tectonic controls on cenozoic foreland basin development in the

Tectonic controls on Cenozoic foreland basindevelopment in the north-eastern Andes,ColombiaMauricio Parra,n Andres Mora,n,w Carlos Jaramillo,z Vladimir Torres,w Gerold Zeilingern andManfred R. Streckern

nInstitut fˇr Geowissenschaften, Universit�t Potsdam, Potsdam, GermanywInstituto Colombiano del Petro¤ leo, Ecopetrol Bucaramanga, ColombiazSmithsonianTropical Research Institute, Balboa, Ancon, Republic of Panama

ABSTRACT

In order to evaluate the relationship between thrust loading and sedimentary facies evolution, weanalyse the progradation of £uvial coarse-grained deposits in the retroarc foreland basin system of thenorthern Andes of Colombia.We compare the observed sedimentary facies distributionwith thecalculated one-dimensional (1D) Eocene to Quaternary sediment-accumulation rates in theMedinawedge-top basin andwith a three-dimensional (3D) sedimentary budget based on the interpretationof �1800 km of industry-style seismic re£ection pro¢les and borehole data. Age constraints arederived from a new chronostratigraphic framework based on extensive fossil palynologicalassemblages.The sedimentological data from theMedina Basin reveal rapid accumulation of £uvialand lacustrine sediments at rates of up to �500mmy�1during theMiocene. Provenance data basedon gravel petrography and paleocurrents reveal that theseMiocene £uvial systems were sourced fromUpper Cretaceous and Paleocene sedimentary units exposed to the west in the Eastern Cordillera.Peak sediment-accumulation rates in the upper Carbonera Formation and theGuayaboGroup occurduring episodes of coarse-grained facies progradation in the early and lateMiocene proximalforedeep.We interpret this positive correlation between sediment accumulation and gravel depositionas the direct consequence of thrust activity along the Servita¤ ^Lengupa¤ faults.This contrasts with oneclass of models relating gravel progradation in more distal portions of foreland basin systems toepisodes of tectonic quiescence.

INTRODUCTION

Grain-size trends and the basinwide distribution ofcoarse-grained strata in foreland basins have been used tointerpret the tectonic and climate-related controls on fore-land basin accumulation (e.g., Flemings & Jordan, 1990;Heller & Paola, 1992; Paola et al., 1992). In general, the ba-sin’s stratigraphic architecture is a function of the relativeimportance between sediment discharge and the rate ofcreation of accommodation space (e.g., Schlunegger et al.,2007). Multiple mechanisms have been proposed to ac-count for the progradation of coarse-grained sedimentsin foreland basins: (1) uplift of the source areas by eitherincreased tectonic activity in the fold-and-thrust belt(e.g., Burbank et al., 1988; Schlunegger et al., 1997a, b; Hor-ton et al., 2004) or erosionally driven isostatic rebound

(e.g., Burbank, 1992); (2) an increase in the e⁄ciency oferosion triggered by global climatic oscillations (e.g.,Mol-nar, 2004) or by orographic e¡ects (Ho¡man&Grotzinger,1993;Masek etal.,1994); (3) tectonic quiescence favouring adecrease in subsidence and progradation of coarse-grained sediments to the distal part of the basin (e.g., Hel-ler et al., 1988; Flemings & Jordan, 1990; Burbank, 1992;Heller & Paola, 1992); and (4) increase of erosion rates andsediment discharge due to a decrease in the resistance toerosion of the source areas, (e.g., DeCelles et al., 1991; Car-roll et al., 2006). Numerical modelling has been used toevaluate the role of each of these controlling factors onthe overall distribution of coarse-grained facies in basinswith di¡erent £exural rigidities (e.g., Flemings & Jordan,1989; Flemings & Jordan, 1990; Sinclair et al., 1991; Paolaetal.,1992). A critical factor determining a basin’s sedimen-tary response to the aforementioned changes, however, in-volves an improved knowledge of the time scales overwhich variations in the external forcings occur comparedwith an inherent background level of erosional and deposi-tional processes in the basin. While considerable debatehas existed on the role of each of these factors in the distalportion of foreland basins (Burbank etal.,1988;Heller etal.,

EAGE

Correspondence: Mauricio Parra, Institut fˇr Geowissenschaf-ten, Universit�t Potsdam, Karl-Liebknecht-Strasse 24, Haus 27,14476 Potsdam, Germany. E-mail: [email protected] address: Department of Geological Sciences, JacksonSchool of Geosciences, University of Texas at Austin, Austin,TX 7871-0254, USA

BasinResearch (2010) 22, 874–903, doi: 10.1111/j.1365-2117.2009.00459.x

r 2010 The AuthorsBasin Researchr 2010 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists874

mailto:[email protected]

mailto:[email protected]

1988), combined ¢eld evidence and numerical models havedemonstrated that coarse-grained sediments accumulatein the proximal part of foreland basins, irrespective of pre-cise tectonic and climatic regimes (e.g., Flemings& Jordan,1990; Jones et al., 2004). These inherent characteristicscomplicate a rigorous assessment of the role exerted byeach of these competing factors on the accumulation ofcoarse-grained sediments in proximal sectors of forelandbasins. Provided su⁄cient temporal control, tectonicallyand climatically controlled forcing factors re£ected in de-positional characteristics may be deconvolved for an im-proved understanding of the spatiotemporal trends intectonic and sedimentary processes of convergent oro-genic belts. Such is the case in the northern Andes, wherecontractional deformation and orogenic growth have beenlinked with reactivated long-lived basement anisotropies(Mora et al., 2008; Parra et al., 2009b) that have fundamen-tally in£uenced the loci of tectonic deformation, erosionand sediment dispersal.

In the eastern Andes of central Colombia, the sedimen-tary record of subduction-related orogenesis is preservedin Late Cretaceous to Cenozoic basins that extend east ofthe Central Cordillera, the present-day magmatic arc(Fig.1).Mesozoic rifting occurred in the area presently oc-cupied by the Eastern Cordillera north of �21N (e.g.,Campbell & Bˇrgl, 1965;Mora et al., 2006; Sarmiento-Ro-jas etal., 2006). In the course of Cenozoic contraction,ma-jor inherited extensional faults became the locus forpreferential accommodation of thrust loading and defor-mation (Mora et al., 2006, 2008; Parra et al., 2009a, b), andtherefore prevented a signi¢cant eastward advance of theorogenic front.This con¢guration has led to the unroo¢ngof progressively older structural levels from the eastern£ank of the Eastern Cordillera and resulted in the coevalaccumulation of sediments in the Llanos Basin to the east.Plio^Pleistocene eastward advance of the foreland fold-and-thrust system to the present-day frontal structure(Mora, 2007) has incorporated only the proximal part ofthis Mio^Pliocene foredeep into the orogen in the formof the Medina wedge-top basin. This exhumed foredeepo¡ers a unique, yet areally limited locationwhere the sedi-mentary record of the late stages of Andean uplift and ex-humation are well exposed. However, because of theabsence of radiometrically datable minerals and the pau-city of published biostratigraphic markers, the sedimen-tary evolution and its relation with Andean tectonic andclimatic evolution are still unclear.

In this study, we present re£ection data unreleased pre-viously that re¢ne earlier estimates on the age of thrust in-itiation and help unravel the tectono-sedimentaryevolution along the eastern margin of the Eastern Cordil-lera. We also present new ¢eld-based sedimentologicaland provenance data and the ¢rst systematically acquiredbiostratigraphic dataset, based on detailed palynology, forthe proximal foredeep deposits in this area. In order to dis-criminate among multiple potential forcing factors on thebasin architecture, we compare one-dimensional (1D)Eo-cene to Pliocene sediment-accumulation rates in the

Medina Basinwith a three-dimensional (3D) sedimentarybudget for an area of �5000 km2, based on the interpreta-tion of �1800 km of industry-style depth-migrated,mul-tichannel seismic re£ection pro¢les and borehole data tiedto a newbiostratigraphic framework. Importantly, our datashow that episodes of coarse-grained sedimentation arecoeval with rapid subsidence throughout the basin history,illustrating that increased tectonic activity in the EasternCordillera has exerted a dominant control on the geome-try andpattern of sediment distribution.Our work has im-plications for the understanding of the response time ofsurface processes to tectonic forcing.

GEOLOGIC BACKGROUND

Geodynamic and structural setting

The Medina Basin is a 90� 25 kmwedge-top depocentrelocated atop the most external east-verging thrust-sheet along the eastern margin of the Eastern Cordilleraof the Colombian Andes (Fig.1).The Eastern Cordillera isthe easternmost branch of a retroarc fold-and-thrust beltrelated to Late Cretaceous to Cenozoic shortening, result-ing from the interaction between the Nazca, Caribbeanand SouthAmerican plates (e.g., Cooper etal., 1995;Taboa-da et al., 2000; Go¤ mez et al., 2005; Parra et al., 2009a). LateCretaceous ( �80Ma) oblique accretion of relicts of a Pa-ci¢c oceanic plateau (e.g.,Kerr &Tarney, 2005;Vallejo etal.,2006) constituted the Western Cordillera and triggeredcrustal shortening and thickening and initial mountainbuilding within the present-day Central Cordillera (e.g.,Cooper et al., 1995). The tectonic loading exerted by thisrange created a foreland-basin system, east of the CentralCordillera (Cooper et al., 1995; Go¤ mez et al., 2005). Subse-quent deformation compartmentalized the foreland basinin a nonsystematic manner due to the selective reactivationof crustal anisotropies inherited from Proterozoic and Pa-laeozoic collision and subduction episodes (e.g., Restrepo-Pace et al., 1997; Cediel et al., 2003, and references therein),and more importantly extensional structures generatedduring Mesozoic rifting (e.g., Cooper et al., 1995; Moraet al., 2006; Sarmiento-Rojas et al., 2006). In this context,initial middle Eocene tectonic inversion of Mesozoic riftbasins in the area of the present-day Eastern Cordilleradisrupted the once contiguous foreland basin and formedtwo principal Cenozoic basins: theMagdalenaValleyBasinto the west, and the Llanos basin to the east (e.g., Go¤ mezet al., 2003; Parra et al., 2009a). During inversion, signi¢ -cant rockuplift in theEasternCordillera occurred throughthe reverse slip along the formerly rift-bounding faults.These major faults include the east-dipping Bituima^LaSalina faults to the west, and the west-dipping Servita¤ ^Lengupa¤ faults to the east (Fig. 1a). This process has re-sulted in the formation of a bivergent, thick-skinnedfold-and-thrust belt with the loci of maximum exhuma-tion coinciding with the proximal hanging-wall blocks ofinverted Mesozoic normal faults whose orientation was

r 2010 The AuthorsBasin Researchr 2010 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 875

Controls on foreland-basin sedimentation, Colombia

Fig.1. (a)Geologic map of theEasternCordillera (Mora etal., 2008; Parra etal., 2009a) showing main structures in the northern sector ofthe QuetameMassif and the adjacentMedina (black box) and Llanos basins. Locations of seismic lines (white lines) andwells areindicated. Inset map denotes the location of theWestern (WC), Central (CC) and Eastern (EC) cordilleras within the ColombianAndes.White box in inset indicates the location of the main map. (b) Structural cross section (Location in a) showing the main structures acrossthe double-vergent EasternCordillera and the adjacentMagdalena andLlanos basins. (c)Geological map of theMedina Basin (locationshown in a). Locations of growth strata (Fig. 3) and measured stratigraphic sections are shown.


M. Parraet al.

favourable for the accommodation of compressional stres-ses during the Andean orogeny (Mora et al., 2006). Promi-nent examples of the more deeply exhumed sectors in theEastern Cordillera are the Villeta Anticlinorium on thewestern £ank of the range, and the Floresta and Quetame

basement massifs in the axial and eastern sectors. Outsideof the Mesozoic rift domain, e¤ n-echelon, northeastwardstepping, thin-skinnedCenozoic thrust sheetswith oppo-site vergence are thrust over theMagdalena andLlanos ba-sins above detachment levels within mechanically weak

Fig.1. Continued



Cretaceous and Palaeogene strata (e.g., Butler & Schamel,1988; Cooper et al., 1995; Go¤ mez et al., 2005; Mora et al.,2008).

Stratigraphy of the Eastern Cordillera

Pre-Devonian low-to medium-grade phyllites, quartzitesand schists, and sparse Palaeozoic intermediate to acid in-trusives comprise the basement of the Eastern Cordillera(Segovia, 1965; Ulloa & Rodr|¤ guez, 1979; Ulloa & Rodr|¤ -guez, 1982; Jime¤ nez, 2000).These basement rocks are un-conformably overlain by up to �4 km of Devonianmarginal marine mudstones and sandstones andCarboni-ferous nonmarine red beds (e.g., Ulloa &Rodr|¤ guez,1979).These units are in turn superseded by Mesozoic rift-re-lated units including: (1) up to 2 km ofLower toUpper Jur-assic lacustrine and volcanoclastic rocks, locally depositedin narrow half-graben basins in the western half of therange (e.g., Kammer & Sa¤ nchez, 2006; Sarmiento-Rojaset al., 2006); (2) up to �5 km of Early Cretaceous (Berria-sian to Aptian) synrift platformal units, deposited in awider rift basin whose limits approximately coincide withthe margins of the present-day mountain range (Moraet al., 2006, 2009a); and (3) up to 2 km of postrift, shallowmarine rocks, deposited within and beyond the structural

limits of the rift, including areas of theLlanos andMagda-lena basins (e.g., Cooper et al., 1995; Mora et al., 2006; Sar-miento-Rojas et al., 2006). In the Eastern Cordillera, theseunits include decimetric layers of glauconitic sandstonesin the Une, Chipaque and Guadalupe formations (Guer-rero & Sarmiento, 1996; Vergara & Rodr|¤ guez, 1996), fora-minifera-bearing siliceous siltstones and phosphaticsandstones (GuadalupeGroup, e.g., F˛llmi etal., 1992;Ver-gara & Rodr|¤ guez, 1996), which constitute importantlithologic markers that help constrain the provenance ofCenozoic sedimentary units.

The onset of nonmarine sedimentation in the EasternCordillera is recorded by the up to 1100-m-thick coastalplain, estuarine and £uvial sedimentary rocks of the upperMaastrichtian^lower Palaeocene Guaduas Formation(Sarmiento, 1992). This unit is interpreted as the distalequivalent of coarse-grained, westerly sourced synoro-genic deposits of the Magdalena Valley (Go¤ mez et al.,2005). In the Medina Basin, only �60m of the GuaduasFormation (Guerrero & Sarmiento, 1996) are preservedbelow a regional unconformity associated with forebulgeerosion (Go¤ mez et al., 2005; Fig. 2). Here, the overlyingCenozoic units comprise two megasequences of late Pa-laeocene and Eocene^Pliocene age, respectively, that pro-gressively onlap eastward to the Mesozoic substratum of

FO

RE

LAN

D B

AS

IN

AXIAL EASTERN CORD. MEDINA BASIN LLANOS BASIN

GUAYABO

PO

ST

-RIF

TS

YN

-RIF

T

USME

LEÓN

CHIPAQUE

TILATÁ

GUADALUPE GROUP

UNE

FÓMEQUE

LAS JUNTAS

MACANAL

GUAVIO

BUENAVISTA

EPOCH

Lacustrine siltstone

Alluvial-fanconglomerates

Nonmarinemudstones

Facies change

Angularunconformity

Shallow-marinesandstones

Delta and coastal-plain sandstones

Nonmarinesandstones

Delta and coastal-plain mudstones

Shallow-marinemudstones

Shallow-marinecarbonates

CARBONERA

MIRADOR

REGADERA

BOGOTÁ LOS CUERVOS

Glauconite-bearing units

Fig. 2. Chronostratigraphic diagram ofthe Late Jurassic^Cenozoic strata in theeastern £ank of the Eastern Cordillera(afterGo¤ mez, E. etal., 2005;Mora, A. etal.,2008b; Parra et al., 2009a). Grey shadingrepresents lithostratigraphic units withglauconitic sandstones used to evaluatethe unroo¢ng of the source areas (see text).


M. Parraet al.

the Llanos Basin (Cooper et al., 1995; Fig. 2). The ¢rst se-quence consists of up to �700m of estuarine and coastalplain deposits of the Barco and Los Cuervos formations(Cooper et al., 1995; Guerrero & Sarmiento, 1996; Cazieret al., 1997; Jaramillo & Dilcher, 2000). The second se-quence comprises a �5-km-thick lower Eocene to Neo-gene strata that thins eastward and rests unconformablyupon progressively older Palaeocene and Cretaceous unitstoward the east in the Llanos Basin. This sequence com-prises up to �250m of estuarine valley- ¢ll and coastal-plain deposits (Cazier et al., 1997) of the lower to middleEocene Mirador Formation that are superseded by up to�3 km of estuarine and locally marine deposits of the lateEocene^earlyMioceneCarbonera Formation (Cazier etal.,1995, 1997; Cooper et al., 1995; Bayona et al., 2008; Parraet al., 2009a). The Carbonera Formation consists of eightmembers (the C1^C8members) of interlayered sandstone-and mudstone-dominated deposits (e.g., Cooper et al.,1995). Rapid sediment accumulation within an eastward-thinning sedimentary wedge with pronounced facieschanges in the upper Eocene^Oligocene lower part of theCarbonera Formation can be inferred for the western partof theMedina Basin and has been related to the initial up-lift of the axial Eastern Cordillera (Parra et al., 2009a, b).These strata are overlain by the approximately 500-m-thick Leo¤ n Formation (Cooper et al., 1995), which recordslacustrine deposition with short-lived marine incursions(Bayona et al., 2008). Overlying the Leo¤ n Formation, pro-tracted nonmarine sedimentation is represented by distalto proximal alluvial deposits of the Lower Guayabo andUpper Guayabo formations (Cooper et al., 1995). TheUpper Guayabo Formation intersects the present-dayerosion surface and has an exposed minimum thicknessof �700m. Our ¢eld observations and geologic mappingshow that the spatial distribution of the upper CarboneraFormation and younger units displays numerous facieschanges, leading to a signi¢cantly di¡erent stratigraphiccolumn for the eastern and western sectors of the MedinaBasin. In this study, we focus on the Miocene^Pliocenestratigraphy of this foreland basin.

Structure

The Medina Basin constitutes the hanging wall of a thin-skinned thrust sheet that extends �40 km east of theTesa-lia fault (Fig. 1c). Here, the Guavio anticline is a broadfault-bend fold related to the Guaicaramo thrust. Subsur-face data and structural interpretations suggest that thisthrust splays at depth from theTesalia fault (Mora et al.,2006) and propagates along two decollement levels withinthe Lower Cretaceous Macanal Formation and the UpperCretaceous Chipaque Formation (Rowan & Linares,2000). The thrust propagates to a higher decollementabove an underlying normal fault and, farther east has gen-erated a fault-propagation fold (Limones anticline) in itshanging wall (Fig. 1c). In the northern part of the MedinaBasin, west of the Guavio anticline, the Nazareth synclineis a highly asymmetric, east-verging fold that forms the

westernmost structure in the area. The western limb isoverturned and constitutes the northern extent of the wes-ternMedina syncline.The steepening of the western limbof theMedina syncline occurs where the deformation stylechanges at the eastern margin of the Quetame massif; inthe south, it is primarily accommodated by thrustingalong the Servita¤ fault, whereas in the north deformationhas resulted in fault-propagation folding (Fig. 1c). Farthereast, in the footwall of the Guaicaramo thrust, follows theLlanos Plain in the modern foredeep depozone. Here, de-formation is minor and results primarily from the south-ward propagation of the Cusiana fault and the associatedhanging-wall La Florida anticline (Fig.1c), a structure cor-responding to a more frontal depocentre within the e¤ n-echelon segments of the eastern fold-and-thrust belt. Toa lesser degree, deformation is associatedwith minor nor-mal faulting within the Cenozoic deposits, imaged in seis-mic re£ection pro¢les and interpreted to be related toforebulge extensional faulting (Bayona et al., 2008).

Chronology of foreland-basin deformation

Crustal thickening in theCentral Cordillera since the LateCretaceous time ( �75^80Ma) led to initial foreland-ba-sin development in central Colombia (e.g., Cooper et al.,1995;Go¤ mez etal., 2005). Eastward advance of the orogenicfront has occurred episodically, with stages of fast advanceassociated with the disruption of the basin through an in-itial bivergent inversion of the Eastern Cordillera. Subse-quently, stagnation of the deformation front has resultedfrom contractional deformation being preferentially ac-commodated along crustal inhomogeneities inheritedfrom previous tectonic events (Mora et al., 2008; Parraetal., 2009a). In particular, this pattern has been documen-ted for the contractionally reactivated Servita¤ ^Lengupa¤faults along the eastern limit of the Quetame basementhigh.There, zircon ¢ssion-track data constrain the mini-mum age of initial deformation-related exhumation asso-ciated with the slip along this fault at �20^25Ma (Parraet al., 2009b). Subsequent deformation and erosion of theCenozoic strata in the vicinity of the trace of the Lengupa¤fault have prevented the determination of tighter age con-straints on the initial deformation. However, despite theselimitations, growth-strata relations in lower Mioceneunits of the lower Carbonera Formation (described below)support previous interpretations of an early Oligocene toearly Miocene onset of thrusting along this fault (Parraet al., 2009b). Protracted deformation and tectonic loadingalong this structure is inferred from aMiocene increase inthe tectonic subsidence of the Medina (Parra et al., 2009a)andLlanos (Bayona etal., 2008) basins.Moreover, the sub-horizontal strati¢cation (i.e. the absence of growth strata)inMiocene^Pliocene strata in the footwall of the Guaicar-amo thrust, as deduced from the analysis of industry seis-mic lines (Mora, 2007), provides a maximum age of�5Ma for the initial thrusting along this fault.This pat-tern thus demonstrates Miocene^Pliocene stagnation ofthe deformation front along the long-lived crustal aniso-



tropy of the Servita¤ ^Lengupa¤ faults and subsequent post-Pliocene migration of the deformation front to its presentlocation along the Guaicaramo thrust (Mora, 2007; Parraet al., 2009b).

METHODS

Detailed mapping of an area of �1500 km2 provides thebasis for stratigraphic pro¢ling of Neogene foreland basinstrata. We conducted sedimentary facies interpretation,and palaeocurrent and provenance determinations onnine measured sections totalling �5.3 km of the Carbo-nera Formation and the Guayabo Group exposed in theMedinaBasin (Fig.1).Correlations betweenmeasured sec-tions are based either on the tracing of laterally continuouslithostratigrahic units over distances of a few kilometreswithin individual limbs of folds or on subsurface extrapo-lation of outcrop exposures in seismic re£ection pro¢leswhere surface correlation is precluded by erosion. Suchan extrapolationwas carried out by correlating seismic re-£ectors with the surface geology, as derived from our de-tailed mapping. A new chronological framework ofsediment accumulation is provided on the basis of a paly-nological study performed on �500 samples.

Palaeocurrent directions were derived from imbricatedclasts, channel-axis orientations and trough cross-bed-ding. Sixteen conglomerate clast counts were conductedto reveal the unroo¢ng history of the source areas. A mini-mum of 100 clasts were counted in individual, clast-sup-ported conglomerate layers, using a 10-cm grid.Conglomerate petrography data are reported inTable 1.

We evaluate the spatial and temporal patterns of sedi-ment accumulation in the Medina and Llanos basins byassessing both 1D and volumetric sedimentary budgets.First, we reconstruct1D, decompacted sediment-accumu-lation rates along a composite stratigraphic section in theMedina wedge-top basin. A composite stratigraphic sec-tion of the Upper Cretaceous^Pliocene units of theMedi-na Basin was constructed by combining the measuredpro¢les of Mio^Pliocene units presented in this studyand sections for older units presented by Parra et al.(2009a) and Jaramillo & Dilcher (2000). Sediment-accu-mulation rates are estimated using thickness and age con-straints based on the palynological biozonation. In orderto account for anomalies in measured stratigraphic thick-nesses derived from the progressive loss of porosity withburial depth and inhomogeneous compaction of mechani-cally di¡erent lithologies, we used a porosity-depth rela-tion to estimate decompacted thicknesses (Sclater &Christie,1980).Decompaction parameters and detailed re-sults are presented in Table S1. Second, we estimate thevolume of sedimentary strata accumulated for speci¢c in-tervals in theMedinaBasin and the proximal, western sec-tor of the Llanos Basin by interpreting an extensive grid of�1800 km of 2D industry seismic re£ection data (Fig.1a).Mapped units were identi¢ed in the grid of seismic linesby a direct extrapolation of surface outcrops in theMedina

Basin. In addition, seismic re£ectors were tied to the stra-tigraphy with data from seven hydrocarbon-explorationboreholes, especially in areas with no surface exposurewithin the footwall of the Guaicaramo thrust. The accu-mulated (compacted) volume of rock was estimated fromthe seismic data by converting the vertical time axis intodepth.We used check-shot surveys from seven boreholesacross the Medina and Llanos basins to evaluate the seis-mic velocities in the Cenozoic strata.The near-surface ve-locity gradient decreases eastward from �3770m s�1 inthe Medina Basin (Coporo-1Well) to �2840m s�1 in theeasternmost part of the study area in the Llanos Basin(Upia-1Well; see Fig. S1).We therefore derived di¡erentdepth^time relations for the hanging and footwalls of theGuicaramo thrust by combining information from avail-able wells in each fault block. For the hanging-wall block,combined data from the Coporo-1 andMedina-1wells re-sult in an average velocity of 3650m s�1, whereas data fromtheGuacavia-1, SanPedro-1,Chaparral-1andUpia-1wellsyield a value of 3050m s�1. We thus estimate that maxi-mum errors in depth conversion due to averaging datafrom various wells are up to �4% in the Medina Basinand up to �7% in the Llanos Basin.

RESULTS

Growth strata and growth unconformities

First, growth-strata relationships and unconformities arewell preserved in di¡erent stratigraphic levels along thewestern margin of the Medina Basin. In the seismic lineMVI-1020, a package of divergent re£ectors in the strataequivalent to the lower part of the C5^C2 members of theCarbonera Formation occurs within the western limb oftheMedina syncline (Fig. 3). Awestward decrease in stratalthickness and onlap geometries suggests contempora-neous sedimentation and tilting of the forelimb of thefault-propagation Farallones anticline (e.g., Riba, 1976).These geometries constrain a minimum, early Mioceneage for the initiation of folding associated with slip alongthe Lengupa¤ fault. Second, in the northwestern part ofthe basin, growth strata and growth unconformities inthe strata of the upperMiocene^PlioceneGuayaboGroupexist on the western £ank of the Nazareth syncline at�41400N (Mora, 2007).This geometry reveals continueddeformation through the tilting of the Farallones anticlineforelimb. Overall, these syncontractional stratal geome-tries in di¡erent stratigraphic intervals of the Mio^Plio-cene units of the western Medina Basin documentinversion, protracted reverse faulting and fault-relatedfolding associatedwith the long-lived Lengupa¤ fault.

Sedimentary facies architecture of theMedina Basin

The upper Eocene^Pliocene basin ¢ll of theMedina Basinis reconstructed on the basis of 13 stratigraphic sectionstotalling �7.4 km of strata, which constitute the Carbo-


M. Parraet al.

Tab

le1.

Gravelp

etrography

dataforE

ocene-Pliocene

unitsofmedinabasin

Sample

Long(1W)

Lat(1N)

Thickness(m

)nUnit

Vein

quartz

Chert

Mud

-stone

Sand

-stone

Siliceous

siltston

eGlaucon

itic

sand

ston

ePh

osph

atic

sand

ston

eSh

ale

Total

MP603

73.22359

4.80948

1840

C7

6436

00

00

00

100

64%

36%

0%0%

0%0%

0%0%

100%

MP605

73.22125

4.80544

2538

C7^C5

6136

30

00

00

100

61%

36%

3%0%

0%0%

0%0%

100%

MP607

73.21739

4.80495

2658

C7^C5

7027

21

00

00

100

70%

27%

2%1%

0%0%

0%0%

100%

MP608

73.21604

4.80566

2687

C7^C5

6525

80

20

00

100

65%

25%

8%0%

2%0%

0%0%

100%

MP610

73.19455

4.81072

3743

C1

3713

1432

01

00

9738%

13%

14%

33%

0%1%

0%0%

100%

MP642

73.36852

4.58595

3925

C1

2012

460

20

20

100

20%

12%

4%60

%2%

0%2%

0%100%

MP611

73.19

678

4.80565

4008

C1

3915

532

112

00

104

38%

14%

5%31%

11%

2%0%

0%100%

MP612

73.19

920

4.79959

4353

C1

118

219

1915

20

7614%

11%

3%25%

25%

20%

3%0%

100%

MP613

73.19

947

4.79635

4615

C1

154

364

104

00

100

15%

4%3%

64%

10%

4%0%

0%100%

MP615

73.19

999

4.79340

4816

Low

erGuayabo

138

1065

44

00

104

13%

8%10%

63%

4%4%

0%0%

100%

MP641

73.34744

4.57738

4905

Low

erGuayabo

00

0100

00

00

100

0%0%

0%100%

0%0%

0%0%

100%

MP616

73.19732

4.78996

4995

Low

erGuayabo

02

4102

00

00

108

0%2%

4%94%

0%0%

0%0%

100%

MP626

73.17091

4.78835

5136

Low

erGuayabo

52

1453

226

00

102

5%2%

14%

52%

2%25%

0%0%

100%

MP618

73.19704

4.78743

5246

Low

erGuayabo

00

070

030

00

100

0%0%

0%70%

0%30%

0%0%

100%

MP623

73.19790

4.78494

5380

Low

erGuayabo

02

486

08

00

100

0%2%

4%86%

0%8%

0%0%

100%

MP627

73.17641

4.79269

5507

Upp

erGuayabo

31

469

320

02

102

3%1%

4%68%

3%20%

0%2%

100%

MP630

73.18

339

4.79121

6188

Upp

erGuayabo

00

256

042

00

100

0%0%

2%56%

0%42%

0%0%

100%

nStratalthicknessinCom

positeSe

ction(Figs8

and9)



nera (C8^C1members), Leo¤ n, Lower Guayabo and UpperGuayabo formations. The characteristics of the lower2.1km of this record, comprising the C8^C5 members,

was presented by Parra et al. (2009a) based on four strati-graphic pro¢leswith facies associations that represent tid-ally in£uenced deltaic, lacustrine, alluvial plain and

424212

396198

369184

341170

CDPSP

674337

646323

619309

591295

563281

535267

507253

480240

452226

–2500

–3000

Tw

o-w

ay tr

avel

time

(ms)

C6 (Guaicarama)

Gacenera

B

250 m250 m

C5-C2 members

1758879

1647823

1535767

1424712

1313656

1202601

1091545

980490

869434

758379

646323

535267

424212

313156

CDPSP

–1000

–2000

–3000

–4000

–5000

Tw

o-w

ay tr

avel

time

(ms)

León

C1

C2 (Huesser)

Gacenera

C6 (Guaicarama)

Mirador

Medina Syncline Limones Anticline

Gua

icar

amo

thru

st

1 km1 km

Tesa

lia fa

ult

Fig. 3. (a) Time-migrated seismic lineMVI-1997-1020 across the southern sector of theMedina Basin (see Fig.1), depicting the tops ofinterpretedCenozoic units (see Figs 5, 6 and 8). For an approximate vertical scale bar, the vertical axis is based on a velocity correction of4 km s�1.The vertical exaggeration is 1.7X. (b) Detail of growth-strata geometries in the lowerMiocene C5^C2members of theCarbonera Formation in theMedina syncline.


M. Parraet al.

braided £uvial sedimentary environments.These four fa-cies associations (FA1^FA4;Table 2) are also present in theupper members of theCarboneraFormation and overlyingunits that are the subject of this work.Here, we build uponour previous work by complementing the description offacies associations FA1^FA4with newdata from the upperCarbonera and Leo¤ n formations. In addition, we add thedescription and interpretation of facies associations FA5and FA6, which occur in the Upper Guayabo Formation.Facies associations are described on the basis of the recog-nition of 16 lithofacies, following Parra et al. (2009a).Thesedescriptions are included in the supplementary material(Table S2). In the following section, we focus on the inter-pretation of the spatial distribution of the six identi¢ed fa-cies associations based on the measured stratigraphicpro¢les (Figs 4 and 5), and the associated depositonal pro-cesses and sedimentary environments.

Interpretation of lithofacies associations

Facies association 1 (coarsening-upward laminated sandsto-nes). Thin interbedded sandstone^mudstone coupletswith lenticular and £aser lamination suggest tidal in£u-ence (Reineck&Wunderlich,1968).The extensive areal ex-tent of laterally continuous sandstone bodies with a varietyof wavy, lenticular, £aser and cross lamination, as well asassociated coal laminae and coal seams, suggests deposi-tion in a transition zone between £uvial^marine and tide-dominated estuarine systems (e.g., Dalrymple et al., 1992;Dalrymple & Choi, 2007). The thickening- and coarsen-ing-upward sequences evolve from laminated dark mud-stones to wavy, lenticular and ¢nally to cross-strati¢edsandstones, suggesting an increase in current velocity(Collinson et al., 2006) and possibly indicating a decreasein water depth through time. This is compatible eitherwith eustatically controlled parasequences (e.g., Mitchum&VanWagoner, 1991) or, alternatively autogenic prograda-tional successions, such as those observed in delta-frontdeposits (e.g., Tye & Coleman, 1989; Coleman et al., 1998).Although dewatering structures, soft- sediment deforma-tion and growth faults indicate a rapid deposition (Lowe,1975; Owen, 1996) typical for deltaic environments (Cole-man et al., 1998; Dalrymple et al., 2003), some allogeniccontrol cannot be ruled out.

Facies association 2 (massive and laminated mudstones). A mar-ine in£uence is indicated bydiscrete thin levels with abun-dant microforaminiferal linings and dino£agellates,including Homotryblium £oripes, Cordosphaeridium inodes,Polysphaeridium subtile, Achomosphaera and Spiniferites. Thelaterally continuous, dark-grey mudstone-dominated fa-cies associatedwith thin, £aser-laminated sandstones andcoal beds suggest deposition in a mud £at environment.This interpretation is supported by the presence of the bi-valves Pachydon, Anondondites and Mytilopsis (Go¤ mez et al.,2009; Parra et al., 2009a), which have been associated withfresh-water lacustrine systems (e.g., Nuttall, 1990; Wesse-

lingh et al., 2002; Anderson et al., 2006;Wesselingh&Mac-sotay, 2006), as well as the occurrence of gastropodsSheppardiconcha (Go¤ mez et al., 2009; see also Fig. 6a). Thefragmentation, corrosion and abrasion of gasteropodaand disarticulated bivalve shells re£ect reworking in an en-vironment with moderate energy. Finally, the sporadic oc-currence of mudstone with negligible bioturbation isindicative of rapid accumulation (e.g., Dalrymple & Choi,2007). Collectively, these observations suggest depositionin a transitional environment between freshwater lakesand estuaries.

Facies association 3 (channelized sandstones and conglomerates).Laterally restricted sandstone bodieswith basal scours arecharacteristic of stream- £ow deposition (e.g., Bridge,2003). The lenticular morphology of the sandstone bedsand the presence of erosive basal scour marks and muddyintraclasts suggest transport by traction (e.g., Collinsonet al., 2006). The poorly de¢ned large-scale, low-angleplanar cross- strati¢cation, absence of well-de¢ned nor-mal grading and ubiquituous £oating pebble clasts mayrepresent deposition in braided £uvial channels (e.g.,Miall, 1985; Bridge, 2003).

Facies association 4 (overbank mudstones and siltstones). Later-ally continuous variegated mudstone and siltstone unitsrepresent deposition by suspended load in overbank areas.Pervasive mottling and root traces (Fig. 6b) indicatepalaeosoil development in a £uvial £oodplain environ-ment (e.g., Bridge, 1984). Dessication cracks (Fig. 6c) andsporadic ferruginous nodules (Fig. 6d) re£ect pedogenesisduring intermittent £ooding and subaerial exposure and£uctuating wet^dry soil conditions (e.g., McCarthy et al.,1997; Kraus, 1999). In this scenario, the thin, wedge-shaped, ¢ning-upward sandstone bodies may have beendeposited as crevasse channel ¢lls (e.g., Bridge, 1984).

Facies association 5 (granule to pebble conglomerates and con-glomeratic sandstones). Clast- supported, granule and peb-ble conglomerates with horizontal strati¢cation or, rarelylow-angle through cross-strati¢cation, lack of muddy ma-trix and dominantly sharp, non-erosive basal contacts(Fig. 6e) indicate waterlaid deposition by uncon¢nedstream £ows (e.g., Blair, 1999b).These features may repre-sent deposition by sheet£oods (e.g. Hogg,1982).The inter-bedding of this facies with facies association FA3 likelyindicate deposition in the distal sectors of waterlaid allu-vial-fans (e.g., Blair, 1999b).

Facies association 6 (cobble and boulder conglomerates). Clast-supported, crudely strati¢ed, pebble-to-boulder con-glomerates (Fig. 6e) with a ribbon-like geometry representdeposition by high-energy stream£ows in moderately towell-con¢ned channels (e.g., Blair, 1999b). Occasional,very poorly sorted, matrix-supported conglomerates or-ganized in subtabular beds are diagnostic of debris- £owdeposits (Nemec & Postma, 1993; Blair, 1999a). Taken to-



Tab

le2.

Summaryoffacies

associations

FaciesAssociation

Description

Stratigraphicoccurrence

Interpretation

FA1(coarsening-up

ward

laminated

sand

ston

e)Upto8m

thickthickening

-andcoarsening

-up

wards

intervalsoftabu

larsandstone

with

minor

thin

interbedsofmud

ston

e.Sand

ston

ebeds

presentn

onerosivebasalcon

tactsandare

frequentlybioturbated.Lam

inae

rich

inorganic-matter,plantrem

ains.T

hinpebb

leconglomeratecommonlycapintervalsattop.

Typicallitho

faciespatterninclud

es,frombase

totop,Fm,F

l,Sw

,Sf,Slc,Sr

andGco.

Dew

ateringstructures,convolutebedd

ingand

grow

thfaultsoccur.Sand

ston

e-mud

ston

ecoup

letswithwavy(Sw),lenticular(Slc),£aser

(Sf)andoscilltorycurrentripplelamination

(Sr)

Predom

inantlitho

facies

inC7,C5andC3

mem

bersin

theeasternmarginofthebasin.

Com

mon

inlower

partofC1

Tidallyin£u

enceddeltaicenvironm

ent.

Growth

faults,convolutebedd

ingandwater-

escape

structures

suggestrapid

accummulation.Possibleallogeniccontrol

FA2(m

assive

andlaminated

dark

mud

ston

e)Thick

intervals(up

to100m)ofd

ark-gray

togreenish

mud

ston

e.Occasionalm

inor

bioturbation.L

imited

interbedsoftrou

ghcross-laminated

sand

ston

e,andup

to30

-cm-

thickcoalseam

s.Occasionalthin,disorganized

bivalve-bearingshell-beds.L

ocal

microforaminife

rallinings

anddino

£agellates.

Fragmentedanddisarticulated

bivalves

belong

ingtothegenu

sPachydon,Anondondites

andMytylopsis,and

gastropo

dsSheppardioncha

(Fig.6a).D

iscretelevelsw

ithmicroforaminife

ral

linings

anddino

£agellatesinclud

ing

Hom

otryblium£orip

es,C

ordosphaeridiuminodes,

Polysphaeridiumsubtile,A

chom

osphaera,and

Spiniferites

Intheeasternmarginofthebasindo

minant

facies

inC8,C6,C4andC2mem

bersandin

LeonForm

ation

Mud

£atinadeltaicplain.Coalind

icateshu

mid

clim

ate.Fragmentedfreshw

ater

mollusks

suggesta

high

-energyenvironm

ent.

Dino£agellatesandmicroforaminife

rallinings

indicatelocalm

arinein£u

ence

FA3(chann

elized

sand

ston

esandconglomerates)

Medium-to

thick-bedd

ed,m

edium-tocoarse

grained,andpebb

lysand

ston

e.Gravellagsand

mud

ston

eintraclastscommon

atbase

ofindividu

albeds.B

edsh

aveerosivebasesabove

mottled

sand

ymud

ston

esandsiltston

es,and

extend

laterally

uptofewtens

ofmeters.

Com

monly£o

atingpebb

leclastsoccur.

Granu

leandpebb

lestring

erslooselyde¢n

ing

large-scaleplanarcrossstrati¢cationoccur

InterbeddedwithFA

4inUpp

erCarbonera

(C5^C2mem

bers)tothewest,andinC1and

Low

erGuayabo

everyw

here.Interbedd

edwith

FA5andFA

6inUpp

erGuayabo

Form

ation

Stream

£owdepo

sits.L

ooselyde¢n

edlarge-

scale,low-angleplanarcross-strati¢cation,

absenceofwellde¢nedno

rmalgradingand

frequent

£oatingpebb

lessuggestd

epositionin

braided£u

vialchannels


M. Parraet al.

Tab

le2.

(Con

tinu

ed)

FaciesAssociation

Description

Stratigraphicoccurrence

Interpretation

rarely.Frequently,

stratagradeup

wardinto

variegated

mud

ston

e(FA4)

FA4(overbank¢n

es)

Reddish

tobrow

n,massive

tocrud

elystrati¢ed

sand

ymud

ston

eandsiltston

e.Ubiqu

itou

smottlingandroottraces(Fig.6b).S

poradic

mud

cracks

(Fig.6c)andiron

nodu

les(Fig.6d).

Lenticular,no

rmallygraded,thinsand

ston

einterbeds

Dom

inantfaciesinC5^C2mem

berstothewest

andinC1andLow

erGuayabo

totheeast.L

ess

frequent

inUpp

erGuayabo

Fluvial£o

odplainenvironm

ent.Dissecation

cracks,pervasive

mottlingandferrug

inou

sno

dulesindicatepedo

genesisdu

ring

£uctuating

wet^drycond

itions

FA5(granu

leto

pebb

leconglomerates

and

sand

ston

es)

Medium-tothick-bedd

ed,clast-sup

ported

granuleandpebb

leconglomerates.Ind

ividual

beds

have

sheet-likeandlenticulargeom

etry,

poor

developedsubh

orizon

talstrati¢cation

and

rarelylow-anglethroughcross-strati¢cation

andclastimbrication.Mod

eratesorting.Flat,

nonerosionalbases

arecommon

(Fig.6e).

InterbeddedwithFA

4andFA

3

Occasionally

inC1mem

ber;Frequent

inup

per

partofLow

erGuayabo,and

Upp

erGuayabo

Stream

£owdepo

sitsin

subaerialallu

vialfans

FA6(m

assive,coarse

conglomerates

and

sand

ston

es)

Upto10

m-thick,dom

inantly

clast-supp

orted

cobb

leandpebb

leconglomerate(Fig.6f).

Subangular

towellrou

nded

clasts.Ind

ividual

beds

have

ribb

on-likegeom

etry,latterally

continuo

usfortens

ofmetes

anddisplaybasal

scou

rs.M

oderateto

poor

sorting,un

graded

toreversegradingandcrud

eim

brication.

Lenticularsandstone

interbeds.Lith

ofacies

Gcd,G

co,S

m.Interbedd

edwithFA

3andFA

Exclusivelyinup

perpartofLow

erGuayabo,

andUpp

erGuayabo

Gravelbarsin£u

vialchannelson

alluvialfan



gether, these features represent deposition in the proximalsector of alluvial fans.

Facies distribution

Both ¢ne- and coarse-grained strata constitute the sedi-mentary ¢ll of the Medina Basin. Coarse-grained £uvialdeposits are commonly con¢ned to the western border ofthe basin and appear in units as old as laterally equivalentstrata to theC6^C5units of the eastern margin (Parra etal.,2009a; Pro¢les 1^4; Fig. 7). Conversely, ¢ne grained lacus-trine and marginal marine deposits are almost exclusivelyrestricted to the eastern margin of the basin.There, the ba-sin ¢ll can be subdivided into two coarsening-upwards cy-cles, largely delineated by the eastward progradation ofbraided stream deposits that constitute the C1member ofthe Carbonera Formation over areas dominated previouslyby estuarine systems (C5^C2 members; Fig. 7). Spatiallyextensive freshwater-lake deposition punctuated by short-lived marine incursions (facies association FA 2) of theCarbonera and Leo¤ n formations exclusively occur in theeasternmost distal part of the basin.This facies associationin theLeo¤ n Formatio¤ n is 450m thick andmarks the begin-ning of the uppermost coarsening-upward cycle. Progres-

sively coarser grained braided stream and sheet£ooddeposits become more abundant upsection. Finally, coarsealluvial-fan conglomerates prograded eastward and be-yond the eastern margin of the basin toward the Llanosplains. A coeval increase in tectonic deformation rates inthe Eastern Cordillera is suggested by the ¢rst occurrenceof growth unconformitieswithin the coarse conglomeratesof the Upper Guayabo Formation (Mora, 2007).

Palaeocurrent indicators re£ect a predominant easterlytransport, locally varying fromNE to SE directions (Figs 4and 5).This palaeo£ow pattern, the trend toward an east-ward change of facies from alluvial to estuarine strata, andthe presence of growth strata within the early lower Mio-cene to Pliocene strata clearly indicate syntectonic sedi-mentation related to the uplift and denudation ofmountainous terrain to the west of the basin.

Age constraints

We build upon our chronostratigraphic framework re-leased previously for the Late Cretaceous ^ Oligocenefrom theMedina area (see Jaramillo &Dilcher, 2000; Parraet al., 2009a, and references therein) by providing a newbiozonation based on palynomorphs for the Carbonera

0

100

200

500

5. Maya

C6

C5

C4

6. Humea0

100

200

300

400

C2

C3

C4

C5

7. Bellavista0

100

200

C2

8. Gazaunta south0

100

200

300

400

500

600

700

C1

9. Gazaunta north

100

200

300

0

C1

10. Gazatavena-Gazamumo

0

100

200

300

400

500

León

Fig.4. Measured straigraphic pro¢les of the Carbonera (C6^C1members) and Leo¤ n formations in the southeastern sector of theMedina Basin (locations in Fig.1b), including lithostratigrahic correlation based on ¢eld-based and remote-sensing observations,interpreted facies associations and palaeocurrent measurements. Locations of facies photographs of Fig. 6 are shown.


M. Parraet al.

0

100

300

200

400

500

700

600

11. Tontogüe 1

Car

bone

ra C

1

11. Tontogüe 2

1200

1100

1000

900

800

Car

bone

ra (

C1)

+ L

eón12. Tontogüe 3

0

100

200

300

400

13. Portones

1000

900

800

700

600

500

400

300

200

100

0

Upp

er G

uaya

boLo

wer

Gua

yabo

Fig. 5. Measured straigraphic pro¢les of the Carbonera Formation and the Guayabo Group in the northwestern sector of theMedinaBasin (locations in Fig.1b), including lithostratigrahic correlation based on ¢eld-based and remote-sensing observations, interpretedfacies associations and palaeocurrent measurements. Locations of facies photographs of Fig. 6 are shown.



Fig. 6. Photographs of representative sedimentary facies from theMio^Pliocene sedimentary units of theMedinaBasin. Locations areindicated inFigs 5 and6. (a)Mollusc-rich horizon at the top of theC2member (HuesserHorizon) in Pro¢le 7 (Bellavista).Approximately1.5m-thick shell bed formed by densely packed specimens of the gasteropodSheppardiconcha (shown in inset) and molds of thin-shelledbivalveAnondonites (Go¤ mez, A. et al., 2009) embedded in muddy matrix (facies association FA1). Dip direction is to the left. (b)Well-developedmudcracks and root traces in pedogenically altered £oodplain deposits (facies association FA4) of theC1member in Pro¢le11(Tontogˇe 2). See pencil for scale. Dessication cracks (c) and ferruginous nodules (d) in massive, pedogenically altered £oodplainsiltstones and silty sanstones of theC1member inPro¢le11 (Tontogˇe1). (e) 2m-thick, subtabular, granule-to-pebble conglomerate bedwith nonerosive basal contact (facies association FA5) overlying £oodplain siltstones in the Lower Guayabo Formation along Pro¢le12(Tontogˇe 3). (f)View toward theNof massive cobble and block conglomerates of theUpperGuayaboFormation inPro¢le13 (Portones).Bedding dip is �121 toward the SW (left).


M. Parraet al.

(C5^C1 members), Leo¤ n, Lower Guayabo and UpperGuayabo Group, which follows standard biostratigraphicmethods (Traverse, 1988). Palynological zones were cali-brated with foraminiferal (Diaz de Gamero, 1977a, b, 1985,1988, 1989, 1997; Wozniak & Wozniak, 1987; Diaz de Ga-mero & Linares, 1989; Rey, 1990), vertebrates (Linares,2004) and magnetic stratigraphic information (Herrera,2008) from the Urumaco Basin in westernVenezuela.

Our main results are summarized in Fig. 8. Units C5 toUpper Guayabo were deposited within palynologicalzones T-12 to T-18 (biozones after Jaramillo & Rueda,2004), corresponding to early Miocene to Pliocene time(Fig. 4). The pollen zone ZoneT-12 is de¢ned at the baseby the last appearance datum (LAD) of Cicatricosisporitesdorogensis, and at the top by the ¢rst appearance datum(FAD) of Echitricolporites maristellae. This zone is dated asthe lower part of the early Miocene.The top of ZoneT-13

is de¢ned by the FAD ofGrimsdalea magnaclavata and cor-responds to the upper part of the earlyMiocene.The FADof Crassoretitriletes vanraadshooveni de¢nes the top of ZoneT-14, which is dated as the upper part of the earlyMioceneto middleMiocene.The top of middleMiocene ZoneT-15is marked by theFAD ofFenestrites spinosus.TheFAD ofCy-atheacidites annulatusmarks the top ofZoneT-16,which cor-responds to the upper part of the middle Miocene to lateMiocene.The ZoneT-17 is de¢ned at the top by the LADof Lanagiopollis crassa, and is dated as late Miocene to ear-liest Pliocene.Finally, theZoneT-18 encompasses the Plio-cene to modern times.

Unroofing of Eastern Cordillera source areas

We evaluate the unroo¢ng history of the basement-coreduplift of the Quetame Massif by tracking the occurrence

Alluvial-fan deposits

Braided fluvial deposits

Estuarine deposits

?

1. Piñalerita2. Guadualera

C7-

C6

3. Gacenera

C6-

C5

4. Guaicarama

5. Maya

6. Humea

7. Bellavista

8. Gazaunta sur

Car

bone

ra (

C1)

9. Gazaunta norte

10. Gazatavena-Gazamumo

León

Car

bone

ra (

C1)

11. Tontogüe 1

12. Tontogüe 2

C1

+ L

eón

L. G

uaya

bo

12. Tontogüe 3

13. Portones

Upp

er G

uaya

bo

Base C7Base C7

Gacenera horizon

Guaicarama horizon

Huesserhorizon

~ 300 m estimated inseismic lines

NW SE

~5 km

Fig.7. Scheme of facies distribution in theMedina Basin based on a simpli¢ed representation of measured stratigraphic pro¢les 5^13(this study) and1^4 (Parra,M et al., 2009a). Locations of pro¢les are shown in Fig.1. Easterly sourced coarse-grained £uvial strata in theCarbonera Formation occur mainly along the western sector of the basin and grade eastward to temporarily marine-in£uencedlacustrine deposits.The distribution of facies delineate two main coarsening upward cycles. See text for discussion.



Los

Cue

rvos

C8

C7

- C

5

Thickness(km)

Bar

coG

uada

lupe

462

502

104

89

207

Epoch

Age (Ma)

Palynozone

Reference

65 60 55 50 45 40 253035

Paleocene Eocene Oligocene Miocene.

80 75 70 20 15 510

Late CretaceousSub-epoch / Stage

Plio.

Channelized sandstone

Variegated mudstone

Interlayered sandstone and mudstone

Sandstone

Mudstone

Conglomerate

119

25

94

163

2927

66

7234

207

96

Uni

t

Bio

zone

C3

C2

C1

León

Upp

er G

uaya

bo

6000

5000

4000

3000

2000

1000

Fig. 8. Palynological biozonation and composite stratigraphic section of the Late Cretaceous^Pliocene strata of theMedina Basinbased on stratigraphic sections by Jaramillo &Dilcher (2000), Parra etal., (2009a), and this study.The assignment of biozones is based onJaramillo & Rueda (2004), Jaramillo et al. (2005) and Jaramillo et al. (2009) (references coded in zonal scheme with numbers1, 2 and 3,respectively).The slope of the curve indicates sediment-accumulation rates. Rates based on decompacted thickness are indicated (seetext and Table S1).Time scale fromGradstein et al., 2004).


M. Parraet al.

of distinct conglomeratic clasts in the Oligo^Miocenestrata of the Medina Basin. Conglomerate clasts are com-posed of two main lithologies: vein quartz and sedimen-tary lithic fragments, with an increasing abundance of thelatter upsection (Fig.9). Conglomerate petrography docu-ments a ¢rst appearance of diagnostic Upper Cretaceousglauconitic sandstone fragments in the lower Miocene C1Member of the Carbonera Formation (Fig. 9). Glauconiticsandstone gravel is present throughout theMiocene sedi-ments and constitutes as much as �40%of the bulk sedi-mentary composition. However, within the Mioceneunits, we identi¢ed two peak intervals in the occurrenceof glauconitic sandstone clasts. The ¢rst peak occurs inthe upper part of the C1Member, and the second peak inthe Guayabo Group. These zones are separated by inter-vals witho �4% of glauconitic sandstone clasts. In addi-tion, a small amount of phosphatic sandstones (up to 3%),and as much as 25% of siliceous siltstones are associatedwith the ¢rst peak of glauconitic sandstones in the uppersections of theC1Member.Finally, redbed clasts indicativeof the Upper Palaeozoic sedimentary units are absent inthe Oligocene^Miocene sedimentary record of the basin.The ¢rst appearance of Palaeozoic red sandstone clasts isobserved in the undated lower alluvial terrace levels of theHumea and Gazaunta rivers of the inferred Quaternaryage (see Fig.1 for location).

In the source area, the Upper Cretaceous glauconiticsandstone-bearing units (the Une, Chipaque and Guada-lupe formations) have been completely eroded from thebasement-cored Farallones anticline and only crop outalong its £anks (Fig. 1). From the gravel petrography data,we interpret a normal unroo¢ng sequence that can besummarized as follows (Fig.9): (1) an absence of glauconi-tic sandstone clasts in the conglomerates below the C1Member suggests a source dominated by Palaeogenerocks; (2) during earlyMiocene accumulation of the upperportion of the C1 Member, an important fraction of thesource area included glauconitic and phosphatic sand-stones, and siliceous siltstones indicative of erosion of theUpper Cretaceous Guadalupe Group; (3) conglomeratesin the uppermost portion of the C1 Member and thecoarse-grained strata laterally equivalent to the lower partof the Leo¤ n Formation are devoid of glauconitic sand-stones and siliceous siltstones, suggesting an exposure ofthe mudstone-rich Cretaceous Chipaque Formationrather than theGuadalupeGroup; and (4) the renewed oc-currence of glauconitic sandstone likely re£ects unroo¢ngof the Cretaceous Une Formation during the late Mio-cene^Pliocene accumulation of the GuayaboGroup.

We thus derive approximate denudation rates on the ba-sis of the thickness of the reconstructed erosion window(e.g., DeCelles et al., 1991) and the time of denudation asconstrained by the statigraphic age of the appearance ofparticular clasts. Upper Cretaceous glauconite-bearingunits typically have a thickness of �1.5 to 2 km in the East-ern Cordillera (Mora et al., 2006). An upper limit for thethickness of the eroded rock is provided by the absence ofPalaeozoic clasts in the investigated Oligocene^Miocene

strata. Such a scenario limits the maximum value oferoded thickness to that of the overlying Cretaceous sec-tion (6.5 km). We, therefore, estimate an apparent long-term 1D denudation rate of as much as 0.3mmyr�1 forthe area of the Quetame massif in the interval between�23 and 2Ma. Similar apparent exhumation rates havebeen inferred from thermochronological data in the East-ern Cordillera (Parra et al., 2009b).

Sediment-accumulation rates

1D analysis

Sediment accumulation in the Medina Basin re£ects athree-stage history characterised by an Eocene^earlyOligocene episode of slow sediment accumulation withrates of 30^70mmy�1 that separates two periods offaster accumulation during Late Cretaceous^Paleocene( �100mmy�1) and late Oligocene^Pliocene time( �220mmy�1), respectively. In agreement with the dis-tribution of Cenozoic sedimentary facies and unconfor-mities in the southern Middle Magdalena Valley Basin(Go¤ mez et al., 2005), plausible explanations for these sedi-ment accumulation trends and tectonic subsidence rateshave been explored by Parra et al. (2009a). They interpretthis pattern as the result of alternating episodes of craton-ward and orogenward migration of the orogen-basin pair.

Here, we particularly consider the signi¢cance of thelate Oligocene^Pliocene episode of rapid accumulation.A ¢rst increase in the rate of sediment accumulation oc-curs at the base of BiozoneT-10, at �30Ma, which corre-sponds to the base of the C7 member of the CarboneraFormation in the Guadualera pro¢le (Fig. 8). On the basisof a comprehensive examination of exhumation patternsderived from thermochronology and of other indicatorsof deformation in the Eastern Cordillera, such an episodewas likely associatedwith an eastwardmigration of the tec-tonic loads to the present-day axial sector of the EasternCordillera, toward the Soapaga fault (Parra et al., 2009b).Our new data reveal a second, more pronounced increasein rates of sediment accumulation at the beginning of theearly Miocene Biozone 31 ( �23Ma). During the earlyMiocene, spanning �7my, accumulation of �3350m ofsediments implies peak mean accumulation rates of�480mmy�1. Our data further suggest that middleMiocene accumulation rates decline to values of �100mmy�1during accumulation of theLeo¤ n andLowerGuaya-bo formations. However, we interpret this result with ex-treme caution, as this part of the composite section isbased on lithostratigraphic correlation of £uvial depositsin theTontogˇe section, in the northwest of the basin, withtheir distal, laterally equivalent lacustrine units in thesoutheast, spanning a distance of �35 km along the struc-tural termination of the Guavio anticline (Figs 1 and 7).Despite these unavoidable di⁄culties imposed by thelocation of the best-exposed sections in the densely vege-tated area, our correlation suggests that rapid sediment ac-cumulation prevailed throughout theMiocene^Pliocene.



Channelized sandstone

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DETRITAL MODES

Fig.9. Compositional trends in Eocene^Pliocene conglomerates of theMedina Basin. Black circles denote the stratigraphic position ofconglomeratic samples. Clasts of Upper Cretaceous glauconitic sandstone, phosphatic sandstone and siliceous siltstone occur inMiocene strata of the Carbonera Formation andGuayabo Group, documenting progressive unroo¢ng of the Eastern Cordillera (rightpanel). Raw data and recalculated modes are reported inTable1. See text for discussion.


M. Parraet al.

3D sedimentary budget

In order to account for potential lateral thickness variationsand to provide a more regionally meaningful assessment ofthe sedimentary budget, we interpreted an extensive gridof �70 2D industry multichannel seismic re£ection pro-¢les totalling �1800 km.We translated six mapped lithos-tratigraphic limits into the seismic pro¢les intersecting theoutcrop exposures close to the measured sections. Wechose horizons that constitute clearly traceable markerseither exposed in the basin or in the seismic pro¢les.Wetrace stratigraphic limits southward along the hanging wallof the Guaicaramo thrust toward the area where the thrustloses displacement and ¢nally terminates, allowing the in-terpreted horizons to be extended eastward toward thefootwall.We further tied seismic re£ectors with the strati-graphy based on interpreted depths of well-de¢ned hori-zons from borehole reports. We interpret ¢ve rock unitsbounded by the following horizons (Table 3): (1) the top ofthe Eocene Mirador Formation sandstones; (2) the top ofthe C6member, de¢ned by the appearance of the fossilifer-ousGuaicarama horizon; (3) the top of the C2member, de-¢ned by the outcrop of the fossiliferous Huesser horizon;(4) the top of the C1member, de¢ned by the change of £u-vial overbank deposits and channelized sandstones tomonotonously bedded, organic-rich mudstones of theLeo¤ n Formatio¤ n; and (5) the top of the Leo¤ n Formationmudstones. Stratigraphic ages for these horizons are inter-polated from the palynological zonation.

Based on interpolations between seismic re£ectors foreach of these horizons, we constructed surfaces in a two-way travel time for the hanging- and footwall blocks ofthe Guaicaramo thrust. In order to avoid errors in areas ofpoor seismic-re£ection coverage, we exclude the area inthe footwall beneath the thrust sheet. Depth conversionwas carried out for surfaces of each blockusing seismic ve-locities obtained from check-shots surveys from the Co-poro-1 and Medina-1 wells in the hanging-wall block ofthe Guaicaramo fault, and from the Guacav|¤ a-1, Chapar-ral-1, San Pedro-1 and Up|¤ a-1 wells in the footwall (seedepth^time relations in Fig. S1). Finally, to reveal spatio-temporal variations in sediment-accumulation rates, wecalculated an average sediment-accumulation rate (com-pacted) for each of the ¢ve interpreted rock-units, com-puted by dividing the volume of rock between successivedepth-converted surfaces (evaluatedwithin the 2Dprojec-tion area of the smaller, usually upper surface of each rockpackage), by the area of that 3D surface. In addition, wegeneratedmaps of sediment-accumulation rates, obtainedby dividing isopach thicknesses by the geologic time re-presented by each unit. Parameters for volumetric calcula-tions and results are reported inTable 3.

Figure10 shows the middleEocene^Holocene history ofvariation in sediment-accumulation rates.The sediment-budget pattern for the entire area resembles that of the1Dreconstructed basin history, albeit with a broader resolu-tion resulting from the larger time windows into whichthe thickness data are binned. After limited late Eocene^

early Oligocene accumulation, rapid sediment depositionat rates of100^350mmy�1 (compacted thickness) have oc-curred since theMiocene,with an absolute minimumdur-ing accumulation of the middleMioceneLeo¤ n Formation.The more regionally meaningful, 3D reconstruction of se-diment accumulation reveals patterns not captured in the1D reconstruction.First, as a result of the lower resolution,the onset of rapid sediment accumulation at �30Ma isnot portrayed in the 3D sedimentary budget. Second, theabsolute maxima in mean sediment-accumulation ratesoccur during the youngest history of the basin, repre-sented here as late-Miocene to Holocene. This pattern isindependently captured in the sedimentary budget of eachblock of theGuaicaramo fault (Fig.10).Third, higher sedi-ment-accumulation rates occur in the western, hanging-wall block of the thrust throughout the basin history. Fi-nally, a local maximum in sedimentation rates restrictedto the hanging-wall block of the thrust occurs during de-position of the C1member.

The spatial distribution of sediment-accumulationrates for the ¢ve time windows analysed displays a consis-tent pattern of eastward-decreasing rates without majornorth^south variations along the strike (Fig.11). Our ana-lysis also portrays widespread increases in sedimentationrates during the accumulation of the lower Miocene C5^C2 members of the Carbonera Formation and enhancedthe accumulation in the proximal, western part of the ba-sin during deposition of theC1member.Likewise, an over-all increase in sedimentation rates is a characteristic sincethe late Miocene, but is slightly more pronounced in thenorthern part of the basin.

DISCUSSION

Early Miocene basin evolution

Integration of the multiple datasets presented in thisstudy allows a correlation of tectonic episodes in the oro-genwith the distribution and rates of sediment accumula-tion in the adjacent basin.The earlyMiocene represents aminimum age for folding associated with the initial mo-tion on the Lengupa¤ fault west of the basin, as supportedby growth^strata relationships in rocks equivalent to theC5^C2 members. An independent assessment of the tim-ing of thrust-related exhumation in the Eastern Cordillerais available from the mineral cooling ages derived fromapatite and zircon ¢ssion-track data from the eastern £ankof the EasternCordillera.This thermochronological infor-mation documents the initial exhumation and uplift dur-ing the middle-Eocene toOligocene (40^30Ma) in its axialsector (Floresta Massif) and during the late Oligocene^early Miocene (25^20Ma) along its eastern £ank (Toro,1990; Parra et al., 2009b).

An abrupt two- to four-fold increase in sediment-accu-mulation rates is observed at the base of the lower Mio-cene lower Carbonera Formation (C6^C5 member) in theMedina Basin (Figs 8, 10 and 11), which followed a period



of rapid accumulation that commenced in the late Oligo-cene ( �30Ma). Sediment-accumulation rates are primar-ily a¡ected by tectonic subsidence in foreland basins whereaccommodation space provided by lithospheric £exure isnearly ¢lled or over¢lled with sediments (Burbank et al.,1988; Jordan, 1995). In the foreland of the Colombian An-des, a dominant orogen-perpendicular, eastward palaeo-current direction observed in Miocene units and the lackof evidence of Miocene forebulge erosion (e.g., Bayonaetal., 2008) point toward a ¢lled-to-over¢lled foreland ba-sin. However, nonmarine basins do have a topographicgradient and the fan apex might be as high as several hun-dreds of metres (e.g., Blair & McPherson, 1994). In such acase, sediment accumulation rates may overstimate subsi-dence rates. For theMedina basin, we infer that maximumelevation at the fan apex never exceeds the present-dayelevation of �300m at the outlets of main rivers towardthe Llanos basin alluvial plain (e.g., the Humea River, Fig.1c). This represents only a minor overestimation, andhence permits using sediment-accumulation rates as aproxy for tectonic subsidence. Foreland basin models thatconsider crustal accommodation on a elastic plate (e.g.,Flemings & Jordan, 1989; DeCelles & Giles, 1996) suggest

that an upsection increase in tectonic subsidence re£ectsthe relative shift of the depositional site toward a moreproximal sector within the foredeep depozone, in responseto the migration of the £exural pro¢le accompanyinggrowth and forward propagation of the orogenic wedge.However, the e¡ect of this tectonically enhanced accom-modation space is attenuated toward the distal part of thebasin.As a result, farther away from the deformation front,such an increase in sedimentation rates may be delayedwith respect to thrusting (e.g., Flemings & Jordan, 1990;Jones et al., 2004). Alternatively, other £exural models(Quinlan & Beaumont, 1984; Beaumont et al., 1988) predictthat, under the presence of static loads or even tectonicquiescence, such deepening and narrowing of the basinmay result from stress relaxation on long-time scales in aviscoelastic plate. In the eastern £ank of the eastern Cor-dillera, synchroneity between independently constrainedearly Miocene ages of thrustbelt advance toward the Len-gupa¤ fault (Parra et al., 2009b) and an increase of tectonicsubsidence in the Medina Basin suggest a causal relation-ship between these phenomena. Although our data cannotcompletely rule out viscoelastic relaxation of the SouthAmerican plate, such synchroneity can be explained with

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Fig.10. Reconstructed one-dimensional sediment-accumulation rates for theMedina Basin (green and black dotted lines), and three-dimensional (3D) sedimentary budgets for theMedina and proximal Llanos basins (blue and red lines, respectively). Errors in 3Daccumulation rates are indicated by the shaded areas. Episodes of faster accumulation are coeval to deposition of coarse-grained faciesin both the western and eastern sectors of the basin, as indicated by lithologies in the upper panel (colour shading as in Fig. 5). Plioceneeastward progradation of coarse conglomerates is contemporaneous with faster shortening rates in the eastern £ank of the EasternCordillera. See text for discussion.


M. Parraet al.

73°0′W73°30′W

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mean sediment accumulation rate:hanging wall: 104 m/myfootwall: 77.5 m/my

mean sediment accumulation rate:hanging wall: 195 m/myfootwall: 173 m/my




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Fig.11. Spatial distribution of sediment-accumulation rates (compacted thickness) for ¢ve interpreted Eocene to Holocenestratigraphic units in the hanging wall (MedinaBasin) and footwall (LlanosBasin) of theGuaicaramo thrust.Towns and the present-daysur¢cial trace of the thrust and indicated for reference. Inset shows the location of the mapped area.



a purely elastic plate. Furthermore, in-phase thrustingand rapid sediment accumulation �20 km away of the de-formation front suggest an almost immediate responsetime between crustal loading and the increase of sediment£ux, which ultimately results in coeval thrusting in thehinterland and gravel progradation into the basin. Such ascenario is di¡erent from the predictions of numericalmodels (e.g., Flemings & Jordan, 1990; Sinclair et al., 1991),and from the results of case studies (e.g., Schlunegger etal.,2007) that document a response time of several millionyears between crustal thickening and enhanced sedimenttransfer.We suggest that the close proximity of theMedinabasin to the location of the Early Miocene deformationfront in the QuetameMassif may have conditioned such arapid response, which is di¡erent from the larger responsetime of sediment transport from a sediment source lo-cated beyond the wavelength of the orogen/basin interac-tion. In addition, such a shorter response time than thatpredicted by numerical models that consider di¡usion asthe mechanism governing sediment transfer suggests thatit can rather be dictated by advection, as recent modelssuggest.

In the Medina Basin, the C6^C1 members of theCarbonera Formation comprise a lower Miocenecoarsening upward cycle approximately 2500m thick.Wecalculate an average 1D decompacted accumulation rateof �480mmy�1 over a span of �7Myr (Figs 8 and 10),which iswithin the upper limit of long-term accumulationrates determined for most nonmarine foreland basins(Burbank et al., 1988; Meigs et al., 1995; Schlunegger et al.,1997b; Echavarria et al., 2003; Uba et al., 2007). Sedimen-tary facies and provenance analysis of the Carbonera For-mation document the accumulation of westerly sourcedsediments derived from Mesozoic and Palaeogene sedi-mentary rocks from the Eastern Cordillera along an east-ward-sloping alluvial plain that transitioned to a low-energy, tidally in£uenced estuarine system.The distribu-tion of sedimentary facies reveals an earlier accumulationof £uvial deposits along the western margin of the basin(upper part of C7^C5 members; Guadualera^Gacenerapro¢le; seeFig.7 and alsoParra etal., 2009a) comparedwiththe east (Guaicaramo and Maya sections; Figs 4 and 7).Subsequent forelandward migration of coarser-grained,£uvial deposits resulted in the accumulation of the C1member at the eastern margin of the basin (Gazaunta sec-tion; Figs 4 and 7).Our volumetric sedimentary budget re-veals that such a progradation of coarser-grained facieslikely occurred during a period of peak accumulation andsubsidence rates, whichwas restricted to theMedinaBasinin the latest early Miocene ( �19^16Ma). Farther east, inthe footwall of the Guaicaramo fault, accumulation ratesduring deposition of the C1 member are slightly lowercompared with those of the underlying lower Mioceneunits (Figs10 and11).

In light of the reconstructed position of the orogenicfront,we interpret such a pattern as the result of an episodeof tectonic thickening along a stationary deformation frontlocated immediately to the west of the basin. Overall, the

facies distribution and the reconstructed earlyMiocene ac-cumulation history of the Medina Basin support thoseforeland-basin models that predict coeval thrusting andaccumulation of coarser grained facies in the proximal partof the foredeep depozone (e.g., Burbank et al., 1988; Schlu-negger et al., 1997a). Our data suggest that this mechanismmay have been active at multiple time scales. First, coarse-grained facies of theC6^C1members of theCarboneraFor-mationwere deposited in the proximal foredeep during anepisode of fast subsidence lasting �7m.y. during the earlyMiocene. Second, peak eastward progradation of thesecoarse-grained sediments towards the easternmost sectorof the present-day Medina wedge-top basin (C1 member)appears to have occurred during episodes of maximum lo-cal subsidence associatedwith active thrusting.

Middle-Miocene basin evolution

A second coarsening-upward cycle corresponds to the ac-cumulation of the middle Miocene Leo¤ n Formation andthe lateMiocene^PlioceneGuayaboGroup (Fig.7). Similarto the underlying coarsening-upward cycle of the uppermembers of theCarboneraFormation, this coarsening-up-ward pattern is more pronounced in the eastern sector ofthe basin, where the Leo¤ n Formation comprises tidally in-£uenced lacustrine deposits punctuated by short-livedmarine incursions. This mud-dominated sequence pro-gressively changes westward to laterally equivalent £uvialdeposits in the eastern limb of the Nazareth syncline (Fig.7). Such a lateral facies change caused the lower portion ofthis cycle to directly overlie similar £uvial deposits of theC1 member, thus partially obscuring the coarsening-up-ward pattern (Fig. 7). A similar pattern of a westward in-crease in the sand-to-mud ratio in the Leo¤ n Formationoccurs approximately 100 km to the north along the wes-tern margin of the Llanos basin (Cooper etal., 1995). Poten-tial causes of such an anomalous widespread accumulationof ¢ne-grained sediments in temporarily marine-in£u-enced, primarily lacustrine environments in proximal sec-tors of the foredeep may have included several factors.These entail eustatic sea level change (e.g., Van Wagoner,1995), reduced erosion rates in the source area due to anarid (e.g., Paola et al., 1992; Schlunegger & Simpson, 2002)or stable (Molnar, 2004) climate, and an exposure of ero-sion-resistant lithologies in the source area causing a gen-eral decrease in erosion rate and sediment supply leading tounder¢lling or sediment starvation in the basin (e.g.,Schlunegger & Simpson, 2002; Carroll et al., 2006). Alter-natively, accumulation of ¢ne-grained sediments may haveresulted from waning tectonics (e.g., Jordan et al., 2001) orfrom the exposure of nonresistant, ¢ne-grained lithologiesthat are less likely to generate coarser sediments (DeCelleset al., 1991). Below, we explore each of these scenarios.

First, recently published sea-level curves (e.g., Kominzet al., 2008; and references therein) do not show any signif-icant increase in eustatic level between early and middleMiocene time, arguing against a causal link between mid-dle Miocene deposition of marine-in£uenced ¢ne-


M. Parraet al.

grained sediments in the foreland of theColombianAndesand eustatic sea-level changes (Cooper et al., 1995; Go¤ mezet al., 2003). Second, whether regional climatic changedrove the accumulation of ¢ne-grained sediments in themiddle Miocene is unclear due to the paucity of detailedreconstructions of pre-middle Miocene climate condi-tions. However, high-resolution palaeoclimate proxies innorthern South America for the last 13m.y. suggest that awetter-than-present-day climate punctuated by intermit-tent aridity promoted rapid erosion during the middleMiocene and Pliocene times (Harris &Mix, 2002). In con-trast, an opposite climatic pattern (i.e., a relatively wet andstable climate) prevailed during late Miocene time, whichmay have resulted in low erosion rates (Harris & Mix,2002). It is hard to envision that this post-middleMioceneclimatic pattern of diminishing wetness and increasingstability could have controlled the observed late Miocenecoarsening-upward trend in the deposits of the MedinaBasin. If climate had played an important role, such a pat-tern would have resulted in diminished rates of sedimentsupply, and thus would have prompted a contradictory ¢n-ing-upward trend in the middle to upper Miocene strata.Third, our unroo¢ng estimates document an early Mio-cene to Pliocene erosion window in the Eastern Cordilleraencompassing up to �2 km of Upper Cretaceous glauco-nitic-bearing units (Fig. 9). Clast composition suggeststhat, within this unroo¢ng sequence, a relatively highercontribution from the mud-rich, glauconitic-poor Chipa-que Formation characterised the accumulation of most ofthe ¢ne-grained middle Miocene Leon Formation. Over-all, gravel petrography data do support a correlation be-tween the erodability of source-area lithologies andgrain-size trends. However, had a high erodability of thesource areas exherted the dominant role on sedimenta-tion, accumulation of ¢ne-grained sediments of the LeonFormation should have accompanied an increase in sedi-ment accumulation rates as a consequence of an increasein sediment supply to the basin (e.g., Carroll et al., 2006;Korup & Schlunegger, 2009). On the contrary, both 1Dand 3D reconstructions of the post-early Miocene sedi-ment accumulation history suggest that sedimentationrates declined during middle Miocene deposition of ¢ne-grained sediments of the Leo¤ n Formation. Subsequently,an increase in sediment accumulation accompanied theprogradation of coarse-grained facies of the middle Mio-cene to Pliocene GuayaboGroup (Figs 7 and10).This pat-tern resembles the syntectonic origin of coarse-grainedfacies progradation in the proximal foredeep observed inother areas (e.g., Burbank et al., 1988; Paola et al., 1992;Schlunegger et al., 1997b; Horton et al., 2004), and hencesuggests that variability in tectonics, rather than climate,erodability or eustasy exerted the de¢ning control on sedi-ment accumulation trends.

Plausible tectonic scenarios that explain the decrease inaccumulation (and subsidence) rates accompanying thedeposition of ¢ne-grained strata of the Leo¤ n Formationinclude either backward stepping of thrust loads (e.g., De-Celles & Giles, 1996) or waning tectonic activity (e.g.,

Flemings & Jordan, 1990). Geologic evidence supportsthe notion that both mechanisms are not mutually exclu-sive and may have operated virtually synchronously. Back-ward stepping of the tectonic loads by out-of-sequencethrusting in the interior of the orogen is supported by thecross-cutting relationships of theSoapaga andPesca faultsin the axial sector of the EasternCordillera (Fig.1a).There,the Oligocene Concentacio¤ n Formation east of the Flores-taMassif is truncated to the west by the east-verging Soa-pagaFault. Early andMiddleMiocene apatite ¢ssion-trackages in the hanging-wall block (Parra et al., 2009b) mostlikely re£ect a synchronous slip along this fault. Similarout-of-sequence reactivation in the interior of the EasternCordillera has been documented �200 km farther north(Bayona et al., 2008). On the other hand, Flemings & Jor-dan (1990) andSinclair etal. (1991) demonstrated thatwan-ing tectonic activity in the thrustwedge generates forelandbasin accumulations with lens-shaped geometries, as op-posed to wedge-like geometries developed during activethrusting. Based on subsurface data in the Llanos basin,Cooper et al. (1995) show that middle Miocene mudstones(their sequence T80) extend farther eastward to theGuya-na shield than any of the older foreland basin strata. Inaddition, our sedimentary budget shows that mean accu-mulation rates between the hanging and footwall blocksof the Guaicaramo fault were more similar during theaccumulation of the Leo¤ n Formation (104 vs. 78mmy�1,respectively) than previously, during accumulation of theC5^C1members (207 vs. 108mmy�1, respectively; seeTa-ble 3 and Figs 10 and11). Such patterns re£ect a more uni-form distribution of tectonic subsidence along theforeland basin during accumulation of the Leo¤ n Forma-tion than before, and thus are compatible with the lens-shaped foreland strata.Taken together with the decline inrates of sediment accumulation, this suggests an episodeof diminishing tectonic loading in the Eastern Cordillera.

Late Miocene to Pliocene basin evolution

A major increase in grain size characterises the secondcoarsening-upwards cycle at the base of the upper Mio-cene Lower Guayabo Formation. There, sediment accu-mulation in braided £uvial and alluvial-fan settings aboveestuarine and bayhead deposits of the Leon Formationdocuments an event of sediment progradation. East ofMedina, the Leon^Guayabo transition in the Llanos basincorresponds to a major change in the seismic characterfrom a seismic sequence exhibiting continuous re£ectors,which include the Carbonera and Leon formations, to aunit with irregular discontinuous subparallel re£ectors(Fig. 12). In contrast to the facies progradation episodethat occurs at the transition between the Early MioceneC2^C1members in the Medina basin, susburface data in-dicate that the late Miocene progradational event wasmuch more regionally extensive. Upper Miocene^Plio-cene coarse-grained £uvial and alluvial-fan conglomerateswere primarily sourced fromUpperCretaceous units fromthe Eastern Cordillera and accumulated in the Medina



Table 3. Results from three-dimensional sedimentary budget of Eocene-Holocene units in theMedina and Llanos basins

UnitVolume(km3)

2DArea attop (km2)

3DArea attop (km2)

Age attop (Ma)

Acc. ratemin (mmy�1)

Acc. ratemean (mmy�1)

Acc. ratemax (mmy�1)

Guaicaramo hanging wall (Medina Basin)Guayabo 1177 744 755 3.6 � 1.8 144 195 300Leon 347 734 759 11.6 � 1.0 71 104 190C1 522 876 924 16.0 � 1.0 103 188 1129C2^C5 1102 1165 1284 19.0 � 1.5 101 156 344C6^C8 1020 1462 1526 24.5 � 1.5 32 38 48Mirador 42.0 � 2.0C1^C5 1624 876 924 16.0 � 1.0 160 207 293Guaicaramo footwall (western Llanos Basin)Guayabo 6793 3382 3386 0 159 173 189Leon 1153 3291 3380 11.6 � 1.0 53 78 142C1 949 3351 3339 16.0 � 1.0 52 95 569C2^C5 2118 3376 3387 19.0 � 1.5 74 114 250C6^C8 939 3472 3628 24.5 � 1.5 12 15 18Mirador 42.0 � 2.0C1^C5 3067 3351 3339 16.0 � 1.0 84 108 153

60041001

5879939

5754876

5629814

5504751

5379689

5254626

5129564

5004501

4879439

4754376

4629314

4504251

4379189

4254126

412964

40052

CDPSP

–1000

–2000

–3000

–4000

–5000

–6000

Tw

o-w

ay tr

avel

time

(ms)

1 km1 km

60041001

5879939

5754876

5629814

5504751

5379689

5254626

5129564

5004501

4879439

4754376

4629314

4504251

4379189

4254126

412964

40052

CDPSP

–1000

–2000

–3000

–4000

–5000

–6000

Tw

o-w

ay tr

avel

time

(ms)

Cus

iana

thru

st

Cus

iana

thru

st

La Florida anticline

Upía 1

1 km1 km

La Florida anticline

Fig.12. Seismic re£ection pro¢le CO-1995-10 across the western sector of the Llanos Basin (see Location in Fig.1a).Tops oflithostratigraphic units are indicated. A major change in seismic facies occurs at the top of the Leon Formation, where a seismicsequence characterised by continuous re£ectors is superseded by a unit with discontinuous re£ectors.This change represents theregional eastward progradation of alluvial deposits of the upperMioceneGuayaboGroup to deltaic and estuarine deposits of the Leonformation. See text for discussion.


M. Parraet al.

Basin during an episode of increasing sediment-accumu-lation rates (Figs 7, 10 and 11). As in the underlying coar-sening-upward cycle of the Carbonera Formation, such acoupled increase in both grain size and tectonic subsi-dence indicates a syntectonic origin for gravel prograda-tion (e.g., Burbank et al., 1988; Heller & Paola, 1992;Schlunegger et al., 1997b).

The reconstructed kinematic history of the EasternCordillera reveals stagnation of the orogenic wedge tipalong the Servita and Lengupa¤ faults, west of Medina,since late Oligocene (Mora et al., 2008; Parra et al., 2009b).We thus hypothesize that a renewed increase in the thrust-ing rates following proposed tectonic quiecense along thisfront triggered the increase in accomodation space in theproximal foredeep. Indeed, the development of growthunconformities in the Upper Miocene^Pliocene strata ofthe Upper Guayabo Formation in the Nazareth synclinesuggests that the uplift rates associated with fault-relatedfolding of the Farallones anticline were greater than localsediment-accumulation rates (Mora, 2007). In addition,regional eastward progradation of £uvial systems to theLlanos Basin, in the course of stagnation of the deforma-tion front, also suggests that sediment supply from the up-lifting Eastern Cordillera must have increased in the lowerlateMiocene (e.g., Schlunegger et al., 1997a). Plausible me-chanisms for the enhancement of sediment supply mayhave included an increase of exhumation rates, an increasein the topographic gradient induced by surface uplift inthe source area or encroachment of the catchments. En-hanced exhumation associated with orographically fo-cused erosion in the Quetame Massif area is documentedfor Pliocene times (Mora et al., 2008). However, availablethermochronometric data do not support a similar patternof increase in exhumation rates for the lower early Mio-cene (Parra et al., 2009b). On the contrary, widespread lateMiocene cooling ages in the Eastern Cordillera (Moraet al., 2009b; Parra et al., 2009b) suggest ubiquitous exhu-mation through the widening of the actively deformingareas. A coeval increase in sediment-accumulation ratesin a ¢lled-to-over¢lled basin re£ects an increase in tec-tonic subsidence, and hence suggests that tectonics musthave exerted a major control on the distribution ofcoarse-grained strata. Finally, rapid subsidence duringthe accumulation of the late Miocene^Pliocene UpperGuayabo Formation, besides re£ecting faster tectonicrates, may also have been favoured by enhanced sedimentloading. Models predict that widening and deepening ofthe basin may occur when su⁄cient sediment £ux fromthe orogen is coupled with e⁄cient mass transport in thebasin, which produces signi¢cant sediment loading andgenerates additional subsidence, even beyond the £exuralwave (Flemings & Jordan, 1989).

SUMMARYAND CONCLUSIONS

The stationary condition of the orogen-basin pair im-posed by the inherited structural fabrics of the eastern

border of the Eastern Cordillera provides an outstandingscenario for directly linking a long-lived history of exhu-mation of the Eastern Cordillera with the sedimentary re-cord of the adjacent proximal foredeep. 1D and 3Dreconstructions of sediment accumulation reveal thatcoarsening-upward trends in sedimentary facies occurprimarily as a result of increased tectonic activity in theEastern Cordillera. Climate, eustasy and di¡erential erod-ability of the source areas have played aminor role in deter-mining the large-scale trends of facies distribution in theproximal sector foreland basin system of the ColombianAndes during most of the Neogene.

ACKNOWLEDGEMENTS

This study was supported by grants and fellowships fromthe German Academic Exchange Service (DAAD) to MParra and A. Mora, the German Research Foundation(DFG), Str 373/19-1 to M. Strecker, funds from the Leib-niz Center for Earth Surface and Climate Studies at Pots-damUniversity, the project ‘Cronolog|¤ a de laDeformacio¤ nen las Cuencas Subandinas’ at the Instituto Colombianodel Petro¤ leo (Ecopetrol/ICP), and Universidad Nacionalde Colombia (Beca deHonor toM. Parra). Additional sup-port was provided by the Smithsonian Tropical ResearchInstitute (STRI).The seismic data used in this work werereleased by an agreement with the Colombian NationalHydrocarbons Agency (ANH).We are grateful to J. Cardo-na at ANH for his help in providing data for this study.Seismic interpretation was carried out using the PET-RELt software package through an academic licensekindly provided bySchlumberger. J. Sayago at the PotsdamUniversity is thanked for her help and advice during seis-mic interpretation. C. Caldana is greatly acknowledged forher help with the graphic work.The ideas presented herebene¢ted from informative discussions with B. Horton,P. Ballato and T. Gaona.We thank S. Moro¤ n, L. Quiroz,A. Rodr|¤ guez and O. Romero for their help during ¢eldwork.The manuscript was improved by the very construc-tive reviews of Paul Heller, Jaume Verge' s and Fritz Schlu-negger, and the Editor, Peter van der Beek.

SUPPORTING INFORMATION

Additional Supporting Information may be found in theonline version of this article:

Table S1. Input parameters for decompaction of theCenozoic Strata of theMedina Basin.

Table S2.Description and interpretation ofLithofacies(afterMiall, 1996; Einsele, 2000).

Fig. S1.Velocity models for seven boreholes in theMed-ina andLlanos basins. Locations ofwells are shown in Fig.1a. Models are based on check-short surveys measuringthe seismic travel-time from the surface to known depths.Since velocity gradient decreases eastward toward the



Llanos basin, we used an average values for each block ofthe Guacaramo thrust. See text for further explanation.

Please note:Wiley-Blackwell are not responsible for thecontent or functionality of any supporting materials sup-plied by the authors. Any queries (other than missing ma-terial) should be directed to the corresponding author forthe article.

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Manuscript received 22 July 2009;Manuscript accepted3 December 2009.



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