Early Jurassic Rift Structures Associated With the Soapaga

7/29/2019 Early Jurassic Rift Structures Associated With the Soapaga

1/11

Early Jurassic rift structures associated with the Soapagaand Boyaca faults of the Eastern Cordillera, Colombia:

Sedimentological inferences and regional implications

Andreas Kammer *, Javier Sanchez

Departamento de Geociencias, Universidad Nacional de Colombia, Apartado Aereo, 14490 Bogota, Colombia

Received 1 October 2004; accepted 1 February 2006

Abstract

The NW-trending Bucaramanga fault links, at its southern termination, with the Soapaga and Boyaca faults, which by their NWtrend define an ample horsetail structure. As a result of their Neogene reactivation as reverse faults, they bound fault-related anticlinesthat expose the sedimentary fill of two Early Jurassic rift basins. These sediments exhibit the wedge-like geometry of rift fills related towest-facing normal faults. Their structural setting was controlled further by segmentation of the bounding faults at approximately 10 kmintervals, in which each segment is separated by a transverse basement high. Isopach contours and different facies associations suggestthese transverse anticlines may have separated depocenters of their adjacent subbasins, which were shaped by a slightly different subsi-dence history and thereby decoupled. The basin fill of the relatively narrow basin associated with the Soapaga fault is dominated byfanglomeratic successions organized in two coarsening-upward cycles. In the larger basin linked to the Boyaca fault, the sedimentaryfill consists of two coarsening-upward sequences that, when fully developed, vary from floodplain to alluvial fan deposits. These EarlyJurassic rift fills temporally constrain the evolution of the Bucaramanga fault, which accommodated right-lateral displacement during the

early Mesozoic rift event. 2006 Elsevier Ltd. All rights reserved.

Keywords: Early Jurassic rift structures; Bucaramanga fault; Giron Formation; Horsetail structure; Transverse basement high

1. Introduction

The Eastern Cordillera of Colombia, through its fore-land position and separation from the rest of the NorthAndean mountain system by the Magalena Valley, formsa distinct physiographic feature that shares the characteris-

tics of a bivergent structure, elevated basement-cored axialzone, and moderate deformations with other intracontinen-tal chains (e.g., Rodgers, 1987; Teixell et al., 2003). Geolo-gists attempting to synthesize its structural evolutionrepeatedly have emphasized the importance of inheritedfaults for controlling Andean structures. In studying theimportance of marginal faults for Mesozoic basin evolu-

tion, some researchers view the Eastern Cordillera as agiant inverted rift structure (Cooper et al., 1995; Roederand Chamberlain, 1995; Villamil, 1999), whereas othersdifferentiate individual grabens or blocks in an EarlyMesozoic rift phase (Fabre, 1983). Two major rift eventshave been postulated as initiators of Mesozoic basin evolu-

tion: a Late TriassicMiddle Jurassic event associated withrift basins (Mojica and Kammer, 1995) and a Tithonian/Neocomian event that established the conditions for anextensive marine Cretaceous backarc basin until the Maas-trichthian (Dorado, 1984; Mojica et al., 1996).

The Early Mesozoic evolution of the North Andeanrealm is closely related to an active continental margin,as evidenced by an extensive magmatic arc and a corre-sponding volcanic roof complex, the remnants of whichoccupy the western flank of the Central Cordillera, includ-ing the San Lucas Range and Sierra Nevada de Santa

0895-9811/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jsames.2006.07.006

* Corresponding author.E-mail address: [email protected] (A. Kammer).

www.elsevier.com/locate/jsames

Journal of South American Earth Sciences 21 (2006) 412422
mailto:[email protected]:[email protected]


2/11

Marta (Fig. 1; Tschanz et al., 1974). This intrusive-volcaniccomplex contains intercalations of sedimentary sequences,among which the Late Triassic Payande Formation (Cedielet al., 1980) and the Early Jurassic Morrocoyal and LosIndios formations (Geyer, 1973) provide a chronostrati-graphic framework for regional evolution (Figs. 1 and 2).By their position east of the magmatic arc and their marinefossil content, they designate a backarc setting (Bayonaet al., 1994). In its more continental position, the Bucara-manga fault plays a crucial role in the Early Mesozoic rif-ting of the North Andean basement, as evidenced by themany small rift basins that straddle its western hangingwallblock (Ward et al., 1973). These basins are partially filledor overlain by continental redbeds of the Giron Formation(Cediel, 1968), from which plants and ostracodes of Juras-

sicEarly Cretaceous age have been retrieved (Remy et al.,

1975; Rabe, 1977). North of Bucaramanga City at Bocas(Fig. 1), the Giron Formation overlies a lacustrine to fluvi-al sequence related to a small rift structure bounded by asplay of the Bucaramanga fault (Figs. 1 and 2). The mainlylacustrine Bocas has yielded a pre-Toarcian age (Remyet al., 1975; Rabe, 1977), so far the most precisely datedEarly Jurassic rift fill of the Eastern Cordillera.

Late TriassicEarly Jurassic activity of the Bucaraman-ga fault also may be inferred from the structural relationsand age data of the intrusive suite of the Santander Massif(Fig. 1). This igneous suite consists of elongated batholithsand stocks that occupy the eastern footwall domain of theBucaramanga fault and attest to a synkinematic originthrough an elongate apophysis emplaced along the faulttrace (Ward et al., 1973). Late TriassicLower Jurassic

age data have been obtained for this suite (Dorr et al.,

Fig. 1. Synoptic map view of the Bucaramanga fault, including the Boyaca and Soapaga splay faults, at its southern termination. Geographic inset locatesthis figure and Fig. 3.

A. Kammer, J. Sanchez / Journal of South American Earth Sciences 21 (2006) 412422 413


3/11

1995; Ward et al., 1973). The rift structures examined in

this article occur at the southern termination of the Bucara-manga fault; according to the regional findings, they likelyhave an Early Jurassic origin. They branch from a rectilin-ear, NNW-trending fault segment into the western (or foot-wall) block; define splay faults of a major horsetail; andindicate, according to a NNE-trending fracture array ofthe magmatic arc (Fig. 1), a regional NWSE extensionfor the Late TriassicEarly Jurassic rift phase. The Bucara-manga fault displays the characteristics of an oblique-slipfault with an important right-lateral component. Amongthe formations that constitute the rift fill of the basins atthe southern termination, only the lacustrine Montebel(Fig. 2) yields fauna of a RhaetianEarly Jurassic age(Geyer, 1973; Trumpy, 1943).

This article analyzes the structural framework of the tworift structures located at the southern termination of theBucaramanga fault and examines whether their sedimentaryfill compares to rift-related depositional systems (Prosser,1993). The two rift fills supply complementary informationbecause of their different sizes: A relatively narrow basinrelated to the Soapaga fault, referred to as the Floresta rift,contains redbeds associated with the Giron Group,whereas a larger basin (the Arcabuco-Guantiva rift)subsided along the Boyaca fault displays a fluviolacustrinesuccession in its lower part (Fig. 2). Both faults underwent

Neogene inversion that controlled the formation of the

Floresta and Arcabuco-Guantiva basement highs. Anadequate understanding of the evolution of these rift struc-tures will help establish a framework for other, poorlyexposed Late TriassicEarly Jurassic rock units of the East-ern Cordillera and, at the regional scale, offer a scenario forintracontinental rifting that preceded the Middle Jurassic

breakup of Pangea (Pindell, 1985). This analysis is limitedby deficient knowledge about a chronostratigraphic frame.Ages are assessed from published data or regional inference.

2. Geologic setting of the Floresta and Guantiva anticlines

The Neogene reactivation of the Boyaca and Soapagafaults gave rise to open folds in their hangingwall blocks,which expose the inherited structures in detail. Deformedfossils indicate a shortening that did not exceed 10% ingently inclined flanks (Kammer, 1996), so the present-daytectonic frame represents a valid proxy for paleotectonicconsiderations. The asymmetric Floresta anticline in the

hangingwall block of the Soapaga fault displays a verticalto overturned eastern flank and exposes Devonian sedi-mentary units, as well as a low-grade metasedimentarybasement in its core (Fig. 3). Jurassic redbeds of the GironFormation overlay the pre-Mesozoic basement in bothflanks, displaying a pronounced wedge, with thicknessesranging from a few meters near their western pinch outbelow the Cretaceous cover to more than 500 m at the Soa-paga fault that delimits them to the east (Figs. 3 and 5a).The ancestral Soapaga fault was therefore a W-facingplane. At Nobsa, the Floresta anticline ends in a periclinalstructure as the Soapaga fault assumes an EW trend

(Fig. 3).The hangingwall domain of the Boyaca fault is divided

by the transverse Rio Negro syncline into the Arcabucoand the Guantiva anticlines (Fig. 3). The Guantiva anti-cline core contains a southern extension of the Late Trias-sicEarly Jurassic Mogotes Batholith, which is linked tothe igneous suite of the Santander Massif farther north(Figs. 1 and 3). North of the study area, this batholithintrudes the Boyaca fault and forms a sheet-like geometrythrough its emplacement along the disconformity betweenthe metamorphic basement and Upper Paleozoic sediments(Fig. 3). Jurassic sediments are exposed all over the Arca-buco and in the southern part of the Guantiva anticline.They also display a wedge-shaped geometry and pinchout in the western flank of the Guantiva anticline. Accord-ingly, the Boyaca fault represents a W-facing normal fault.The Arcabuco anticline ends in a periclinal terminationwest of the town of Duitama, similar to the Floresta Massiffarther east (Fig. 3).

A particular feature of this geologic setting refers to aLate Paleozoic rift event, the faults from which are maskedby elongate intrusive stocks and plugs dated at 471 and 394Ma (Ulloa and Rodrguez, 1982; Cordani, in Etayo et al.,1983). The Soiqua fault east of the culmination of theFloresta anticline has the same polarity as the Soapaga

fault and displays a slice of high-grade metamorphic

LowerMagdalenaValley &

San LucasRange

EasternCordillera

Boyaca

SantanderMassif

Sierra Nevadade Santa Marta

T

rias

Jurassic

Cretaceous

Early

MiddletoLate

Mogotes

Morrocoyal

Tablazo

La Rusia

La Mojana

Los SantosArcabuco

ConglomerateSandstoneShalesLimestone

Palermo

Noren

El Sudan

Golero

Guatapuri

Corual

Los Indios

Intermediate to acidigneous rock

Acid flowSubaqueous

flow

Pyroclastic

deposit

CogolloGroup

Montebel

Bocas

Girn

Tiburn

Jordan

Pescadero

Fig. 2. Stratigraphic correlation chart of units deposited in rift basinsalong the Bucaramanga fault (Eastern Cordillera and Santander Massif)and close to an early Mesozoic magmatic arc or its adjacent backarc basin(Lower Magdalena Valley, Sierra Nevada de Santa Marta).

414 A. Kammer, J. Sanchez / Journal of South American Earth Sciences 21 (2006) 412422


4/11

basement in its footwall (designated Bunta Orthogneiss byUlloa et al., 1998) in contact with low-grade units of theanticlinal core. Together with the Tutasa fault farther west,the Soiqua fault limits a minor symmetric graben filledwith the basal Devonian Tibet Formation (Fig. 4; Mojicaand Villarroel, 1984). In the western flank of the Guantivaanticline, a Late Paleozoic fault is sealed by Jurassic sedi-ments, which suggests a lack of reactivation during theMesozoic and Cenozoic, despite its favorable attitude andpolarity. The Middle Devonian Floresta and Cuche forma-tions recover Upper Paleozoic fault blocks and therebymark the end of this rift event.

3. Late TriassicEarly Jurassic rift structure settings

A W-facing polarity for the Soapaga and Boyaca faultsis deduced from the wedge-shaped basin fills of their hang-ingwall blocks. A uniform structural setting of these faults

and a splay fault linked to the Servita fault farther north

(Fig. 3) also is indicated by regular segmentation, 1040 km long, created by linked, curved fault traces, eachof which displays the downthrown block on its concaveside. For the Soapaga fault, this segmentation is highlight-ed by relays spaced at 1222 km. The relays represent sitesfor a structural reorganization of the eastern flank of theFloresta Massif, as exemplified at Beteitiva (Fig. 5a), wherea southern fault-related fold gives way to a northernreverse fault associated with reverse drag folding (Kam-mer, 1996). The Boyaca fault displays one of the mostprominent relay structures west of Duitama (Figs. 3 and5a). Relay patterns farther north are speculative, becausethe Guantiva Massif is deprived of its sedimentary cover.

To explore Jurassic fault-related folding and its controlon the depositional history further, we compiled an iso-pach map for the Giron Formation of the Floresta andthe Rusia Formation of the Arcabuco-Guantiva rift struc-tures (Fig. 5a). Thicknesses were taken from measured

columns (Figs. 6 and 7) and complemented with data

Fig. 3. Geologic setting of the Arcabuco-Guantiva and Floresta anticlines with the Soapaga and Boyaca faults deduced from present-day fault arrays.



5/11

from structural sections drawn at a scale of 1:25,000.Each data set involves inherent uncertainties. On theone hand, stratigraphic columns A, B, C, and G(Fig. 5) extend over much of the eastern flank of theArcabuco-Guantiva anticline and thus cross isopach con-tours. On the other hand, outcrop conditions and the pre-liminary nature of our base map impede precise locationof formation boundaries in the Arcabuco-Guantiva anti-cline. Along the Soapaga fault, however, lithological lim-

its are mapped out accurately enough for this purpose. Ineach sedimentary wedge, the pinch-out line is constrainedby just one point (Figs. 3 and 5a). In the Arcabuco-Guan-tiva anticline, the base of the Jurassic sediments isexposed only in the northern part, and isopach contoursare established for the lithologic break between the Mon-tebel and Rusia formations (Fig. 2).

By equating the isopach contours with structural con-tours of the disconformity between Jurassic sedimentsand a pre-Mesozoic basement, we develop a fairly simplepicture of the structures of Jurassic wedges. Accordingly,we divide the rift structures into different subbasins alongtransverse basement highs tied to fault relays. A primeexample of a transverse basement high coincides with theNeogene Rio Negro syncline. It connects to the fault relaywest of Duitama and serves, in size and well-constrainedoutline, as a prototype for interpreting less evolved struc-tures, as displayed at fault relays of the Soapaga fault.

A transverse section (Fig. 4) connecting two depocentersof the Floresta and Guantiva subbasins documents uni-form block tilting for the pre-Jurassic basement, whichevokes domino-style faulting. Considering along-strikethickness and facies variations a fault growth modelaccounts more appropriately for the segmented geometryof these rift basins however (Schlische, 1991; Gawthorpe

and Leeder, 2000).

4. Stratigraphic considerations

4.1. Floresta graben

The Floresta graben is composed exclusively of redbedsassigned to the Giron Formation. Despite their uniformappearance, they can be divided into fluvial and gravityflow deposits related to alluvial fans or bajadas that strad-dle the Soapaga fault and alluvial slope deposits that blan-

ket the tilted hangingwall block. Deposits along the troughaxis are completely eroded. Four facies types dominatethese alluvial fan deposits, as indicated in Table 1.

Conglomeratic clasts are composed of fine- to medium-grained sandstones of the Cuche (3040%), felsic igneousrocks (20%), and metamorphic basement (locally 40%).The felsic igneous components attest to local intrusiveactivity, as also evidenced by felsic dikes. In one case, adebris flow unit caps a felsic dike. This synsedimentaryigneous activity is limited to the fault trace; similar felsicdikes have not been observed in the metamorphic unitsof the Floresta Massif. In massive redbeds of facies associ-ation G4, white, distorted, angular fragments stem fromkaolinitized feldspar crystals and attest to the presence oftuffaceous material.

Lopez and Mesa (1997) present lithologic columns ofthe central and peripheral parts of the southern subbasin,reproduced here as Fig. 6. The two successions displaycomparable sequential organizations, despite their dissimi-lar thickness, and overlie the nonconformity with the meta-morphic basement by a massive cobble-pebbleconglomerate (facies association G1). This basal unit issucceeded by a coarsening- and thickening-upwardsequence, dominated in its lower part by conglomeraticlenses embedded in a sandy matrix (G2/G4) and grading

at the top into thick-bedded cobble conglomerates (G3).

meters

Fig. 4. Transverse section through the Arcabuco-Guantiva and Floresta rift structures, as restored to a postrift stage. For location, refer to Fig. 5a.



6/11

This sequence is interpreted in terms of a prograding fansystem (Blair, 1986), with the basal conglomeratic unit des-ignating coarse-grained, possibly uncoalesced talus cones

near a fault scarp or mass flows at the very proximal

reaches of an alluvial fan. The succeeding conglomeraticto sandy deposits (G2) are unconfined and thereby revealan elevated aggradation rate; thus, we posit that they rep-resent a depositional environment below the intersectionpoint of the incised feeder channels of an alluvial slope.The thick-bedded cobble conglomerates at the top of this

succession record a proximal facies association (G3), dom-inated by channel deposits above or near the intersectionpoint. Finally, a sequence of sandy to conglomeratic lenses(G2) embedded in a finer-grained matrix (G4) initiates asecond coarsening-upward cycle, which displays faciesassociations G2 and G4 at its base, which we interpret ascharacteristics of the intermediate reaches of an alluvialfan. This depositional change might reflect fault-inducedsubsidence.

Alluvial slope deposits are exposed on the western flankof the Floresta anticline near the pinch-out line of the gra-ben fill. The sequence starts with a pebble conglomerate(G1), which bears lithologic and granulometric similaritiesto the fan deposits sourced from the Soapaga fault scarp.This conglomerate is overlain by a succession of lenticular,coarse-grained sandstone bodies of less than 1 m and fine-grained redbeds (G2/G4). This facies association representsa floodplain environment dissected by minor channels,which designates consequent drainage on the tilted hang-ingwall block of the Floresta graben and has been referredto as an alluvial slope deposit (Smith, 2000).

4.2. Arcabuco-Guantiva graben

The sedimentary fill of the southern subbasin discloses a

clear tripartite division, according to the lithological suc-

a

b

c

d

Fig. 5. Isopachs and facies associations compiled on a base map of pre-Cretaceous units. (a) Numbers associated with lines refer to thicknessestimates of the Rusia Formation in the Arcabuco-Guantiva anticline orGiron Formation in the Floresta anticline. Numbers associated with filledcircles indicate thicknesses of measured sections. Contour lines for theRusia (Arcabuco-Guantiva) or Giron (Floresta) formations are indicatedin intervals of 200 m. (bd) Close-ups of (a, rectangle) showing the distri-bution of facies associations, as determined in measured sections AH(Fig. 7). Abbreviations of facies associations are listed in Table 2. (d)Paleocurrent directions are plotted for four localities.

a

b

Fig. 6. Two lithologic profiles of the southern subbasin of the Florestarift. (a) Southern termination of the Floresta Massif; (b) central part.Modified from Lopez and Mesa (1997).



7/11

cessions of the Palermo, Montebel, and Rusia formations(Fig. 2). In the northern subbasin, the Palermo was notmeasured. The Palermo of the southern subbasin containsa basal conglomerate of a few meters, with pebbles derivedfrom redbeds of the Upper Paleozoic Cuche Formation(Renzoni, 1967). Otherwise, the sequence begins with blackshales and becomes coarser grained, following a successionof siltstones and fine- to medium-grained sandstones. Aconspicuous horizon of black shales at the base of theMontebel initiates a second coarsening-upward cycle, con-sisting again of a succession of silt- and sandstones thatbecome variegated at higher stratigraphic levels. From itsbasal part, plant remnants and a freshwater mollusk faunaof probable RhaetianLiassic age were retrieved (Langen-heim, 1961; Benedetto and Odreman, 1977; Geyer, 1973).The upper third of this rift fill contains conglomeraticand sandy redbeds assigned to the Rusia Formation. Thepredominant facies associations of this sequence and theircorresponding interpretations appear in Table 2.

A lithostratigraphic correlation between the differentsequences of the two subbasins and their intervening base-ment high is hampered by the absence of mapable key hori-

zons. The limit between the Montebel and Rusia

formations, regardless of its utility for establishing theispoach contours of the Rusia Formation, is transitionaland should be used with caution for correlative purposes.The Montebel Formation shows different stacking patternsin the two subbasins. In the southern, it indicates a coars-ening-upward sequence, with shaly lacustrine to floodplaindeposits (M1) predominant in its lower part. In its upperpart, it includes increasing distributary channel and sandybraidplain deposits (M2 and M3; Fig. 7). Floodplaindeposits are silty. In contrast, in the columns of the north-ern subbasin, braidplain and channel deposits (M3 andM2) and silty floodplain deposits (M1) dominate the lowerpart and are succeeded by shaly lacustrine to floodplaindeposits (M1) below the transitional contact with the RusiaFormation (Fig. 7), defining a fining-upward sequence.Figs. 5b and c contrast these facies associations in a mapview. The predominance of fine-grained facies associationslikely reflects periods of increased subsidence, if the drain-age system of the rift structures was relatively closed,whereas coarsening-upward trends record periods of tec-tonic quiescence (Blair, 1988). Accordingly, these litholog-ical differences may suggest episodes of fault activity that

affected the two subbasins. In support of a fault growth

A

B

D

F

E

G

C

H

Fig. 7. Lithologic profiles of the Arcabuco-Guantiva rift. For location, refer to Figs. 5bd. Modified from Baez and Sanchez (2003).



8/11

model, the transverse anticline at their relay may haveacted as a seismogenic barrier during episodic growth, ashas been demonstrated for the Wasatch fault zone (Mach-ette et al., 1991).

Facies associations of the Rusia Formation show a morehomogeneous distribution (Fig. 5d). Proximal mass flowdeposits of talus cones (R1) straddle the fault trace in thetwo subbasins. This fringe is narrower along the transverseanticline. This facies association is succeeded by channel(R2 and R3) and overbank (R4) deposits. Paleocurrentdirections are transverse to parallel with respect to the bor-der fault (Fig. 5d). Near the transverse high, they display aNNW trend, possibly mimicking a relay ramp (Gawthorpeand Hurst, 1993).

5. Discussion

Current views on the evolution of segmented faultarrays during the early stages of a rift evolution adhereto two models based on displacement length profiles andnumerical simulation. They involve the formation of (1)isolated faults in a diffuse deformation zone and subse-quent competitive growth of certain faults through linking(Gupta et al., 1998; Cowie et al., 2000; Gawthorpe and

Leeder, 2000) or (2) a coherent segmented fault array, in

which fault segments are linked after their initiation to asingle fault at depth (Walsh et al., 2003). For the rift basinsexamined herein, displacement length profiles are con-strained only by isopach maps and do not offer sufficientresolution to quantify trends and skewness patterns. Wefavor the model of a coherent fault development (i.e., tiedto a single fault), because the fault segments are alignedprincipally along two strands (Boyaca and Soapaga faults).The only case of a soft-linked synthetic interference zoneoccurs at the southern termination of the Servita fault,where a segmented fault string cuts into the footwall ofthe Soapaga fault (north of Sativa Norte in Figs. 5 and3). The setting of the two major fault strings is independentof the location of Late Paleozoic faults, though some werefavorably orientated in their strike and dip for an earlyMesozoic reactivation. Tentatively, we explain the lockingof the Paleozoic faults during the Early Mesozoic rift eventaccording to their limitation to a shallow crustal level,which implies the Jurassic faults nucleated independentlyand more deeply.

In discussing the depositional patterns of the two riftbasins, we follow a conceptual filling model of continen-tal rift basins outlined by Schlische (1991) and Schlischeand Olson (1990), who presume a constant volumetric

sedimentation and subsidence rate. In a fault growth

Table 2Compilation of facies associations and respective interpretations ofArcabuco-Guantiva graben

Faciesassociation

Lithology Interpretation

Palermo and Montebel Formations

M1 Massive silty mud or mud Standing freshwater bodies

and wetlands of adjacentfloodplains with incipient soildevelopment

Laminated or cross-laminated silt, mud or finesandCalcrete horizons

M2 Planar cross-stratified,coarse to fine sand inisolated lenses

Minor channel fills formedby ephemeral streams onalluvial plains

M3 Massive or horizontallystratified, medium to finesand

Transverse and linguoidbedforms formed onbraidplains by unconfinedflash floods and crevassesplays

Rusia Formation

R1 Matrix-supported gravel

and coarse sand

Channelized or little confined

mass flow deposits (sheetfloods) of talus conesMassive coarse to fine sand

R2 Clast-supported, crudelystratified gravel

Longitudinal bedforms andlag deposits; sandyoutwashes during waningfloods

Horizontally stratified,coarse to fine sand

R3 Clast- and matrix-supported gravel

Sinuously crested andlinguoid bedforms associatedwith minor channelsPlanar and trough

cross-stratified, coarseto fine sand

R4 Massive or laminated siltor mud

Overbank deposits

Table 1Compilation of facies associations and respective interpretations consti-tuting the Floresta graben

Faciesassociation

Lithology Interpretation

Giron Formation

G1 Massive, thick to very thick

beds of matrix-supportedcobble-pebble conglomerates

Gravity flows associated

with proximal fan reaches ortalus cones

G2 Medium to thick beds of pebble-cobbleconglomerates, organized inlenses, with slightly scouredbases and convex-upwardforms

Little channelized debris orsheet flows on proximal fanreaches, grading into lobesof sieve deposits

Massive to horizontallystratified conglomeratic,grain-supported sands

G3 Crudely to horizontallystratified conglomerates inplanar beds

Longitudinal bars andminor channel fills formedby ephemeral streams onmiddle fan reaches or

alluvial plains

Cross-bedded sandy

conglomerates, grading intosand, with convex-up formsPlanar horizontally stratifiedconglomeratic sand, finingupward

G4 Horizontally stratified,coarse to fine sand, in thinplanar beds

Flash flood deposits ofephemeral streams andoverbank deposits

Massive sandy mud, withlithic components



9/11

model, which considers both an increase in fault throwand length, the change in accommodation space is posi-tive through time, which predicts evolution from a filledto an undersaturated basin. This tendency is reversed inthe postclimax rift stage of waning fault activity (Prosser,1993), during which the basin fill tends to return to sat-

urated conditions. Cyclic behavior can be traced for eachsubbasin. In the southern subbasin of the Arcabuco-Guantiva rift, the sedimentation pattern displays twocoarsening-upward cycles, the lower for the Palermoand the upper for the Montebel and Rusia formations,that were initiated by extensive floodplain deposits. Themodel conditions outlined previously are satisfied hereby two discrete events followed by a waning fault activ-ity. The evolution of the Montebel in the northern sub-basin is contrasted by a fining-upward sequence,presupposing ongoing tectonic activity according to thepreceding predictions, though the departure from a satu-rated basin, as evidenced by a major standing water

body, would not have been significant.In the smaller subbasin of the Floresta rift, two coarsen-

ing-upward cycles may be interpreted in terms of prograd-ing fan conditions, which again implies waning faultactivity for each cycle. The predominance and lateral con-tinuity of coarse-grained facies associations favors the exis-tence of coalesced fan deposits or bajadas and, thus,oversaturated sedimentary conditions. Transverse paleo-current directions in both rifts indicate the importance offootwall-derived sedimentary input. Nothing is knownabout a regional drainage system. The only argument per-taining to a local base level applies to the Arcabuco-Guan-

tiva rift, where the Montebel and Rusia formations extendto a common pinch-out line. This situation may beexplained by a fixed outlet during the corresponding riftepisode.

With regard to their regional significance, the subsidencehistory of the two examined rift basins constrains ancestralactivity of the Bucaramanga fault to the Early Jurassic.The fault accommodates displacement transfer along twofault strings (Fig. 1; Ujueta, 2003), which suggests thatfault events would manifest themselves along considerablelengths. An interaction between rifting and magmaticactivity may be inferred from the following arguments:North of the city of Bucaramanga, the two rectilinearstrands of the Bucaramanga fault are displaced along aright-stepping relay, which should correspond to a releas-ing relay for a right-lateral displacement. This relaydomain is matched by an S-shaped termination of theRio Negro Batholith (Fig. 1) which resembles a majorrhombochastic structure. The emplacement of the batho-lith was thus concurrent with the displacement transferon these two fault segments. The longitudinal extensionof the Bucaramanga fault also coincides with the presenceof the intrusive suite of the Santander Massive in its foot-wall, evidence of a relationship between faulting and crust-al weakening induced by the infiltration of magmatic fluids

(Hollister and Crawford, 1986).

From these regional considerations, we postulate ahard-linked origin of the Soapaga and Boyaca faults toa mid-crustal detachment. In this scenario, the Bucaraman-ga fault constitutes an oblique ramp that separates anattenuated crust on its western side (including the EarlyTriassicLower Jurassic backarc basin) from a relatively

unaffected block on the eastern side. The sedimentary fillof the examined rift structures, together with those locatedalong its eastern hanging block, such as at Bocas (Figs. 1and 2), provide insight into the evolution of this first-ordercrustal discontinuity.

6. Conclusions

First, the Soapaga and Boyaca faults branch off of arectilinear fault strand of the Bucaramanga fault andform the splay faults of a conspicuous horsetail struc-ture. The two faults bound rift basins filled with redfanglomerates (Soapaga fault) or lacustrinefluvial

sequences (Boyaca fault), which display the wedge-likegeometry of a rift-related depositional system. The EarlyJurassic age of these sediments documents Early Meso-zoic activity of the Bucaramanga fault. The Soapagaand Bucaramanga faults compose a horsetail structurethat, in combination with the rhombochastic structureat a right-stepping relay of the Bucaramanga fault,establishes a right-lateral displacement transfer for thiscrustal-scale discontinuity.

Second, the Boyaca and Soapaga faults are divided intocurved segments at intervals of 1040 km. Their relaydomains define sites of distinct transverse basement highs

that controlled the sedimentation in adjacent subbasins.This segmentation conforms to a fault growth model thatcombines the accumulation of dip-slip displacements witha longitudinal fault expansion.

Third, the sediments of the relatively narrow basin asso-ciated with the Soapaga fault are composed of two coars-ening-upward cycles that may be accounted for by analluvial fan progression. These fanglomeratic successionsinclude sheetflood to channel deposits.

Fourth, in the larger basin linked to the Boyaca fault,a distinct transverse basement high separates two subba-sins characterized by different sedimentary stackingpatterns. The southern subbasin comprises two coarsen-ing-upward cycles, each of which was initiated bylacustrine to floodplain deposits. In the northern subba-sin, this stacking pattern is reversed in the basal partby a fining-upward cycle. The subsidence history of thesebasins therefore was decoupled to some degree, whichillustrates that the transverse high acted as a barrierduring the seismogenic and geomorphic evolution ofadjacent fault segments.

Acknowledgements

The manuscript benefited greatly from the constructive

comments of two anonymous reviewers. We thank Victor



10/11

Ramos and James Kellogg for their editorial efforts andtheir decisive help in improving the manuscript.

References

Baez, L.A., Sanchez C.J., 2003, Un escenario paleo-ambiental para unatectonica extensional jurasica mediante la caracterizacion de las

formaciones La Rusia y Montebel, al nor-occidente de la Falla deBoyaca, en cercanas a los municipios de Paipa, Duitama, Cerinza yBelen. Unpublished thesis, Universidad Nacional de Colombia(Bogota), 83 p.

Bayona, G.A., Garcia, D.F., Mora, G., 1994, la Formacion Saldana:producto de la actividad de estratovolcanes continentales en undominio de retroarco. Estudios Geologicos del Valle Superior delMagdalena, I-1-21, Universidad Nacional de Colombia, Bogota.

Benedetto, G., Odreman, R.O., 1977, Nuevas evidencias paleontologicasen la formacion la Quinta, su edad y correlacion con las unidadesaflorantes en la Sierra de Perija y Cordillera Oriental de Colombia.Memoria, V Congreso Geologico Venezolano, tomo 1, Ministerio deEnerga y Minas, Sociedad Venezolana de Geologos, Caracas, 87106.

Blair, T., 1986. Tectonic and hydrologic controls on cyclic alluvial fan,fluvial, and lacustrine rift-basin sedimentation, Jurassic lowermostCretaceous Todos Santos formation, Chiapas, Mexico. Journal ofSedimentary Petrology 57, 845862.

Blair, T., 1988. Development of tectonic cyclothems in rift, pull-apart andforeland basins: sedimentary response to episodic tectonism. Geology16, 517520.

Cediel, F., 1968, El grupo Giron, una molasa mesozoica de la CordilleraOriental. Boletn Geologico, INGEOMINAS, Bogota, 16, pp. 596.

Cediel, F., Mojica, J., Macia, C., 1980. Definicion estratigrafica delTriasico en Colombia, Sudamerica, Formaciones Luisa, Payande ySaldana. Newsletters on Stratigraphy 9, 73104.

Cooper, M.A., Addison, F.T., Alvarez, R., Coral, M., Graham, R.H.,Hayward, A.B., Howe, S., Martinez, J., Naar, J., Penas, R., Pulham,A.J., Taborda, A., 1995. Basin development and tectonic history of theLlanos Basin, Eastern Cordillera, and the Middle Magdalena Valley,Colombia. American Association of Petroleum Geologists Bulletin 79,14211443.

Cowie, P.A., Gupta, S., Dawers, N.H., 2000. Implications for fault arrayevolution for synrift depocenter development: insights from a numer-ical fault growth model. Basin Research 12, 241261.

Dorado, J., 1984, Contribucion al conocimiento de la estratigrafa de laFormacion Brechas de Buenavista, Oeste de Villavicencio, Meta.Unpublished thesis, Universidad Nacional de Colombia (Bogota),150 p.

Dorr, W., Grosser, J.R., Rodrguez, G.I., Kramm, U., 1995. Zircon U-Pbage of the Paramo Rico tonalite-granodiorite, Santander Massif(Cordillera Oriental, Colombia) and its geotectonic significance.Journal of South American Earth Sciences 8, 187194.

Etayo, F., Barrero, L.B., Lozano, Q.H., Espinosa, B.A., Gonzalez, I.H.,

Orrego, L.A., Ballesteros, T.I., Forero, O.H., Ramrez, Q.C.,Zambrano-Ortiz, F., Duque-Caro, H., Vargas, H.R., Nunez, T.A.,

Alvarez, A.J., Ropain, U.C., Cardozo, P.E., Gavis, G.N., Sarmien-to, R.L., 1983. Mapa de terrenos geologicos de Colombia:Publicaciones Geologicos Especiales, INGEOMINAS (Bogota),141, 235 p.

Fabre, A., 1983. La subsidencia de la cuenca del Cocuy (CordilleraOriental de Colombia) durante el Cretaceo y el Terciario Inferior.Primera parte: Estudio cuantitativo de la subsidencia. GeologaNorandina 8, 4961.

Gawthorpe, R.L., Hurst, J.M., 1993. Transfer zones in extensional basins:their structural style and influence on drainage development andstratigraphy. Journal of the Geological Society, London 150, 11371152.

Gawthorpe, R.L., Leeder, M.R., 2000. Tectono-sedimentary evolution of

active extensional Basins. Basin Research 12, 195218.

Geyer, O.F., 1973, Das prakretazische Mesozoikum von Kolumbien.Geologisches Jahrbuch, B5, Hannover, 1156.

Gupta, S., Cowie, P.A., Dawers, N.H., Underhill, J.R., 1998. Amechanism to explain rift basin subsidence and stratigraphic patternsthrough fault-array evolution. Geology 26, 595598.

Hollister, L.S., Crawford, M.L., 1986. Melt-enhanced deformation; amajor tectonic process. Geology 14, 558561.

Kammer, A., 1996. Estructuras y deformaciones del borde oriental delMacizo de Floresta. Geologa Colombiana No. 21, 6580.

Langenheim, J.H., 1961, Late Paleozoic and early Mesozoic fossils fromthe Cordillera Oriental of Colombia and correlation to the Gironformation. Boletn de Geologa, INGEOMINAS, Bogota, 8, 95132.

Lopez, A., Mesa, J.E., 1997, Estratigrafa y ambientes de deposito de laFormacion Giron en el Macizo de Floresta, Departamento de Boyaca.Unpublished thesis, UniversidadNacional de Colombia (Bogota),190p.

Machette, M.N., Personius, S.F., Nelson, A.R., Schwartz, D.P., Lund,W.R., 1991. The Wasatch fault zone, Utah segmentation and historyof Holocene earthquakes. Journal of Structural Geology 13, 137149.

Mojica, J., Kammer, A., 1995. Eventos jurasicos en Colombia. GeologaColombiana 19, 165172.

Mojica, J., Kammer, A., Ujueta, G., 1996. El Jurasico del sectornoroccidental de Suramerica. Geologa Colombiana 21, 340.

Mojica, J., Villarroel, C., 1984, Contribucion al conocimiento de lasunidades Paleozoicas del area de Floresta (Cordillera OrientalColombiana, Depto. de Boyaca) y en especial al de la FormacionCuche. Geologa Colombiana 13, 5580.

Pindell, J.L., 1985. Alleghenian reconstruction and subsequent evolutionof the Gulf of Mexico, Bahamas, and Proto-Caribbean. Tectonics 4,139.

Prosser, S., 1993, Rift-related linked depositional systems and their seismicexpression. In: Williams, G.D., Dobb, A. (Eds.), Tectonics and seismicsequence stratigraphy, Geological Society Special Publication, vol. 71,pp. 3566.

Rabe, E., 1977, Zur Stratigraphie des ostandinen Raumes von Kolumbien:Die Abfolge Devon bis Perm der Ostkordillere, nordlich von Bucara-manga, 146, Giessener Geologische Schriften, Lenz Verlag, Giessen.

Remy, W., Remy, R., Pfefferkorn, H. W., Volkheimer, W., Rabe, E., 1975,Neueinstufung der Bocas-Folge (Bucaramanga, Kolumbien) in denUnteren jura anhand einer Phlebopteris branneri und Classopollis-Flora. Argumenta Palaobotanica., Munster, 4, 5577.

Renzoni, G., 1967, Geologa del Cuadrangulo J-12, Tunja: INGEOMIN-AS, Boletn Geologico, v. 24, 2, 3148.

Rodgers, J., 1987. Chains of basement uplifts within cratons marginal toorogenic belts. American Journal of Science 287, 661692.

Roeder, D., Chamberlain, R.L., 1995. Eastern Cordillera of Colombia:Jurassic-Neogene crustal evolution. In: Tankard, A.J., Suarez Soruco,R., Welsink, H. (Eds.), Petroleum Basins of South America, 62.American Association of Petroleum Geologists, Memoir, pp. 633645.

Schlische, R.W., 1991. Half-graben filling models: new constraints oncontinental extensional basin development. Basin Research 3, 123141.

Schlische, R.W., Olson, P.E., 1990. Quantitative filling model forcontinental extensional basins with applications to the early Mesozoic

rifts of eastern North America. Journal of Geology 98, 135155.Smith, G.A., 2000. Recognition and significance of stream flow-dominatedpiedmont facies in extensional basins. Basin Research 12, 399411.

Teixell, A., Arboleya, M.-L., Julivert, M., 2003, Tectonic shortening andtopography in the central High Atlas (Morocco), Tectonics 22-5, 6-16-13.

Trumpy, D., 1943. Pre-Cretaceous of Colombia. Geological Society ofAmerica, Bulletin 54, 12811304.

Tschanz, C., Marvin, R., Cruz, J., Mehnert, H., Cebulla, G., 1974.Geologic evolution of the Sierra Nevada de Santa Marta, NorthernColombia. Geological Society of America, Bulletin 85, 273284.

Ujueta, G., 2003, La Falla de Santa Marta-Bucaramanga no es una solafalla; son dos fallas diferentes: La Falla de Santa Marta y la Falla deBucaramanga. Geologa Colombiana, No. 28, p. 133153.

Ulloa, C.E., Guerra, A., Escovar, R., 1998, Geologa de la plancha 172

Paz del Ro, INGEOMINAS, Bogota.



11/11

Ulloa, C.E., Rodrguez, C., 1982, Intrusivos acidos ordovcicos y post-Devonicos en La Floresta (Boyaca). IV Congreso Colombiano deGeologa, Resumenes, 1718.

Villamil, T., 1999. Campanian-Miocene tectonostratigraphy, depocenterevolution and basin development of Colombia and western Venezuela.Paleogeography, Paleoclimatology, Paleoecology 153, 239275.

Walsh, J.J., Bailey, W.R., Childs, C., Nicol, A., Bonson, C.G., 2003.Formation of segmented normal faults: a 3-D perspective. Journal ofStructural Geology 25, 12511262.

Ward, D.E., Goldsmith, R., Cruz, B.J., Restrepo, A.H., 1973, Geologa delosCuadrangulos H-12 Bucaramanga y H-13 Pamplona, Departamentode Santander: INGEOMINAS, Boletn Geologico, v. 21, 13, 132 p.


Early Jurassic Rift Structures Associated With the Soapaga

Documents

Transcript of Early Jurassic Rift Structures Associated With the Soapaga