Middle Eocene deformation and sedimentation in the Puna– Eastern Cordillera transition (23°–26°S): Control by preexisting heterogeneities on the pattern of initial Andean shortening F. Hongn Consejo Nacional de Investigaciones Científi cas y Técnicas–Universidad Nacional de Salta, C. del Papa Buenos Aires 177, 4400 Salta, Argentina J. Powell Consejo Nacional de Investigaciones Científi cas y Técnicas–Universidad Nacional de Tucumán, Miguel Lillo 205, 4000 Tucumán, Argentina I. Petrinovic Consejo Nacional de Investigaciones Científi cas y Técnicas–Universidad Nacional de Salta, Buenos Aires 177, 4400 Salta, Argentina R. Mon Consejo Nacional de Investigaciones Científi cas y Técnicas–Universidad Nacional de Tucumán, V. Deraco Miguel Lillo 205, 4000 Tucumán, Argentina ABSTRACT (Elger et al., 2005), as well as models interpreting eastward propagation of The Quebrada de los Colorados Formation, at the north end of deformation (DeCelles and Horton, 2003), focused on the Bolivian Alti- Calchaquí Valley in Salta Province, northwest Argentina, preserves plano. A late Eocene–Oligocene uplifting guided by preexisting structures evidence of syndepositional deformation since the middle Eocene was described along the Puna–Eastern Cordillera transition (Coutand (ca. 40 Ma) that includes (1) an angular unconformity with the under- et al., 2001; Deeken et al., 2006). This work presents new structural, strati- lying Salta Group (Paleogene), and (2) internal unconformities and graphic, and paleontologic evidence for a middle Eocene deformation and changes in vertical facies succession and provenance. Its fossil record sedimentation in the Puna–Eastern Cordillera transition between 24° and [mammalian (notoungulates), middle Eocene] is correlatable to the 26°S. Our results, along with others from the western Puna border (e.g., Casa Grande Formation, which also unconformably overlies the Salta Mpodozis et al., 2005), suggest that preexisting crustal heterogeneities Group; both units record middle Eocene deformation along the eastern infl uenced the localization of the fi rst manifestations of the Andean short- border of the Puna Plateau and outline previous fi rst-order mechanical ening from the middle Eocene in the southern portion of the plateau. Our heterogeneities related to the Cretaceous rift basin border. Along the data confi rm that distribution of the initial shortening and sedimentation western margin of the Puna, Eocene deformation coincides with ther- in the Central Andes was irregular, and that the classic models for Andean mal (magmatic arc) and mechanical (basin inversion) heterogeneities. evolution postulating diachronous and eastward progressive deformation Thus, the distribution of Eocene deformation followed an irregular pat- should be revised, because heterogeneities controlling shortening were tern as a consequence of the heritage of preexisting heterogeneities. activated along both plateau margins by the middle Eocene. Our data underscore a new key area to understand the initial evolution of the Andes, Keywords: Eocene deformation, initial foreland, preexisting hetero- the archetype of a major noncollisional orogen. geneities, Central Andes. GEOLOGICAL FRAMEWORK INTRODUCTION The Argentine Eastern Cordillera is a bivergent fold-thrust belt affect- The geological peculiarities of the Central Andes and their plateau ing the oldest rocks known in the region. The North Calchaquí Valley (a plate tectonics paradox; e.g., Allmendinger et al., 1997) have made (Fig. 1A) is limited by faults that thrust Neoproterozoic–Paleozoic base- this cordillera one of the most studied. Most previous work has been ment on the Salta Group (Upper Cretaceous–Paleogene; rift basin infi ll- devoted to Miocene evolution because this was the time of main shorten- ing) and Payogastilla Group (foreland basin infi lling). The Quebrada de ing and volcanic activity that distinguished the Western Cordillera, plateau los Colorados Formation is the lowest unit of the Payogastilla Group and ( Altiplano-Puna), Eastern Cordillera, and Subandean Ranges (see synthe- its age was considered to be Neogene (Díaz and Malizzia, 1984), although sis in Allmendinger et al., 1997). In contrast, Paleogene evolution (espe- some workers recognized that sedimentation might have started in the cially in basin research) received less attention, and as a consequence there Paleogene (Marrett, 1990; Starck and Vergani, 1996). are still many unanswered questions concerning (1) the age, intensity, and distribution of the initial Andean shortening (Kennan et al., 1995; Jordan EOCENE SYNTECTONIC SEDIMENTATION et al., 1997; Elger et al., 2005); (2) the map of the earliest foreland basins Our structural, sedimentological, and paleontological analyses con- (Horton, 2005); and (3) Paleogene deformation as an extra source of short- centrated on two areas of the northern Calchaquí Valley, Saladillo and La ening for compensating the defi cit between calculated and actual crustal Poma (Fig. 1A). Sedimentological and structural additional data on syn- thickness of the Central Andes (Kley and Monaldi, 1998). tectonic sedimentation are available in the GSA Data Repository1. The lack of direct evidence for an Incaic unconformity (Steinmann, 1929) in south Bolivia and north Argentina led to hypotheses postulat- Stratigraphic Evidence: Sequences and Growth Strata ing a progressive and diachronous southward and eastward propagation The 1500-m-thick Quebrada de los Colorados Formation consists of deformation in the Central Andes. Thus, shortening is thought to have of an upward-thickening and upward-coarsening continental succession begun in the Paleogene in the Bolivian Altiplano, and in the Neogene in (Fig. 1B). The internal stratigraphic arrangement includes three irregu- the Puna (Jordan et al., 1997). Nevertheless, available ages for the uplift larly distributed sequences, locally bounded by unconformities. Next to (e.g., Carrapa et al., 2005) and regional stratigraphic analyses (Boll and Hernández, 1986; Mpodozis et al., 2005) delineate Paleogene deformation 1GSA Data Repository item 2007058, Table DR1 (characteristic features of the sedimentary sequences) and Figures DR1–DR3 (main features of syntec- south of the Bolivian Altiplano. tonic strata) and Appendix with fossil description, is available online at www. Models regarding a control by preexisting (thermal or mechanical) geosociety .org/pubs/ft2007.htm, or on request from [email protected] or heterogeneities on localizing the plateau margins as zones of shortening Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. © 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, March March 2007; 2007 v. 35; no. 3; p. 271–274; doi: 10.1130/G23189A.1; 2 fi gures; Data Repository item 2007058. 271 the Toro Muerto fault (Fig. 1A), younger unit III overlies older ones with variable angular relations (Figs. 1C, 1D, 1E). 0 0 0 SALADILLO 5 5 0 0 5 9 7 959 A B Sequence I consists of fi ne-grained overbank, shallow sheet-like, 3 N 39 3 3 66°13’ and fi xed isolated channel facies and paleosols deposited in fl ood plains 42004 44504 2 45 00 Saladillo 0 and high-sinuosity rivers. Sequence II is composed of overbank, shallow LA POMA fi xed channels, and bars and/or channel facies of fl uvial sandy braided III 3450345 0 Faul 24°35’ and fl ood-plain environments. Analyses of provenance and paleofl ows mation t indicate source areas to the north, composed of low-grade metamorphic For s 37003 and granitic rocks. Sequence III comprises mudfl ow deposits, sandy and Saladillo 7 III 0 0 gravelly channel-fi ll facies, and fi ne-grained overbank deposits of alluvial t fan environment. In Saladillo, paleofl ows indicate a provenance from the north and northwest (Fig. 1B). In addition to these compositions, con- Faul II II Q. Los Colorado 323 í 3 2 glomerate clasts include Paleozoic volcanic rocks and Cretaceous sand- 4 0 5 0 0 250m aqu stones and limestones. In La Poma, paleofl ows indicate provenance from h lc I I the southwest (Fig. 1B), and the composition is Paleozoic sandstones and Ca Fault r 0 volcanic rocks. o e t iv r S f mc f m S f mc f m In Saladillo, intraformational deformation is recognized in sequences SS CL SS CL Muer I and II (Figs. 1C, 1D, 1E) and involves fi ne-grained deposits except next ESE Sequenc WNW C to the growth folds, mainly related to the Saladillo and Calchaqui faults, eI oro haquí I T Apparent dip where channelized gravelly sandstones were deposited at the lowest topo- Calc graphic levels. Toward the crest of anticline, strata show fan geometries N defi ned by onlap relations, decreasing in dip upsection and thinning. A 0 3 0 La Poma 3 7 7 700 37003 0 reduction in deformation rate generated stratal overlap and reestablish- 0 Sequence I ment of fi ne-grained sedimentation. Seq. II (n=11) Seq. I (n=24) Stratal stacking patterns suggest a period of a high rate of intrabasinal 10 m Cerro deformation that caused local low accommodation for sequences I and II, ’ t 0 Bayo l 0 N ault D followed by a period of high accommodation that allowed progradation of 2 F 42004 66°13 35 Fau middle to proximal alluvial fans (sequence III). A o 85 52 0 40 67 A’ 3700370 37003 30 35 The described features suggest that the tectonic-sedimentary evolu- 7 33 44 32003 0 Muert 55 30 25 2 0 27 9 2 tion along the northern valley was not simple, but was conditioned by fault 5 0 30 34 Cer 0 0 15 44 32 chaquí 32003200 Toro 20 slip within a complex paleogeography. ro al 14 Ba 44 C 24°35’ 29 18 yo B 30 50 B’ Fault 10 20 30 45 Structural Indicators: Growth Faults and Growth Folds 88 30 44 60 ’ 80 20 40 40 The Saladillo and Cerro Bayo faults branch from the Calchaquí fault 85 10 33 22 Saladillo 20 (Fig. 1A). The west limbs of related anticlines are either vertical or over- 66°13 turned, suggesting west-vergent faults. These are subvertical faults due to 4000 4000 m.a.s.l.