Home » Projects » Research Themes » Research Theme G8

Subproject G8: Neogene evolution of the Chaco foreland basin, Bolivia and Argentina

Research Field: Stratigraphy, sedimentology

Staff

Working area of the subproject G8

g8_working_area

Fig. 1: Working area of project G8

The Miocene to Recent Chaco foreland basin records flexural subsidence and sedimentary fill, largely east of the Andean deformation front but also in the Subandean Belt, the Santa Barbara System, and the Sierras Pampeanas in southern Bolivia and northernmost Argentina, respectively. Location, sequence, and style of Andean shortening (shallow detachment vs. basement-cored uplifts) are reflected in stratigraphic age, thickness, facies, provenance and geometry of basin fill, and in its timing of subsequent incorporation into the deformed belt. Age and thickness of basin fill sediments also relate to flexural and thermal parameters of Andean lithosphere. Facies, lithologic maturity, and provenance are possibly directly related to climatic change. We propose to study the Neogene basin fill of the Chaco foreland, including seismic stratigraphy tied to key wells, biostratigraphic and radiometric age dating, characterization of depositional systems using seismic, logs, cores, and outcrop sampling along selected transects normal to the strike of the basin in order to contrast and compare Neogene basin evolution and its relationship to Andean deformation.

Objectives, methods, work plan, and schedule

Objectives

Present-day sedimentation in the northern and southern parts of the Chaco foreland basin is characterized by climatically controlled differences. To the north, high precipitation supports an efficient removal of rock exposed through uplift. Sandstones and pelites dominate the Tertiary and Quaternary basin fill which is characterized by low-gradient fluvial (and possibly lacustrine) deposits. Adjacent to the Sierras Pampeanas and Santa Barbara System to the south, much lower precipitation rates hinder effective mass transport. There, sedimentary systems are characterized by steep gradients, coarse-grained conglomeratic material, frequent mass wasting, intermittent transport, and internal drainage (Sobel et al., 2000; Strecker et al., 2000). However, these sedimentary processes are also a product of tectonic style. The fold-and-thrust belt to the north permits the development of large, in part strike-parallel interconnected drainages, while blockfaulting in the Sierras Pampeanas to the south tends to generate small, internally-drained watersheds.

The proposed project will examine Neogene stratigraphy and sedimentation in the Chaco basin. It will relate the Neogene uplift and shortening history along the east side of the Andes (e.g. Adelmann, 2001) to the stratigraphic record of the Chaco foreland basin. Sedimentological and stratigraphic evidence for the style of tectonic deformation in the deformed belt adjacent to the basin will be examined as to tectonic and climatic signals. Because the interrelationship between climatic and tectonic factors in the geologic past is not clear, the present-day distribution of climate and tectonic style will used as a baseline. As a working hypothesis, comparisons to the present will be drawn from the geologic record. The project will also link the subsidence history of the basin to tectonophysical models of shortening and basin loading and provide stratigraphic information which complements the discontinuous record from coeval outcrop sections further west.

Did the Andean plateau grow eastwards in leaps (and by backfilling its basins) or through slow continuous growth? Did plateau uplift occur gradually or rapidly? Was aridization causally linked to the uplift or was it related to global climatic change (Van der Voort et al., 1995? A thorough comparison between the stratigraphic record of the Neogene Altiplano basin fill and the Chaco foreland basin will immediately allow at least a qualitative assessment of these hypotheses through the methods described below. Bio- and chronostratigraphic data will be used to correlate depositional systems of equal age on either side of the Cordillera Oriental.

"Out-of-sequence" uplift of the Cordillera Oriental should result in significant steepening of the gradient, rapid input of coarse clastics into the foreland, episodic climate change leading eventually to aridization, rapid facies changes, higher depositional rates, lack of paleosols, and immaturity of in-situ weathering reactions. By contrast, continuous eastward growth of the plateau should produce foreland basin strata which display a more gradational coarsening, predominance of fluvial transport, more numerous paleosols, lack of contrast between the sediments east and west of the Cordillera Oriental, and a greater residence time of sediment in fluvial systems, resulting in a higher degree of diagenetic changes and compositional maturity. Global climatic change should produce sediments which, all other factors being equal, display similar features on both sides of the orogen; climatic effects of the orogen would produce asymmetric distributions through time.

There is no one-on-one match of the processes sketched above and sedimentary response, and same causes may yield different results. For instance, a pelitic or slate source rock will produce substantially less of an alluvial fan than a volcanic or metamorphic rock, even if all other factors (climate, distance, mode of transport, relief) are equal. Vice versa, different causes may still yield the same product. Quartz-rich sandstones may be produced through short- or long-distance transport, depending on the moving agent (wind, water), climate, and source rock. However, a combination of characteristics collected over a sufficient spatial and temporal interval may be adequate to eliminate these ambiguities. This requires a thorough effort on collecting data and in integrating across techniques and disciplines.

Methods

The objectives are achieved best through a combination of a subsurface stratigraphic study along several key transects based on seismic data, well data, and well logs, and an outcrop study of selected Neogene sections in the Eastern Cordillera and the Subandean belt. The former will yield information on basin geometry and style of basin fill (onlap relationships, regional correlation, location of depocenters through time, growth strata, unconformities); the latter will provide critical lithologic, petrographic, biostratigraphic, and geochronologic data to constrain composition and timing of basin fill.

Seismic stratigraphy is available through modern industry reflection seismic lines in the Subandean Belt and the Chaco Basin. The petroleum industry is primarily interested in deep structural traps; the overlying Neogene foreland basin stratigraphy, however, is relevant for thermal basin modelling and hydrocarbon maturation and migration studies. Seismic sections will be interpreted and digititized, either on a workstation or on paper sections. Flattening on individual successive horizons will allow the generation of estimates of subsidence and sediment input volumes through time. Pulses of shortening will be apparent through subtle internal unconformities. Internal stratigraphic surfaces (onlap-, offlap, downlap etc.) will yield information on depocenter migration, direction of basin fill, and progradational style. The majority of the seismic-stratigraphic study will be performed under supervision of Dr. Krawczyk at the GFZ Potsdam.

Basin analysis from decompacted sediment age- thickness curves is a powerful tool to reconstruct the geodynamic evolution of sedimentary basins (Steckler and Watts, 1978; Beaumont, 1981). To date, only one basin analysis study, based on limited data, has been performed for the Chaco basin (Coudert et al., 1995). We hypothesize that the timing and geometry of Chaco basin fill, as a function of differential shortening through two end-member styles of contractional deformation and also as a function of differing climatic conditions, will differ substantially from north to south. This will be expressed in dissimilar subsidence curves. Methods and results from our work in the Chaco basin will be closely coordinated with similar methods in the Puna-margin basins by subproject G6 (Strecker et al.) in the Sierras Pampeanas.

Provenance analysis from sandstones in cores (if available), cuttings, and outcrop will complement existing data sets from the Santa Barbara System (Reynolds et al., 2000) and from analyses to be performed in subproject G6 in the adjacent basins of the Puna margin. Because the results of thin-section point counting on sandstones is a complex function of source rock, relief, tectonics, climate, transport, and diagenesis, large data sets need to be acquired and studied (Dickinson, 1970, 1984; Zuffa, 1980; Pettijohn et al., 1987). Because of their limited availability, thin-section point counts from Tertiary sandstones in the subsurface of the Chaco basin will not be sufficient to provide a valid data sets on their own; however, they will deliver important extensions to the data sets from the surface outcrops of the Subandean Ranges and the Puna margin.

In order to contrast the similarities and differences between the northern and southern Chaco basin, it is planned to assign one Ph.D.-student each to work on a cross-strike seismic section in the North (Subandean to Brazilian Shield) and in the south (Santa Barbara System to Brazilian Shield). These seismic sections need to be integrated with well data and related seismic and outcrop data (e.g., middle Miocene Rio Honda deposits of eastern Cordillera (McFadden et al., 1990), especially in the poorly dated Miocene strata of the western Subandean zone (Tariquia and Guandacay Fms.; Gubbels et al., 1993; Marshall et al., 1993; Jordan et al., 1997)).

References

Adelmann, D. (2001): K?nozoische Beckenentwicklung des zentralandinen Punaplateaus (NW- Argentinien) - Das Gebiet um den Salar de Antofalla und ein Vergleich zur n?rdlichen Puna: Ph.D. Dissertation (unpublished), Free University Berlin, Germany, 142 p.

Beaumont, C. (1981): Foreland basins: Geophysical Journal of the Royal Astronomical Society, 65, p. 291-329.

Coudert, L., Frappa, M., Viguier, C., and Arias, R. (1995): Tectonic subsidence and crustal flexure in the Neogene Chaco Basin of Bolivia : Tectonophysics, 243, p. 277-292.

Dickinson, W.R. (1970): Interpreting detrital modes of greywacke and arkose: Jouranl of Sedimentary Petrology, 40, p. 695-707.

Dickinson, W.R. (1984): Interpreting provenance relations from detrital modes of sandstones; in Zuffa, G.G., ed., Provenance of arenites: Dordrecht (Reidel), p. 333-362.

Pettijohn , F.J., Potter, P.E., and Siever, R. (1987): Sand and Sandstone: Springer (New York), 553 p.

Sobel, E., Strecker, M., and Haselton, K. (2000): A possible genetic link between ponded sedimentary basins, rain shadows, and lateral plateau growth: EOS (Supplement), Transactions, American Geophysical Union, Program and Abstracts, 81 (48), p. F1084.

Steckler, M.S., and Watts, A.B. (1978): Subsidence of the Atlantic-type continental margin off New York: Earth and Planetary Science Letters, 41, p. 1-13.

Strecker, M., Haselton, K., Hilley, G., and Trauth, M. (2000): Limits of lateral plateau growth and ephemeral sedimentary basins at eastern Puna margin, NW Argentina: EOS, Transactions, American Geophysical Union, Program and Abstracts, 81 (48), p. F1136.

Vandervoort, D.S., T.E. Jordan, P.K. Zeitler, and R.N. Alonso (1995): Chronology of internal drainage and uplift, southern Puna plateau, Argentine central Andes: Geology 23, p. 145-148.

Zuffa, G.G. (1980): Hybrid arenites: Their composition and classification: Journal of Sedimentary Petrology, 50, p. 21-29.

Work plan and Schedule

1st year (2002):

Visit to industry offices in Santa Cruz, Bolivia; selection of seismic and well data. Limited field work (sampling of sandstones, clays, paleosols?) on selected outcrop sections in Bolivia and Argentina. Setup of workstation hardware and software, loading and QC of data. Preparation of thin sections.

2nd year (2003):

Visit to industry offices in Santa Cruz, Bolivia and Buenos Aires, Argentina, supplementary field work in Bolivia and Argentina, seismic-stratigraphic work, lithologic and log correlation from wells, age determination (Rb-Sr) on volcanic tuffs, biostratigraphy (mostly palynology) and sandstone petrography on cuttings (if possible) or cores (where available). Construction of cross sections, basin analysis (subsidence models).

3rd year (2004):

Continuation of basinwide basin analysis, correlation, connection to well data. Generation of basinwide transects for Neogene basin fill, backstripping, geohistory analysis. Calculation of flexural response through time. Development of a model for Chaco basin evolution. Presentation of results at international congresses and publication of results in international journals.

Collaboration with external research groups

Names of important partners in the host countries

Nigel Robinson, Oscar Aranibar, Eloy Martinez (Chaco S.A., Santa Cruz, Bolivia), Estanislao Koslowski, Doug Tasker (PanAmerican Energy, Buenos Aires, Argentina): Advice on regional geology of Chaco basin, selection of seismic data and well logs, quality checking of data, joint field work.

Publications

Posters

The SFB 267 poster index is available on the poster page

For this project/subproject are no data in the publications database available.