtQTtQTtQTtQTtQTtQTtQTtQTtQTtpCpCpCpCpCpCpCTbTbTlTlTlTbTbTbTbTbTbTgmTgm7540 40606080251024261735 262512081020 203545586060 30 1825 16 14 15757170 6521162020704051201006112520101030105012210204040710100808150606061507153409153945326520 4125044632652030 4720 35253628163230322840253275207.5 m separation12.0 ± 0.5 Ma7.82 ± 0.3 Ma6.93 ± 0.2 Ma12.2 ± 0.5 Ma5.71 ± 0.4 Ma11.8 ± 0.4 Ma11.6 ± 0.5 MaBasin Compartmentalization in the Foreland:  El Cajon Basin, Southwestern ArgentinaJonathan R. Pratt - The Ohio State University,   Lindsay Schoenbohm - The Ohio State University,   Estelle Mortimer - University of LeedsBasin compartmentalization in a thick-skinned set-ting:  In a thin-skinned setting there is a fairly accuratemodel for basin compartmentalization, where thrustspropagate in sequence utilizing bedding planes in sed-imentary rocks.  However, in thick-skinned settingswhere reverse faults occur at higher angles, this modelis compromised by the importance of pre-existing weaknesses.Approach:  Analyze the spatial and temporal evolut-ion of one basin, the El Cajon Basin of Northwest Argentina as an example of a basin compartmentaliz-ation in a thick-skinned setting.Data:  As part of a collaborative effort, several data are combined to understand the process and controls that compartmentalized the El Cajon Basin. 1. Geologic mapping of the northern basin, (Fig 2a),  produced by L. Schoenbohm. 2. Stratigraphic analysis (Fig. 1), produced by E.  Mortimer. 3. Rigorous cross sections (Fig 3a,b), produced  by J. Pratt. 4. Seismic data from southern basin (Fig 3c)   produced by E. Mortimer. 5. U-Pb dating of intercalated tuff beds (Fig 1 &   Fig 2a), produced by J. PrattEE’FF’Sierra de Chango RealSierrra de QuilmesSanta Maria Basin -><- Puna Plateau4545Qal: Quaternary alluviumQt: Quaternary terrace depositsQTt: Quaternary-Tertiary Tortoral Fm.Tpz: Tertiary Playa del Zorro Fm.Tl: Tertiary lacustrineTf: Tertiary coarse fan depositsTpa: Tertiary Penas Azules Fm.Tb: Tertiary basaltT: Undifferentiated TertiarypC: Precambrian (?) basementQl: Quaternary lacustrineQls: Quaternary landslide depositsThrust fault with dipThrust fault- concealedNormal fault with dipThrust reactivated as normal faultSyncline axisAnticline axisFold axis- approximateContactContact- approximateContact- concealedThrust fault- approximateStrike and dip of beddingStrike and dip of overturned bedding4545Richard Yuretich 11.8 ± 0.4 Ma5.00 ± 0.14 Ma0s1s2sARG01553SQSSeq. 1Seq. 2Seq. 3Seq. 4TWTTSierra AconquijaSierra de QuilmesChango RealSanta Maria BasinHualfin BasinEl Cajon BasinNPuna Plateau26ºS67ºW 66ºW27ºS2 0 2 4 6 KILOMETERS~13 Ma~10 Ma~6 MaChango Real uplifts, creating accomodation spaceSierra de Quilmes begins uplifting in the north, beginningcompartmentalization.Sierra de Quilmes propagates further south and the Sierra Aconquijabeings to uplift, effectively compartmentalizing the foreland.Geologic Background:The El Cajon basin is one of several intermontane basins along the southeastern margin of the Puna Plateau (Fig. 2b). This part of the Andes is an active, foreland-propagating, thick-skinned fold and thrust belt, which is superimposed on the Cretaceous Salta Rift.  Once part of a continuous foreland stretching from the Hualfin basin to the southwest to the Santa Maria basin to the east (Fig. 2b), the El Cajon was compartmentalized by the uplift of the Sierra de Quilmes. The El Cajon basin contains three Miocene to Pliocene alloformations:1. Penas Azules: a coarsening-upwards alluvial sequence between11.8-13.6 Ma. Deposited before significant deformation.2. Playa del Zorro: a coarsening-upwards alluvial sequence show-ing growth structures and containing a lacustrine unit. Syntectonic, between 6.93-11.8 Ma. in age.3. Tortoral: Massive fan conglomerate, regionally widespread and sits above an unconformity in the El Cajon. Upper age of 5.00 Ma.Sierra deFiambalaSierra de AftoHuasiSierra de HualfinSierra deAconquijaOrogenic FrontSierras PampeanasPunaFarallon NegroVolcanicsRegional Cross-section from just south of the El Cajon. The area encompassing the El Cajon’s east-west extent is boxed in red.  Published by V.A. Ramos et. al.Fig. 1Fig. 2aFig. 2bF F'distance (kilometers)1 32 4 5 6 7 8 9 10 11 12 13elevation (meters)3500300025002000150010005000-500TpzTpzTpapCpCTbTfHigh angle, basement faultsAlluvial fan along basin marginGrowth strata within Playa del Zorro fm.Fig. 3aFig. 3celevation (meters)distance (kilometers)E E'1 32 4 5 6 7 8 9 10 11 12 13 143500300025002000150010005000-500pCpCTpaTpzQTtTlTbFig. 3bFig. 4Fig. 5High angleblind reverse faultFault propagation foldAngular unconformity- Middle El Cajon- Tpa thins eastward- Less deformation   along the Chango   Real than in the   north.- Fault pair younger   than in the north.- Northern El Cajon- Tpa thins eastward- Eastern margin gives   constraint on Tpa   thickness. Potential   pre-existing topo-   graphy.- Faults are most likely   reactivated normal   faults.Thin Playa del Zorro fm.over Penas Azules fm.Landslide depositLacustrine unitDiscussion:By using the basal age of the sedimentary sequence of 13.6 Mato approximate the onset of loading along the Chango Real, we can estimate that this is when significant uplift along the fault began.The lacustrine bed serves as an indication of a switch from foreland drainage to internal drainage, which was most likely caused by uplift of the Sierra de Quilmes basement anticline asit propagated southward reactivating Salta Rift structures. The age of the lacustrine bed is bracketed between ~8 & ~12 Ma  therefore the uplift of the Sierra de Quilmes is also bracketed between these ages.The Sierra Aconquija is east of the El Cajon basin and is theeastern bound of the Santa Maria basin.  Previous work by Strecker has dated the Aconquija uplift at 5.5 Ma.The evolution of the El Cajon basin is therefore an in-sequencecompartmentalization following a propagating thrust-wedgemodel with deformation strongly controlled by pre-existing structures as is shown simplified in Fig. 5.1231 Mortimer et al, 2007, Fragmentation of a foreland basin in response to out-of-sequence basement uplifts and structural reactivation: El Cajon-Campo del Arenal basin, NW Argentina: GSA Bulletin: May/June 2007, p.637-653 2 Ramos, V.A., et al, 2002, The Pampean flat-slab of the Central Andes: Journal of South American Earth Science v. 15, p.59-783 Sobel, E.R. & M.R. Strecker, 2003, Uplift, Exhumation and Precipitation: Tectonic and Climatic Control of Late Cenozoic Landscape Evolution in the Northern Sierras Pampeanas, Argentina: Basin Research, p.1-21-Seismic section from south of the El Cajon subbasin-Shows both high angle reverse faults and growth strata  as exist in the north.-SQS stands for South Quilmes Structure, which is the buried extension of the Sierra de Quilmes.-Tpa thins, but to a lesser extent than it does in the north.Produced by E. Mortimer and modified by J. PrattProduced by L. Schoenbohm and modified by J. PrattAbove:Geologic map of the El Cajoncontaining locations of theU-Pb dated tuff samples andthe cross-section lines corresp-onding to Fig. 3a and 3b.The ashes were dated usingan ion microprobe at the Univer-sity of  Los Angeles with theaid of Dr. Axel Schmidt.Left:Stratigraphic column showingthe rock types and interpreteddepositional environments.U-Pb ages are placed in thecolumn and are shown in red.Of note is the bracketing of thelacustrine unit between 7.82 &12.0 Ma.13.6 ± 0.5 Ma10.7 ± 1.7 Ma5.00 ± 0.14 Ma

Basin Compartmentalization in the Foreland: El Cajon Basin, Southwestern Argentina

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Basin Compartmentalization in the Foreland: El Cajon Basin, Southwestern Argentina

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