coarsening-up alluvial shallow braided alluvial plain Reworked basalt clasts Palaeosols braided alluvial plain phyllite bivalves in green-grey silt Lake and ephemeral streams distal alluvial plain reworked basalt clasts Palaeosols interbedded with gravel sheets, rare channels red-brown alluvial plain sediments basalt coarsening-up alluvial massive conglomerate debris fan Channellised alluvial plain 7.82 ± 0.32 Ma 6.93 ± 0.15 Ma 13.6 ± 0.5 Ma 12.2 ± 0.5 Ma 12.0 ± 0.5 Ma QTt: Quaternary-Tertiary Tortoral Fm. Tpz: Tertiary Playa del Zorro Fm. Tl: Tertiary lacustrine Tpa: Tertiary Penas Azules Fm. Tb: Tertiary basalt pC: Precambrian (?) basement Tgm: Tertiary green mudstone marker Penas Azules Formation ~ Sections ARG1 - 6 Section ARG8 Section ARG 9 Playa del Zorro Formation Totoral Formation 300 m Basement Sequence 1 Sequence 2 Sequence 3 Sequence 4 3000 3000 4000 3500 3000 3000 3500 2500 3000 3000 ? Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Ql Ql Ql Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qt Qls Qls Qls Qls Qls Qls Qls Qls Qls Qls T T T Tpa Tpa Tpa Tpa Tpa Tpa Tpa Tpa Tpa Tpa Tpa Tpa Tf Tpz Tpz Tpz Tpz Tpz Tpz Tpz Tpz Tpz Tpz Tpz Tpz QTt QTt QTt QTt QTt QTt QTt QTt QTt QTt QTt QTt QTt pC pC pC pC pC pC pC Tb Tb Tl Tl Tl Tb Tb Tb Tb Tb Tgm Tgm 75 40 40 60 60 80 25 10 24 26 17 12 08 10 20 20 35 45 58 60 60 30 18 25 16 14 15 75 71 70 65 21 16 20 20 70 40 51 20 10 06 25 20 10 10 30 10 50 12 21 02 04 04 07 10 10 08 08 15 06 06 06 15 07 15 34 09 15 39 45 32 65 20 41 25 04 46 32 65 20 30 47 20 35 25 36 28 16 32 30 32 28 40 25 32 75 20 12.0 ± 0.5 Ma 7.82 ± 0.3 Ma 6.93 ± 0.2 Ma 12.2 ± 0.5 Ma 5.71 ± 0.4 Ma 11.8 ± 0.4 Ma 11.6 ± 0.5 Ma Basin Compartmentalization in the Foreland: El Cajon Basin, Southwestern Argentina Jonathan R. Pratt - The Ohio State University, Lindsay Schoenbohm - The Ohio State University, Estelle Mortimer - University of Leeds Basin compartmentalization in a thick-skinned set- ting: In a thin-skinned setting there is a fairly accurate model for basin compartmentalization, where thrusts propagate in sequence utilizing bedding planes in sed- imentary rocks. However, in thick-skinned settings where reverse faults occur at higher angles, this model is 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. Pratt E E’ F F’ Sierra de Chango Real Sierrra de Quilmes Santa Maria Basin -> <- Puna Plateau 45 45 Qal: Quaternary alluvium Qt: Quaternary terrace deposits QTt: Quaternary-Tertiary Tortoral Fm. Tpz: Tertiary Playa del Zorro Fm. Tl: Tertiary lacustrine Tf: Tertiary coarse fan deposits Tpa: Tertiary Penas Azules Fm. Tb: Tertiary basalt T: Undifferentiated Tertiary pC: Precambrian (?) basement Ql: Quaternary lacustrine Qls: Quaternary landslide deposits Thrust fault with dip Thrust fault- concealed Normal fault with dip Thrust reactivated as normal fault Syncline axis Anticline axis Fold axis- approximate Contact Contact- approximate Contact- concealed Thrust fault- approximate Strike and dip of bedding Strike and dip of overturned bedding 45 45 Richard Yuretich Richard Yuretich Richard Yuretich Richard Yuretich Richard Yuretich Richard Yuretich Richard Yuretich 11.8 ± 0.4 Ma 5.00 ± 0.14 Ma 0s 1s 2s ARG01553 SQS Seq. 1 Seq. 2 Seq. 3 Seq. 4 TWTT Sierra Aconquija Sierra de Quilmes Chango Real Santa Maria Basin Hualfin Basin El Cajon Basin N Puna Plateau 26ºS 67ºW 66ºW 27ºS 2 0 2 4 6 KILOMETERS ~13 Ma ~10 Ma ~6 Ma Chango Real uplifts, creating accomodation space Sierra de Quilmes begins uplifting in the north, beginning compartmentalization. Sierra de Quilmes propagates further south and the Sierra Aconquija beings 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 between 11.8-13.6 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 de Fiambala Sierra de Afto Huasi Sierra de Hualfin Sierra de Aconquija Orogenic Front Sierras Pampeanas Puna Farallon Negro Volcanics Regional 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. 1 Fig. 2a Fig. 2b F F' distance (kilometers) 1 3 2 4 5 6 7 8 9 10 11 12 13 elevation (meters) 3500 3000 2500 2000 1500 1000 500 0 -500 Tpz Tpz Tpa pC Tb Tf High angle, basement faults Alluvial fan along basin margin Growth strata within Playa del Zorro fm. Fig. 3a Fig. 3c elevation (meters) distance (kilometers) E E' 1 3 2 4 5 6 7 8 9 10 11 12 13 14 3500 3000 2500 2000 1500 1000 500 0 -500 pC pC Tpa Tpz QTt Tl Tb Fig. 3b Fig. 4 Fig. 5 High angle blind reverse fault Fault propagation fold Angular 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 deposit Lacustrine unit Discussion: By using the basal age of the sedimentary sequence of 13.6 Ma to 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 as it 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 the eastern 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-sequence compartmentalization following a propagating thrust-wedge model with deformation strongly controlled by pre-existing structures as is shown simplified in Fig. 5. 1 2 3 1 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.59-78 3 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. Pratt Produced by L. Schoenbohm and modified by J. Pratt Above: Geologic map of the El Cajon containing locations of the U-Pb dated tuff samples and the cross-section lines corresp- onding to Fig. 3a and 3b. The ashes were dated using an ion microprobe at the Univer- sity of Los Angeles with the aid of Dr. Axel Schmidt. Left: Stratigraphic column showing the rock types and interpreted depositional environments. U-Pb ages are placed in the column and are shown in red. Of note is the bracketing of the lacustrine unit between 7.82 & 12.0 Ma. 13.6 ± 0.5 Ma 10.7 ± 1.7 Ma 5.00 ± 0.14 Ma