Drainage Reorganization Research Articles

Climate vulnerability of ecosystems and landscapes on Alaska’s North Slope

Alaska’s North Slope is especially vulnerable to climatic change because higher latitudes are subject to positive snow- and sea ice-atmosphere feedbacks under warming conditions and because the dynamics of frozen seascapes and landscapes are tightly determined by thermal regime. Shifts in timing and magnitude of freeze–thaw processes are observed to have or expected to have non-linear, threshold-crossing impacts on sea ice, landforms, and biota. Observed changes in North Slope surface air temperatures and precipitation were non-monotonic over the last century, but have trended upward for the last several decades. These changes are linked to hemispheric climate dynamics, reflected in North Pacific and Arctic Oscillation circulation indices. Projected anthropogenic climate changes—with the possibility of continued warming, increased storm frequency and intensity, and decreased insulating snow cover—portend an uncertain future for this domain. Current or foreseen physical system shifts include: (1) declining seasonal and permanent sea ice extent and character, (2) rapid coastal erosion due to storm exposure over a longer near-shore ice-free season, (3) deeper soil active layer over warmer permafrost, along with altered thermokarst processes—contributing to thaw lake expansion, surface drainage re-organization, and hillslope instability. Biogeophysical responses encompass (1) modified surface-atmosphere energy balance from snow cover, vegetation, and hydrologic change and (2) shifted soil and wetland biogeochemical dynamics, including accelerated carbon efflux. Climate-driven plant community shifts on the North Slope result from the interplay of climate, vegetation response, and landscape processes. Some transitions involve stabilizing, others destabilizing plant-permafrost feedbacks. Impacts on caribou, migratory avifauna, and freshwater biota are through direct effects of climate on organism physiology and reproductive biology and indirectly through disruption of habitat mosaics (including along migratory routes) and shifts in competition and trophic linkages. The North Slope’s physical and biological vulnerabilities to shifting climate and observed leading indicators of change are compelling reasons for land managers to consider climatic instability as a threat in conjunction with other known stressors while seeking strategies for protection of this domain’s natural heritage and ecosystem services.

Climate-induced formation of a closed basin: Great Divide Basin, Wyoming

Large closed basins are often associated with regions of active tectonics. In contrast, we provide the example of the Great Divide Basin, in southern Wyoming, where climate change, through its impact on erosion and fl exure, provides the primary mechanism of basin closure. Two- and three-dimensional fl exural models that incorporate the effects of local basin abandonment due to aridity as well as minor extrabasinal extension demon- strate that basin closure with 40 m of base- ment relief (tilt) could have been achieved in the Great Divide Basin under reasonable assumptions of sediment rock density and fl exural rigidity. The primary drive for ba- sin abandonment was insuffi cient discharge that retarded river downcutting. Continued erosion in the surrounding drainage led to fl exural rebound, against which erosion by the outlet from the Great Divide Basin was unable to compete. Eventually, the basin became detached and isolated from the sur- rounding drainage, and a few tens of meters of differential tilt developed between the ba- sin fl oor and spillover point. An extra amount of tilt, up to a few meters, could have been at- tained by rift shoulder effects associated with minor nearby extension outside of the basin to the north. Hence, closure of the Great Di- vide Basin took place with no internal fault- ing, nor did it require any extrabasinal tec- tonic activity to force basin closure. Hence, basin closure and/or drainage reorganization need not record tectonic activity in all cases. Differential erosion provides an alternative hypothesis that does not refl ect local tectonic timing. Our model of the evolution of the Great Divide Basin illustrates a mechanism by which basins can become closed by cli- matic effects alone.

Diachronous isotopic and sedimentary responses to topographic change as indicators of mid‐Eocene hydrologic reorganization in the western United States

ABSTRACTEarly Cenozoic terrestrial deposits in the western United States represent well‐preserved archives of climatic and tectonic processes that together shaped the Earth's surface during the demise of a large continental plateau. This study examines a Cenozoic terrestrial sedimentary sequence in the central part of the Cordilleran orogen (Montana) using sedimentologic and geochemical techniques. At ∼49 Ma, we observe rapid major shifts in oxygen, carbon and strontium isotope records that are too large to directly reflect changes in meteoric water composition due to simple orographic rainout. The transition to low‐δ18O values in pedogenic carbonate in concert with changes in the composition of clastic material at ∼49 Ma points to the input of evolved meteoric water to the hydrological cycle due to a change in the source of waters reaching Cordilleran intermontane regions in southwestern Montana. This drainage reorganization coincides with the initiation of magmatism and extension to the west in what is now Montana and Idaho. The sedimentological record shows evidence that depositional gradients increased in the study area ∼46 Ma, ∼3 Myr after the drainage reorganization occurred. This interval is most likely the time it took for extensional deformation to propagate to the study area itself. Evidence of freshening events in Laramide Basins to the southeast suggests that this drainage reorganization diverted waters to progressively fill these basins and highlights the impact of post‐plateau extension‐related landscape reorganization on river networks and lake dynamics. This study also emphasizes the importance of using multiple tools in deciphering topographic history through the study of terrestrial basin deposits, in that interpretation based on any single method employed would have compromised our ability to successfully identify the regional evolution of topography and drainage networks.

Stewart Mountain Terrace: A New Salt River Terrace with Implications for Landscape Evolution of the Lower Salt River Valley, Arizona

Stream terraces of the Salt River form the interpretive backbone of Plio-Pleistocene landscape evolution of central Arizona, because they represent the base level of all tributary streams. This paper presents a new addition to T.L. Péwé' s Salt River Terrace sequence (in decreasing topographic position and age: Sawik, Mesa, Blue Point, and Lehi) that has been unrefined for the last 30 years. The existence of an older, higher terrace was predicted by research suggesting that the lower Salt River originated by lake overflow from an ancestral Pliocene lake in the Tonto Basin. Field reconnaissance, aerial photo interpretation, and sedimentological analysis revealed this terrace on the north side of the Salt River, named here the Stewart Mountain Terrace (SMT). Where exposed, the fluvial sediments of SMT overlay Tertiary basin fill unconformably. SMT sediments are characterized by ∼50 m thick fluvial gravels found more than 70 m above remnants of the Sawik Terrace. Although the gravels are distinctly Salt River in origin, Stewart Mountain gravels differ from the lower and younger Salt River Terraces. The clast sizes are much larger on average and host a significantly different lithology. Because of these differences the SMT has profound implications for the understanding of regional drainage reorganization after basin and range extension. The existence of this terrace and its distinct gravels are consistent with, but do not prove, a lake overflow mechanism for the initiation of through flowing drainage in the Salt River Valley.

Quaternary incision rates and drainage evolution of the Uncompahgre and Gunnison Rivers, western Colorado, as calibrated by the Lava Creek B ash

ABSTRACT The Quaternary erosional history of western Colorado is documented in terraces of the Colorado, Gunnison, and Uncompahgre Rivers that contain the Lava Creek B ash (0.64 Ma). This paper reports an important new ash locality that dates ca. 100-m-high river gravels associated with the paleo-confluence of the Gunnison and Uncompahgre Rivers upstream from Grand Junction. Provenance analysis reveals paleo-Gunnison River gravels (containing granite and gneiss clasts) and paleo-Uncompahgre River gravels (containing Uncompahgre Group quartzite and San Juan volcanic field rocks). The paleo-Uncompahgre River gravels are 3 m directly beneath Lava Creek B ash, and the areal distribution of terraces indicates that this area was the paleo-confluence between the Gunnison and Uncompahgre Rivers. This confluence has shifted 11 km to the east since 0.64 Ma due to events related to stream piracy and drainage reorganization. Gunnison terrace straths near the paleo-confluence are estimated to be 106 m above the modern strath, giving an estimated incision rate of 165 m/Ma. Because of excellent age and geologic control, this is one of the best incision-rate data points in the upper Colorado River system. It is similar to previously reported regional rates, but is substantially lower than upstream incision rates in the Black Canyon of the Gunnison River. This dated Gunnison River terrace anchors the projection of Lava Creek B-bearing Grand Mesa pediment surfaces (e.g., Petrie Mesa) to regional base level and helps constrain a regional reconstruction of the 0.64-Ma profile of the paleo-Gunnison River. This reconstruction shows dramatic differences in incision rate in the Gunnison River system since 0.64 Ma, and that a transient knickpoint migrated past Sawmill Mesa prior to 0.64 Ma. This incision data point has important implications for evaluating major Quaternary changes in the configuration of this part of the Rocky Mountain drainage system. It also provides evidence for a young, disequilibrium drainage system that is responding to base-level changes downstream driven by a stream capture event, which in turn may have been driven by tectonic or climatic perturbations.

Quaternary evolution of the Pecora River (southern Tuscany, Italy): paleohydrography and sediments provenance

The catchment of the Pecora River is located between the towns of Follonica and Massa Marittima (Southern Tuscany, Italy) and is characterized by Neogene-Quaternary strata resting unconformably on a pre-Neogene substratum. A critical geological re-interpretation of this area indicates that the Quaternary succession, composed by a relatively thin veneer of terrigenous deposits and by calcareous tufas, is the result of five major stages of drainage reorganization. The data from this study indicate that, similarly to present, the paleo-catchment of the Pecora River included the Arsenti Mount area, located few km eastward from Massa Marittima, and that was constantly separated from the nearby Bruna River. This new re-interpretation of the Quaternary evolution of the Pecora River area has several implications, in particular on the nature of the arsenic anomaly known in this area and mainly confined to the fluvial terrigenous deposits. Specifically, the erosion and dispersal of As-bearing mineralization emplaced between the late Pliocene and the Quaternary in fractures and faults affecting the bedrock of the Arsenti Mount area, was possibly driven by the sedimentary development of the Pecora River catchment resulting in the landforms and deposits described in this study. As a consequence, the As anomaly known in this area may have resulted from fully natural phenomena such as relief denudation and sediment dispersal.

The effect of drainage reorganization on paleoaltimetry studies: An example from the Paleogene Laramide foreland

Using multiple isotope systems, we examine the complex effects of drainage reorganization in the Laramide Foreland in the context of stable isotope paleoaltimetry. Strontium, oxygen and carbon isotopic data from lacustrine carbonates formed in the southwestern Uinta Basin, Utah between the Late Cretaceous and late Middle Eocene reveal a two stage expansion in the drainage basin of Lake Uinta beginning at ~ 53 Ma culminating in the Mahogany highstand at 48.6 Ma. A marked increase in 87Sr/ 86Sr ratios of samples from the Main Body of the Green River Formation is interpreted as the result of water overflowing the Greater Green River Basin in Wyoming and entering Lake Uinta from the east via the Piceance Creek Basin of northwestern Colorado. This large new source of water caused a rapid expansion of Lake Uinta and was accompanied by a significant and rapid increase in the O isotope record of carbonate samples by ~ 6‰. The periodic overspilling of Lake Gosiute probably became continuous at ~ 49 Ma, when the lake captured low- δ 18O water from the Challis and Absaroka Volcanic Fields to the north. However, evaporation in the Greater Green River and Piceance Creek Basins meant that the waters entering Lake Uinta were still enriched in 18O. By ~ 46 Ma, inflows from the Greater Green River Basin ceased, resulting in a lowstand of Lake Uinta and the deposition of bedded evaporites in the Saline Facies of the Green River Formation. We thus show that basin development and lake hydrology in the Laramide foreland were characterized by large-scale changes in Cordilleran drainage patterns, capable of confounding paleoaltimetry studies premised on too few isotopic systems, samples or localities. In the case of the North American Cordillera of the Paleogene, we further demonstrate the likelihood that (1) topographic evolution of distal source areas strongly influenced the isotopic records of intraforeland basins and (2) a pattern of drainage integration between the hinterland and foreland may correlate in space and time with the southward sweep of hinterland magmatism.

Benthic foraminiferal evidence of sediment supply changes and fluvial drainage reorganization in Holocene deposits of the Po Delta, Italy

Detailed benthic foraminiferal analyses performed on the Holocene subsurface deposits of modern Po Delta evidenced a complex palaeogeographic evolution. Hierarchical R- and Q-mode cluster analyses allowed to distinguish four assemblages indicative of different marine environments and sub-environments. Temporal and spatial distribution patterns of benthic foraminifers reflect changes in Po River discharge during delta evolution. The capability of foraminiferal assemblages to track nutrients and sediment supply oscillations permitted to recognize four main evolutionary phases (A–D), which took place after the maximum marine transgression (ca. 5500 cal yr BP). Prodelta and delta front deposits are accumulated in the western part of the study area between the first stages of progradation and the late Middle Ages (ca. 10th century AD); these are paralleled, in a more distal position, by open-marine sedimentation, replaced upward by prodelta sediments (phases A and B). Owing to the combined effect of a major avulsion (Rotta di Ficarolo) and the Adriatic circulation patterns, an organic-rich zone developed in the eastern part of the present Po Delta at the transition to the Modern Age (phase C), around 600 cal yr BP. For the fist time a fossil mud-belt, although its marginal fringe, along with its peculiar microfossil content ( Nonionella turgida assemblage), is recorded beneath a modern delta plain. During the recent past an outstanding rate of sedimentation affected the Po Delta, as clearly evidenced by the deposition of 14 meters fine-grained prodelta sediments between 600 and 380 cal yr BP. Prodelta deposits were finally replaced by delta front sands after the Porto Viro cut (ca. 350 cal yr BP), which marked the onset of Modern Delta (phase D).

Evolving east Asian river systems reconstructed by trace element and Pb and Nd isotope variations in modern and ancient Red River‐Song Hong sediments

Rivers in east Asia have been recognized as having unusual geometries, suggestive of drainage reorganization linked to Tibetan Plateau surface uplift. In this study we applied a series of major and trace element proxies, together with bulk Nd and single K‐feldspar grain Pb isotope ion probe isotope analyses, to understand the sediment budget of the modern Red River. We also investigate how this may have evolved during the Cenozoic. We show that while most of the modern sediment is generated by physical erosion in the upper reaches in Yunnan there is significant additional flux from the Song Lo, draining Cathaysia and the SW Yangtze Block. Nd isotope data suggest that 40% of the modern delta sediment comes from the Song Lo. Carbonates in the Song Lo basin make this a major control on the Red River Sr budget. Erosion is not a simple function of monsoon precipitation. Active rock uplift is also required to drive strong erosion. Single grain Pb data show a connection in the Eocene between the middle Yangtze and the Red River, and probably with rivers draining the Songpan Garze terrane. However, the isotope data do not support a former connection with the upper Yangtze, Mekong, or Salween rivers. Drainage capture appears to have occurred throughout the Cenozoic, consistent with surface uplift propagating gradually to the southeast. The middle Yangtze was lost from the Red River prior to 24 Ma, while the connection to the Songpan Garze was cut prior to 12 Ma. The Song Lo joined the Red River after 9 Ma. Bulk sample Pb analyses have limited provenance use compared to single grain data, and detailed provenance is only possible with a matrix of different proxies.

Drainage reorganization during breakup of Pangea revealed by in-situ Pb isotopic analysis of detrital K-feldspar

Pb isotopes in detrital K-feldspar grains provide a powerful provenance tracer for feldspathic sandstones. Common Pb isotopic compositions show broad (hundred-kilometer scale) regional variation, and this signature can survive weathering, transport, and diagenesis. The feldspar Pb signature can be measured rapidly using laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS), and careful targeting can avoid inclusions and altered regions within grains. Here, we combine a new Pb domain map for the circum–North Atlantic with detrital K-feldspar Pb isotopic data from Triassic and Jurassic sandstones from basins on the Irish Atlantic margin. The Pb isotopic compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern. Triassic sandstones originated from distant Archean and Paleoproterozoic rocks, probably in Green-land, Labrador, and the Rockall Bank to the NW, implying long (>500 km) transport across a nascent rift system. Later, Jurassic sandstones had a composite Paleo- and Mesoproterozoic source in more proximal sources to the north (

Late Neogene and Quaternary landscape evolution of the northern California Coast Ranges: Evidence for Mendocino triple junction tectonics

A landscape records the surface response to tectonics at time scales intermediate between short time-scale information derived from seismic imaging and global positioning systems and the long-term geologic record. We link late Neogene and Quaternary deposits and landforms in the northern California Coast Ranges to the tectonics of the Mendocino triple junction. In the northern California Coast Ranges, the Mendocino crustal conveyor geodynamic model describes crustal thickening, thinning, and dynamic topography that produce a pattern of uplift that migrates northward with the Mendocino triple junction. The tectonics are manifest in the drainage system and elevation pattern of the Coast Ranges. At long wavelengths, the elevation pattern closely matches the predicted double-peaked shape of Mendocino crustal conveyor topography, and the high points of uplift control the location of drainage divides. Presently, the divide between the Russian and Eel Rivers and the divide between the Eel and Van Duzen Rivers approximately correspond to the peaks of uplift predicted by the Mendocino crustal conveyor model. As the triple junction migrates northward, the double-humped pattern of uplift and subsidence migrates, and the Coast Ranges emerge. Smaller drainages develop and evolve by stream capture and flow reversal, and the two main divides migrate in concert with the triple junction. In contrast to the systematic development of the small streams, the largest trunk streams can maintain grade through regions of high uplift, and coastal river mouths remain stationary despite the uplift moving north. Before ca. 2 Ma, the majority of the Coast Range drainage flowed to a southern coastal outlet near the present mouth of the Russian River. At 2 Ma, facilitated by headwater stream capture at key locations, the drainage direction reversed, and the majority of Coast Range rivers now drain into the north-flowing Eel River. The major drainage reorganization at 2 Ma highlights the potential for complexity in geomorphic response to tectonics.

River spacing and drainage network growth in widening mountain ranges

ABSTRACTDrainage networks in linear mountain ranges always display a particular geometrical organisation whereby the spacing between the major drainage basins is on average equal to half the mountain width (distance from the mountain front to the main drainage divide), independent of climate and tectonics. This relationship is valid for mountains having different widths and is thus usually thought to be maintained by drainage reorganisation during mountain belt widening. However, such large‐scale systematic drainage reorganisation has never been evidenced. In this paper, we suggest an alternative explanation, namely that the observed drainage basin relationships are an inherent property of dendritic river networks and that these relationships are established on the undissected, lowland margins outside mountain ranges and are progressively incorporated and quenched into uplifted topography during range widening. Thus, we suggest that the large‐scale geometry of drainage networks in mountain ranges is mainly antecedent to erosion. We propose a model in which the large‐scale drainage geometry is controlled mainly by the geometrical properties of the undissected surfaces (in particular, the ratio of the regional slope to the local slope related to roughness) over which rivers are flowing before uplift, and is therefore independent of climate and tectonics.

Geomorphology and structure of the Jid right-lateral strike-slip fault in the Mongolian Altay mountains

We describe the late Cenozoic structure and evolution of the Jid right-lateral strike-slip fault in the northern Mongolian Altay mountain chain. Parts of the Jid fault have a vertical component of motion, introduced by bending along the fault. Some locations show additional structural complexity at the Earth's surface, with parallel strands having strike-slip, normal and reverse faulting, probably indicating a spatial separation of dip-slip and strike-slip components of motion in cross-section. The Jid fault shows indications of recent movements at scales ranging from drainage re-organisation at the kilometre scale, to ground deformations likely to result from movements during the last earthquake, which occurred ∼870–980 years ago based on OSL dating of deposits exposed in trenching. From the scale of the ruptures the earthquake would have had a magnitude of ∼7.5 with an estimated right-lateral slip of ∼5 m. Numerous horizontal offsets of about 10–15 m, and vertical scarps of ∼5 m in the youngest alluvial deposits show the likely cumulative movement over the last ∼15,000 years, and suggest a maximum slip-rate of roughly 1 mm/yr. The minimum cumulative right-lateral slip on the Jid fault is 1 km, which would accumulate in ∼1 Ma at present-day rates of slip. The relatively small vertical component of motion on the Jid fault is not sufficient to generate topography in the adjacent Kharkhiraa and Türgen Uul mountains, with relief of over 2000 m. The high mountains may therefore result from localised uplift on thrust faults rather than by oblique slip on the Jid fault.

Tectonic controls on drainage evolution and development of terminal alluvial fans, southern Pyrenees, Spain

AbstractAn understanding of how drainage patterns respond to tectonics can provide an insight into past deformational events within mountain belts and where sediment flux is supplied to depositional basins. The transverse rivers draining the Spanish Pyrenees show sudden diversions to axial courses and the capture of lateral systems producing large trunk rivers that break through the thrust front at structurally controlled points. The drainage reorganization from the initial regularly spaced pattern in the Late Eocene caused by the growth of thrust‐controlled topography influenced the location of outlets into the Ebro basin. The headward capture and merger of rivers as a result of structural diversion formed two large terminal fan systems during the Oligo‐Miocene along the Pyrenean thrust front. The early structural topographic controls on drainage evolution will have long‐term effects on sedimentation and stratigraphic architecture of foreland basins. This will only be maintained as long as there is tectonic uplift and the river systems strive to re‐attain a regular drainage spacing across the orogenic belt as partly achieved in the Pyrenees.

Faunal and environmental change in the late Miocene Siwaliks of northern Pakistan

The Siwalik formations of northern Pakistan consist of deposits of ancient rivers that existed throughout the early Miocene through the late Pliocene. The formations are highly fossil- iferous with a diverse array of terrestrial and freshwater vertebrates, which in combination with exceptional lateral exposure and good chronostratigraphic control allows a more detailed and tem- porally resolved study of the sediments and faunas than is typical in terrestrial deposits. Conse- quently the Siwaliks provide an opportunity to document temporal differences in species richness, turnover, and ecological structure in a terrestrial setting, and to investigate how such differences are related to changes in the fluvial system, vegetation, and climate. Here we focus on the interval between 10.7 and 5.7 Ma, a time of significant local tectonic and global climatic change. It is also the interval with the best temporal calibration of Siwalik faunas and most comprehensive data on species occurrences. A methodological focus of this paper is on controlling sampling biases that confound biological and ecological signals. Such biases include uneven sampling through time, differential preservation of larger animals and more durable skeletal elements, errors in age-dating imposed by uncertainties in correlation and paleomagnetic timescale calibrations, and uneven tax- onomic treatment across groups. We attempt to control for them primarily by using a relative-abun- dance model to estimate limits for the first and last appearances from the occurrence data. This model also incorporates uncertainties in age estimates. Because of sampling limitations inherent in the terrestrial fossil record, our 100-Kyr temporal resolution may approach the finest possible level of resolution for studies of vertebrate faunal changes over periods of millions of years. Approximately 40,000 specimens from surface and screenwash collections made at 555 localities form the basis of our study. Sixty percent of the localities have maximum and minimum age esti- mates differing by 100 Kyr or less, 82% by 200 Kyr or less. The fossils represent 115 mammalian species or lineages of ten orders: Insectivora, Scandentia, Primates, Tubulidentata, Proboscidea, Pholidota, Lagomorpha, Perissodactyla, Artiodactyla, and Rodentia. Important taxa omitted from this study include Carnivora, Elephantoidea, and Rhinocerotidae. Because different collecting methods were used for large and small species, they are treated separately in analyses. Small spe- cies include insectivores, tree shrews, rodents, lagomorphs, and small primates. They generally weigh less than 5 kg. The sediments of the study interval were deposited by coexisting fluvial systems, with the larger emergent Nagri system being displaced between 10.1 and 9.0 Ma by an interfan Dhok Pathan sys- tem. In comparison to Nagri floodplains, Dhok Pathan floodplains were less well drained, with smaller rivers having more seasonally variable flow and more frequent avulsions. Paleosol se- quences indicate reorganization of topography and drainage accompanying a transition to a more seasonal climate. A few paleosols may have formed under waterlogged, grassy woodlands, but most formed under drier conditions and more closed vegetation. The oxygen isotopic record also indicates significant change in the patterns of precipitation be- ginning at 9.2 Ma, in what may have been a shift to a drier and more seasonal climate. The carbon isotope record demonstrates that after 8.1 Ma significant amounts of C 4 grasses began to appear and that by 6.8 Ma floodplain habitats included extensive C4 grasslands. Plant communities with predominantly C3 plants were greatly diminished after 7.0 Ma, and those with predominantly C4 plants, which would have been open woodlands or grassy woodlands, appeared as early as 7.4 Ma. Inferred first and last appearances show a constant, low level of faunal turnover throughout the interval 10.7-5.7-Ma, with three short periods of elevated turnover at 10.3, 7.8, and 7.3-7.0 Ma. The three pulses account for nearly 44% of all turnover. Throughout the late Miocene, species richness declined steadily, and diversity and richness indices together with data on body size imply that community ecological structure changed abruptly just after 10 Ma, and then again at 7.8 Ma. Be- tween 10 and 7.8 Ma the large-mammal assemblages were strongly dominated by equids, with more balanced faunas before and after. The pattern of appearance and disappearance is selective with respect to inferred habits of the animals. Species appearing after 9.0 Ma are grazers or typical of more open habitats, whereas many species that disappear can be linked to more closed vege- tation. We presume exceptions to this pattern were animals of the mixed C

Quantification of river-capture-induced base-level changes and landscape development, Sorbas Basin, SE Spain

Abstract The Aguas/Feos river system of the Sorbas Basin, SE Spain was captured by an aggressive subsequent stream c. 100 ka. The consequence of the capture event was twofold: (1) basin-scale drainage reorganization via beheading of the southward flowing Aguas/Feos system and re-routing the drainage eastwards into the Vera Basin; and (2) the creation of a new, lower base level and associated upstream propagation of a wave of incision. The sequence of pre- and post-capture events are well established from previous studies of the Quaternary terrace record. Using these studies, this paper makes the first attempt to quantify the impact of river capture in terms of spatial and temporal variations in rates of incision, sediment flux and surface lowering. This was carried out through construction of 43 valley cross-sections from the ‘captured’ (Upper Aguas), ‘beheaded’ (Feos) and ‘capturing’ streams (Lower Aguas) within the central-southern parts of the Sorbas Basin. Dated pre- and post-capture terrace and corresponding strath levels were plotted on to the valley cross-sections enabling incision amounts, rates and valley cross-sectional areas to be calculated. Sediment fluxes were calculated using a mean valley section method. Surface lowering calculations were made through reconstruction of the top basin-fill surface and subtraction from the modern contour values. The lowering of base level has resulted in a dramatic increase in incision upstream of the capture site by a factor of 4 to 20. This in turn has been associated with significant pre- and post-capture changes in valley shape. The increased incision resulted in dramatic post-capture increases in valley erosion upstream and downstream of the capture site by a factor of 2 to 9 which can be related to changes in associated stream power as a function of increased gradient and discharge. In excess of 60% of the landscape change can be accounted for by valley-constrained erosion as opposed to overall surface lowering.

Drainage evolution in south-central Africa since the breakup of Gondwana

The drainage system in south-central Africa has undergone major reorganisations since the disruption of Gondwana. Isopachs of the Kalahari sequence and a variety of geomorphological features can be used to pinpoint abandoned drainage lines. Continental fluvial sediments of Mesozoic-Cenozoic age reflect river systems which existed prior to and immediately following continental break-up. The east coast sedimentary sequence documents changes in the location of major supplies of terrigenous sediments, and provides a framework for establishing the timing of changes in drainage configuration. This evidence indicates that during the upper Jurassic to Cretaceous, the Okavango, Cuando and Zambezi-Luangwa rivers formed the headwaters of the proto-Limpopo. The lower-Zambezi-Shire formed a separate graben-bound river system with a discharge point into the Indian Ocean in the vicinity of mouth of the present-day Zambezi. A third major drainage entered the Indian in the vicinity of the modern Save mouth. End Cretaceous uplift along the Okavango-Kalahari-Zimbabwe Axis severed the links between the Limpopo and the Okavango, Cuando and Zambezi-Luangwa. This resulted in a senile endoreic drainage system which supplied sediment to the Kalahari basin. However, the uplift rejuvenated the lower Zambezi, initiating headward erosion and progressive capture of the Luangwa, upper Zambezi and Kafue. Predatory headward extension of the Zambezi is still active, and this river will eventually capture the Okavango. The model developed for drainage reorganisation provides a framework for interpreting kimberlitic heavy mineral dispersion patterns. It also forms the basis for explaining fish and plant dispersion patterns, and understanding recent water level fluctuations in the Makgadigadi pans system in Botswana.

Paleomagnetic evidence for multiple Late Pliocene - Early Pleistocene glaciations in the Klondike area, Yukon Territory

The Late Pliocene - Early Pleistocene terraces of the Klondike area provide a conformal record of sedimentation which marks the transition from preglacial to glacial conditions, and is one of the most complete records of glaciation and interglaciation in the northern Cordillera. Preglacial sedimentation is recorded in the Lower White Channel gravel that contains a reverse-normal polarity sequence. A re-aggradation of the nonglaciated valleys of the goldfields in response to Late Pliocene cooling is recorded by the Upper White Channel gravel, which is characterized by a dominant normal polarity with a lower reversal likely associated with the first ice-wedge casts. Klondike gravel interfingers with Upper White Channel and is normally magnetized, indicating an equivalent magnetostratigraphic unit. This glaciofluvial gravel provides evidence of extra-basinal clasts, marking a major reorganization of drainage associated with the first advance of the Cordilleran Ice Sheet in western Yukon Territory correlated with the Gauss chron (>2.58 Ma). An intermediate terrace in Klondike valley is overlain by 8 m of fluvial and glaciofluvial gravel named the Midnight Dome gravel. This gravel is mantled by 15 m of loess and hillslope deposits, named the Midnight Dome loess, which preserve at least three interglacial pollen assemblages, and a reverse-normal-reverse-normal polarity sequence assigned to the late Matuyama chron, including Jaramillo subchron (1.07-0.99 Ma), and early Brunhes chron (<0.78 Ma). The oldest glaciations here, based on the preferred interpretation, extend well into the Pliocene (>2.58 Ma) making these among the oldest glacial sediments in North America.

Simple models of drainage reorganisation on a tectonically active ridge system

We have studied the reorganisation of drainage patterns in response to the tectonically driven emergence of a small ridge (South Rough Ridge) along the flank of a larger ridge (Rough Ridge) as described by previous workers in the Central Otago region of New Zealand. We compare the results of a numerical model of landscape evolution (CASCADE) in which stream incision is linearly related to local slope, discharge (hence catchment size), and sediment load, to present‐day drainage patterns in Central Otago in order to infer possible scenarios for the evolution of the stream network. We conclude that the observed drainage patterns along the flanks of Rough Ridge are most likely to have resulted from the lateral propagation of South Rough Ridge. Our conclusion is also supported by simple mathematical derivations which imply that the periodic patterns observed in the modern fluvial network—the regular spacing of wind gaps and stream valleys—may have been created by a constant tectonic driving force. A dimensionless number is found that best describes stream evolution as a function of tectonic uplift, erosion, and slope. We also demonstrate how the shape of a landform may be used to constrain the sequence of events that have led to its formation.

VOLCANIC MAARS, LONG QUATERNARY SEQUENCES AND THE WORK OF THE INQUA SUBCOMMISSION ON EUROPEAN QUATERNARY STRATIGRAPHY

Abstract For several years the INQUA Subcommission on European Quaternary Stratigraphy (SEQS) has been actively fostering debate on the stratigraphy and typification of the Middle Pleistocene in Europe. Palynological investigations of deep sediment cores from maar lakes in Italy and the French Massif Central have now provided a stratigraphical framework conclusively correlated with the deep ocean record, confirming the occurrence of two clearly temperate intervals between the Holsteinian and Eemian Interglacials. This places interglacial deposits from localities such as Schoningen, Pritzwalk (Domnitz), Wacken and Maastricht-Belvedere into a clearer perspective. All major Saalian glacial advances probably relate to Oxygen Isotope Stage 6. Reasons for the relative paucity of continental interglacial deposits equivalent to Oxygen Isotope Stages 7 and 9 are discussed. These Stages were not preceded by major glacial events in northern Europe, so suitable basins of deposition were largely absent; in contrast fluvial erosion was very active, with much downcutting and drainage re-organisation. Finally evidence from the sea-surface temperature record of the Norwegian Sea does suggest a less powerful pattern of oceanic circulation in the North Atlantic during these interglacial intervals within the Saalian Complex Stage, when compared to those of the Eemian and Holocene. This difference would certainly have affected environments on the adjacent European continent.