Incised Valleys Research Articles

Evaluation and assessment of the northern shelf adjacent to the Apodi River incised valley, Rio Grande do Norte (RN), Brazil

This study was conducted in the area adjacent to the Apodi River Incised Valley, near the city of Areia Branca, RN, Brazil. The objective was to evaluate the responses of foraminiferal associations to environmental aspects. Ten surface sediment samples were collected from the northern shelf of the state of Rio Grande do Norte, along with the measurement of abiotic parameters. Granulometry (represented by silt content) was found to have the least influence on the dispersion of foraminiferal species, while physical water variables, mainly depth, showed a greater impact. An important observation in the northern region of the state was the absence of a single species dominating all stations. Instead, some stations were dominated by Quinqueloculina lamarckiana, while others were dominated by Amphistegina gibbosa.

Incised valleys drive distinctive oceanographic processes and biological assemblages within rhodolith beds

Continental shelves encompass gently sloped seascapes that are highly productive and intensively exploited for natural resources. Islands, reefs and other emergent or quasi-emergent features punctuate these shallow (<100 m) seascapes and are well known drivers of increased biomass and biodiversity, as well as predictors of fishing and other human uses. On the other hand, relict mesoscale geomorphological features that do not represent navigation hazards, such as incised valleys (IVs), remain poorly charted. Consequently, their role in biophysical processes remains poorly assessed and sampled. Incised valleys are common within rhodolith beds (RBs), the most extensive benthic habitat along the tropical and subtropical portions of the mid and outer Brazilian shelf. Here, we report on a multi-proxy assessment carried out in a tropical-subtropical transition region (~20°S) off Eastern Brazil, contrasting physicochemical and biological variables in IVs and adjacent RBs. Valleys interfere in near bottom circulation and function as conduits for water and propagules from the slope up to the mid shelf. In addition, they provide a stable and structurally complex habitat for black corals and gorgonians that usually occur in deeper water, contrasting sharply with the algae-dominated RB. Fish richness, abundance and biomass were also higher in the IVs, with small planktivores and large-bodied, commercially important species (e.g. groupers, snappers and grunts) presenting smaller abundances or being absent from RBs. Overall, IVs are unique and vulnerable habitats that sustain diverse assemblages and important ecosystem processes. As new IVs are detected by remote sensing or bathymetric surveys, they can be incorporated into regional marine management plans as conservation targets and priority sites for detailed in situ surveys.

Transgressive Architecture of Coastal Barrier Systems in the Ofanto Incised Valley and Its Surrounding Shelf in Response to Stepped Sea-Level Rise

Coastal deposits/barriers react to sea-level rise through rollover or overstepping. Preserved coastal deposits/barriers allow us to examine coastal responses to sea-level rise, an important aspect within the context of climate change. This study identifies the Ofanto incised valley and examines the possible factors that caused the considerable difference in shape between this valley and adjacent valleys: the Carapelle and Cervaro incised valley and Manfredonia incised valley. In addition, this study assesses the response of transgressive units to stepped sea-level rise with a focus on the evolution of palaeo-barriers/shorelines on the continental shelf and within the infill of Ofanto incised valley. We identified the traces of two slowstands in sea-level rise: the first, short-lived at a centennial scale, interrupted Meltwater Pulse 1A; the second is referable to part of Bølling-Allerød and Younger Dryas. During these two slowstands, two barrier-shoreface/estuarine-backbarrier systems formed. Meltwater Pulse 1A and Meltwater Pulse 1B led to overstepping and partial preservation of these systems in the form of aligned topographic highs. The second slowstand gave rise to continuous landward backstepping of the coastal barrier system; during the following Meltwater Pulse 1B (MWP-1B), landward rollover of the coeval barrier/backbarrier system occurred.

Late Quaternary landscape evolution of the buried incised valley of Concordia Sagittaria (Tagliamento River, NE Italy): A reconstruction of incision and transgression

While recent landscape changes can be usually easily read through accessible morphological features, relatively ancient processes can be difficult to detect as the associated morphologies and deposits are often buried below younger sediments. With the aim of understanding the evolution of the distal Venetian-Friulian Plain (NE Italy) after the Last Glacial Maximum (LGM), this work presents the analysis performed on a dataset of ca. 2300 mechanical and hand-made cores, which allowed a detailed reconstruction of the formation and evolution of an incised valley, now almost completely filled and with little to absent morphologic expression. Such valley, up to 1.2 km wide, with a depth of 20 m below the LGM alluvial plain and traced for a length of 25 km, is the result of the complex interplay between minor spring-fed rivers and the Tagliamento River. The detailed characterization of the infilling allowed to identify two main phases in the valley evolution. The first one is related to the activity of the paleo Tagliamento River and led to the deposition of a 10 m thick gravelly unit (ca. 19–9.5 ka cal BP). The second phase, which followed a disconnection of the Tagliamento, is linked to the Holocene marine transgression and led to the formation of a lagoon environment within the valley and to the deposition of a ca. 15 m thick unit of lagoon muds (ca. 8 ka cal BP - historic time). This latter unit lays on top of gyttja deposits, indicating for the first time in this area the presence of widespread lacustrine environments in the Early Holocene. This work presents an in-depth analysis on the evolution of a distal plain incised valley, from its formation to its final filling, providing at the same time the means to describe the development of the entire alluvial plain landscape, spanning from the end of the LGM to the middle Holocene. We present new data on the paleoenvironmental and morphologic evolution of the Venetian-Friulian Plain area as a consequence of the interplay between autogenic forces and sea-level rise. Our study allowed to understand the importance of both Alpine rivers and groundwater-fed streams in the formation of large incised valleys in the coastal sector of the whole Venetian-Friulian Plain. Finally, as during the transgressive phase the upstream sediment input in the valley was almost absent, this study provides insights into facies and architecture of a rare example of downstream-controlled filling of an incised valley.

Palynostratigraphy and lithostratigraphy of Upper Cretaceous and Paleogene outcrop sections, Mérida Andes (Maracaibo Basin), Western Venezuela

Detailed palynologic and lithostratigraphic studies of outcrop sections on the western flank of the Mérida Andes in Western Venezuela were undertaken to establish a framework for interpreting and correlating regional and local unconformities from the outcrop to subsurface data. The Upper Cretaceous to lower Paleogene section of the Maracaibo Basin consists of laterally continuous rock units attributed to the Mito Juan and Barco/Catatumbo formations, which are characterized by marine sediments deposited relatively close to shore. The continuity of plant and dinoflagellate species across the Cretaceous-Paleogene boundary in this area may have potential significance to K/Pg studies elsewhere. The lower to upper Paleogene Los Cuervos, Mirador, and Carbonera formations were predominantly deposited in non-marine environments, though palynologic and stratigraphic data from each unit show the variable influence of marine and lacustrine conditions. A new chronostratigraphic framework for the study area (the “Venezuelan Composite Standard”) was developed based on first and last appearances of spores, pollen, and dinoflagellates, and three pollen acme events and calibrated to the latest time scale. Graphic correlation plots of the most complete outcrop sections at Río Lobaterita, Río Lora, and Río Chama indicate the presence of significant hiatuses in time, due to erosion and non-deposition (or a combination of the two) reflecting the complex structural and depositional history of the southeastern Maracaibo Basin. A regionally significant upper Paleocene to lower Eocene unconformity, as well as several local and regional intra middle Eocene unconformities appear to have been caused by a drop in base level (sea level plus structural uplift) forming incised valleys over much of the study area. Unconformities are also present in the upper middle Eocene to Oligocene and represent ravinement surfaces caused by transgressive marine reworking of pre-existing units. These surfaces are often marked by the presence of dinoflagellates within the upper Eocene Carbonera Formation. Depositional environments during much of the Upper Cretaceous to Paleogene succession vary from fluvial-floodplain to marginal marine, with newly noted periods of widespread lacustrine deposition in all geologic units. Temporal differences between similar Paleogene lithostratigraphic units in northeastern Colombia and southwestern Venezuela are likely due to the influence and timing of structural events on regional and local depositional environments.

“Continuous” Backstepping of Holocene Coastal Barrier Systems into Incised Valleys: Insights from the Ofanto and Carapelle-Cervaro Valleys

Two recently recognised incised valleys in the Manfredonia Gulf are described. The first (CCV) is correlated with the current Carapelle and Cervaro streams. The second (OSFV) is correlated mostly with the current Ofanto River. Six seismic facies and seven unconformity-bounded seismic units have been identified, which infilled CCV and OSFV. In CCV, during the sea-level ranges from −29 to −18 and from −18 to −4.7 m b.s.l., two barrier/spit-backbarrier systems formed in the most landward sector of the valley. The lower system was attributed to a time interval between 9.2 ka BP and ca. 8.3 ka BP, chronologically constrained by the ZS2 borehole. In OSFV, during the sea level ranges from −39 to −29, and from −29 and to −18 m b.s.l., two beach/spit-backbarrier systems, arranged in a “continuous” landward backstepping pattern, formed. The phase that contributed most to the beach/spit-backbarrier systems formation is that which is coeval with the formation of the sapropel S1 in the Mediterranean. The conservation of barrier/spit-backbarrier systems arranged in a “continuous” landward backstepping pattern, is due to a strong and continued sediment supply that occurred during the sapropel S1 formation, coupled with low-gradient settings and a regime of slow sea-level rise.

Discovery of late Tortonian incised valleys in the Saouaf Formation (northeastern Tunisia) - Evidence of high-frequency sea-level variations in the central Mediterranean

During the late Miocene, the connection between the Atlantic Ocean and the Mediterranean Sea was progressively restricted. Despite many discoveries of Messinian incised valleys and canyons in the peri-Mediterranean basins, the Tortonian incised valleys are poorly described. The Tortonian series exposed in the northeastern end of the Cap Bon peninsula (northeastern Tunisia, central Mediterranean) provide an excellent example of complex and extensive incised valleys system traceable for up to 5 km (entire length of the outcrop). Detailed facies and sedimentological analysis permitted the recognition of five distinct phases of incision. These valleys (approximately 300 m wide and 10 m to 30 m deep) were incised during sea-level fall and were filled during subsequent sea-level rise. These valleys are characterized by the lack of fluvial deposits at their bases. During sea-level rise, these valleys were filled mainly with cross-bedded estuarine sandstone forming multistorey channels and point-bar deposits. Tidal sand flat deposits are locally well developed at the margins of the valleys fills. These tide-dominated estuarine deposits, corresponding to the transgressive systems tracts, are arranged into fining-upward retrogradationnal parasequences. The highstand systems tracts correspond to wave-dominated deltaic deposits comprising offshore, offshore-transition and shoreface deposits. These successions are typically arranged in thickening and coarsening-upward parasequences. The upper surfaces of the highstand systems tracts are incised by later valleys. The formation and the filling of these valleys record the complex interplay of different allocyclic factors: (1) tectonic uplift due to the major compressive Atlassic phase, (2) high-frequency sea-level variations, (3) warm and humid climate, and (4) high river sediment input. The results of this study provide important data, such as incised valleys geometries, dimension, extension and connectivity. The results of this study emphasis the potential link between high-frequency sea-level variation and progressive restriction of the Mediterranean during the late Tortonian.

Neogene-Quaternary onshore record in the lower Ebro river incised palaeovalley (Ebro margin, Catalan Coastal Range, NE Iberia)

The lower Ebro is a bedrock-alluvial mixed incised valley with a persistent degradational stacking architecture developed from latest Serravallian(?) to Holocene. This degradational pattern was probably controlled by isostatic rebound in NE Iberia and punctuated by major relative sea level changes that temporally accentuated or attenuated the palaeovalley entrenchment and sediment retention. Six allostratigraphic units in the palaeovalley constitute the onshore record of its evolution and the opening and connection of the Ebro Basin with the Mediterranean. This paper deals with the analysis and reinterpretation of these units in order to precise the sequence of events that took place on the onshore part of the Catalan continental margin during the Ebro River drainage entrenchment. Plausible chronology and palaeogeographic evolution of the Neogene-Quaternary drainage incision in the lower Ebro are also proposed. The early evolutionary stages of the incised palaeovalley (Latest Serravallian?-Tortonian-Early Messinian, from 11.63–9? to near 5.6Ma) were dominated by entrenchment and intense sediment transfer from the onshore to the offshore zones (erosion surface S2). These processes were only punctuated by the sedimentation of the alluvial palaeovalley unit M2 (late Messinian). The polygenetic onshore erosion surfaces S2 and S3 are linked here with the onshore erosive processes that fed the sedimentation of the terrigenous shelf-slope system of the offshore Castellon group and considered coeval with the offshore Messinian erosion surfaces (reflectors M and m). During a further evolutionary stage (Pliocene to Early Pleistocene from 5.3 to approximately 2Ma) the Early Pliocene major Mediterranean reflooding caused some sediment retention in the incised palaeovalley (sedimentation of unit P) but sediment transfer into the offshore remained very effective. In the last evolutionary stage (Early Pleistocene-Holocene, from 2Ma to present) the palaeovalley became again mainly degradational (generation of erosion surfaces S4 to S6 and sedimentation of stepped alluvial terraces Q1-2 to Q4). The onshore stratigraphic record, including the allostratigraphic units P and Q1-2 to Q4 and the related bounding surfaces S3 to S6, is correlated with the sedimentation of the terrigenous shelf-slope system of the offshore Ebro group.

Transition from incised valley to barrier island systems during MIS 5e in the northern Chiba area, Kanto Plain, central Japan

Facies analysis, together with palynostratigraphic studies of the Kioroshi Formation, which lies beneath the northern Chiba area of the Kanto Plain of central Japan, reveal the sedimentary processes in the fluvial to coastal settings during marine isotope stage (MIS) 5e in this region. The depositional sequence of the Kioroshi Formation comprises a lower incised valley system and an upper barrier island system. The incised valley system consists of fluvial, estuary, and central basin facies with sandbar facies in the baymouth area. In the study area, the overlying barrier island system includes flood tidal delta, spit, and lagoon facies. These facies form part of the barrier island system known as Paleo-Tokyo Bay, which covered the present-day Kanto Plain. On the basis of palynostratigraphic correlation with the oxygen isotope curve in an offshore core (MD01-2421), the fluvial and estuary facies, together with the lower half of the central basin facies of the incised valley system are interpreted to have accumulated during a sea level rise in the early stage of MIS 5e. The baymouth sandbar also developed and retrograded with the rising sea level and was abruptly covered by the central basin facies during the middle stage of MIS 5e, probably due to the migration of the sandbar to the outer-bay area. The abrupt change corresponds with the initial stage of development of the barrier island system. The landward (westward) retreating bay ravinement erosion resulted in the transition to the barrier island system during middle MIS 5e. Maximum transgression also occurred during middle MIS 5e. The back-barrier lagoon was infilled during the late stage of MIS 5e. The sedimentary processes of the Kioroshi Formation are almost in accordance with the marine isotope curve.

Controls On the Geometry And Evolution of Humid-Tropical Fluvial Systems: Insights From 3D Seismic Geomorphological Analysis of the Malay Basin, Sunda Shelf, Southeast Asia

Abstract High-resolution three-dimensional (3D) seismic reflection data from the Pleistocene-to-Recent succession of the Malay Basin (Sunda Shelf, SE Asia) have been used to evaluate the geometry, dimensions, distribution, and evolution of humid-tropical fluvial channel systems. Based on their seismic expression and variations in sinuosity and depth, we identify six fluvial channel types, which occur in eight, seismically defined, 18–145-m-thick depositional units. Each unit is characterized by a consistent vertical change in channel-body geometry and dimensions as follows (bottom to top): (1) relatively large (300–3000 m wide and 15–45 m deep), straight to low-sinuosity channels and/or large incised valleys at their bases, and (2) smaller (75–250 m wide and 8–23 m deep), highly sinuous channels at their tops. This cyclical stratigraphic architecture is interpreted as a mainly climatically driven change in fluvial discharge and sediment supply, rather than by eustatically or tectonically driven changes in relative sea level based on (i) the study area was located far upstream (ca. 700 km) of the coeval shoreline, and (ii) tectonically quiescent nature of the basin during the Pleistocene to Recent. Although the upstream controls were likely the main controls on the stratigraphic organization (Units 2–4 and 6), the large incised valleys (Units 5 and 7) are interpreted to be formed due a relative sea-level fall during the Last Glacial Maximum (LGM). This study demonstrates how an improved understanding of the temporal and spatial variability of fluvial channel systems can be obtained through detailed analysis of regionally extensive, high-resolution 3D seismic reflection data. In particular, regional constraints on channel geometry, scale, and orientation can be more confidently determined than is typically permitted by deeper, lower-resolution subsurface data. Furthermore, we have provided quantitative constraints on the dimensions, geometries, and paleohydrology of humid-tropical fluvial systems, which form significant, but generally less well-imaged, hydrocarbon-bearing reservoirs, in the more deeply buried parts of the Malay Basin and in many other basins around the world.

Form, evolution and controls of a Jurassic incised valley‐fill: middle part of the Western Sichuan Depression, China

The second member of the Middle Jurassic Shaximiao Formation in the rejuvenated Western Sichuan Foreland Basin contains tens of metres of thick incised valley successions. Incised valleys have been widely discussed in traditional models; however, quantitative and detailed analyses of geometric variations and their interpreted controls on ancient incised valleys have rarely been attempted in the past. Using an integrated dataset of seismic profiles, well logs and cores, this study quantifies the geometric parameters of incised valleys and investigates the smaller‐scale depositional patterns. (1) During this period, incised valleys (IV) were 5–17 km wide, 20–60 m deep and traceable for 120 km along their axis, placing these valleys among the longest seismically imaged incised valleys in the world. (2) Three nested incised valley successions IV‐1, IV‐2 and IV‐3 can be identified. Except for IV‐2, the incised valleys decrease in width and depth downstream. (3) In the downstream segment, the role of tectonism diminishes gradually, and periodic base‐level changes control the form and evolution of the incised valleys. A downstream decrease in the stream power appears to be responsible for the decreasing trend in the incised valley geometry. Copyright © 2016 John Wiley & Sons, Ltd.

Facies transition and depositional architecture of the Late Eocene tide-dominated delta in northern coast of Birket Qarun, Fayum, Egypt

Late Eocene successions in the Fayum Depression display notable facies transition from open-marine to brackish-marine realms. Stratigraphic and sedimentologic characteristics of the depositional facies are integrated with ichnological data for the recognition of four facies associations (FA1 to FA4). The transition from open-marine sandstones (FA1) to the brackish-marine deposits (FA2) heralds a transgressive – regressive dislocation. The shallowing- and coarsening-upward progradation from the basal prodelta mudstone-dominated facies (FA2a) to deltafront heterolithics (FA2b) and sandstone facies (FA2c) are overlain by finning-upward delta plain deposits which are expressed by the delta plain mudstone (FA2d) and erosive-based distributary channel fills (FA4). Prodelta/deltfront deposits of FA2 are arranged in thinning- and coarsening-upward parasequences which are stacked in a shallowing-upward progressive cycle. Shallow-marine fossiliferous sandstones (FA3) mark the basal part of each parasequence. Stratigraphic and depositional architectures reflect a tide-dominated delta rather than an estuarine and incised valley (IV) model. This can be evinced by the progressive facies architecture, absence of basal regional incision or a subaerial unconformity and the stratigraphic position above a maximum flooding surface (MFS), in addition to the presence of multiple tidally-influenced distributary channels. Stratigraphic and depositional characteristics of the suggested model resemble those of modern tide-dominated deltaic systems. Accordingly, this model contributes to our understanding of the depositional models for analogous brackish-marine environments, particularly tide-dominated deltas in the rock record.

Sequence stratigraphy and seismic geomorphology application of facies architecture and sediment-dispersal patterns analysis in the third member of Eocene Shahejie Formation, slope system of Zhanhua Sag, Bohai Bay Basin, China

An important hydrocarbon reservoir is hosted by the third member of the Shahejie Formation (Es3) in the Zhanhua Sag of the Bohai Bay Basin. Using lithology, wire-line logs and three-dimensional (3-D) seismic data in the southern slope of Zhanhua Sag, we demonstrated the utility of stratal slice images for analyzing facies architecture and sediment-dispersal characters of complex depositional systems in Es3. The Es3, a para-second-order sequence, can be subdivided into three third-order sequences (from base to top: SQ1, SQ2, and SQ3). The facies architecture was analyzed by using the seismic geomorphology approach based on 3-D seismic data. Sediments of the Es3 sequences were derived from the southern Chenjiazhuang Uplift via six major incised valleys, four of them associated with slope-break belt in the southern zone, and others in the southwestern zone. Seismic stratal slices reveal different characteristics of the channels and lobes between south and southwest from source to sink. On the basis of an integrated analysis of well log, core data, seismic facies based on multi-seismic attributes, three depositional environments (e.g., “fan-delta”, “shallow lacustrine” and “turbidite” facies) have been recognized. Stratal slices indicate that the depositional environments of these sequences evolved from gravel- or sand-rich fan delta and turbidite to lacustrine mud, and lastly to mixed sand-mud fan delta systems. The results of high resolution 3-D depositional systems analysis showed that the third-order sequence located in different systems tracts of the para-second-order sequence would contribute to their systems tracts with the different types and scales of depositional systems. Types of slope belts, subsidence rate, sediment provenance and their evolution jointly control the sediment-dispersal characters in deposition area. The proposed sediment dispersal patterns may aid in the prediction of potential reservoir distribution.

Dating Pleistocene deltaic deposits using in-situ 26Al and 10Be cosmogenic nuclides

The present study aims at testing the possibility of using the in-situ cosmogenic burial dating technique on deltaic deposits. The sequence analyzed is exposed along the Ligurian coast (north-west Italy) and is made of proximal marine and continental deposits previously considered Pliocene or Plio-Quaternary in age. In the study area two allostratigraphic units were recognized. The lower unit represents the evolution of a small coarse-grained delta developed in a fjord or embayment environment. The coarsening/shallowing upward trend observed within the sections, from bottom to top, suggests that the delta prograded rapidly in the landward portion of the canyon placed opposite to the paleo-river outlet. Within the deltaic sequence the transgressive and highstand system tracts were recognized. The unit 2 is composed by several alluvial fan systems deposited in small incised valleys developed within the previously, uplifted deltaic deposits and successively incised by a braided river system. In-situ produced cosmogenic nuclides were used in order to date the age of the deposition of the deltaic deposits. Results suggest that the studied deltaic sediments belonging to the unit 1 were deposited between 1,300,000 and 200,000 year ago thus during the Lower to Middle Pleistocene, whereas the unit 2 was deposited during the Middle Pleistocene as a consequence of a tectonically driven uplift phase. Furthermore samples collected within the prograding part of the delta show the higher denudation rates. The obtained results demonstrate that burial ages and related erosion rates inferred from cosmogenic nuclides concentrations can be considered as a very useful tool to reconstruct the sea level changes over the past 1 million year.

The investigation of a mixed carbonate-siliciclastic shelf, NE Brazil: side-scan sonar imagery, underwater photography, and surface-sediment data

Side-scan sonar imagery and underwater photography, coupled with bathymetric and sedimentologic data, revealed noteworthy features and the interplay of multiple processes on the north-eastern Brazilian continental shelf. The high-resolution data cover a 500 km2 area of the shallow (~70 m) and narrow (~40 km) shelf, which comprises the submerged portion of the Potiguar Basin; the area includes the partially filled incised valley of the ancient Acu River.The seafloor imagery was integrated with 673 surface sediment samples to distinguish boundaries within the mixed siliciclastic-carbonate region. The sediment grain size increases from nearshore, muddy, siliciclastic sediments to biogenic, coarse sand and gravel on the outer shelf. Carbonate-rich sediment predominates, except on the eastern margin of the Acu Incised Valley (AIV), the nearshore zone and estuary mouths. The AIV is filled mainly with muddy sediments reworked from the latest transgressive deposits. Bedform configurations indicate bottom currents associated with tidal and wave-driven currents.Mixing processes are controlled by the interplay of high hydrodynamic energy regime and shelf physiography. Special attention was paid to the middle and outer shelves, where a large field of newly found reefs occurs between 20 and 50 m depths. The reef substrate is composed of limestones, probably belonging to the Guamare Formation.SCUBA dive inspections revealed benthic colonies (corals: Montastraea cavernosa, Porites astreoides, Siderastrea stellata; sponges: Scopalina ruetzleri, Callyspongia vaginalis, Ectyoplasia ferox, Ircinia sp., Spirastrella sp., Aiolochroia sp.) over fossil reefs. A preliminary examination of the foraminiferal species showed a Caribbean-type reef community, including Amphisorus hemprichii, Amphistegina gibbosa, Archaias angulatus, Heterostegina antillarum, Homotrema rubra, Peneroplis carinatus and P. proteus.

Mapping and quantifying groundwater inflows to Deep Creek (Maribyrnong catchment, SE Australia) using 222Rn, implications for protecting groundwater-dependant ecosystems

Understanding groundwater inflows to rivers is important in managing connected groundwater and surface water systems and for protecting groundwater-dependant ecosystems. This study defines the distribution of gaining reaches and estimates groundwater inflows to a 62 km long section of Deep Creek (Maribyrnong catchment, Australia) using 222Rn. During summer months, Deep Creek ceases to flow and comprises a chain of ponds that δ18O and δ2H values, major ion concentrations, and 222Rn activities imply are groundwater fed. During the period where the river flows, the relative contribution of groundwater inflows to total river discharge ranges from ∼14% at high flow conditions to ∼100% at low flows. That the predicted groundwater inflows account for all of the increase in discharge at low flow conditions lends confidence to the mass balance calculations. Near-continuous 27 week 222Rn monitoring at one location in the middle of the catchment confirms the inverse correlation between river discharge and relative groundwater inflows, and also implies that there are limited bank return flows. Variations in groundwater inflows are related to geology and topography. High groundwater inflows occur where the river is at the edge of its floodplain, adjacent to hills composed of basement rocks, or flowing through steep incised valleys. Understanding the distribution of groundwater inflows and quantifying the contribution of groundwater to Deep Creek is important for managing and protecting the surface water resources, which support the endangered Yarra pygmy perch.

Delta growth and river valleys: the influence of climate and sea level changes on the South Adriatic shelf (Mediterranean Sea)

Incised valleys across continental margins represent the response of fluvial systems to changes in their equilibrium dynamics, mainly driven by base level fall forced by glacial–eustatic cycles. The Manfredonia Incised Valley formed during the last glacial sea level lowstand, when most of the southern Adriatic shelf was sub-aerially exposed but the outer shelf remained under water. The pronounced upstream deepening of the valley is ascribed to river incision of the MIS5e highstand coastal prism and related subaqueous clinoform under the influence of MIS5-4 sea level fluctuations, while the downstream shallowing and narrowing mainly reflects the impact of increased rates of sea level fall at the MIS3-2 transition on a flatter mid-outer shelf. Until 15 ka BP, the valley fed an asymmetric delta confined to the mid-outer shelf, testifying that continental and deep marine systems remained disconnected during the lowstand. Sea level rise reached the inner shelf during the Early Holocene, drowning the valley and leading to the formation of a sheltered embayment confined toward the land: at this time part of the incision remained underfilled with a marked bathymetric expression. This mini-basin was rapidly filled by sandy bayhead deltas, prograding from both the northern and southern sides of the valley. In this environment, protected by marine reworking and where sediment dispersal was less effective, the accommodation space was reduced and autogenic processes forced the formation of multiple and coalescing delta lobes. Bayhead delta progradations occurred in few centuries, between 8 and 7.2 ka cal BP, confirming the recent hypothesis that in this area the valley was filled during the formation of sapropel S1. This proximal valley fill, representing the very shallow-water equivalent of the cm-thick sapropel layers accumulated offshore in the deeper southern Adriatic basin, is of key importance in following the signature of the sapropel in a facies-tract ideally from the shoreline to the abyss.

Controls On Lower-Coastal-Plain Valley Morphology and Fill Architecture

Abstract Small rivers that form at the apex of the coastal prism are common along many passive margins. They typically have drainage basins < 1000 km2, and their headwaters are isolated to low-relief areas of the previous highstand shoreline. As sea-level fall exposes the apex of the coastal prism, incision and drainage-basin evolution of those small rivers should be strongly influenced by coastal paleotopography, but previous work suggests that paleodischarge is the dominant control on incised-valley dimension. To examine the controls on the morphologic evolution of small incised valleys, we examined 20 incised valleys in North Carolina situated close to the last interglacial shoreline, including coastal-prism-incised valleys, which have well-developed dendritic drainage networks on the highstand-shoreline topography, and tributary incised valleys, which form adjacent to and merge with larger piedmont incised valleys. Coastal prism (n = 18) and tributary (n = 2) incised-valley width, measured at the common bayhead-delta location, is proportional to drainage-basin area (long-term discharge); however, that comparison location overlaps with the knickzone only for the larger river systems. The knickzone of small river systems is located down dip from the modern bay-head delta, which is likely due to slower rates of knickpoint migration. We use geophysical and lithologic data from three incised valleys to highlight differences in regional valley morphology that can be used to distinguish between coastal-prism and tributary incised valleys. The Newport and North coastal-prism incised valleys connect to small Sangamonian drainage networks of 210 km2 and 50 km2, respectively. Those valleys widen and deepen at the knickzone, which is also where smaller tributary systems merge with the main trunk valley. The Scuppernong tributary incised valley has a drainage basin of 400 km2 and can be differentiated from other incised valleys based on the absence of a compound fill and continuous seaward deepening and widening from the upstream limit of the knickzone. The knickzone is the junction of the Scuppernong with the adjacent larger Roanoke incised valley, the depth of which controls base level for the Scuppernong. The dramatic Scuppernong valley widening, in a down-dip direction, is principally due to highly erosive bay-ravinement processes associated with the adjacent much larger drowned Roanoke incised valley. These varying modes of valley evolution, specifically controls of paleotopography on valley incision and ravinement, explain the morphologies of small drowned river-mouth estuaries and improve understanding of basin-scale sediment-transport pathways.

Subglacial extensional fracture development and implications for Alpine Valley evolution

AbstractBedrock stresses induced through exhumation and tectonic processes play a key role in the topographic evolution of alpine valleys. Using a finite difference model combining the effects of tectonics, erosion, and long‐term bedrock strength, we assess the development of near‐surface in situ stresses and predict bedrock behavior in response to glacial erosion in an Alpine Valley (the Matter Valley, southern Switzerland). Initial stresses are derived from the regional tectonic history, which is characterized by ongoing transtensional or extensional strain throughout exhumation of the brittle crust. We find that bedrock stresses beneath glacial ice in an initial V‐shaped topography are sufficient to induce localized extensional fracturing in a zone extending laterally 600 m from the valley axis. The limit of this zone is reflected in the landscape today by a valley “shoulder,” separating linear upper mountain slopes from the deep U‐shaped inner valley. We propose that this extensional fracture development enhanced glacial quarrying between the valley shoulder and axis and identify a positive feedback where enhanced quarrying promoted valley incision, which in turn increased in situ stress concentrations near the valley floor, assisting erosion and further driving rapid U‐shaped valley development. During interglacial periods, these stresses were relieved through brittle strain or topographic modification, and without significant erosion to reach more highly stressed bedrock, subsequent glaciation caused a reduction in differential stress and suppressed extensional fracturing. A combination of stress relief during interglacial periods, and increased ice accumulation rates in highly incised valleys, will reduce the likelihood of repeat enhanced erosion events.

Large-scale single incised valley from a small catchment basin on the western Adriatic margin (central Mediterranean Sea)

The Manfredonia Incised Valley (MIV) is a huge erosional feature buried below the Apulian shelf, on the western side of the Adriatic margin. The incision extends more than 60km eastward, from the Tavoliere Plain to the outer shelf, not reaching the shelf edge. High-resolution chirp sonar profiles allow reconstruction of the morphology of the incision and its correlation at regional scale. The MIV records a single episode of incision, induced by the last glacial–interglacial sea level fall that forced the rivers draining the Tavoliere Plain to advance basinward, reaching their maximum extent at the peak of the Last Glacial Maximum. The valley was filled during a relatively short interval of about 10,000yr during the Late Pleistocene–Holocene sea level rise and almost leveled-off at the time of maximum marine ingression, possibly recording the short-term climatic fluctuations that occurred. The accommodation space generated by the lowstand incision was exploited during the following interval of sea level rise by very high rates of sediment supply that allowed the preservation of up to 45m of valley fill. High-resolution chirp sonar profiles highlight stratal geometries that are consistent with a typical transgressive valley fill of an estuary environment, including bay-head deltas, central basin and distal barrier-island deposits, organized in a backstepping configuration. The highest complexity of the valley fill is reached in the shallowest and most proximal area, where a kilometric prograding wedge formed during a period dominated by riverine input, possibly connected to high precipitation rates. Based on the depth of the valley margins during this interval, the fill was likely isochronous with the formation of sapropel S1 in the Mediterranean region and may have recorded significant fluctuations within the hydrological cycle.