Autogenic Mechanisms Research Articles

Magnitude and timing of transient incision resulting from large‐scale drainage capture, Sutlej River, Northwest Himalaya

AbstractFew studies have constrained the magnitudes and timescales associated with large‐scale drainage captures (areas >103 km2), even though these constraints are crucial to reconstruct sediment budgets, assess the potential for drainage reorganization to be preserved in the rock record, and determine the extent to which environmental signals (i.e., structures, composition and fossil assemblages within sedimentary rocks that are influenced by sediment supply and transport) are representative of conditions during deposition. In this work, we characterize the Pleistocene capture of the Zhada Basin, an ~23 000 km2 extensional basin in southern Tibet, by the Sutlej River, a prominent tributary to the Indus River. We quantify the magnitudes and timescales of capture‐driven erosion using knickpoint celerity modelling, paleotopographic reconstructions, 10Be‐derived denudation rates, and topographic analyses of drainage divides. We find that capture has removed 2010 ± 400 km3 of sediment from the Zhada Basin, increasing sediment supply to the Sutlej network by 17%–29% since 735 ± 269 ka. This work represents a crucial step towards reconstructing the Pleistocene sediment budget of the Indus sedimentary system and identifying potential impacts from sediment redistribution. We also identify several plausible tectonic or autogenic mechanisms that may have facilitated capture of the Zhada Basin, including: (1) preferential erosion of weak lithologies along active faults, (2) headward erosion in response to prior capture of the Spiti River and (3) headward erosion generated by breaching of a structural culmination downstream (the Kullu‐Rampur Window). This provides a framework to assess the mechanistic links between arc‐parallel extension, large‐scale drainage capture, landscape evolution and orogenic wedge deformation.

Tracing marine flooding surface equivalents across freshwater peats and other wetland deposits by integrated sedimentological and pollen data

Large volumes of peatland deposits characterise the Holocene stratigraphy of the Po Plain. A combination of sedimentological and pollen-based paleoenvironmental analyses enables recognition and stratigraphic correlation of small-scale (2–5 m thick) packages of peat-bearing strata, stacked rhythmically in a retrogradational to progradational set and bounded by chronostratigraphically significant surfaces. Across these repetitive lithofacies successions, the proportion of facies-controlled palynomorphs is used as a diagnostic signature to characterise marine flooding surface equivalents (helophytes and hydrophytes) and shoaling-upward (terrestrialization) trends (trees and mesophilous herbs) that record systematic variations in groundwater table associated with increasing/decreasing accommodation. Paludification surfaces at the base of peats delineate the updip (freshwater) equivalents of brackish/marine flooding surfaces recognized at seaward locations atop peat layers (give-up transgressive surfaces). Peat beds exhibit maximum thickness in aggradational strata of the lowermost highstand systems tract, above the maximum flooding surface (MFS). An extrinsic control due to eustatic rise can be inferred for peat development in transgressive deposits: peats, in particular, reveal warmer climates at flooding surfaces (specifically around the MFS) that invariably coincide with rapid shifts to deeper depositional environments. Under highstand conditions, autogenic mechanisms affected base-level changes in the paralic swamps. At this stratigraphic level, peat-bearing strata primarily reflect subtle changes in accommodation due to distributary-channel avulsion, subsidence, and peat autocompaction. Detailed patterns of Holocene peat distribution on millennial timescales can help decipher multiple high-resolution accommodation cycles developed in the rock record on sub-seismic scales, resulting in an improved stratigraphic analysis and prediction of chronologically less constrained non-marine successions.

Cyclic stacking pattern, architecture and facies of the turbidite lobes in the Macigno Sandstones Formation (Chattian-Aquitanian, northern Apennines, Italy)

This study presents a detailed facies analysis of an 800 m-thick stratigraphic succession in the basinal turbidites of the Macigno Sandstones Formation (MSF) recording the first foredeep in the northern Apennines (Italy). This stratigraphic succession shows an evident stacking pattern that, besides the basal part (200 m thick) dominated by fine grained and thin beds, is characterized by seven magnificently exposed sandstone lobes (from 90 to 19 m thick) separated by well-developed fine-grained interlobe deposits. Thanks to the detailed facies analysis, not only a well-defined architectural hierarchy has been identified, but a facies scheme has also been proposed, whereby cyclic recurring of three facies associations has in turn been identified. Each facies association records a well-determined evolutionary stage of lobe development, where an early depositional stage of the lobe building phase is followed by a flow impact and bypass stage recording flow decelerations and hydraulic jumps induced by the morphology created by the underlying depositional lobe. The latter phase promotes a switching and channel feeder avulsion that heralds the third phase of abandonment with the deposition of fine-grained interlobes. The facies stacking pattern and the evolutionary model of the MSF - also supported by other similar field and experimental works - are intended as a contribution to the understanding of intrinsic autogenic mechanisms at the basis of sand-rich depositional lobe development.

Invasive ecosystem engineers threaten benthic nitrogen cycling by altering native infaunal and biofouling communities

Predicting the effects of invasive ecosystem engineering species in new bioregions has proved elusive. In part this is because separating biological effects from purely physical mechanisms has been little studied and yet could help predict potentially damaging bioinvasions. Here we tested the effects of a large bio-engineering fanworm Sabella spallanzanii (Sabella) versus worm-like structures (mimics) on gas and nutrient fluxes in a marine soft bottom sediment. Experimental plots of sediment in Hauraki Gulf (New Zealand) were used to test the hypothesis that ecosystem engineers negatively influence benthic ecosystem function through autogenic mechanisms, facilitating activity by biofouling organisms and competitive exclusion of native infauna. Enhanced physical structure associated with Sabella and mimics increased nitrogen fluxes, community metabolism and reduced denitrification from 23 μmol m−2 h−1 to zero at densities greater than 25 m2. Sabella plots on average had greater respiration (29%), NH4 release (33%), and greater NO3 release (52%) compared to mimics, suggesting allogenic (biological) mechanisms occur, but play a secondary role to autogenic (physical) mechanisms. The dominance of autogenic mechanisms indicates that bio-engineers are likely to cause significant impacts when established, regardless of fundamental differences in recipient regions or identity of the introduced bio-engineer. In the case of Sabella spallanzanii, compromised denitrification has the potential to tip the balance of net solute and gas exchanges and cause further ecological degradation in an already eutrophic system.

Autogenic Erosional Surfaces in Fluvio-deltaic Stratigraphy from Floods, Avulsions, and Backwater Hydrodynamics

AbstractErosional surfaces set the architecture of fluvio-deltaic stratigraphy, and they have classically been interpreted in terms of changes in boundary conditions such as climate, tectonics, and base level (allogenic forces). Intrinsic dynamics of sedimentary systems (autogenic dynamics) can also create a rich stratigraphic architecture, and a major knowledge gap exists in parsing the relative roles of autogenic versus allogenic processes. Emerging theoretical and experimental work suggests that backwater hydrodynamics play an important role in driving transient channel incision in river deltas, even those experiencing net aggradation. Here, we identify and quantify two autogenic mechanisms that produce broad erosional surfaces in fluvio-deltaic stratigraphy, namely, floods and avulsions. Using a simple mass-balance model for single-threaded delta channel systems, we show that flood-induced scours begin near the shoreline, and avulsion-induced scours begin at the avulsion site, and both propagate upstream over a distance that scales with the backwater length, bed slope, and bed grain size. We also develop scaling relationships for the maximum scour depths arising from these mechanisms, which are functions of characteristic flow depth and formative flood variability. We test our theoretical predictions using a flume experiment of river delta evolution governed by persistent backwater hydrodynamics under constant relative sea level. Results indicate that autogenic dynamics of backwater-mediated deltas under conditions of constant base level can result in stratigraphic surfaces and shoreline trajectories similar to those often interpreted to represent multiple sea-level cycles. Our work provides a quantitative framework to decouple autogenic and allogenic controls on erosional surfaces preserved in fluvio-deltaic stratigraphy.

Anatomy and evolution of a dynamic arroyo system, Kanab Creek, southern Utah, USA

Abstract Many alluvial valleys in the American Southwest are entrenched within continuous arroyos, and stratigraphic evidence indicates that these fluvial systems experienced repeated periods of entrenchment and aggradation during the mid- to late-Holocene. Previous research suggests arroyo dynamics were regionally quasi-synchronous, implying that they were driven by allogenic forcing due to hydroclimatic fluctuations. However, several of these interpretations rely on records with limited age control and include distal correlations across the American Southwest. While hydroclimatic variability must exert some role, autogenic mechanisms related to catchment-specific geomorphic thresholds are hypothesized to partially control the timing of arroyo dynamics. If driven by autogenic processes, episodes of arroyo cutting and filling may not be regionally contemporaneous. Recent improvements in dating methods permit more detailed reconstructions of the timing and evolution of arroyo dynamics, allowing for a more nuanced assessment of these competing hypotheses. Here we present a uniquely large and focused chronostratigraphic data set from two alluvial reaches of Kanab Creek, located in the Grand Staircase region of southern Utah. Episodes of prehistoric arroyo cutting and filling are reconstructed from 27 sites through recognition of soils and buttressed unconformities in the arroyo-wall stratigraphy, and age control derived from 54 optically stimulated luminescence (OSL) ages and 50 radiocarbon ages. Our chronostratigraphic data set indicates five periods of channel aggradation occurred since ca. 6.0 ka, with each interrupted by an episode of arroyo entrenchment. Repeated aggradation to a similar channel elevation suggests attainment of a threshold profile, and comparison of the pre-entrenchment longitudinal profile with the modern arroyo channel demonstrates that changes between end-member entrenched and aggraded states are expressed in channel concavity and slope. We propose that arroyo dynamics are partially driven by sediment supply and the rate of channel aggradation, and that these systems must approach complete re-filling before they become sensitive to incision. Entrenchment itself appears to be associated with rapid transitions from pronounced decadal-scale aridity to pluvial (wetter) periods. Not all such hydroclimatic fluctuations are associated with arroyo entrenchment, which highlights the importance of threshold controls on the behavior of these systems. The collective period of “dynamic instability” characterized by epicycles of arroyo entrenchment and aggradation did not initiate until the mid-Holocene, when a climatic shift toward warmer and drier conditions likely increased fine-grained sediment supply to the fluvial system.

Sea-level control on the submarine fan architecture in a deepwater sequence of the Niger Delta Basin

Abstract Submarine fan architecture has long been considered much complicated as controlled by both allogenic and autogenic mechanisms. However, to which extent the allogenic sea-level change controls the submarine fan architecture is still unclear. This study uses integrated 3-D seismic, well-log and core data to characterize the submarine fan architecture in a deepwater sequence of the Niger delta basin. Correlation of the timing of architecture units to high-frequency sea-level curve helps to investigate the sea-level control on architecture distribution and evolution of submarine fan systems. The studied sequence is a 3rd-order sequence that formed during 12.5–10.5 Ma and contains a lowstand systems tract (LST) and a transgressive-highstand systems tract (TST-HST). We find that the LST develops a single channel system that is terminated by a lobe system; whereas the TST-HST develops two mutually incised channel systems and mudflow deposits, which are distinct from the conventional condensed sections and interpreted to result from the repeated 4th-order sea-level cycles during the TST-HST of the 3rd-order sequence. Single submarine fan systems, which last for ∼0.3–0.6 Myr, are assumed to form in response to single 4th-order sea-level cycles since they are well correlated with each other. Each single submarine fan system contains several vertically stacked complex sets, which are well correlated with 5th-order sea-level cycles that last for ∼0.1–0.2 Myr, suggesting their potential genetic links. Different complex sets in a single submarine fan system exhibit variable architectural features, which are interpreted to result from the evolution of gravity-flow types as the 4th-order sea-level fluctuates. Thus for the Niger system, at time scales >0.1 Myr, the allogenic sea-level change is considered to have played a major role in controlling the submarine fan architecture at the scale of submarine fan systems and complex sets, whereas autogenic effects on the submarine fan architecture may increase and become dominant on time scales

Plio-quaternary deposits in the Eastern Rharb (Nw Morocco): Electrosequential characterization

The Rharb basin, of which our study area is part, is located at the western extremity of the south Rif corridor. It corresponds to a subsiding zone that appeared in the Upper Miocene, between two major structural domains: the Rif to the north and east and the Meseta domain to the south. The eastern part of this basin is characterized by a Plio-Quaternary continental fill. Its geographical position and its structural and paleo-environmental contexts are reflected by a great facies heterogeneity.This work aims to image the subsurface structure and to characterize the distribution mode of Plio-Quaternary deposits of the eastern Rharb. The use of a database consisting of geo-electrical cross sections, hydrogeological drilling and wireline logging integrated in a Geographic Information System (GIS) allowed us to establish a new three-dimensional model of the top of the Mio-Pliocene substratum, new geo-electrical cross sections, as well as the isopach maps of lower, intermediate, upper and superficial geo-electric interval.This approach allowed us to characterize the Plio-Quaternary deposits of the study area and to highlight the effects of the tectonic regime and the relative sea level fluctuations on the sequential organization of these deposits. Our new model shows the development of prograding, aggrading and retrograding parasequences denoting the existence of autogenic mechanisms in the organization of plio-quaternary deposits of the eastern part of the Rharb basin. Therefore, it opens new perspectives for the exploration of water resources and monitoring their quality throughout the basin.

Lower Permian fluvial cyclicity and stratigraphic evolution of the northern margin of Gondwanaland: Warchha Sandstone, Salt Range, Pakistan

During the Lower Permian (Artinskian), fluvial conditions prevailed in what is now the Salt Range of northern Pakistan. Deposits of the Warchha Sandstone are characterised by a range of fluvial facies and architectural elements that together record both the proximal and distal parts of a meandering river system that drained the northern margin of Gondwanaland. Stratigraphic units are arranged into vertically stacked fining-upward cycles represented by thin accumulations of channel-lag deposits at their bases, and sandstone-dominated channel fill and thicker accumulations of overbank mudstone at their tops. Sedimentary cyclicity records fluvial system development on a variety of spatial and temporal scales. Overall, the Warchha Sandstone preserves a series of three to ten vertically stacked fining-upward cycles that form part of a larger-scale, third-order sequence that is itself bounded by regionally extensive and laterally correlatable unconformities that were generated in response to combined tectonic and eustatic changes. The sequence-stratigraphic architecture reflects regional palaeogeographic development of the Salt Range region. The small-scale fluvial cycles originated through autogenic mechanisms, predominantly as a result of repeated channel avulsion processes that occurred concurrently with on-going subsidence and the progressive generation of accommodation. Each erosively based fining-upward fluvial cycle is divided into three parts: a lower part of trough cross-bedded conglomerate and coarse sandstone; a middle part of tabular cross-bedded, ripple cross-laminated and horizontally laminated sandstone; and an upper part of predominantly horizontally laminated and massive mudstone. Overall, the Warchha Sandstone records the progradation of a wedge of non-marine strata into a previously shallow-marine depositional setting. The underlying marine Dandot Formation is terminated by a major unconformity that represents a type-I sequence boundary associated with a regional relative sea-level fall and a significant regression of the Tethyan shoreline. The overlying Warchha Sandstone represents the onset of the subsequent lowstand systems tract in which a northward-flowing meandering river system redistributed clastic detritus derived from a tectonically-active source area (the Aravalli and Malani ranges) that lay to the south. This episode of fluvial sedimentation was terminated by a widespread marine transgression recorded by an abrupt upward transition to estuarine and shallow-marine deposits of the overlying Sardhai Formation. This change marks the transition from lowstand deposits to a transgressive system tract.

Autogenic dynamics of alluvial fans in endorheic basins: Outcrop examples and stratigraphic significance

AbstractAlluvial fans are relatively simple depositional systems, due to the direct coupling of sediment sources and adjacent accumulation areas. Nonetheless, general models of alluvial‐fan evolution and stratigraphy remain elusive, due to the great sensitivity of such systems to allogenic controls and their strongly case‐specific responses. Autogenic processes intrinsic to alluvial‐fan dynamics can complicate stratigraphic architectures, with effects not easily distinguishable from those of allogenic forcing. A distinction is made here between lateral autogenic dynamics, tied to spatial sediment distribution over fan surfaces, and vertical autogenic dynamics, related to independent incision‐aggradation cycles. Autogenic mechanisms have been highlighted recently by modelling studies, but remain poorly constrained in field‐based studies. Examples are presented here from the margins of the Cenozoic Teruel and Ebro basins (Spain), where alluvial fans accumulated thick successions during phases of basin topographic closure and endorheic drainage which promoted forced aggradation. Fan successions consist of conformable architectures of stacked clastic sheets, laterally continuous and with no evidence of internal unconformities, inset architectures, fan segmentation or preserved incised channels. Continuous aggradation in these closed basins strongly inhibited ‘vertical’ autogenic dynamics in the form of fan head and through fan incision, due to the forced rise in geomorphic base level and the creation of positive accommodation. Furthermore, the lack of incised channels favoured widespread sediment transport and aggradation over broad fan sectors in relatively short time spans, in contrast to the typical occurrence of active lobes and abandoned fan surfaces caused by ‘lateral’ autogenic dynamics. Stratigraphic records of alluvial fans developed in endorheic basins are essentially complete and largely unaffected by autogenic processes. The latter characteristic implies that they can be more unambiguously interpreted in terms of allogenic forcing, because stratigraphic signatures are not complicated by the effects of complex fan autodynamics.

Erosion dynamics modelling in a coupled catchment–fan system with constant external forcing

Recent alluvial fan models have suggested that deep alluvial fan entrenchment could occur without any change in sediment and water influx. Moreover, other studies have shown that the evolution of a fan could strongly depend on feedback between the fan and the mountain catchment. We evaluate if natural entrenchment still occurs in a coupled catchment–fan system, and we evaluate its possible impact on the evolution of mountain erosion. We use a landscape evolution model where the mountain corresponds to an uplifting block and where fans form over an initial horizontal surface. Our experiments confirm that deep entrenchment at the fan apex can occur without a change in climate or in uplift rate under two conditions: (1) the transport threshold (critical shear stress) is significant and (2) the downstream boundary condition corresponds to an open boundary with fixed elevation, which stops the progradation of alluvial fans. A stable entrenchment occurs when sediments reach this limit. The entrenchment can be explained by a strong nonlinear dependence of the transport law on the slope when the shear stress is close to critical. A fan slope increase occurs when sediments reach the model limit, and this drives entrenchment. Fan entrenchment drives strong erosion in the mountain with intensity and a response time similar to those observed for the initial mountain uplift. These results indicate that determining how much of natural erosion is from autogenic mechanisms is essential in order to link landscape entrenchment with past external changes.

Distinguishing allogenic from autogenic causes of bed elevation change in late Quaternary alluvial stratigraphic records

Allogenic and autogenic mechanisms both cause changes in the bed elevation of rivers and thereby influence the characteristics of alluvial stratigraphic records (ASRs). Allogenic forcing mechanisms can be grouped into five categories whose relative influence varies with timescale: climate, tectonism, base level, land use/land cover and direct human modification of channels. Late Quaternary ASRs are influenced by the greatest range of allogenic forcing variables with climate among the most important. Autogenic mechanisms of bed elevation change are ubiquitous throughout fluvial systems and are always time-transgressive. Autogenic bed elevation change propagates through drainage networks at predictable rates and results in a time-space envelope within which its effects are capable of operating. ASRs that can be correlated over geographical areas large enough and time intervals small enough to exist outside this envelope most likely result from allogenic forcing. This formulation represents a quantitative and geographic set of threshold criteria for distinguishing between autogenic and allogenic mechanisms. Over late Quaternary timescales (10 2 to 10 5 years) in tectonically stable regions climate change is the dominant allogenic mechanism and, therefore, the first-order control on the morphology, sedimentology, pedologic characteristics and chronology of alluvial stratigraphic records that meet or exceed the threshold criteria for demonstrating allogenic causality.

Sedimentary evolution of the Late Eocene Vernet lacustrine system (South-Central Pyrenees). Tectono-climatic control in an alluvial-lacustrine piggyback basin

A complete third-order tectono-sedimentary cycle filled the Vernet lacustrine piggyback basin during the late Eocene. The depositional sequence can be subdivided into five units or systems tracts with distinct architectural frameworks, which developed a progressive unconformity. Sedimentary infilling of the piggyback basin began with retrogradational alluvial fans and fluvio-alluvial deposits being deposited, arranged in two thinning- and fining-upward cycles during the uplifting and emplacement of the southernmost Pyrenean thrusts, and with three thickening- and coarsening-upward progradational floodplain and lacustrine cycles, during the quiescence of the southernmost thrusts, at the same time as the emplacement of several northernmost thrusts of the southern Pyrenees. The facies associations identified represent different depositional environments in a sheetflow-dominated system, including playa-lake deposits, floodplain deposits, sheetflow fluvial deposits and sheet deltas in ephemeral to shallow lacustrine settings. Fourth and fifth order arid/semi-arid climatic cycles can be recognized, driven either by orbital obliquity and precessional forcing (Milankovitch cyclicity), insolation cycles or possibly subprecessional autogenic mechanisms related to the emplacement of northern thrusts and the consequent changes in the drainage basin. Arid periods are recorded by sheetflood fluvial, ephemeral lacustrine or terminal splay deposits, with abundant medium-to-coarse-grained sandstone packages in red-to-grey mudstones, a product of unconfined or low-confined sheetflows or hyperconcentrated flows, with bedload and suspended load, during flood episodes, at times of lake lowstand. Semi-arid periods are recorded as relative highstand lacustrine deposits, including grey-to-white mudstones deposited in prodelta environments and fine-grained sheet sandstone units sedimented in a delta front (with minor channels and extensive sheet lobes) and prodelta environments in shallow lakes, from sheetflood events.

Phytoplankton structure and dynamics at a daily temporal scale: Response to the thermal overturn

We examined phytoplankton structure and dynamics at the daily scale in a temperate lake during overturn (75 days). Fifty species of phytoplankton inhabited the oligo-mesotrophic Las Madres Lake (Spain) throughout the study period, demonstrating different strategies and resulting distribution patterns. The distribution of dominant taxonomic groups changed through the overturn, varying within four dendrogram sets of dates. Biomass was affected by changes in species composition as a result of the overturn process (the rate of change been markedly greater at the beginning and end). Autogenic mechanisms in the density of the main algae were highlighted by the diversity. The diversity index trajectory changed at start of overturn and in the middle of the complete mixing period. A Principal Components Analysis showed four different groups of dates, achieved by overturn process and by endogenous changes in the unperturbed period. Time series analyses (auto- and cross-correlations) showed delays of between a few days to some weeks, depending on species strategies (body size-related) and changes in the biomass trajectory at three weeks. Therefore, four alternate persistent states or alternate structures of phytoplankton were observed and described at the daily scale. We suggest that there were three mechanisms which drove the changes of state, namely overturn, grazing and competition. Population strategies may suggest how community structure changes at a down-scale level. Algal assemblage responses to daily changes were slower than population ones and showed greater persistence. The emergent assemblage behaviour was well exhibited by structural dynamics and biomass. In other words, changes in a lower level of ecological hierarchy (populations) led to some stability in an upper level (assemblage, community) when the latter was studied at the population time scale.

The habitat diversity and fish reproductive function of floodplain ecosystems

Fish reproduction in floodplain ecosystems, based on relative abundance and total biomass of 0+ juveniles, was studied using the synchronic approach to typological analysis in conjunction with Point Abundance Sampling by modified electrofishing. In 3 different flood plains of the Upper Rhone River, 1015 point samples yielding 4573 juveniles (0+) from 21 species were collected from 48 ecosystems of various geomorphological origin. The results demonstrate the lotic-to-lentic succession of floodplain ecosystems to be a series of non-sequential reproductive zones, with spawning conditions being reflected by the specific composition and guild structure of the YOY fish assemblages. The habitat diversity and the fish reproductive potential of floodplain ecosystems are strongly influenced by geomorphological origin and by past and present hydrological conditions. The YOY assemblages of autogenically driven ecosystems (usually of anastomose or meander origin) tend to differ both in composition and in quantity from those found in allogenically driven ecosystems (generally of braided origin). Ecosystems of intermediate character, and fish reproduction thereof, occur as the result of either ecosystem rejuvenation or senescence: autogenically driven ecosystems by allogenic mechanisms, or allogenically driven ecosystems by anthropic and/or autogenic mechanisms, respectively. Because of co-occurrence of ecosystems at similar and at different successional status, the flood plain as an entity is seen as ‘stable’ with respect to fish reproduction.