Geometry and kinematics of the North Monagas thrust belt (Venezuela)

Geometry and kinematics of the North Monagas thrust belt (Venezuela) F. R o u r e *

IFP, BP 311, 92506 RueiI-Malmaison Cedex, France

J. O. Carnevali INTEVEP, SA, Apo/o 76343, Caracas 1070A, Venezuela

Y. Gou BE/C/P, BP 213, 92502 Ruei/-Ma/maison Cedex, France

T. Subieta INTEVEP, SA, Apo/o 76343, Caracas I070A, Venezuela Received 14 September 1992; revised 27 May 1993;accepted 30 May 1993 The complex geometry and structural evolution of the North Monagas thrust belt were investigated and new exploration trends in this part of eastern Venezuela considered. The Pirital Thrust and related structures of the Maturin Basin are used to discuss the sequence of thrusting and to illustrate the constant balance between erosion and thrust reactivation in a recent fold and thrust belt. A careful analysis of the chronostratigraphy and the internal geometry of the foredeep and piggy-back syntectonic deposits has been used to constrain balanced cross-sections and to establish the kinematics of each individual structure. Basement involvement is postulated for the Pirital Thrust, which remobilizes previously emplaced cover structures such as the El Furrial or Orocual structural units. It thus probably relates to the pinch-out of a shallow d~collement level (coal measures of the Barranquin Formation, older Late Jurassic to Lower Cretaceous evaporites, or other Mesozoic plastic horizons) and to the reactivation of a deeper basement fault. The westward emergence of the Pirital and El Furrial Thrusts in a complex transfer area (the Urica Fault) is also analysed. According to transverse profiles, the surficial Urica Fault does not connect directly to a basement fault, but instead acts as a lateral ramp for the El Furrial cover unit and as a tear fault for the Pirital Thrust. Reactivation of ancient thrust faults also occurs at the rear of the Pirital Thrust in the west, in areas where the topographic profile of the eastern Venezuelan Ranges has recently been modified by the Pirital Thrust emplacement. These out of sequence thrusts resulting from fault reactivation in the inner parts of the prism are discussed in terms of the preservation of the critical taper. They effectively relate directly to erosional processes which significantly decrease the load applied on this portion of the wedge. The influence of the sequence of thrusting on the potential plays is considered.

Keywords: structural evolution; thrusting; North Monagas thrust belt, Venezuela

Various thrust emplacement modes have been observed in fold and thrust belts around the world, or have been reproduced in laboratory experiments (Davis et al., 1983; Malavieille, 1984; Mulugeta, 1987; Liu and Dixon, 1990; Colletta et al., 1991). The conventional mode of deformation implies a progression of thrusts towards the foreland in a piggy-back sequence. Anomalies in thrust propagation relate to changes in the boundary conditions, which are best controlled in analogue models. Case studies describing near-surface phenomena, such as out of sequence thrusting, are numerous (Morley, 1988; Mugnier et al., 1990; Sage et al., 1991), but only a few examples with reasonably well constrained deep *Correspondence to Dr F. Roure

geometry have been published, making it difficult to decipher the various mechanisms involved in this mode of deformation. The excellent grid of seismic reflection lines and the large number of exploration wells in the Maturin Basin (North Monagas, eastern Venezuela) are used here to compile numerous profiles starting south of the deformation front and successively crossing in the north the El Furrial and Orocual ramp-anticlines, the Pirital complex structure and a Late Miocene to Quaternary piggy-back basin, the Morichito Basin (Figure 2). A careful analysis of the chronostratigraphy and the internal geometry of the foredeep and piggy-back syntectonic deposits shows an anomaly in the thrust propagation, as the Pirital Thrust remobilizes previously emplaced structures and thus appears to be m part out of sequence. The kinematics of the Pirital

0264-8172/94/030/347-16 ©1994 Butterworth-Heinemann Ltd M a r i n e and P e t r o l e u m G e o l o g y 1994 V o l u m e 11 N u m b e r 3

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North Monagas thrust belt (Venezuela): F. Roure et al. Thrust emplacement and its lateral changes are Bajos Fault and Trinidad (Salvador and Stainforth, modelled along two depth-converted sections, which 1968; Persad, 1978; Woodside, 1981; Leonard, 1983). are balanced with the computer-assisted LOCACE The Maturin Basin itself is separated from the technique (Moretti and Larrere, 1989) and restored to outcrops of the Serrania del Interior by the Pirital their initial precompressive state as well as subsurface high, a complex structure that is described intermediate configurations. Finally, the causes and in detail in this paper. Southward, the Maturin Basin consequences of the Pirital thrusting are discussed in extends to the Orinoco River, which marks the relation to the possible pinch-out of an intra-Mesozoic northern limit of the basement outcrops of the Guyana ddcollement level, and to the reactivation of a deeper Shield. Filled with up to 5 km of Neogene sediments, basement fault. Its effects on the critical taper in the the Maturin Basin results from the combination of eastern Venezuelan Ranges are also discussed, as well oblique convergence of the Caribbean and South as its influence on potential petroleum plays• American plates, and loading of the allochthonous units which progressively forced the South American continental lithosphere to flex northward between the Geological and geophysical background Guyana Shield and El Pilar Fault. South of the El Pilar right-lateral transform fault The northern and deepest part of the basin is (Figure 1), the eastern Venezuelan Ranges consist of characterized by the occurrence of ENE trending thrust the surface outcrops of the Serrania del Interior and a folds that involve pre-Tertiary rocks (El Furrial and buried fold and thrust belt that extends southward Orocual trends) and which are largely overthrust by the beneath the Maturin Basin (Hedberg, 1950; Creole Pirital allochthon (Figure 2) (Carnevali, 1988a; Poti6, Petro, 1965; Gonzales de Juana et al., 1980; Rossi, 1989; Margherita and Riart, 1989; Fiume and Graterol, 1990; Hernandez, 1990; Lander et al., 1990). 1985; Rossi et al., 1985; Vivas, 1986; Carnevali, 1988b; Southward, the compressive structures die out rapidly Poti6, 1989). The eastern Venezuelan Ranges are along intra-Neogene ddcollements and are locally dissected by north-west trending dextral faults, the associated with shallower diapiric structures. Urica Fault in the west, the San Francisco Fault in the centre and the Los Bajos Fault in the east, which delineate individual structural blocks, the Santa-Rosa, Major lithostratigraphic units and d~collement Bergantin, Caripe and Trinidad blocks, respectively, levels whose elevation progressively decreases eastward In this study, foreland exploration wells and outcrops (Figure 1) (Wilson, 1968; Rosales, 1972; Munro and of the Serrania provide enough data to reconstruct the Smith, 1984). The highest peak, Mount Turimiquire palaeogeographic and sedimentological evolution of (2500 m), lies in the Bergantin block, whereas eastward the Tethyan margin of the South American continent the thrust belt remains below sea level between the Los

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Structural map of eastern Venezuela

Marine and Petroleum Geology 1994 Volume 11 Number 3

+

North Monagas thrust belt (Venezuela): F. Roure et al. i

Figure 2

Structural map of the Maturin Basin and the Serrania del Interior. Locations of the two balanced cross-sections are indicated (AA', BB'), as well as the locations of the seismic profiles imaged in Figures 6 - 11. Open circles correspond to selected wells used for the calibration of the seismic sections

since the beginning of Cretaceous times (Figure 3). Late Jurassic ophiolites are currently exposed north of the El Pilar Fault (e.g. on Margarita Island; Chevalier et al., 1985; 1988) and a passive margin sequence is clearly defined along all of the northern border of the South American craton (Fourcade et al., 1991). Southward, near the Guyana shield, Cretaceous beds lie directly on top of the Precambrian basement, which is composed of various schists and amphibolitic and granitic complexes. No local data is available on the Jurassic or older (Palaeozoic) strata, which are assumed to extend northward beneath the passive margin deposits, as suggested by the occurrence of very deep seismic reflectors beneath the drilled Cretaceous horizons of the Maturin Basin. Pre-Cretaceous beds have been recognized elsewhere, either westward in the Espino Graben or Guarico Basin (where Late Jurassic to Early Cretaceous layers overlie thick Palaeozoic strata; Hedberg, 1950; Beck, 1978; Fiorillo, 1982; Bartok, 1993), or north-eastward in the Gulf of Paria and the Trinidad fold and thrust belt (where three wells have penetrated into evaporites and major tectonic units are thus probably detached above these horizons; Kugler, 1953: Beck, 1978; Arnstein et al., 1985; Chiock, 1985). The age of the rifting is thus well constrained to the Late Jurassic (Burke, 1988; Eva et al., 1989). However, as stated earlier, only the Berriasian to Oligocene shallow water carbonates, shales and sandstones of this margin have as yet been sampled in this area, and these can reach a thickness of 3 km. Coal measures are fairly common in the littoral Berriasian-Aptian Barranquin Formation (Figure 3A) (Van der Osten, 1957; Guillaume et al., 1972) and could act locally as a potential ddcollement level. However, the most obvious source rock for oil remains the Cenomanian black shales (Querecual Formation;

Figure 3A) (Hedberg, 1937), which also constitutes a local ddcollement horizon. A Late Cretaceous carbonate horizon (El Cantil Formation; Figure 3A) is clearly imaged with strong reflections on the seismic profiles. Palaeogene deltaic sandstones constitute the major potential reservoir for oil, especially in the El Furrial area. In addition to a general increase in thickness from south to north, no synsedimentary fault or any significant thickness variation of the Cretaceous sequence can be detected from the surface in the Serrania. Nevertheless, seismic profiles in the Maturin Basin show that a number of south-facing normal faults and related half-grabens are sealed by the Cretaceous beds, attesting for an earlier (Late Jurassic or Early Cretaceous) rifting event. Thus thermal subsidence alone probably accounts for the Cretaceous and younger sedimentation of the passive margin sequence. The area later evolved into a convergent margin. Tectonic loading due to thrust emplacement in the inner parts of the belt induced a progressive flexure of the South American lithosphere, coeval with the deposition of deep turbiditic facies in the resulting foreland basin. The age of the first terrigenous sequences (Carapita Formation) (Sulek, 1961; Stainforth, 1971) is thus older to the north and is progressively younger southward. An Early Miocene age is assumed here for the initiation of flexural deformation (Figure 3B). Owing to the rapid sedimentary infilling of the foredeep, argillites of the basal Carapita Formation remained undercompacted until the initiation of thrusting, providing a potential decoupling horizon that commonly acted as the major detachment level. Dewatering processes in the foredeep sediments may also account for the wide occurrence of mud diapirs, which are closely related to the frontal area of major thrusts. In inner portions of the belt the fluids were

Marine and Petroleum Geology 1994 Volume 11 Number 3

349

North Monagas thrust belt (Venezuela)." F. Roure et el. Potential levels

A)

decollement

sequence)

__Jb.. Oligocen Eocene Paleocer MaaastTicht~ Santoniao

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to

Allan Berriasian to

0

L.Aptian ~ ~

A ' ? Evaporites or other infra-Barranquin ~" ~ plastic horizons

Jurassk:

Jurassic(and/orPaleozoic) synrift sequence

Sandstone

Cherl

Pellite

Limestone

Coal

Evaporite

Figure 3a Composite lithostratigraphic columns of the passive margin sequences

confined and circulated preferentially laterally along the decoupling horizons, as recognized by ODP drilling in the Barbados wedge (Moore, Mascle et al., 1988). In contrast, in frontal areas, where the lithostatic loading was insufficient, fluids tended to escape vertically to the surface, creating the mud diapirs or mud volcanoes observed (Figure 8).

B)

S

FOREDEEP BASIN

PNTAL HIGH

Timing of the deformation and sequence of thrusting The timing of thrust emplacement can be established by careful analysis of the synorogenic deposits. It is usually well constrained between the age of the last conformable synflexural deposit and the age of the basal unconformity of the piggy-back basins. North of the Pirital High, the unconformable shallow water Upper Carapita Formation (Chapapotal Member, Late Miocene) and the continental Morichito Formation (Pliocene) (Lamb and de Sisto, 1963) infill a slightly tilted piggyback basin (Figure 3b). First deformation thus occurred during the sedimentation of the Carapita Formation, as part of this formation is of deep marine foredeep origin, whereas its upper part is composed of shallow water facies representing piggy-back basin deposits (Figure 3b). Nevertheless, shortening still occurred during deposition of the Morichito Formation, as the piggy-back basin also shows evidence of a migration of its depocentres during this time interval. Effectively, the age of the unconformity at the bottom of the Morichito Basin varies gradually from south to north, and simultaneously the amount of eroded material increases northward. As a result, Late Miocene littoral facies of the Upper Carapita Formation lie almost conformably above Palaeogene sequences near the Pirital High, whereas Upper Miocene or even Pliocene continental beds of the Morichito Formation lie directly and unconformably on top of Cretaceous sequences along the northern flank of the Morichito piggy-back basin. In the north, i.e. in the Serrania, deformation probably started earlier as the first synorogenic deposits (Naricual Formation) appear to be older than the Carapita beds. South of the deformation front and above the El Furrial and Orocual folds, the offshore and nearshore La Pica and Las Piedras Formations (Late Miocene to Quaternary) (de Sisto, 1960) constitute the upper part of the synorogenic sequence. All deformation now seems to be inactive, being generally sealed by the Mesa Formation (Pleistocene).

PIGGY- BACK BASIN

N

PLEISTOCEr, PLIOCENE LATE MIOCENE MIDDLE MIOCENE PALEOGENI: to EARLY CRETACEOUS LATEJURASS~ PALEOZOIC

AUTOCHTHON

Figure 3b Major tectonostratigraphic units and Neogene synorogenic s e q u e n c e s of the Maturin Basin and the Serrania del Interior. Note the southward increase of the sedimentary gap between the synorogenic flexural sequences and the passive margin sequences, as a result of erosion on top of the southward migrating flexural bulge

350

Marine and Petroleum Geology 1994 Volume 11 Number 3

North Monagas thrust belt (Venezuela): F. Roure et al. Despite the fact that all the compressive deformation observed in the Maturin Basin appears to post-date the Middle Miocene Carapita Formation, it is clear from the obliquity between the folds and thrusts that the last movements in the Pirital High post-date the El Furrial thrust emplacement. Effectively, the Pirital Thrust was still active very recently, even during the deposition of the Las Piedras and Mesa Formations (Figures 3b, 6 and 7). However, according to the internal attitude of bedding in the synorogenic deposits of the piggy-back basin, most of its shortening occurred simultaneously with the deposition of the upper Carapita and Morichito Formation, a time interval when the El Furrial and Orocual Thrusts were also active (Figure 3B). Effectively, distinct d~collement levels were activated successively during the deformation; the thin-skinned tectonic units of El Furrial and Orocual were detached along a pre-Lower Cretaceous (infraBarranquin) layer that merges southward with shallower intra-Carapita d6collement horizons. In contrast, the Pirital Thrust connects directly to a basement fault and cuts through deeper infraBarranquin horizons (i.e. Jurassic and/or Palaeozoic and/or basement). To establish the mode and timing of thrusting along the Pirital Thrust, we restored two distinct regional cross-sections in their pre-orogenic form as well as in their intermediate geometry between the Present and the Early Miocene (before the tilting of the Morichito piggy-back basin).

Subsurface data used to constrain the two initial sections From the thousands of kilometres of unmigrated and migrated seismic reflection profiles that cover the Maturin Basin, we selected two north-south oriented regional lines extending from outcrops in the Serrania to the Orinoco River. The quality of the lines is excellent, providing information down to 6 s TWT. Additional transverse or parallel lines have been used at various points to help calibrate the seismic reflections observed along the regional lines with data arising from projected wells. They are also used to constrain the lateral evolution of complex structures such as the Pirital High or the Orocual and Manresa thrust fronts. There are numerous exploration wells crossing the entire synorogenic sedimentary sequence down to the Oligocene, or even the Cretaceous (Figure 2), thus providing good calibration for the geological interpretation. Velocities deduced from well surveys (Table 1) were used for the construction of two depth-converted cross-sections in two distinct parts of the Maturin Basin, one in the west (Figure 4) and the other in the east (Figure 5). The two depth-converted cross-sections were then balanced using the computer-assisted LOCACE technique (Moretti and Larr6re, 1989) and restored either in their initial precompressive configuration or in an intermediate stage that preserves the Morichito piggy-back basin in a horizontal position by just unfolding the Pirital basement-involved thrust. Ray-tracing with the Sierra software was also used to test different geometries at depth beneath the E1 Furrial structure and to choose between the hypothesis of a velocity pull-up (Figure 4) and the possible occurrence of a deeply buried duplex (Figure 4B).

Table 1 Velocities deduced from well surveys and used for the construction of the depth-converted sections Stratigraphic intervals

Seismic velocity (m/s)

Plio-Quaternary Mesa/Las Piedras La Pica Morichito

2200 2400 2800

Miocene (Carapita)

2900

Palaeogene (Merecure)

4000

Cretaceous

4200

Pre-Cretaceous

4400

Deep structure of the sections Both seismic sections clearly image the Pirital Thrust (Figure 6), which merges near the surface into a very complex structure, the Pirital High, which is described in the following. The Pirital Thrust itself is outlined by a very strong north-dipping reflection, and synorogenic deposits of the Morichito piggy-back basin or of the foredeep are imaged as well delineated horizons with internal truncations (tilted onlaps along erosional surfaces, or downlaps at the periphery of major structures). The Pirital Thrust is seen to separate two distinct domains at depth: to the north, the Pirital allochthon and the overlying Morichito piggy-back basin (Figure 3), and to the south, the E1 Furrial or Orocual thin-skinned structures, which are deeply buried beneath the foredeep deposits. At depth, the seismic nature of the sections differs strongly from one side of the Pirital Thrust to the other. North of the fault, the seismic profiles rapidly become transparent beneath 2 or 3 s TWT, i.e. underneath the well bedded and highly reflective Cretaceous sedimentary sequence. In contrast, well organized horizontal reflections extend as far as 6 s TWT underneath the Pirital Thrust, and most likely correspond to the northward extension of Cretaceous beds in the El Furrial or Orocual tectonic units, whose cutoffs more or less coincide with the northern end of the seismic sections. We interpret this change in the seismic nature and the lack of good reflections between 3 and 6 s TWT as being due to the involvement of either non-reflective pre-Cretaceous sedimentary rocks (i.e. Jurassic or Palaeozoic) or even basement rocks in the hangingwall of the Pirital Thrust. Owing to the lack of deeper seismic profiles (the present processing does not provide any information deeper than 6 s TWT), it is not yet known whether the basal detachment of the El Furrial and Orocual units remains in the sedimentary cover (coal measures of the Barranquin Formation, or conjectural infra-Cretaceous evaporitic horizon, or any other Mesozoic plastic levels) or reaches the basement. South of El Furrial, steep normal faults involving the basement have recently been inverted, thus indicating the activation of very deep levels. Nevertheless, transverse profiles around the Orocual fold may be explained by thin-skinned duplexes and the mobilization of a shallow detachment (infra-Cretaceous). This question of basement involvement is of economic importance underneath the El Furrial fold, depending on whether there is one or two superposed structures along this

Marine and Petroleum Geology 1994 Volume 11 Number 3

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North Monagas thrust belt (Venezuela): F. Roure et al. A)

S

N Present

stage

El Furrial

Tilted piggy-back

Plio-Quaternary

Marine portion of the piggy back basin thrust Miocene (Carapita Formation)

Upper Cretaceous-Paleogene

20KI 0

B

10

20

30

Intermediate

0

~

_

40

50

60

70

80Km

Lower Cretaceous

Jurassic and/or Paleozoic

stage

EL FURRIAL : . . : . . ........

_

CAICARA~OROCUAL

Crystalline basement Initial piggy back

tO o.

0 (.

10 Initial

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10

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20

30

40

50

60

70

80

90

100

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stage

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i

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20Km , 25

)

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125

N

150

175Km

Plio-Quaternary Marine portion of the Piggy- back basin

Present

stage

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Miocene (Carapita Formation)

0

Upper Cretaceous to Paleogene Lower Cretaceous Jurassic and/or Paleozoic

,F.5~ Crysta,i,e~sement 20 Krn

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i 20

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~ 70

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20

30

40

50

60

70

80

~

active I

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inactive I

stage

O-

10-

20Km~ 0

10

90

100

110

120

130

140

150

160 Km

Figure 4 Western (BB') balanced cross-section (see Figure 2 for location). (A) Hypothesis w i t h a velocity pull-up beneath t h e El Furrial anticline. (B) Hypothesis with a basement-involved duplex beneath the El Furrial anticline

trend. The time sections image a compressive structure, which apparently affects the Mesozoic horizons of the autochthon, and could thus be interpreted as evidence of the occurrence of a deep duplex related to recent basement-involved shortening. Nonetheless, structural modelling with the Sierra ray-tracing software suggests that the antiformal image shown in time sections could be related to a velocity pull-up due to the lateral juxtaposition of poorly compacted and slow velocity foredeep deposits along with compacted fast velocity Mesozoic beds of the overlying El Furrial anticline or Pirital High (Figure 4 and Plate 1). Unable to favour one or the other solution, we 352

decided to restore both geometries in their initial shape (Figure 4A and 4B). In the first hypothesis, the Pirital basement-involved thrust post-dates the E1 Furrial emplacement and was the youngest active front in the area. In the second hypothesis the deep basement-involved duplex that underlies E1 Furrial was emplaced more recently than the Pirital Thrust itself, and thus appears to be in sequence with Pirital with respect to an intrabasement d6collement level. The main discrepancy observed at depth between the two balanced cross-sections (Figures 4 and 5) results from the obliquity of the Pirital Thrust compared with the orientation of the more external El Furrial and

M a r i n e and P e t r o l e u m G e o l o g y 1994 V o l u m e 11 N u m b e r 3

North Monagas thrust belt (Venezuela)." F. Roure et al. N

S

Rio-Quaternary Present

stage EL FURRIAL

PIRITAL THRUST

OROCUAL

Miocene (Carapita Formation)

0•

Upper Cretaceous to Paleogene

. . ! ............................

Lower Cretaceous

10Km-

0

I

I

10

20

[

30

40

50

60

i

70

Jurassic and/or Paleozoic

8OKm

Crystalline basement Intermediate

stage

EL FURRIAL

OROCUAL

I active

MANRESA

Thrust faults

inactive

O-

1OKm, 0

10

20

30

40

50

I

I

[

I

60

70

ao

90

]

t0OKm

stage

Initial

EL FURRIAL

OROCUAL

i

MANRESA

:FUTURE.R,TAL I

0

10

20

30

40

50

60

70

80

90

100

110

Figure 2 f o r

12o Km

Figure 5 Eastern (AA') balanced cross-section. Present/intermediate

a n d i n i t i a l s t a g e s (see

Orocual trends. Therefore the western cross-section images a single tectonic unit in the footwall of the Pirital Thrust (i.e. the El Furrial unit), whose northern extension is now to be found in the Pirital hangingwall. In contrast, the eastern cross-section shows a second structure intercalated between the Pirital and E1 Furrial fronts (i.e. the Orocual unit). In both sections the Pirital hangingwall contains older tectonic units, with the best imaged being the Manresa Thrust (Figures 2 and 4). Between the Manresa Thrust front and the Pirital High, Mesozoic strata have been temporarily assigned to a new tectonic unit, referred to as the Caicara Unit on the structural map (Figure 2).

unit overthrust on top of the E1 Furrial structure, very similar to the Orocual unit of the eastern profile. Identified as the Caicara unit on the structural map, this pre-Pirital structure can be interpreted as the western extension of the Orocual thrust fold, which had been progressively uplifted and wedged out along the western section during the Pirital Thrust emplacement. All the movement along the Pirital Thrust is thus recorded in the synchronous infill of the Morichito Basin. The Pirital Thrust emplacement induced a progressive northward migration of the depocentres in the Morichito piggy-back basin, and the tilting of early synorogenic deposits (the bottom of the Morichito Basin still preserves marine sediments of the upper Carapita Formation; Figures 2 and 6). By laterally and temporally comparing the evolution of the Pirital wedge (Figures 2 and 4), it appears that shortening was active longer and to a greater extent in the west than in the east. In the east, near the San Francisco Fault (Figure 8), only incipient basement shortening has occurred and, besides a frontal diapir (Figure 8), no structural high has developed along the Pirital Thrust front. As a result, no tilt of the piggy-back basin or any related wedging is observed in this area. In the central area, however, deep basement shortening induced the 'out of sequence' Pirital Thrust. Early emplaced thin-skinned units such as the Orocual thrust fold (alias the Caicara unit) were then disrupted, with part left behind in the Pirital footwall (para-autochthon), and part being incorporated into the Pirital hangingwall. Owing to early compaction, these remobilized units acted as an indenter that

Shallow effects of the Pirital Thrust Near the surface, the Pirital Thrust merges into a fishtail network with both north-directed back-thrusts and south-directed structures that create a complex tectonic wedge known as the Pirital High (Figures 6 and

7, Plate 1). To constrain the kinematics of the growth of the Pirital High, we progressively restored the structures of the western section to an intermediate configuration, i.e. before the basement-involved Pirital Thrust emplacement and related wedging and back-thrusting (Figure 4). In so doing, we obtained a simple geometry for the western section, where the initial deposits of the piggy-back basin are not yet tilted and the Morichito Basin is still not elevated, and where the incompetent material of the Pirital High forms a coherent tectonic

location)

Marine and Petroleum Geology 1994 Volume 11 Number 3

353

North Monagas thrust belt (Venezuela)." F. Roure et al.

,

.~@_s _

.

~ _6

STWT

v

srWT

MORICHITO

PIGGY BACK

STWr

N -- 0

Basal

o 1,k=

STVVT

Figure 6 Seismic profile imaging the Pirital Thrust (see Figure 2 for location). Note the wedging and back-thrusting associated with the Pirital High, and the Recent emplacement of the fault, which even affects the Plio-Quaternary beds

progressively pinched out the undercompacted sediments of the foredeep, creating either back-thrusting or complex fishtail structures in the Pirital High, which are characterized by the plastic deformation of incompetent Neogene beds. The timing of the Pirital Thrust emplacement is well constrained to the upper Carapita-Morichito time intervals, with a recent reactivation during the Las Piedras or even La Mesa deposition. It is thus in part synchronous with the El Furrial and Orocual thrust emplacements, which predate the Las Piedras deposition (Figure 3B). As part of the Orocual material is now incorporated in the hangingwall of the Piritat Thrust, the Pirital Thrust itself must be considered as out of sequence. However, although most of the deformation induced by the basement indenter was transferred into the Pirital out of sequence thrust, the frontal thrust propagation (El Furrial) was still activated simultaneously. Alternatively, successive flips between forward (piggy-back) and out of sequence thrust propagations occurred in response to the permanent modification of the equilibrium of the tectonic wedge (with the progressive surficial erosion of the Pirital High constantly modifying the boundary conditions and the taper of the prism). Westward, the Pirital Thrust front branches onto the N W - S E oriented Urica strike-slip fault that underwent some tens of kiiometres of displacement (Munro and 354

Smith, 1984). Only the southern continuation of the Utica Fault has been studied here. Transverse (east-west) seismic profiles (Figure 10) help us to understand the relations at depth between basement-involved structures such as the Pirital Thrust and the underlying thin-skinned structures such as E1 Furrial. Both the Pirital and El Furrial units result from the structural inversion of a thick sedimentary basin that was thinning progressively westward. In fact, the surface trace of the Urica Fault is not directly related at depth to a basement structure, but acts as a lateral ramp for the inverted E1 Furrial unit, and as a tear fault for the more recently emplaced Pirital Thrust. East-dipping normal faults are still preserved beneath the basal detachment of El Furrial east of the Utica Fault, thus outlining the precompressive geometry of the basin (Figure 10). Normal throws are also imaged on transverse profiles along shallower tear faults that dissect the Pirital allochthon from side to side (Figure

9). Whereas the Orocual and El Furrial thin-skinned thrust fronts are perfectly parallel ENE-WSW structures, the S W - N E oriented basement-involved Pirital Thrust front is oblique in relation to these external trends, accounting for a slight amount of anticlockwise rotation during basement inversion. The Pirital Thrust has a decreasing displacement to the east and no longer affects the Manresa-Orocual tectonic

Marine and Petroleum Geology 1994 Volume 11 Number 3

North Monagas thrust belt (Venezuela): F. Roure

e t al.

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S

A) WESTERN SECTION (Hypothesis with a velocity pull up beneath the EL FURRIAL anticline) t ~--

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Marine and Petroleum

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North Monagas thrust be/t (Venezuela): F. Roure et al. pile. The two cross-sections have also been extended northward across the Serrania del Interior (Figure 12), where, surprisingly, they portray two very distinct topographic profiles. In the west, where the Pirital out of sequence thrust has been more active, the topography shows a break along the piggy-back basin. A careful analysis of the seismic section here shows the present reactivation of the Manresa and Cerro Jimenez Thrusts (Figt~re 11). Along the northern border of the Morichito piggy-back basin, early unconformities are cut by more recent displacements along these faults, which can thus also be viewed as out of sequence thrusts as they were reactivated at a time when the external E1 Furrial and Orocual thrust fronts were no longer active. To the east, however, where the Pirital Thrust was almost inactive, there is no break in the topography and no older fault has been reactivated. Apart from its relations with the successive activation of deeper detachment levels, out of sequence thrusting may also be related to variations in the topography of the belt.

Thrust reactivation and out of sequence thrusting

Two sets of out of sequence or reactivated thrusts are shown along the cross-sections modelled. To the south, the Pirital Thrust relates to recent basement involvement in compressive deformation. Northward, the Manresa and related thrusts result from a surficial reactivation of thin-skinned structures. From chronostratigraphic evidence (internal geometry of the Morichito piggy-back basin), it appears that the initiation of the Pirital Thrust (and the initial tilt of the Morichito infill as well as the sedimentation of the upper Carapita Formation) predates the reactivation along the Manresa thrust, which cuts through the basal unconformity of the Morichito piggy-back basin. If the two sets of out of sequence thrusts are directly associated (in space and time), the fault reactivation of shallow internal structures such as the Manresa Thrust appears to be a second-order feature, induced by the emplacement of the Pirital Thrust. Therefore, two different explanations for the two sets of thrusts are proposed here.

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Marine and Petroleum Geology 1994 V o l u m e 11 N u m b e r 3

Figure2for

North Monagas thrust belt (Venezuela): F. Roure et al. 0

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Figure 8 Seismic profile outlining mud diapirism at the eastern front of the Pirital Thrust (see Figure 2 for location)

Pirital out o f sequence m o v e m e n t Late Jurassic to Early Cretaceous evaporitic layers are known to occur both westward in the Guarico Basin (Hedberg, 1950; Beck, 1978) and eastward in the Trinidad fold and thrust belt; they are a possible candidate to explain the shallow detachement of E1 Furrial, Orocual or other thin-skinned structures of the eastern Venezuelan Ranges and Maturin Basin. Other infra-Barranquin Mesozoic plastic layers or even intra-Barranquin coal measures can also be inferred to account for these thin-skinned deformations. In such a context, the Pirital out of sequence thrust appears to be a basement-involved structure that cross-cuts this early mobilized d6collement level. An explanation for this change in the thrust sequence could be the pinch-out of the shallow plastic horizons south of El Furrial, which would preclude further fault propagation. In fact, more

externally sited exploration wells passing through the entire sedimentary pile down to the pre-Cretaceous basement have not encountered any potential detachment level. When the shallow (infra-Cretaceous) detachment level had been entirely mobilized, more internal basement shortening could no longer propagate southward into thin-skinned deformations. It instead emerges directly at the surface through previously deformed shallow structures, or reactivates older thrusts.

Manresa and related reactivated thrusts According to numerical modelling (Davis et al., 1983), each tectonic prism tends to reach an equilibrium with a specific geometry characterized by an angle (critical taper) controlled by the cohesion of the materials

Marine and Petroleum Geology 1994 Volume 11 Number 3

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North Monagas thrust belt (Venezuela): F. Roure et al. o

~'-_t

~-4:'e 5

STWT

STWT

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Figure 9 Seismic profile outlining tear faults in the Pirital hangingwall (see Figure 2 for location)

involved in the deformation and friction along the basal detachment level. In such a context, basement involvement along the Pirital Thrust has modified the initial taper of the prism. Thus the two topographic profiles differ strongly across the eastern Venezuelan Ranges, where the Pirital Thrust has either been or not been mobilized (Figure 12); we can observe a break in the topographic profile in the west, precisely where the Pirital High and the resulting Morichito piggy-back basin have been emplaced. The reactivation of the Manresa and related thrusts occurred along the northern border of the Morichito Basin, exactly at the place where the topographic break occurs. Therefore, it seems that this fault reactivation was an attempt to restore the critical taper of the prism. To the east, where the Pirital Thrust was almost inactive, there is no break in the topography, the critical taper is maintained and there is no fault reactivation. Nonetheless, another simpler explanation would not require a complex balance of the critical taper constraints. The fault reactivation could also be related to erosion, which locally forced a decrease in the lithostatic load of the allochthon applied on the basal detachment of the prism. Under such conditions it becomes easier for a 358

thrust to propagate upward rather than to propagate horizontally toward the front of the prism (Colletta and Bal6, 1991; Malavieille et al., 1991). In the eastern Venezuelan Ranges, the Pirital Thrust could have induced sufficient erosion to initiate the fault reactivation observed. As a result, two or more thrust fronts could have been active simultaneously; this was associated with incipient deformation at the front of the prism (deep El Furrial duplex or more external basement-inverted structures), a major shortening along the Pirital basement involving an out of sequence thrust, which in turn induced the reactivation of an inner front along the Manresa thrust. The break observed in the western topographic profile would thus be better interpreted as the superposition of two competing wedges, with each of them being characterized by its own critical taper. South of the Morichito Basin, a fossil wedge relates to frontal accretion along the E! Furria[ and Pirital deep basal detachment, whereas to the north a shallower active wedge relates to the Manresa thrust front. This reactivation of the Manresa Thrust, together with the sedimentation in the Morichito piggy-back basin and erosion in the Serrania, constitutes an attempt to

Marine and Petroleum Geology 1994 Volume 11 Number 3

North Monagas thrust belt (Venezuela): F. Roure et al. restore the global equilibrium of the tectonic prism.

Discussion and conclusions The same kind of relations between shallow structures and deep basement involvement have been postulated for the St Chinian fold and thrust belt north-east of the Pyrenees, where out of sequence thrusting may be related to the pinching out of a Triassic evaporite layer (Roure et al., 1990a). In the Apennines, late deep basement involvement in the inner zones may have also induced shallow detachment levels in the external zones (Sage et al., 1991). Nevertheless, one of the most common features of late basement shortening remains an indenter effect associated with back-thrusting, either at the top of the remobilized units (this study) or along the basement cover interface (southern Apennines, Roure et al., 1991).

Other fold and thrust belts may also account for different fronts that were active almost simultaneously• This is the case in the western Alps (Mugnier et al., 1990; Roure el al., 1990b), where the Jura Mountains were folded at the same time as the reactivation of the Penninic front. There, deep wedging and cryptic back-thrusting connect the two active fronts at depth. This solution could also been applied here; however, no deep seismic profile is available as yet underneath the Serrania, and therefore it is not yet clear how the Manresa and Pirital Thrusts are linked up at depth (Figure 13). The occurrence of a deep crustal wedge overlain by a zone of active back-thrusting can also be proposed for the Eastern Venezuelan Ranges, as discussed elsewhere (Figure 13; Passalacqua et al., in press). In addition to the disappearance of shallow detachment levels and the possible link with back-thrusting and wedging, maintenance of a critical

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Marine and Petroleum Geology 1994 Volume 11 Number 3

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North Monagas thrust belt (Venezuela): F. Roure et al. I

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i

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i

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Figure 12 Topographic profiles across the Serrania del Interior and the Maturin Basin, with location of major 'out of sequence' structures (see F/~qure I for location) 368

Marine and Petroleum Geology 1994 Volume 11 Number 3

North Monagas

t h r u s t b e l t ( V e n e z u e l a ) : F. R o u r e et al.

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Figure 13 Crustal scale section of the Maturin Basin and Serrania outlining the possible connection at depth between the various competing thrust fronts (modified after Passalacqua et a/., in press) (see Figure 1 for location)

taper has also been postulated to explain thrust reactivation in the inner parts of a prism. In fact, as expressed here and in numerous orogenic belts (i.e. the Carpathians; Roure et al., 1993) or even in analogue models (Colletta and Bale, 1991; Malavielle et al., 1991), the competition between different active fronts is a common p h e n o m e n o n that can be explained by the effects of erosion, which permanently modify the equilibrium of the prism. The influence of the thrust sequence is also crucial in the evolution of petroleum systems in fold and thrust belts because it progressively modifies the burial and maturation history of potential source rocks as well as the migration paths. The identification of the thrust sequence can thus help to identify new petroleum plays. Before the formation of the frontal structures such as the El Furrial or Orocual trends, early generated oils probably migrated over long distances from the inner parts of the belt towards the Orinoco to fill the traps of the Faja Petrolifera (Gallango and Parnaud, 1993). The El Furrial and Orocual trends were formed later, providing closer traps for the oil generated north of the present Pirital High, at a time when the Querecual Formation in these external units was still immature. More recently, the Pirital basement-involved structure was emplaced and migration paths are no longer possible between the inner zones and the El Furrial and Orocual trends. However, this recent overburial of the external units has induced the maturation of the Querecual Formation south of the Pirital High, allowing a very young but short migration of oil towards the frontal structures. Such multiphase episodes of oil generation and migration, directly in connection with the thrust propagation history, are probably the rule for fold and thrust belt petroleum systems (e.g. the Carpathians, Lafargue et al., 1994).

Acknowledgements We are indebted to PDVSA for access to and use of the

seismic profiles. We had helpful discussions with A. Bally and B. Colletta. Two anonymous reviewers provided helpful comments.

References Arnstein, R., Cobrera, E., Russomanno, F. and Sanchez, H. (1985) Revision estratigrafica de la cuenca de Venezuela oriental In: Vl Conf. GeoL Venez., Mere., Vol. 1, 41-69 Bartok, P. (1993) Prebreakup geology of the Gulf of MexicoCaribbean: its relation to Triassic and Jurassic rift systems of the region Tectonics 12, 441-459 Beck, C. M. (1978) Polyphasic Tertiary tectonics of the Interior Range in the Central part of the western Caribbean chain, Guarico State, northern Venezuela Geol. Mijnb. 57, 99-104 Burke, K. (1988) Tectonic evolution of the Caribbean Annu. Rev. Earth Planet. Sci. 16, 201-203 Carnevali, J. O. (1988a) El Furrial Oil field, northeastern Venezuela: first giant in foreland fold and thrust belts of western hemisphere Am. Assoc. Petrol. Geol. Bull. 72, 168 Carnevali, J. O. (1988b) Venezuela nor-oriental: exploracion en frente de montana. In: III Simposio Bolivariano (Eds A. G. Bellizzia, A. C. Escoffery and I. Bass), Soc. Venezolana de Geol., Caracas, 69-89 Chevalier, Y., Stephan, J. F., Blanchet, R., Gravetle, M. and Bellizzia, A. (1985) L'fle de Margarita et ta peninsule d'Araya (Vendzudla); jalons d'une collision Crdtace sup&ieur sur la paldo-fronti&e caraibes (Am&ique du Sud). In: Symp. Geodyn. des Cara~bes, Paris, 1985 (Ed. A. Mascle), Editions Technip, Paris Chevalier, Y., Stephan, J.F., Darboux, J. R., Gravelle, M., Bellon, H., Bellizzia, A. and Blanchet, R. (1988) Obduction et collision pretertiaire dans les zones internes de la chaTne caraibe vdnezudlienne sur le transect Tie de Margarita-Peninsule d'Araya C. R. Acad. SCL Paris 307, 1925-1932 Chiock, M. (1985) Cretaceo y Paleogeno en el subsuelo de Monagas. In: VI Confr. Gaol Venez., Caracas 1985, Vol. I, 350-383 Colletta, B. and Bale, P. (1991) Thrust propagation experiments Cinematheque IFP Colletta, B., Letouzey, J., Pinedo, R., Ballard, J. F. and Bale, B. (1991) Computerized X-ray analysis of sandbox models: examples of thin-skinned thrust systems Geology 19, 1063-1067 Creole Petroleum Corporation (1965) Geological Maps D10 and D l l , 1/100.000 scale Davis, D., Suppe, J. and Dahlen, F. A. (1983) Mechanics of foldout-thrust belts accretionary wedges J. Geophys. Res. 88, 1156-1172

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North Monagas thrust belt (Venezuela)." F. Roure et al. Eva, A., Burke, K., Mann, P. and Wadge, G. (1989) Four-phase tectonostratigraphic development of the southern Caribbean Mar. Petrol. GeoL 6, 9-21 Fiorillo, G. (1982) Exploracion y evaluacion de la Faja Petrolifera del Orinoco. In: Symposium Exploracion Petrolera en /as Cuencas Subandinas de Venezuela, Colombia, Ecuador y Peru, Asociacion Colombiana de Geologia y Feofisica de Petroleo, Bogota, 45pp Flume, G. and Graterol, O. (1990) Procesamiento e interpretacion de datos aerogravimetricos y aeromagneticos en area de interes exploration en la region PedernalesQuiriquire. In: V Venezuelan Geophysical Congress, Caracas, 458-465 Fourcade, E., Azema, J., Cecca F., Bonneau, M., Peybern~s, B. and Dercourt, J. (1991) Essai de reconstitution cartographique de la paleogeographie et des pal~o-environements de la Tethys au Tithonique sup#rieur (138 & 135 M.a.) Bull. Soc. GdoL Fr. 162, 1197-1208 Gallango, O. and Parnaud, F. (1993) 2-D computer modelling of oil generation and migration in a transect of the Eastern Venezuela basin [abstractl Am. Assoc. Petrol. GeoL, Caracas Meeting, 48 Gonzales de Juana, C., Arozenia, J. and Picard-Cadillat, X. (1980) Geologica de Venezuela y de sus cuencas petroliferas; Foninves Ed., Caracas, 2 vols, 1051 pp Guillaume, H. A., Bolli, H. M. and Backmann, J. P. (1972) Estratigrafica del Cretaceo inferior en la Serrania del Interior, oriente de Venezuela. IV Cong. Geol. Venez., Mem., Vol. 3, BoL GeoL 5 1619-1655 Hedberg, H. D. (1937) Stratigraphy of the Rio Querecual section of northern Venezuela Geol. Soc. Am. Bull. 48, 1971-2024 Hedberg, H. D. (1950) Geology of the eastern Venezuela basin (Anzoutegui-Monagas-Sucre-eastern Guarico portion) GeoL Soc. Am. Bull. 61, 1173-1216 Hernandez, O. (1990) Exploracion en areas estructuralmente complejas, norte de Monagas. In: V Venezuela Geophysical Congress, Caracas, 534-537 Kugler, H. G. (1953) Jurassic to Recent sedimentary environments in Trinidad Bull. Ver. Scheveiz. Petrol. Geol. Ing. 20, 27- 60 Lafargue, E., EIIouz, N. and Roure, F. (1994) Thrust controlled exploration plays in the Outer Carpathians and their foreland (Poland, Ukraine and Romania) First Break 12, 69-79 Lamb, J. L. and de Sisto, J. (1963) The Morichito Formation of northern Monagas Assoc. Ven. GeoL, Min. Petrol., Bol. Inform. 6, 269-276 Lander, R., Hernandez, V., Fuentes, J. and Dos Santos, S. (1990) Metadologia integrala para la interpretacion estructural de areas geologicas complejas: Basque, norte de Monagas, Cuenca oriental de Venezuela. In: V. Venezuelan Geophysical Congress, Caracas, XX Leonard, R. (1983) Geology and hydrocarbon accumulations, Columbus basin, offshore Trinidad Am. Assoc. Petrol. Geol. Bull. 1081-1093 Liu, S. and Dixon, J. M. (1990) Centrifuge modelling of thrust faulting: strain partitioning and sequence of thrusting in duplex structures. In: Deformation Mechanisms, Rheology and Tectonics (Eds R. J. Knippe and E. H. Rutter), Spec. PubL Geol. Soc. London No. 54, 431-444 Malavieille, J. (1984) Modelisation exp~rimentale des chevauchements imbriqu~s: application aux chaTnes de montagnes Bull. Soc. GeoL Fr. 7, 129-138 Malavieille, J., Calassou, S., Larroque, C. and Lallemand, S. (1991) Experimental modelling of accretionary wedges Terra Abstr. 367 Margherita, L. and Riart, F. (1989) Sismica y modelaje estructural en una region al Norte del corrimiento de Pirital. In: VII Congresso Geologicao Venezolano, Barquisimieto, 496-513 Moore, J. C., Mascle, A., et al., (1988) Tectonics and hydrogeoIogy of the northern Barbades Ridge: results from ocean drilling program leg 110 GeoL Soc. Am. Bull. 100, 1578-1593 Moretti, I. and Larr~re, M. (1989) LOCACE computer-aided construction of balanced geological cross-sections Geobyte 4, 16-24

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Morley, C. K. (1988) Out-of-sequence thrusts Tectonics 7, 539-561 Mugnier, J. L., Guellec, S., Menard, G., Roure, F., Tardy, M. and Viallon, P. (1990) Crustal balanced cross-sections through the external Alps deduced from the ECORS profile. In: Deep Structure of the Alps (Eds F. Roure, P. Heitzmann and R. Polino) Mem. Soc. GeoL Ft. No. 156, Soc. Geol. Suisse, Soc. Geol. It. 1,203-216 Mulugetta, G. K. H. (1987) Modelling the geometry of Coulomb thrust wedges J. Struct. GeoL 10, 847-859 Munro, S. E. and Smith, F. D. Jr (1984) The Urica fault zone, northeastern Venezuela GeoL Soc. Am. Mem. No. 162, 213-215 Passalacqua, H., Fernandez, F., Gou, Y. and Roure, F. Deep architecture and strain partitioning in the eastern Venezuelan Ranges. In: South American Basins (Ed. A. J. Tankard), Am. Assoc. Petrol. GeoL Mem. in press Persad, K. M. (1978) Hydrocarbon potential of the Trinidad area, 1977 GeoL Mijnb. 57, 272-285 Potie, G. (1989) La Serrania del Interior oriental sur le transect Cumana-Urica et le bassin de Maturin (Venezuela) PhD Thesis, Universit~ Bretagne Occ., Brest, 240 pp Rosales, H. (1972) La falla de San Francisco en el Oriente de Venezuela. In: IV Congres. GeoL Venez., Caracas, 1969, Mem. BoL GeoL, Caracas 5 2322-2336 Rossi, T. (1985) Contribution & I'~tude g~ologique de la frontiere sud-est de la plaque Carai'be: la Serrania del Interior oriental (Venezuela) sur le transect Cariaco-Maturin PhD Thesis, Universite Bretagne Occ., Brest, 338 pp Rossi, T., Stephan, J. F., Blanchet, R. and Hernandez, G. (1985) Etude geologique de la Serrania del Interior oriental sur le transect Cariaco-Maturin. In: Caribbean Geodynamics (Ed. A. Mascle), Editions Technip, Paris Roure, F., Howell, D. G., Guellec, S. and Casero, P. (1990a) Shallow structures induced by deep seated thrusting. In: Petroleum Tectonics in Mobile Belts (Ed. J. Letouzey), Editions Technip, Paris, 15-30 Roure, F., Polino, R. and Nicolich, R. (1990b) Early Neogene deformation beneath the Po Plain: constraints on postcollisional Alpine evolution. In: Deep Structure of the Alps (Eds F. Roure, P. Heitzmann and R. Polino), Mem. Soc. GeoL Fr. No. 156, Soc. Geol. Suisse 1, Soc. Geol. It. 1,309-322 Roure, F., Casero, P. and Vially, R. (1991) Growth processes and melange formation in the Southern Apennines accretionary wedge Earth Planet. Sci. Lett. 102, 395-412 Roure, F., Roca, E. and Sassi, W. (1993) Neogene deformations of the outer Carpathians (Poland, Ukraine and Romania) Sedim. Geol. 86, 177-201 Sage, L., Mosconi, A., Moretti, I., Riva, E. and Roure, F. (1991) Cross-section balancing in the Central Apennines: an application of LOCACE Am. Assoc. Petrol. GeoL Bull. 75, 832-844 Salvador, A. and Stainforth, R. M. (1968) Clues in Venezuela to the geology of Trinidad and vice-versa. In: Transactions, IV Caribbean Geological Conference, 1965, Trinidad, 31-40 de Sisto, J. (1960) Submarine sliding in the La Pica Formation Assoc. Ven. GeoL Min. Petrol BoL Inform. 3, 270-281 Stainforth, R. M. (1971) La formation Carapita de Venezuela oriental. In: IV Cong. Geol. Venez., Caracas, BoL Geol. 1, 433-463 Sulek, J. A. (1961) Miocene correlations in the Maturin subbasin Assoc. Venez. GeoL Min. Petrol. 4, 131-139 Van der Osten, E. (1957) Lower Cretaceous Barranquin Formation of northwestern Venezuela Am. Assoc. Petrol GeoL Bull.

41,679-708 Vivas, V. (1986) Contribucion al estudio de la Serrania del Interior oriental (Venezuela); estratigraphia y tectonica de la region de Bergantin - Santa Ines Thesis, Universit~ Bretagne Occ., Brest Wilson, C. C. (1968) The Los Bajos Fault. In: IV Caribbean Geological Conference Trinidad, 1965, 87-89 Woodside, P. R. (1981) Petroleum geology of Trinidad and Tobago Oil Gas J. 364-389

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