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- Volume 8, Issue 3, 1996
Basin Research - Volume 8, Issue 3, 1996
Volume 8, Issue 3, 1996
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Interactions of growing folds and coeval depositional systems
Authors Douglas Burbank, Andrew Meigs and Nicholas BrozovićResponses of both modern and ancient fluvial depositional systems to growing folds can be interpreted in terms of interactions among competing controlling variables which can be incorporated into simple conceptual models. The ratio of the rate of sediment accumulation to the rate of structural uplift determines whether a fold develops a topographic expression above local base level. The balance between (a) stream power and rates of upstream deposition vs. (b) bedrock resistance and rates of crestal uplift and of fold widening determines whether an antecedent stream maintains its course or is defeated by a growing structure. Modern drainage configurations in actively folding landscapes can often be interpreted in terms of these competing variables, and through analysis of digital topography, detailed topographic characteristics of these folds can be quantified. Modern examples of growing folds display both defeated and persistent antecedent rivers, deflected drainages and laterally propagating structures. The topography associated with a defeated antecedent river at Wheeler Ridge, California, is consistent with a model in which defeat results from forced aggradation in the piggyback basin, without the need to vary discharge or uplift rate.
Reconstruction of the long‐term interplay between a depositional system and evolving folds requires a stratigraphic perspective, such as that provided by syntectonic strata which are directly juxtaposed with ancient folds and faults. Analysis of Palaeogene growth strata bounding the Catalan Coastal Ranges of NE Spain demonstrates the synchronous growth and the kinematic history of multiple folds and faults in the proximal foreland basin. Although dominated by transverse rivers which crossed fold crests, palaeovalleys, interfan lows, structural re‐entrants and saddles, and rising anticlines diverted flow and influenced local deposition. In the ancient record, drainage‐network events, such as avulsion or defeat of a transverse stream, usually cannot be unambiguously attributed to a single cause. Examination of ancient syntectonic strata from a geomorphological perspective, however, permits successive reconstructions of synorogenic topography, landscapes and depositional systems.
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Axial–transverse fluvial interactions in half‐graben: Plio‐Pleistocene Palomas Basin, southern Rio Grande Rift, New Mexico, USA
Authors M. R. Leeder, G. H. Mack and S. L. SalyardsAccurate magnetostratigraphic dating of Plio‐Pleistocene alluvium in the Palomas half‐graben permits correlation of transverse and axial deposits, thus enabling analysis of the movement of alluvial facies belts in time and space for the first time. Northern areas show evidence for basinward progradation of footwall‐sourced Matuyama‐age alluvial fan deposits over axial channel belt deposits of the ancestral Rio Grande, despite both deposits having similar deposition rates. This gradual ‘forced’ westward migration of the axial belt was in opposition to ongoing eastward growth of hangingwall‐sourced fans and tectonic tilt imposed by the bounding Caballo normal fault. Fan growth was coincident with a recently proposed gradual climatic shift that may have increased sediment flux out of transverse catchments. It is also possible that continuing tectonic footwall uplift and divided retreat caused catchment areas to increase, contributing to these trends.
Southern areas of the Palomas half‐graben feature late Gilbert/early Gauss deposits indicative of rapid westwards progradation of large low‐gradient, footwall‐sourced fans over axial deposits. This ‘forced’ migration of the ancestral Rio Grande may have occurred due to footwall catchment and fan growth consequent upon initiation and growth of the Red Hills Fault. Subsequent eastward movement of the axial channel belt in late Gauss and Matuyama times overwhelmed these large fans. We attribute this to continued tilting on the Red Hills Fault and to development of the Jornada Fault to the south‐east, the axial river belt avulsing north and eastwards through a developing Red Hills/Jornada crossover transfer zone.
We conclude generally that facies architecture of axial and transverse elements in half‐graben must reflect both climatic influences and the effects of fault development. Careful field mapping, accurate dating and palaeoclimatic studies are all necessary to determine the relative importance of these controls. Although adequate as broad guides, previous purely ‘fixist’ tectonosedimentary models allow for no fault growth, decay or climatic modulation of facies trends and are thus generally inadequate to explain important aspects rift basin stratigraphy.
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Palaeohydraulics revisited: palaeoslope estimation in coarse‐grained braided rivers
Authors Chris Paola and David MohrigMethods for estimating palaeoslope from fluvial deposits have been available for some time, but new data and improved understanding of the relevant physical processes afford the possibility of improving existing methods, and the emerging field of quantitative stratigraphy provides a new context for the results. Here we focus on deriving palaeoslope estimates for coarse‐grained fluvial deposits. These estimates can be used in basin analyses to constrain the magnitude of the slope change necessary for a given deflection of palaeocurrents, to constrain temporal and spatial variation in basin subsidence rate, and to provide a surface datum for use in sediment‐backstripping calculations. The algorithm we derive to estimate palaeoslope applies to rivers that self‐adjust through variations in channel width to maintain a temporally and spatially averaged bed shear stress equal to some constant multiple of the critical shear stress for initial motion of bed sediment. Data from modern coarse‐grained rivers with minimal bank cohesion and form resistance suggest that this boundary shear stress is equal to about 1.4 times the critical shear stress for movement of the median‐sized clast of the surface layer. The key sedimentological criteria for recognition of systems appropriate for this type of analysis are: (1) field relations suggesting that channel banks formed in effectively noncohesive gravel (i.e. free of clay‐size sediment and plant roots); (2) absence of significant volumes of dune‐derived cross‐stratification; and (3) absence of indicators of extremely rapid, flash‐flood‐type deposition. The basic input data for a palaeoslope calculation are spatially averaged estimates of palaeodepth and median grain size. The most important aspect of data collection is that the depth and grain‐size estimates should be determined independently by random sampling over the whole outcrop. Joint analysis of data from appropriate modern rivers and of errors associated with palaeodepth and grain‐size estimates indicates that in coarse‐grained braided‐river deposits, palaeoslope can be estimated to within a factor of 2.
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Macrogeomorphic evolution of the post‐Triassic Appalachian mountains determined by deconvolution of the offshore basin sedimentary record
Authors Frank J. Pazzaglia and Mark T. BrandonA perplexing macrogeomorphic problem is the persistence of topography in mountain ranges that were initially formed by orogenic events hundreds of millions of years old. In this paper, we deconvolve the post‐Triassic macrogeomorphic history of a portion of one of these ranges, the central and northern Appalachians, using a well‐documented offshore isopach sedimentary record of the US Atlantic margin.
Topography is an important signature of tectonic, eustatic and/or geomorphic processes that produces changes in the erodible thickness of the crust (etc). We define etc as the total thickness of crust that would have to be consumed by erosion to reduce the mean elevation of a landscape to sea level. We use the term ‘source flux’, designated by ν˙, to describe the rate of change in etc attributed to deep‐seated geological processes such as crustal thickening, crustal extension, magmatic intrusions or dynamic flow in the mantle. In a mountain belt, the rate of change of mean elevation with respect to a base level, designated by ż′, can be represented as ż′ = c(ν˙ − kd z′ −; ėc ) −& hairsp;l˙, where kd is a proportionality constant relating the mean mechanical erosion rate to mean elevation, ėc is the mean chemcial erosion rate, c is a compensation ratio (held constant for Airy isostasy at 0.21) and l˙ is the rate of eustatic sea‐level change. This equation describes the sum of constructive source terms, two destructive erosion terms and a eustatic sea‐level term.
We use this simple linear equation to develop a landscape evolution model based on the concept of a unit response function. The unit response function is analogous to a unit hydrograph which describes the relationship between input (rainfall) and output (discharge) in a hydrological system. In our case, we solve for the general relationship between the source flux into the mountain belt and the erosional flux out of the belt. Offshore sediment volumes are used to determine the erosional flux. Drainage basin area is treated as either a constant (pinned drainage divide) or as increasing through time (migrating drainage divide). We use a third‐order polynomial fit to a global sea‐level curve to account for long‐term eustatically driven changes in etc and in drainage basin area. Chemical erosion is treated as a constant fixed at 5 m Myr−1.
We consider two end‐member models. The first is a ‘tectonic’ model in which the source flux is allowed to vary while kd is assumed to be constant over geological time and equal to its mean Pleistocene value of about 0.07 Myr−1. The second is an ‘erodibility’ model in which kd is allowed to vary, reflecting changes in climatic, climatic variables and rock‐type erodibility, while the source flux is held constant at zero. The ‘tectonic’ model reveals four important increases in the source flux, ranging from 200 to 2000 m Myr−1 that occur over short (<10 Myr) time spans, followed by a protracted period (>25 Myr) where ν˙ drops below zero to values of −1000 to −6000 m Myr−1. The ‘erodibility’ model produces a topography that decays in a step‐like fashion from an initial mean elevation that ranges between ∼1800 and 2300 m.
These models cannot unequivocally distinguish the relative importance of tectonic vs. climatic processes in the macrogeomorphic evolution of the post‐rift Appalachians, but they do provide some first‐order quantitative prediction about these two end‐member options. In light of existing stratigraphic, geological and thermochronological data, we favour the tectonic model because most of the events correlate well in time and form with known syn‐ and post‐rift magmatic events. Nevertheless, the most recent episode of increased sediment flux to the offshore basins during the Miocene remains difficult to explain by either model. Limited evidence suggests that this event may reflect asthenospheric flow‐driven uplift and the development of dynamically supported topography at a time when mechanical erosion rates were increasing in response to a cooling terrestrial climate.
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Late Quaternary sedimentation on the Leidy Creek fan, Nevada–California: geomorphic responses to climate change
Well‐dated surface and subsurface deposits in semiarid Fish Lake Valley, Nevada and California, demonstrate that alluvial‐fan deposition is strongly associated with the warm dry climate of the last two interglacial intervals, and that fans were stable and (or) incised during the last glaciation. Fan deposition was probably triggered by a change from relatively moist to arid conditions causing a decrease in vegetation cover and increases in flash floods and sediment yield. We think that this scenario applies to most of the other valleys in the southern Basin and Range.
Radiocarbon, tephra, and a few thermoluminescence and cosmogenic ages from outcrops throughout Fish Lake Valley and from cores on the Leidy Creek fan yield ages of >100–50 ka and 11–0 ka for the last two periods of alluvial‐fan deposition. Mapping, coring and shallow seismic profiling indicate that these periods were synchronous throughout the valley and on the proximal and distal parts of the fans. From 50 to 11 ka, fan deposition ceased, a soil formed on the older alluvium and the axial drainage became active as runoff and stream competence increased. Slow deposition due to sheet flow or aeolian processes locally continued during this interval, producing cumulic soil profiles. The soil was buried by debris‐flow sediment beginning at about 11 ka, coincident with the onset of relatively dry and warm conditions in the region. However, ground‐water discharge maintained a large freshwater marsh on the valley floor throughout the Holocene. Pulses of deposition during the Holocene are recorded in the marsh and fan deposits; some pulses coincided with periods of or transitions to warm, dry climate indicated by proxy climate records, whereas others may reflect local disturbances associated with volcanism and fires. Within the marsh deposits, much of the clastic material is probably desert loess. In addition, the deposition of coppice dunes within the fan deposits coincides with two dry periods during the late Holocene.
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The kinematics and pattern of escarpment retreat across the rifted continental margin of SE Australia
Authors Michele A. Seidl, Jeffrey K. Weissel and Lincoln F. PratsonRifted continental margins generally display an interior, low‐relief, highly weathered upland area and a deeply incised, high‐relief coastal area. The boundary between the two zones is commonly demarcated by an abrupt, seaward‐facing escarpment. We investigate the rate and pattern of escarpment erosion and landscape evolution along the passive margin of south‐east Australia, in the region of the New England Tableland. The process of rifting is shown to initiate an escarpment across which rivers flow, resulting in an escarpment that takes the form of dramatic, elongated gorges. Using a mass balance approach, we estimate the volume/unit length of continental material eroded seaward of the escarpment to be between 41 and 68 km2, approximately an order of magnitude less than the 339 km2 of terrigenous sediments calculated to have been deposited offshore, but consistent with earlier denudation estimates based on apatite fission track data. On the bedrock rivers draining the New England Tableland region, the escarpment is manifested as a series of sharp knickpoints punctuating the river longitudinal profiles. The knickpoints are situated the same distance upstream along the different channels and uniform escarpment retreat rates on the order of 2 km Myr−1 are estimated, despite some differences in bedrock lithologies. Gorge head migration appears to be very important as a bedrock incision mechanism. Field observations indicate a coupling between escarpment retreat and knickpoint propagation, bedrock channel incision, and hillslope development.
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Estimating palaeorelief from detrital mineral age ranges
Authors Jonathan D. Stock and David R. MontgomeryWe propose a method that uses the increase in mineral age with elevation in some bedrock landscapes to quantify palaeotopographic relief from the age range of detrital minerals in coeval sediment. We use the rate at which mineral age changes with elevation (its age‐gradient, dt/dz) and its age range (Δt) in the sediment to invert for relief: Δz=Δt/(dt/dz). Relief inversion requires a single‐grain dating precision high enough that detrital grains originate from resolvably different elevations (e.g. laser microprobe 40Ar/39Ar fusion). The technique assumes that there is no change in mineral age during erosion and transport, that sediment is mixed well enough and (or) sampled sufficiently to capture the extrema of mineral ages, and that isochrons were horizontal during erosion. Subject to these constraints, inversion of the age range of individual grains in synorogenic sedimentary sequences allows quantitative estimation of relief development for eroded mountain ranges. This method provides the only direct quantitative measure of palaeorelief, a poorly constrained, but important aspect of many geological, geomorphological and geodynamic models.
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Predicting sediment flux from fold and thrust belts
Authors Gregory E. Tucker and Rudy SlingerlandThe rate of sediment influx to a basin exerts a first‐order control on stratal architecture. Despite its importance, however, little is known about how sediment flux varies as a function of morphotectonic processes in the source terrain, such as fold and thrust growth, variations in bedrock lithology, drainage pattern changes and temporary sediment storage in intermontane basins. In this study, these factors are explored with a mathematical model of topographic evolution which couples fluvial erosion with fold and thrust kinematics. The model is calibrated by comparing predicted topographic relief with relief measured from a DEM of the Central Zagros Mountains fold belt. The sediment‐flux curve produced by the Zagros fold belt simulation shows a delay between the onset of uplift and the ensuing sediment flux response. This delay is a combination of the natural response time of the geomorphic system and a time lag associated with filling, and then subsequently uplifting and re‐eroding, the proximal part of the basin. Because deformation typically propagates toward the foreland, the latter time lag may be common to many ancient foreland basins. Model results further suggest that the response time of the bedrock fluvial system is a function of rock resistance, of the width of the region subject to uplift and erosion, and, assuming a nonlinear dependence of fluvial erosion upon channel gradient, of uplift rate. The geomorphic response time for the calibrated Zagros model is on the order of a few million years, which is commensurate with, or somewhat larger than, typical recurrence intervals for episodes of thrusting. However, model experiments also highlight the potential for significant variations in both geomorphic response time and in sediment flux as a function of varying rock resistance. Given a reasonable erodibility contrast between resistant and erodible lithologies, model sediment flux curves show significant sediment flux variations that are related solely to changes in rock resistance as the outcrop pattern changes. An additional control on sediment flux to a basin is drainage diversion in response to folding or thrusting, which can produce major shifts in the location and magnitude of sediment source points. Finally, these models illustrate the potential for a significant mismatch between tectonic events and sediment influx to a basin in cases where sediment is temporarily ponded in an intermontane basin and later remobilized.
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Tectonic control of fan size: the importance of spatially variable subsidence rates
Authors Kelin X. Whipple and Carolyn R. TraylerWe study the geophysical controls on the size of alluvial fans. Simple relationships between catchment characteristics, sediment yield, subsidence patterns and fan size are developed. As predicting fan size is essentially a conservation of mass problem, our analysis is general, applying to all types of fan landform. The importance of spatially variable subsidence rates has gone largely unrecognized in previous studies of modern fans. Here we stress that the distribution of subsidence rates in the depositional basin is a primary control on relative fan size. Both free coefficients in the oft‐cited power‐law correlation of fan area and catchment area can be shown to be set primarily by the tectonic setting, taken to include source area uplift rate and the subsidence distribution in the depositional basin. In the case of a steady‐state landscape, relative fan size is shown to be independent of both climate and source lithology; only during times of significant departure from steady state can relative fan size be expected to vary with either climate or source lithology. Transients associated with (1) a sudden increase in rock uplift rate, (2) a sudden change in climate and (3) the unroofing of strata with greatly differing erodibilities may produce variation of relative fan areas with both climate and source lithology. Variation of relative fan size with climate or lithology, however, requires that catchment–fan system response to perturbations away from steady state is sensitive to climate and lithology. Neither the strength of transient system responses nor their sensitivity to climate or lithology are known at present. Furthermore, internal feedbacks can significantly dampen any climatic or lithological effect. Thus theoretical considerations of the importance of climatic and lithological variables are inconclusive, but suggest that climatic and lithological effects are probably of secondary importance to tectonic effects. Field data from an unsteady landscape in Owens Valley, California, support and illustrate theoretical predictions regarding tectonic control of fan size. Field data from Owens Valley allow, but do not prove, a secondary dependence on source lithology. In addition, the Owens Valley field data indicate no relationship between relative fan size and climate. Headward catchment growth and enhanced sediment bypassing of fans during times of increased sediment yield (glacial) are put forward as plausible explanations.
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Volumes & issues
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Volume 36 (2024)
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Volume 35 (2023)
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Volume 34 (2022)
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Volume 33 (2021)
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Volume 32 (2020)
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Volume 31 (2019)
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Volume 30 (2018)
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Volume 29 (2017)
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Volume 28 (2016)
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Volume 27 (2015)
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Volume 26 (2014)
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Volume 25 (2013)
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Volume 24 (2012)
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Volume 23 (2011)
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Volume 22 (2010)
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Volume 21 (2009)
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Volume 20 (2008)
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Volume 19 (2007)
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Volume 18 (2006)
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Volume 17 (2005)
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Volume 16 (2004)
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Volume 15 (2003)
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Volume 14 (2002)
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Volume 13 (2001)
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Volume 12 (2000)
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Volume 11 (1999)
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Volume 10 (1998)
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Volume 9 (1997)
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Volume 8 (1996)
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Volume 7 (1994)
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Volume 6 (1994)
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Volume 5 (1993)
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Volume 4 (1992)
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Volume 3 (1991)
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Volume 2 (1989)
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Volume 1 (1988)