Distal Foreland Research Articles

Terminal glacial overdeepenings: Patterns of erosion, infilling and new constraints on the glaciation history of Northern Switzerland

Glacially overdeepened basins are a common landform of subglacial erosion. However, the controlling erosional–depositional processes and their age remain poorly understood on a global scale. Terminal overdeepenings near the former glacier margins are critical for the understanding of subglacial processes and their development over time. This study examines the geomorphology and sedimentology of buried terminal overdeepenings eroded below the Rhein Glacier and adjacent lobes in the distal northern foreland of the European Alps. The evolution of erosion and infilling in the overdeepened troughs over time is investigated using high-quality drill cores (∼1463 m of core in total) that were logged for lithofacies, petrophysical, geotechnical and compositional properties. The drill data is integrated with 2D-reflection seismics (∼41 km in total) and supplementary subsurface data. This extensive dataset reveals that the studied overdeepened basins include twelve typical facies associations (partially emplaced in characteristic sequences) and characteristic architectural elements. These categories serve as effective tools to reduce the high facies variability and facilitate easier comparison and correlation of the valley fill. Our analysis shows that the formation of terminal overdeepenings on soft sedimentary bedrock (Molasse) is the result of a combination of erosional processes. Subglacial water erosion and evacuation are the dominant processes and active during periods of glacier–bed decoupling and flushing. Direct subglacial erosion occurs during glacier–bed coupling and is documented by bedrock glacitectonites. The valley fill architecture shows that the studied overdeepenings typically undergo a multiphase evolution, with several phases of overdeepening erosion, deposition and partial re-erosion (or re-activation). The combined dataset (including geochronological data) suggests that the overdeepenings were eroded during many, if not all, extensive glaciations during the Middle–Late Pleistocene that reached the distal foreland. These are findings relevant for the large number of overdeepenings known from the Northern Alpine foreland and overdeepened features worldwide. They corroborate the importance of overdeepenings as archives for the paleoenvironmental change and landscape evolution during the Quaternary.

Chronostratigraphy and tectono‐sedimentary history of the Eastern South Pyrenean foreland basin (Ripoll Syncline, North‐East Spain)

AbstractThis paper contributes to an understanding of the tectono‐sedimentary evolution of the South Pyrenean foreland system by reviewing the chronostratigraphic framework of the basin infill in its eastern sector. Six sections are analysed and cross‐correlated to build a 6.5 km thick composite magnetostratigraphy that represents the complete record of the Cadí Nappe in the Ripoll Syncline. New and previous magnetostratigraphic sections are integrated with available biostratigraphy to provide a new age calibration of the sedimentary succession of the Cadí Nappe, encompassing from Palaeocene to Middle Lutetian age. The proposed correlation with the Geomagnetic Polarity Time Scale aims at best reconcile magnetostratigraphic data with the regional biochronology built on the marine Shallow Benthic Zonation (SBZ biozones), the continental mammalian biochronology (MP levels) and the newly collected charophyte data. A subsidence analysis was performed on the calibrated composite succession, resulting in two well‐defined intervals bounded by a hiatus. A Palaeocene to Early Eocene interval with low (11–21 cm/kyr) total subsidence rates, and an Early to Middle Eocene interval, characterised by high (70–75 cm/kyr) total subsidence rates. The detailed trends in both subsidence and sedimentation mark the development and evolution of the foreland depozones, from distal foreland depozones to foredeep and wedge‐top depozones, relative to the emplacement of the Pedraforca Nappe and Cadí Thrust Nappe. The most pronounced sedimentary shift in the Cadí Nappe occurred at 49 Ma, with the rapid drowning of the carbonate platforms and its transition into talus and deep basinal environments. Carbonate platforms collapsed and resedimented on the talus of the elongated trough, newly formed parallel to the orogenic front. This marked the onset of tectonic subsidence triggered by the submarine emplacement of the Lower Pedraforca Nappe. The emersion of the orogenic wedge drove the entry of siliciclastics, lagged by 1 Myr, into the Ripoll Trough. The foredeep filled rapidly (5.5 km thickness in less than 7 Myr) compared to other South Pyrenean regions, favoured by its semi‐enclosed palaeogeography. The emplacement of the Vallfogona Thrust as early as the Middle Lutetian (43 Ma) brought the Cadi Nappe into a wedge‐top setting. However, the Ripoll growth syncline continued acting as a temporary sink for alluvial sedimentation while a foredeep developed further south in the autochthonous Ebro Basin. The flexural response of the Iberian plate to the tectonic thickening of the Axial Zone counterbalanced for a period the local uplift of the Cadi Nappe, providing accommodation space for the top sediments filling the Ripoll Syncline.

Cenozoic subsidence‐driving mechanisms in the southernmost Patagonian basins of Tierra del Fuego and SW Atlantic

AbstractForeland basins are ideal laboratories to examine and quantify forces that contribute to Earth's topography. The interaction of these driving mechanisms (atmospheric, lithospheric and asthenospheric) affects the accumulation and preservation of strata in marine or terrestrial depocentres. For foreland basins that cover thousands of kilometres along orogens, geodynamic processes or lithospheric structure might differ and/or overlap differently along or across strike. The Magallanes‐Austral basin in the southernmost Patagonia serves as a good analogue to analyse the interactions between subcrustal forces and foreland sedimentation. While to the northern part of southern Patagonia, Cenozoic basins were predominantly terrigenous and above sea level; at the southernmost end of Patagonia, sedimentation in the island of Tierra del Fuego was mostly submarine. We analysed in this contribution the southernmost foreland of Patagonia by combining backstripping with reconstruction of flexural and dynamic subsidence. These results were compared with terrestrial records exposed further north of southern Patagonia. We found that, in addition to crustal contributions (as deformation and sedimentation), subcrustal forces are required to accommodate the proximal and distal foreland strata and explain the palaeoenvironmental and subsidence discrepancies that resulted after our analysis. When our models are compared with dynamic topographic curves, strong correlations are observed during the Palaeogene, whereas strong topographic differences occurred in the Neogene. Dynamic topography models in the Neogene have reproduced clear uplift, whereas our residual topography results show equilibrium (close to the orogen) to subsidence values (to the distal foreland). We propose that changes in the lithospheric mantle had to work together with the rest of the tectonics and dynamic forces to match 1‐D backstripping and flexural curves. This suggests that foreland basins in southern Patagonia were controlled differently along strike the southern Andes and that crustal deformation, asthenospheric flows and a heterogeneous lithospheric mantle structure affected the Cenozoic basin evolution.

Platform-to-Basin Evolution of a Tectonically Indistinct Part of a Multiple Foreland—Analysis of a 3D Seismic Block in the Northern Adriatic Sea (Croatian Offshore)

The Aiza research area covers over 650 km2 of the northern Adriatic offshore, a common Adriatic foreland of the older Dinarides on the NE, and the younger Apennines on the SW. High-quality 3D reflection seismic data were used to investigate the area’s Mesozoic to Cenozoic tectono-stratigraphic evolution. Four main seismo-stratigraphical horizons were recognized: Base of Carbonate Platform (BCP), Top of Carbonate Platform (TCP), Messinian Erosional Surface (MES), and a Plio-Quaternary horizon (PlQh), as well as the dominant faults. The results depict the geological setting and tectonic evolution of the area. A long-lasting (Jurassic to Cretaceous) stable NW-SE striking platform margin evolved probably along the inherited Triassic normal fault. The marginal belt of the platform was affected during the Late Cretaceous to Palaeogene by extension and opening of the intra-platform basin, probably on the southern limb of the then developing Dinaric forebulge. The transverse fault system (Kvarner fault) was probably reactivated as a strike-slip zone during the late Miocene tectonic reorganization. The area was tilted to the SW during the Pliocene, in the distal foreland of the progressively northward propagating Northern Apennines. Sub-horizontal late Quaternary cover of Dinaric and Apenninic structures could imply active subsidence of the foreland in between nowadays sub-vertically exhuming neighboring orogenic belts.

Geochronology of reworked ash and its implications for accommodation space variations in distal foreland basins, McMurray Formation, Alberta, Canada

A bentonite bed consisting of reworked volcanic ash from the Barremian to Aptian lower McMurray Formation is dated using chemical abrasion thermal ionization mass spectrometry (CA-TIMS) on zircon. The bentonite bed is from a cored interval in the McKay Paleovalley in the northern Athabasca Oil Sands Region of Alberta, Canada. Deposition of the lower McMurray Formation took place in the distal portion of the Western Canada Sedimentary Basin early in the Cretaceous-to-Paleocene phase of widespread contractional deformation in the Canadian Cordillera. A 30 g sample of volcanic ash yielded >10,000 very small (<50 µm) zircon grains with the vast majority (~99.7%) being highly abraded and clearly detrital. Twenty-six sharply faceted grains were sufficiently large to be mounted for laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis, which yielded seven Lower Cretaceous dates. These Lower Cretaceous grains were further analyzed using CA-TIMS. The five youngest CA-TIMS dates agree with a weighted mean of 121.51 ± 0.11 Ma which is conservatively interpreted as a maximum depositional age (MDA). The good agreement between the five zircon dates indicates the MDA is probably the depositional age. This study demonstrates that with careful analysis of zircon populations, high-confidence MDAs can be derived even from reworked ash beds that are dominated overwhelmingly by detrital zircon. The date from this study overlaps a previously published MDA of 121.39 ± 0.27 Ma for the top of the lower McMurray Formation in Firebag Tributary in the northeastern Athabasca Oil Sands Area. The overlapping dates establish a widespread chronostratigraphic surface within the northern Athabasca Oil Sands Area. Both ages are from ash beds preserved in coal seams, suggesting that the coal-bearing interval is a regional marker bed and that the ash beds may form an intraformational datum. Further, the occurrence of the ash beds in the McMurray Formation in close proximity to regional flooding surfaces suggests that tectonic activity in the emerging Canadian Cordillera influenced accommodation space variations in the distal portions of the adjacent foreland basin.

The southern extension of the Eocene Andean orogeny: New sedimentary record of the foreland basin in the southern Central Andes at 32° S

AbstractThe Eocene compressional phase is well known to have contributed to the construction of the Andes orogen at latitudes north of 30° S, but its extension to the south has not been fully studied. Moreover, synorogenic deposits of Eocene age across the foreland are scarce. The Cenozoic Manantiales Basin records the unroofing sequence of the Andes at 32° S. This study focuses on the basal infill of the Manantiales Basin, informally called the Areniscas Chocolate, which has received less attention than the upper infill until now. Sedimentological, geochronological and provenance studies were carried out on this unit. Here, we present the first ages for the Areniscas Chocolate sequence, of ca. 35–39 Ma (maximum depositional age, MDA). This MDA is interpreted as close to its depositional age, which together with their stratigraphic characteristics, allow us both to separate it from the overlying Miocene Chinches Formation and to propose it as an independent lithostratigraphic unit called Río de los Patos Formation (nov. den.). The provenance analysis of the Río delos Patos Formation indicates a sediment input from western sources located in the Coastal Cordillera and Western Principal Cordillera. Facies associations suggest that the Río de los Patos Formation represents the distal synorogenic deposits during the construction of an Eocene relief to the west. Therefore, the Manantiales Basin started during the late Eocene as a distal foreland basin, indicating that Eocene compression reached latitudes as far south as 32° S. Our results shed light into the characterization of the earliest infill of the Manantiales Basin, as well as into the tectonic evolution of the basin.

A composite terrain-and-provenance analysis tied to specific times of a basement-foreland drainage system and its gravel-sand-clay deposits (Bohemian Massif, SE Germany)

Provenance analysis and terrain analysis backed by radiometric age dating have been combined for the first time and conducted as a composite in a foreland-basement transition zone with mutual benefits for both disciplines. A meticulous provenance analysis enables a fine-tuning during studies of the landscapes and relief generations, while a detailed terrain analysis is essential for the disentanglement of lithological-geodynamic processes that often are telescoped into each other by sedimentological processes. The basic drivers for the denudation, transport and deposition processes of the various clast communities (gravel, sand, clay) are the climate change from a tropical (Neogene) to a glacial/temperate Earth and the brittle neotectonics along the margin of the study area (Rhine Graben Rift N-S, Eger/Ohře Graben Rift E-W). As a result of this composite approach the landscape formation in the study area is described as follows: (1) (Paleogene)-Neogene peneplanation in the basement, (2) Quaternary straight to low sinuosity drainage in the basement, (3) Late Pleistocene to Holocene high-sinuosity drainage in the proximal foreland and (4) Holocene (to Late Pleistocene) meandering fluvial rivers evolving in the distal foreland. In additions to these geomorphological results progress can be made in the geodynamic evolution of the study/source area. Five Proterozoic to Late Paleozoic lithological units are identified: (1) Para-metamorphic rocks, (2) greenstones, (3) basic volcano-sedimentary rocks, (4) sedimentary units abundant in chert and (5) felsic volcanics. By means of these sedimentological finds a more detailed picture of the Variscan orogeny can be presented.

Causes of Variable Shortening and Tectonic Subsidence During Changes in Subduction: Insights From Flexural Thermokinematic Modeling of the Neogene Southern Central Andes (28–30°S)

AbstractThe Andes of western Argentina record spatiotemporal variations in morphology, basin geometry, and structural style that correspond with changes in crustal inheritance and convergent margin dynamics. Above the modern Pampean flat‐slab subduction segment (27–33°S), retroarc shortening generated a fold‐thrust belt and intraforeland basement uplifts that converge north of ∼29°S, providing opportunities to explore the effects of varied deformation and subduction regimes on synorogenic sedimentation. We integrate new detrital zircon U‐Pb and apatite (U‐Th)/He analyses with sequentially restored, flexurally balanced cross sections and thermokinematic models at ∼28.5–30°S to link deformation with resulting uplift, erosion, and basin accumulation histories. Tectonic subsidence, topographic evolution, and thermochronometric cooling records point to (a) shortening and distal foreland basin accumulation at ∼18–16 Ma, (b) thrust belt migration, changes in sediment provenance, and enhanced flexural subsidence from ∼16 to 9 Ma, (c) intraforeland basement deformation, local flexure, and drainage reorganization at ∼12–7 Ma, and (d) out‐of‐sequence shortening and exhumation of foreland basin fill by ∼8–2 Ma. Thrust belt kinematics and the reactivation of basement heterogeneities strongly controlled tectonic load configurations and subsidence patterns. Geo/thermochronological data and model results resolve increased shortening and combined thrust belt and intraforeland basement loading in response to ridge collision and Neogene shallowing of the subducted oceanic slab. Finally, this study demonstrates the utility of integrated flexural thermokinematic and erosion modeling for evaluating the geometries, rates, and potential drivers of retroarc deformation and foreland basin evolution during changes in subduction.

Zircon (U-Th)/(He-Pb) double-dating constraints on the interplay between thrust deformation and foreland basin architecture, Sevier foreland basin, Utah

AbstractThe Cretaceous Cordilleran foreland basin strata exposed in the Book Cliffs of eastern Utah and western Colorado have motivated important concepts linking thrust belt deformation and foreland basin evolution largely on the basis of sequence stratigraphy, stratal architecture, and sediment provenance evolution. However, these methods and approaches generally cannot provide critical insights into the temporal or causal linkages between foreland basin architecture and thrust belt deformation. This is in part due to discrepancies in age resolution and lack of evidence with which to directly couple sediment supply and basin-fill evolution to thrust belt unroofing. New detrital zircon (DZ) geothermochronometric data from Upper Cretaceous proximal to distal foreland basin strata in the Book Cliffs provide new quantitative insights into sediment origin and dispersal in relation to thrust belt deformation and exhumation. Detailed DZ U-Pb and (U-Th)/He double dating reveals that the Book Cliffs foredeep detritus was mainly delivered by transverse routing systems from two major sources: (1) Neoproterozoic and Lower Paleozoic strata from the central Utah Sevier thrust belt, and (2) Permian–Jurassic and synorogenic Cretaceous strata recycled from the frontal part of the thrust belt. A dramatic increase in Sierran magmatic arc and Yavapai-Mazatzal DZ U-Pb ages, as well as Paleozoic DZ He ages, in the deeper marine portions of the foreland basin points to axial fluvial and littoral sediment input from the Sierran magmatic arc and Mogollon highland sources. Both transverse and axial transport systems acted contemporaneously during eastward propagation of the Late Cretaceous thrust belt. DZ He depositional lag time estimates reveal three distinct exhumation pulses in the Sevier thrust belt in the Cenomanian and Campanian. The exhumation pulses correlate with shifts in sediment provenance, dispersal style, and progradation rates in the foreland basin. These new data support conceptual models that temporally and causally link accelerated exhumation and unroofing in the thrust belt to increases in sediment supply and rapid clastic progradation in the foreland basin.

Sequence stratigraphy of the Niobrara Formation: Implications for age-constraining tectonic events and stratigraphic complexities in the Denver-Julesburg Basin, United States

The Upper Cretaceous Niobrara Formation in the Denver-Julesburg (D-J) Basin, United States, records late Turonian–early Campanian transgressive–regressive cycles that governed the rhythmic character of pelagic and detrital sedimentation along the former carbonate ramp of the Western Interior seaway (WIS). The general stratigraphic and architectural complexity of the full Niobrara section has long been recognized; however, fine-scale subdivision of the formation from a sequence stratigraphic aspect has been lacking and is critical for basin-scale analysis. The timing and controls of significant changes in architectural and sediment-accumulation patterns have important implications for the Niobrara petroleum system. As part of this study, we present a sequence stratigraphic framework appropriate for the distal carbonate ramp of the WIS. We project published biozone mapping and radiometric ages into basin-wide well control to establish the timing of chronostratigraphic surfaces. Age-constrained isochore maps reveal a chronology of dynamic paleoseafloor morphology that is critical to understanding the stratigraphic and architectural evolution of the Niobrara. At a regional scale, thickness patterns suggest that the north-south–oriented flexural forebulge in the Sevier foreland basin had migrated eastward to the position of the future Rocky Mountains, a location coincident with the western edge of dominantly pelagic carbonate deposition in the backbulge of the WIS basin. At the D-J Basin scale, chronostratigraphic isochore maps of four third-order sequences, and their finer-scale subintervals, record two broadly distinct periods of depositional influence in the Niobrara. Between ca. 90.0 and ca. 85.7 Ma (upper Turonian–lower Santonian), relatively even deposition was dominated by a pattern of differential sediment accumulation. Systematic reversals of thicks and thins through time are indicative of compensational infilling patterns with no influence of tectonic uplifts along the seafloor. In contrast, from ca. 85.7 to ca. 81.7 Ma (lower Santonian–lower Campanian), a reorganization of earlier architecture was driven by the influence of sublinear uplifts along the trend of the emergent Transcontinental arch. As a result, sediment accumulation was dominated by patterns of draping over the long-lived paleohighs. Absolute timing of the architectural changes of the Niobrara suggests a link among Sevier thrusting events, a migrating flexural forebulge, and uplifts at reactivated basement shear zones in the distal foreland. Thus, both active tectonics and the inherited Proterozoic basement fabric of the WIS influenced the evolution of the Niobrara Formation. This example from the Cretaceous WIS shows that age-constraining sequence stratigraphy for regional isochore mapping can reveal relationships among the structural response to far-field tectonic events, oceanographic variations, and stratigraphic architecture.

Absolute Age and Temperature Constraints on Deformation Along the Basal Décollement of the Jura Fold‐and‐Thrust Belt From Carbonate U‐Pb Dating and Clumped Isotopes

AbstractDuring its late‐stage evolution, the European Alpine orogen witnessed a northwest‐directed propagation of its deformation front along an evaporitic basal décollement into the foreland. This resulted in the decoupling of the northern Alpine Molasse Basin from its basement and the formation of the Jura fold‐and‐thrust belt. Here, we present the first absolute age and temperature constraints on deformation along this basal décollement using carbonate U‐Pb LA‐ICP‐MS dating and clumped isotope thermometry. We analyzed calcite veins associated with a thrust fault branching off from the basal décollement in the distal Molasse Basin and slickenfibers from thrusts and strike‐slip faults in the eastern Jura Mountains. Our U‐Pb data provide evidence for tectonic activity related to Alpine contraction between 14.3 and 4.5 Ma ago. According to the oldest deformation ages, the propagation of Alpine deformation into the distal foreland along the basal décollement occurred earlier than previously inferred by biostratigraphy, at Middle Miocene (Langhian) times at the latest. Younger deformation ages between 11.3 and 4.5 Ma correspond very well in time with shortening in the Subalpine Molasse and the Central Alps, proving simultaneous tectonic activity along both thrust fronts; i.e., the Jura Mountains and the Subalpine Molasse. Clumped isotopes reveal vein calcite precipitation at temperatures between 53 and 104 °C from fluids with oxygen isotope compositions between −6.2‰ and +9.5‰ reflecting distinct burial settings. Combined, our U‐Pb and clumped isotope data show that the burial conditions in the studied area remained constant between 14.3 and 4.5 Ma indicating that large‐scale foreland erosion did not initiate before 4.5 Ma.

Mixed carbonate-siliciclastic sequence development influenced by astronomical forcing on a distal foreland, Miocene Dam Formation, eastern Saudi Arabia

Given increasing importance of tertiary reservoirs in the Middle East, the Miocene (Upper Aquitanian to mid-Langhian) Dam Formation in eastern Saudi Arabia was cored along a 40 km transect, to develop a high-resolution sequence framework, and examine the role played by astronomical forcing, moderate glacio-eustasy (20- to 40-m sea-level changes), and tectonic deformation in this distal foreland setting. The Dam Formation (up to 80 m thick) contains updip siliciclastics which interfinger downdip with marine carbonates. Siliciclastic facies include paleosol-capped, red mudrock (mudflat), green and gray mudrock (near-shore siliciclastic lagoon), rare mud clast–bearing sandstone (fluvial), and massive very fine to medium sandstone (terrestrial sand sheets, rare tidal channels). Carbonate facies include brecciated carbonates (paleosols), lime/clay-clast quartz wackestone (lag gravel), carbonate laminites and microbial heads and mounds (tidal flat and shallow subtidal), argillaceous quartzose marl (near-shore lagoon), peloid mudstone-wackestone (restricted lagoon), variably quartzose skeletal peloid mudstone to packstone (open lagoon), oolitic grainstone (hypersaline beach, tidal channel), mollusk packstone (hypersaline lagoon border), and foram-mollusk packstone-grainstone (seagrass meadows). The Dam Formation contains eight sequences, each of which contains 2 to 4 parasequence sets or individual parasequences. Sequences and individual parasequences are capped by paleosols and/or erosion surfaces, and microbial laminite caps are rare. Bases of parasequences locally are veneered with ravinement gravel lags or siliciclastics beneath shallowing-upward subtidal carbonates, and lack intercalated deeper water facies. Statistical analysis of core-gamma ray (API) vs depth (m) shows that accumulation rates were ~ 3 to 4 cm/kyr, considerably faster than long-term accommodation rates (~ 1.2 cm/kyr), indicating deposition occurred for only 40% of the time, the remainder of the time being non-depositional during many short-term lowered sea levels, evidenced by the coeval oceanic δ18O curve. Spectral analysis of the core-gamma ray logs show astronomical forcing within the eccentricity, obliquity, and precessional bands. Spectral analysis indicates that the eight Dam sequences (average 10 m thick, range < 5 to 18 m) are a mix of long-term obliquity (~ 1.2 Myr) and long-term eccentricity (400 k.y.) cycles. The 3- to 4.4-m sets/parasequences are short term (~ 100 kyr) cycles, and the 0.7- to 1.25-m parasequences are obliquity and precessional cycles. Given the updip position of the study area, repeated glacio-estutatic sea-level changes caused repeated shallow flooding (< 20 m) and emergence, which generated paleosol-bounded or erosionally capped sequences and parasequences. Although the distal foreland was undergoing synsedimentary deformation and slow but locally variable subsidence, which influenced variable thickness of units, astronomically driven glacio-eustasy was the dominant influence on timing of sequence and parasequence development.

Variations of thick-skinned deformation along tumuxiuke thrust in Bachu uplift of Tarim Basin, northwestern China

The curved Tumuxiuke thrust is the northeastern boundary of the Bachu uplift in the Tarim Basin. Based on the interpretation of 2D seismic reflection sections, thick-skinned deformation along the Tumuxiuke structural belt is characterized by monoclinal fault-propagation folds that alternate with push-up structures. The localization and patterns of thick-skinned deformation were controlled by pre-existing normal faults. The forward trishear modeling process further revealed the existence of short-cut thrusts branched from pre-existing normal faults and tested their effect on folding deformation. Monoclines decapitated by low-angle short-cut thrusts coexist with anticlines decapitated by high-angle inverted faults due to the selective inversion of pre-existing normal fault segments. Furthermore, the fault propagation to fault slip ratio (P/S) and the combined effect of multiple propagating thrusts determine the variations in the fold amplification rate and the growth-stratal architecture. The coeval activity of the western Kunlun orogen and the Tumuxiuke thrust in the Cenozoic indicates the far-field transmission of orogenic stress into the distal foreland. Under the approximately N–S-directed compressive stress, inverted structures in nearly W-E-striking segments exhibit monoclinal fault-propagation folds controlled by forethrusts; for the oblique NW-SE-striking segments, the transpressional deformation was partitioned into the frontal contractional deformation zone and the backlimb strike-slip zone.

The Jáchal river cross-section revisited (Andes of Argentina, 30°S): Constraints from the chronology and geometry of neogene synorogenic deposits

In the southern Central Andes, between 27 and 33°S, a flab-slab subduction has been established since the last 10 Ma. This particular subduction geometry has been proposed to control the eastward migration of the deformation, with the Precordillera fold-and-thrust belt development and the uplift of the foreland Sierras Pampeanas basement blocks. In this study, we present a 190 km-long structural section, across the easternmost Cordillera Frontal, the Precordillera and the westernmost sector of the Sierras Pampeanas, and revise both previous kinematic models, and the relationship between the establishment of the flat-slab and deformation in the foreland. For this purpose, we integrate published sedimentological data with a new structural analysis. Our forward kinematic model is fed back by: (i) the geometries of Neogene deformed beds throughout the Precordillera, such as sedimentary wedges and angular unconformities; (ii) flexural subsidence due to the topographic and sedimentary loads for each deformation step, constrained by measured stratigraphic sections; (iii) a revised chronostratigraphy and correlation chart of the Cenozoic sedimentary units; and, (iv) recent constraints on the ages of activity of the faults. This allows us to build a model that considers the timing of deformation for each main structure and its relationship with sedimentation, producing a model with better geometrical, sedimentological and paleogeographical constraints than previous proposals.The results of this modelling are presented in six different evolutionary stages. During the first stage, between 19 and 15 Ma, the eastern Cordillera Frontal is uplifted, associated with subsidence in the Rodeo-Iglesia proximal foreland basin, and low subsidence in the Bermejo distal foreland basin. In the second stage, between 15 and 12 Ma, uplift of the Western Precordillera took place, associated with the creation of a shallow detachment beneath the Rodeo-Iglesia basin. The third stage, between 12 and 8 Ma, was the beginning of the Central Precordillera uplift and the first pulse of uplift in the Valle Fértil range. During the next stage, between 8 and 5 Ma, the Central Precordillera rapidly deformed. Between 5 and 2 Ma, deformation in the Western and Central Precordillera almost ceased, and the focus of contractional deformation shifted to the Eastern Precordillera. The last 2 Ma phase is characterized by strike-slip deformation in the Rodeo-Iglesia basin and contractional deformation in the Eastern Precordillera and the Valle Fértil range.The key results are that the amount of total shortening, 66 km (35%), calculated for the cross-section, is significantly less than previously proposed, and the detachment of the Precordillera fold-and-thrust belt is shallower (~6.5 km vs 16 km) than previous models. This last point allows us to connect the Cordillera Frontal with the Precordillera, with a ramp-plane geometry of the detachment, as previously suggested, but allowing at the same time the subsidence and the generation of the observed geometries in the Rodeo-Iglesia basin.

Astronomically forced sequence development of terrestrial massive sand-sheet, inland sabkha and palustrine units, Lower Miocene Hadrukh Formation, eastern Saudi Arabia

The Lower Miocene (lower Aquitanian) Hadrukh Formation (30–65 m thick), Al-Lidam area, Eastern Province, Saudi Arabia, was cored to examine sequence development within a non-marine siliciclastic-prone coastal plain in a semi-arid distal foreland setting. Facies are dominated by structureless quartz sands (terrestrial sand-sheets/shrub-coppice dunes or nebkhas), along with gypsiferous/calcareous quartz sands and gypsites (inland sabkhas), brecciated and sheet-cracked sandy carbonates/calcareous sands (palustrine), and some coarse lithic-clast quartz sands (fluvial) and associated paleosol-capped red and green-gray mudrock (interfluves). However, they lack the intercalated eolian laminated and cross-bedded sands of this hyper-arid region today, reflecting semi-arid Early Miocene conditions.Spectral analysis of core gamma-ray logs shows astronomical forcing within the eccentricity, obliquity and precessional bands. Tuning indicates the Hadrukh Formation is a composite sequence (~1.3 myr duration) containing three (400 kyr) major sequences and eight minor sequences. Fluvial facies are locally developed at the base and near the top of the composite sequence. Massive sand sheet facies dominate the sequences, and sandy evaporitic facies and palustrine sandy carbonates are best developed in upper parts of sequences but interfinger within short distances with other facies. Subsidence and differential warping caused thickness and facies variation but the succession shows a strong astronomical signal. Astronomical forcing controlled semi-arid vs. more humid phases of deposition, as well as the position of base level, elevation of the ground-water table and presumably density of shrub cover. The three major sequence boundaries developed during 400 kyr eccentricity minima and cooling, while minor sequence boundaries formed during short eccentricity (100 kyr) cooling events.

Late oligocene to quaternary tectonic evolution of the extra andean basins of the pampean plain, Argentina

The extra-Andean Pampean plain of central Argentina shows a complex evolution history. Three basins were identified in the subsurface (Salado, Macachín and Laboulaye) generated during the opening of the South Atlantic Ocean in the Mesozoic with a continuous subsidence history from the Oligocene to the Quaternary.The purpose of this contribution is to analyze the Cenozoic sedimentary filling of the Pampean plain basins with the goal of interpreting the tectonic evolution and understand the subsidence history. The study was based on the correlation and integration of subsurface data. Isopach maps were constructed from boreholes logs for each stratigraphic unit to infer depositional patterns, temporal and spatial thickness distributions, and possible depocenter migrations. The data was integrated in order to analyze the subsidence history of the basins and evaluate the potential mechanisms of post-rift tectonic reactivation of the passive margin.The Cenozoic sedimentary record was divided into three intervals for purposes of analysis with contrasting distribution and mechanisms. The late Oligocene-early Miocene (∼27-19 My) interval started before and continued during the earliest stage of the Andean orogeny; the second interval (early Miocene-middle Miocene 19-15 My) that coexisted with the initial stages of Andean uplift and the third interval (middle Miocene-Quaternary, 15 My-Quaternary) developed during the last stages of the Andean orogeny.The subsidence during the late Oligocene-early Miocene was geographically restricted to the pre-existent basins, so an extensional reactivation of the Mesozoic depocenters is proposed. It is considered to be associated with an Oligocene generalized extension responsible for basin development before the beginning of the Andean orogeny, which later shifted into transpressional conditions. The early Miocene sedimentation (19-15 My) is here interpreted as the result of subsidence during the beginning of the Andean orogeny due to a less common but lengthy process that may lead to foreland rifting or transtension in the three analyzed basins.The middle Miocene-Quaternary (15 My-Present) interval is characterized by a blanket-like subsidence resulting in monotonous thicknesses with reduced-scale accumulation, both in the basins and over the relative highs. This sedimentation is interpreted as the result of accumulation in an extensive distal foreland basin with the foredeep located in the Alvear basin, the forebulge in the Pampa Central block, and the backbulge in the Laboulaye, Macachín and Salado basins. Located far east from the Andean tectonic loads, this distal foreland basin cannot be solely explained by short-wave subsidence patterns. Thus, dynamic subsidence is here proposed as an additional mechanism.

Petroleum tectonic comparison of fold and thrust belts: the Zagros of Iraq and Iran, the Pyrenees of Spain, the Sevier of Western USA and the Beni Sub-Andean of Bolivia

Abstract The genetic analysis of fold and thrust belts is facilitated by tracking the evolution of their organic endowment (petroleum tectonics). Petroleum tectonic analysis of convergent orogenic systems provides an audit of the processes that control the deformation and kinematics of orogenic belts. The distribution and deformation paths of the organic endowment intervals are key factors in determining the petroleum system evolution of fold and thrust belts. This comparison of orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper, mechanical stratigraphy and inherited architecture on the creation, preservation and destruction of petroleum accumulations. The Zagros, Pyrenees, Sevier and Beni Sub-Andean convergent systems share key characteristics of fold and thrust belts, with major differences in scale, degree of incorporation of organic endowment in evolution of the fold and thrust belt and its foreland, and preservation of fold and thrust belt wedge-top deposits. The Zagros is an orogen dominated by flexural processes that is a perfect storm for hydrocarbon generation and preservation. Its multiple stacked sources ensure continuous hydrocarbon generation while stacked detachments foster a low taper and thick wedge-top basins. The Pyrenees is also a flexurally dominated orogen, but the early consumption of its source rocks led to minimal survival of hydrocarbon accumulations during exhumation in a long lasting, high-taper orogenic wedge. The Sevier was initially a flexural orogen that was later dominated by dynamic uplift of the fold and thrust belt and distal foreland subsidence with foreland deformation. The consumption of its pre-orogenic sources during the early low-taper phase indicates a probable robust petroleum system at that time. However, the late high-taper phase exhumed and destroyed much of the early petroleum system. The addition of syntectonic foreland sources to be matured by both local and dynamic subsidence created an additional later set of petroleum systems. Post-orogenic events have left only remnants of world-class petroleum systems. The Beni segment of the Sub-Andean Orogen is a flexural system with probable dynamic overprints. Its most robust petroleum system probably occurred during its early low-taper flexural phase, with dynamic subsidence enhancement. Its late high-taper phase with possible dynamic uplift shuts down and stresses the petroleum systems. Comparison of these orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper kinematics, mechanical stratigraphy, distribution of source rocks relative to shortening and inherited architecture on the creation, preservation and destruction of petroleum accumulations in fold and thrust belts.

Sediment provenance, sediment-dispersal systems, and major arc-magmatic events recorded in the Mexican foreland basin, North-Central and Northeastern Mexico

ABSTRACTThe Late Cretaceous-Paleogene Mexican foreland basin (MFB), defined herein, represents the southern continuation of the late Mesozoic Cordilleran foreland basin. Sandstone petrography, new detrital-zircon (DZ) U-Pb geochronology, and paleocurrent data indicate that much of the sedimentary fill of the basin was derived from an active Late Cretaceous-Paleogene magmatic arc, termed here the Mexican Cordilleran arc, on the western continental margin of Mexico. The oldest known strata of the proximal foreland basin in the Mesa Central consist of Cenomanian-Turonian turbidites. Sampled sandstones are compositional volcanic litharenites with abundant neovolcanic grains and a dominant, approximately syndepositional DZ age group ranging ~98–92 Ma that records a major magmatic event in the Mexican Cordilleran arc. Santonian-Campanian strata in the distal MFB consist of carbonate pelagites with abundant interbedded tuffs and tuffaceous sandstones. Represented by the Caracol and San Felipe formations deposited in the forebulge and back-bulge depozones, respectively, these strata form an arcuate outcrop belt ~700 km in length. DZ ages ranging ~85–74 Ma in the arc-derived tuffaceous strata record a second prominent magmatic event.Two principal transport mechanisms delivered volcanogenic sediment to the MFB from multiple, simultaneously active arc sources during Late Cretaceous time: (1) Cenomanian-Turonian east-directed transverse fluvial systems transported volcanic-lithic sand rich in young zircon grains; and (2) airborne ash clouds transported Santonian-Campanian zircon grains to the distal foreland basin in prevailing Late Cretaceous northwesterly winds. Axial transport of sediment derived from active arc sources, Proterozoic basement and derivative sedimentary rocks in northwestern Mexico, in addition to transverse transport from the thrust orogen itself, represents a younger sediment-routing system, modified by advance of the foreland fold-thrust belt, to the Maastrichtian-Paleogene foreland of northeastern Mexico.

U[sbnd]Pb and Sm[sbnd]Nd constraints on miocene units in the Ischigualasto-Villa Unión foreland basin, Sierras Pampeanas, Argentina: Sedimentary provenance, landscape evolution coupling flat-slab subduction

Foreland basin systems are intimately linked to thrust belts and activation and reactivation of structures in association with convergent plate dynamics, capable of controlling regional uplifts, sedimentation, as well as drainage patterns among other processes. The Ischigualasto-Villa Unión basin (IVB) is a Miocene broken foreland, occurring in-between the Famatina-Sañogasta, Valle Fértil and Maz Ranges in the Western Sierras Pampeanas, Argentina, Central Andes. We conducted U–Pb geochronology by SHRIMP in igneous zircons and LA-ICPMS in detrital zircons as well as whole rock Sm–Nd analyses in the Quebrada del Médano and Desencuentro Formations – the uppermost units of the IVB. The new and compiled data provide new insights on the influence of flat-slab on the uplift of the Sierras Pampeanas and Precordillera and consequent shift of the drainage patterns in the basin infill. The SHRIMP age for the youngest zircons from a volcanic tuff of the middle portion of the Desencuentro Formation yielded a mean 206Pb/238U age of 8.49 ± 0.33 Ma whereas the other grains gave concordant ages from 10 to 1433 Ma. The detrital zircon ages from both Formations indicated that different sourced materials contributed to the basin infill, through time and space. The major age modes in the detrital populations were: 1) 1250-950 Ma, (2) 850-600 Ma, (3) 540-500 Ma, (4) 490-460 Ma, (5) 350-320 Ma, (6) 300-250, (7) 40 Ma and (8) 16-8 Ma. From a statistical point of view, we defined two similar aged groups with distinct major contribution provenance: a first cluster with main Cambrian-Ordovician ages and a second one with main Permian and Mesoproterozoic ages. The bulk picture suggests a critical change in the IVB provenance history since the mid-late Miocene, roughly consistent with the Pampean flat-slab subduction model that controls uplifts of the Western and Eastern Sierras Pampeanas and associated regional drainage shifts in Central Andes, as like geodynamic processes in Western North America. We interpret the statistic change in the distribution of the IVB zircons as product of: (1) the presence of axial rivers flowing parallel to the basement arches in the more distal foreland; (2) a disconnection between the IVB and the Bermejo basin related to the uplift of the IVB western margin; (3) the uprising of the IVB eastern ranges. Our findings are also consistent with the acquired TDM ages (1.2–0.9 Ga) and the coupled εNd(t) constraints (−6 to −5) for samples collected along the stratigraphy of both IVB Formations that indicated recycled Grenville-aged sources were likely overload by sourced rocks with high lithologic erodibility.

Post-collisional deposits in the Zagros foreland basin: Implications for diachronous underthrusting

Detailed sedimentology of the Neogene foreland basin deposits is investigated and classified into 11 lithofacies associations with respect to their paleo-sedimentary environments. The foreland deposits reveal a single coarsening-upward mega-sequence with continuous passage from back-bulge to forebulge, foredeep, and wedge-top sedimentary environments. The Gachsaran deposits form the base of the foreland strata and consist mainly of three different lithofacies associations including fluvial, marine, and sabkha deposits in the eastern Zagros in Fars, and are typically dominated with evaporites toward the west in the Dezful and Kirkuk embayments. The Mishan Formation has three different shallow-marine lithofacies associations in a vertical succession representing foredeep deposits in the eastern Zagros, which tapers toward the Dezful embayment and disappears in Iraq. The Agha Jari distal wedge-top deposits also contain three different lithofacies associations including delta deposits mostly in the Fars, tidal flat deposits in Dezful and Mesopotamia basin, and continental fluvial deposits across the entire Zagros. The uppermost synorogenic Bakhtiari Formation represents proximal wedge-top deposits and consists mainly of two main lithofacies associations including shallow marine and fluvial deposits, within which the fluvial succession is divided into three sub-lithofacies associations with respect to distance from the mountain front and hydraulic power of the river networks. Synthetizing sedimentary facies association with age constraints of the old foreland deposits near the Zagros suture in the High Zagros area suggests that a considerable part of the Arabian plate has been removed at the northern edge by underthrusting and erosion. Moreover, preservation of the young distal foreland deposits near the suture in the western Zagros implies that the magnitude and rate of removal of the proximal foreland deposits have been inconstant along-strike the belt and decreases toward the east.