Excavation Of Valleys Research Articles

Late Quaternary sedimentary record of estuarine incised-valley filling and interfluve flooding: The Manfredonia paleovalley system (southern Italy)

Multiple paleovalley systems of late Quaternary age have been widely explored in previous research from the Gulf of Manfredonia on the basis of seismic data, but only limited information is available on their proximal (onshore) segments. Through an integration of sedimentary, paleoecological (mollusks, benthic foraminifers, ostracods), and geochemical analyses from three 30-50 m-long onshore cores and a sequence-stratigraphic framework chronologically constrained by 25 radiocarbon data, we document the stratigraphic architecture of three contiguous paleovalley systems (Candelaro, Cervaro and Carapelle rivers) and their sedimentary response to Late Pleistocene valley excavation, Holocene filling and basinwide interfluve flooding. Above a prominent sequence boundary formed during the Late Pleistocene relative sea-level fall, fluvial-channel gravels and sands (lowstand systems tract) are overlain by a deepening-upward, valley-fill succession of inner-estuarine (freshwater) to outer-estuarine (brackish) muds (lower transgressive systems tract – TST). The individual valley fills are overlain by laterally extensive bay deposits (upper TST) and by a progradational succession of prodelta/delta front and offshore/shoreface deposits that reflect normal regression under highstand conditions (HST). The transition from fluvial to inner-estuary (fluvial-dominated) and outer-estuary (wave-influenced) deposits records the progressive shift in sediment provenance from Southern Apennine source rocks to a mixed composition that reflects increasing alongshore sediment contribution from northern sources via the SE-directed Western Adriatic Current. The stratigraphic surface that demarcates the transition from estuarine to bay depositional systems (lower/upper TST boundary) represents the physical prolongation of interfluvial terrace surfaces, as valley margins were flooded during the Early Holocene transgression. This surface, typically recognizable on seismic profiles at the boundary between low-amplitude and overlying high-amplitude reflectors, has no obvious lithologic signature in core. However, it can be readily identified on the basis of paleoecologic features (sharp increase in species diversity and in the proportion of marine taxa) that reveal the abrupt transition from laterally confined to unconfined settings.

The ups and downs of the Missouri River from Pleistocene to present: Impact of climatic change and forebulge migration on river profiles, river course, and valley fill complexity

AbstractThe Missouri River is a continent-scale river that has thus far escaped a rigorous reporting of valley fill trends within its trunk system. This study summarizes evolution of the lower Missouri River profile from the time of outwash in the Last Glacial Maximum (LGM) until establishment of the modern dominantly precipitation-fed river. This work relies on optically stimulated luminescence (OSL) dating, water-well data, and a collection of surficial geological maps of the valley compiled from U.S. Geological Survey EDMAP and National Science Foundation Research Experience for Undergrads projects. Mapping reveals five traceable surfaces within valley fill between Yankton, South Dakota, USA, and Columbia, Missouri, USA, that record two cycles of incision and aggradation between ca. 23 ka and ca. 8 ka. The river aggraded during the LGM to form the Malta Bend surface by ca. 26 ka. The Malta Bend surface is buried and fragmented but presumed to record a braided outwash plain. The Malta Bend surface was incised up to 18 m between ca. 23 ka and ca. 16 ka to form the Carrolton surface (ca. 16 ka to ca. 14 ka). The Carrollton surface ghosts a braided outwash morphology locally through overlying mud. Aggradation followed (ca. 14 ka to ca. 13.5 ka) to within 4 m of the modern floodplain surface and generated the Salix surface (ca. 13.5 to ca. 12 ka). By Salix time, the Missouri River was no longer an outwash river and formed a single-thread meandering pattern. Reincision at ca. 12 ka followed Salix deposition to form the short-lived Vermillion surface at approximately the grade of the earlier Carrolton surface. Rapid aggradation from ca. 10 ka to ca. 8 ka followed and formed the modern Omaha surface (ca. 8 ka to Present). The higher Malta Bend and Omaha profiles are at roughly the same grade, as are the lower Carrolton and Vermillion surfaces. The Salix surface is in between. All surfaces converge downstream as they enter the narrow and shallow bedrock valley just before reaching Columbia, Missouri. The maximum departure of the profiles is 18 m near Sioux City, Iowa, USA, at ∼100 km downstream from the James Lobe glacial input near Yankton, South Dakota. Incision and aggradation appear to be driven by relative changes in input of sediment and water related to glacial advance and retreat and then later by climatic changes near the Holocene transition. The incision from the Malta Bend to the Carrolton surface records the initial breakdown of the cryosphere at the end of the LGM, and this same incisional event is found in both the Ohio and Mississippi valleys. This incisional event records a “big wash” that resulted in the evacuation of sediment from each of the major outwash rivers of North America. The direction and magnitude of incision from the LGM to the modern does not fit with modeled glacioisostatic adjustment trends for the Missouri Valley. Glaciotectonics likely influenced the magnitude of incision and aggradation secondarily but does not appear to have controlled the overall timing or magnitude of either. Glaciotectonic valley tilting during the Holocene, however, did likely cause the Holocene channel to consistently migrate away from the glacial front, which argues for a forebulge axis south of the Missouri Valley during the Holocene and, by inference, earlier. This is at least 200 km south of where models predict the Holocene forebulge axis. The Missouri Valley thus appears to reside in the tectonic low between the ice front and the forebulge crest. The buffer valley component of incision caused by profile variation could explain as much as 25 m of the total ∼40 m of valley incision at Sioux City, Iowa. The Missouri Valley also hosted a glacial lobe as far south as Sioux City, Iowa, in pre-Wisconsinan time, which is also a factor in valley excavation.

Salt‐induced crestal faults control the formation of Quaternary tunnel valleys in the southern North Sea

Tunnel valleys are major features of glaciated margins and they enable meltwater expulsion from underneath a thick ice cover. Their formation is related to the erosion of subglacial sediments by overpressured meltwater and direct glacial erosion. Yet, the impact of pre‐existing structures on their formation and morphology remains poorly known. High‐quality 3D seismic data allowed the mapping of a large tunnel valley that eroded underlying preglacial delta deposits in the southern North Sea. The valley follows the N–S strike of crestal faults related to a Zechstein salt wall. A change in downstream tunnel valley orientation towards the SE accompanies a change in the strike direction of salt‐induced faults. Fault offsets indicate important activity of crestal faults during the deposition of preglacial deltaic sediments. We propose that crestal faults facilitated tunnel valley erosion by acting as high‐permeability pathways and allowing subglacial meltwater to reach low‐permeability sediments in the underlying Neogene deltaic sequences, ultimately resulting in meltwater overpressure build‐up and tunnel valley excavation. Active faults probably also weakened the near‐surface sediment to allow a more efficient erosion of the glacial substrate. This control of substrate structures on tunnel valley morphology is considered as a primary factor in subglacial drainage pattern development in the study area.

Drumlinized tunnel valleys in south-central Ontario

A network of Late Wisconsin valleys deeply incise thick successions (up to 200 m) of Quaternary sediment and Paleozoic bedrock across south-central Ontario. The valleys have previously been interpreted as an integrated network of tunnel channels recording catastrophic releases of subglacial meltwater across the bed of the Laurentide Ice Sheet (LIS). Recent geologic investigations in the western part of the valley network integrate information gained from surficial sediment and landform mapping, continuously-cored boreholes that penetrate the entire Quaternary sediment succession in valleys as well as intervening uplands, and both water-borne and land-based geophysical surveys. Together, these data provide new insights and a refined interpretation of the genesis, timing and paleoglaciological significance of these valleys.Valleys within the study area are some of the largest observed within the regional network – up to 30 km long, 7 km wide and >175 m deep. They are oriented both north-south and in a radial pattern, varying from NE-SW in the southern part of the study area, to ESE-WNW in the north. An undulating Late Wisconsin till sheet (Newmarket Till) caps the regional uplands and is commonly observed along the flanks and floors of the valleys – all of which show evidence of a drumlinization phase that postdates excavation of most of the valleys. At least two generations of valleys are interpreted from cross-cutting relationships, with abrupt heads and locally perched downflow ends of some valley forms. Radiocarbon age determinations of organic material from both below and above the Newmarket Till constrain the timing of valley excavation to between 28.05 and 12.81 14C kyr BP. Borehole and outcrop data within the valleys reveal four stratigraphic units (SU1-SU4) that form the floor and sediment infill of the valleys. The Newmarket Till (SU1) is commonly underlain by deformed substrates. The till is absent in parts of some valleys and the lowermost parts of the valley fill commonly consist of coarse-grained gravels (SU2) that pass upwards into sands (SU3). The bulk of the sediment infill consists of glaciolacustrine silt-clay rhythmites (SU4) that pass upwards into organic- and mollusk-bearing nearshore and fluviodeltaic deposits that record drainage of proglacial lakes from the study area during deglaciation.It is proposed that these features are a previously unrecognized type of tunnel valley that developed subglacially during Late Wisconsin ice cover. Meltwater surpluses would be generated at the ice-bed interface due to the low-transmissivity substrate in the study area with flow preferentially routed into lows on the bed. Downward incision into confined and pressurized aquifers likely facilitated rapid headward erosion of the valleys enhanced by piping. Waning meltwater flow velocities/discharges allowed ice creep to re-occupy newly-formed valleys permitting renewed till deposition and subsequent drumlinization of the valleys. The valleys also provided efficient routing for meltwater draining the retreating ice marginal zone during deglaciation and allowed localised sand and gravel deposition as eskers and ice-proximal subaquatic fans in the Oak Ridges Moraine and within proglacial lakes that developed during ice retreat. However, there is no evidence for regional-scale catastrophic release of subglacial meltwater in the valley infills. This integrated dataset represents one of the most complete characterizations of subglacial valleys in a terrestrial setting in North America.

Speleothem U-series constraints on scarp retreat rates and landscape evolution: an example from the Severn valley and Cotswold Hills gull-caves, UK

Modelling landscape evolution requires quantitative estimates of erosional processes. Dating erosional landscape features such as escarpments is usually difficult because of the lack of datable deposits. Some escarpments and valley margins are associated with the formation of mass-movement caves, sometimes known as ‘gull’ or ‘crevice’ caves, which are typically restricted to within 0.5 km of the valley margin or scarp edge. As in other caves, these mass-movement cavities may host speleothems. As gull-caves develop only after valley incision, uranium-series dating of speleothems within them can provide a minimum age for the timing of valley excavation and scarp formation. Here we present data from several gull-caves in the Cotswold Hills, which form the eastern flank of the Severn valley in southern England. U-series ages from these gull-caves yield estimates for both the minimum age of the Cotswold escarpment and the maximum scarp retreat rate. This is combined with data from geological modelling to propose a model for the evolution of the Severn valley and the Cotswold Hills. The data suggest that the location of the escarpment and regional topography is determined not by valley widening and scarp retreat, but by the in situ generation of relief by differential erosion.

Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall

Many high-latitude fluvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced base-level fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development. The Le Sueur River meanders as it incises, so we coupled the best-fit incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fine sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased flows in rivers, leading to higher sediment loads, not just from field erosion, but from increased bank and bluff erosion in the deeply incised valleys.

Erosional unloading, hillslope geometry, and the height of the Cascade Range, Washington State, USA

AbstractPeaks in the Cascade Range in northern Washington State are on average ∼800 m higher than in southern Washington. The influences of differential valley excavation and variations in hillslope length and average slope on these altitudinal trends were tested using a 3‐dimensional model for isostatic rock uplift and calculations of hillslope length and slope respectively. The magnitude of isostatic peak uplift calculated by the model is highly dependent on the flexural rigidity (D) and the related effective elastic thickness (Te) of the crust of this region. Crustal rigidity was constrained using published estimates and by estimating the depth of the seismogenic zone in the area (D > 1 × 1023 Nm and Te > 24 km). With these constraints, isostatic compensation due to differential erosion added < 700 m and 300 m, or < 25% overall, of height to peaks in the northern and southern Cascades, respectively. Deeper valley incision in the northern Cascades accounts for < 300 m of the 800 m difference in peak altitudes between north and south. Similarly, variation in valley spacing and slope account for < 350 m of the difference in mean altitude between northern and southern regions. Hence, at least several hundred m difference in altitude between the northern and southern regions of the Cascades in Washington must be due to tectonic, geologic, or geophysical factors rather than surficial and geomorphic effects like isostatic response to valley incision and hillslope geometry. Copyright © 2009 John Wiley & Sons, Ltd.

Glaciation, erosion, and landscape evolution of Iceland

Stratigraphic and sedimentological studies indicate that Iceland has experienced over 20 glaciations during the last 4–5 Myr, in reasonable agreement with the number of glaciations reconstructed from the ∂18O record in deep-sea sediment. The pattern of glacial erosion was to a large part controlled by constructive volcanic processes resulting in increased topographic relief after 2.5 Myr. Between 2.5 and 0.5 Ma valleys up to 400 m deep were excavated into the Tertiary basalts of eastern and south Iceland with an average erosion rate of 10–20 cm ka−1. During the last 0.5 million years rates of erosion increased to 50–175 cm ka−1, with an additional 200 to over 1000 m of valley excavation. Previous estimates of the rate of landscape erosion during the Holocene vary widely, from 5 to 70,000 cm ka−1. We present new studies that define the rates of landscape denudation during the major part of the Holocene (the last 10,200 years): one based on the Iceland shelf sediment record, the other from the sediment record in the glacier-fed lake, Hvítárvatn. Both studies indicate average Holocene erosion rates of about 5 cm ka−1 similar to our erosion rate estimate for 4–5 Ma old strata that has not been subjected to regional glaciation.

Valley formation by fluvial and glacial erosion

Cross-valley profiles from the west slope of the Olympic Peninsula, Washington, are used to investigate the relative effects of fluvial and glacial erosion on valley formation. Unlike most ranges where glaciers and rivers sequentially occupied the same valleys, neighboring valleys in the Olympic Mountains developed in similar lithologies but were subject to different degrees of glaciation, allowing comparison of the net effect of glacial and fluvial processes integrated over many glacial cycles. Upslope drainage area was used to normalize comparisons of valley width, ridge-crest-to-valley-bottom relief, and valley cross-sectional area as measures of net differences in the mass of rock excavated from below ridgelines for 131 valley-spanning transects. Valley width, relief, and cross-sectional areas are similar for glaciated, partly glaciated, and unglaciated (fluvial) valleys with drainage areas of ,10 km 2 , but diverge downslope. Glaciated valleys draining .50 km 2 reach two to four times the cross-sectional area and have to 500 m greater relief than comparable fluvial valleys; partly glaciated valleys have intermediate dimensions. At distances of .5 km from valley heads, the cumulative upstream volume of rock removed to form valleys is two to four times greater in glacially incised valleys than in fluvial valleys. The finding of strong differences in the net result of valley excavation by fluvial and glacial erosion supports the interpretation that alpine glaciers are more effective erosional agents than are rivers and implies that large alpine valleys deepened and enlarged in response to Pleistocene glaciation.

Genetically related alluvial deposits across active fault zones: an example of alluvial fan-terrace correlation from the upper Quaternary of the southern Po Basin, Italy

Integrated studies of alluvial fan systems and related intramontane valley deposits provide potentially useful tools for understanding the character of alluvial sedimentation and its control in fault-controlled piedmont zones. An example is reported from the uppermost Quaternary alluvial deposits of the northern Apennines and the southern margin of the Po Basin. A distinctive cyclicity characterizes both the outcropping and the buried units in the study area. In intramontane valleys individual cycles consist of regular alternations of sections displaying high and low terrace preservation. In the subsurface, a rhythmic alternation of coarse-grained bodies and laterally extensive pelitic horizons occurs at various scales and represents the basic cyclic motif of the depositional system. The correlation between the terrace sequences and the depositional cycles in the subsurface allows the differentiation of distinct sequences with a characteristic internal architecture. In the lower part of sequences, poorly preserved terrace deposits are laterally correlatable with alluvial fan gravel bodies, whereas highly preserved terrace deposits are correlated laterally with predominantly floodplain sediments in the upper part of sequences. Reliable correlation lines have been constructed using 14C dating. Thrust tectonics and climate exerted a major control on sequence development. Deep valley excavation, with subsequent low terrace preservation, occurred in response to relief rejuvenation during successive thrusting events. On the other hand, decreasing erosion related to periods of minor tectonic activity led to terrace preservation in the intramontane valleys. The coeval episodes of alluvial fan growth and abandonment, recorded at the basin margin, are interpreted to reflect changes in the efficiency of the drainage system, due to variations of tectonic uplift in the adjacent areas. Alternating phases of deglaciation and renewed glaciation, however, are inferred to have played a major role in controlling the amount and type of sediment delivered to the alluvial fan complexes.

Late Cenozoic surficial deposits and valley evolution of unglaciated northern New Jersey

Multiple alluvial, colluvial, and eolian deposits in unglaciated northern New Jersey, and the eroded bedrock surfaces on which they rest, provide evidence of both long-term valley evolution driven by sustained eustatic baselevel lowering and short-term filling and excavation of valleys during glacial and interglacial climate cycles. The long-term changes occur over durations of 10 6 years, the short-term features evolve over durations of 10 4 to 10 5 years. Direct glacial effects, including blockage of valleys by glacial ice and sediment, and valley gradient reversals induced by crustal depression, are relatively sudden changes that account for several major Pleistocene drainage shifts. After deposition of the Beacon Hill fluvial gravel in the Late Miocene, lowering of sea level, perhaps in response to growth of the Antarctic ice sheet, led to almost complete dissection of the gravel. A suite of alluvial, colluvial, and eolian sediments was deposited in the dissected landscape. The fluvial Bridgeton Formation was deposited in the Raritan lowland, in the Amboy-Trenton lowland, and in the Delaware valley. Following southeastward diversion of the main Bridgeton river, perhaps during Late Pliocene or Early Pleistocene glaciation, northeastward drainage was established on the inactive Bridgeton fluvial plain. About 30 to 45 m of entrenchment followed, forming narrow, incised valleys within which Late Pleistocene deposits rest. This entrenchment may have occurred in response to lowered sea level caused by growth of ice sheets in the northern hemisphere. Under periglacial conditions in the Middle and Late Pleistocene, valleys were partially filled with alluvium and colluvium. During interglacials slopes were stabilized by vegetation and the alluvial and colluvial valley-fill was excavated by gullying, bank erosion, and spring sapping. During Illinoian and late Wisconsinan glaciation, the lower Raritan River was diverted when glacial deposits blocked its valley, and the Delaware River was partially diverted down the isostatically-steepened lower Millstone valley.

Fluvial Ribbon Deposits of Channel and Valley Origin and Their Relationship to Petroleum Reservoir Development: Glauconite Member (Lower Cretaceous), Southern Alberta: ABSTRACT

Stratigraphic fluvial ribbon deposits within the subsurface Glauconite Member (Mannville Group) have been mapped over a 60 x 30-km area of southern Alberta. These ribbons truncate regional markers and have fills comprised of discontinuous sandstone bodies isolated by shales. Ribbons of two distinct types are recognized. Type 1 ribbons are up to 2 km wide and 25 m thick and have relatively simple fills with pod-shaped sandstone bodies distributed preferentially along the ribbon margins. These ribbons are interpreted as superimposed paleochannels which incised into preexisting sediments during river or distributary avulsion. Sandstone bodies represent lateral bars deposited within active channels, whereas the intervening shales were deposited during abandonment. Type 2 ribbons are up to 7 km wide and 40 m thick and have complex fills. Sandstone bodies are pod or ribbon-shaped, occur in lateral or mid-ribbon positions, and locally show vertical or lateral amalgamation. These ribbons are interpreted as paleovalleys excavated in response to base level drop associated with northward regression of the Lower Cretaceous Boreal sea. Valleys were filled with deposits from several distinct, confined, underfit streams resulting in highly complex internal facies relationships. Paleosols are present within strata laterally adjacent to the top of Type 2 ribbons andmore » formed during subaerial exposure associated with valley excavation. Similar surfaces have not been recognized in association with Type 1 ribbons.« less

I.—On the Origin of Valleys

A Controversy apperas to be carried on between geologists as to the causes of the excavation of valleys, and the degree in which marine denudation on the one hand, or the eroding power of rain and rivers on the other, have been concerned in the process. To this controversy Mr. D. Mackintosh has contributed two papers, in recent numbers of this Magazine. I am desirous of recalling to the recollection of its readers certain facts which demonstrate, beyond the possibility of doubt, how vast is the error of those writers who, like Mr. Mackintosh, speak of “the impotence of rain as a denuding agent.”

XXXV. Notice on the excavation of valleys

XXXV. <i>Notice on the excavation of valleys</i>

VIII.—On the Excavation of Valleys by diluvian Action, as illustrated by a succession of Valleys which intersect the South Coast of Dorset and Devon.

My intention in the following communication is to consider the general causes to which valleys owe their origin, and particularly such as occur in horizontal and undisturbed strata within the limits of their escarpments. That portion of the coast of Dorset and Devon which lies on the east of Lyme and on the east of Sidmouth, affords some of the best examples of such valleys with which I am acquainted: I beg therefore to present to the Society two geological views of that coast, drawn at my request some years since by Hubert Cornish, Esq., which will tend also further to illustrate the description given of it in a preceding paper by Mr. De la Beche. Many valleys may be ascribed to the elevation or depression of the strata composing the adjacent hills, by forces acting at very remote periods from within the body of the earth itself, and to similar forces we may principally refer the high inclination and contortions of strata that occur in the highest mountains, and sometimes also in minor hills: other valleys have been occasioned by the strata having been originally deposited at irregular levels, and others by some partial slip or dislocation of portions of the strata. But at different periods of time intermediate between the deposition of the most ancient and the most recent formations, the irregularities of level arising from the preceding causes hare been variously modified by the action of violent inundations hollowing out portions of the surface and removing the

LXXVIII. Geological observations on the excavation of valleys, and local denudations of the strata of the earth in particular districts, &amp;c., in reply to Mr. John Carr's Letter in the last Number, p. 385

LXXVIII. <i>Geological observations on the excavation of valleys, and local denudations of the strata of the earth in particular districts, &amp;c., in reply to Mr.</i> J<scp>ohn</scp> C<scp>arr's</scp> <i>Letter in the last Number, p.</i> 385