(3) YPFB-GXG, cc 1659, Santa Cruz, Bolivia. RÃSUMÃ: L'évolution de la subsidence dans le bassin du Chaco (Bolivie) est étudi& h l'aide de sismique réflexion ...
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SUBSIDENCEANDCRUSTALFLEXUREEVOLUTION THENEOGENECHACOFORELANDBASIN(BOLIVIA)
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Laurent COUDERT(~), Thierry SEMPERE(~), Michel FRAPPA(l), Claude VIGUIER(l),and Rafael (1) Laboratoire d'Études et de Recherche en Géotechniques et Géophysique Appliquk, 351 Coursde la Libération, 33405 Talence, France. (2) Orstom (UR lH),Convention YPFB-ORSTOM, Santa Cruz, Bolivia. Present address: Dkpartement de Géologie sédimentaire(LGTE), case 119, UniversitéParis VI,75252 Paris cedex 05, France. (3) YPFB-GXG, cc 1659, Santa Cruz, Bolivia.
RÉSUMÉ: L'évolution de la subsidence dansle bassin du Chaco (Bolivie) est étudi&h l'aide de sismique réflexion, de gravimetrie et de diagraphies diff&r&s.Les courbes de subsidence depuisle Mioche superieur mettent en évidence Fois &tapestectoniques avec des taux de subsidence compris entre 0.1 et 0,4 km/Ma. L'application d'un modèlede flexion, avecune rigiditk de flexionde 1OD N.m et une épaisseur de la croûte de 30-31 km,permet de décrire l'kvolutionde la géométrie et dela topographie du bassin depuis M 10a. KEY WORDS: Bolivia, foreland basin, crustal flexure, tectonic subsidence, geophysics, rncdel.
INTRODUCTIONANDGEOLOGICAL
SETTINC
During the last decade, important progress has been made in understanding the subsidence rnechanisms of foreland basins. Several modelling studies in continental regions have shown that the flexural responseto tectonic loading canbe represented by an elastic plate overlyinga weak fluid (Turcotte and Schubert, 1982; Flemings and Jordan, 1989). The purposeof this paper is to quantitatively estirnate the evolution of tectonic subsidence and crustal flexure in the Chaco basin of Bolivia (lat 19O-2OoS)through study of geological, seismic reflexion, gravity and log-welling data (mostly unpublished and borrowed from the Bolivian State Oil Company YPFB). The study was carriedout in the Subandean beltof southern Bolivia (Rio Grande-Parapeti area;lat 19'20"s and long 62O-63'S; Fig. 1). The Subandean belt is bounded in the Westby the Main Frontal Thrust (CFF,"Cabalgamiento Frontal Principal"), and the Subandean deformation dies out toward the east into the Chaco plain (Herail et al.. 1990; Baby et al., 1992). During the Neogene, the width of the foreland basin has apparently varied €rom 100 to 120 km in general. The basin is filled by late Oligocene to recent sediments (Sempere et al., 1990) with a maximum thicknessof 3000 m in the study Brea.The Tertiary deposits consist of conglomerates, sandstones, siltstonesand mudstones, and are subdivided in the Petaca. Yecua, Tariquia, Guandacay and Ernbomzd formations (see Marshall and Sempere, 1991; Marshall et al., 1993). RESULTS Wireline data and cutting descriptionsfrom two Wells (further on referred to as well 1 and well2) were used to build the two corresponding synthetic stratigraphic columns. Structural rnaps and seismic reflexion lines, in which four Tertiary sequences bounded by reflectors T,, T,, T3 and T4- were selected, were interpreted and correlated with these two logs (Fig. 2). Ages tentatively attributed to these reflectors are respectively: T4 =10 Ma (beginning of local onlap of Neogene deposition), T3~ 7 . 5Ma (top of Yecua Formation; see Marshall et al., 1993), T, 4 . 5 Ma (by interpolation), T, =1 Ma (major out-of-sequence
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reactivation of h e CFP system;Sempre. unpublished). Future precisions on the a g a of these reflectors may obviously l a d to modificationsof some of the results presented hereafter. In order to approachthe effects of subsidence, the "back-stripping" method (Stakler and Watts, 1978)wiis used. The effect of compaction was incorporatedfor computation of tetonic subsidenee. Paleobathyrnetry and sa-level changes were neglectedbecause the depositionalenvironment remained fluvial. The porosity of argillaceous m d s and mudstones as a funetion of depth was deduced from wnic bol (BHC) and density toolW C ) using the matrix parameters and asurning a l i n a relation between porosity and well logs(Steckler and Watts, 1978). Three stages were identified (Fig.3). During stage T,-T, (=10-=7.5 Ma), the tectonic subsidenea rate was ~0.4 km/Myr.During stage T3-T1(=7.5-=1, Ma), this rate decreased ts =0.1 km/Myr. Stage Tl-T,, (from =1 Ma to the present) apparentlyshows an increase in subsidence rateto 50.4 kWA4yr (Coudert, 1992). The Bsuguer anomaly regularly inereases toavard the est, Le. toward the "Alto de Izozog" forebulge (YPFB, unpublished data). In srder to understand the distribution of the successive loadson the continental lithosphere, thepower spectrum of gravity anomalies (Karner and Watls. 1983) was determined. Discontinuities evidencedthrough this method are locatedat the top of and within the basement,Le. bdow the base of the sedimentary pile (2-2.2 km-thick). The deepest discontinuity, at 31-32 km, apparently corresponds to the Moho. This crusta1 thicknessis used for estimation of the flexural rigidity, whieh appears t~ be -lOB N.m (Coudefi, 1992). The evolution of crustal flexure using an elastic modal (Turcotte and Schubert. 1982; Flemings and Jordan, 1989) is shown in Fig. 4. The best Fit betwwn csmpuhtional results and seismic and well data is obtained when a flexural parameter (=horizontal extension of the flexure)of 4'0 !an is chosen. In addition, the position (xd and height (wb)of the forebulge weree s h a t e d for each period Pis. 4). A migration of the forebulge OF about 90 km is observed for the lasb -7.5 MF, which indieateo am average migration velwity of =9-10 h / M y r .
Analysis of multiple geophysieal data in the late Miwene Chaco continental foreland basinof Bolivia permits to sketeh out its evolution and to tentatively describe its successive geomevies and topographies since 10 Ma. At least three tectsnic stages are identified, which include CUI interval of relatively low subsidence (49.1 h m y r ; -7.5-1 Ma) intercalated between intervalsof higher subsidence ( ~ 0 . 4h / M y r ) . Models used a m wilh a flexmd rigidity of =1023N.m and a crustal thicbess of40-31 km.
Aeknowledgrnents This work was funded by the French Research Administration. the Institut Francais de Recherche Scientifique pour le D6veloppement en Coop5ration (Orstom), and Yacimientos Petroliferos Fiscales Boliviarnoa (YPFB). We t h d the Gerencia de Exploraci611 of YPFB for granting permission to publish this papep.
REFERENCES Baby, P., Hbrail, G.. Salinas, R.. and Sempre. T., 1992. Geometry and kinematicevolution of passive roof duplexes deduced from cross-section balancing: Exmple from the foreland thrwt system of the Southem Bolivim Subandean Zone. Twtonics, 11: 523-536. Coudert, L.. 1992. Apports de ln sismique et des diagraphies diffbr6e.s h 1'Btude stratigrsphiqua du bmsim du Chaco de Bolivia (Rio Grande-Parapeti) Traitements statistiques et mod6lisatiom. "%&se de Doctorat de I'Universitk de Bordeaux 1, Bopp. Remings. PB., and Jordan, T.E.,1989. A synthetic stratigraphie mode1 of foreland b a i n development. Journal of Geophysical Research, 94: 3851-3866. Hkrail, G., Baby, P.. Oller. J., L6pez. M., M p z . O., Salinas, R.. Sempere, T., Beccar, G., and Toledo, H., 1990. Structure and Enematic ~ o l ~ t ofi othe~ Subandean thrust system of Bolivia. International Symposium on Andean Geodynamics, lst, Grenoble: 179-182. Kamer, G.D.. and Watts. A.B., 1983. Gravity anomalies and flexure of the lithosphere ot mountain ranges. Journal of Geophysical Research, 88: 10449-10477. Marshall, L.G.,and Sempere, T., 1991. The Eocene to Pleistocene vertebrates of Bolivia and thair stratigraphie context: Q review. In: F6siles y Facies de Bolivia, R. Su&ez-Somco (ed.), Revista Tdcnica de YPFB, Santa Cm.?.. 12: 631-652.
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Marshall, L.G.,Sernpere. T., and Gayet, M., 1993. The Peraca (lateOligocene-rniddleMiocene) and Yecua (late Miocene)formations of the Subandean-Chaco basin, Bolivia, and their tectonic significance. Documents du Laboratoire de géologie de Lyon, 125: in press. Sempere, T., Herail, G.. Oller, J., and Bonhomme, M. G., 1990. Late Oligocene-early Miocene major tectonic crisis and related basins in Bolivia. Ceology, 18: 946-949. Steckler, M.S., and Watts. A.B., 1978. Subsidence of the Atlantic-type continental margin off New-York. Earth and Planetary Sciences Letters, 41: 1-13. Turcotte, D.L..and Schubert, G., 1982. Geodynamics: Applications of Continuum Physics to Geological Problems. J. Wiley and Sons, New York, 650 pp.
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Fig. 1
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Formations Lithology
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Sequences
Fig. 2 Schematic lithology and stratigraphy o f well 1, with indication of stratigraphiclocation of reflectors T, through T,. Wireline logs: G R = gamma ray, PS = spontaneouspotential,RILD = inductionresistivity,Rho B = density tool (FDC). Lithology:sandstonesareinwhite;mudstonesareinblack.
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T 1 O e
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TlME (rnyrl 6
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2
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'?,
I
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Fig. 3 Tectoaicsubsidence curves since IO Ma for wdls 1 and 2. Dashed line: observed subsidenee;dotted line: subsidence after decompaction;solid line: eomputed basemeal subsidence.
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Successive depths of the top of the basemernt at "tirnes" T3, Tz, Tl, and "To" (presemst). Points: observed seismic data; solid line: somputed flexure profile; dotted liue: compensated topography. Fig. 4
