Base-level Changes Research Articles

The Brahmaputra tale of tectonics and erosion: Early Miocene river capture in the Eastern Himalaya

The Himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion. Within the broader orogen, fluvial drainages partly reflect surface uplift, different climatic zones and a response to crustal deformation. In the eastern Himalaya, the unusual drainage configuration of the Yarlung Tsangpo–Brahmaputra River has been interpreted either as antecedent drainage distorted by the India–Asia collision (and as such applied as a passive strain marker of lateral extrusion), latest Neogene tectonically-induced river capture, or glacial damming-induced river diversion events.Here we apply a multi-technique approach to the Neogene paleo-Brahmaputra deposits of the Surma Basin (Bengal Basin, Bangladesh) to test the long-debated occurrence and timing of river capture of the Yarlung Tsangpo by the Brahmaputra River. We provide U–Pb detrital zircon and rutile, isotopic (Sr–Nd and Hf) and petrographic evidence consistent with river capture of the Yarlung Tsangpo by the Brahmaputra River in the Early Miocene. We document influx of Cretaceous–Paleogene zircons in Early Miocene sediments of the paleo-Brahmaputra River that we interpret as first influx of material from the Asian plate (Transhimalayan arc) indicative of Yarlung Tsangpo contribution. Prior to capture, the predominantly Precambrian–Paleozoic zircons indicate that only the Indian plate was drained. Contemporaneous with Transhimalayan influx reflecting the river capture, we record arrival of detrital material affected by Cenozoic metamorphism, as indicated by rutiles and zircons with Cenozoic U–Pb ages and an increase in metamorphic grade of detritus as recorded by petrography. We interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen. Whole rock Sr–Nd isotopic data from the same samples provide further support to this interpretation. River capture may have been caused by a change in relative base level due to uplift of the Tibetan plateau. Assuming such river capture occurred via the Siang River in the Early Miocene, we refute the “tectonic aneurysm” model of tectonic–erosion coupling between river capture and rapid exhumation of the eastern syntaxis, since a time interval of at least 10 Ma between these two events is now demonstrated. This work is also the first to highlight U–Pb dating on detrital rutile as a powerful approach in provenance studies in the Himalaya in combination with zircon U–Pb chronology.

Geomorphic evidence for enhanced Pliocene–Quaternary faulting in the northwestern Basin and Range

Mountains in the U.S. Basin and Range Province are similar in form, yet they have different histories of deformation and uplift. Unfortunately, chronicling fault slip with techniques like thermochronology and geodetics can still leave sizable, yet potentially important gaps at Pliocene–Quaternary (∼105–106 yr) time scales. Here, we combine existing geochronology with new geomorphic observations and approaches to investigate the Miocene to Quaternary slip history of active normal faults that are exhuming three footwall ranges in northwestern Nevada: the Pine Forest Range, the Jackson Mountains, and the Santa Rosa Range. We use the National Elevation Dataset (10 m) digital elevation model (DEM) to measure bedrock river profiles and hillslope gradients from these ranges. We observe a prominent suite of channel convexities (knickpoints) that segment the channels into upper reaches with low steepness (mean k sn = ∼182; θref = 0.51) and lower, fault-proximal reaches with high steepness (mean k sn = ∼361), with a concomitant increase in hillslope angles of ∼6°–9°. Geologic maps and field-based proxies for rock strength allow us to rule out static causes for the knickpoints and interpret them as transient features triggered by a drop in base level that created ∼20% of the existing relief (∼220 m of ∼1050 m total). We then constrain the timing of base-level change using paleochannel profile reconstructions, catchment-scale volumetric erosion fluxes, and a stream-power–based knickpoint celerity (migration) model. Low-temperature thermochronology data show that faulting began at ca. 11–12 Ma, yet our results estimate knickpoint initiation began in the last 5 Ma and possibly as recently as 0.1 Ma with reasonable migration rates of 0.5–2 mm/yr. We interpret the collective results to be evidence for enhanced Pliocene–Quaternary fault slip that may be related to tectonic reorganization in the American West, although we cannot rule out climate as a contributing mechanism. We propose that similar studies, which remain remarkably rare across the region, be used to further test how robust this Plio–Quaternary landscape signal may be throughout the Great Basin.

Applications of chemostratigraphy in Cretaceous sediments encountered in the North Central Rub’ al-Khali Basin, Saudi Arabia

The following study was undertaken on inorganic geochemical data acquired for 832 core and cuttings samples taken from Cretaceous sediments encountered in 11 wells in the North Central Rub’ al-Khali Basin, Saudi Arabia. The study sections extend from Khafji Member at the base of the Wasia Formation to the basal part of the Aruma Formation. The principal objective of the study was to produce a chemostratigraphic scheme for these wells, though a comparison was also made with a previous lithostratigraphic scheme.Although ICP–OES (Inductively Coupled Plasma–Optical Emission Spectrometry) and ICP–MS (Inductively Coupled Plasma–Mass Spectrometry) were used to acquire data for 50 elements, the chemostratigraphic scheme is based on changes in the following ‘key’ elements and ratios: Ca, Al, Zr, P/Y, Zr/P, Zr/Y, Zr/Nb, Mo and Zr. The scheme comprises a hierarchical order of five zones, 18 subzones and four divisions, with the zones labelled C1, C2, C3, C4 and C5 in ascending stratigraphic order. The definition of these zones are mainly based on changes in Ca and Al reflecting variations in bulk lithology as a response to changes in base level. Zone C1 is dominated by elevated Al and low Ca concentrations and represents calcareous mudrocks deposited in a lagoonal–prodelta environment. An increase in base level led to the formation of highly calcareous rudistic limestones deposited in a shallow marine environment, with a further increase in base level resulting in the deposition of calcareous mudrocks with high Al values in zone C3. Zone C4 comprises rudistic limestones (with elevated Ca and low Al) deposited in shallow marine conditions, while C5 is dominated by calcareous mudrocks and argillaceous limestones deposited in a prodeltaic paleoenvironment. Although the definition of zones mainly relates to changes in base level/bulk lithology, more subtle changes in provenance are modelled using the P/Y, Zr/P, Zr/Y and Zr/Nb ratios which are used to define most subzones and divisions. Variations in this ratio are almost entirely dependent on changes in source/provenance.Although all of the zones and most of the subzones are correlative between the study wells it is noted that different criteria are used to define the zone C2 and C4 subzones in the eastern and western wells of the study area. This suggests that separate sediment sources were responsible for the delivery of clastic material to these carbonate ‘reefs’ during the deposition of these subzones.One important conclusion from this study is that there are close links between the chemostratigraphic and lithostratigraphic correlations but the former offers a much higher level of resolution.

Floodplain Change in the Glenmalure Valley, South-east Leinster

The Glenmalure Valley is one of several U-shaped valleys which drain the eastern flanks of the Wicklow Mountains, 45km south west of Dublin. Five channel and floodplain reaches are recognised along a 25km stretch of the Glenmalure River on the basis of spatial changes in stream power and sediment load characteristics. The downvalley sequence of floodplain types within the river is atypical of that commonly associated with decreasing slope and sediment calibre downstream. Radiocarbon dating together with the use of anomalous concentrations of heavy metals, which are used as stratigraphic indicators of alluvial deposition post 1800, also reveal temporal changes in the rate of Holocene floodplain development within the valley. Downstream changes in floodplain processes within the valley are explained, therefore, in terms of spatial and temporal changes in the river's ability to adjust its slope, form and bed texture within the constraints of geological and Pleistocene geomorphic inheritance. The presence of a bedrock basin in the valley floor profile, infilled by Pleistocene lacustrine deposits, is believed to have caused a local change in base level leading to a significant reduction in valley slope and local stream power in the middle reaches of the valley. The subsurface bedrock form and the Pleistocene palaeolake also had a significant impact on coarse sediment transport, storage and transfer through the river system and hence on the ability of the Holocene river to regrade its slope and laterally rework Pleistocene valley fills for floodplain construction.

Modelling long‐term (300 ka) upland catchment response to multiple lava damming events

AbstractLandscapes respond in complex ways to external drivers such as base level change due to damming events. In this study, landscape evolution modelling was used to understand and analyse long‐term catchment response to lava damming events. PalaeoDEM reconstruction of a small Turkish catchment (45 km2) that endured multiple lava damming events in the past 300 ka, was used to derive long‐term net erosion rates. These erosion rates were used for parameter calibration and led to a best fit parameter set. This optimal parameter set was used to compare net erosion landscape time series of four scenarios: (i) no uplift and no damming events; (ii) no uplift and three damming events; (iii) uplift and no damming events; and (iv) uplift and three damming events. Spatial evolution of net erosion and sediment storage of scenario (iii) and (iv) were compared. Simulation results demonstrate net erosion differences after 250 000 years between scenarios with and without dams. Initially, trunk gullies show less net erosion in the scenario with damming events compared with the scenario without damming events. This effect of dampened erosion migrates upstream to smaller gullies and local slopes. Finally, an intrinsic incision pulse in the dam scenario results in a higher net erosion of trunk gullies while decoupled local slopes are still responding to the pre‐incision landscape conditions. Sediment storage differences also occur on a 100 ka scale. These differences behaved in a complex manner owing to different timings of the migration of erosion and sediment waves along the gullies for each scenario. Although the specific spatial and temporal sequence of erosion and deposition events is sensitive to local parameters, this model study shows the manner in which past short‐lived events like lava dams have long‐lasting effects on catchment evolution. Copyright © 2014 John Wiley & Sons, Ltd.

Geomorphic response and 14C chronology of base-level changes induced by Late Quaternary Caspian Sea mobility (middle Kura Valley, Azerbaijan)

Recent geomorphological studies in Azerbaijan reveal the importance of climato-eustatic influences on landscape changes in the Caspian Sea Basin during the last 40ka. Radiocarbon-dated fluvial landforms show that the paleohydrosystem of the middle Kura valley and tributaries responded to regressive and transgressive phases of the Caspian Sea. Chrono-sequences and landforms secured by 32 radiocarbon ages show strong correspondence between fluvial sediment accumulation and oscillations of base level in the Caspian Sea.Six phases of valley floor aggradation (e.g. 27,000–13,000; 11,000–3200; 2900–2100; 1600–1000; 400–150cal.yr BP) and seven phases of fluvial incision (>37,000; 37,000–27,000; 13,000–11,000; 3200–2900; 2100–1600; 1000–400; 150–0cal.yr BP) are recorded. Morphodynamics of the fluvial landscape show a strong correspondence with fluctuation in sea levels in the Caspian Sea: the major Upper Pleistocene–Late Glacial Khvalynian (c.a. 26,000 to 12,400cal.yr BP) and various Holocene transgressions as well as the Mangyshlak (c.a. 12,000cal.yr BP) and Derbent (6th–12th century AD) regressions. These data contribute to refining the Caspian relative sea-level curve for the last 30ka using the chronology of changes in the aggradation or incision in the connected fluvial system.

Case Study of Variation of Sedimentation in the Yellow and Wei Rivers

The Sanmenxia Dam built in 1960 in the middle reach of the Yellow River has experienced severe sedimentation problems, not only in the reservoir area itself but also extending hundreds of kilometers in the backwater region. Morphological responses in the middle Yellow River and its tributary, the Wei River, upstream of the dam have generally lagged behind the causal changes in upstream and downstream controls. Based on the geometrical characteristics of a wedge-shaped sedimentation body, a method was derived for calculating equilibrium values of the accumulated sedimentation volume Ve in the channel reaches. Combining the method for calculating Ve with rate law models for channel morphological adjustment, a method for calculating the accumulated deposition volumes was derived. The results showed that the proposed method can accurately simulate sedimentation processes in the study reaches during 1960 and 2011, in the response to the integrated impacts of base level fluctuations and changes in incoming water and sediment discharges. The characteristics of the potential adjusted (equilibrium) morphology was analyzed and it is shown that while the nature of the adjusted (or equilibrium state) of the fluvial system varies dynamically with external conditions, it generally acts as the target for the observed channel adjustments, although the actual values of variables defining the fluvial system seldom adjust to their corresponding equilibrium values due to the effects of multiple disruptions, continuously varying water and sediment inputs, and delayed responses in the fluvial system.

Reciprocal Changes In Foreset To Bottomset Facies In A Gilbert-Type Delta: Response To Short-Term Changes In Base Level

Abstract Results of a detailed sedimentological study of the mid-Pleistocene Gilbert-type Ilias delta at the southern coast of the Corinth Rift, Greece, are reported. The study indicates that the development of a turbidite-dominated facies assemblage (TFA) of delta foreset deposits is associated with an oblique delta-brink geometry, which signifies a deficit of delta-front accommodation due to the falling or stable base level. A debrite-dominated assemblage (DFA) of foreset deposits forms when the base level is rising and the delta-brink geometry becomes sigmoidal, signifying an increase in delta-front accommodation. The comparison of coeval foreset and toeset to bottomset deposits in the delta indicates a reverse pattern of reciprocal changes in facies assemblages, with the foreset TFA deposits corresponding to delta-foot DFA deposits, and with the foreset DFA deposits corresponding to delta-foot TFA deposits. This reciprocal alternation of TFA and DFA facies assemblages is attributed to changes in the delta-front accommodation driven by short-term base-level changes, with a degree of “noise” in the facies record due to the system autogenic variability and regional climatic fluctuations. The study sheds a new light on the changing pattern of subaqueous sediment dispersal in Gilbert-type deltaic systems in response to base-level changes. It also indicates a new possibility for the recognition of a hidden record of such changes in other similar ancient deltas on the basis of their detailed facies analysis.

To what extent can intracrater layered deposits that lack clear sedimentary textures be used to infer depositional environments?

Craters within Arabia Terra, Mars, contain hundreds of meters of layered strata showing systematic alternation between slope- and cliff-forming units, suggesting either rhythmic deposition of distinct lithologies or similar lithologies that experienced differential cementation. On Earth, rhythmically deposited strata can be examined in terms of stratal packaging, wherein the interplay of tectonics, sediment deposition, and base level (i.e., the position above which sediment accumulation is expected to be temporary) result in changes in the amount of space available for sediment accumulation. These predictable patterns of sediment deposition can be used to infer changes in basin accommodation regardless of the mechanism of deposition (e.g. fluvial, lacustrine, or aeolian). Here, we analyze sedimentary deposits from three craters (Becquerel Crater, Danielson Crater, Crater A) in Arabia Terra. Each crater contains layered deposits that are clearly observed in orbital images. Although orbital images are insufficient to specifically determine the origin of sedimentary deposits, depositional couplets can be interpreted in terms of potential accommodation space available for deposition, and changes in the distribution of couplet thickness through stratigraphy can be interpreted in terms of changing base level and the production of new accommodation space. Differences in stratal packaging in these three craters suggest varying relationships between sedimentary influx, sedimentary base level, and concomitant changes in accommodation space. Previous groundwater upwelling models hypothesize that layered sedimentary deposits were deposited under warm climate conditions of early Mars. Here, we use observed stacking patterns to propose a model for deposition under cold climate conditions, wherein episodic melting of ground ice could raise local base level, stabilize sediment deposition, and result in differential cementation of accumulated strata. Such analysis demonstrates that a first-order understanding of sedimentary deposition and accumulation—despite a lack of textural information that inhibits interpretation of depositional mechanism—can provide insight into potentially changeable depositional conditions of early Mars.

The role of waterfalls and knickzones in controlling the style and pace of landscape adjustment in the western San Gabriel Mountains, California

Bedrock rivers set the pace of landscape adjustment to tectonic and climatic forcing by transmitting signals of base-level change upstream through the channel network and ultimately to hillslopes. River incision is typically modeled as a monotonic function of bed shear stress or stream power, modulated by sediment tools and cover effects, but these models do not apply in channels with steep or vertical bedrock reaches due to changes in flow dynamics, hydraulic geometry, and bed cover. Here, we investigate how such knickzones (oversteepened channel reaches often containing waterfalls) influence the propagation of slope-break knickpoints that separate relict from adjusting topography, and thus the response times of landscapes to external forcing. We use a conceptual long-profile model to explore the consequences of waterfalls and knickzones on channel response and compare predictions to light detection and ranging (LiDAR) topography, field observations, and cosmogenic radionuclide data from Big Tujunga Creek, a 300 km2 watershed in the San Gabriel Mountains, California. Three prominent knickzones along Big Tujunga Creek, characterized by numerous waterfalls, show contrasting behavior. For the upper knickzone, waterfalls align with bands of harder rock exposed on adjacent hillslopes, and between waterfalls, the channel is mantled by large (>2 m) boulders, indicating knickzone retreat is slow compared to predictions of slope-break knickpoint retreat from stream-power models, enhancing the preservation of an upstream relict landscape. The middle knickzone shows evidence for both fast and slow knickzone retreat, as well as significant deviations from predictions of uniform tributary knickpoint elevations derived from stream-power models. The lower knickzone is characterized by a waterfall and knickzone within an incised inner gorge that provide evidence of rapid retreat relative to background channel incision. Overall, we find a pattern of decreasing knickzone and waterfall retreat rate with distance upstream of the range front, beyond decreases predicted by simple area-dependent celerity models. Our results highlight that waterfalls and knickzones can both enhance and inhibit landscape adjustment, leading to divergent controls on the pace of landscape evolution.

Erosion Triangular Facets as Markers of Order in an Open Dissipative System

The complexity and non-linearity of the morphogenetic system which is responsible for shaping the Earth’s surface have been widely recognised by many authors who have documented the fractal nature of erosion. In this paper, two peculiar kinds of landforms are compared to point out ordered structures, i.e. triangular facets that arise in different geomorphic systems, due to the principle of morphologic convergence. Occurrence of triangular facets has been documented in mountainous areas in relation to base level changes and hydrographical network evolution; similarly shaped landforms are present even in recent tectonic uplift areas along faults. The spatial distribution of the two kinds of facets has been investigated in two river basins located in Liguria (northern Italy) and in a mountainous area in Oman. The results of this analysis document the different spatial features of the two kinds of facets.

Morphotectonics of the Upper Tiber Valley (Northern Apennines, Italy) through quantitative analysis of drainage and landforms

We present a geomorphological analysis of the recent extensional tectonics of a Quaternary continental basin in the Northern Apennines (Italy). The study area is focused on Upper Tiber Valley (UTV), a basin elongated for 70 km in NNW-SSE direction hosting the Tiber River. The area is characterized by a series of features that make it an excellent case study: (i) homogeneity of lithology (ii) active faults, and (iii) strong morphogenetic activity. In this study, 36 hydrographical basins, tributaries of Tiber River, have been analysed. A preliminary qualitative geomorphological setting was outlined pointing out that the drainage river network shows meaningful evidence of tectonic control, such as abrupt changes in stream directions, knickpoints and steepness anomalies alignments along meaningful length in adjacent basins. Besides, the tectonic control is well marked in base level changes and consequent tectonically induced downcutting. Signs of neotectonics are highlighted by structural landforms too. The entrenchment of alluvial fans, the triangular facets and the fault planes are mapped by field survey and aerial photo interpretation. In addition, a quantitative analysis was also performed. Linear, areal and volumetric indexes related to drainage basins and river networks are taken into account. The geometry of the escarpments delimiting the basin and the landforms detected along the adjacent piedmont are investigated. The ranges of values, according to the existing literature, confirm a condition of wide-ranging morphological disturbance. In the central part of the study area, while the western basins are almost in equilibrium, the eastern ones reveal clear signs of disequilibrium, this is particularly evident along the distal segment of the river network. These data, joined with the characteristics of the escarpment and piedmont junction, confirm that the neotectonic activity, in the centre and in the eastern side of the basin, is the main factor controlling the morphological system.

Geomorpho-tectonic evolution of the Jamaican restraining bend

This work applies recent advances in tectonic geomorphology in order to understand the geomorphic evolution of the Jamaican restraining bend located along the Caribbean–Gonâve–North American plate boundary. We propose a classification of landscapes according to their erosional stages. The approach is mainly based on the combination of two DEM-based geomorphic indices: the hypsometric integral which highlights elevated surfaces, and the surface roughness which increases when the relief is incised by the drainage network. River longitudinal profiles were also analyzed as the drainage network responds quickly to base-level change triggered by external forcing such as tectonics. Anomalies in river profiles (knickpoints and convex segments) were mapped using stream length-gradient (SL) and normalized steepness (ksn) indices. The results provide new insights for understanding the complex evolution of the Jamaican restraining bend. Three main morphotectonic regions were identified in Jamaica: (1) the Blue Mountain–Wagwater unit located at the eastern tip of the island, (2) the Jamaican highlands plateau which covers most of the northern and central areas and (3) the tilted block province located along the southern part of Jamaica. Each region has a specific morphological signature which marks a different stage in the Late Miocene to present evolution of the Jamaican restraining bend. The evolution of the bend is mainly associated with the western propagation of major E-trending strike-slip faults and NW-trending thrusts. In the western and central parts of Jamaica the present-day motion between the Caribbean plate and the Gonâve microplate is broadly distributed along several structures, while in the easternmost part of the island this motion seems to be almost completely accommodated along the Blue Mountain range and the Plantain-Garden Fault.

Geomorphic response of submarine canyons to tectonic activity: Insights from the Cook Strait canyon system, New Zealand

Active margins host more than half of submarine canyons worldwide. Understanding the coupling between active tectonics and canyon processes is required to improve modeling of canyon evolution and derive tectonic information from canyon morphology. In this paper we analyze high-resolution geophysical data and imagery from the Cook Strait canyon system (CS), offshore New Zealand, to characterize the influence of active tectonics on the morphology, processes, and evolution of submarine canyons, and to deduce tectonic activity from canyon morphology. Canyon location and morphology bear the clearest evidence of tectonic activity, with major faults and structural ridges giving rise to sinuosity, steep and linear longitudinal profiles, cross-sectional asymmetry, and breaks in slope gradient, relief, and slope-area plots. Faults are also associated with stronger and more frequent sedimentary flows, steep canyon walls that promote gully erosion, and seismicity that is considered the most likely trigger of failure of canyon walls. Tectonic activity gives rise to two types of knickpoints in the CS. Gentle, rounded and diffusive knickpoints form due to short-wavelength folds or fault breakouts. The more widespread steep and angular knickpoints have migrated through canyon-floor slope failures and localized quarrying and/or plucking. Migration is driven by base-level lowering due to regional margin uplift and deepening of the lower Cook Strait Canyon, and is likely faster in larger canyons because of higher sedimentary flow throughput. The knickpoints, nonadherence to Playfair’s Law, linear longitudinal profiles, and lack of canyon-wide, inverse power law slope-area relationships indicate that the CS is in a transient state, adjusting to perturbations associated with tectonic displacements and changes in base level and sediment fluxes. We conclude by inferring unmapped faults and regions of more pronounced uplift, and proposing a generalized model for canyon geomorphic evolution in tectonically active margins.

Transient river response, captured by channel steepness and its concavity

Mountain rivers draining tropical regions are known to be great conveyor belts carrying efficiently more than half of the global sediment flux to the oceans. Many tropical mountain areas are located in tectonically active belts where the hillslope and stream channel morphology are rapidly evolving in response to changes in base level. Here, we report basin-wide denudation rates for an east–west transect through the tropical Andes. Hillslope and channel morphology vary systematically from east to west, reflecting the transition from high relief, strongly dissected topography in the escarpment zones into relatively low relief topography in the inter-Andean valley. The spatial pattern of differential denudation rates reflects the transient adjustment of the landscape to rapid river incision following tectonic uplift and river diversion. In the inter-Andean valley, upstream of the wave of incision, slopes and river channels display a relatively smooth, concave-up morphology and denudation rates (time scale of 104–105a) are consistently low (3 to 200mm/ka). In contrast, slopes and river channels of rejuvenated basins draining the eastern cordillera are steep to very steep; and the studied drainage basins show a wide range of denudation rate values (60 to 400mm/ka) that increase systematically with increasing basin mean slope gradient, channel steepness, and channel convexity. Drainage basins that are characterised by strong convexities in their river longitudinal profiles systematically have higher denudation rates. As such, this is one of the first studies that provides field-based evidence of a correlation between channel concavity and basin mean denudation rates, consistent with process-based fluvial incision models.

High-resolution stratigraphy of Holocene lagoon terraces of Southern Brazil

The northern portion of the coastal plain of the Rio Grande do Sul State (southernmost Brazil) comprises an outer sandy barrier that protects a complex lagoon system formed during the Holocene. The terraces of three different lagoons (Gentil, Malvas and Pinguela) formed along their margins record the depositional processes and the relative base level changes over the past 5000yr. Therefore, our main objective was to characterize and quantify base level fluctuations from the study of these terraces, to correlate them to sea-level changes and to describe the depositional architecture related to the distinct sea-level stages (high-resolution sequence stratigraphy). Satellite images, topographic and GPR profiles, auger holes and radiometric dating were used. The main results indicate a close relationship between relative base level and relative sea-level changes, a stillstand period just after the last transgressive maximum (4840–4650calyr BP) and a subsequent overall relative sea-level fall of about 3m. Both a normal (highstand systems tract) and a forced regression (falling-stage systems tract) controlled the geological record preserved in the terraces. The highstand (older terrace) is characterized by agradational bedding, whereas the falling stage comprises three progradational sets (terraces) bounded by erosive surfaces related to smaller-scale sea-level drops.

Very long-term incision dynamics of big rivers

Constraining large-scale incision dynamics of shield and post-rift margin domains is key to understanding the sediment routing system over the overwhelming part of the continental surface. Based on dated and regionally correlated incision markers from West Africa, we reconstruct for the first time the entire paleo-long profiles of big rivers such as the Niger at ca. 24, 11 and 6 Ma, as well as the Eocene topography those rivers have dissected. The results provide boundary conditions and calibration for surface process models and paleodrainage dynamics. Though spatially and temporally variable, incision remained mostly below 10m/my with a mean around 5m/my. The spatial stability of both the river outlets and divides imposed maintenance or increasing concavity of the river long profiles through time, resulting from spatially contrasted adjustment of river segments bounded by recurrent lithogenic knickzones that persisted since 24 Ma. Drainages evolved preferentially by very slow slope decrease or uniform incision in between the stationary knickzones of evolving amplitude, with apparently no relation to base level change. Therefore, knickzone height or position may not simply reflect the transient response of big rivers to base level fall as predicted by stream-power incision river models. This may also challenge uplift histories of deep continental interiors retrieved from river long profiles inversion relying on such models. Very slow incision allowed amplification of the Hoggar hot spot swell and flexural uplift of the continental margin to be recorded by river long profiles, emphasizing the potential of big non-orogenic rivers as gauges of dynamic topography.

Tectonics from fluvial topography using formal linear inversion: Theory and applications to the Inyo Mountains, California

AbstractTectonic activity generates topography, and the variability of tectonic forcing is responsible for topographic patterns and variability of relief in fluvial landscapes. Despite this basic relation, the inverse problem, by which features of the topography are used for inferring tectonic uplift rates, has proven challenging. Here we develop formal linear inversion schemes to infer a record of the rate of relative uplift as a function of space and time from the long profiles of rivers. The relative uplift rate is the difference between the rates of rock uplift and of the base level change. The inversion schemes are based on a closed‐form analytic solution to the transient linear stream power model, and to increase model resolution they make use of the multiplicity of information made available by multiple rivers and their tributaries. The distribution of the fluvial response time to tectonic perturbations is a key component of the inversion scheme, as this determines which tectonic events are preserved in the topography. We develop two inversion parameterizations that differ in their assumptions about the tectonic forcing: space‐invariant and time‐space variability with an assumed spatial distribution. The inversion schemes are applied to the Inyo Mountains, an uplifted block along the western boundary of the Basin and Range Province in California. Inversion results indicate that the range has been experiencing an acceleration of the relative uplift in the past ∼2‐3 Ma. We use the inversion results to constrain the paleotopography and paleo‐erosion rate along the range and to recover the throw rate history along the fault that bounds the Inyo range.

Comparison between experimental and numerical stratigraphy emplaced by a prograding delta

Abstract. A one-dimensional model that is able to store the stratigraphy emplaced by a prograding delta is validated against experimental results. The laboratory experiment describes the migration of a Gilbert delta on a sloping basement into standing water, i.e., a condition in which the stratigraphy emplaced by the delta front is entirely stored in the deposit. The migration of the delta front and the deposition on the delta top are modeled with total and grain-size-based mass conservation models. The vertical sorting on the delta front is modeled with a lee-face-sorting model as a function of the grain size distribution of the sediment deposited at the brinkpoint, i.e., at the downstream end of the delta top. Notwithstanding the errors associated with the grain-size-specific bedload transport formulation, the comparison between numerical and experimental results shows that the model is able to reasonably describe the progradation of the delta front, the frictional resistances on the delta top, and the overall grain size distribution of the delta top and delta front deposits. Further validation of the model in the case of variable base level is currently in progress to allow for future studies, at field and laboratory scale, on how the delta stratigraphy is affected by different changes of relative base level.

Integrated diagenetic and sequence stratigraphy of a late Oligocene–early Miocene, mixed-sediment platform (Austral Basin, southern Patagonia): Resolving base-level and paleoceanographic changes, and paleoaquifer characteristics

A condensed (~20-m-thick) marine transgressive-highstand succession comprises the upper San Julián Formation (upper Oligocene–lower Miocene) of the northern retroarc Austral Basin, southern Patagonia. Mixed-sediment facies identify a shelf-interior setting, part of an overall warm-temperate regional platform of moderate energy. Giant oyster-dominated skeletal–hiatal accumulations along the maximum flooding surface and forming high-energy event beds in the highstand succession preserve relict micrite in protected shelter porosity, and identify periods of reduced sediment accumulation. The stratigraphic distribution of marine-derived glaucony and diagenetic carbonates is spatially related to sequence development. Depositional siderite coincides with prominent marine transgression, defining transient mixing of marine and meteoric waters across coastal-plain deposits. Chemically evolved autochthonous glaucony coincides with periods of extended seafloor exposure and transgressions that bracket the marine succession, and within the oyster-dominated skeletal accumulations. Seafloor cement, likely once magnesian calcite, formed in association with an encrusting/boring biota along the maximum flooding surface in concert with incursion of cool (11–13°C) water. The cement is present locally in skeletal event beds in the highstand succession suggesting a possible association with high-order base-level change and cooler water. As the highstand succession coincides with elevated global sea level in the late Oligocene–early Miocene, the locally marine-cemented glauconitic skeletal event beds in the highstand succession may identify higher order glacio-eustatic control. Local stratal condensation, however, is best explained by regional differences in basement subsidence. In the burial realm, carbonate diagenesis produced layers of phreatic calcrete coincident with skeletal-rich deposits. Zeolite (clinoptilolite-K) cement is restricted to the lowermost marine transgressive interval probably due to initial elevated metastability of reworked weathered silicates. Clay (illite)-cement is restricted to siliciclastic-rich intervals wherein skeletal carbonate did not buffer pore-water pH. Diagenetic carbonate geochemistry (Sr, Na, and δ18O and δ13C) shows that, with burial, the transgressive and highstand system tracts developed as distinct paleoaquifers resulting from different proximities to meteoric recharge zones.