Base-level Changes Research Articles

Unraveling the tectonic and climatic influences on River Erosion: A late Quaternary case from the Dongda River, northeast tibet

Rivers adjust their equilibrium profiles to base-level changes induced both by climatic fluctuations and tectonic movements. It is crucial, yet challenging, to differentiate their distinct roles in shaping the evolution of fluvial systems when both forces are involved. In contrast to the broader, regional-scale impact of climatic changes, the various tectonic deformation at local-scale can be recorded as various terrace deformation patterns. The Dongda River, located on the northeastern edge of the Tibetan Plateau, crosses active folds and thrusts, providing a unique opportunity to isolate the effects of climatic and tectonic forces. In this study, we identified five river terraces along Dongda River, designated as T1, T2, T3, T4a, and T4b, from lowest to highest. Radiocarbon and OSL dating results reveal abandonment ages of 4.2 ± 0.3 ka, 6.1 ± 0.5 ka, 12.4 ± 2.5 ka, 16.4 ± 0.2 ka, and 27.4 ± 2.5 ka, showing rhythmic alignment with climatic fluctuations since the late Pleistocene. Elevation profiles of the terraces demonstrate base-level aggradation at the footwall of the Fengle fault between T4a/b and T2, highlighting a tectonically dominated process of river incision at its hanging wall. Differential uplift rates were estimated to be 2.2 ± 0.3 mm/a at the North Qilian Shan hinterland, 1.1 ± 0.2 mm/a in the Huangcheng Basin, 0.2–0.4 mm/a in the Yangxiang Basin, and 1.1 ± 0.2 mm/a within the foreland fold-and-thrust belt of Dahuang Shan, respectively. In contrast, an accelerated incision rate (>1.8 mm/a) has been observed throughout the entire basin since the abandonment of T2, coinciding with the adjustments in the Asian monsoon and westerlies systems since the Mid-Holocene. This study provides insights into the complex interplay between climatic variability and tectonic activity in fluvial systems, enhancing our understanding of how these competing forces have shaped the evolution of the fluvial landscape over time.

The interplay of lithology, tectonics and climate in the morphology of Corsica

The morphology of the landscape on the island of Corsica in the western Mediterranean Sea is transient with a clear topographic asymmetry. The geology of the island can be divided into two units – Hercynian Corsica and Alpine Corsica – with contrasting lithology, climate and structural controls between them. We used topographic analysis to assess the influence of each of these factors on landscape form. Disequilibrium in the landscape is illustrated by analysing χ and Gilbert metrics, which indicate active migration of the drainage divide, along with a new metric that quantifies tributary heights above the main stem channel. The climate in the region has fluctuated dramatically from the Messinian salinity crisis through to Quaternary glaciation. However, analysis of the size and distribution of knickpoints across the island does not reflect specific base-level changes. Instead, the basement structures control valley orientation and the conditioning of the landscape by glaciation exerts strong controls on drainage evolution.

Tectonic controls on the morphometry of alluvial fans in an arid region, northeast Iran

ABSTRACT The morphology of alluvial fans provides valuable information about changes in tectonic, climate, base level, and drainage basin characteristics. This study aims to evaluate the relationship between active tectonics and morphometric characteristics of alluvial fans in the less studied southern region of the Joghatay mountain, northeast Iran. The study area includes 13 alluvial fans and their corresponding catchment basins in a region with semi-arid climate. Morphometric characteristics of alluvial fans and their corresponding drainage basin including fan area, fan slope, sweep angle, fan toe length, fan radius, basin area, basin slope, fan’s width-to-length ratio, asymmetry factor, and basin shape were measured. Our results revealed a negative relationship between the slope of alluvial fans and their corresponding basin area. The weak correlation between the drainage basin area and the fans area may be attributed to the lithology and active tectonics in the study area. The small sweep angle values of the studied alluvial fans can be attributed to the activity of thrust faults and presence of an uplifting anticline that resulted in formation of elongated alluvial fans in our study area. Analysis of the relationship between the morphometric characteristics of the fans and their corresponding basins indicated a negative correlation, which can be attributed to the sediment transport efficiency of the basins.

Influence of the last (de)glaciation on a complex cave system: Grønli-Seter cave system, Northern Norway

Cave studies enable us to unravel valuable information on landscape evolution. In glacial settings, the glacial cycles induced fluctuations in the underground hydraulic regime, sediment availability and base level in adjacent caves. Here, we present our work on the Grønli-Seter cave system in Northern Norway, which comprises >8 km of passage length at an elevation between 250 m and present sea level. By linking cave morphology and deposits to shifting glacier configurations and external base level changes during the last deglaciation and the Early Holocene, we establish how the hydraulic conditions in the cave system shifted in pace with glacier fluctuations. Maze sections and phreatic loops appear in a hydraulic confined setting close to the interface between the calcite marble and the overlying mica schist. Scallops in the cave walls demonstrate slow, ascending water flow, evidence of subglacial speleogenesis under wet-based, topographically constrained glaciers with a gentle surface slope. Moreover, cave morphology and clastic deposits indicate various hydrological phases of descending water flow, stagnant conditions, and excessive flow rates. Our work establishes that the last glaciation and deglaciation had an insignificant effect on the cave system's solutional development. In contrast, the cave sediments reveal diverse hydraulic conditions that may be related to shifting glacier configurations. This research offers valuable insights into glacier ice-contact speleogenesis and landscape development in glacial terrains.

Reservoir identification of marine facies carbonate shoals in the Middle Permian in the Sichuan Basin

Controlled by the base level change cycle, the carbonates exposed from the periodically shallowed water body at the top of the high-frequency sequence develops multiple sets of thin reservoirs. Such reservoir identification has always been tricky in the prediction of carbonate reservoirs. Based on the Middle Permian strata from the Sichuan Basin, this paper established a forecasting approach of high-frequency-sequence-controlled carbonate thin reservoir. The steps are as follows: (1) Combine the measured results of samples from the typical outcrops with changes of carbon and oxygen isotopes, thus to construct a division program for the 3rd-order sequence of the Middle Permian strata. (2) Compare the carbon and oxygen isotope changes and the global sea level change trend, determine the controlling factors of the 3rd-order sequences. (3) Carry out spectrum analysis based on the natural gamma energy spectrum logging (ln (Th/K)) curves of Wells Long-17 and Anping-1 to establish a high-frequency sequence framework division program, and then apply the logging curve characteristics to correct the top interface positions of the high-frequency sequences, which is the potential location of carbonate reservoir. The research results suggest that the Middle Permian in the Sichuan Basin can be divided into three 3rd-order sequences, which are PSQ1, PSQ2 and PSQ3 from bottom to top. Among which, PSQ1 is mainly affected by global sea level (GSL) changes, and the changes originated from its tuning curve can be directly applied to the division of high-frequency sequences and reservoir prediction. PSQ2 and PSQ3 are jointly controlled by global sea level changes and regional tectonics. In cope with the influence of regional tectonics, the positions of the four-level sequence interface established according to the tuning curve need to be fully considered, and good results were achieved through systematical correction. This method can effectively reduce the risk of reservoir prediction caused by marine carbonate heterogeneity, and provide scientific foundation for the prediction of high-frequency cyclic reservoirs of platform facies and platform margin subfacies in other marine carbonate sedimentary basins.

Integrated uplift, subsidence, erosion and deposition in a tightly coupled source‐to‐sink system, Pagliara basin, northeastern Sicily, Italy

AbstractHow tectonic forcing, expressed as base level change, is encoded in the stratigraphic and geomorphic records of coupled source‐to‐sink systems remains uncertain. Using sedimentological, geochronological and geomorphic approaches, we describe the relationship between transient topographic change and sediment deposition for a low‐storage system forced by rapid rock uplift. We present five new luminescence ages and two terrestrial cosmogenic nuclide paleo‐erosion rates for the late Pleistocene Pagliara fan‐delta complex and we model corresponding base level fall history and erosion of the source catchment located on the Ionian flank of the Peloritani Mountains (NE‐Sicily, Italy). The Pagliara delta complex is part of the broader Messina Gravel‐and‐Sands lithostratigraphic unit that outcrops along the Peloritani coastal belt as extensional basins have been recently inverted by both normal faults and regional uplift at the Messina Straits. The deltas exposed at the mouth of the Pagliara River have constructional tops at ca. 300 m a.s.l. and onlap steeply east‐dipping bedrock at the coast to thickness between ca. 100 and 200 m. Five infrared‐stimulated luminescence (IRSL) ages collected from the delta range in age from ca. 327 to 208 ka and indicate a vertical long‐term sediment accumulation rate as rapid as ca. 2.2 cm/yr during MIS 7. Two cosmogenic 10Be concentrations measured in samples of delta sediment indicate paleo‐erosion rates during MIS 8–7 near or slightly higher than the modern rates of ca. 1 mm/yr. Linear inversion of Pagliara fluvial topography indicates an unsteady base level fall history in phase with eustasy that is superimposed on a longer, tectonically driven trend that doubled in rate from ca. 0.95 to 1.8 mm/yr in the past 150 ky. The combination of footwall uplift rate and eustasy determines the accommodation space history to trap the fan‐deltas at the Peloritani coast in hanging wall basins, which are now inverted, uplifted and exposed hundreds of metres above the sea level.

Age-depth model for uppermost Ndutu Beds constrains Middle Stone Age technology and climate-induced paleoenvironmental changes at Olduvai Gorge (Tanzania)

Olduvai Gorge in northern Tanzania is part of a globally important archeological and paleoanthropological World Heritage Site location critical to our understanding of modern human evolution. The Ndutu Beds in the upper part of the geological sequence at Olduvai Gorge represent the oldest unit to yield modern Homo sapiens skeletal material and Middle Stone Age technology. However, the timing of the deposition of the Ndutu Beds is poorly constrained at present, which limits our understanding of the paleoenvironments critical for contextualizing H. sapiens and related technologies in the Olduvai Basin. Using a suite of 15 luminescence ages of sedimentary core samples, combined with Bayesian statistics, this study provides a new higher-resolution age-depth model for the deposition of the uppermost Upper Ndutu and Naisiuiu Beds cored by the Olduvai Gorge Coring Project. The luminescence and modeled ages are presented as ±1 σ uncertainties. The Ndutu Beds intersected by the Olduvai Gorge Coring Project cores are dated to between 117.1 ± 17.9 and 45.3 ± 4.2 ka (between 125.9 ± 26.5 and 45.8 ± 8.2 ka modeled ages), while a probable overlying layer of Naisiusiu Beds dates to 23.7 ± 10.9 to 12.1 ± 1.7 ka (25.7 ± 18.9 ka and 12.0 ± 3.4 ka modeled age). Time-averaged accretion rates are derived during this time: (1) initially low rates (<5 cm ka−1) from the bottom of the core at 117.1 ± 17.9 ka up to 95.3 ± 11.1 ka (125.9 ± 26.5 to 95.5 ± 23.3 ka modeled ages); (2) the middle section spanning between 95.3 ± 11.1 and 62.7 ± 5.7 ka (95.5 ± 23.3 to 61.9 ± 10.4 ka modeled ages) with mean rates above 15 cm ka−1; and (3) the last 62.7 ± 5.7 ka (61.9 ± 10.4 ka modeled age) where the accretion rate reduces to below 5 cm ka−1. This reduction can be explained by the evolution of the gorge system that was likely driven by subsidence of the Olbalbal depression and changes in climate, particularly precipitation and resulting lake and base level changes. Older Upper Ndutu and Lower Ndutu Beds are contained within proto-gorges within the modern gorge system.

Characterization of tight sandstone and sedimentary facies using well logs and seismic inversion in lacustrine gravity-flow deposits

Lacustrine gravity-flow deposits are common in sedimentary basins, and sand bodies formed in the basin center mingled with organic rich mudstone may form excellent tight reservoirs of hydrocarbons. However, the complex architecture and intensive heterogeneities of lacustrine gravity-flow deposits commonly pose great risks for hydrocarbon exploration and development. To address this problem, a new workflow for predicting tight sandstone is proposed in this work. The workflow combines well log analysis, intelligent seismic inversion and 3D geological modelling to improve the prediction effectiveness of tight sandstone and sedimentary facies. The original 3D seismic data was firstly decomposed into three components with low, middle and high frequencies to improve the resolution of seismic inversion, and a nonlinear relationship between well logs and the decomposed 3D seismic data was then established by using an algorithm of support vector machine. A 3D lithofacies model was subsequently established integrating well logs and the seismic inversion result. The distribution of sand thickness was imaged relying on the lithofacies model. Correlations between the actual and predicted sand thickness based on the seismic attributes and the seismic inversion result was improved from 0.66 to 0.79. The prediction of sand bodies was further improved through the 3D lithofacies modelling, especially for these thin sandstone layers. Sedimentary facies of the Chang_7 member in the Qingcheng Oilfield, Ordos Basin, were characterized based on the distribution of sand bodies. Fan-shaped sand bodies are mapped in each zone of the Chang_7 member, revealing that the entire Chang_7 successions are of sub-lacustrine fan in origin. The Chang_7 member mainly contains six sedimentary architectural elements: main channel, branch channel, lobe, lobe edge, slump body and inter lobe/inter channel. The distribution of sedimentary facies is different in different zones, and appears to be largely related to the change of base-level. A horizontal well was designed according to the prediction of sand bodies, which show a higher probability of drilling into sandstone by approximately 10% compared with that of other horizontal wells.

Identification of morphostructures by utilizing Knickpoint Ranks, a method to avoid the camouflage effect in knickpoint analysis

Knickpoints are relief features characterized by a convex-up abrupt change in the equilibrium profile of a given river. In the field they can be recognized as rapids or waterfalls. Their presence indicate geological conditions or processes that modify erosive rates, such as contrasting lithologies, base-level changes, or active structures, and, therefore, especially relevant in tectonic geomorphology studies. The computation of knickpoints for classification purposes is made dividing the Slope-Extension Ratio (RDE) by the Total Slope-Extension Ratio (RDT), which are derived from the Stream-Gradient index (SL). Traditionally, the classification of knickpoints has been treated in two ways, (1) by the simple division into first and second order, according to a limit value of RDE/RDT of 10, and (2) by the statistical division of values into classes, mainly with natural breaks. Recent studies have used both divisions and correlated the RDE/RDT values with the size of the knickpoints in the field. However, there are studies on morphometric analysis that point to the existence of gaps that occur in specific cases, such as the so-called knickpoint camouflage. In order to eliminate these gaps and with the aim of improve the classification technique, we propose the determination of fixed intervals of distribution of knickpoints which, based on the division according to the RDE/RDT values, use the strengths of the statistical divisions as a means to define limits of RDE/RDT values. The objective of this methodological development is to prevent errors during the classifications as well as to facilitate the interpretation of the obtained knickpoints. The new classification method was applied in 20 hydrographic basins with their RDE/RDT values checked in 44 knickpoints in the field. The analyses carried out point to the coherence of the new proposed classification and confirm the effectiveness of both the fixed intervals and the tools used in the cartography of knickpoints.

Paleoenviromental evolution of the Cenozoic foreland basin to intermontane basins in the Eastern Cordillera, North-Western Argentina

The stratigraphic and sedimentological characteristics of the Cenozoic deposits are very important aspects to investigate the tecto-sedimentary evolution of the Andean foreland basin to intermontane hinterland basins in north-western Argentina. These deposits lie in an unconformity or paraconformity relationship over the Salta Group (Cretaceous-Paleocene) or older deposits. During the middle Eocene-Pleistocene, the changes in the sedimentary paleoenvironment and its evolution are the direct result of tectonic effects, a change in base level and accommodation space. This study provides the results of the integrated analysis of the middle-upper Eocene to Plio-Pleistocene basins in the Eastern Cordillera, in north-western Argentina. These basins that have been generated in the Central Andes region in north-western Argentina during the Cenozoic tectonic convergence, we divided in four sequences in the evolution from middle to upper Eocene to Plio-Pleistocene. They present fluvial deposits of different topographic slopes, with development of ephemeral fluvial systems associated with dune fields, meandering fluvial systems in arid climates, interlaced sandy-gravelly fluvial systems, sinuous sandy-gravelly fluvial systems of intermediate to high sinuosity, with the development of lagoons and swamps of tropical climate during ∼10 to ∼5 Ma, and the change to arid - semiarid claimed condition from ∼5 to ∼2.5 Ma. The sedimentary record of the Plio-Pleistocene hinterland basin in the Eastern Cordillera and the adjacent Andean Puna Plateau in the Central Andes of NW Argentina constitutes geological archive that provide spatio-temporal insights into geological evolution and associated surface uplift.These basins, probably connected at their onset during the middle Eocene, are characterized by different subsidence histories, differences in sedimentary paleoenvironment evolution, variations in topographic slopes, changes in provenance and paleocurrents, resulting in differences in tecto-sedimentary history.

Cenozoic tectonic reactivation and its implications for landscape transience in southeastern Brazil

AbstractIntraplate settings far from tectonic plate boundaries are normally thought to be tectonically inactive regions in which the landscape evolves via the slow downcutting of fluvial systems. When identified, tectonic reactivations are thought to be of too low an amplitude and not cause any change in the landscape. However, in the absence of datable markers of tectonic deformation, the landscape can be used as a passive marker of intraplate tectonic activity. In this study, we investigate the northern boundary of the Ponta Grossa Arch in the southeastern portion of the Brazilian passive margin. We mapped and studied the spatial and temporal (relative) relationships between brittle structures and geomorphic markers of base level changes. Drainage anomalies (knickpoints and high ksn values) are correlated with structural/tectonic lineaments in which we recognize strike‐slip and transtensional faults with right‐lateral and left‐lateral kinematics. Through mapping and analysis of structural data, we constrained the underlying structural framework. The fault data demonstrate the counterclockwise rotation of the maximum stress field, which moved from the NE–SW orientation (at the Cretaceous‐Paleogene boundary) to N‐S to reach the current WNW‐ESE position. Together, the data suggest that, though subtle and low‐magnitude, the evolution of intraplate stress field forms a dynamic forcing of intracontinental landscape change.

Assessment of check dams and afforestation in mitigating debris flows based on dendrogeomorphic reconstructions, field surveys and semi-empirical models

Check dams and afforestation are two common measures for debris-flow mitigation; however, there is a lack of methods allowing quantitative evaluation of check dam and afforestation effects on debris flow motion and deposition. Laogan Gully is an active debris-flow system with a large landslide on the right hillslope, currently representing the main source of debris flows in the catchment. Several debris flows have occurred since 1985 which led to the construction of check dams and the plantation of silver wattle (Acacia dealbata) trees between 1991 and 1993. These measures significantly reduced the volume of debris flows reaching downstream portions of the system. At the same time, large amounts of sediment were trapped on the gully bed and thereby increased the base level of the torrent, which again tended to stabilize the large landslide on the right hillslope. This paper analyzes sedimentation of debris-flow material on the gully bed and on the fan using 12 trial pits behind the check dams and 6 incised geological sections on the fan. The timing and extent of debris flow motion and deposition was reconstructed using scars and growth suppression events in the annual growth ring series of 306 Acacia trees trapped in the newly formed deposits. The gravitational potential energy of the sediment of the large landslide is calculated according to the change in the erosion base level behind the check dams. On the basis of these results, we constructed a semi-empirical vegetation coverage–landslide energy–debris flow volume model to quantitatively evaluate the effects of the check dams and afforestation on debris-flow mitigation. The model exhibited predictive capabilities for estimating the volume of debris flow generated by the reactivation of landslides. Check dams are found to play a major role in debris-flow mitigation in the early stages, whereas the Acacia trees play an increasingly important role once the check dams are filled. The partial resurgence of the landslide caused a sudden increase in the gravitational potential energy of the sediment source, which in turn affected the effectiveness of the debris-flow mitigation measures.

Geological Assessment of Turonian - Coniacian terrestrial sedimentation records during climatic recovery, Moreno Hill Formation, Zuni Basin

Understanding of Upper Cretaceous terrestrial sediments within the Western Interior Basin is advancing; however, the Turonian–Coniacian transition remains enigmatic. Recent chronometry of the Moreno Hill Formation indicates that sediment deposition took place during this interval of geodynamic upheaval and climatic recovery immediately following the peak of the Cretaceous Thermal Maximum (CTM). To decipher these effects, the sedimentary record of the Moreno Hill Formation was reassessed near the type section, and near Quemado, New Mexico (USA) using facies analysis and architectural reconstruction. Seven facies types (thirteen lithofacies codes), eleven architectural elements, and three facies associations were identified. Sedimentation within the floodplain of the lower Moreno Hill Formation was affected by the east-migrating forebulge of the Western Interior Basin. Furthermore, increasingly bedload-rich multi-story channel complexes and a transition from near-coastal to alluvial coals reflect gradual climatic cooling and overall regression of the Western Interior Seaway (with interruption by a regional [T2–R2] transgressive-regressive sequence). This is consistent with more subaerial conditions indicative of continued regression reflected within the floodplain sediments of the upper Moreno Hill Formation. Whilst diversion of westerly fluvial feeder systems by ongoing forebulge migration also affected sediment transport and deposition, a return to more suspended-load-rich single-story channels and thin coals are tied to an intervening (T3) transgression. Repetitive paleosol sequences throughout the Moreno Hill Formation indicate groundwater fluctuation in response to these base level changes. Together with detrital zircon-based geochronology, these slight sedimentary differences support a revised subdivision from three into two members: lower and upper. Beyond feeding the seaward Gallup Delta, the newly defined lower member correlates to the Toreva, Straight Cliffs (Smoky Hollow member), Ferron Sandstone, Funk Valley and Frontier formations (Dry Hollow Member) and the upper member to the Wepo and Straight Cliffs (John Henry Member) formations within the Kaiparowits, Notom, Last Chance and Vernal fluvio-deltaic systems. Landward sediments of the Cardium Formation (Canada) correlate with the lower and upper members of the Moreno Hill Formation.

Diversity of depositional architecture and sandbody distribution of sublacustrine fans during forced regression: A case study of Paleogene Middle Sha 3 Member in Dongying Sag, Bohai Bay Basin, East China

Currently, the differences in gravity flow deposits within different systems tracts in continental lacustrine basins are not clear. Taking the middle submember of the third member of Paleogene Shahejie Formation (Sha 3 Member) in the Shishen 100 area of the Dongying Sag in the Bohai Bay Basin as an example, the depositional architecture of sublacustrine fans during forced regression and the impact of the fourth-order base-level changes on their growth were investigated using cores, well logs and 3D seismic data. Sublacustrine fans were mainly caused by hyperpycnal flow during the fourth-order base-level rise, while the proportion of slump-induced sublacustrine fans gradually increased during the late fourth-order base-level fall. From rising to falling of the fourth-order base-level, the extension distance of channels inside hyperpycnal-fed sublacustrine fans reduced progressively, resulting in the transformation in their morphology from a significantly channelized fan to a skirt-like fan. Furthermore, the depositional architecture of distributary channel complexes in sublacustrine fans changed from vertical aggradation to lateral migration, and the lateral size of individual channel steadily decreased. The lobe complex’s architectural patterns evolved from compensational stacking of lateral migration to aggradational stacking, and the lateral size of individual lobe steadily grew. This study deepens the understanding of depositional features of gravity flow in high-frequency sequence stratigraphy and provides a geological foundation for the fine development of sublacustrine fan reservoirs.

Accumulation of thick fluvial sediments in the Shinano River incised-valley fills: implications for sequence stratigraphy and alluvial architecture

ABSTRACTThe relationship between fluvial systems and base-level changes, controlled mainly by sea-level fluctuations and tectonic activity, on time scales of 103–104 years, is important for evaluating and improving nonmarine sequence stratigraphy and numerical and physical alluvial architecture models. However, there are not enough quantitative data available from field studies, especially for periods of rapid base-level rise. The Echigo Plain, Japan, is a fluvial–coastal lowland where the tidal range is very small and where a very deep valley was incised by the Shinano River during the latest Pleistocene. The depth of the incised valley near the shoreline, 160 m below mean sea level, is greater than the amount of sea-level fall during the Last Glacial Maximum (LGM) because of tectonic subsidence. We studied an approximately 100-m-long borehole sediment core obtained from the landward part of the plain, where thick fluvial deposits have accumulated since the LGM. Sediment facies, diatom assemblages, sulfur content, carbon to sulfur ratios, and radiocarbon age dates indicate that most of the core sediments were deposited in a fluvial environment during the past 12 kyr. Sediment accumulation rates were 10–15 m/kyr between 12 and 8 cal kyr BP, and relatively low (less than 5 m/kyr) subsequently, mainly because of glacio-eustatic sea-level rise. The timing of the reduction in the accumulation rate is one possible indicator of the position of the maximum-flooding surface in a sequence composed only of fluvial sediments. The presence of many thin layers of very fine to fine-grained sand (20–80 cm thick), interpreted as crevasse-splay deposits, that formed between 11 and 8 cal kyr BP, when the base level was rising rapidly, implies that crevasse-splay deposits are characteristic of aggrading fluvial sediments and the transgressive systems tract in nonmarine sequence stratigraphy. Tectonic subsidence at approximately 2 m/kyr has contributed markedly to the larger accumulation rate of fluvial sediments of the highstand systems tract on the Echigo Plain, compared with rates in other fluvial–coastal lowlands, since 5–6 cal kyr BP, when the eustatic sea-level rise had mostly ceased.

A window into development of a complex ice-marginal lake prior to the Late Glacial Maximum (LGM) in Austria

Abstract Although the retreat process of glaciers from the Late Glacial Maximum (LGM) is well documented, high-resolution insight into conditions prior to the maximum is lacking in the eastern European Alps, resulting in a gap in our understanding about the processes associated with this important climatic tipping point. We describe an outstanding sand and gravel outcrop at Gröbming in the Enns Valley (Ennstal), Austria, that represents the development of a delta complex that debouched into a large body of water that we name paleo-Lake Gröbming, fed by a major valley glacier. The succession consists of sands bearing climbing ripples, parallel laminations, and supercritical bed forms (bottomsets) overlain by meter-scale gravelly foresets. Topsets comprise gravels. We argue that sustained accumulation of supercritical bed forms required a jet efflux feeder mechanism best explained by a direct glacial meltwater source. Complex vertical and lateral repetition of this tripartite succession is observed, with sequence stratigraphic analysis permitting us to argue that stratal complexity is attributable to base-level changes in paleo-Lake Gröbming resulting from dam breaches of the lake. Thus, application of well-established sequence stratigraphic approaches to Quaternary ice-marginal successions in the Alps has significant potential to yield unprecedented insights into conditions prior to the LGM.

Distal fluvial to lacustrine/palustrine carbonate facies of the El-Ris Formation (Late Miocene?) at the Bahariya Depression, Western Desert, Egypt: Depositional model and controls

This article aims to discuss the distal fluvial to lacustrine/palustrine carbonate facies of the El-Ris Formation (Late Miocene?) in the Bahariya Depression (Western Desert, Egypt), in terms of their depositional evolution and factors (base-level, climate, tectonics) controlling deposition. The El-Ris Formation disconformably overlies the Lower Cenomanian Bahariya Formation. It is up to 11 m thick, and includes siliciclastic facies, followed upward by carbonate facies. The siliciclastic facies consist of greenish-white, fine-grained calcareous sandstones interbedded with subordinate yellowish-brown mudrocks having insects ichnofossils reflecting deposition in a distal fluvial distributary system. The carbonate facies include the fossiliferous (mostly ostracods and a few dispersed gastropods, Lanistes carinatus,) fine crystalline lacustrine/palustrine carbonates, and calcretes-dolocretes. The lacustrine/palustrine beds are grey to pale-yellow colored massive dolomitic beds, with exposure features, reflecting deposition in a shallow, low-gradient (marginal) lake with frequent base-level changes. Calcretes-dolocretes are nodular to massive orange beds, developed at the expense of siliciclastic facies, that formed a fringe between the distal alluvial and central lacustrine-palustrine environments. Palustrine carbonate and duricrust (calcrete/dolocrete) facies increase progressively toward the top of the studied rock unit. The development of silica nodules in the palustrine facies is a result of intense evaporation just above the water table. Silica could have been released by silicate dissolution from the host alluvial deposits into the groundwater. δ13C and δ18O data indicate a hydrologically closed palustrine/lacustrine environment fed by meteoric waters, that originally precipitated calcite. Progressive evaporation of freshwater in this closed system led to dolomitization. The upward increase of carbonates in the El-Ris Formation reflects a progressive decrease of a terrigenous clastic supply in the source area and a dominance of carbonate development in the study area. During the deposition of the El-Ris Formation, the climate had marked more arid seasons with fewer intermittent humid periods. Also, the tectonic influence cannot be ruled out, since tectonic instability led to the creation of the basin depression and subsequent accommodation space that made the deposition of the El-Ris Formation.

Facies and stratigraphy of the basal Ediacaran cap carbonate, Naukluft Mountains, Namibia

The termination of the Marinoan Snowball Earth glaciation constitutes a dramatic interval of climate change. This field study seeks to investigate this climate transition by characterizing the basal-Ediacaran cap carbonate succession of the Tsabisis Formation overlying the Blässkranz Formation glacial deposits exposed in the Naukluft Mountains of Namibia by using sedimentologic and stratigraphic field observations supplemented by chemostratigraphic analyses. Measured sections and lateral bed/unit tracing delineate stratigraphic stacking patterns and regional facies changes. In downdip areas Marinoan diamictite transitions upward into dolostone intermixed with sandstone and extrabasinal clasts that is gradually overlain by fine grained laminated dolostone. Updip localities show the diamictite is overlain by intercalated sandstones, gravels, and shales before an abrupt change to laminated dolostone of the cap carbonate. A succession of stromatolites, which become strongly elongate upward, prograde into the laminated dolostone in the updip localities. The stromatolites are overlain by laminated dolostone that grades upward into rhythmite with intercalations of shale. Near the top of the cap, rhythmites may be reworked into tabular intraclast conglomerate, locally intercalated with hummocky cross stratified sandstone, which passes upward into the shale and limestone members of the Tsabisis Formation. The Tsabisis Formation is overlain by the Noab Formation, dominated by sedimentary breccia, laminated dolostone, and sandstone. The lateral and vertical distribution of facies indicate a retreat of the shoreline and glacially sourced siliciclastics near the base of the cap carbonate, a shallowing succession to fair-weather wave base at the top of the stromatolite facies, and a second shallowing succession to storm wave base near the top of the cap carbonate. Maximum flooding occurred soon after the initiation of carbonate deposition and two sequence boundaries mark higher stratigraphic levels within the cap carbonate. The depositional patterns of the Tsabisis Formation cap carbonate resemble that of other Precambrian mixed carbonate-siliciclastic platforms and shows multiple base-level fluctuations that could be attributed to a range of mechanisms. Some of the base-level changes may have been directly coupled to events driven by deglaciation but distinguishing these from conventional controls on accommodation space in the rock record is challenging and multiple depositional models are feasible.

The delta-fluvial evolution of a lacustrine basin with gentle slope and low sedimentation rate: A case study of the Fudong Slope, Junggar Basin, Northwest China

Gentle slope belts of lacustrine basins are important areas for the generation of stratigraphic petroleum reservoirs controlled by sedimentary systems. Based on coring, well logging, 3D seismic and analytical test data, the sedimentary system and sandbody distribution characteristics of the Jurassic Toutunhe Formation (J2t) in the Fudong slope of the Junggar Basin were analysed. The results indicated that the Fudong slope showed the characteristics of low sediment input. Sediments mainly originated from the Shaqi uplift, and the sediment input gradually weakened from the early to the late J2t stages. The sedimentary system evolved from a delta to a meandering river, resulting in a delta front facies in the 1st member (J2t1) and a meandering river facies in the 2nd (J2t2) and 3rd (J2t3) members of the Toutunhe Formation. A sedimentary evolution pattern for the gentle slope with low sediment input was established. Thick deltaic sandbodies developed locally in the J2t1 stage. However, meandering rivers with high sinuosities predominantly developed and resulted in a striped geometry in the J2t2 and J2t3 members. Moreover, the meandering sandbodies exhibited an anastomosing morphology after multistage superposition. The channel and point bar facies in J2t2 were more developed than those in J2t3 because of sediment input differences. Under low-sediment input conditions, the sedimentary environment of the gentle slope belt in the lacustrine basin was controlled by changes in the palaeoclimate, palaeogeomorphology, gradient (slope) and base level. Palaeoclimate was the main factor controlling the evolution of the sedimentary environment. Palaeogeomorphology and slope controlled the distribution of sedimentary microfacies, channel sinuosity and sandbodies. Base-level change restricted the sandbody superimposition style. Under the low sediment input conditions of the Fudong slope, the subaqueous distributary channels of the delta environment and the point bars of the meandering river environment in the transitional belt of the Fudong slope are the main potential targets for stratigraphic petroleum reservoirs, whilst the 2nd member of the Toutunhe Formation (J2t2) is the main favourable interval for stratigraphic reservoir exploration.

Incision history of the Mekong River valley revealed by spatially differential exhumation

The evolution of the Mekong River valley offers a unique record of the rock uplift and climate history of southeastern Tibet. Faster erosion rates along the valley than those on the flanks caused valley incision and local incision rates may reflect a mix of driving mechanisms, migrating knickpoints linked to a change in base level, and large-scale surface uplift. The thermochronometric-constrained erosion rates in the 2–3 km deep river channel compared to rates outside of the channels provide unique constraints on the driving mechanisms of the river incision. In particular, the onset of river incision at various points along the Mekong River helps identify whether exhumation signals are the result of knickpoint migration that would generate a younger incision upstream, or the outgrowth of the plateau, which would produce younger incision downstream. We analyze new and published apatite (UTh)/He ages collected along two elevation transects in the upper Mekong River: downstream at Tu'Er and upstream at Deqin. The data and thermokinematic modeling predict that the exhumation rates of the valley near Tu'Er and Deqin increased more rapidly than those of adjacent high-elevation, low-relief surfaces at ~11 Ma and ~ 4 Ma, respectively. Since then, exhumation variations have caused an increase of >2 km in the depth of the valley, indicating that the major incision of the Mekong River valley began at ~11 Ma near Tu'Er and approximately 4 Ma near Deqin. The results indicate that a major phase of the Mekong River valley incision may have developed and progressed upstream since the Miocene.