Rates Of Landscape Evolution Research Articles

Comparison of U–Th, paleomagnetism, and cosmogenic burial methods for dating caves: Implications for landscape evolution studies

Caves are useful in landscape evolution studies because they often mark the level of previous water tables and, when dated, yield incision rates. Dating caves is problematic, however, because their ages are only constrained by the oldest deposits contained within, which may be far younger than the cave itself. We dated cave deposits in the Sierra Nevada using U–Th dating of speleothems, paleomagnetic dating of fine sediment, and cosmogenic 26Al/ 10Be burial dating of coarse sediment. The sampled caves formed sequentially as the water table lowered, providing an important stratigraphic test for the dating methods. Large discrepancies between deposit ages from similar cave levels demonstrate that, even when accurately determined, deposit ages can seriously underestimate the timing of cave development. Drip-type speleothems are most prone to this minimum age bias because they can accumulate long after caves form, and because the U–Th method is limited to ∼400 ka. Paleomagnetic dating requires correlation with the global reversal chronology, and is hindered by a lack of continuous stratigraphy. The fine sediment analyzed for paleomagnetism is also highly susceptible to remobilization and deposition in cave passages well above base level. Cosmogenic 26Al/ 10Be dates bedload material deposited when caves were at or very near river level, and can date material as old as ∼5 Ma. In the Sierra Nevada, speleothem U–Th ages and sediment burial ages from the same cave levels differ by as much as an order of magnitude. These results suggest speleothem ages alone may significantly underestimate cave ages and thereby overestimate rates of landscape evolution. Cosmogenic burial dating of coarse clastic sediment appears to be the most reliable method for dating cave development in mountainous regions.

Relief-rejuvenation and topographic length scales in a fluvial drainage basin, Napf area, Central Switzerland

This paper explores how, and to what extent, a phase of relief-rejuvenation modifies the mode of surface erosion in an approximately 63 km 2 drainage basin located at the northern border of the Swiss Alps (Luzern area). In the study area, the retreat of the Alpine glaciers at the end of the Last Glacial Maximum (LGM) caused base level to lower by approximately 80 m. The fluvial system adapted to the lowered base level by headward erosion. This is indicated by knickzones in the longitudinal stream profiles and by the continuous upstream narrowing of the width of the valley floor towards these knickzones. In the headwaters above these knickzones, processes are still to a significant extent controlled by the higher base level of the LGM. There, frequent exposure of bedrock in channels and especially on hillslopes implies that sediment flux is to a large extent limited by weathering rates. In the knickzones, however, exposure of bedrock in channels implies that sediment flux is supply-limited, and that erosion rates are controlled by stream power. The morphometric analysis reveals the existence of length scales in the topography that result from distinct geomorphic processes. Along the tributaries where the upstream sizes of the drainage basins exceed 100,000–200,000 m 2, the mode of sediment transport and erosion changes from predominantly hillslope processes (i.e., landsliding, creep of regolith, rock avalanches and to some extent debris flows) to processes in channels (fluvial processes and debris flows). This length scale reflects the minimum size of the contributing area for channelized processes to take over in the geomorphic development (i.e., threshold size of drainage basin). This threshold size depends on the ratio between production rates of sediment on hillslopes, and export rates of sediment by processes in channels. Consequently, in the headwaters, erosion rates and sediment flux, and hence landscape evolution rates, are to a large extent limited by weathering processes. In contrast, in the lower portion of the drainage basin that adjusts to the lowered base-level, rates of channelized erosion and relief formation are controlled mainly by stream power. Hence, this paper shows that base-level lowering, headward erosion and establishment of knickzones separate drainage basins in two segments with different controls on rates of surface erosion, sediment flux and relief formation.

Observed and modelled distributions of channel and gully heads—with examples from SE Spain and Belgium

In soil erosion by overland flow, sediment discharge is driven by discharge and gradient against the resistance of the soil and vegetation. The form of the relationship, which is taken as a sum of power law terms, may then be inferred from several data sources: (1) ephemeral gully head location, (2) permanent channel head location, (3) slope profile form, (4) consistency with evolutionary models and (5) erosion plot data. The paper focuses on (1) to (4), in order to understand how far these data constrain the form of the dependence on discharge and gradient. Field data from mapping a set of ephemeral gully heads after storms show a weak inverse relationship between channel head gradient and catchment area, which flattens towards an upper threshold gradient of about 25%. These relationships are compared with those theoretically obtained for a range of sediment transport laws which have been previously used, and which are analysed to forecast both overall slope profile form and the location of stream heads. The theory predicts that the location of ephemeral gully heads produced in a single storm will differ from the long-term average stream head location. There is a fairly good relationship between theoretical and observed locations, associated with the form of the sediment transport law. For a single event, the location depends on storm magnitude which cannot be well defined from the data sets, creating a spread of data with an envelope curve defined for the largest events. The empirically acceptable sediment transport equations have also been used to analyse families of slope profiles in a small area, and used within a landscape evolution model to generate landscapes which are compatible with those observed in the field, and provide confirmation of the long-term viability of the expressions obtained to forecast the direction, style and rates of landscape evolution in response to land use or climate drivers.

Datation de surfaces geomorphologiques reperes par le 10 Be produit in-situ; implications tectoniques et climatiques

Abstract The evolution of continental landforms is mainly modulated by the impact of climatic and tectonic processes. Because of their distinctive morphology and the periodicity of their deposition, climatically induced landforms such as alluvial fans or terraces are well suited to infer rates of tectonic and continental climatic processes. Within tectonically active regions, an important step consists in dating displaced geomorphic features to calculate slip rates on active faults. Dating is probably the most critical tool because it is generally much more simpler to measure deformation resulting from tectonic activity than it is to accurately date when that deformation occurred. Recent advances in analytical chemistry and nuclear physics (accelerator mass spectrometry) now allow quantitative abundance measurements of the extremely rare isotopes produced by the interaction of cosmic rays with surface rocks and soils, the so-called in situ-produced cosmogenic nuclides ( 3 He, 10 Be, 21 Ne, 26 Al, 36 Cl), and allow to directly date the duration that a landform has been exposed to cosmic rays at the Earth's surface [Lal, 1991; Nishiizumi et al., 1993; Cerling and Craig, 1994; Clark et al., 1995]. In fact, the abundance of these cosmonuclides is proportional to landscape stability and, under favorable circumstances, their abundance within surface rocks can be used as a proxy for erosion rate or exposure age. These cosmonuclides thus provide geomorphologists with the opportunity to constrain rates of landscape evolution. This paper presents a new approach that combines cosmic ray exposure (CRE) dating using in situ-produced 10 Be and geomorphic as well as structural analyses. This approach has been applied on two active strike-slip and reverse faults located in the Andean foreland of western Argentina. These two case studies illustrate how CRE dating using in situ-produced 10 Be is particularly well suited for geomorphic studies that aim to estimate the respective control of climate and tectonics on morphogenesis.

Soil production on a retreating escarpment in southeastern Australia

The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil production function) has been widely recognized as essential to understanding landscape evolution, but was quantified only recently. Here we report soil production rates for the first time at the base of a retreating escarpment, on the soil-mantled hilly slopes in the upper Bega Valley, southeastern Australia. Concentrations of 10 Be and 26 Al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth. These data define a soil production function with a maximum soil production rate of 53 m/m.y. under no soil mantle and a minimum of 7 m/m.y. under 100 cm of soil, thus constraining landscape evolution rates subsequent to escarpment retreat. The form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes. Spatial variation of soil production shows a landscape out of dynamic equilibrium, possibly in response to the propagation of the escarpment through the field area within the past few million years. In addition, we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10 Be and 26 Al on the surfaces of emergent tors.

Post‐Palaeozoic uplift history of southeastern Australia revisited: Results from a process‐based model of landscape evolution

Developments over the last decade in the debate concerning the geomorphic evolution of the southeastern Australian highlands are reviewed, taking as a starting point the model of passive denudation and isostatic rebound of a Palaeozoic mountain belt presented by Lambeck and Stephenson (1986). This model has been popular in the geomorphological community because it provides a quantitative framework to explain the very low rates of landscape evolution inferred from most geomorphological studies. The model has, however, also been criticised for its treatment of erosion as being linearly dependent on elevation, as well as for its predictions of regional uplift and denudation patterns that are not in accord with inferences from fission‐track thermochronological data. First results of a new physical process‐based model for large‐scale, long‐term landscape evolution in the southeastern highlands are presented. These show that the denudation history and drainage development of southeastern Australia can be explained to a first order without invoking large‐scale mid‐Cretaceous or Tertiary uplift events. The model predicts drainage patterns in southeastern Australia to have evolved by rearrangement of an initially northwesterly directed drainage net, caused by drops in base‐level during Mesozoic rifting along the southern and eastern margins of the study region. The geomorphology and available fission‐track data in the Snowy Mountains region (and possibly also in the Bathurst—Blue Mountains region) do require renewed (mid‐Cretaceous?) uplift to have taken place. The model results are discussed in the light of recent controversies surrounding the southeastern highlands—their uplift history, denudation rates, depth of denudation of the coastal strip and inferred Mesozoic drainage patterns.

ESTIMATING RATES OF DENUDATION USING COSMOGENIC ISOTOPE ABUNDANCES IN SEDIMENT

We propose, as a testable hypothesis, a basin-scale approach for interpreting the abundance of in situ produced cosmogenic isotopes, an approach which considers explicitly both the isotope and sediment flux through a drainage basin. Unlike most existing models, which are appropriate for evaluating in situ produced cosmogenic isotope abundance at discrete points on Earth’s surface, our model is designed for interpreting isotope abundance in sediment. Because sediment is a mixture of materials, in favourable cases derived from throughout a drainage basin, we suggest that measured isotope abundances may reflect spatially averaged rates of erosion. We investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed.

Using in situ produced cosmogenic isotopes to estimate rates of landscape evolution: A review from the geomorphic perspective

The application of in‐situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade. At least 57 papers and numerous abstracts have been published since the mid‐1980s when the first mass‐spectrometric measurements of terrestrially produced cosmogenic isotopes were made. Taken at face value, these studies provide quantitative information about rates of landscape evolution and landform age; however, the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data, the validity of assumptions inherent to these models, and the geologic surroundings in which the samples were collected. This paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted. At present, the establishment of high‐precision, cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces, the erosion of rock from sampled boulders, the potential for isotope inheritance from previous exposure, and the uncertainty of isotopic measurements. Uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques. Estimation of erosion rates at single points on the landscape gives useful small‐scale information. Extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites. However, because erosion rates are so poorly constrained at present, even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists.

Eolian landscape evolution and soil formation in the Chaco dune field, southern Colorado Plateau, New Mexico

Eolian deposits of the Chaco dune field cover a broad expanse of the southeastern Colorado Plateau in New Mexico and comprise a large part of the late Quaternary sedimentary record in this region. The primary eolian landforms include sand sheets, parabolic and modified parabolic dunes, barchan-barchanoid dune complexes, and ridge dunes. Many of the eolian landforms in the Chaco dune field have remained stable over a sufficiently long time to permit soil development. Weakly to moderately developed calcic soils, which formed in eolian deposits of mid-Holocene and latest Pleistocene age, possess clay-enriched, weak argillic B horizons. These soils have been buried progressively by younger eolian sediments, forming regionally correlative bounding surfaces over the Chaco dune field and recording relict latest Pleistocene and middle Holocene eolian landscapes. Soil-geomorphic relations and radiocarbon-dated material within the eolian deposits have been useful in (1) delineating three regionally correlative soil-stratigraphic units and their associated landforms and (2) studying the types and rates of landscape evolution and climatic changes within this portion of the Colorado Plateau. The most significant period of eolian landform construction is inferred to have occurred during the latest Pleistocene glacial-to-interglacial climatic change, forming widespread sand sheets and modified parabolic dunes. Eolian deposition during the middle and late Holocene were dramatically influenced by the presence of previously developed latest Pleistocene to early Holocene (buried) soils which may have enhanced sand-sheet and parabolic-dune development due to the binding effect of the soils. Middle Holocene eolian deposition may be related primarily to neoglacial climatic changes which enhanced ground-surface moisture and promoted sand-sheet and parabolic-dune formation. The youngest period of wolian deposition occurred after 1.5 ka and resulted in the formation of the most diverse suite of eolian landforms and deposition of volumetrically more eolian sediments than the mid-Holocene event. In contrast to earlier interpretations, the classic mid-holocene or “altithermal event” apparently did not play a major role in the formation of eolian landscapes in the Chaco dune field.

Stream Profile Change and Longterm Landscape Evolution: Early Miocene and Modern Rivers of the East Australian Highland Crest, Central New South Wales, Australia

Early Miocene basaltic lavas that flowed into river systems in central eastern New South Wales are still preserved as sinuous hill-top cappings. They permit reconstruction of many of the characteristics of the Early Miocene river systems and allow for comparison with the adjacent modern counterparts of these rivers. The long profiles of both the Early Miocene and modern valleys are rather similar and apparently exhibit adjustment to the lithologies of their channel substrates, as would be proposed under Hack's condition of dynamic equilibrium. Slightly less post-Early Miocene incision in the rivers' headwaters indicates, however, that the landscape has not experienced uniform catchment-wide downwasting, as would also be proposed by Hack. Traditional geomorphic models which propose declining relief and decreasing importance of lithological influences over long time periods also are inapplicable to the area. On balance, the most suitable model of the post-Early Miocene evolution of this area would incorporate slow rates of landscape evolution, the persistence, rather than waning, of lithological influences on river systems, and minor increases in relief. Such a model is probably applicable to other humid areas which are of moderate relief and have experienced tectonic stability.

The Geomorphic Evolution of the Taylor Black Prairie Between the Trinity and Colorado Rivers: ABSTRACT

The Taylor Black Prairie is an interesting and complex area that has been, heretofore, relatively ignored as a distinct geomorphic unit. Therefore, the purpose of this study is to describe the region as it exists at the present time and to speculate on its overall evolutionary development. Based on descriptive geomorphic variables including geology, landform morphology, soil type and distribution, surface gravure, vegetation type and distribution, and land use, the Taylor Black Prairie may be subdivided into three north-to-south trending geomorphic areas. These are, from west to east, the Lower Taylor Prairie, the Wolfe City scarp, and the Upper Taylor Prairie. There is also present, in the southern region of the study area, a relatively distinct small exposure of high gravels located between Little River and Brushy Creek. Because these gravels are geographically restricted within the study area, and because they are dissimilar petrologically from the Cretaceous strata upon which the north-to-south prairies are developed, they constitute a distinct geomorphic area. The dominant active processes in the Taylor Black Prairie are soil erosion and mass wasting. These are acting under present climatic conditions to shape and modify and topography of this region. The geomorphic evolution of the Taylor Black Prairie is related to the deposition of Tertiary Uvalde Gravel in the central Texas region. These sediments were carried by major rivers from the edge of the southern High Plains eastward through valleys entrenched in Paleozoic and Comanchean rocks. The less-resistant Gulfian and Tertiary rocks allowed valley widening, thus forming braided alluvial streams that deposited humid alluvial fans. End_Page 517------------------------------ Topographic development was further enhanced by changes in climate and fluvial response during glacial and interglacial periods. Glacial periods were characterized by increased rainfall which resulted in downcutting and erosion of the divides by major rivers. Interglacial periods were characterized by a rise in regional base level accompanied by fluvial aggradation. The net effect has been a reversal of topography in which the Uvalde Gravel, which occupied the bottom of river valleys at the time of deposition, now caps the highest divides. Studies of modern erosion rates indicate that man's activities, mainly agricultural, have greatly increased the rate of landscape evolution. End_of_Article - Last_Page 518------------