Chaco Foreland Research Articles

Onset of the Miocene Chaco foreland basin: depositional conditions, provenance and paleogeographic implications of the Tranquitas Formation (Argentina)

Onset of the Miocene Chaco foreland basin: depositional conditions, provenance and paleogeographic implications of the Tranquitas Formation (Argentina)

Tectonic and paleoclimatic controls on the composition of inland wetland deposits, Chaco foreland basin, Central Andes

ABSTRACTIn recent decades, the growing interest in wetlands highlights the fundamental role of these complex ecosystems. Integrated sedimentological and geochemical studies that refer to wetlands appear more frequently in the literature, since such approaches provide a window into understanding the functions they have played through geologic time. The Miocene Yecua Formation (Chaco foreland basin, Central Andes, Bolivia) presents sedimentological features that show how siliciclastic, chemical, and biological processes lead to lithofacies and compositional features that are distinctive of wetlands related to back-bulge settings. The succession is composed of shallowing-upward parasequences with various compositional characteristics controlled by tectonic and climatic factors.Data integration obtained from paleocurrents, petrography, and clay mineralogy show the influence of the Central Andes fold and thrust belt located to the west of the basin as a principal source area. The high compositional maturity of sandstones reflects the tectonic control as the main allocyclic variable associated with reworking of already mature sediment sources and transport factors in relation to the distal position of the basin (back-bulge), combined with climatic and paleoenvironmental conditions. XRD and SEM-EDS analyses show that illitic clays represent the main mineral phase of the clay assemblages, and have a detrital origin related to physical weathering processes in the source area. Al-rich smectitic minerals have an authigenic origin and were likely formed by alteration of volcanic detritus as well as from precursor minerals such as illite and feldspars. Kaolinite is determined to be detrital and likely derived from the erosion of floodplains with soil development. Throughout the parasequences, illite–smectite–kaolinite minerals are arranged in different proportions, showing a general vertical trend (from base to top) in which the illitic clays decrease at the expense of the increase in Al-rich smectite and kaolinite. The increase in smectite is accompanied by mixed siliciclastic–carbonate sedimentation, which points to a reduction in the clastic contribution linked to drier conditions. This vertical compositional arrangement reveals a paleoclimatic control, influenced by middle-term wet and dry periods. This work demonstrates that source-rock composition and transport distance (tectonic processes) are the key factors influencing quartz-rich sandstones and illite-rich clay mineral suites in distal positions of continental foreland basins. The long-term tropical to subtropical climate favored the development of inland wetlands and played a secondary role in the final composition of these depositional systems.

Reconciling Spatial and Temporal Patterns of Cenozoic Shortening, Exhumation, and Subsidence in the Southern Bolivian Andes

The Bolivian Andes are an archetypal convergent margin orogen with a paired fold-thrust belt and foreland basin. Existing chronostratigraphic constraints highlight a discrepancy between unroofing of the Eastern Cordillera and Interandean Zone fold-thrust systems since 40 Ma and the onset of rapid sediment accumulation in the Subandean Chaco foreland after 11 Ma, previously attributed to Miocene climate shifts. New results from magnetostratigraphic, backstripping, erosional volumetric calculations, and flexural modeling efforts are integrated with existing structural and thermochronologic datasets to investigate the linkages between shortening, exhumation, and subsidence. Magnetostratigraphic and backstripping results determine tectonic subsidence in the Chaco foreland basin, which informs flexural models that evaluate topographic load and lithospheric parameters. These models show that Chaco foreland subsidence is consistent with a range of loading scenarios. Eroded volumes from the fold-thrust belt were sufficient to fill the Chaco foreland basin, further supporting the linkage between sediment source and sink. Erosional beveling of the Eastern Cordillera, local intermontane sediment accumulation after 30–25 Ma, and regional development of the high-elevation San Juan del Oro geomorphic surface from 25 to 10 Ma suggest that the western Eastern Cordillera did not store the large sediment volume expected from erosion of the fold-thrust belt, which arrived in the Subandean Zone after 11 Ma. Eocene to middle Miocene foreland basin accumulation was likely focused between the Eastern Cordillera and Interandean Zone, and has been almost completely recycled into the modern Subandean foreland basin. The delay between initial fold-thrust belt exhumation (early Cenozoic) and rapid Subandean subsidence (late Cenozoic) highlights the interplay between protracted shortening, underthrusting, and foreland basin recycling. Only with sufficient crustal shortening, accommodated by eastward advance of the fold-thrust belt and attendant underthrusting of Brazilian Shield lithosphere beneath the Subandes, did the Subandean zone enter proximal foreland basin deposystems after ca. 11 Ma. Prior to the late Miocene, the precursor flexural basin was situated westward and not wide enough to incorporate the distal Subandean Zone. These results highlight the interplay between a range of crustal and surface processes linked to tectonics and Miocene climate shifts on the evolution of the southern Bolivian Andes.

A singular Hegetotheriinae (Notoungulata, Typotheria) from the late Oligocene-Early Miocene of the Subandean Region of Bolivia

Abstract Geological studies in the northern sector of the Chaco foreland Basin, Bolivia, yielded new fossils coming from late Oligocene-late Miocene of the Petaca Formation. Few fossil mammals were known from the Subandean Region of Bolivia. We report a partially complete mandible of a hegetotheriid Hegetotheriinae (Notoungulata, Typotheria) from Abapo (Rio Grande River). The specimen (YPFB-LIT-PAL-005) is very close in size and dental morphology to the late Oligocene (Deseadan South American Land Mammal Age, SALMA) — Santacrucian (early Miocene) Prohegetotherium schiaffinoi (Kraglievich 1932) from Fray Bentos (Uruguay and Argentina), Salla (Bolivia), Divisadero Largo-Potrerillos and Quebrada Fiera (Argentina). However, mandibular characteristics, as the shape with a marked change in height along its length, increasing towards the back, a prominent masseteric crest, a deep mandibular groove, and a remarkable thickening of the ventral rim of the mandible, indicate differences between this specimen and Prohegetotherium schiaffinoi and the other Hegetotheriinae. The affinity of YPFB-LIT-PAL-005 with P. schiaffinoi suggests a late Oligocene to early Miocene for the upper bearing horizon of the Petaca Formation (in Abapo), an older age than previously assigned to the top of this unit (late Miocene), and confirms the taxon distribution between ∼ 36 ° to ∼ 17 ° south latitude.

Facies model of a sedimentary record for a Pantanal‐like inland wetland

AbstractDespite recent advances in wetland studies, ancient wetland deposits are still not well documented, and their facies characteristics are poorly registered. Sedimentary facies and sequence stratigraphic analysis of the Miocene Yecua Formation (Chaco foreland basin, Central Andes, Bolivia) and their comparison to Pantanal‐like modern wetlands provide an insight into their variability, suggesting a facies model for large inland wetlands that developed in a tropical–subtropical climate. Sedimentological features show that clastic, chemical and biological processes in these environments lead to distinguishable lithofacies variations. Six architectural elements are described: (i) muddy sublittoral (FA1); (ii) mixed siliciclastic–carbonate shoreface (FA2); (iii) sand‐flat (FA3) and (iv) mud‐flat (FA4) deposits; (iv) floodplain (FA5); and (vi) simple channel deposits (FA6). The succession is composed of shallowing‐upward parasequences with different facies characteristics caused by climatically‐driven changes of the water level over three orders of magnitude and cyclicity. These cyclic changes reflect a climatic control on the sedimentation. A palaeoclimatic interpretation suggests a generally warm, humid climate with marked rainfall regime changes. The aim of this article is to use this dataset to improve the understanding of depositional elements, lithologies and stratigraphy in tropical–subtropical large inland wetlands. The proposed criteria will help in recognizing inland wetland deposits in other sedimentary basins.

An Integrated Sedimentary Systems Analysis of the RíO Bermejo (Argentina): Megafan Character in the Overfilled Southern Chaco Foreland Basin

Abstract: The Chaco foreland is well known for its spectacular fluvial megafans, and this basin frequently serves as a modern analog for studies that use the geological record to investigate tectonic processes in convergent orogenic systems. Yet little sedimentary data has been recovered from the southern Chaco, particularly at the latitude of northern Argentina, which limits both its utility as an analog and our understanding of Andean megafan development in the subtropics. The southern Chaco is particularly amenable for a modern time-slice sedimentary systems analysis, as environmental gradients controlled by tectonics and climate are relatively well documented. Deposition in the southern Chaco foreland is dominated by the Rio Bermejo, which was studied by integrating geomorphological observations made using geographic information systems with analyses of composition, texture, and U-Pb ages of modern detrital sediments. Remote-sensing data reveal changes in Rio Bermejo channel width, sinuosity, and bifurcation from west to east across the foreland, which are compatible with aspects of geomorphic models for distributive fluvial systems. The Rio Bermejo fully spans the foreland and connects with the south-flowing Rio Paraguay; fluvial sediments and reworked loess bury and obscure any surficial expression of a flexural forebulge. Therefore, the Rio Bermejo megafan (RBM) stands as an important example of a megafan in an overfilled foreland basin. Variability in RBM morphology and landforms appears to have only subtle linkages to flexural partitioning of the Chaco, which is likely a consequence of sediment supply overwhelming available accommodation. Trend-surface analysis highlighted topographic contrasts in RBM architectural elements and indicates the presence of distributary channels with variable surficial expression. Shallow accommodation, marked by ponds, is most pervasively developed at the eastern end of the RBM; this pattern is consistent with model-predicted locations of a back-bulge depozone. Although three environmentally distinct hinterland areas contribute sediment to the RBM, sands from the foreland cannot distinguish these zones. Rather, provenance is controlled chiefly by parent lithologies and transport distance. Sands in the RBM plot within the recycled orogen provenance field (mean QtFL = 77\6\17), and compositional maturity increases with distance from the thrust front, such that quartzose sublitharenites dominate in the distal back-bulge. Distinct intra-foreland gradients are captured in sand texture and clay-mineral abundances. Rio Bermejo sand sizes become progressively finer towards the forebulge and back-bulge (mean = ∼ 98 μm), whereas mean particle sizes in the wedgetop and foredeep are ∼ 280 and 195 μm, respectively. Detrital clays suspended in the Rio Bermejo consist of illite, smectite, chlorite, and kaolinite (in rank order of abundance), with smectite concentrated in the foredeep due to reworking of the semiarid floodplain by water and wind. The results of this study provide novel sedimentary ground truth for distributive fluvial systems in overfilled retroarc forelands that can be used to improve facies models and interpretations of the geological record.

Influence of subduction history on South American topography

The Cenozoic evolution of South American topography is marked by episodes of large-scale uplift and subsidence not readily explained by lithospheric deformation. The drying up of the inland Pebas system, the drainage reversal of the Amazon river, the uplift of the Sierras Pampeanas and the uplift of Patagonia have all been linked to the evolution of mantle flow since the Miocene in separate studies. Here we investigate the evolution of long-wavelength South American topography as a function of subduction history in a time-dependent global geodynamic model. This model is shown to be consistent with these inferred changes, as well as with the migration of the Chaco foreland basin depocentre, that we partly attribute to the inboard migration of subduction resulting from Andean mountain building. We suggest that the history of subduction along South America has had an important influence on the evolution of the topography of the continent because time-dependent mantle flow models are consistent with the history of vertical motions as constrained by the geological record at four distant areas over a whole continent. Testing alternative subduction scenarios reveals flat slab segments are necessary to reconcile inferred Miocene shorelines with a simple model paleogeography. As recently suggested, we find that the flattening of a subduction zone results in dynamic uplift between the leading edge of the flat slab segment and the trench, and in a wave of dynamic subsidence associated with the inboard migration of the leading edge of flat subduction. For example, the flattening of the Peruvian subduction contributed to the demise of Pebas shallow-water sedimentation, while continental-scale tilting also contributed to the drainage reversal of the Amazon River. The best correlation to P-wave tomography models for the Peruvian flat slab segment is obtained for a case when the asthenosphere, here considered to be 150 km thick and 10 times less viscous than the upper mantle, is restricted to the oceanic domain.

Composition and Provenance History of Late Cenozoic Sediments in Southeastern Bolivia: Implications for Chaco Foreland Basin Evolution and Andean Uplift

Abstract The Chaco foreland basin marks the eastern edge of the Andean orogenic system and contains up to 7.5 km of mostly nonmarine siliciclastics, thus providing an opportunity to study the detrital record of late Cenozoic Andean geodynamics and paleogeography. Sandstones from the southern Bolivian Chaco foreland basin testify to variable contributions from the craton and the Andean foreland fold-and-thrust belt in the context of an eastward-advancing orogen. Sandstone and conglomerate petrography of the Oligocene to Recent strata exposed in the Chaco Basin show an overall trend towards reduced mineralogical maturity, corresponding to diminishing transport distance, increasing topographic gradient and changing depositional environment from arid continental plain (Oligocene to middle Miocene Petaca sandstone) through marginal marine (middle Miocene to late Miocene Yecua sandstone) and braided fluvial settings (late Miocene Tariquia sandstone) to semihumid alluvial-fan environments (late Miocene to Recent Guandacay and Emborozu formations). Paleocurrent patterns of pre-tectonic sediments record westward transport. The late Miocene transitional-marine Yecua Formation sediments show a complex shoreline-related pattern. Syntectonic strata (Tariquia, Guandacay, and Emborozu formations) filling the foreland basin indicate dominant east-directed sediment transport from the advancing Andean orographic front. Because the Eastern Cordillera provided a drainage barrier to Altiplano erosion, sandstones of the Chaco Basin do not directly record Andean volcanism or Altiplano uplift; however, climatic inferences from changing facies and petrographic composition suggest major uplift and formation of orographic barriers between 14 and 7 Ma.

Flexural isostasy in the Bolivian Andes: Chaco foreland basin development

The Chaco foreland basin was initiated during the late Oligocene as a result of thrusting in the Eastern Cordillera in response to Nazca–South America plate convergence. Foreland basins are the result of the flexural isostatic response of an elastic plate to orogenic and/or thrust sheet loading. We carried out flexural modelling along a W–E profile (21.4°S) to investigate Chaco foreland basin development using new information on ages of foreland basin strata, elastic and sedimentary thicknesses and structural histories. It was possible to reproduce present-day elevation, gravity anomaly, Moho depth, elastic thicknesses, foreland sedimentary thicknesses and the basin geometry. Our model predicted the basin geometry and sedimentary thicknesses for different evolutionary stages. Measured thicknesses and previously proposed depozones were compared with our predictions. Our results shed more light on the Chaco foreland basin evolution and suggest that an apparent decrease in elastic thickness beneath the Eastern Cordillera and the Interandean Zone could have occurred between 14 and 6 Ma.

Plio‐Quaternary sediment budget between thrust belt erosion and foreland deposition in the central Andes, southern Bolivia

ABSTRACTEstimates of the physical boundary conditions on sediment source and sink regions and the flux between them provide insights into the evolution of topography and associated sedimentary basins. We present a regional‐scale, Plio‐Quaternary to recent sediment budget analysis of the Grande, Parapeti and Pilcomayo drainages of the central Andean fold‐thrust belt and related deposits in the Chaco foreland of southern Bolivia (18–23°S). We constrain source‐sink dimensions, fluxes and their errors with topographic maps, satellite imagery, a hydrologically conditioned digital elevation model, reconstructions of the San Juan del Oro (SJDO) erosion surface, foreland sediment isopachs and estimated denudation rates. Modern drainages range from 7453 to 86 798 km2 for a total source area of 153 632 km2. Palaeo‐drainage areas range from 9336 to 52 620 km2 and total 100 706 km2, suggesting basin source area growth of ∼50% since ∼10 Ma. About 2.4–3.1 × 104 km3 were excavated from below the SJDO surface since ∼3 Ma. The modern foredeep is 132 080 km2 with fluvial megafan areas and volumes ranging from 6142 to 22 511 km2 and from 1511 to 3332 km3, respectively. Since Emborozú Formation deposition beginning 2.1 ± 0.2 Ma, the foreland has a fill of ∼6.4 × 104 km3. The volume and rate of deposition require that at least ∼40–60% of additional sediment be supplied beyond that incised from below the SJDO. The data also place a lower limit of ≥0.2 mm year−1 (perhaps ≥0.4 mm year−1) on the time‐ and space‐averaged source area denudation rate since ∼2–3 Ma. These rates are within the median range measured for the Neogene, but are up to 2 orders of magnitude higher than some observations, as well as analytic solutions for basin topography and stratigraphy using a two‐dimensional mathematical model of foreland basin evolution. Source‐to‐sink sediment budget analyses and associated interpretations must explicitly and quantitatively reconcile all available area, volume and rate observations because of their inherent imprecision and the potential for magnification when they are convolved.

Evolution of the late Cenozoic Chaco foreland basin, Southern Bolivia

ABSTRACTEastward Andean orogenic growth since the late Oligocene led to variable crustal loading, flexural subsidence and foreland basin sedimentation in the Chaco basin. To understand the interaction between Andean tectonics and contemporaneous foreland development, we analyse stratigraphic, sedimentologic and seismic data from the Subandean Belt and the Chaco Basin. The structural features provide a mechanism for transferring zones of deposition, subsidence and uplift. These can be reconstructed based on regional distribution of clastic sequences. Isopach maps, combined with sedimentary architecture analysis, establish systematic thickness variations, facies changes and depositional styles. The foreland basin consists of five stratigraphic successions controlled by Andean orogenic episodes and climate: (1) the foreland basin sequence commences between ∼27 and 14 Ma with the regionally unconformable, thin, easterly sourced fluvial Petaca strata. It represents a significant time interval of low sediment accumulation in a forebulge‐backbulge depocentre. (2) The overlying ∼14–7 Ma‐old Yecua Formation, deposited in marine, fluvial and lacustrine settings, represents increased subsidence rates from thrust‐belt loading outpacing sedimentation rates. It marks the onset of active deformation and the underfilled stage of the foreland basin in a distal foredeep. (3) The overlying ∼7–6 Ma‐old, westerly sourced Tariquia Formation indicates a relatively high accommodation and sediment supply concomitant with the onset of deposition of Andean‐derived sediment in the medial‐foredeep depocentre on a distal fluvial megafan. Progradation of syntectonic, wedge‐shaped, westerly sourced, thickening‐ and coarsening‐upward clastics of the (4) ∼6–2.1 Ma‐old Guandacay and (5) ∼2.1 Ma‐to‐Recent Emborozú Formations represent the propagation of the deformation front in the present Subandean Zone, thereby indicating selective trapping of coarse sediments in the proximal foredeep and wedge‐top depocentres, respectively.Overall, the late Cenozoic stratigraphic intervals record the easterly propagation of the deformation front and foreland depocentre in response to loading and flexure by the growing Intra‐ and Subandean fold‐and‐thrust belt.

Depositional setting of the Middle to Late Miocene Yecua Formation of the Chaco Foreland Basin, southern Bolivia

Middle–Late Miocene marine incursions are known from several foreland basin systems adjacent to the Andes, likely a result of combined foreland basin loading and sea-level rising. The equivalent formation in the southern Bolivian Chaco foreland Basin is the Middle–Late Miocene (14–7 Ma) Yecua Formation. New lithological and paleontological data permit a reconstruction of the facies and depositional environment. These data suggest a coastal setting with humid to semiarid floodplains, shorelines, and tidal and restricted shallow marine environments. The marine facies diminishes to the south and west, suggesting a connection to the Amazon Basin. However, a connection to the Paranense Sea via the Paraguayan Chaco Basin is also possible.

Tectonic subsidence and crustal flexure in the Neogene Chaco basin of Bolivia

We examined the evolution of the geometry of the Chaco foreland basin of Bolivia (Río Grande-Parapetíarea, latitude 18°50′–20°S) using seismic reflection, gravity and well-log data. Subsidence curves enabled us to identify three tectonic stages with a subsidence rate which ranged from 0.1 to 0.4 km/Myr during the late Miocene. We used the Bouguer gravity anomaly and the power spectrum of the gravity anomalies to estimate the mean depth of the compensating interfaces. In order to understand the evolution of the crustal flexure, we used an elastic model with a flexural rigidity of ≈ 10 23 N.m and crustal thickness of ≈30–31 km. We observed a migration of the forebulge during the Neogene of about 90 km. Analysis of multiple geophysical data in the Chaco foreland basin of Bolivia permits us to sketch out its evolution and to tentatively describe its successive geometries and topographies since 10 Ma.