Development of the Colombian Foreland-basin System_Gomez2005

Geological Society of America Bulletin

doi: 10.1130/B25456.1 2005;117, no. 9-10;1272-1292Geological Society of America Bulletin

Elas Gmez, Teresa E. Jordan, Richard W. Allmendinger and Nestor Cardozo

diachronous exhumation of the northern AndesDevelopment of the Colombian foreland-basin system as a consequence of

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ABSTRACT

This study addresses multiple controls on foreland-basin accommodation and con-tributes to enhanced understanding of the evolution of the northern Andes. The Middle Magdalena Valley Basin (MMVB), Eastern Cordillera, and Llanos Basin are part of a Late CretaceousCenozoic foreland-basin system, east of the Colombian Central Cordil-lera. Mechanical modeling indicates that the primary control on complex distributions of sedimentary thicknesses, facies, and uncon-formities was lithospheric fl exure in response to crustal loads from the Central and Eastern Cordilleras. Shorter-wavelength folding and paleoaltitude determined the local character of strata. Our mechanical modeling consists of the application of orogenic and sedimen-tary loads extracted from geologic data on a continuous elastic lithosphere. The results validate two major basin confi gurations. The fi rst confi guration was a Maastrichtianearly Eocene foreland basin coupled with Central Cordillera uplift. Growth strata record con-tinuous sedimentation in the Eastern Cordil-lera, whereas regional unconformities in the Llanos Basin (distal foreland basin) refl ect isostatic adjustments of the basins amplitude and wavelength to Central Cordillera episodic uplift and tectonic quiescence. The second major basin confi guration was characterized by Central Cordillera erosion since middle Eocene times recorded by a regional pediment surface. In the absence of Central Cordil-lera effective loading, loads from onlapping sediments and Eastern Cordillera piggyback sub-basins provoked postmiddle Eocene

accommodation in the MMVB and Llanos Basin. Intensifi ed Eastern Cordillera uplift during the Neogene produced basinal tilting recorded by unconformities in the MMVB. This study highlights the importance of assess-ing the causes of tectonic accommodation as a foundation for interpretation of the evolution of large foreland and intermontane basins.

Keywords: basin analysis, subsidence, uncon-formity, paleogeographic controls, Colombia, Northern Andes.

INTRODUCTION

The Middle Magdalena Valley Basin (MMVB), the now-uplifted Eastern Cordil-lera area, and the Llanos Basin belong to a large Andean foreland-basin system east of the Central Cordillera (Fig. 1), which formed in response to Late CretaceousCenozoic conver-gent-margin tectonics. This region provides an opportunity to study various scales of tectonic controls on accommodation of sedimentary basins coupled to large orogens. In this context, the main objective of this paper is to explain the mechanical causes of the complex distributions of Maastrichtian-Cenozoic facies and unconfor-mities that resulted from diachronous exhuma-tion of the Central and Eastern Cordilleras. We integrate numerous data sets and use mechanical models to test the validity of interpretations of tectonic accommodation. This study contributes two main sets of results: fi rst, knowledge of the controls on basinal wavelengths and amplitudes, which can be applied to foreland and intermon-tane basins elsewhere; and second, enhanced understanding of the evolution of the northern Andes region, as the chronologies of interaction between tectonic subsidence, sedimentation, and mountain deformation proposed in this paper are substantially different from those envisioned by previous works.

The attributes of the Colombian foreland-basin sedimentary fi ll, locally reaching 10 km in thickness, can be grouped into two main scale categories. The fi rst category involves features and stratigraphic changes that occur over hori-zontal scales of hundreds of kilometers, which refl ect regional isostatic responses to tectonic and sedimentary loading and erosion. This group includes an eastward change from con-tinental sedimentation in the MMVB to coastal environments that predominated over a large portion of the Llanos Basin history and regional unconformities in the MMVB and Llanos Basin region. The second scale of attributes involves features with extents of kilometers to tens of kilometers such as growth unconformities and local distributions of facies, associated with shorter-wavelength synsedimentary folding.

This study is the continuation of recent MMVB papers (Gmez et al., 2003, 2005), summarized in a later section, which provide data necessary to reconstruct the links between sedimentary fi ll and mountain evolution. The MMVB contains the best record of dual development of the Central and Eastern Cordil-leras. The strata in this basin record Maastrich-tianearly Eocene eastward propagation of the Central Cordillera mountain front, followed by a record of middle EoceneNeogene erosion of the Central Cordillera and simultaneous Eastern Cordillera deformation. In order to investi-gate the effects of tectonic loading and stratal accumulation on the fi nal accommodation histories from the MMVB to the Llanos Basin across the Eastern Cordillera, we performed the following basin-analyses steps, whose results are described sequentially through this paper: (1) quantitative assessment of subsid-ence derived from backstripped sedimentary columns; (2) analyses of accommodation pat-terns, as retrieved from sedimentary thicknesses and facies, and interpretations of mechanisms of subsidence and genesis of unconformities;

Development of the Colombian foreland-basin system as a consequence of diachronous exhumation of the northern Andes

Elas Gmez

Teresa E. JordanRichard W. AllmendingerNestor CardozoDepartment of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA

GSA Bulletin; September/October 2005; v. 117; no. 9/10; p. 12721292; doi: 10.1130/B25456.1; 17 fi gures; 1 table; Data Repository item 2005148.

Present address: Shell International Exploration and Production Inc., E&P Solutions, 200 North Dairy Ashford, Houston, Texas 77079, USA; e-mail: [email protected].

1272


Aura Cuervo GmezResaltado

DEVELOPMENT OF THE COLOMBIAN FORELAND-BASIN SYSTEM AND NORTHERN ANDES

Geological Society of America Bulletin, September/October 2005 1273

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GMEZ et al.

1274 Geological Society of America Bulletin, September/October 2005

(3) testing of hypotheses by means of two-dimensional mechanical modeling of subsid-ence; and (4) extrapolation of two-dimensional modeling results to investigate broader regional evolution of the northern Andes.

GEOLOGIC SETTING

The basins considered in this paper lie on Proterozoic to Paleozoic continental basement of South America, which is bounded to the west by the Romeral fault system along the western fl ank of the Central Cordillera (Etayo-Serna et al., 1983, Fig. 1A herein). Mesozoic and Ceno-zoic calc-alkaline plutons intrude older meta-morphic complexes of the Eastern and Central Cordilleras as well as accreted oceanic terranes to the west of the Romeral fault (Etayo-Serna et al., 1983). Lithospheric stretching characterized the MMVB and Eastern Cordillera areas during the Mesozoic (Etayo-Serna et al., 1983). Trias-sic to Jurassic synrift red beds are exposed at the core of Eastern Cordillera anticlines (Fig. 1B). Three main rifted sub-basins persisted during the Cretaceous to the east of the Central Cor-dillera. The Magdalena-Tablazo and the Cocuy sub-basins (Fabre, 1983a, 1983b) were located to the west and east of the present Santander Massif and merged toward the south into the Cundinamarca sub-basin at the present location of Bogot and the Villeta anticlinorium (Fig. 1B herein, Sarmiento, 1989). A marine transgres-sion during the Cretaceous deposited a trans-gressive-regressive megasequence of mainly shales and limestones (Macellari, 1988).

The Late CretaceousTertiary exhumation of the Central and Eastern Cordilleras was linked to the evolution of the western active margin of South America. Late CretaceousEocene oblique accretion of the Western Cordillera caused northward propagation of uplift of the Central Cordillera (Campbell, 1968; Etayo-Serna et al., 1983). Compressional deformation and tectonic inversion of Mesozoic grabens in the Eastern Cordillera area were also initi-ated at that time and continued throughout the Cenozoic (Julivert, 1963; Gmez, 2001). The most intense pulse of Eastern Cordillera uplift started at 12.9 Ma and has been attributed to accretion of the Panam-Baud arc (Dengo and Covey, 1993).

No estimates of orogenic shortening exist for the Central Cordillera. This range may represent a crustal-scale, positive fl ower structure (D. Barrero, 1989, 1999, personal commun.). A signifi cant component of compressional defor-mation generated important uplift. The Central Cordillera basement reaches altitudes of 3500 m in places where it is not overlain by Pliocene volcanoes. As explained later, the age of major

deformation of the Central Cordillera at the latitude of the MMVB is premiddle Eocene, as evidenced by a major unconformity, the Middle Magdalena Valley unconformity (MMVU).

The Eastern Cordillera increases its width from south to north (1N to 7N, Fig. 1A). North of 7N, the Eastern Cordillera structural trend changes to NNW, and it also splits into a NW branch, the Santander Massif and Perij Range, and a NE branch, the Mrida Andes. The Eastern Cordillera south of 7N is characterized by oppositely verging fold and thrust belts, which overthrust the MMVB and Llanos Basin (Figs. 1B and 1C). Discrepant values of shorten-ing between 68 and 170 km have been obtained from regional cross sections at the same locality and refl ect different interpretations of the rela-tive importance assigned to basement-involved faulting versus thin-skinned styles of deforma-tion (Figs. 1B and 1C; Colletta et al., 1990; Dengo and Covey, 1993; Cooper et al., 1995; Roeder and Chamberlain, 1995; Taboada et al., 2000). The orientation of these cross sections (120SE) is perpendicular to the structural trend of the Eastern Cordillera. The timing of Eastern Cordillera deformation derived from these cross sections is late Miocene to Pliocene. However, growth strata provide strong evidence of Late Cretaceousearly Miocene Eastern Cordillera deformation, as discussed later.

The Bucaramanga Fault along the western sides of the Santander and Santa Marta Massifs is a left-lateral strike-slip fault with a component of west-verging, reverse movement (Fig. 1B). Left lateral movement along this fault started during the late Oligocene and has been accom-modated by reverse faulting in the Santander Massif and Eastern Cordillera (Toro, 1991). Estimates of the total amount of sinistral dis-placement range between 100 and 115 km (e.g., Pindell et al., 1998); offset features include crys-talline and Mesozoic rock units of the Central Cordillera relative to the Santa Marta Massif and the Cesar-Ranchera Basin relative to the MMVB (Campbell, 1968; Fig. 1B).

CONSTRAINTS ON SEDIMENTARY FILL EVOLUTION

This major section describes impor-tant aspects of the sedimentary fi ll of the MMVB, Eastern Cordillera, and Llanos Basin (Figs. DR1DR31), which are essential for our reconstructions of basin confi gurations in later sections of this paper.

Middle Magdalena Valley Basin

Here we synthesize conclusions of MMVB studies by Gmez et al. (2003, 2005). An out-standing feature in the MMVB and adjacent Eastern Cordillera foothills is the MMVU (Figs. DR1A and DR1B, see footnote 1). This surface bevels deformed pre-Eocene rocks and is overlain by onlapping middle Eocene to Neogene strata. Stratigraphic features of the southern MMVB reveal that two long-lasting events produced the MMVU (Fig. DR1A, see footnote 1): (1) Late Cretaceousearly Eocene eastward migrating Central Cordillera uplift, and (2) consequent formation of a westward-expanding pediment zone, a process still active in the present Central Cordillera slope. Zircon and apatite fi ssion track data point to erosion of 713 km of Central Cordillera rocks since the Late Cretaceous, which has translated into large sediment supply to the basins to the east (Gmez et al., 2003, 2005).

In the southern MMVB, the record of Cen-tral Cordillera uplift is a Maastrichtian-Paleo-cene sequence of marine to alluvial fan facies (Fig. DR1A, see footnote 1) with paleofl ow and petrography indicative of Central Cordillera provenance. Plutonic granitic clasts appear in the Paleocene Hoyn conglomerates and indi-cate early Tertiary erosion of deep levels of the Central Cordillera. Northward propagation of Central Cordillera uplift is recorded by a change to continental facies and Central Cordillera provenance, both of which occur in the Paleo-cene Lisama Formation of the northern MMVB (Fig. DR1B, see footnote 1; Campbell, 1968). MMVB Paleocene deposits were partially eroded during continued early Eocene eastward propagation of Central Cordillera uplift.

The Central Cordillera has been erosionally beveled since middle Eocene times. Its boundary with the MMVB has moved westward since then as alluvial deposits onlapped over the residual pediment (the MMVU, Fig. DR1A) and local paleohighs (Fig. DR1B, see footnote 1). Eastern Cordillera folding controlled middle EoceneNeogene sedimentation and was diachronous along the MMVB as indicated by the ages of associated growth strata (Figs. 2A, 2B, DR1A, and DR1B, see footnote 1). Specifi cally, middle EoceneOligocene deformation of the Villeta anticlinorium, east of the southern MMVB, is indicated by growth strata in the lower and middle parts of the San Jun de Ro Seco Forma-tion (Figs. 2A, 2B, and DR1A, see footnote 1). In the northern MMVB, younger deformation of the Los Cobardes, Provincia, and Lisama anticlines (Fig. 1B) is recorded by growth strata of late Oligoceneearly middle Miocene age equivalent to the upper part of the Mugrosa and Colorado

1GSA Data Repository item 2005148, Figures DR1DR5 and Tables DR1DR3, is available on the Web at http://www.geosociety.org/pubs/ft2005.htm. Requests may also be sent to [email protected].




N

Figure 2. (A) Geologic map of the southern Middle Mag-dalena Valley Basin (MMVB) showing the locations of the seismic lines in Figures 2B and 2C. (B) Portion of a depth-converted seismic line across the Guaduas syncline. The lower and middle San Jun de Ro Seco synorogenic wedge is made of smaller-scale syntec-tonic units, which are bounded by growth unconformities (GU). Overall divergence of strata and lateral offsets of the anticlinal axial surface along the growth unconformities were produced by pulses of synsedimentary deformation (e.g., Ford et al., 1997). The overlying portion of the San Jun de Ro Seco Formation and the Santa Teresa Forma-tion completely overlapped this Eastern Cordillera uplift. (B) Seismic line across the southern MMVB west of the Guaduas syncline. Strata of the San Jun de Ro Seco Forma-tion truncate against the base of the Honda Group, and Neo-gene strata truncate against the surface of the southern MMVB. These confi gurations were produced by basin rota-tion in response to loading pulses from the Eastern Cor-dillera. See Gmez et al. (2003, 2005) for full description and interpretation of MMVB seis-mic information.


GMEZ et al.


Formations (Fig. DR1B, see footnote 1). Three Cenozoic fossil horizons record adjustments of the MMVB alluvial plain profi le (Fig. DR1, see footnote 1), two of which correlate with eustatic highstands (Los Corros and La CiraSanta Teresa units, e.g., Haq et al., 1988).

Eastern Cordillera deformation also produced regional-scale unconformities due to fl exural tilting. Two of these surfaces occur to the west of the Eastern Cordillera thrust and fold belt. The older is an unconformity at the base of the Honda Group in the southern MMVB; the youngest is the present surface of the MMVB (Figs. 2A and 2C). Both surfaces merge with the MMVU (i.e., slope of the Central Cordil-lera) and both truncate progressively older strata toward the west. These unconformities indicate eastward rotation of the basin in response to episodes of increased tectonic loading from the Eastern Cordillera combined with Central Cordillera erosional unloading (Gmez et al., 2003, 2005).

Eastern Cordillera

The Upper CretaceousPaleogene Eastern Cordillera sedimentary record is composed of three sequences in the Bogot and Boyac regions (Fig. DR2, see footnote 1). The fi rst sequence, Upper Cretaceouslower Paleocene Guadalupe Group and Guaduas Formation, shallows upward, recording sea withdrawal from this region (Fabre, 1983b; Sarmiento, 1994). The Guadalupe-Guaduas sequence is overlain by two fi ning-upward sequences of upper Paleocenelower Eocene and middle EoceneOligocene ages, both with fl uvial sandstones at their bases (Cuervo and Ramrez, 1985; Acosta and Beltrn, 1987). Nonmarine conditions were fi rmly established in the Sabana de Bogot area during the second sequence but marine infl uence increased toward the north (Fig. DR2, see footnote 1). In the third sequence (middle EoceneOligocene), fl uvial sandstones of the Regadera Formation (Hoorn, 1988) and equivalent Picacho Formation are transitional upward into mudstones with foraminifera and oolitic iron of the Usme and Concentracin Formations, which record renewed marine infl uence in the Eastern Cordillera (Hubach, 1957; Acosta and Beltrn, 1987). The Eastern Cordillera Paleogene rocks are unconformably overlain by upper Miocene to Pliocene alluvial and lacustrine deposits (Marichuela and Tilat Formations). They formed in intramontane set-tings during the most intense phase of Eastern Cordillera uplift (Helmens, 1990).

The Usme syncline provides detailed infor-mation to reconstruct Late CretaceousPaleo-gene relations typical of Eastern Cordillera

folds (Figs. 1B and 3; Julivert, 1963). Upper CretaceousOligocene strata are conformable at the core of this syncline but become thinner and unconformable toward the eastern fl ank, where angular relations locally reach 90. In addition

to an absence of Maastrichtian beds (Gp4 and Gs in Figs. 3B and 3C) on the eastern fl ank, sev-eral other unconformities occur within the over-lying Paleogene section. We interpret that these geometries resulted from progressive rotation of

Figure 3. Upper CretaceousOligocene growth strata of the Usme syncline. (A) Geometry of the Usme syncline extracted from structural data provided by Julivert (1963). (B) Detailed mapping of the eastern fl ank of the Usme syncline by Julivert (1963) reveals the occurrence of unconformities. (C) Reconstruction of the geometry of strata and unconformities in Figure 3B results in a westward-thickening wedge of strata with internal angular uncon-formities, which become conformable surfaces toward the west. Upper Cretaceouslower Oligocene strata are thicker and conformable at the core of this syncline. (D) The geometries of growth strata of the Pyrenees associated with progressive limb rotation (Ford et al., 1997) are similar to the stratal confi gurations found in the Usme syncline and MMVB.




the eastern limb of the Usme syncline, as in the case of similar syntectonic strata found in the MMVB (Fig. 2B) and in the Pyrenees (Ford et al., 1997, Fig. 3D herein). Julivert (1963), Raas-veldt (1956), and Laverde (1989) also document variable patterns of unconformities, thicknesses, and facies in other folds located south of the Bogot region and along the Upper Magdalena Valley Basin, which are evidence of broader Paleogene Eastern Cordillera synsedimentary deformation.

In contrast with the MMVB, there are no physical or chronological bases to postulate the occurrence of an unconformity equivalent to the MMVU in central areas of the Eastern Cordil-lera, as previous works did (e.g., Cooper et al., 1995; Pindell et al., 1998; Villamil, 1999). The vertical bars in Figure 4 describe the maximum time duration of each Maastrichtian to Cenozoic stratigraphic unit of the Eastern Cordillera per-missible by data. Maastrichtianearly Paleocene ages are based on ammonites, foraminifera, and pollen (Etayo-Serna, 1964, 1985; Sarmiento, 1994). The age assignments of the overlying Paleogene units rely on palynology and are coarse due to incomplete palynologic sampling (Hubach, 1957; Van der Hammen, 1957; Hoorn, 1988), owing in part to the occurrence of growth unconformities at sampling sites (e.g., eastern fl ank of the Usme syncline; Hoorn, 1988). Despite incomplete palynologic sampling, the ages of successive Paleogene units overlap

(Fig. 4), which indicates that major-rank uncon-formities do not exist in the Sabana de Bogot region. In contrast, the duration of the time gap associated with the MMVU in the Eastern Cordillera western foothills is on the order of 1015 m.y. (Fig. DR1, see footnote 1).

The preservation of Upper CretaceousOligo-cene Eastern Cordillera growth strata (Fig. 3) required continuous accumulation of sediment over a time scale (~1 m.y.) substantially less than the total duration of each unit (Fig. 4; e.g., Crowley, 1984; Anders et al., 1987; Anadn et al., 1986; Ford et al., 1997). We emphasize that although minor temporal gaps exist within Upper CretaceousPaleogene strata fl anking Eastern Cordillera folds, the mechanisms that produced these unconformities were associated with local synsedimentary folding rather than with regional uplift and pedimentation of the Central Cordil-lera, as in the case of the MMVU.

Llanos Foothills and Llanos Basin

The Late CretaceousCenozoic stratigraphy of the Llanos foothills and Llanos Basin has been comprehensively studied by Cooper et al. (1995; Fig. DR3, see footnote 1). The main sediment source areas of these regions were the Guyana Shield, during the Late Cretaceous to early middle Miocene, and the Eastern Cordil-lera since the late middle Miocene (Cooper et al., 1995). The MMVU unconformity of

the MMVB correlates with two major time-transgressive unconformities in the Llanos foothills, which merge into a single composite unconformity further east in the Llanos Basin (Fabre, 1983b; Cooper et al., 1995). The older surface truncates deeper levels of the Guaduas Formation and Guadalupe Group from west to east (Sarmiento, 1994). The Guaduas Forma-tion and upper Guadalupe Group are absent in the Llanos Basin, where upper Paleocene sandstone-rich stuarine and coastal plain mud-stones of the Barco and Los Cuervos overlie Campanian rocks (Cooper et al., 1995). A sec-ond major unconformity separates these units from the overlying upper Eocene to Neogene eastward onlapping megasequence. The upper Eocenelower Miocene strata of the Llanos Basin consist of alluvial plain, coastal plain, and estuarine valley-fi ll deposits; the continental character of these deposits increases toward the east. Sedimentation initiated with deposition of the Mirador Formation sandstones and contin-ued with deposition of four eastward stepping, coarsening upward sequences in the Carbo-nera Formation (Fig. DR3, see footnote 1). The Carbonera is capped by the Len Shale, which marks a major marine transgression. Major Eastern Cordillera uplift is indicated by provenance of the middle MiocenePliocene Guayabo Formation (Cooper et al., 1995).

GEOHISTORY ANALYSIS OF THE MMVB AND EASTERN CORDILLERA

As a fi rst step to extract the signal of tectonic subsidence to the east of the Central Cordil-lera, we carried out one-dimensional geohis-tory analyses of three columns exposed in the southernmost MMVB (Villeta anticlinorium and Guaduas syncline), northern MMVB (Los Cobardes anticline and Nuevo Mundo syn-cline), and the Cocuy anticlinorium (Boyac region, Figs. 1B, 5A5C; Fabre, 1983b, 1985; Sarmiento, 1989; Gmez, 2001). The backstrip-ping techniques are described by Allen and Allen (1992). The sedimentary columns were decompacted assuming a depth-dependent, exponentially decreasing porosity function. In order to estimate the tectonic subsidence, the subsidence driven by the load of sediment was subtracted from the total (decompacted) subsid-ence; corrections for varying water depth and long-term eustatic sea level (Haq et al., 1988) were applied. Summary tables of stratigraphic attributes and physical parameters used for backstripping are available (Tables DR1DR3, see footnote 1). Constraints on Tertiary paleo-altitudes are poor (see error bars in Fig. 6). We have assumed an average altitude between 0 m and 500 m for the MMVB, which corresponds

Figure 4. Paleontologic age assignments of Upper CretaceousPaleogene units of the Eastern Cordillera are overlapping in the Bogot and Boyac (names within parentheses) regions. Therefore, there is no chronological basis to postulate the occurrence of a regional unconformity equivalent to the MMVU in central areas of the Eastern Cordillera. Wider bars indicate our preferred interpretation of depositional ages. The palynologic ages of Paleogene units of the Boyac region (gray bars) are broadly similar to the ages of their equivalents in the Bogot area (black bars). Most important sources of ages: Hubach (1957), Van der Hammen (1957), Germeraad et al. (1968), Acosta and Beltrn (1987), Hoorn (1988), Helmens (1990), and Cspedes and Pea (1995).


GMEZ et al.


to the present altitude limits of the MMVB alluvial plain. Facies in the Cocuy region and Llanos Basin refl ect eustatic infl uence and indi-cate lower altitudes.

Mesozoic Subsidence

The backstripped sections illustrate the sub-sidence history of the Mesozoic Magdalena-Tablazo, Cocuy, and Cundinamarca extensional sub-basins (Fig. 1B). Mesozoic synrift subsid-ence in these localities is independently recorded by unconformity-bounded units, rotated-block morphology, spatially variable thicknesses, bimodal subaerial volcanogenic strata, and mafi c intrusives (Julivert, 1958; Fabre and Delaloye, 1982). Lithospheric extension is expressed on the backstripped curves as rapid and temporally vari-able tectonic subsidence (Fig. 5). Subsequently, smooth decline of the subsidence curves indi-cates postrift thermal contraction during litho-spheric cooling (Allen and Allen, 1992).

The synrift tectonic subsidence of the north-ern MMVB lasted until the end of the Jurassic (2993 m, 59 m/m.y., Fig. 5A). Synrift subsid-ence in the Villeta anticlinorium (Cundinamarca sub-basin) until the Coniacian is indicated by the stair-shaped synrift tectonic subsidence until 87 Ma (2892 m, 56 m/m.y. average rate, Fig. 5B). This history correlates well with Hauterivian to Coniacian gabbroic sills of the northern part of the Villeta anticlinorium (Fabre and Delaloye, 1982; Rodrguez and Ulloa, 1994a, 1994b), which attest to extension and injection of mafi c magmas due to partial adia-batic melting (Turcotte and Schubert, 1982). This prolonged synrift subsidence explains the accumulation of ~7 km of Cretaceous strata in

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Figure 5. Geohistory plots of the MMVB and Eastern Cordillera extracted from backstrip-ping of sedimentary columns. (A) Northern MMVB (Los Cobardes anticline and Nuevo Mundo syncline). (B) Southern MMVB (Vil-leta anticlinorium and Guaduas syncline). (C) Cocuy region of the Eastern Cordillera, modifi ed after Fabre (1983b). See Figure 1B for locations. Facies, ages, and water depths of Cretaceous units from Sarmiento (1989), Fabre (1985), and Etayo-Serna (personal commun., 2000). Late CretaceousMesozoic parts of the subsidence curves are enlarged in Figure 6, with error bars. Curves of sub-sidence corrected for sediment load nearly overlap the fi nal tectonic subsidence curves because the correction for long-term eustatic change is small.




the Villeta anticlinorium area, the thickest in the Eastern Cordillera. The synrift subsidence in the Eastern Cordillera Cocuy region (1960 m, 73 m/m.y.) spans the Berriasian to Aptian time interval (144117 Ma, Fig. 5C), according to subsidence history and mafi c magmatism (Fabre, 1983b).

Thermal sagging spanned the Cretaceous in the northern MMVB (933 m, 12 m/m.y.), the late ConiacianMaastrichtian in the southern MMVB (195 m, 10 m/m.y.), and the AptianMaastrichtian in the Cocuy region (893 m, 18 m/m.y.; Figs. 5A5C). Thermal subsidence curves are suggestive of stretching factors () of 1.4, 1.2, and 1.6, respectively (Gmez, 2001). However, these values most likely underes-timate , given the heat loss during the long rifting (Allen and Allen, 1992). Previously, it was suggested that thermal sagging of a passive margin produced subsidence east of the Central Cordillera during most of the Cretaceous (Pin-dell, 1993; Roeder and Chamberlain, 1995). However, the temporally and spatially variable subsidence histories described in this section most likely correspond to back-arc subsidence behind a magmatic arc in the Central Cordillera (e.g., Aspden et al., 1987; Cooper et al., 1995).

Late Cretaceous to Neogene Subsidence

The Late CretaceousCenozoic subsidence curves (Figs. 6A6C) highlight two aspects of regional accommodation. First, continuous sub-sidence characterized the MaastrichtianPaleo-cene and middle EoceneNeogene times in the MMVB and the Late CretaceousOligocene in the Cocuy region. Tectonic subsidence was larger than the contemporaneous oscillations of eustatic sea level. The resultant long-term high accommodation explains the preserva-tion of abundant muddy deposits in the Upper CretaceousCenozoic sedimentary record. For the two MMVB sites, the intervals of subsid-ence are separated by a time of uplift. Second, sediment loading was the most important force driving total subsidence and amounts for ~70% of the total Tertiary subsidence, as illustrated by the difference between total subsidence and tectonic subsidence in Figure 5.

Independent of manipulation of error bars in the subsidence plots, tectonic subsidence rates seem to have increased during the late Maas-trichtianPaleocene in the southern MMVB (Fig. 6B, 433 m, 37 m/m.y.) and during the Paleocene in the northern MMVB (Fig. 6A, 290 m, 32 m/m.y.) relative to the Cretaceous thermal subsidence. The tectonic subsidence curve of the Cocuy region (Fig. 6C, 413 m, 11 m/m.y.) does not show this behavior. The increased rates of tectonic subsidence of the

southern and northern MMVB correlate with Central Cordillera uplift and appear to sup-port interpretations of foreland basin subsid-ence triggered by Central Cordillera loading (Dengo and Covey, 1993). Because tectonic

subsidence rates were much higher than the rate of contemporaneous long-term sea-level drop (1 m/m.y., Haq et al., 1988), one might suppose that the basin would have remained fl ooded by marine waters. The regressive facies

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A

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Figure 6. Late Cretaceousearly Miocene tectonic subsidence of the (A) northern MMVB, (B) southern MMVB (Gmez, 2001), and (C) Eastern Cordillera (Cocuy region, modifi ed after Fabre, 1983b). Vertical and horizontal error bars represent uncertainties in paleoalti-tude and stratigraphic ages, respectively. See text for discussion.


GMEZ et al.


(Fig. DR1, see footnote 1) clarify that this was not true. Thus, under conditions of enhanced accommodation relative to Late Cretaceous times, the MMVB MaastrichtianPaleocene sea withdrawal is explained by the large input of sediment from the Central Cordillera. The combined enhanced tectonic subsidence and facies patterns refute the interpretation that Paleocene regression was caused by decreasing accommodation resulting from Central Cordil-lera deformation combined with the drop of eustatic sea level (e.g., Villamil, 1999).

Early Eocene kilometer-scale uplift in the MMVB is indicated by apatite-fi ssion-track parameters from both the Central Cordillera basement and Mesozoiclower Paleogene sedimentary rocks of the MMVB (Gmez et al., 2003, 2005). Middle Eocene to early middle Miocene subsidence rates of the south-ern MMVB (468 m, 15 m/m.y.) and northern MMVB (649 m, 21 m/m.y.) are lower than MaastrichtianPaleocene values.

RECONSTRUCTION OF LATE CRETACEOUSCENOZOIC BASIN GEOMETRIES AND HYPOTHESES OF MECHANISMS OF LONG-TERM SUBSIDENCE

In this section, we describe the construction of a palinspastically restored base (Fig. 7), which serves as a template for reconstruction of two major basin confi gurations of Late Cre-taceousearly Eocene and middle EoceneNeo-gene ages (Figs. 811). We also correlate the previously summarized information from the MMVB, Eastern Cordillera, and Llanos Basin along the cross sections in Figures 9 and 11.

Palinspastic Reconstruction

As previously described, deformation has modifi ed the original geometries of the Colombian basins, and therefore palinspastic reconstruction is required to reconstruct the

wavelength of regional subsidence. Retro-deformation perpendicular to the Eastern Cordillera structural grain, a condition of pure shear, should also restore the original continu-ity between the Central Cordillera and the Santa Marta Massif and between the northern MMVB and the Cesar-Ranchera Basin across the Bucaramanga fault (e.g., Fig. 1B). This constraint favors structural models that predict at least 150 km of Eastern Cordillera crustal shortening (e.g., Dengo and Covey, 1993; Roeder and Chamberlain, 1995). Dengo and Coveys (1993) cross section (Fig. 1C) is well suited for restoration because they describe their supporting data and validate their section with a gravity profi le. Their restoration of East-ern Cordillera shortening also best restores the ~110 km of sinistral offset along the Bucara-manga fault. We used this restoration to move the Paleogene and Jurassic outcrops of the northern part of the Eastern Cordillera and the Central CordilleraMMVB block to their position in the early Eocene (Fig. 7). Retrode-formation generates an opening between the present trace of the Bucaramanga fault and the northern MMVB, whose partial fi lling requires counterclockwise rotation of the Santander and Santa Marta Massifs by ~12. The remaining unfi lled space may represent compressional deformation in the MMVB and Santa Marta Massif. A long-term history of pedimentation indicates that no important Central Cordil-lera shortening has occurred since the middle Eocene (Gmez et al., 2003). Thus, we treat the Central Cordillera and MMVB as a single rigid block, which is transported toward the NW during restoration. No estimates of Late Cretaceousearly Eocene Central Cordillera shortening are available. Thus, no further retrodeformation of this range is attempted. The Llanos Basin area, overlying the Guyana Shield, is assumed to remain fi xed during restoration. The palinspastically restored base of Daz (1994), based on numerous balanced cross sections, was used to restore the southern portion of the Eastern Cordillera at the latitude of Bogot. Restoration of the Eastern Cordil-lera foothills is also constrained by cross sec-tions across the MMVB, Llanos foothills, and the Perij range (Fig. 7).

Palinspastic restoration is an exercise with considerable assumptions and error. Simple shear conditions may lead to more accurate restored maps, but they require oblique fault displacement and rotation of thrust slices, which are unconstrained. Our restoration is simple, meets major regional constraints, and offers a better approximation of pre-Neogene basin dimensions for the subsequent basin analyses than do modern spatial relations.

NM

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VA: Villeta anticlinorium

Figure 7. Early Paleocene palinspastic base map constructed by combining Dengo and Coveys (1993) and Dazs (1994) reconstructions with restored sections from other authors (numbered lines). This retrodeformation, perpendicular to the Eastern Cordillera structural strike, also restores the continuity between stratigraphic units of the Central Cordillera and Santa Marta Massif and between the MMVB and the Cesar-Ranchera basin across the Bucaramanga fault. See text for discussion and other sources of data. Features marked as reference outcrops form a template visible in the paleogeographic maps of Figures 8, 10, 1517, DR4, and DR5 (see footnote 1). See also Figure 1B for present location of outcrops.




Late Cretaceous to Early Eocene Basin Geometry and Hypothetical Causes

The restored isopach map and cross section of Maastrichtianlower Eocene strata to the east of the Central Cordillera reveal an asymmetric distribution of sedimentary thicknesses (Fig. 8). The reconstructions of geometric and temporal relations among strata are shown by the restored cross section and accompanying chronostrati-graphic diagram in Figures 9A and 9B. Thick-nesses decrease toward the east from the Eastern Cordillera to the Llanos Basin (Figs. 8 and 9A). Maastrichtian to Paleocene rocks in the southern MMVB have a maximum thickness of 1897 m. However, they were truncated beneath the MMVU, and their initial total thickness was greater; thermal history parameters indicate erosion of a lower Paleogene sedimentary sec-tion up to 3 km thick in the Guaduas syncline (Gmez et al., 2003). The maximum thickness preserved of the Maastrichtianlower Eocene section is in the western side of the Sabana de Bogot region (~2400 m). Diminished thick-nesses toward the east resulted from diminished eastward accommodation and the presence of two regional unconformities. The thickness of Maastrichtianlower Eocene strata in the Cocuy region is 1109 m (Fabre, 1985) and diminishes to zero toward the Llanos Basin region. Geohis-tory analyses suggest that Maastrichtianearly Eocene tectonic subsidence decreased eastward from 500 m in the southern MMVB to 205 m in the Cocuy region (Fig. 9A, inset). The deposi-tional topographic slope likely decreased from alluvial-fan gradients (>2) in the west to lower alluvial- and coastal-plain gradients in the East-ern Cordillera and Llanos Basin (

GMEZ et al.


late Paleocene tectonic quiescence, isostatic rebound of the Central Cordillera, and ensu-ing depression of the Llanos peripheral bulge. Similarly, the Danian and early Eocene stages of basin narrowing may refl ect times of Central Cordillera shortening and uplift and westward displacement of sedimentation to the locus of maximum subsidence. The area of maximum thickness in the Sabana de Bogot region marks the fi nal early Eocene position of the zone of maximum accommodation. Continuous sedi-mentation characterized this region, although the foreland basin was disrupted by small synsedimentary folds (e.g., Usme Syncline) that produced growth unconformities of limited extent (not shown in Fig. 9).

Middle Eocene to Neogene Basin Geometry and Hypothetical Causes

Regional deformation at the latitude of the MMVB changed from the Central Cordillera to the Eastern Cordillera during the middle EoceneNeogene. Pedimentation was the domi-nant process in the Central Cordillera, whereas widespread Eastern Cordillera deformation is revealed by thermal-history and provenance data of the MMVB and by growth strata fl anking the Usme and Fusagasug synclines and the Vi lle ta, Provincia, and Lisama anticlines (Fig. 1B; Gmez et al., 2003, 2005). Basin geometry also

differed during the middle Eocene to Neogene, compared to the previous patterns, as shown by the distribution of thicknesses of units of these ages (Fig. 10). These strata thin regionally toward both the west and east due to sedimen-tary onlap onto the Central Cordillera pediment surface and the Guyana Shield (e.g., Figs. 10, DR1, and DR3, see footnote 1). Maximum preserved thicknesses of middle EoceneNeo-gene strata are in the MMVB (~7000 m) and Llanos foothills area (~4500 m). Stratigraphic thicknesses are not well constrained in the Eastern Cordillera area (gray-shadowed area in Fig. 10A). Scarce outcrops of these ages in this region have maximum preserved thicknesses on the order of ~1500 to ~1700 m.

The cross sections in Figure 11 synthesize our interpretation of middle EoceneOligocene basin geometry. Eastern Cordillera anticlines segmented the region to the east of the Central Cordillera, as revealed by growth strata in the Usme and Guaduas synclines. Conglomer-atic piedmont facies indicate a high-gradient topographic profi le in the southern MMVB, which decreased eastward across the synclinal basins of the Eastern Cordillera to coastal-plain gradients in the Llanos Basin. The gradient of the residual slope of the Central Cordillera (MMVU) was persistently steeper than the top-ographic gradient of the Guyana Shield to the east, which resulted in faster onlap further east.

There is no evidence that Central Cordillera tectonic thickening in the middle EoceneNeo-gene caused basinal subsidence. Unlike during the Maastrichtianearly Eocene, during this later time the MMVB strata passively onlap westward onto the Central Cordillera, sug-gesting progressive decrease in its elevation (Gmez et al., 2003). Erosional denudation of the Central Cordillera would have contributed fl exural uplift to the MMVB rather than subsid-ence. Isostatic adjustment to erosion explains the net decrease of Central Cordillera altitude. The explanation for tectonic subsidence in the MMVB and Llanos Basin rests ultimately with Eastern Cordillera thickening. A small amount of tectonic subsidence was amplifi ed into a larger amount of total subsidence (e.g., Fig. 5) because sediment was trapped between the Cen-tral Cordillera and uplifts in the Eastern Cordil-lera and between these uplifts and the Guyana Shield (Fig. 11). Onlap of these sediments toward the west and east caused additional loading and fl exural subsidence. Sediment compaction also contributed a modest amount of space (~6% of total subsidence according to backstripping) and increased toward the east in the MMVB and toward the west in the Llanos Basin because the sedimentary fi ll was thicker in those directions.

The contribution of Eastern Cordillera tec-tonic loading to subsidence increased through

Figure 9. (A) Palinspastically restored cross-section confi gu-ration of the Maastrichtian to early Eocene foreland basin adjacent to the Central Cor-dillera. Vertical exaggeration ~65. (B) Corresponding chron-ostratigraphic section. See text for discussion.




time as this range evolved into a continuous and wide topographic feature. For example, Neogene pulses of Eastern Cordillera uplift have clear manifestations in the MMVB where Cenozoic strata are regionally tilted toward the east (e.g., Fig. 2C). The mechanisms of MMVB Neogene tilting and erosional truncation of strata are similar to those that generated Paleo-gene unconformities in the Llanos Basin area, the difference being that the Llanos Paleogene surfaces were produced by fl exure under Cen-tral Cordillera loading.

MECHANICAL MODELING: TESTING HYPOTHESES OF BASIN SUBSIDENCE

In this section, we test the mechanical viabil-ity of our interpretations of subsidence mecha-nisms. The simplest test is to compute in two dimensions the fl exural deformation produced by Central and Eastern Cordillera crustal loads on an infi nite, elastic lithosphere of constant thickness (Turcotte and Schubert, 1982). If the mechanical controls are as previously hypoth-esized and the geologic constraints are accurate, this modeling should reproduce the basin con-fi gurations that were interpreted from geologic data; failure of the mechanical models to fi t the observations would invalidate our hypotheses.

Numerical experiments were conducted according to procedures described by Car-dozo and Jordan (2001). The two-dimensional modeling strategy involves the conversion of tectonic and sedimentary loads into rectangles of equal width (w), but different height (hi) and density (i) (Fig. 12). The defl ection profi le [u

i(x)] of each one of these elements (i g hi)

is then computed based on differential equations that describe the fl exure of an elastic lithosphere (Turcotte and Schubert, 1982). The total dis-placement profi le [u(x)] is equal to the sum of all the defl ection profi les of the rectangular ele-ments. The sum of the load profi le [h(x)] and the displacement profi le [u(x)] is the relative topo-graphic profi le [rt(x)], which displays elevation relative to an initially undeformed reference datum (Fig. 12).

We selected four cases to simulate the fl ex-ural response to crustal loads of the Central and Eastern Cordilleras. Discrete sedimentary loads (w = 10 km, Fig. 13) were extracted from the cross sections in Figures 9A and 11A. Discrete sedimentary thicknesses were decompacted by a factor of 6%, an average extracted from back-stripping. In all cases, we evaluated the fi t of the model approximations of paleotopography by comparing it to the facies pattern in the basin. The fi rst three experiments (Maastrichtian, early, and late Paleocene confi gurations) test the Late Cretaceousearly Eocene foreland

basin interpretation. This hypothesis relies on the assumption that there was crustal thicken-ing of the Central Cordillera. But because there are not accurate structural data of this range demonstrating the amount of crustal shortening, the paleoaltitude of the Central Cordillera is the best record of crustal thickness changes. In our experiments, the Central Cordillera tectonic load profi les are the best choices after several

iterations. As discussed earlier, kilometer-scale paleoaltitudes of the Central Cordillera can be inferred from its remnant topography, its degree of denudation as revealed by thermal history parameters of basement rocks, the early unroof-ing of Mesozoic granitic plutons revealed by provenance data of lower Tertiary conglomer-ates, and effects on MMVB facies (Gmez et al., 2003, 2005). Iterative testing showed that

N

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Figure 10. (A) Palinspastically restored distribution of thicknesses of middle EoceneNeogene strata in the MMVB, Eastern Cordillera, and Llanos Basin. See text for discussion. Outlines of the Central Cordillera, Santa Marta Massif, restored positions of the Eastern Cordillera boundaries, and some Eastern Cordillera outcrops provided for reference (e.g., Figs. 1B and 2). (B) Cross-section view of stratal-thickness variation. Vertical exaggeration, 2.


GMEZ et al.


the best choice of lithospheric elastic thickness (t) for modeling was 35 km. This value is inter-mediate between elastic thicknesses associated with rifting (030 km) and cold continental lithospheres (4590 km), which represent extreme values for the Colombian lithosphere during Mesozoic and late Tertiary times, respec-tively (Turcotte and Schubert, 1982; Allen and Allen, 1992; see also Kellogg et al., 1995; Roeder and Chamberlain, 1995).

Modeling Results

The three fi rst experiments evaluate the fl ex-ural responses to uplift and tectonic quiescence of the Central Cordillera (Figs. 13 and 14). Experi-ments 1 and 2 (Figs. 14A and 14B) simulate Maastrichtianearly Paleocene eastward-advanc-ing uplift of this range. The third experiment simulates the redistribution of crustal loads associated with late Paleocene Central Cordil-lera tectonic quiescence and erosion (Fig. 14C). The reference datum is a middle Maastrichtian surface at the bases of the Cimarrona Formation and Tierna sandstone (Fig. 9A).

Our fi rst model (Fig. 14A) evaluates the Maastrichtian confi guration beneath the La Seca and Guaduas Formations (Fig. 9A). The maximum modeled Central Cordillera paleoalti-tude is 2000 m. The depositional profi le of this model is steeper on the western side of the basin and gentler in the eastern side, as predicted by piedmont and coastal facies, respectively. A topographic low between 100 and 200 km in the horizontal distance axis of Figure 14A is consis-tent with the locus of sedimentation of shallow marine mudstones (Daz, 1994). Most of the accommodation space is produced by fl exure, with a maximum thickness of 250 m being accommodated above the undeformed reference datum in the western side of the basin.

In the second experiment (early Paleocene, Fig. 14B), we increased the height of the Cen-tral Cordillera loads and moved the boundary between this range and the sedimentary basin toward the east (e.g., Fig. 9). Total subsidence next to the Central Cordillera is amplifi ed more than two times relative to Maastrichtian subsidence (compare Figs. 14A and 14B). The modeled position of the forebulge is displaced toward the west relative to its Maastrichtian position, which is consistent with the forma-tion of a Late CretaceousDanian unconformity along the eastern side of the basin. The con-tinuation of this surface in the modeled basin is delineated by the dashed line in Figure 14B, representing the defl ected position of what had been the Maastrichtian topography. The fl at topography of the eastern side of the basin, east of 320 km in the horizontal-distance axis

Figure 11. Palinspastically restored cross-section confi guration of (A) late EoceneOligo-cene and (B) late Oligocene Colombian basins to the east of the Central Cordillera. Long-term subsidence was produced by loading from Eastern Cordillera folds and onlapping sediments. See text for explanation.

Figure 12. Conceptual description of our two-dimensional mechanical models after Cardozo and Jordan (2001). Loading of an elastic lithosphere overlying a semifl uid asthenosphere (left) is solved analytically using the analogy of an infi nite beam on an elastic foundation (middle and right). See text and Table 1 for explanation of constants and variables. Dark and light gray denote mountain and sedimentary material, respectively.




of Figure 14B, truncates westward-tilted Maas-trichtianearliest Paleocene beds. Toward the west, the slope of the depositional profi le is the steepest in the region adjacent to the Central Cordillera, where alluvial fan deposition is documented. This model suggests that a total subsidence of 1700 m and paleoelevations on the order of 300 m next to the Central Cordillera provided enough space to accommodate most of the lower Paleocene alluvial sediments sourced by the Central Cordillera (Fig. 14B). Erosion and sediment bypass dominated in the eastern sector of the basin. Paleocene paleofl ow in the Sabana de Bogot region was oriented toward the NNE (Laverde, 1989), which refl ects the effects of a northward slope gradient and intra-basinal folding. These features are ignored in these two-dimensional models.

In the third experiment (Fig. 14C), we reduced the height of the Central Cordillera loads to simulate late Paleocene tectonic quiescence and erosion. The boundary between Central Cordillera and the foreland basin is kept at the same position as in the early Paleocene model. The interpreted loads of quiescent-phase strata correspond approximately to the Cacho and lower Bogot and the Barco and Los Cuervos Formations (Fig. 9). Erosion produces isostatic rebound of the Central Cordillera and proximal depocenter, but the redistributed sedimentary load produces displacement of the forebulge toward the east relative to its early Paleocene position (compare Figs. 14B and 14C). Thus a broader late Paleocene basin is created. The elevation of the depositional profi le diminishes from ~300 m in the western side of the basin to 0 m at the eastern side. Most of the upper Paleo-cene quiescent-phase strata are accommodated between the undeformed reference datum and the graded topographic profi le in the western side of the basin (west of 300 km). In the east-ern part of the basin (east of 300 km), however, most of these strata (e.g., Barco and Los Cuer-vos) are accommodated below the undeformed reference datum in the space provided by fl ex-ural subsidence. This confi guration is consistent with a corresponding west-to-east change from continental to coastal-plain facies.

In our fourth experiment, we chose the late Eoceneearly Oligocene basinal confi guration in Figure 11A to test whether Eastern Cordillera tectonic and sedimentary loading explain subsid-ence in the absence of Central Cordillera load-ing. This basin confi guration postdated the fi nal episode of uplift and eastward expansion of the Central Cordillera during the early Eocene. For lack of constraints, we disregard the paleoeleva-tion of the Central Cordillera slope in the west-ern margin of the late Eoceneearly Oligocene basin and assume a fl at initial reference datum

TABLE 1. MECHANICAL PARAMETERS FOR 2-D FLEXURAL MODELING

E 70 Gpa 0.25t 35 kmi 2400 kg/m3 (sedimentary rocks at shallow burials)i 2700 kg/m3 (crystalline rocks, consolidated sedimentary rocks)m 3300 kg/m3g 9.8 m/s2

Note: EYoungs modulus, GpaGigapascals, Poissons ratio, telastic thickness, icrust density, mmantle density, gEarths gravity.

Figure 13. (A) Maastrichtian, (B) early Paleocene, and (C) late Paleocene discrete load con-fi gurations [h(x)] derived from Figure 9A. (D) Early Oligocene load confi guration derived from Figure 11A. See text for discussion.


GMEZ et al.


that corresponds to the surface of the Late Creta-ceousearly Eocene foreland basin. The loads of Eastern Cordillera sedimentary fi ll and folds in Figure 13D produce the deformed confi guration in Figure 14D. The fi nal confi guration of the early Oligocene defl ection profi le in the west-ernmost part of the basin (100160 km in hori-zontal scale of Fig. 14E) includes an assumed paleoelevation of the Central Cordillera, which restores the westward onlapping confi guration of strata. Thicknesses up to 400 m are accom-modated between the undeformed reference datum and the graded depositional profi le in the western part of the basin. The gradient of the depositional profi le decreases toward the east, while fl exural subsidence accommodates all the sediment in the easternmost sector of the basin (east of 500 km, Fig. 14E). In general, the combination of fl exural subsidence and lower paleoaltitudes explains the persistent eustatic signature of upper Eocenelower Miocene strata in the Llanos Basin area (e.g., Figs. 14 and DR3, see footnote 1).

INTEGRATED INTERPRETATION OF BASIN DEVELOPMENT AND EXHUMATION OF THE CENTRAL AND EASTERN CORDILLERAS

The successful fi t between the results of the two-dimensional mechanical models and the patterns of sedimentary thicknesses and facies east of the Central Cordillera indicate that the fi rst-order characteristics of the sedimentary fi ll were simply controlled by fl exural variations. We can now extrapolate these results and assess the degree to which regional distributions of unconformities and facies were also determined by similar mechanisms or by secondary controls such as eustatic variation. The pictures of evo-lution in Figures 1517, DR4, and DR5 (see footnote 1) illustrate this discussion.

Late CretaceousEarly Eocene Foreland Basin

Expanding Central Cordillera uplift created a wedge-shaped foreland basin to the east with regional drainage oriented toward the NE to the Maracaibo Basin (Figs. 15 and 16; Campbell, 1968). Central Cordillera deformation was episodic, which produced shifting positions of fl exural subsidence and marine-infl uenced sedi-mentation. At all times, the basin topographic axis lay east of the maximum accommodation areas, and it migrated toward the Central Cor-dillera during times of enhanced deformation (Figs. 15 and 16, cross sections). Two periods of uplift during the MaastrichtianDanian and the early Eocene enhanced subsidence next to

Figure 14. Modeled relative topography profi les [rt(x)] for (A) Maastrichtian, (B) early Paleocene, (C) late Paleocene, and (D) and (E) early Oligocene times. These simulations sat-isfactorily replicate the subsidence mechanisms and basinal confi gurations fi rst interpreted from geological data sets. See text for discussion.




Fig

ure

15. S

ynth

esis

of (

A) M

aast

rich

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GMEZ et al.


N

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Fig

ure

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GMEZ et al.


the Central Cordillera, while erosive uncon-formities developed in the eastern sector of the basin (Figs. 15A, 15B, and 16B). Maastrichtian-Danian sea withdrawal was caused by increased sediment supply from the Central Cordillera and from exposed areas of the Guyana Shield (Figs. 15A and 15B). Late Paleocene redis-tribution of sedimentary loads during Central Cordillera tectonic quiescence created fl exural space for renewed coastal sedimentation in the Llanos area (Fig. 16A), which fi lled an irregular topography incised during the previous ero-sional period (e.g., Cazier et al., 1995). Ample tectonic subsidence generated a narrower basin in the early Eocene and facilitated the impact of a sea-level rise (Haq et al., 1988) to be felt to the north (upper Socha, Fig. 16B).

The Late CretaceousDanian unconformity of the eastern sector of the foreland basin was attributed previously to reverse faulting in the Llanos foothills (Sarmiento, 1994; Villamil, 1999). However, this erosional surface has a large geographic extent and cuts older strata monotonically toward the Guyana Shield. Such characteristics are diagnostic of regional-scale controls of the kind exerted by regional fl exure rather than local faulting. The stratigraphic hia-tuses associated with the MMVU of the MMVB and the Llanos Paleogene unconformities decrease toward the Eastern Cordillera, where there are age-equivalent strata, albeit the strata contain minor temporal gaps related to local folding. Previous models correlated the MMVU with the younger Paleogene unconformity of the Llanos Basin across the Eastern Cordillera and postulated a period of regional uplift and erosion of all the Colombian territory during the middle Eocene (Cooper et al., 1995; Pindell et al., 1998; Villamil, 1999). However, the magni-tude and duration of such an event are not con-sistent with preserved Maastrichtian-Oligocene growth strata of the Eastern Cordillera, whose formation required continuous sedimentation. The sweeping regional-uplift interpretation also makes it very diffi cult to explain the mechanical causes of subsequent subsidence of such a vast crustal uplift, which accommodated km-scale thick piles of younger Cenozoic strata through-out the Colombian territory.

Middle Eocene to Neogene: Effects of Eastern Cordillera Diachronous Uplift

No foreland basin coupled to the Central Cor-dillera has existed since middle Eocene times; long-term subsidence in the MMVB and Llanos Basin resulted from Eastern Cordillera sedimen-tary and tectonic loading. Crustal loading by Eastern Cordillera anticlines and westward and eastward onlapping sediments caused long-term,

regional subsidence. The middle Eocene to early Oligocene reconstructions (Figs. 17A and DR4, see footnote 1) highlight the coexistence of two different sedimentary systems, each with NE-directed drainage. Sedimentation in the west (MMVB) was continental, a likely result of a higher paleoaltitude and larger supply of sedi-ment from the Central Cordillera. Farther to the east, a marine transgression is recorded by facies of the Eastern Cordillera and the Llanos Basin (Cooper et al., 1995). The spatial parallelism of these drainages is explained by an intervening zone of deformation along the western half of the Eastern Cordillera. The southern part of this drainage divide was the Villeta anticlinorium, as revealed by fl anking middle EoceneOligocene growth strata (Gmez et al., 2003).

During the late Eocene, shallow marine con-ditions were established to the east of the Eastern Cordillera divide (Fig. DR4, see footnote 1). The peak of this transgression broadly correlates with a global sea-level highstand at the Eocene-Oligo-cene boundary (ca. 3334 Ma, Haq et al., 1988). Its effect propagated into the northern MMVB (Los Corros fossil horizon) from the Maracaibo basin around the northern part of the Eastern Cordillera divide (Fig. DR4, see footnote 1). No physical evidence of this transgression exists in the southernmost MMVB because this area was at a higher altitude, and the growing Villeta anticlinorium formed a barrier. Oligoceneearly middle Miocene sedimentation of the Llanos areas expanded toward the east (Fig. 17B) and remained close to sea level (Carbonera Forma-tion) due to the combination of low paleoalti-tudes and creation of fl exural accommodation by tectonic and sedimentary loading.

Major changes in basin confi guration hap-pened during the late Oligoceneearly middle Miocene due to deformation to the NE of the MMVB (Gmez et al., 2005; Fig. 17B herein). In palinspastically restored position, the Los Cobardes, Provincia, and Lisama anticlines are part of a larger morphostructural unit whose northeastward continuation was the Perij Range. Simultaneous deformation of the Perij Range is documented by structural studies (Kel-logg, 1984). We interpret that the Eastern Cor-dilleraPerij RangeSantander Massif struc-tural barrier further raised the MMVB base level and forced the MMVB rivers to fl ow toward the Llanos Basin region across the Eastern Cordil-lera region (Gmez et al., 2005). In the south-ern MMVB, lower to lower middle Miocene sediments overlapped the Villeta anticlinorium (Gmez et al., 2003, Fig. 17A herein). Another global eustatic sea-level rise (Haq et al., 1988) probably contributed to the early to early middle Miocene tectonically enhanced accommodation of the Colombian basins. The ensuing highstand

is recorded by the Len Formation (Llanos Basin) and the slightly brackish deposits of the Santa Teresa Formation (southern MMVB) and the La Cira fossil horizon (northern MMVB). The MMVB paleodrainage returned to the north during the late middle to late Miocene (Fig. DR5, see footnote 1) due to Eastern Cor-dillera continued uplift and sedimentary overlap of the Cchira Arch (older northern boundary of the MMVB, Gmez et al., 2005). Neogene pulses of Eastern Cordillera uplift also caused episodes of MMVB fl exural tilting and partial erosion of Neogene deposits (e.g., Fig. 2C).

CONCLUSIONS AND DISCUSSION

The Colombian foreland basin system east of the Central Cordillera overlapped a Mesozoic rift province, and subsequent inversion of the rift system in the Eastern Cordillera modifi ed the nature of the foreland basin. Long-term tectonic accommodation was controlled by isostatic adjustments to variable distributions of crustal tectonic loads, in combination with sediment supply from tectonic highlands, and paleoaltitude of depositional profi les. Eustasy was a secondary factor, and its geographic distribution was controlled by fl exural accom-modation and local deformation. Although mechanically related, at least four different types of unconformities can be recognized in the foreland basin system, which refl ect a range of scales of variation in the wavelength of tec-tonic accommodation. First, the MMVU in the western sector of the foreland system resulted from eastward-migrating Late Cretaceousearly Eocene Central Cordillera uplift and consequent long-term erosion since the middle Eocene. Sec-ond, Paleogene unconformities formed on the distal eastern side of the Late Cretaceousearly Eocene foreland basin (e.g., Llanos area) due to lithospheric fl exure and erosional beveling dur-ing periods of Central Cordillera uplift. Third, similar unconformities, but formed by eastward tilting under the load of the Eastern Cordillera, are found in the Neogene MMVB. Fourth, shorter wavelength folding produced local growth unconformities and modifi ed drainage patterns in the subsiding basin between the MMVB and Llanos area. Growth strata are the most important evidence of Late Creta-ceousNeogene diachronous Eastern Cordillera deformation, prior to massive Pliocene uplift.

The two-dimensional mechanical models seem to explain variations in accommodation at a broad regional level in the palinspastically restored area east of the Central Cordillera, which suggests that mechanical controls were relatively simple during the Cenozoic. Thus, there is no need to utilize alternative modeling


Aura Cuervo GmezResaltado



techniques that invoke broken plates or thermal weakening of the lithosphere. These models are better suited for cases in which simple fl exural approaches cannot explain complex distributions of sedimentary thicknesses and facies, as exem-plifi ed by foreland basins fl anking the Central Andes (e.g., Cardozo and Jordan, 2001).

This paper highlights the importance of assessing the mechanical causes of accom-modation and unconformities to understand the evolution of tectonic basins. Time correlations based on sequence stratigraphy, if they assume simultaneous base-level variations that are sensi-tive to choice of datum, are not appropriate for basin analysis at scales comparable to the dis-tances over which tectonic controls act. For the MMVBEastern CordilleraLlanos system, we fi nd that there are four different scales of tectonic control on unconformities. At each of these spa-tial scales, in relatively distal basin areas far from zones of active deformation, it may be diffi cult to recognize the tectonic control on unconformities and correlative conformities. It would be just as easy to ascribe these surfaces to eustasy even if that explanation were erroneous. This discussion illustrates the importance of placing the detailed local studies within a thorough regional study, at which scale the tectonic controls are clear.

ACKNOWLEDGMENTS

This study was supported by grants and fellow-ships from the Petroleum Research Fund (ACS-PRF no. 32818-AC8), the National Science Foundation (Faculty Award to Women in Science and Engineering award GER-9022811 to T.E. Jordan), and the Instituto Colombiano para el desarrollo de la Ciencia y la Tec-nologa (Colciencias). Cornell University, the Geologi-cal Society of America, the American Association of Petroleum Geologists, Shell Oil Company Foundation, Shell E&P Solutions, and Ecopetrol also contributed funding to this research. We also thank geologists Matthew Burns and Francisco Gmez for helpful discussions. Critical reviews by Rebecca Dorsey, Brian Horton, Ken Ridgway, Allen Glazner, Cynthia Evinger, Frdric Mouthereau, Yildirim Dilek, and an anonymous reviewer helped us to improve this paper.

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Development of the Colombian Foreland-basin System_Gomez2005