Clastic deposition, provenance, and sequence of Andean thrusting in the frontal Eastern Cordillera...

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Clastic deposition, provenance, and sequence of Andean thrusting in the frontal Eastern Cordillera and Llanos foreland basin of Colombia

Alejandro Bande1,†, Brian K. Horton1,2,§, Juan C. Ramírez3,4, Andrés Mora4, Mauricio Parra1,4, and Daniel F. Stockli1,5

1Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA2Institute for Geophysics, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA3Escuela de Geología, Universidad Industrial de Santander, Bucaramanga, Colombia4Instituto Colombiano del Petróleo, Ecopetrol, Bucaramanga, Colombia5Department 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 fi gures; 1 table; Data Repository item 2011300.

†Present address: Institut für Erd- und Umweltwissenschaften, Universität Potsdam, 14476 Potsdam, Germany.§E-mail: [email protected]

ABSTRACT

Sedimentological, provenance, and detrital thermochronological results for basin fi ll 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- infl uenced sedimentation of the distal to medial foreland (foredeep) in the Carbonera and León Formations, to anastomosing fl u-vial and distributive braided fl uvial 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 defi ne 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 infl uence 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 fi rst-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 infl uence 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 fl exural 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 signifi cant effort, tracing the timing of deformation has proven to be diffi cult and

Bande et al.

60 Geological Society of America Bulletin, January/February 2012

complex in the Eastern Cordillera. To date, no consensus exists on the onset of deformation, with timing estimates ranging from the mid-Cretaceous to late Miocene (Cooper et al., 1995; Jaimes and de Freitas, 2006; Bayona et al., 2008; Parra et al., 2009b; Horton et al., 2010a). This disagreement relates in part to the diffi culty of distinguishing among diverse source areas in western (orogenic) regions. Specifi cally, in the thickest, best-exposed Cenozoic succession, located along the transition between the Eastern Cordillera and Llanos basin, a western prov-enance could either be derived directly from the Central Cordillera or from recycled sediments of the Eastern Cordillera. Thus, our understanding of the history of shortening deformation along the eastern fl ank of the Eastern Cordillera would benefi t from integrated assessments of deposi-tional environments and provenance using sedi-mentological, mineralogical, and low- and high-temperature geochronological approaches.

This paper seeks to evaluate the history of thrust deformation in the frontal region of the

Eastern Cordillera through diverse provenance techniques coupled with fi eld-based depositional systems analysis. These methods include detri-tal zircon U-Pb geochronology and (U-Th)/He thermochronology integrated with physical sed-imentology, paleocurrent analyses, sandstone petrography, and conglomerate clast composi-tions. Despite their widespread usage, many of these methods have been applied to a limited degree in the Eastern Cordillera of Colombia.

GEOLOGIC BACKGROUND

In northwestern South America, the Ama-zon and Orinoco lowland drainages in the east and the Andean highlands in the west domi-nate the physiography of Colombia (Fig. 1). At 2°N–8°N, the major ranges forming the Andes (the Western, Central, and Eastern Cordilleras) are the result of complex interactions among the Nazca, Caribbean, and South America plates. In the east, Precambrian basement of the Guyana Shield has defi ned a stable region throughout

6°N

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NAZCAPLATE

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Figure 1. Map of northwestern South America illustrating the major tectonomorphic provinces (WC—Western Cordillera; CC—Central Cordillera; EC—Eastern Cor-dillera; SMB—Santa Marta block; MA—Merida Andes; MV—Magdalena Valley), after Mora et al. (2006). Dashed boxes denote map locations of Figures 2 and 3.

Phanerozoic time (Cordani et al., 2000). The Western Cordillera is composed of oceanic and arc terranes accreted to the stable continent since Late Cretaceous time (e.g., McCourt et al., 1984). In contrast, the Central Cordillera consists of Paleozoic metamorphic rocks over-printed by intrusive and extrusive igneous rocks of a Jurassic–Cretaceous magmatic arc (Aspden et al., 1987). The Eastern Cordillera represents a Cretaceous extensional basin system that was tectonically inverted during Cenozoic shortening (Colletta et al., 1990; Cooper et al., 1995; Mora et al., 2006; Sarmiento-Rojas et al., 2006). This inversion process partitioned a previously con-tiguous foreland basin east of the Central Cordil-lera (Cooper et al., 1995; Horton et al., 2010b).

Rifting took place in the pre-Andean back-arc area during Late Triassic to Early Creta-ceous time (Roure et al., 2003; Sarmiento Rojas et al., 2006). Up to 3 km of synrift deposits (Jurassic Girón Formation) were deposited in fault-bounded, approximately N-trending gra-bens (Kammer and Sánchez, 2006; Mora et al., 2009). Late-stage extension promoted fur-ther accumulation in the area now occupied by the Eastern Cordillera (Sarmiento-Rojas et al., 2006), leading to the deposition of up to ~5 km of Lower Cretaceous shallow-marine units. These units are covered by 1.5–2 km of Upper Cretaceous, quartz-rich marine strata, deposited during postrift thermal subsidence (Sarmiento-Rojas et al., 2006). The Upper Cretaceous section has two glauconite-rich units useful for tracking provenance and unroofi ng of the Eastern Cordillera: the Albian Une Formation and Campanian Guadalupe Group. In the Paleo-gene, a foreland basin system evolved to the east of the uplifted Central Cordillera (Cooper et al., 1995; Gómez et al., 2005b). More than 5 km of Cenozoic sediments were deposited in this fl ex-ural depocenter, recording the transition from marginal marine to nonmarine conditions (Parra et al., 2010). Subsequent shortening and uplift of the Eastern Cordillera divided this contiguous foreland basin into a western hinterland (Middle Magdalena Valley) basin and eastern foreland (Llanos) basin, with both basins accommodat-ing 3–5 km of Neogene clastic sediments (Hor-ton, 2011; Saylor et al., 2011).

In the Eastern Cordillera, reactivation of Mesozoic basement-involved normal faults and growth of fi rst-generation shortening structures produced a doubly vergent, ~200-km-wide fold-and-thrust belt (Colletta et al., 1990; Dengo and Covey, 1993; Cooper et al., 1995; Toro et al., 2004; Mora et al., 2006, 2010). Between the high topography of the axial Eastern Cordillera (including Soapaga fault and Floresta basin) and fl at lowlands of the Llanos basin, there lies the frontal fold-and-thrust region informally


Geological Society of America Bulletin, January/February 2012 61

referred to as the eastern foothills (Fig. 2). This zone constitutes the surfi cial expression of thin-skinned thrusts that commonly correspond to footwall shortcuts of reverse faults that origi-nally formed the master normal faults bound-ing Mesozoic extensional basins (Mora and Parra, 2008). A key structure within the eastern foothills is the NE-trending Nunchía syncline, bounded by the Guaicaramo fault to the west and Yopal thrust to the east (Figs. 2 and 3). The Guaicaramo fault was activated during Ceno-zoic shortening to produce a complex fault-bend fold to duplex system along the western

fl ank of the SW-plunging Nunchía syncline. The corresponding eastern fl ank is formed by the Yopal thrust, the modern topographic front of the fold-and-thrust belt (Bayona et al., 2008). East of this structure, an ~6-km-thick Cenozoic sedimentary succession underlies the relatively undeformed Llanos foreland basin.

STRATIGRAPHY AND SEDIMENT SOURCES

Cenozoic strata of the Eastern Cordillera onlap eastward onto the Mesozoic substratum

of the Llanos basin. A basal Paleocene suc-cession consists of up to ~700 m of estuarine and coastal-plain deposits of the Barco and Los Cuervos Formations (Cooper et al., 1995). The overlying Eocene–Pliocene succession along the Eastern Cordillera–Llanos basin boundary involved deposition of an upward-coarsening, ~5-km-thick succession (Fig. 4). At the base, the Eocene Mirador Formation (Jaramillo et al., 2009) consists of an ~200-m-thick interval of sandstone and subordinate mudstone concor-dantly overlying Paleocene strata in sharp con-tact. The Mirador was described in the Virgen

Fig.3

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Upper Cretaceous Jurassic

Triassic intrusive

Quaternary alluvium

FLO

RESTABASIN

Yopal

Tunja

Pre-Devonian

Devonian-Carbonif.

Figure 2. Geologic map of the Eastern Cordillera of the Colombian Andes at ~5°N–6.5°N latitude, modifi ed from Mora et al. (2010), showing regional structures and locations of U-Pb samples (green rings), (U-Th)/He samples (blue rings), and both U-Pb and (U-Th)/He samples (red rings). Dashed box denotes map location of Figure 3.

Bande et al.


measured section along the eastern limb of the Monterralo anticline (Fig. 3).

The overlying uppermost Eocene–lower Miocene Carbonera Formation is subdivided into eight informal units in the Llanos basin: members C1 to C8, with odd numbers assigned to sandstones and even numbers to mudstones (e.g., Parra et al., 2009b). The base of the 1500–2500-m-thick formation contains a basal member (C8) assigned to the late Eocene (Jara-millo et al., 2009). Lower mudstone members form a regional seal present in numerous oil and gas fi elds in the Llanos basin (Cazier et al., 1995). A localized unconformity at the base of the C5 member has been proposed on the basis of palynological data and subsurface structural relationships (Martinez, 2006). Two main strati-graphic sections (Tocaría and Buenavista) were measured in the upper Carbonera Formation, including the C1 to C4 members, with a total thickness of ~900 m (Fig. 3). The sections were

correlated using the Huesser fossiliferous hori-zon described in the Medina basin (Gómez et al., 2009) and its northern age-equivalent hori-zon in the C2 member of the Tocaría section.

A continuously exposed, 650-m-thick sec-tion of the León Formation was measured along the Tocaría River (Fig. 3). This middle Miocene section consists of dark laminated mudstone and shale that recorded tidally infl uenced lacustrine deposition with short-lived marine incursions (Bayona et al., 2008; Parra et al., 2010). The top of the León Formation refl ects the fi nal marine infl uence in the system.

Approximately 3000–3500 m of clastic sedi-ments were deposited from late Miocene to Pliocene time in the Llanos basin (Cooper et al., 1995). The Guayabo Formation (Hubach, 1957) includes varicolored mudstone, lithic sandstone, and conglomerate, with coarser lithologies dominant toward the top (Bayona et al., 2008). Four stratigraphic sections of the fi ner-grained

lower Guayabo Formation were measured and correlated near the axis of the Nunchía syncline, using the León-Guayabo contact as a correla-tion horizon (Fig. 3). The maximum thickness of the exposed sections is 900 m. For the con-glomeratic upper Guayabo Formation, only one section (Rincón del Soldado) could be measured because of the nearly horizontal bedding and limited areal extent (Fig. 3). According to Parra et al. (2009a), the unit has an exposed mini-mum thickness of ~700 m in the Medina basin ~100 km along strike to the southwest.

Numerous palynological assemblages pro-vide Cenozoic age constraints for Eastern Cordillera deposits. The base of the Mirador Formation corresponds to pollen zone T05, yielding an age of ca. 55 Ma (Jaramillo et al., 2009). Parra et al. (2009a) assigned ages of ca. 36 and ca. 23 Ma to the C8 and C6 members, respectively, of the Carbonera Formation. Simi-larly, Parra et al. (2010) assigned ages of ca. 19,

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Mirador Formation (Em)

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Quaternary deposits (Qal)

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Figure 3. Geologic map of the eastern foothills region of the Eastern Cordillera showing the main structures, regional stratigraphy, and location of six measured stratigraphic sections.



ca. 16, ca. 10, and ca. 5 Ma to the C2 member of the Carbonera, base of the León Formation, base of the lower Guayabo Formation, and base of the upper Guayabo Formation, respectively.

Sediment source areas in the northern Andes include rocks ranging from Proterozoic to Cenozoic age. Intrusive and metamorphic ages for granitic and gneissic basement rocks of the Guyana Shield are reported as 1600–1450, 1350–1250, and 1100–900 Ma (Teixeira et al., 1989; Goldstein et al., 1997), with the Grenvil-

lian ages (1100–900 Ma) related to assembly of Rodinia (Dalziel, 1991; Ramos, 2010). In the Eastern Cordillera, basement outcrops toward the eastern margin (Santander and Floresta massifs) and along the southern Llanos foot-hills segment (Quetame and Garzón massifs) exhibit Mesoproterozoic, Grenville-age signa-tures (Restrepo-Pace et al., 1997; Cordani et al., 2005; Cardona et al., 2010). However, fi eld relationships between intrusive and Paleozoic rocks (Irving, 1975; Suarez, 1990) and recent geochronological data (Horton et al., 2010b) from the Quetame massif at ~4°N support early Paleozoic (520–420 Ma) magmatism potentially linked to subduction. The Central Cordillera is formed by subduction-related, calc-alkaline igneous arc rocks of Jurassic (170–150 Ma), mid- to Late Cretaceous (120–70 Ma), and Paleogene (60–40 Ma) age overprinting Paleo-zoic metamorphic rocks (Aspden et al., 1987; Restrepo-Pace, 1992). Sediment recycling potentially complicates interpretation of detri-tal age signatures and compositional trends. In Colombia, many Phanerozoic sedimentary units have experienced some degree of reworking during the tectonic episodes that affected the northern Andes. Although the present study is focused on the sedimentary history of Cenozoic shortening and exhumation in the Eastern Cor-dillera, it is critical to recognize that an earlier cycle of sedimentation affected the region dur-ing Mesozoic extension and basin evolution.

DEPOSITIONAL SYSTEMS

The Eocene–Pliocene succession of the east-ern foothills is exposed in synclines adjacent to the main thrust faults. At ~6°N, the Nunchía syncline contains key exposures in the footwall of the Guaicaramo fault (Figs. 2 and 3). We documented sedimentological characteristics from six measured stratigraphic sections total-ing 4.8 km in thickness (Fig. 3). These sections provide the foundation for descriptions of sedi-mentary lithofacies and interpretations of depo-sitional processes. In total, 20 lithofacies modi-fi ed from Miall (1996) and seven lithofacies associations (this study; Table 1), detailed in the GSA Data Repository item (Figs. DR1–DR3),1 are defi ned on the basis of lithology, texture, grain size, bedding geometry and thickness, stacking patterns, and sedimentary structures.

Lithofacies were identifi ed as follows. For fi ne-grained deposits, three mudstone lithofa-

Upper

Lower

Figure 4. Generalized middle Eocene– Pliocene stratigraphic section of the eastern foothills of the Eastern Cordillera and Lla-nos basin (modifi ed from Parra et al., 2009a).

1GSA Data Repository item 2011300, Supple-mental data for lithofacies, point-count, U-Pb, and (U-Th)/He analyses, is available at http://www.geo-society.org/pubs/ft2011.htm or by request to [email protected].

cies were defi ned: massive claystone (Fm), laminated siltstone (Fl), and siltstone with paleosols (Fps). In total, 12 sandstone lithofa-cies were distinguished. Lenticular bedded sandstone (Sle), wavy bedded sandstone (Sw), and fl aser bedded sandstone (Sf) can be grouped into a heterolithic suite. Two types of massive sandstone were determined based on their bed geometry: massive lenticular sandstone (Sml) and massive tabular sandstone (Smt). Cross-stratifi cation patterns helped distinguish fi ve additional lithofacies: horizontally stratifi ed sandstone (Sh), ripple cross-stratifi ed sand-stone (Sr), planar cross-stratifi ed sandstone (Sp), trough cross-stratifi ed sandstone (St), and swaley cross- stratifi ed sandstone (Ssw). Scour surface (Ss) and bioturbated sandstone (Sb) lithofacies were also defi ned. Four conglomer-atic lithofacies were identifi ed: clast-supported massive conglomerate (Gcm), clast-supported imbricated conglomerate (Gci), planar cross-stratifi ed conglomerate (Gp), and trough cross-stratifi ed conglomerate (Gt). Finally, we also identifi ed a coal-bearing lithofacies (C).

Lithofacies are grouped into seven lithofacies associations (Table 1; Table DR1 [see footnote 1]) attributed to different depositional environ-ments. Lithofacies association 1, 2, and 7 are interpreted as components of fl uvial systems. Association 1 is composed of mudstones with interbedded sandstones representing overbank deposits. Amalgamated channelized sandstones of association 2 are interpreted as channel deposits of medium-energy streams. Associa-tion 7 consists of conglomerates and interbed-ded cross-stratifi ed sandstones interpreted as bed-load dominated, poor- to well-confi ned channels. Lithofacies associations 3–6 are inter-preted as part of a coastal environment. Litho-facies 3 is composed of upward-coarsening cross-stratifi ed quartzose sandstones interpreted as wave-dominated coastal deposits. Upward-fi ning bioturbated sandstones of association 4 represent estuarine distributary channels. Mud-stones of association 5 are interpreted as pro-delta deposits. Finally, association 6 contains upward-coarsening sandstones representing dis-tal to medial sand bar/ridge systems.

Depositional Synthesis

On the basis of observed lithofacies, the Mirador Formation is considered to represent deposition in a distal fl uvial to marginal marine environment. In the lower Mirador Formation, the organization of lithofacies associations 1 and 2 shows a high degree of amalgamation and near absence of overbank deposits, suggesting relatively high channel migration and deposi-tion in a low-accommodation basin. Warren and

Bande et al.


Pulham (2002) described nonmarine palyno-morphs consistent with a fl uvial setting for the lower Mirador Formation. In the upper Mirador Formation, above a proposed intraformational unconformity (Villamil, 1999; Warren and Pul-ham, 2002), association 3 is consistent with coastal wave-dominated deposition. The verti-cal facies organization of upward- coarsening, progradational sandstone intervals tens of meters thick represents episodes of shoreline regression capped by thin transgressive inter-vals (Van Wagoner et al., 1990; Hampson and Storms, 2003).

The upper Carbonera and León Formations are interpreted as deltaic facies and wave- dominated shorelines superseded by back-stepping tidally infl uenced estuaries. Accord-ing to Willis (2005), tide-dominated deltas are a consequence of bayhead delta progradation over estuarine systems, but that transition may be quite gradual. The lithofacies and vertical arrangement of facies associations 4, 5, and 6 characterize a clastic tongue that gradually built outward in a series of regressive-transgressive cycles. Progradation of active distributaries generates a gradational, upward-coarsening, and shallowing succession that passes from prodelta mudstone into delta-front sandstone and subordinate mudstone (Dalrymple, 1992). This context is appropriate for the numerous upward-coarsening sequences observed in mea-

sured sections. Finally, in a coastal setting, these deposits are erosionally overlain by distributary channels and fi ne-grained deposits of a tidally infl uenced estuary (Dalrymple, 1992), consis-tent with association 4. Back-stepping estuarine facies favor the introduction of marine/brackish traces into the estuary mouth more easily than in a deltaic setting, because of the common strong fl ood tidal currents.

The coastal environment switches to a non-marine setting with the accumulation of fl uvial sediments of the Guayabo Formation. This unit represents the Miocene onset of nonma-rine deposition in the Llanos basin, a trend that has continued to present. Lenticular amalgam-ated sandstones tens of meters thick overlain by fi ner-grained overbank and sandy crevasse splay deposits indicate an overall anastomos-ing river system (Miall, 1996) for the lower Guayabo Formation. The lack of lateral channel migration suggests signifi cant channel stabil-ity, vertical aggradation, and relatively straight channel planform geometries (Uba et al., 2005). The vertical stacking of sand bodies could be due to aggradation with minor shifting of chan-nel bars associated with channel switching (Bridge, 1993). The presence of paleosols sug-gests that channels were relatively stable prior to avulsion and migration. Facies of the upper Guayabo Formation suggest deposition in high-energy bed load–dominated fl uvial channels

and corresponding overbank areas. The upward- coarsening progradational character, avulsive channel behavior, lack of matrix- supported con-glomerate, and coupled channel and overbank facies suggest deposition in a fl uvial mega-fan or distributive fl uvial system (Horton and DeCelles, 2001; Hartley et al., 2010).

SEDIMENT DISPERSAL

Methods

Paleocurrent indicators were measured wher-ever possible in the stratigraphic sections span-ning a 20 × 40 km distance in the Nunchía syn-cline (Fig. 3). Sedimentary structures suitable for these measurements are rare in the Carbo-nera and León Formations because of the fi ne grain size, high bioturbation, and poor exposure of these units in the eastern foothills. The paleo-current database consists of 669 trough-cross limb measurements (method I of DeCelles et al., 1983) at 38 locations and 50 conglomerate clast imbrications at three sites.

Results and Interpretation

Paleocurrent indicators for the Eocene Mira-dor Formation show a clear NW-directed trans-port direction (Fig. 5A), implying a sediment source in the Amazonian craton. Up-section,

TABLE 1. LITHOFACIES ASSOCIATIONS AND RELATED LITHOFACIES

Facies association Lithofacies Description Thickness (m)

Interpretation Occurrence

FA 1. Mudstones with interbedded sandstones

Fm, Fl, Fps, Smt, Sml, Sr

Laterally extensive, tabular, laminated or massive mudstones interbedded with sandstone beds. If present, nodular blocky horizons to the top of the association. Upward-fi ning lenticular and sheet-like, sharp-based fi ne sandstones intercalated.

5 to 20 Floodplain deposits produced by avulsion of the main stream. Sandstone beds represent crevasse splay and levee deposits.

Lower Mirador and lower and

upper Guayabo

FA 2. Channelized amalgamated sandstones

Ss, St, So, Sh, Sml, Sl

Fining-upward stacked sandstone successions. The beds show lenticular erosive surfaces, and unit grades upward into FA 6. Sedimentary structures: small- to medium-scale trough and planar cross-stratifi cation, massive and horizontal cross-lamination.

5 to 20 Main channel deposits of a medium-energy stream.

Lower Mirador and lower and

upper Guayabo

FA 3. Upward-coarsening cross-stratifi ed quartzose sandstones

Smt, Sw, Sh, Fl

Upward-coarsening siltstone to sandstone sequences. Sedimentary structures: horizontal lamination and swaley cross-stratifi cation. Cruziana and Skolithos ichnofacies burrows.

15 to 20 Wave-dominated coastal deposits. Upper Mirador

FA 4. Upward-fi ning bioturbated sandstones

Sml, Sb, St, Fl, Fm, C

Upward-fi ning sandstone deposits intercalated with laminated siltstones. Most of the beds have lenticular shapes and erosive bases. Bioturbation degree is higher to the top of the beds. Organic matter and coal fragment layering in the fi nes.

<20 Distributary channels and interdistributary areas on a deltaic plain subenvironment.

León - Carbonera

FA 5. Mudstones Fm, Fl, Sb, Sf, Smt, Sml

Dark-gray mudstones with no visible structures. Intercalations of fl aser laminated sandy units in lenticular to tabular-shaped beds. Rare Thalassionoides burrows. Molluscan-bearing marine horizon.

>100 Prodelta deposits. León - Carbonera

FA 6. Upward-coarsening sandstones

Sh, Sle, Sf, Sw, Sr, Sp, Fm, Smt, C

Upward-coarsening and thickening, fi ne to medium sandstones. Lower units can have current ripple, horizontal, fl aser, wavy or lenticular lamination. Planar cross-stratifi cation is abundant up-section in the sequence Mud drapes and plant fragments are common. Bioturbation is rare.

20 to 80 Prograding sand bar/ridge systems in a delta front, tidally infl uenced.

León - Carbonera

FA 7. Conglomerate and interbedded cross-stratifi ed sandstone

Gci, Gcm, Gt, Gp, Sh, St, Sp

Erosive-based, imbricated, normally graded, clast-supported and cross-stratifi ed conglomerates. Intercalated cross-stratifi ed coarse sandstones.

<20 Shallow, gravelly, poor- to well-confi ned channels of fl uvial megafan system.

Upper Guayabo



an important change occurs in paleocurrent patterns: the Carbonera and León Formations present more variable directions but display a general eastward trend (Fig. 5B), implying an Andean source area to the west. We note that the limited density of measurements in these units and coastal depositional environment generally promote greater paleocurrent variability. Never-theless, a broad eastward fl ow is suggested.

Correlation of four measured sections (Fig. 6), with thicknesses between 500 and 900 m, shows eastward paleofl ow for the lower ~150 m of the lower Guayabo Formation (Fig. 5C). This trend diverges in younger beds, with an average fl ow toward the south for the rest of the lower Guayabo section (Fig. 5D). This change from eastward to southward fl ow is attributed to the growth of a fl uvial drainage divide, probably controlled by surface uplift along the eastern fl ank of the Nunchía syncline. Finally, in the upper Guayabo Formation, there is a return to generally eastward paleofl ow (Fig. 5E), interpreted as fl uvial transport trans-verse to the evolving deformation front with suffi cient stream power to erode through topog-raphy generated by the frontal thrust.

PROVENANCE

Sandstone Modal Compositions

MethodsSandstone modal framework compositions

were collected from 49 standard petrographic

thin sections of samples obtained from mea-sured sections across the Nunchía syncline (Fig. 3). Thin sections were stained for potas-sium and calcium feldspar, and 450 grains per thin section were counted according to the Gazzi-Dickinson method (Ingersoll et al., 1984). Framework grains (>0.0625 mm) were classifi ed using the petrographic parameters listed in Table DR2 (see footnote 1). Recalcu-lated compositional modal data are shown in Table DR3 (see footnote 1).

ResultsResults of petrographic modal analyses are

presented in ternary diagrams depicting total quartz–feldspar–lithic fragment (Qt-F-L) and quartz–feldspar–lithic fragment (Q-F-L) pro-portions (Fig. 7) with single-sample point counts arranged in stratigraphic order (Fig. 8). Mean sandstone compositions and 1σ error polygons are depicted for each unit. The entire sample set contains >60% quartz, possibly due to the extremely high weathering conditions associated with the tropical climate of Colombia (Johnsson et al., 1991).

Quartz is classifi ed as monocrystalline (Qm), polycrystalline (Qp), and foliated polycrystal-line (Qpf) grains. Lithic fragments (Ls) are primarily siltstone (Lsi) and chert, with minor amounts of claystone (Lc) and metamorphic fragments (Lm). Volcanic grains are very rare. Feldspar is rare, and it is extremely weathered where present, precluding consistent accurate distinction between potassium feldspar and pla-

N

E

S

W

n=704 stations

vector mean: 314°

A. MiradorN

E

S

W

n=1297 stations

vector mean: 92°

B. Carbonera-LeónN

W E

S

n=1106 stations

vector mean: 107°

C. Lower Guayabo I

N

W E

S

n=32921 stations

vector mean: 174°

D. Lower Guayabo IIN

E

S

W

n=1185 stations

vector mean: 148°

E. Upper Guayabo

Figure 5. Paleocurrent data for the (A) Mirador, (B) upper Carbonera and León, (C, D) lower Guayabo, and (E) upper Guayabo Formations. Mean paleocurrent vector, number of measurements, and stations are listed for each rose diagram.

gioclase. Notable accessory minerals include micas, amphibole, pyroxene, and detrital (non-authigenic) glauconite.

Sandstones from the Eocene Mirador For-mation consist of moderately sorted, well-cemented quartzarenites (Fig. 7A). Monocrys-talline quartz (Qm) is the dominant constituent, with subordinate polycrystalline quartz (Qp). Lithic grains are dominated by chert fragments with feldspars composing less than 2% of the modal composition.

The overlying Carbonera sandstones con-sist of both quartzarenites and sublitharenites (Fig. 7A). Sorting is more variable, and samples are less well cemented. The lower Carbonera (C8–C5 members) samples are quartzarenites with Qm as the main component and secondary amounts of Qp. In contrast, the upper Carbonera (C4–C1 members) is composed of sublitha-renites, also with Qm as the main constituent, but with minor Qp and sedimentary lithic (Ls) fragments (up to 8% of the modal composi-tion). Feldspar grains represent less than 3% of counted grains. Importantly, these upper Carbo-nera samples record the fi rst appearance of glau-conitic sandstone grains (Fig. 8).

Middle Miocene León sandstones are clas-sifi ed as sublitharenites and help defi ne an up- section trend toward more lithic-rich com-positions (Fig. 7A). Here, Qm is the major component, representing >76%, whereas Ls constitutes up to 10% in some samples and Qp up to 15% of the total modal composition. Glau-conitic grains are still present (Fig. 8).

Guayabo sandstones are composed of mod-erately sorted, subrounded sublitharenites and litharenites that continue the overall trend toward increased lithic content (Fig. 7A). Qm is the main component of lower Guayabo samples, always exceeding 66%. Ls represents as much as 14% and feldspar up to 5% in some samples. An abrupt decrease in the quantity of glauconitic grains is observed within the lower Guayabo Formation (Fig. 8). For the middle Guayabo Formation, Qm remains the main component, constituting ~75% of the total composition. Upper Guayabo samples range from sublithar-enites to litharenites (Fig. 7A). Upper Guayabo sandstones are intercalated with conglomeratic beds and are moderately sorted and cemented. Ls constitutes up to 29% in some samples, but Qm remains the major component.

InterpretationExtremely Qm-rich sandstones of the Mira-

dor and lowermost Carbonera Formations are consistent with derivation from a cratonic source (Fig. 7). These units have relatively low amounts of sedimentary lithic fragments (Ls), in contrast to typical recycled orogen provenance

Bande et al.


South-directedpaleoflow

East-directed paleoflow

HURON 1500 m

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n=15

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n=19

n=17

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RINCÓN DEL SOLDADO

n=24

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Base of Guayabo Fm.

PALEOCURRENT SHIFT

cs fmc gpcb

cs fmc gpcb

cs fmc gpcb

cs fmc gpcb

Scour and fill structuresRoots - paleosolsBurrows - bioturbation

PALEOCURRENT DATAn = number of measurements

GRAIN SIZE

n

cs fmc gpcb

c - clay s - silt f - fine sand m - medium sand c - coarse sand g - granule p - pebble c - cobble b - boulder

SEDIMENTARY STRUCTURES

Figure 6. Measured stratigraphic sections of the middle Miocene León and lower Guayabo Formations in the Nunchía syn-cline. Stratigraphic correlations are based on the top of the León Formation. Horizon-tal gray band represents the stratigraphic position of major paleocurrent shift.



(Fig. 7B) (e.g., Dickinson, 1985). Upper Car-bonera sandstones represent the lowest unit containing compositional evidence for ero-sion and recycling of the axial Eastern Cordil-lera (Fig. 7B). This is supported by an abrupt increase in Ls and appearance of detrital glau-conitic grains (Fig. 8). The lowest stratigraphic occurrence of glauconitic grains in Carbonera sandstones likely marks initial erosion of the glauconite-bearing, Upper Cretaceous section in the axial Eastern Cordillera.

Modal compositions for León sandstones are further consistent with derivation from a fold-and-thrust belt source (e.g., Dickinson, 1985). However, the sedimentary lithic con-tent is less than in underlying strata (Fig. 8). This trend may refl ect unroofi ng of the Lower Cretaceous section (Berriasian–Aptian) com-posed of mudstone-dominated units (Macanal and Fómeque Formations). In addition, severe tropical weathering (e.g., Johnsson et al., 1991) likely prevented unstable grains from reaching the distal Llanos basin to the east.

Guayabo sandstones continue the trend toward less mature composition with an over-all increase in lithic content (Fig. 7B), consis-tent with continued erosion and exhumation of the Eastern Cordillera sedimentary succession. The abrupt Ls increase (Fig. 8) together with the up-section decrease of glauconitic grains in the middle and upper Guayabo Formation may suggest recycled sedimentary material in

a new sediment source closer to the study area, thus reducing transport distances and weather-ing effects. In this context, uplifted basin fi ll in intermontane regions of the Eastern Cordillera (e.g., Floresta basin; Fig. 2) may have provided recycled sediment to the Llanos basin. The new source could be explained by exposure and recy-cling of the Oligocene–Lower Miocene Car-bonera Formation. In this case, the up-section decrease in glauconitic content for the lower Guayabo Formation could be related to ero-sional removal of the glauconite-bearing lower Carbonera Formation and tapping into deeper, Paleocene–Eocene levels of the Cenozoic suc-cession. Finally, for the upper Guayabo Forma-tion, sandstone compositions show a relative increase in sedimentary lithic fragments and an absence of glauconitic grains (Fig. 8). This pat-tern is consistent with nearly complete erosion of the Cenozoic section west of the Nunchía syncline and exposure of the Upper Cretaceous Guadalupe Group.

Conglomerate Clast Compositions

MethodsCollection of conglomerate compositional

data was conducted at eight outcrop localities. Square grids of at least 30 cm length were drafted on the outcrop, and 100 clasts were counted per location. Compositional clast count data are available from the middle and

upper Guayabo Formation. Although many clasts cannot be identifi ed with confi dence at the formation level, some are indicative of specifi c intervals. The lower part of the Cretaceous (Berriasian–Aptian) section in the Eastern Cordillera is mostly fi ne grained with thick black mudstone in the Macanal and Fómeque Formations. Paleogene fi ne-grained lithologies from the muddy Los Cuervos and Carbonera Formations are possible sources for the siltstone fragments observed in clast counts. On the other hand, quartzarenites in the eastern foothills are typically derived from the Albian to Campanian section (Gua-dalupe Group), which is mostly composed of shallow-marine sandstones originating from the Guyana Shield.

ResultsThe principal conglomerate clasts in the

middle Guayabo Formation (Fig. 9) are quartza-renite (48%), with minor chert (19%), siltstone (13%), and litharenite (13%). Glauconitic aren-ite is rather limited (6%), and micaceous quartz-arenite occurs in trace amounts (1%).

In the upper Guayabo Formation (Fig. 9), the majority (71%) of the conglomerate clasts are quartzarenite, with litharenite constituting ~10% of the composition. The percentages of chert and siltstone are substantially diminished, at 8% and 3%, respectively. Glauconitic clasts continue to occur in limited quantities (<1%).

Mirador - lowerCarbonera (n=12)

Lower Guayabo (n=13)

Middle Guayabo (n=4)

Upper Guayabo (n=3)

Upper Carbonera - León (n=17)

Recycled orogen

Qt

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Feldspathic litharenite

Lith

ic a

rkos

e

Feld

sare

nite

Quartz- arenite A

50% Q

Subf

elds

aren

ite Sublitharenite

Figure 7. (A) Q-F-L and (B) Qt-F-L ternary diagrams with a 50% quartz baseline for 49 sandstone samples from the Eocene–Pliocene section. Geometric symbols show mean values for different stratigraphic units, and polygons represent unit 1σ standard deviation errors. Sandstone nomenclature fi elds (Folk, 1980) and tectonic provenance fi elds (Dickinson, 1985) are included for classifi cation purposes.

Bande et al.


10080604020

Number of grains0

MON0617091-3

MON0617096

MOR0612091

MOR0612093

MOR0612095

MOR0613091

TOC0610091TOC0610093TOC0610095TOC0610096TOC0610098

TOC0610099TOC0108094

RSO0111096RSO0111098

RSO01110910RSO01110911RSO0112091RSO0112093

RSO0112094

RSO0112095

RSO0112097

CHI0119091

CHI0119092

VCB0114094

ANT0614091

MON0617094-5

Glauconitic grains

Sedimentary lithics

Upper

Lower

Figure 8. Eocene–Pliocene section of the eastern foothills showing 30 sandstone samples arranged stratigraphically, including glauconitic and sedimentary lithic compositional trends. Horizontal axis represents total number of grains based on 450 points counted per sample.

InterpretationAccording to the different sources proposed

for the conglomerate clasts, we interpret a principally Cenozoic source for the middle Guayabo Formation. The relatively high pro-portions of fi ne-grained clast lithologies could be explained by unroofi ng of the Paleogene section (Los Cuervos Formation) west of the Nunchía syncline, in agreement with the sand-stone modal compositions. In contrast, con-glomerate clast compositions for the upper Guayabo Formation suggest a provenance from the quartzarenite-dominated Upper Cre-taceous section (Guadalupe Group), which is most pronounced in the hanging wall of the Guaicaramo fault. This up-section shift in clast compositions is consistent with continuous late Miocene–Pliocene erosional exhumation of the Paleogene and then underlying Upper Cretaceous succession, potentially in a single thrust sheet in the fold-and-thrust belt west of the Nunchía syncline.

DETRITAL ZIRCON U-Pb GEOCHRONOLOGY

Methods

U-Pb geochronological analyses were conducted on detrital zircons separated from four new samples collected from Cenozoic sandstones of the Nunchía syncline and unconsolidated sand from modern rivers in the area. Samples were processed using stan-dard procedures described by Gehrels (2000), Gehrels et al. (2008), and Dickinson and Geh-rels (2008). Analyses were conducted using the multicollector–laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) at the University of Arizona LaserChron Center. Approximately 100 individual zircon grains were analyzed from each sample. Zir-cons were selected randomly from all sizes and shapes, although grains with signifi cant cracks or inclusions were avoided. In-run analyses of fragments of a large zircon crystal with known age of 564 ± 4 Ma (2σ error) were conducted every ~5 measurements in order to correct for inter- and intra-element fraction-ation. The uncertainty resulting from the cali-bration correction is generally 1%–2% (2σ error) for both 206Pb/207Pb and 206Pb/238U ages. The analytical data are reported in Table DR4 (see footnote 1). Details of the operating con-ditions and analytical procedures are provided by Gehrels et al. (2008). Analyses exhibiting >20% uncertainty, >30% discordance (by comparison of 206Pb/238U and 206Pb/207Pb ages), or >5% reverse discordance are omitted from further consideration.

Middle Guayabo Upper Guayabo

n=400 n=400

A B

Figure 9. Conglomerate clast compositional data for the (A) Middle Guayabo and (B) Upper Guayabo Formation.



In total, 330 new zircon ages from four samples are reported here. Also included in this discussion are 498 zircon ages from fi ve additional samples previously reported by Horton et al. (2010b). The preferred ages are shown on normalized relative age-probability diagrams (Fig. 10); these dia-grams show each age and its uncertainty as a nor-mal distribution, summing all ages and uncertain-ties from a sample into a single age-distribution curve. Because inclusion of slightly discordant analyses will add some degree of scatter to the age spectra, individual age peaks are considered robust only if defi ned by three or more analyses (see Dickinson and Gehrels, 2008).


Nine samples ranging from middle Eocene basin fi ll to modern river sands show varia-tions in age spectra refl ective of progressive Andean deformation in the Eastern Cordillera. The Eocene Mirador Formation has prominent age peaks at 1450, 1550, and 1750 Ma, with an absence of statistically signifi cant ages younger than ca. 1200 Ma (Fig. 10A). The Mirador age distribution is assigned to a dominant eastern source of Neoproterozoic and Mesoproterozoic rocks in the Guyana Shield, consistent with pre-vious studies (e.g., Cooper et al., 1995; Roure et al., 2003; Horton et al., 2010b).

A major shift in provenance is recorded dur-ing Oligocene sedimentation. In the lower Car-bonera Formation (C7 member), a signifi cant U-Pb age peak at 200 Ma (Fig. 10B) represents the introduction of west-derived (Andean) grains in the eastern foothills. Magmatic rocks in the Central Cordillera would seem the most logical source for Jurassic to Paleogene ages (Nie et al., 2010). However, the presence of Paleoproterozoic ages (peaks at 1650 and 1800 Ma) in the analyzed C7 sample requires an additional source. The composite signal could refl ect mixing of two direct sources: the Central Cordillera in the west and the craton in the east. Alternatively, the shift could refl ect recycling of craton- and arc-derived zircons from Paleocene–Eocene basin fi ll in the axial Eastern Cordillera (e.g., Floresta basin; Fig. 2). We favor the second option, which is consistent with eastward paleofl ow (Bayona et al., 2008;

Figure 10. Detrital zircon U-Pb ages for nine samples of Eocene–Pliocene strata in the Nunchía syncline. Normalized age probabil-ity plots (black lines) and age histograms (gray bars) are arranged in stratigraphic order. Data in A, D, E, F, and G are from Horton et al. (2010a).

(A) Eocene Mirador, n=93

(E) Lower Miocene Carbonera C1, n=1(E) Lower Miocene Carbonera C1, n=1(E) Lower Miocene Carbonera C1, n=114

(C) Lower Miocene Carbonera C5, n=91

(B) Oligocene Carbonera C7, n=82

(D) Lower Miocene Carbonera C2, n=78

(F) Middle Miocene lower Guayabo, n=109

(H) Holocene Cravo Sur River, n=71

(G) Pliocene upper Guayabo, n=104(G) Pliocene upper Guayabo, n=104(G) Pliocene upper Guayabo, n=104

(I) Holocene Cusiana River, n=86

2000 400 600 800 1000 1200 1400 1600 1800 2000

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Bande et al.


Parra et al., 2010) and independent evidence of initial uplift along the western and axial zones of the Eastern Cordillera (Gómez et al., 2003; Parra et al., 2009b; Nie et al., 2010; Saylor et al., 2011). Thus, we suggest that the earliest detrital evidence of shortening-induced denudation in the Eastern Cordillera is recorded by the mid-Oligocene C7 member.

Lower Miocene strata of the upper Carbo-nera Formation (members C5 and C2) show age spectra similar to the lower Carbonera (C7), with signifi cant peaks at 50–150, 1500, and 1800 Ma, and a minor peak at 900–1100 Ma (Figs. 10C and 10D). These samples refl ect continued ero-sion and recycling of the Paleogene section in the axial Eastern Cordillera. In the uppermost Carbonera member (C1), Mesozoic–Cenozoic peaks are statistically absent, and a major peak at 900–1100 Ma is observed with subordinate peaks at 1350 and 1500 Ma (Fig. 10E). This pattern suggests further exhumation in the axial Eastern Cordillera, with exposure of a Creta-ceous succession rich in Grenvillian zircons (900–1100 Ma; Horton et al., 2010b) by the lat-est early Miocene.

Age spectra for the middle Miocene lower Guayabo Formation are governed by signifi -cant peaks at 50, 90–1100, 1250–1350, and 1550 Ma (Fig. 10F). The reappearance of a Mesozoic–Cenozoic peak suggests erosion of a new Andean source. We attribute this trend to eastward advance of the deformation front, activating a new frontal fault (Guaicaramo fault) between the axial Eastern Cordillera and the eastern foothills, prompting erosion of the Oligocene–lower Miocene (Carbonera) section and/or Paleocene–Eocene (Barco, Los Cuervos, Socha, Picacho, and Concentración Formations) section west of the Nunchía syncline. This interpretation is supported by sandstone petro-graphic data, which reveal an abrupt increase in sedimentary lithic fragments and decrease in glauconitic grains.

The Pliocene upper Guayabo Formation has major age peaks at 900–1100, 1200, 1500–1600, and 1750–1850 Ma (Fig. 10G). The age spectra show no Phanerozoic ages. Here, we suggest that the Cenozoic sedimentary cover had been largely stripped off the main sediment source west of the Nunchía syncline, leaving Creta-ceous and older units as the principal source for the distal Llanos basin. The importance of the Grenvillian-age peak (900–1100 Ma) is con-sistent with substantial contribution from the Lower Cretaceous section with possible contri-butions from the lowest exposed levels (Jurassic and Paleozoic) of the Phanerozoic succession.

U-Pb results from modern river sand sam-ples show limited departures from the upper Guayabo age spectra (Figs. 10H and 10I).

Dominance of 900–1100 Ma peaks lends fur-ther support to continued exhumation of Gren-villian grains derived from Cretaceous and older strata in the Eastern Cordillera. Moreover, the increased proportion of two Mesoproterozoic peaks at 1500–1600 and 1150–1250 Ma repre-sents a good match to age spectra reported from Lower Cretaceous, Upper Jurassic, and Devo-nian–Carboniferous sandstones of the Eastern Cordillera (Horton et al., 2010b).

DETRITAL ZIRCON (U-Th)/He THERMOCHRONOLOGY

Methods

Detrital zircon (U-Th)/He ages are presented for composite samples from six representa-tive horizons within the Eocene–Pliocene suc-cession of the Nunchía syncline. The data set (Table DR5 [see footnote 1]) incorporates 14 new with 55 individual zircon (U-Th)/He ages previously reported by Horton et al. (2010a). Zircon (U-Th)/He thermochronology is an established technique involving a closure tem-perature of ~180–200 °C (e.g., Reiners, 2005). Because the sampled Eocene–Pliocene succes-sion has experienced limited Cenozoic heating

during 0–5 km of burial, we regard the measured (U-Th)/He ages of most samples as records of principally exhumational cooling associated with detrital zircon grains originally depos-ited in the Eastern Cordillera region. Detrital (U-Th)/He age determinations were carried out in the (U-Th)/He laboratory at the University of Kansas, following procedures described in Biswas et al. (2007). All ages were calculated using Fish Canyon and Durango zircon age standards, and alpha-ejection corrections based on morphometric analyses (Farley et al., 1996). Reported age uncertainties refl ect the reproduc-ibility of replicate analyses of the two standards, with estimated analytical uncertainties of ~8% (2σ) for zircon (U-Th)/He ages (Reiners, 2005). Results are grouped into stratigraphic units and plotted against stratigraphic age (Fig. 11) in order to visualize up-section changes in domi-nant populations.


Detrital zircon (U-Th)/He results show a dras-tic up-section shift in cooling ages. Whereas the Eocene–Oligocene Mirador and lower Carbonera Formations show principally Pre-cambrian–Paleozoic (850–300 Ma) ages, the

Eoc

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Zircon (U-Th)/He Age (Ma)100

upper Guayabo (N=8)lower Guayabo

(N=23)León (N=9)Carbonera C2 (N=10)Carbonera C7 (N=9)Mirador (N=9)

Figure 11. Detrital zircon (U-Th)/He data from the eastern foothills. The vertical axis rep-resents the stratigraphic age of the host formation, and the horizontal axis represents the (U-Th)/He age, with error bars, of each detrital grain. Arrows denote three anomalously young ages (see text for further details).



Miocene–Pliocene levels of the upper Carbo-nera, Leon, and Guayabo Formations are domi-nated by Cretaceous–Cenozoic (<150 Ma) ages (Fig. 11). The pre-Cretaceous age signatures are attributed to long residence time at upper-crustal levels in the stable Guyana Shield or recycled zir-cons from the Eastern Cordillera that never were buried to depths below the ~180 °C isotherm. In contrast, the Cretaceous–Cenozoic age signa-tures are interpreted as the product of recycling in the Eastern Cordillera of shallowly buried sedi-ments (<5 km) that originated from Andean arc and thrust-belt sources to the west. We link the stratigraphic shift in detrital zircon (U-Th)/He ages near the Oligocene–Miocene boundary to a major provenance change from eastern (cratonic) to western (orogenic) sources (e.g., Horton et al., 2010a), consistent with initial uplift of the eastern fl ank of the Eastern Cordillera.

Although the detrital zircon (U-Th)/He ages provide important provenance constraints, they also have the potential to identify key phases of Andean exhumation or magmatism. We con-sider three notable age populations (Fig. 11; Table DR5 [see footnote 1]). First, a 130–120 Ma population defi ned by fi ve (U-Th)/He analyses could be a product of unroofi ng during Early Cretaceous extension across the Eastern Cordillera (e.g., Sarmiento-Rojas et al., 2006; Horton et al., 2010b). Second, a 55–45 Ma population defi ned by 11 (U-Th)/He analyses potentially represents nonreset volcanogenic zircons from Paleocene strata of the Eastern Cordillera (e.g., Bayona et al., 2008; Horton et al., 2010a), even though limited detrital volca-nic lithic grains are represented in the sandstone modal compositions. Third, three (U-Th)/He analyses yield anomalously young ages of ca. 20–15 Ma, which are younger than or indis-tinguishable from the depositional ages of the Mirador and upper Carbonera Formations. These analyses could represent reset ages, con-sistent with apatite fi ssion-track evidence (Mora et al., 2010) for rapid early Miocene exhuma-tion in the footwall of the Soapaga thrust and hanging wall of the Pajarito fault (Fig. 2). Fur-ther possibilities include a volcanogenic origin (which would be at odds with the petrographic point counts), sample contamination, or anoma-lously nonretentive zircon grains potentially related to the degree of radiation damage (e.g., Flowers et al., 2009).

DISCUSSION

Sedimentological and provenance results out-line a Cenozoic history of varied sediment accu-mulation in the advancing foreland basin system of the Colombian Andes. Initial craton-derived depositional systems in the distal foreland were

replaced by the earliest signatures of Andean orogenic detritus. Thereafter, a series of new Andean sediment sources was introduced within the Eastern Cordillera, inducing progressively more proximal sedimentation with different compositional and geochronological signatures of sediment provenance. Next, we integrate new and existing data into a reconstruction of Cenozoic thrusting and basin evolution. Key upper-crustal structures and Cenozoic depos-its (observable in Fig. 2) provide the regional context for an interpreted six-step evolution of depositional systems in relationship to fold-and-thrust deformation in the frontal Eastern Cordil-lera and Llanos basin (Figs. 12A–12F).

(1) Results for the Eocene–Pliocene sedi-mentary record in the frontal Eastern Cordil-lera confi rm that the region was occupied by a foreland basin, in agreement with previous studies (e.g., Cooper et al., 1995; Bayona et al., 2008; Parra et al., 2009a). Craton-derived sandstones of the Eocene Mirador Formation record fl uvial to marginal marine deposition in a distal foreland basin, potentially in a back-bulge zone (DeCelles and Giles, 1996), similar to modern fl uvial drainages in the most-distal central Andean foreland basin (Horton and DeCelles, 1997). Sandstone modal composi-tions and paleocurrent indicators document a highly mature, quartz-rich source (Fig. 7) located to the southeast (Fig. 5A). In addi-tion, detrital zircon U-Pb and (U-Th)/He ages (Figs. 10A and 11) show erosion of Precam-brian sources that reached upper-crustal levels in Neoproterozoic–Paleozoic time and were not subsequently buried below the zircon helium partial retention zone (PRZ) of ~120–180 °C (Stockli, 2005). These results clearly implicate the Guyana Shield (northwestern Amazonian craton) as the principal source for Eocene strata along the Eastern Cordillera–Llanos basin transition (Fig. 12A). Although incipient Paleogene fold-and-thrust deforma-tion likely affected the Central Cordillera to westernmost Eastern Cordillera, the lack of western, orogenic sediment sources suggests that west-derived depositional systems did not reach the Llanos basin during Eocene time, potentially due to a low-amplitude forebulge in the eastern part of the Eastern Cordillera (Fig. 12A) (Saylor et al., 2011).

(2) The detrital record suggests that uplift-induced exhumation in the Eastern Cordillera had commenced by Oligocene time. Detrital zircon U-Pb ages for the lower Carbonera For-mation record a signifi cant decrease of cratonic input and initial recycling of Eastern Cordillera cover strata. Age spectra of the C7 member (Fig. 10B) show a mid-Oligocene introduction of west-derived Phanerozoic grains to the basin.

Although this age is unlikely to refl ect the pre-cise onset of Andean shortening in the Eastern Cordillera, it is consistent with an eastward advance of deformation involving middle-late Eocene exhumation along the westernmost Eastern Cordillera (eastern Magdalena Val-ley basin; Nie et al., 2010) and late Eocene– Oligocene deformation in the axial zone of the Eastern Cordillera (Floresta basin; Saylor et al., 2011). Modeled apatite fi ssion-track (AFT) data in the axial Eastern Cordillera constrain initial cooling in the hanging wall of the Soapaga fault (west margin of Floresta basin; Fig. 2) between 50 and 30 Ma (Parra et al., 2009b). However, detrital zircon (U-Th)/He ages for the lower Car-bonera Formation (Fig. 11) show no evidence of rapid Cenozoic exhumation of deeply buried rocks (below the PRZ). Therefore, we suggest that by Oligocene time, only the shallowly bur-ied strata in the hanging wall of the Soapaga thrust—namely, Paleocene–Eocene deposits of the Floresta basin (Fig. 2)—were undergoing uplift and erosion in the axial Eastern Cordil-lera, shedding sediments eastward into the study area (Fig. 12B).

(3) For the early Miocene, compositional and geochronological data from the upper Carbonera Formation indicate further exhu-mation in the axial Eastern Cordillera driven by continued motion along the Soapaga fault and activation of the Pajarito thrust to the east (Fig. 12C). Erosional breaching of the Upper Cretaceous (Guadalupe Group) succession in the Soapaga thrust sheet is evidenced by the fi rst appearance of glauconitic detrital grains (Fig. 8) accompanied by Grenville and other Proterozoic U-Pb ages (Figs. 10C and 10D) indicative of derivation from the Cretaceous section (Horton et al., 2010b). Initial exposure of Paleogene strata in the Pajarito thrust sheet is suggested by a drastic increase in sedimen-tary lithic fragments (Ls) (Fig. 8) together with a new Mesozoic– Cenozoic U-Pb age signal (C2 member; Fig. 10D), emblematic of con-tributions from Paleogene strata (Horton et al., 2010a). Detrital zircon (U-Th)/He analyses for the upper Carbonera Formation also show a drastic decrease in cooling ages by the time of C2 deposition, consistent with sediment source areas undergoing rapid, deep exhumation in the growing orogen (Fig. 11). Paleocurrents are fur-ther consistent with an uplifted Andean source to the west (Fig. 5B).

(4) Exhumation of the Pajarito thrust sheet in the latest early Miocene to middle Mio-cene represents an eastward advance of short-ening, following preexisting structures. We suggest that the Paleogene section exposed by displacement along the Pajarito fault was largely eroded away by middle Miocene time

Bande et al.


A

B

C

D

E

F



(Fig. 12D). This interpretation is supported by evidence for a dominant contribution from the Cretaceous section, including the lack of Phanerozoic U-Pb ages in the uppermost (C1) member of the Carbonera Formation (Fig. 10E), the relative abundance of glauconitic grains in C1 sandstone modal compositions (Fig. 8), and the presence of early Miocene detrital (U-Th)/He ages. Topographic growth in the hanging wall of the Pajarito thrust likely transformed the formerly extensive Floresta basin from an actively accumulating proximal foreland basin (foredeep) to an uplifted piggyback (wedge-top) basin (Fig. 12D). Nevertheless, an east-fl owing transverse river (Fig. 5B) apparently maintained an antecedent course across the growing deformation front, bypassing largely fi ne-grained sediments to the León Formation in the foreland basin depocenter. This fl uvial system eroded large parts of the Cretaceous sec-tion in the Soapaga thrust sheet and most of the Paleogene section uplifted by the Pajarito fault (Figs. 12C and 12D). A few young (U-Th)/He ages of ca. 16 Ma show a continued pattern of principally Cenozoic cooling (Fig. 11), consis-tent with exhumation along the Pajarito hanging wall. Following models for advancing foreland basins systems (Horton and DeCelles, 1997; DeCelles and Horton, 2003; Horton, 2011), we propose that a newly developed foredeep depozone (recorded by the upper Carbonera and León Formations) was situated directly east of

Figure 12. Schematic block diagram show-ing the Cenozoic evolution of depositional systems in relationship to fold-thrust defor-mation in the frontal zone of the Eastern Cordillera, Colombia. Gray dashed line identifi es the Nunchía (Nu) syncline study region. (A) Back-bulge deposition of prin-cipally fl uvial deposits (middle Eocene: Mirador Formation); (B) axial Eastern Cordillera uplift and onset of marine del-taic deposition (Oligocene: lower Carbo-nera Formation); (C) eastward advance of the deformation front and continued deltaic deposition (early Miocene: upper Carbonera Formation); (D) stalled thrust front with lacustrine-lagoonal deposition (early-middle Miocene: León Formation); (E) thrust-front advance and piggyback deposition with axial fl uvial transport par-allel to the growing frontal structure (mid-dle Miocene: lower Guayabo Formation); and (F) fl uvial megafan system with trans-port perpendicular to frontal structures of the fold-thrust belt (late Miocene–Pliocene: upper Guayabo Formation).

the Pajarito fault, in agreement with increased sediment accumulation rates (Parra et al., 2010).

(5) The eastward advance of deformation in the eastern foothills continued during middle–upper Miocene deposition of the lower Guayabo Formation. By this time, a more-proximal sedi-ment source is suggested by the eastward pro-gradational shift from fi ne-grained coastal facies to coarse-grained fl uvial deposits (Fig. DR1 [see footnote 1]) and enhanced proportion of sedi-mentary lithic fragments (Fig. 7). Zircon U-Pb age spectra for the lower Guayabo Formation show an introduction of new Cenozoic ages (Fig. 10F). We attribute these patterns, along with the continued trend of youthful (U-Th)/He ages (Fig. 11), to activation of the Guaicaramo fault (Fig. 12E), which resulted in exposure of Upper Oligocene–Lower Miocene strata in the elevated highlands of the eastern foothills (Fig. 12E). This episode of fault propagation introduced a younger, more-proximal source of sediment (principally recycled Cenozoic deposits) and triggered eastward progradation of deposystems.

(6) Sedimentological and provenance shifts in middle–late Miocene time are linked to thrust imbrication in the footwall of the Guaicaramo fault. The disappearance of Mesozoic– Cenozoic U-Pb ages in the upper Guayabo Formation (Fig. 10G) and in modern river sand (Figs. 10H and 10I) suggests that Paleogene strata have not been major contributors over the past ~10 m.y. In the absence of major along-strike variations, we consider this to be the combined product of erosional unroofi ng of most Paleogene rocks within the Guaicaramo thrust sheet and erosional recycling of the uppermost basin fi ll (upper Car-bonera and León Formations) due to activation of the Yopal imbricate thrust (shortcut fault) east of the Nunchía syncline (Fig. 12F). Sedi-ment dispersal data (Fig. 6) show that the earlier east-fl owing transverse river system diverted its course southward, parallel to the main structural trend. This drainage reorganization and change in basin confi guration denote the formation of an axial dispersal system within a structurally controlled wedge-top depozone bounded by the Yopal thrust (Fig. 12E). The upper Guayabo Formation exhibits the youngest (U-Th)/He ages in the Cenozoic succession, consistent with continued to accelerated rates of exhumation. Upper Guayabo conglomerates show a domi-nantly quartzarenitic composition (Fig. 9) and zircon U-Pb spectra lacking Phanerozoic ages (Fig. 10G). These observations are consistent with continued exhumation along the Guaicar-amo fault, implying nearly complete removal of the Cenozoic section and widespread exposure of the Cretaceous succession in the elevated interior of the Eastern Cordillera (Fig. 12F).

CONCLUSIONS

(1) Sedimentary lithofacies and facies asso-ciations identifi ed in the middle Eocene to Plio-cene succession of the eastern foothills along the Eastern Cordillera–Llanos basin boundary in Colombia indicate a transition from marginal marine to nonmarine clastic deposition within the northern Andean foreland basin system. Accumulation took place in tide-dominated del-taic and coastal environments, then relatively low-energy fl uvial systems, with fi nal deposi-tion in a relatively high-energy fl uvial system, possibly a fl uvial megafan. Up-section shifts to more-proximal facies are consistent with increased proximity to the sediment source area in the growing Eastern Cordillera fold-and-thrust belt.

(2) Compositional provenance information from sandstone petrographic data and conglom-erate clast lithologies show that Cretaceous and Paleogene strata of the Eastern Cordillera were the principal sediment sources for the Oligocene–Pliocene formations of the eastern foothills. The same data set indicates a cratonic provenance for the Eocene Mirador Formation, as supported by paleocurrent orientations. The compositional provenance data reveal a com-plex unroofi ng pattern in which the up-section proximity of the sediment source is expressed by an increase in sedimentary lithic fragments and an increase and then decrease in glauconite fragments. This provenance trend is inconsis-tent with simple unroofi ng of a single thrust sheet but is compatible with the introduction of several thrust sheets containing Cretaceous– Cenozoic strata.

(3) Detrital zircon U-Pb ages from the east-ern foothills reveal an important age population shift during the Oligocene. The Eocene age spectrum is governed by Paleoproterozoic and Mesoproterozoic ages. In contrast, the Oligo-cene age distribution shows the fi rst introduc-tion of west-derived clasts in the eastern foot-hills, implying uplift of the Eastern Cordillera by at least Oligocene time. Additionally, up-section trends in Oligocene–Pliocene samples show the appearance and then disappearance of a Mesozoic–Cenozoic age population. This provenance trend requires the introduction of younger rocks in the source area, probably related to eastward advance of the deformation front. Detrital zircon (U-Th)/He ages indicate a substantial change at the Oligocene– Miocene boundary from principally Precambrian–Paleozoic to Cretaceous–Cenozoic ages. The younger cooling ages identifi ed in Miocene–Pliocene strata are considered to be the product of rapid exhumation in more-hinterland sectors of the Eastern Cordillera.

Bande et al.


(4) Sediment dispersal patterns in the lower Guayabo Formation show intriguing variations that can be related to structural evolution of the deformation front. The lowermost Guayabo Formation was deposited by a transverse river system fl owing to the east, as expected for sed-iment derived from the uplifted Eastern Cordil-lera. In the upper part of the lower Guayabo Formation, however, the river system drained largely southward, parallel to the structure. This suggests that diversion by a topographic barrier had developed east of the Nunchía syn-cline by middle–late Miocene time, probably related to activation of the easternmost struc-ture (Yopal thrust).

(5) The complex up-section shifts in com-position, detrital geochronology, detrital ther-mochronology, sediment dispersal, and east-ward progradation of depositional systems can be attributed to sequential activation of thrust-belt structures within the Eastern Cor-dillera. Oligocene activation of the Soapaga fault in the axial Eastern Cordillera followed a period of cratonic provenance in the eastern foothills. An eastward advance in deforma-tion is recorded in the Lower Miocene section, implying reactivation of the Pajarito fault. The eastward younging of exhumation is further recorded by a later, middle–late Miocene epi-sode involving activation of the Guaicaramo fault and Yopal thrust (footwall shortcuts), ultimately forming a wedge-top (piggyback) basin in the Nunchía syncline along the east-ern foothills of the Eastern Cordillera–Llanos basin transition in Colombia.

ACKNOWLEDGMENTS

Funding was provided by the Instituto Colombiano del Petróleo (ICP), a division of Ecopetrol, and the Jackson School of Geosciences as part of a collabora-tive research agreement between ICP and the Univer-sity of Texas at Austin. The ICP project “Cronología de la deformación en las Cuencas Subandinas” pro-vided valuable information and logistical support during the research. Additional funding was provided by the Jackson School of Geosciences through fellow-ship support from ConocoPhillips and the Ronald K. DeFord Field Scholarship Fund. We thank Jaime Toro, Andrew Meigs, and David Schofi eld for constructive reviews and Joel Saylor, Junsheng Nie, Christopher Moreno, Javier Sánchez, Jorge Rubiano, Germán Bayona, Ronald Steel, and Richard Ketcham for use-ful discussions. Isaid Quintero and Jaime Corredor provided assistance in the fi eld.

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SCIENCE EDITOR: NANCY RIGGS

ASSOCIATE EDITOR: DAVID SCHOFIELD

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Clastic deposition, provenance, and sequence of Andean thrusting in the frontal Eastern Cordillera...

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