Landscape Dissection Research Articles

Geomorphic expression of active tectonics in a rapidly-deforming forearc, Sila massif, Calabria, southern Italy

We examine the tectonic geomorphology of La Sila, Calabria, southern Italy, a high-standing (1500 m) plateau surrounded by crustal extension, with map, DEM, and field data. These data are used to constrain a simple geodynamic model that describes short-wavelength deformation of the plateau rim. Topographic metrics including drainage patterns, river longitudinal profiles, and swath profiles are consistent with field stratigraphic relationships collectively suggesting that local tectonic deformation is embedded within a broader, regional uplift that has increased the mean elevation of the Sila Plateau during the Quaternary. The concavity, steepness, and length-gradient index of modeled river longitudinal profiles proved to be the most useful metrics for recording the effects of locally active faults. A previously unrecognized horst on the northeastern portion of the plateau was identified with these data. Nevertheless, the core of the Sila Plateau remains relatively undeformed with respect to its flanks which are characterized by steep, fault-bounded escarpments. Active extension has dropped base level for and unloaded the plateau rim, causing deep fluvial incision and associated flexural uplift of the footwall. This footwall uplift helps maintain a separate external, and nearly radial drainage away from the plateau along its flanks, and a nearly closed drainage for the plateau interior. A drainage divide roughly correlates with the plateau rim, inhibiting the process of drainage integration through the Sila interior and subsequent landscape dissection. As a result, geomorphic processes driving erosion of the Sila upland are lagging behind the tectonic processes that have driven rock uplift leading to an overall increase in mean surface elevation. These results underscore the importance of the lag times a landscape may endure in transforming from one tectonic and base level condition to another. We conclude that the Calabrian forearc has a similar geomorphic evolution as the more northern Apennines consistent with the emerging view that growth of Apennine topography has been recent (Quaternary), rapid, and possibly decoupled from rock deformation in the orogenic wedge.

Lithology, landscape dissection and glaciation controls on catchment erosion as determined by cosmogenic nuclides in river sediment (the Wutach Gorge, Black Forest)

AbstractCosmogenic nuclides, measured in quartz from recent river bedload, provide a novel tool to quantify catchment‐wide erosion rates at geologically meaningful time scales. Here we present an analysis of the geomorphological evolution of the 350 km2 Wutach catchment in the uplands of the south‐west German Black Forest. The robustness of the method is demonstrated by the fact that, although the area was affected by river capture at 18 kyr bp, the formed gorge is so narrow that spatially averaged erosion rates were not resolvably perturbed. However, because cosmogenic nuclides preserve an erosion memory of several thousand years, the only perturbation introduced was detected in the minor areas that have been subject to the last maximum glaciation. In unglaciated areas, an important relationship between lithology and erosion can by quantified: sandstone lithologies erode at 12–18 mm kyr−1, granite lithologies at 35–47 mm kyr−1 and limestone lithologies (as deduced from river load gauging) at 70–90 mm kyr−1.

Spatial variability of palaeosols across Cretaceous interfluves in the Dunvegan Formation, NE British Columbia, Canada: palaeohydrological, palaeogeomorphological and stratigraphic implications

AbstractA basic sedimentological and palaeopedological framework is now in place for non‐marine sequence models. The variability of interfluve palaeosols has not, however, been systematically documented, nor have the stratigraphic implications of that variability been incorporated into sequence models. Interfluve palaeosol variability in the deltaic Dunvegan Formation, NE British Columbia, Canada, is investigated, for which a detailed allostratigraphic and palaeogeographic framework has been developed, based on abundant well (> 2300 logs) and outcrop (> 60 sections) control. Regionally extensive valley fills and interfluves have been mapped in coastal plain deposits over an area of about 50 000 km2. This palaeogeographic framework permits interfluve surfaces exposed in outcrop to be located in terms of distance from the margins of coeval valleys. The micromorphological, geochemical and mineralogical characteristics of five representative sequence‐bounding palaeosols located from 250 m to more than 15 km from coeval valley margins are described. These interfluve palaeosols are similar to modern Alfisols and all record (i) aggradation on an alluvial/coastal plain; (ii) a subsequent static and/or degradational phase related to valley incision, non‐deposition and soil thickening; and (iii) a final aggradational phase related to valley filling and renewed sedimentation across the coastal plain. Within this overall template, however, variations in palaeosol thickness, redoximorphic features, illuvial clay content and geochemistry suggest developmental control by hydrological characteristics that were influenced by both the nature of the underlying alluvial sediments and distance from the valley margin. The presence of mature interfluve palaeosols with complex developmental histories suggests that landscape dissection may have been related to terrace development associated with valley incision. Palaeosols closer to valley margins are thicker, contain more illuvial clay and display characteristics suggesting better drained conditions relative to those palaeosols that developed further from valley margins. Subsequent deposition on interfluves also reflects distance from valleys, with those sites close to valleys accumulating cumulic soils with evidence of brackish groundwater, whereas far from valleys (> 10–15 km), groundwater was fresh and clastic supply minimal, encouraging peat formation. Variations in drainage and palaeotopography during landscape dissection resulted in different palaeosol development styles on interfluve surfaces that can be shown, on the basis of physical correlation, to have the same geomorphic age. These observations support the concept of the soil‐forming interval as a basis for pedostratigraphic correlation in ancient terrestrial deposits. Palaeosol variability on interfluves is to be expected, and recognition and documentation of this variability is an important prerequisite to palaeogeomorphological, palaeoclimatic and sequence stratigraphic interpretations.

The history of the major rivers of southern Britain during the Tertiary

The evolution of the drainage system of lowland Britain is discussed on the basis of available geological evidence, including that from terrestrial sites and that which has more recently become available from offshore exploration of the North Sea, the English and Bristol Channels, and the Irish Sea. Tertiary stratigraphy throws considerable light on landform and river development. Paleocene destruction of a chalk cover, which seems to have been incomplete in western Britain, was accompanied by basin sedimentation under a tropical climate. The major elements, the Thames, Solent, Hampshire (?proto-Avon) river, Irish Sea river and possibly an early Trent river, existed almost throughout the Cenozoic. The influence of Atlantic rifting and thermal doming in NW Britain appears to have been stronger and more temporally focused than the persistent flexuring that determined and maintained Tertiary drainage lines in the SE. Here also the folded Mesozoic terrains on the surface contrast with the more dominant block-faulted relief of the Palaeozoic ‘oldlands’. The rivers of the SE can be shown to have extended or reduced their lengths in response to relative sea-level change and gentle warping. Drainage antecedence, the destruction of the Solent system and the breaching of the English Channel are also evident. By contrast, the major river systems of the west are now entirely submerged. Long-term stability of the drainage pattern reflects a persistent tectonic regime in the south, with a subdued low-relief landscape having a weathered regolith and dense vegetation cover. Meandering river channels and alluvial styles predominated, although channel forms varied according to sediment load, slope and discharge variability. Coarse gravel-dominated accumulations are rare and localized. Chemically stable lithologies dominate the clastic component throughout. It is apparent that the deeply incised river valleys seen today are related to high, predominantly coarse sediment yields, encouraged by substantial, rapid climate changes in the Pleistocene. This emphasizes the significance of mechanical compared with chemical weathering for the rate and nature of landscape dissection, and the modifications that have arisen as a result of glaciation, frost-climate weathering, rapidly changing climates and sea levels. The stratigraphical evidence here reviewed is at variance with older, largely geomorphologically based landform evolution models (‘denudational chronology’), but gives considerable support to the recent proposals emphasizing the significance of Paleocene erosion, and enduring low-relief landscapes and drainage systems evolving alongside fold development during the Paleogene. Given the depobasin evidence now available, postulated fluvially active episodes can, and must, be linked to contemporaneous deposition. Some at least of the many controversies involving the identity of erosion surfaces, the dating of them using only residual deposits and weathering mantles, and the selection of particular Tertiary episodes as ones of landscape development can now be resolved.

Equation for high-rate gully erosion

An approach to gully erosion is presented in this paper. The approach is based on general equations derived from theoretical considerations. The equations apply to a situation of intense erosion rate, such as at peak discharge during the few critical rainstorms, able to generate or to widen gullies. Equations linking gully widening to gully deepening are derived. They do not depend on the way in which concentrated flow aggressiveness is estimated. The equation expressing gully width/depth relationship was successfully compared with data from the literature. When runoff aggressiveness was estimated through unit stream power and bottom flow shear stress, the width/discharge relationships found were similar to those expected on the basis of previous studies (e.g., Leopold and Maddock [U. S. Geol. Surv. Prof. Pap. 252 (1953) 57 pp.]) even if slope gradient explicitly appears in contrast with empirical evidence. Only threshold conditions for gullies indicate that flow shear stress (for laminar flow conditions) can explain the observed trends. This astonishing result most probably indicates that gully initiation needs more complex contexts to be explained than the one here used (based on a Montgomery and Dietrich [1994. Landscape dissection and drainage area–slope thresholds. In: M.J. Kirkby (ed.), Process Models and Theoretical Geomorphology. Wiley, 221–246] approach). A selection of the proposed equations have been arranged into a research model and an example of the outcome has been given for two situations typical of cropland in southern Tuscany (Italy). The results indicate that the spatial distribution of soil characteristics and of land use influences significantly gully generation and evolution. This further confirms that gully morphological thresholds cannot be explained by simple approaches.

Cenozoic Landscape Development in the Blue Mountains (SE Australia): Lithological and Tectonic Controls on Rifted Margin Morphology

Abstract Cenozoic landscape development on the southeastern Australian rifted margin, as recorded by mid‐Tertiary basalt flows that preserve ancient landforms, is generally considered to be very slow. Eocene‐Miocene basalts of the southeastern Australian highlands flowed down paleovalleys, indicating that landscape dissection was already well under way at the time of their eruption. Within the deeply incised Blue Mountains, however, Miocene basalts cap relatively flat hilltops, suggesting that most incision postdates their emplacement. We have studied the controls on these apparent lateral variations in Cenozoic landscape development using both field observations and numerical models. We have mapped the Blue Mountains basalts in detail to reconstruct the Miocene landscape and to quantify both the amount of subbasalt relief and postbasalt incision rates. New geochemical and K‐Ar geochronological data indicate that most of the Blue Mountains basalts were derived from a common magma source and were erupted i...

Provenance, geochemistry and grain-sizes of glacigene sediments, including the Sirius Group, and Late Genozoic Glaciol history of the southern Prince Albert Mountains, Victoria Land, Antarctica

The southern Prince Albert Mountains, between David and Mawson Glaciers (75°30' to 76°S) in Victoria Land, Antarctica, comprise a series of nunataks with elevations ranging from 800m near the coast to 2300 m ~130 km inland. Geochemical and grain-size analyses of tills from these nunataks reveal three major groups of deposits: (1) coarse to medium sandy tills, found on Glaciolly streamlined summit plateaus of Kirk-pat rick Basalt above 2000 ma.s.l.., with geochemical compositions very similar to those of the underlying jurassic Kirkpatrick Basalt; (2) bimodal silty and sanely tills of the Sirius Group with Ferrar/Beacon-dominated geochemical compositions, at elevations of 1300-1600 m a.s.l. on striated summit plateaus and high-elevation terraces; (3) fine-grained tills with high SiO2 contents from ice-cored moraines at the lee sides of large nunataks. The geochemical composition of sandy tills from the highest summit plateaus suggests that valleys had not yet cut through the Kirkpatrick Basalt and into Beacon and Ferrar rocks at the time of deposition. These tills represent a phase of temperate glaciation prior to deposition of diatom-bearing Sirius Group tills. The latter were deposited after a first phase of landscape dissection as inferred from geochemical data. The fine-grained ice-cored moraines are late-Pleistocene basal tills. The presence of pre-Pliocene Glacial deposits on high mountain summits in the Prince Albert Mountains has implications for the interpretation of high-elevation Sirius Group sediments in other areas of the Transantarctic Mountains. It is possible that the “Sirius debate” has its origin in interpretations of both thin, barren pre-Pliocene deposits on high mountain summits and thick sequences of diatom-bearing deposits in valleys elsewhere in the Transantarctic Mountains. Both types of deposits are associated with the Sirius Group, but they belong to separate Glacial episodes.

Landscape and Soil Development on Monaro Basalt West of Nimmitabel, New South Wales

Prolonged stability of the southern tableland of NSW, developed on early Tertiary Monaro basalt, is evident from weak landscape dissection and numerous small lakes. However, poorly developed soils and weathered zones are inconsistent with prolonged Cainozoic weathering or formation under climates substantially different to the present. Thin Holocene sediments are also at variance with the sizes of the lake basins. Low depositional landforms, termed ‘lake shadows’, occur on the eastern perimeters of most lakes, and comprise clay pellets blown from basin floors during dry phases. Deflation, dominantly during Quaternary glacial periods, and reactivated today, is responsible for the thin soils and sediment cover. The lake basins are partly solutional and partly deflational features. The modern soil cover reflects the modern climate.

Landscape dissection, isostatic uplift, and the morphologic development of orogens

We examine the morphologic factors that determine the isostatic response to landscape dissection in orogenic terrains in order to quantify the relative contributions of active tectonic uplift and passive isostatic uplift in generating mountain topography. We demonstrate that although peaks many times the elevation of an assumed initial plateau can theoretically be generated, only moderate amounts of peak uplift can be explained by isostatic rebound in real terrains. Analysis of topographic data from the European Alps and other orogens indicates that isostatic uplift in response to dissection can account for about one-fourth of the elevation of the highest peaks if local isostatic compensation is assumed and less if the lithosphere has significant flexural rigidity. We conclude that although isostatic uplift can be significant in orogens, high peaks are predominantly a consequence of tectonic processes in convergent settings.

Tectonic, climatic and lithologic influences on landscape fractal dimension and hypsometry: implications for landscape evolution in the San Gabriel Mountains, California

East-west variation in tectonic activity and strong north-south climatic gradients provide a unique opportunity to study tectonic, climatic and lithologic influences on landscape evolution in the San Gabriel Mountains, California. The competing tendencies of constructive tectonic and degradational climatic effects act against lithologic resistance to influence fluvial systems, and thus the nature of adjacent and nested drainage basins. Landscape fractal dimension ( D), a measure of surface roughness over a variety of scales, and the hypsometric integral ( I), a measure of the distribution of landmass volume above a reference plane are useful measures of altitudinal variation with scale. As such, they may provide clues as to the relative influences of tectonism, climate and rock type. Topographic analyses of the San Gabriel Mountains clearly indicate that tectonism strongly influences D at range-wavelength scales, while rock-type variation apparently influences D and I at smaller scales. Tectonism is also shown to influence I across the mountain-piedmont junction at all scales investigated. Tectonic activity shows strong negative correlation with both I and D because tectonically active portions of the mountain front do not allow time for much landscape dissection by lower-order streams. Three-dimensional topographic modeling suggests climatic parameters exert a stronger influence on D than does tectonism. This modeling also suggests an inverse correlation between range-scale D and I with varying climate and uplift rate; a positive correlation is observed in the San Gabriel Mountains. We suggest this difference results from (1) differences in uplift style between the San Gabriel Mountains and the models, and/or (2) variation in rock-type erodibility present in the San Gabriel Mountains but which was not modeled. We postulate that the interaction of tectonic, climatic and lithologic parameters influences the stable I and D to which a landscape evolves. Key questions remaining include: (1) the time required to reach a stable I or D after a climatic or tectonic change; (2) what does the fractal nature of topography tell us about the scaling characteristics of major landforming processes; (3) how does climate influence range-scale I and both range- and small-scale D; and (4) what are the effects on I and D of range-scale lithologic variation, both in the models and in real landscapes?

Channel Initiation and the Problem of Landscape Scale

Since the 1940s it has been proposed that landscape dissection into distinct valleys is limited by a threshold of channelization that sets a finite scale to the landscape. This threshold is equal to the hillslope length that is just shorter than that necessary to support a channel head. A field study supports this hypothesis by showing that an empirically defined topographic threshold associated with channel head locations also defines the border between essentially smooth, undissected hillslopes and the valley bottoms to which they drain. This finding contradicts assertions that landscapes are scale-independent and suggests that landscape response to changes in climate or land use depends on the corresponding changes in the threshold of channelization.

Pedogenic and Groundwater Silcretes at Stuart Creek Opal Field, South Australia

ABSTRACT The opal mining areas of inland Australia have exposures in which a systematic association between near-surface silcrete and one or more silicified horizons at depth is displayed. In the Stuart Creek opal field, the deepest horizons are opalite and glassy quartzites in which all primary sedimentary structures are conserved. Higher in the section, lenses and nodules of quartzite occur in bleached, unconsolidated sands. Near the top of the section, a nodular and columnar silcrete displays numerous illuviation features. At the base of the section, the earliest phase of silicification is the pseudomorphic replacement of sedimentary clay minerals by opal. Subsequently, void linings of micro-laminated opal were formed, and fibrous silica was precipitated in residual cavities. In the middle part of the section, sedimentary clay minerals were replaced by microcrystalline quartz, while silicification of clay-free sands was achieved by overgrowth of detrital quartz grains. In both cases, residual voids were filled with chalcedony and euhedral quartz. In the upper part of the section, silicification produced microcrystalline quartz in the matrix of the host sediment and in titania-rich illuviation laminae at the base of voids and channels. The near-surface silcrete displays many features relating to infiltration and downward percolation of water. Variable rates of water percolation, as well as alternating periods of leaching and deposition, are inferred from macro- and micro-scale structures and fabrics. The presence of microquartz indicates that solutions contained comparatively low silica concentrations, but enough impurity elements to restrict the growth of large crystals. In the deepest horizons, the preservation of sedimentary structures, the occurrence of micro-laminated void cutans of silica, and the horizontal disposition of silicified pans or lenses suggest a relationship with former groundwater tables. Secondary silica is mainly opal, indicating that the precipitating solutions had high silica concentrations. In the middle part of the section, structures are similar to those at depth except that opal is lacking in the matrix and appears to have recrystallized to microquartz. Silicification may have commenced at a groundwater level, but it later proceeded in response to dissolution and recrystallization in the unsaturated zone above the water table. The different silicification processes occurred in the same landscape in response to different mechanisms. Near the surface, in the pedogenic silcrete, solutions appear to have dissolved silica during infiltration and concentrated it through evaporation during dry periods. At depth, there appears to have been a general acidification of the environment leading to destruction of sedimentary clay minerals and the consequent production of silica phases that had a comparatively high solubility. Opal was precipitated in these groundwater environments. During landscape dissection, the water table was lowered and former silicified horizons were stranded in the unsaturated zone. Here, percolating waters with relatively low concentrations of silica dissolved opal and precipitated microquartz. M anwhile, groundwater silicification proceeded at a deeper level at the new water table.

Quaternary hydrological development in the Morecambe Bay karst, northwest England

The distribution of active and abandoned caves in the Morecambe Bay karst is indicative of at least two phases of subsurface drainage development, separated by an episode of valley incision, landscape dissection and surface lowering. It is suggested that the phase of erosion was associated with glaciation by ice from the adjoining Cumbrian massif, and that the earlier phase of drainage development was of at least last interglacial age. This view is supported by the evidence of the age and mode of deposition of the sedimentary infill in the caves, and by estimates of cave palacocatchment area. The caves of the earlier phase of drainage development were formed in valleys that were already deeply incised. It would therefore appear that the last glaciation only succeeded in slightly modifying an existing topography, a view at odds with conventional ideas of landscape development in Cumbria as a whole.

Holocene soils and soil-geomorphic relations in an arid region of southern New Mexico

A study area in an arid region of southern New Mexico is in basin-and-range topography and includes both a river valley and a closed basin. Holocene soils occur in valley fills and low terraces between Pleistocene fans, in and near drainageways on the fan-piedmont, on ridges, and in dunes. Holocene soils suggest the character of initial development in soils that are much older and more complex, and record the beginnings of various soil horizons. Noncalcareous brown or reddish brown B horizons have formed in low-carbonate parent materials of stable sites. Incipient development of the argillic horizon and the Haplargids occurs at stable sites in very gravelly materials that are about 1–2000 yr old. The cambic horizon and Camborthids occur in adjacent low-gravel materials of the same age. The argillic horizon occurs continuously in soils of earliest Holocene, particularly in very gravelly materials. Where soils have been truncated, as in areas affected by landscape dissection, argillic and cambic horizons are usually absent and the soils are Torripsamments, Torriorthents, or Torrifluvents depending on content of sand, gravel, and organic carbon. In high-carbonate parent materials, noncalcareous, reddish brown B horizons have not formed at any time in the Holocene. Most of these soils are Torriorthents or Torrifluvents although an incipient calcic horizon has formed in some of the oldest Holocene soils; the latter are Calciorthids. Horizons of carbonate accumulation are the best and most common pedogenic indicators of soil age. Stage I carbonate horizons are a major feature of pedogenesis in the Holocene. Because of additions of carbonate from the atmosphere, carbonate horizons are morphologically similar whether they have formed in high or low-carbonate alluvium. The carbonate accumulations are illuvial. Some Holocene deposits apparently resulted from changes in climate. Others, such as the youthful deposits of coppice dunes, apparently were caused by man's introduction of cattle and subsequent overgrazing and seed dispersal.

Relations Between Soil Morphology and Water‐Table Levels on a Dissected North Carolina Coastal Plain Surface

AbstractDepth to water table in Typic Paleudults decreases away from the dissected edge of the geomorphic surface in east central North Carolina. This relation is linear in log log form and statistically highly significant for 19 of 22 time periods tested. The greatest changes in water‐table depths and soil morphology are within the first 0.15 to 0.3 km from the surface edge. Deep water tables are associated with thick A2 horizons and fine‐textured B horizons. The shallow water tables are associated with thin A2 horizons, low‐contrast mottling, and presence of Be bodies. This close association between water‐table depths and soil morphology is interpreted as indicating that landscape dissection, acting through its influence on water‐table depths, is one of the driving forces in genesis of North Carolina Coastal Plain soils.