Local Base Level Research Articles

Response of terrace deposit thickness to climate change and tectonic deformation: An example of the Liyuan River in the Northeast Tibetan Plateau

AbstractThe relationships between tectonics, climate and sedimentation are important for understanding landscape evolution. In order to elucidate these relationships, we conducted a detailed survey of the thickness of terrace deposits and the altitude of terrace surfaces along the Liyuan River valley, in the Northeast (NE) Tibetan Plateau. We have dated the formation ages of terraces T3 and T4 to 42 ± 3.2 and 93.7 ± 7.5 ka, respectively, using optically stimulated luminescence (OSL) method. By compiling the deposit thickness of terraces, palaeoclimate records and local tectonism, we found that variations in precipitation account for the temporal differences in thickness/rates of sediment deposition in terraces T4 and T3. However, the spatial distribution of terrace deposit thickness was determined by tectonism, including faulting and folding, which changed the local base level. In order to maintain a foreland‐ward‐dipping gradient, the Liyuan River adjusts its sedimentary processes via spatially differentiated aggradation.

Morphometry and morphology of fan-shaped landforms in the high-Arctic settings of central Spitsbergen, Svalbard

In this study, the distribution, morphometry and morphology of alluvial fans and colluvial cones were investigated in a high-Arctic, periglacial environment. The main aims were to present their surface morphology types, analyse the relationship between the morphometric properties of fans and their catchments, and to investigate potential factors controlling the morphometry of fans. Data from a set of 297 fan-shaped landforms in central Spitsbergen, Svalbard, were presented and analysed to ascertain factors likely to influence their distribution and character. First, based on the dominant morphological processes, the studied landforms were divided into three groups: fluvial-flow-dominated fans, debris-flow-dominated fans, and colluvial fans. Then, to analyse the relationship between fans and their catchments, 20 variables were investigated. Those analyses indicate that small catchments, characterised by a low relief and high Melton's R number, tend to produce small and steep fans. This tendency was most visible for colluvial fans and debris-flow-dominated fans but less so for fluvial-flow-dominated fans. The latter tend to adjust stronger to the local topography. However, in all cases, these associations were weaker than in morphometry studies based on data from other climatic settings. The most important controls on the distribution of the studied fan-shaped landforms seem to be the availability of accommodation space (related to deglaciation time) and local erosion base level.

The impacts of base level and lithology on fluvial geomorphic evolution at the tectonically active Laohu and Hasi Mountains, northeastern Tibetan Plateau

Previous researches had emphasized tectonic impacts on the fluvial system at the tectonically active areas, while the effects of lithology and local base level change have received relatively rare attention. Here we investigated fluvial landforms at different spatial scales, focusing on knickpoints and channel network reorganization from an area affected by the Haiyuan Fault in the northeastern Tibetan Plateau. The geomorphic indices, i.e., drainage pattern and χ anomalies, were calculated and investigated. The results show that two regional radial drainages formed around the Laohu and Hasi Mountains. Within the interior of the radial drainage, tributaries from the southeast side of the Laohu Mountain experienced near 180° direction change. We interpret this as the gradual drainage capture originating from the height difference (∼190 m) of the local base level between the two catchments. Some tributaries from the Hasi Mountain show alternating gorges and broad valleys controlled by lithology. Besides, tectonic uplift and the lowering of base level (from the incision of the Yellow River) triggered an autogenic positivefeedback transition from parallel to dendritic drainage patterns. These observations suggest that base level change and lithology play a crucial role in landscape evolution, even in a tectonically active region.

Active Deformation Analysis in the Dehdasht Structural Basin Based on Geomorphic Features

The Dehdasht Structural Basin, at the boundary of the southern Dezful Embayment and the Izeh Zone, is bordered by large anticlines with Cretaceous to Oligo-Miocene outcrops above hidden basement faults. The drainage system within the Dehdasht Structural Basin and the surrounding is considerably influenced by growth of the Gachsaran diapiric anticlines and salt extrusions and of the boundary anticlines. The Bangestan anticline in the NW of the basin uplifted 655 m since the Maroon River was diverted. Comparing the uplifts recorded in the topography of the large anticlines which are related to the Mountain Front Fault, to their structural amplitudes shows that the Khaviz anticline on the southern Dehdasht Structural Basin has more recent activity than the Bangestan and the Kuh-e-Siah anticlines. Variations of the local base levels along these anticlines with regards to their recent uplift values suggest 30 to 45% contribution of the thrust faults on their forelimb in the active deformation of the structures bounding the Dehdasht Structural Basin.

New Chronological Constraints from Hypogean Deposits for Late Pliocene to Recent Morphotectonic History of the Alpi Apuane (NW Tuscany, Italy)

A sedimentary sequence of fluvial deposits preserved in the Corchia Cave (Alpi Apuane) provides new chronological constraints for the evolution of the cave system and the timing and rate of uplift of this sector of the Alpi Apuane since the late Pliocene. Supported by magnetostratigraphic analysis performed on fine-grained fluvial deposits, and by radiometric dating of speleothems, we suggest that the deposition of fluvial sediments occurred between ~1.6–1.2 Ma. This implies that the host volume of rock was already located close to the local base level, adding key information about the recent tectonic evolution of the Alpi Apuane. A few before ~1 Ma, an erosive phase occurred due to the base-level lowering, followed by continuous speleothem deposition since at least 0.97 Ma. From that time, Monte Corchia uplifted at a maximum rate of ~0.5 mm/year, which is consistent with isostatic uplift mainly driven by erosional unloading. The petrographical study of the fluvial deposits highlights the presence of material derived from the erosion of rocks that today are absent in the cave’s catchment area, suggesting a different surface morphology during the Early Pleistocene. This study highlights the potential of cave sediments as archives for reconstructing the uplift history of mountain ranges.

Features of terraces and the incision rate along the lower reaches of the Yarlung Zangbo River east of Namche Barwa: Constraints on tectonic uplift

Abstract Along the lower reaches of the Yarlung Zangbo River, scattered alluvium sections appear on T1 and T2 terraces. The alluvial deposits on the T1 terrace in Linduo and Ximogou and the T2 terrace in Guoguotang are composed principally of coarse-grained sand particles and rock fragments, with no observable fine-grained components. The T1 terrace alluvium section is dominated by clay and silt and occurs near the town of Dexing, and optically stimulated luminescence dating of sample from this site revealed an age of 18.2 kyear, which indicates that the incision rate of the Yarlung Zangbo River has been 4.7 mm/year since the formation of this section. On the basis of the component characteristics of terraces in Motuo County, the provenance for the terraces is probably related to the breaking of the palaeo-dammed lakes in the middle reaches of the Yarlung Zangbo River. A 430 m elevation difference still exists between the study area and the local base level downstream of the Yalung Zangbo River (Assam Plain), although this river has a strong incision capability (4.7 mm/year), which suggests that tectonic uplift remains very intense east of the Namche Barwa syntaxis.

Late Cenozoic evolution of the Ariège River valley (Pyrenees) constrained by cosmogenic 26Al/10Be and 10Be/21Ne dating of cave sediments

Despite increasing knowledge on the orogenic phases of the Pyrenees, the Neogene evolution of the range remains poorly constrained. The central Pyrenees, particularly the Ariège River valley and its terrace systems and glacial extensions, are key to reconstructing Pyrenean evolution during the Neogene. However, few terrace relics remain on the piedmont edges. To overcome this limitation and temporally extend the dataset obtained from terraces, we focus on alluvium-filled horizontal epiphreatic passages developed in limestone karstic networks. These landforms record the transient position of former local base levels during the process of valley deepening, similar to fluvial terraces. The alluvium fills of the studied caves in the Tarascon-sur-Ariège area, in the transition zone between the upper range and the piedmont, therefore enable the reconstruction of the geodynamic evolution of the Ariège River valley. All studied caves are developed on at least eight levels. Based on burial durations determined by 26Al/10Be and 10Be/21Ne terrestrial cosmogenic nuclide dating of samples from epiphreatic levels, our results indicate Langhian to Messinian (~13–5 Ma) incision rates averaging 48 m Myr−1. However, the obtained record does not allow us to accurately retrace the Pliocene evolution of the area due to the lack of known caves at corresponding levels. Moreover, raised local base levels during glacial phases both make the record more complex and call into question the methodological approach in terms of potential internal sediment remobilization and mixing related to implied re-flooding periods.

Coupling of tectonic factors and precipitation variability as a driver of Late Quaternary aggradation in Northeast Brazil

AbstractThe coupling between tectonic factors and climate processes is a key element in understanding landscape evolution. However, few studies address this issue in the context of unglaciated tropical areas in passive continental margins. Thus, this research aimed to understand the origins and evolution of the geomorphological landscape in the eastern sector of Northeast Brazil along the Late Quaternary, between the Last Interstadial (Marine Isotopic State 3) and the Upper Holocene. The morphostratigraphic approach, coupled with sedimentological analyses and optically stimulated luminescence dating of sediments, showed that the depositional record stored in the landscape ranges from at least 60 000 years BP until the Upper Holocene. Depositional intervals suggest that there were moments of climatic instability followed by moments of relative geomorphological quiescence. Such active moments coincide chronologically with Heinrich and Dansgaard‐Oeschger events, which are likely linked to changes in paleo‐pluviosity that might respond to the dismantling of elluvial covers and colluvial deposition in the area. Likewise, reworking of hillslope materials led to increased deposition in the fluvial realm. In addition, such deposits might have been affected by the neotectonic complexity of the area, responsible for the creation of non‐concatenated accommodation spaces, indicating a dynamics of uplifting and subsidence of blocks typical of passive margin taphrogenic environments. Therefore, the formative processes that led to the Late Quaternary deposition of sediments in a platform margin context reflect a coupling between tectonic and climatic factors. The former, driven by precipitation variability on a regional scale, triggered fluvial and high‐energy hillslope processes that resulted in ubiquitous valley floor and piedmont aggradation, whereas the latter led to the dismantling of local base levels, transforming depositional units into new denudational landforms subject to the current climate dynamics. © 2020 John Wiley & Sons, Ltd.

Coarse sediment dynamics in a large glaciated river system: Holocene history and storage dynamics dictate contemporary climate sensitivity

Abstract The gravel-bedded White River drains a 1279 km2 basin in Washington State, with lowlands sculpted by continental glaciation and headwaters on an actively glaciated stratovolcano. Chronic aggradation along an alluvial fan near the river’s mouth has progressively reduced flood conveyance. In order to better understand how forecasted climate change may influence coarse sediment delivery and aggradation rates in this lowland depositional setting, we assessed the contemporary delivery and routing of coarse sediment through the watershed; this assessment was based on a rich set of topographic, sedimentologic, and hydrologic data from the past century, with a focus on repeat high-resolution topographic surveys from the past decade. We found that most of the lower river’s contemporary bed-load flux originates from persistent erosion of alluvial deposits in the lower watershed. This erosion is a response to a drop in local base level caused by a major avulsion across the fan in 1906 and then augmented by subsequent dredging. The 1906 avulsion and modern disequilibrium valley profiles reflect landscape conditioning by continental glaciation and a massive mid-Holocene lahar. In the proglacial headwaters, infrequent large sediment pulses have accomplished most of the observed coarse sediment export, with exported material blanketing downstream valley floors; during typical floods, transported bed material is largely sourced from erosion of these valley floor floods. Throughout the watershed, we observe decadal-scale coarse sediment dynamics strongly related to the filling or emptying of valley-scale sediment storage over 102–104 yr time scales, often in response to major disturbances that either emplace large deposits or influence their redistribution. Paraglacial responses in large watersheds are suggested to be inherently complicated and punctuated as a result of internal landform interactions and stochastic/threshold-dependent events. We argue that, in combination, Holocene disturbance, storage dynamics, and human flow modification make coarse sediment fluxes in the lower White River relatively insensitive to decadal climate variability. Results highlight the degree to which river sensitivity to contemporary disturbance, climatic or otherwise, may be contingent on local and idiosyncratic watershed histories, underscoring the need to unpack those histories while demonstrating the utility of watershed-scale high-resolution topography toward that end.

An Idealized Landslide Failure Surface and Its Impacts on the Traveling Paths

Numerical scenario simulation may serve as an efficient and powerful tool for hazard assessment, but it often suffers from the lack of a definite failure surface before the occurrence of failure. In the present study, an idealized curved surface (ICS) is proposed for mimicking the sliding surface in the numerical simulation. Different from the conventional Sloping Local Base Level (SLBL) or Scoops3D method, this idealized surface consists of two curvatures, which are defined in the down-slope and cross-slope directions, respectively. Applying this idealized surface to 45 historical landslides of sliding type in southern Taiwan, two specific relations of geometry (length, curvature radius and depth) in the down-slope and cross-slope directions are figured out. These specific relations simplify the complexity of constructing the idealized curved surface for areas prone to landslides for the sake of hazard assessments. That is, once the area with landslide susceptibility is identified and the associated released volume is given, the idealized sliding surface can be uniquely determined with the help of these geometric relations. The proposed method is integrated with a two-phase grain-fluid model and numerically validated against a historical large-scale landslide for investigating its feasibility. Although the idealized failure surface may deviate from that determined by the post-event investigation, it is interesting to note that the major discrepancy is mainly found at the first stage. The differences (both flow thickness and paths) reduce over time, and only minor discrepancies can be identified at the deposition stage. These findings reveal the weak co-relation between the geometry of the failure surface and the flow paths. It also indicates the feasibility of the ICS for predicting the flow paths by scenario investigation, especially when the exact sliding surface of a landslide is not available.

Fluvial knickpoint identification and their characterizations in the drainage basins of Western Ghats, India

The fluvial knickzones, a geomorphic marker of erosion, climate, and tectonics has received much interest in the topographic developmental studies in the recent past. Nevertheless, examining their spatial distribution in passive margins has not been well documented especially for the drainage basins in the humid tropical climatic systems. Here in this study, we presented the spatial distribution of knickzones in the three river basins of Western Ghats, India, namely Chaliyar, Pambar and Muthirapuzha basins. Because the conventional method of knickzone identification using topographic maps or field readings is laborious, we employed a 30 m digital elevation model and TopoToolBox, a Matlab© based tool to extract the knickpoints in the study area. A total of 356 knickpoints were identified by analyzing ~ 2670 km of river network, and the knickzone frequency is estimated as 0.13 km−1. The average height of the knickzones is found to be 83 m, and majority of the knickpoints are located closest to the headwaters. Although we noticed that the streams underlying metamorphic lithology tends to have the highest knickzone frequency, but it can be also attributed to the fact that metamorphic rocks are the most dominant rock type in the study area. The high amount of knickzone frequency in the study area thus may be the result of coupled climatic control caused by heavy summer rainfall, bed rock erosion triggered by high grade metamorphism, and also the local base level changes as indicated by the clustered knickpoints in the stream heads.

80,000 years of geophysical stratigraphic record at the Serra da Capivara National Park, in northeastern Brazil: Uncovering hidden deposits and landforms at a canyon's floor

Acquisition of high-resolution geophysical spatial data at the Serra Branca canyon, in the Serra da Capivara National park, in the semiarid of the Northeastern Brazil, enables depicting deposits and relict landforms that have never been reported before for the area. Ground Penetrating Radar (GPR) and GPS real-time kinematics (RTK) enable uncovering wide cut-and-fill structures, a relict perched river channel, relatively well-stratified alluvial deposits and two relict pediment surfaces that border the current canyon's thalweg. Optically Stimulated Luminescence (OSL) geochronology shows that the stratigraphic archive tends to fit important global climatic markers of the last glacial cycle. The relict river channel was active at least from marine isotopic stage (MIS) 5a to MIS 2, showing low sedimentation rates during about 60,000 years. Between the last glacial maximum (LGM) and the Holocene, the paleochannel was abandoned. The canyon's floor was incised, and the current thalweg was carved out, probably following a local baselevel drop. Well-structured alluvial deposits have accumulated near the outlet of the canyon since at least the Early Holocene. During the Middle Holocene, colluvial and alluvial deposits accumulated in sites near the cliffs that border the canyon. The study's geochronology coincides with the pace of independent paleoclimatic proxies from Northeastern Brazil, suggesting that a humid millennial drift was associated with the incision of local bedrocks and accumulation of alluvial and colluvial deposits during the Early and the Middle Holocene. Abandonment of the Pleistocene river channel, incision of the local baselevel, and creation of the canyon's current thalweg were probably caused by a Late Glacial humid spell that may coincide with Heinrich event 1 (H1), about 16,000 years ago. However, complementary geochronologic and sedimentologic data are crucial for improving knowledge about this major drainage system change at the study site.

Cosmogenic nuclide dating of cave sediments in the Eastern Alps and implications for erosion rates

Karstic caves are created by water eroding and corroding rocks that can be dissolved. Since both the spring areas of caves (normally at the valley bottom) as well as the recharge is controlled by superficial processes, the morphology of the cave bears strong links to these influences. Lowering of local base levels promotes the development of horizontal phreatic cave passages at progressively lower elevations, resulting in the formation of multi-level karst systems. Upon the next lowering of base level, these upper systems become fossilized, and sediment trapped within them may remain preserved for millions of years. Dating these sediments gives clues regarding the time when the passages were last active, and thus may yield age information for old valley floors. The present paper presents cosmogenic nuclide datings of twelve samples from eight caves in the central part of the Northern Calcareous Alps of Austria. Besides three samples that gave no results, most of the obtained ages are at the Mio-Pliocene boundary or within the Pliocene, as was expected before sampling. No multi-level caves could be sampled at different elevations, thus, the obtained valley deepening rates are averages between the age of sediment deposition and the present-day valley floor. However, the valley deepening rates of 0.12 to 0.21 km/Ma are in accordance to previous findings and corroborate a comparatively slow evolution of base level lowering in the Eastern Alps compared to the fast (Late Quaternary) evolution in the Central and Western Alps.

Small catchments evolution on clayey hilly landscapes in Central Apennines and northern Sicily (Italy) since the Late Pleistocene

The study aims to define a possible evolutionary geomorphological model of small catchments (i.e., <40 km2) that characterise the clayey hilly landscape of Central Apennines and Northern Sicily piedmont for the late Pleistocene–Holocene (i.e., the last 20,000 years, from the last glacial stage to the Holocene climate amelioration). The study is based on an integrated approach incorporating (i) geomorphological surveys and mapping, (ii) dating of Quaternary continental deposits, and (iii) topographical and morphometrical processing. It combines the data collected from previous published investigations as well as new data, specifically related to the availability of geo-chronological markers, and helps outline a common evolutionary model. The selected small catchments are tributaries of major rivers in inland areas (small catchments of the inland hills of Tuscany, Marche, and Sicily) or flow directly to the coast (coastal small catchments in Abruzzo). For each area, the geomorphological features (clay dominated bedrock, erosional landforms and fluvial terraces, and erosional/depositional strath terraces) and the dating of Quaternary deposits (from <50kyr to 15kyr) were compared, reconstructing the morphometry of the probable late Pleistocene landscape configuration. The analysis of the results showed that, although currently characterised by different climatic and geo-structural conditions, the different basins underwent a common geomorphological evolution mostly since the late Pleistocene. During the last glacial stage, under cold climate rhexistasy conditions, the small catchments were dominated by low gradient erosional surfaces with the deposition of fluvial, colluvial, or slope deposits, resulting in the formation of uniform slopes and wide minor valleys. The Holocene climate warming, together with marine transgression and tectonic uplift, induced the incision and dissection of the erosional surfaces and the continental deposits via gravitational movements and rapid erosion processes up until the present-day landscape configuration. The control factors of this evolution are most likely linked to the climate changes at the beginning of the Holocene and the interplay with the changes in the local base level of the small catchments since the late Pleistocene, combining late Quaternary tectonic uplift, sea-level rise, and river/coastal incision.

Drainage and erosion of Cambodia’s great lake in the middle-late Holocene: The combined role of climatic drying, base-level fall and river capture

We provide evidence for a large-scale geomorphic event in Cambodia’s great lake, the Tonlé Sap, during the middle Holocene. The present-day hydrology of the basin is dominated by an annual flood pulse where water from the Mekong River raises the lake level by c. 8 m during the monsoon season. We present new subsurface geophysical data, allied to new and past core studies, which unequivocally show a period of major mid-Holocene erosion across the entire Tonlé Sap basin that is coincident with establishment of the lake’s flood pulse. We argue that this widespread erosion, which removed at least 1.2 m of sediment across the lake’s extent, was triggered by up to three, likely interacting, processes: (1) base-level lowering due to mid-Holocene sea-level fall, leading to (2) capture of the Tonlé Sap drainage by the Mekong River, and (3) a drying climate that also reduced lake level. Longer-term landscape evolution was thus punctuated by a rapid, river capture- and base-level fall- induced, lake drainage that established the ecosystem that flourishes today. The scale of change induced by this mid-Holocene river capture event demonstrates the susceptibility of the Tonlé Sap lake to ongoing changes in local base-level and hydrology induced by anthropogenic activity, such as damming and sand mining, within the Mekong River Basin.

The Zeyi Cave Geosite in Northern Ethiopia

Despite the high geoheritage value of caves and karsts, northern Ethiopia’s largest cave at Zeyi (13.5586°N, 39.1454°E) in the Dogu’a Tembien district has received little attention so far. We have studied its geological, geomorphic, socio-cultural and historical dimensions in a holistic way. The basal member of the Antalo Limestone, in which the Zeyi cave is located, consists of grainstone and wackestone with subordinate marly interlayers. Over a length of 364 m, the oval-shaped gallery displays stalagmites, stalactites, five columns, dissolution holes (“bell-holes”) following joints, stalagmitic floors and other concretions or speleothems. In the absence of any dating of the cave, we contrasted its elevation above the current local base level with known average incision rates of the northern Ethiopian highlands to reconstruct its age, which was calculated as at least 2 to 4 million years. The palaeo-environmental information that is archived in the Zeyi cave sediment would hence cover the Pleistocene. The graves in the sediment at the bottom of the Zeyi cave further indicate that the place could be an ancient burial site, which gives scope for archaeological research. Zeyi boosts a unique combination of abiotic, biotic and cultural components: the nineteenth c. church under the overhanging cliff; the unique cave; the speleothems, cliffs and gorges; and the cave’s bat colony which has been genetically confirmed to be composed of three syntopically roosting species. Accounting for a good balance between cave research, community-based geotourism, geoconservation and biodiversity conservation, the Zeyi cave has strong credentials to become a top geotouristic site in northern Ethiopia. However, major work needs to be done, including granting access for women and organising community-based geotourism.

A review of methods used to estimate initial landslide failure surface depths and volumes

This paper presents an overview of existing methods developed to characterize initial landslide failure surfaces and compute volumes based on surface information, when it is buried. This paper reviews the simplest models, such as volume estimation based on ellipsoids, and the information that can be obtained from surface displacement and morphology. A simple method to define the volume of a rockslide using the identified discontinuities is also outlined. The sloping local base level (SLBL), which is able to automatically obtain a possible failure surface on the basis of a digital elevation model (DEM), is described. The balanced cross section methods applied to landslides are illustrated and can give the depth of the failure surface. Using predefined assumptions, such as vector displacement parallel to the failure surface, we can obtain an approximation of the failure surface either along translational or circular slides, and this method can be improved by adding a mass balance equation. Recent work has also shown that using an inverse model, a mass balance equation and assumptions about the rheology enables estimation of the failure surface.These methods are illustrated with examples, and they are applied to case studies and compared when the data available makes it possible. The advantages and disadvantages of the methods are discussed. This paper shows that the use high-resolution topography and interpretation of the morphology is underexploited to extract information about failure surfaces geometry.

Plant trait-based analysis reveals greater focus needed for mid-channel bar downstream from the Three Gorges Dam of the Yangtze River

Abstract As a transitional zone between terrestrial and aquatic ecosystems, riparian ecosystems are critical in maintaining the health of river-ways and watersheds, and particularly sensitive to changes in hydrological regimes. Effects of dams on riparian zone are widely addressed, but less attention focus on riparian plant community of bar habitats. The Jingjiang reach downstream of the Three Gorges Dam has undergone significant changes of water and sediment condition and local base-level due to the Three Gorges Dam. To address responses of riparian plant communities to dams, the riparian plant communities of the mid-channel bars and the point bars in the Jingjiang reach were examined with a trait-based analysis. The results showed a lower species richness and a higher risk of invading by invasive species for mid-channel bars comparing with point bars. Moreover, the species richness of both the mid-channel bars and the point bars were observed increasing with the distance increasing from the dam, but the increasing rate of mid-channel bars was much smaller than that of point bar. Furthermore, functional redundancy of the mid-channel bar represented by the difference between functional diversity and species richness was marked lower than that of the point bars. Thus, we conclude that affected by the regulation of the dam, the mid-channel bars seem to be more hydrologically susceptible and harsher habitats for the assembly of plant community comparing with point bars. Therefore, more research of mid-channel bar downstream the dam were suggest and different emphasis of research and protecting management are needed for different bar types.

Sediment routing system and sink preservation during the post-orogenic evolution of a retro-foreland basin: The case example of the North Pyrenean (Aquitaine, Bay of Biscay) Basins

Abstract We investigated here the evolution of the sediment routing system, i.e. the sediment transport and deposition evolution along successive depositional topographies and environments, and the sink (i.e. deposited sediments) preservation in a foreland basin from the period of mountain belt shortening to its post-orogenic stage. The studied system is the North Pyrenean retro-foreland basin from 50 Ma to today which is, composed of a subsiding platform (the Aquitaine Basin) fed by the erosion of the Pyrenees passing laterally to a slope and a deep-sea plain (the Bay of Biscay deep basin), the ultimate area of deposition. This study is based on a double seismic stratigraphic and structural analysis of an extensive seismic dataset and on an age model of the sediments combining biostratigraphy, orbitostratigraphy and sequence stratigraphy with a time resolution of 0.1 Ma. Four major periods of deformation corresponding to a single or a set of stratigraphic sequence boundaries were characterized. The Pyrenean shortening decrease may be recorded by a basin-scale uplift at 49.8 Ma (Late Ypresian). The paroxysm of the piggy-back shortening and related uplift is dated at 35.8 Ma (Priabonian). The end of the Pyrenees shortening (transition to post-orogenic conditions) is well dated between 27.1 and 25.2 Ma (Chattian). A major West European scale deformation of possible mantle origin uplifted the basin from 16.4 to 10.6 Ma (Late Burdigalian to Early Tortonian). These major periods of deformations controlled the sink preservation at the first order through the ratio between the accommodation space created by subsidence on the Aquitaine platform (Asub) and the siliciclastic sediment supply coming from the erosion of the Pyrenees (Ssc). A general model is proposed. At the time of the foreland basin when ΔAsub ≤ ΔSsc, most of the sediments are preserved on the platform as a progradational-aggradational wedge (up to 25.2 Ma here). At the time of the post-foreland evolution when ΔAsub The continental sediment routing system is mainly provided along nearly flat alluvial plains with extensive lakes and/or humid zones, i.e. the local base levels of the alluvial fans. The Aquitaine retro-foreland was never overfilled.

Reconciling the stratigraphy and depositional history of the Lycian orogen-top basins, SW Anatolia

Terrestrial fossil records from the SW Anatolian basins are crucial both for regional correlations and palaeoenvironmental reconstructions. By reassessing biostratigraphic constraints and incorporating new fossil data, we calibrated and reconstructed the late Neogene and Quaternary palaeoenvironments within a regional palaeogeographical framework. The culmination of the Taurides in SW Anatolia was followed by a regional crustal extension from the late Tortonian onwards that created a broad array of NE-trending orogen-top basins with synchronic associations of alluvial fan, fluvial and lacustrine deposits. The terrestrial basins are superimposed on the upper Burdigalian marine units with a c. 7 myr of hiatus that corresponds to a shift from regional shortening to extension. The initial infill of these basins is documented by a transition from marginal alluvial fans and axial fluvial systems into central shallow-perennial lakes coinciding with a climatic shift from warm/humid to arid conditions. The basal alluvial fan deposits abound in fossil macro-mammals of an early Turolian (MN11–12; late Tortonian) age. The Pliocene epoch in the region was punctuated by subhumid/humid conditions resulting in a rise of local base levels and expansion of lakes as evidenced by marsh-swamp deposits containing diverse fossil mammal assemblages indicating late Ruscinian (late MN15; late Zanclean) age. A second pulse of extension, accompanied by regional climatic changes, prompted subsequent deepening of the lakes as manifested by thick and laterally extensive carbonate successions. These lakes, which prevailed c. 1 myr, later shrank due to renewed progradation of alluvial fans and eventually filled up and dried out, reflected by marsh-swamp deposits at the top of a complete lacustrine succession that contains diverse micro-mammal assemblages indicating a latest Villanyian (MN17; Gelasian) age. A third pulse of tectonic reorganisation and associated extension dissected the basins into their present-day configuration from the early Pleistocene onwards under warm/humid climatic conditions. The new age data provide means to correlate deposits across various basins in the region that help to place the basin development into a regional tectonic framework, which can be attributed to the consequence of the well-articulated regional phenomena of slab-tear/detachment-induced uplift followed by crustal extension and basin formation (late Tortonian), the outward extension of the Aegean arc (early Pliocene) and eventually accompanied by westward extrusion of the Anatolian Plate (early Pleistocene).