For permission to copy, contact editing@geosociety.org © 2012 Geological Society of America Clastic deposition, provenance, and sequence of Andean thrusting in the frontal Eastern Cordillera and Llanos foreland basin of Colombia Alejandro Bande 1,† , Brian K. Horton 1,2,§ , Juan C. Ramírez 3,4 , Andrés Mora 4 , Mauricio Parra 1,4 , and Daniel F. Stockli 1,5 1 Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA 2 Institute for Geophysics, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA 3 Escuela de Geología, Universidad Industrial de Santander, Bucaramanga, Colombia 4 Instituto Colombiano del Petróleo, Ecopetrol, Bucaramanga, Colombia 5 Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA 59 GSA Bulletin; January/February 2012; v. 124; no. 1/2; p. 59–76; doi: 10.1130/B30412.1; 12 figures; 1 table; Data Repository item 2011300. † Present address: Institut für Erd- und Umweltwissenschaften, Universität Potsdam, 14476 Potsdam, Germany. § E-mail: horton@mail.utexas.edu ABSTRACT Sedimentological, provenance, and detrital thermochronological results for basin fill at the modern deformation front of the north- ern Andes (6°N latitude) provide a long-term, Eocene to Pliocene record of foreland-basin sedimentation along the Eastern Cordillera– Llanos basin boundary in Colombia. Litho- facies assemblages and paleocurrent orienta- tions in the upward-coarsening, ~5-km-thick succession of the Nunchía syncline reveal a systematic shift from craton-derived, shallow-marine distal foreland (back-bulge) accumulation in the Mirador Formation, to orogen-sourced, deltaic, and coastal- influenced sedimentation of the distal to medial foreland (foredeep) in the Carbonera and León Formations, to anastomosing flu- vial and distributive braided fluvial megafan systems of the proximal foreland (foredeep to wedge-top) basin in the lower and upper Guayabo Formation. These changes in depo- sitional processes and sediment dispersal are supported by up-section variations in detrital zircon U-Pb and (U-Th)/He ages that record exhumation of evolving, compartmentalized sediment source areas in the Eastern Cor- dillera. The data are interpreted in terms of a progressive eastward advance in fold- and-thrust deformation, with late Eocene– Oligocene deformation in the axial zone of the Eastern Cordillera along the western edge of Floresta basin (Soapaga thrust), early Mio- cene reactivation (inversion) of the eastern margin of the Mesozoic rift system (Pajarito and Guaicaramo thrusts), and middle–late Miocene propagation of a footwall shortcut fault (Yopal thrust) that created the Nunchía syncline in a wedge-top (piggyback) setting of the eastern foothills along the transition from the Eastern Cordillera to Llanos fore- land basin. Collectively, the data presented here for the frontal Eastern Cordillera define a general in-sequence pattern of eastward- advancing fold-and-thrust deformation during Cenozoic east-west shortening in the Colombian Andes. INTRODUCTION Identifying the sequence of deformation in fold-and-thrust belts is essential to monitoring net shortening, crustal thickening, and attendant surface uplift (Schelling and Arita, 1991; Barke and Lamb, 2006; McQuarrie et al., 2008), gaug- ing the influence of fault reactivation (inver- sion) on the time-space evolution of orogen- esis (Hayward and Graham, 1989; Flöttmann and James, 1997), assessing the applicability of critical-taper models (DeCelles and Mitra, 1995; Horton, 1999; Nieuwland et al., 2000), and predicting petroleum maturation and migra- tion histories (Cazier et al., 1995; Echavarria et al., 2003). In addition to fault cutoff relation- ships within the fold-and-thrust belt (Diegel, 1986; Morley, 1988; Schirmer, 1988), the depo- sitional and provenance record of the adjacent foreland basin has been long recognized as an important factor in extracting timing informa- tion on evolving structures (e.g., Wiltschko and Dorr, 1983; Lawton, 1985; Jordan et al., 1993; Sinclair, 1997; DeCelles et al., 1998). Although sediment recycling, diagenetic alteration, com- plex dispersal pathways, and multiple or nonu- nique sediment sources complicate interpreta- tions (Steidtmann and Schmitt, 1988; Schmitt and Steidtmann, 1990), careful consideration of multiple hypotheses commonly leads to well-constrained histories of thrust deformation (e.g., DeCelles, 1988, 1994, 2004; Lageson and Schmitt, 1994; Meigs et al., 1995; Horton, 1998; Reynolds et al., 2000; Echavarria et al., 2003). In the northern Andes of Colombia, the ~200-km-wide Eastern Cordillera marks the foreland zone of regional retroarc fold-and- thrust deformation. Several distinguishing fac- tors make the Eastern Cordillera a key region: a combination of thin- and thick-skinned defor- mation (Dengo and Covey, 1993; Cooper et al., 1995); a series of both first-generation and reac- tivated faults (Colletta et al., 1990; Mora et al., 2006); proposed out-of-sequence thrusts (Mar- tinez, 2006; Bayona et al., 2008); a climatic/ erosional influence on thrust kinematics (Mora et al., 2008); and a petroliferous foothills belt and adjacent foreland basin (Cazier et al., 1995). Numerous previous studies of synorogenic sedi- mentation and basin evolution have considered the frontal (easternmost) zone of shortening in the Eastern Cordillera. These studies have gener- ated new insights into regional basin evolution from flexural modeling, stratigraphic geometries and onlap relationships, one-dimensional (1-D) subsidence histories, three-dimensional (3-D) sediment budgets, conglomerate clast composi- tions, and bedrock low-temperature thermochro- nology (e.g., Gómez et al., 2005a; Bayona et al., 2008; Parra et al., 2009a, 2009b, 2010). Despite significant effort, tracing the timing of deformation has proven to be difficult and