Erosional Terraces Research Articles

Role of flood discharge in shaping stream geometry: Analysis of a small modern stream in the Uinta Basin, USA

This small modern river system is located on a relatively flat (about 1°–2°), unconsolidated sandy pediment surface in the Uinta Basin of Utah, USA, and it is with a scale of about 30 m long and 0.4–0.8 m wide, similar as a natural flume experiment model. The small stream is informally divided into upstream, midstream and downstream. The analysis shows that flood discharge influences channel sinuosity and morphology to produce an initial meandering pattern which is later changed to a braided and then a straight pattern in the downflow direction.The upstream segment has a high sinuous geometry dominated by both erosion (cutbanks) and deposition (point bars). In the resistance of sporadic vegetation rooting in banks, the upstream flood deviates its original direction, which results in the powerful flood intensively eroding the cutbank and accreting clastics to build point bars, and thus producing a high sinuous channel.The midstream is dominated by deposits (many small bars) with a moderate to low sinuosity. Due to the bad drainage of the high sinuous channel in the upstream, the strong flood can cut off the point bar completely or even surmount the levee in the last meandering upstream, which widens the channel suddenly with a quick decreasing current power. Then, the clastics from the upstream are unloaded in the midstream and form many small bars. Unloaded sediments protect the bank, and the low-power current brings a moderate erosion to the bank, which forms a moderate to low sinuous channel in the midstream.The downstream shows multistage erosional terraces in its relatively straight channels. After the midstream water drops its load, it becomes “clear” and reaches downstream, the lower current power is helpless to reform channel geometry. Thus, the downstream channel segment keeps a lower sinuous geometry, even straight partially. Small amounts of fine clastics are deposited, and simultaneously multistage terraces are formed due to regressive flood erosion.This stream example demonstrates the subtleties of stream flow and the importance of flood discharge in shaping the channel geometry. Although it is difficult to scale up this example to a large river system that carves geomorphic landscape, this case shows how river geometries vary from the traditional patterns due to different gradient.

Late Quaternary alluvial fans of Emli Valley in the Ecemiş Fault Zone, south central Turkey: Insights from cosmogenic nuclides

Alluvial fans within the paraglacial Ecemiş River drainages on the Aladağlar Mountains in south central Turkey were studied using geomorphological, sedimentological, and chlorine-36 terrestrial cosmogenic nuclide (TCN) surface exposure dating methods to examine the timing of alluvial fan abandonment/incision, and to understand the role of climatic and tectonic processes in the region. These alluvial fan complexes are among the best-preserved succession of alluvial fans in Turkey and they were offset by the major strike-slip Ecemiş Fault of the Central Anatolian Fault Zone. The alluvial fans are mostly composed of well-lithified limestone cobbles (5 to 25cm in size), and comprise crudely stratified thick beds with a total thickness reaching up to about 80m. TCN surface exposure dating indicates that the oldest alluvial fan surface (Yalak Fan) was likely formed and subsequently abandoned latest by 136.0±23.4ka ago, largely on the transition of the Penultimate Glaciation (Marine Isotope Stage 6, MIS 6) to the Last Interglacial (MIS 5) (i.e. Termination II). The second set of alluvial fan (Emli Fan) was possibly developed during the Last Interglacial (MIS 5), and incised twice by between roughly 97.0±13.8 and 81.2±13.2ka ago. A younger alluvial fan deposit placed on relatively older erosional terraces of the Emli Fan suggests that it may have been produced during the Last Glacial Cycle (MIS 2). These events are similar to findings from other fluvial and lacustrine deposits throughout central Anatolia. The incision times of the Ecemiş alluvial fan surfaces largely coincide with major climatic shifts from the cooler glacial periods to warmer interglacial/interstadial conditions. This indicates that alluvial fans were produced by outwash sediments of paleoglaciers during cooler conditions, and, later, when glaciers started to retreat due to a major warming event, the excess water released from the glaciers incised the pre-existing fan surfaces. An alluvial fan in the study area was also cut by the Ecemiş Fault, highlighting the influence of tectonics on fan development. It was offset vertically 35±3m since at least 97.0±13.8ka, which suggests a 0.36±0.06mma−1 vertical slip-rate of the fault.

Beach development on an uplifted coral atoll: Niue, south west Pacific

Niue is an uplifted coral atoll in the south western Pacific characterised by erosional terraces on its coastal margin. Beaches are found around the island located in pockets at the rear of erosional shore platforms. The beaches in Niue are <100m long, <25m wide and generally less than 0.5m thick. The beaches sit on top of an abrasion ramp that dips seaward at a similar angle to the beach. The morphology, stability and sedimentology of these beaches are investigated through laser surveying, aerial photo analysis and petrographic techniques. Surveying was undertaken in 2008 and 2010 with data compared to previous work conducted in the 1990s in order to assess the controls on sediment deposition on uplifted coral atolls. There is a high potential for sediment transport on the island. The beaches are entirely removed during tropical cyclone events and even under calm conditions sediment is mobile. The restriction of beaches to pockets along the rocky coast suggests that these areas temporally interrupt sediment transport allowing beaches to form. All the beaches are composed of a typical chlorozoan assemblage of carbonate grains dominated by coral (20–50%), coralline algae (18%) and foraminifera (up to 81%). These sediments are produced on the platforms in the immediate vicinity of the beaches with little longshore transport between embayments being evident. The close relationship between source and depositional zones, combined with the high transport potential across the platforms indicates that the beaches are highly vulnerable to any change in either energy conditions or sediment supply.

Extraction of Glacio-Geomorphological Units of Tons River Watershed Based on Remote Sensing Techniques

Tons basin has the maximum share of glaciers, more than 50 glaciers, as well as glacierised area in Uttarakhand and Himachal Pradesh and the majority of the glaciers are of valley type. One of the important features of the glaciers of Tons valley is the presence of a thick mantle of supraglaciers moraine cover which can be attributed to the terrain characteristics, besides, the avalanche fed nature of the glaciers. The present study is the extraction of Glacio-geomorphological unit of Tons River basin based on the visual interpretation of remote sensing data. It was very much difficult in field, to extract all glacio-geomorphological units in glaciated area, but based on the remote sensing data, it becomes easy to identify. With the help of glacio-geomorphological map it has been found that four most important glaciers which fed the Tons River are Bandarpunch Glacier, Jaundar Bamak glacier, Jhajju Bamak and Tilku glacier. The tributaries of Tons River i.e. Harkidun Gad, Rupin Nadi and Supin Nadi are mainly fed by the mountain glaciers, valley glaciers and glacier lakes. The erosional terraces, glacio-fluvial terraces, open “U” shaped valleys, proglacial lake, lateral moraines, terminal moraines, palaeo-cirque and debris/talus cones are well developed in this glaciated regions. Glacio-geomorphic features are very much significant for palaeo-climatic reconstruction, showing variations, temporally and spatially. At the same time, these landforms, which are also altered by processes prevailing during interglacial period, helps in the geo-environment studies and glacier related problems like avalanches, global warming and cloudburst etc.

Shore platform development on an uplifting limestone island over multiple sea-level cycles, Niue, South Pacific

Niue is an uplifted limestone island, which preserves an atoll morphology with a sequence of terraces around its outer margin. A modern terrace fringes much of the island and this paper examines the extent to which this terrace can be considered accretionary and constructed by coral and coralline algae, or erosional and truncated by those processes that form shore platforms. The occurrence of Pleistocene limestone outcropping across this terrace and the continuation of caves and other karst features, dissecting the sub-aerial limestones, onto the forereef indicate the overall significance of erosion. At present live coral cover is restricted to isolated colonies growing in grooves, potholes and karstic channels eroded into the modern terrace surface. Coralline algae coat the outer margin of this terrace, and also veneer prominent surf benches that occur on the windward shore, 1–2m above the terrace surface. Terrace evolution on Niue therefore appears to be primarily erosional in origin. There is little accommodation space for reef growth and, as a result, accretional fringing reefs are absent around most of the island. Erosional processes have also dominated landscape evolution for at least the past few eustatic cycles. Fringing reef growth is absent or severely restricted during the last interglacial and occurs as sporadic small reefs on the seaward margin of an erosional terrace during the penultimate interglacial. The development of terrace features in uplifting reef settings does not necessarily originate through accretional reef processes and may in fact be entirely erosional. Although reef terraces are frequently depositional in other settings, those present on Niue indicate a predominance of erosion, indicating the need to discriminate emergent terraces in terms of the extent to which they are erosional or accretionary when using such features to reconstruct island palaeoenvironments.

Slope channel formation, evolution and backfilling in a wide shelf, passive continental margin (Northeastern Sardinia slope, Central Tyrrhenian Sea)

The sedimentary processes of the Caprera slope channel system (CSCS), located in the northern slope sector of the eastern Sardinian margin, have been reconstructed through the integrated interpretation of multibeam bathymetric data and seismic profiles. The CSCS is composed of three main slope channels (C0, C1, C2), and of a series of smaller V-shaped channel-forms, that show different morphologies and sedimentary infill. The southernmost C0 slope channel, is abandoned and its upper segment is completely buried and healed, whereas its lower segment is still evident in the bathymetry. On the contrary, the upper segments of the northernmost slope channels, C1 and C2, are characterized by an inner thalweg, erosional terraces, and internal levees. The channel bends feature complex erosive geomorphic elements, mainly controlled by the interaction of sediment gravity flows with the channel shape. The lower reaches of C1 and C2 slope channels have a flat floor, which is the result of the deposition of channel-wide sedimentary bodies. The present setting of the CSCS is indicative of a waning phase and a decreased volume of sediment gravity flows following a main waxing phase of channel excavation. This scenario can be interpreted as resulting from the decreased energy flow following the last glacial maximum. During low stand periods, river-fed sediment gravity flows had high energy and excavated the slope channels. During the present-day high stand, the main flows, resulting from the interaction between alongshore and cross-shelf currents had a downbasin depositional behavior and resulted in the complete healing, or partial backfilling of the slope channels.

トカラ列島における中期更新世の酸性海底火山活動

To understand the submarine volcanism surrounding the Tokara Islands, a submarine topographic analysis and 67 dredge samplings were carried out. Prior to the submarine investigations, we reviewed comprehensively geological and geophysical data on this region and confirmed the complexity of both volcanic activity and tectonic setting of the Tokara Islands. In contrast to the homogeneous subaerial volcanic rocks comprising predominantly two-pyroxene andesite lava flows, the dredged samples vary from basaltic andesite to rhyolite in composition. Furthermore, we reveal that dacitic and rhyolitic pumices are abundant and broadly distributed throughout the submarine area. The recovered volcanic rocks were mainly subangular to angular cobble-boulder fragments of lava, scoria, and variably vesiculated pumice. Volcanic rocks with hornblende phenocrysts occur only north of the Tokara strike-slip fault, which is a major tectonic element of volcanism. The pumices can be classified into three categories based on the size and abundance of the phenocrysts: aphyric pumice, fine-grained porphyritic pumice, and coarse-grained porphyritic pumice. Occurrences, such as amount in a dredge, shape without extensive abrasion, large fragment size, and bulk rock chemical compositions of the major pumice fragments suggest that they are in situ, rather than originating as drifted pumice or air fall, exotic pyroclastic fragments derived from the four super-eruptions of Kyushu Island. Because dredged samples contained fresh volcanic glass in the groundmass, and are not covered by iron-manganese oxide crust, they appear to have originated from the Quaternary eruptions. Indeed volcanic islands have developed above the submarine erosional terraces (indicated as knick points at approximately 110 m in depth), which is assumed to have formed during the last glacial age. K-Ar age dating on the representative pumice samples resulted in ages of 0.60 ± 0.20 Ma and < 0.2 Ma, respectively. These newly obtained submarine data support that acidic volcanisms occurred around the submarine calderas during the Mid-Pleistocene age.

Erosional sculpting of the Caprera confined deep-sea fan as a result of distal basin-spilling processes (eastern Sardinian margin, Tyrrhenian Sea)

Deep-water sedimentation is currently a major focus of both academic research and industrial interest. Recent studies have emphasized the fundamental influence of seafloor topography on the growth and morphology of deep-sea fans. In topographically complex slopes, the development of turbidite systems is strongly controlled by pre-existing bounding slopes. The aim of this work is to study the effect of basin confinement on the Caprera deep-sea fan (CDF), the main turbidite system of the Olbia intraslope basin (OIB) located in the eastern Sardinian margin (Central Tyrrhenian Sea). The OIB is the northernmost intraslope basin of the margin, bound seaward by the Etruschi and Baronie seamounts, and landward by the Sardinian continental slope. The OIB is completely filled and sediments can spill toward the lower Tavolara basin (TB) through a system of bypass canyons located in a saddle between the two seamounts. The CDF has been imaged by high-resolution swath-bathymetric dataset acquired by the ISMAR of Bologna during the cruises TIR96 and TIR99. The multibeam data have been integrated with seismic lines in order to understand the shape, the morphology and internal organization of the CDF. The CDF faces a wide shelf sector and as a consequence canyons are presumably stranded from river supply during high stands of sea level. At the base of slope the CDF consists of a wide leveed channel. The increasing channel flank relief moving downfan, the presence of knickpoints, erosional remnants, chutes scouring the levees and erosional terraces show that the Caprera channel, at the present day, is out of grade. In addition, erosional processes also act beyond the channel mouth, where distal erosional channels are present, and in the basin plain, that is scoured by a broad shallow valley. The bypass of sediments, from the completely filled OIB to the lower TB, has promoted the lowering of the ultimate base level for sediment gravity flows which is considered the cause of the disruption of the equilibrium profile of the CDF. However, the seismic data show that, in the CDF phases of deposition also alternate with the mainly erosional ones. These are interpreted to reflect the variability in flow volume during different sea level stands. The depositional style of the CDF does not conform to simple deep-sea fan models, and emphasizes that seafloor topography is a prime control on turbidite system development also in tectonically inactive margins.

Neogene–Recent extension on the eastern flank of Mount Olympus, Greece

The Olympus massif of NE Greece comprises metamorphosed and deformed Triassic and Cretaceous–Eocene carbonates considered to represent part of the passive southwestern margin of Neotethys, which separated the Apulia plate of mainland Greece from southern Europe in the Mesozoic. Ocean closure in the Early Tertiary led to subduction of this passive margin, as a result of which the Olympus carbonates were tectonically overridden in the Eocene by a series of thrust sheets consisting of locally high P/T (blueschist) metamorphosed continental margin sediments, basement granitoid gneisses, and ophiolitic rocks. Following collision, uplift of the Olympus carbonates by ~ 6–8 km was accomplished by Early Neogene ductile and Late Neogene–Recent brittle normal faulting, which exposed the carbonates in the form of a structural window through the overriding thrust stack. The focus of this exhumation occurred on the eastern flank of the massif where a thick calc–mylonite with down-to-the-east sense-of-shear indicators and a prominent zone of NNW- to NW-trending brittle normal faults separate the carbonates from telescoped structurally overlying units. Published 40Ar/ 39Ar microcline ages that indicate a thermal perturbation close to the contact zone in excess of 100–150 °C at 16–23 Ma (Early Miocene) are taken to date the onset of ductile extension, and imply long-term uplift rates of ~ 0.4 to 1 km/m.y. Degradation of Pleistocene glacial features on the Olympus massif and the presence on its eastern piedmont of deeply incised streams, erosional terraces, and fault scarps that offset the sedimentary deposits of three cycles of Pleistocene glacial deposition (Units 1–3), testify to its continuing uplift. A prominent frontal fault offsets Unit 2 by > 150 m and several subsidiary NW-trending normal faults with a cumulative displacement of ~ 130 m offset a paleosol developed on Unit 1. Kinematic indicators indicate normal dip-slip movement. Cumulative fault displacement for Unit 1 exceeds 277 m, yielding a minimum uplift rate of ~ 1.3 mm/yr, given the oxygen isotope stage 7 (Mindel/Riss) age of 210 ka proposed for the Unit 1 paleosol. Cumulative offset of > 196 m for the paleosol developed on Unit 2 yields an uplift rate of ~ 1.6 mm/yr, given its proposed isotope stage 5e (Riss/Würm) age of 125 ka. This estimate is supported by longitudinal valley profile data that document ~ 200 m of stream incision since Unit 2 deposition and show the offset as reflecting uplift of the Olympus massif rather than subsidence of the adjacent Gulf of Thermaikos. Assuming 1.8 Ma for the base of the Pleistocene, these rates yield a total uplift range for the Quaternary of 2.3–4.1 km, consistent with the present 2.6 km height of Olympus above the exposed frontal fault. Neogene uplift of Olympus was likely facilitated by extension accompanying the opening of the Aegean back-arc basin during the Miocene as a result of the southward retreat of the Hellenic subduction zone. For the Pliocene–Recent, uplift was likely facilitated by extension linked to the termination of dextral motion at the western end of the North Anatolian Fault associated with the westward tectonic escape of Anatolia .

Geological evolution of Ares Vallis on Mars: Formation by multiple events of catastrophic flooding, glacial and periglacial processes

Ares Vallis is one of the greatest outflow channels of Mars. Using high-resolution images of recent missions to Mars (MGS, 2001 Odyssey, and Mars Express), we investigated Ares Vallis and its valley arms, taking advantage of 3-dimensional analysis performed using the high-resolution stereo capability of the Mars Express High Resolution Stereo Camera (HRSC). In our view, Ares Vallis is characterized by catastrophic flood landscapes partially superimposed by ice-related morphologies. Catastrophic flood landforms include erosional terraces, grooved terrains, streamlined uplands, giant bars, pendant bars, and cataract-like features. Ice-related morphologies include probable kame features, thermokarstic depressions, and patterned grounds. Our investigations outline that throughout the Hesperian age, Ares Vallis and its valley arms had been sculpted by several, time-scattered, catastrophic floods, originating from Iani, Hydaspis and Aram Chaos. Geomorphological evidence suggests that catastrophic floods were ice-covered, and that climatic conditions of Mars at this time were similar to those of the present day. At the end of each catastrophic flood, ice masses grounded, forming a thick stagnant dead-ice body. Each catastrophic flood was followed by a relatively brief period of warmer-wetter climatic conditions, originated as a consequence of catastrophic flooding. During such periods thermokarstic depressions originated, liquid water formed meandering channels, and ice-contact deposits were emplaced by ice-walled streams. Finally, the climate turned into cold-dry conditions similar to the present-day ones, and ice masses sublimated.

Water input requirements of the rapidly shrinking Dead Sea

The deepest point on Earth, the Dead Sea level, has been dropping alarmingly since 1978 by 0.7 m/a on average due to the accelerating water consumption in the Jordan catchment and stood in 2008 at 420 m below sea level. In this study, a terrain model of the surface area and water volume of the Dead Sea was developed from the Shuttle Radar Topography Mission data using ArcGIS. The model shows that the lake shrinks on average by 4 km(2)/a in area and by 0.47 km(3)/a in volume, amounting to a cumulative loss of 14 km(3) in the last 30 years. The receding level leaves almost annually erosional terraces, recorded here for the first time by Differential Global Positioning System field surveys. The terrace altitudes were correlated among the different profiles and dated to specific years of the lake level regression, illustrating the tight correlation between the morphology of the terrace sequence and the receding lake level. Our volume-level model described here and previous work on groundwater inflow suggest that the projected Dead Sea-Red Sea channel or the Mediterranean-Dead Sea channel must have a carrying capacity of >0.9 km(3)/a in order to slowly re-fill the lake to its former level and to create a sustainable system of electricity generation and freshwater production by desalinization. Moreover, such a channel will maintain tourism and potash industry on both sides of the Dead Sea and reduce the natural hazard caused by the recession.

Geomorphic evidence of major sea-level fluctuations during marine isotope substage-5e, Cape Cuvier, Western Australia

A detailed geomorphologic and morphostratigraphic investigation of raised marine terraces at Cape Cuvier, Western Australia, reveals two morphologically distinct units. A lower, well-developed accretional reef terrace between 3 and 5.5 m above MLWS (mean low-water springs; hereafter denoted as “+”) represents an extended interval of stable sea level. An upper erosional terrace and incipient coralgal rim between + 8.5 to 10.5 m represents a brief sea-level stillstand at this higher elevation. These features suggest the lower and upper terraces developed during discrete sea-level events. In an attempt to better define the timing of emplacement of each marine unit, 20 coral samples collected along vertical and lateral reef growth axis from both terraces were analysed with U-series dating. Unfortunately, all coral samples exhibited elevated δ 234U initial values, suggesting that pervasive uptake of 234U-enriched uranium and 230Th thorium had occurred. Despite the shortcomings of absolute dating, a succession of events can be resolved though morphostratigraphic relationships. Comparison of the facies relationships, coral growth, and morphostratigraphic features between the lower and upper terraces indicates that an early to mid MIS 5e stillstand at + 3 to 5 m was followed by a late rise to + 8.5 to 10.5 m. This agrees with an emerging global view of MIS 5e sea-level history derived from stable carbonate platforms, rejecting the hypothesis that these higher sea-level benchmarks are an artefact of localized tectonic processes.

Micromorphological properties of salt affected soils in Northeast Thailand

A micromorphological study was carried out on salt affected soils from three locations on erosional terraces overlain by wash and alluvium under tropical savanna climate in Northeast Thailand where salinity problem has been acute. This aimed at providing detailed data for a better agricultural land use planning of the area. Five or six profiles at each location were sampled along a linear traverse that included the soils with highest and lowest levels of salt accumulation. Sixteen soil profiles were described, sampled and analyzed using physio-chemical, optical microscope and scanning electron microscope with energy dispersive spectrometer (SEM/EDS) techniques. The highest salt affected profiles show salt crusts, bare surfaces and halophytic plants and these soils have been left idle. All soils are deep, poorly drained, moderately to highly developed and with various salinity levels. They are fourteen Natraqualfs and two Endoaqualfs. Soil textures are sandy for location 1, clayey for location 2 and with a sandy topsoil and clayey subsoil for location 3. The soils at locations 1 and 2 mostly have high values of electrical conductivity (EC) and exchangeable sodium percentage (ESP). Soils at location 3 may have been affected by desalinization and alkalization as is indicated by low EC, high basic cations and high pH. Field morphology, optical micromorphology and SEM/EDS analysis results show clear accumulation of salt in some profiles. Their microstructures are different due to the difference in their texture. Compact grain and bridge grain structures are common in sandy-textured soil whereas channel and subangular blocky structures are common in the clayey soils. Quartz and runi-quartz grains are common as skeleton materials, but are present in different sizes and amounts. Halite occurs in voids in soils at locations 1 and 2 and is abundant in Pedons 1 and 6 where salt crusts exist. Calcite is present only in one profile (Pedon 1). Amorphous pedofeatures are manganiferrous impregnative nodules and iron and manganese oxide impregnated s-matrix. The EDS spectra indicate the presence of barite (BaSO 4) and sylvite (KCl) crystals in voids in some of the soils. The element composition of the soil matrix in a triangle graph of SiO 2: Al 2O 3: Fe 2O 3 is displaced from the ‘kaolin line’ towards the SiO 2 apex, indicating that very fine-grained quartz and/or a Si-rich clay mineral (s) is present in the matrix. The presence of Si-rich clay minerals is supported by XRD results that identified smectite in addition to kaolin and may be an evidence of the resilication process that can occur in salt affected soils. The data points near the Fe 2O 3 apex of the triangle graphs represent iron oxide nodules and their position on the matrix Al 2O 3/SiO 2 line indicates that induration of the matrix with iron oxide has occurred and that the nodules have formed in situ. The normalized element composition of the matrix in triangle graph between SiO 2: Al 2O 3: Na 2O + CaO + MgO indicates that in some instances the salts of Na, Ca and Mg have indurated the soil matrix as data points for salt-rich matrix fall on the clay matrix composition line.

Mega-highstand or megatsunami? Discussion of McMurtry et al. (Elevated marine deposits in Bermuda record a late Quaternary megatsunami: Sed. Geol. 200 (2007) 155–165)

Graded, sorted, rounded, and ponded marine sand and conglomerate deposited in caves and on an erosional terrace at + 20 m on Bermuda, previously interpreted as originating in a eustatic highstand of sea level during the middle Pleistocene, were reinterpreted by McMurtry et al. (2007) as the result of a great 20 m megatsunami at sea, propagating out across the North Atlantic from the Canary Islands, cresting, and rolling up and over the Bermuda platform. However, no middle Pleistocene tsunami deposits have been reported elsewhere on Bermuda or anywhere else around the North Atlantic rim. The tsunami origin is unsupportable whereas the available evidence unequivocally establishes a + 21 m eustatic sea level during the middle Pleistocene MIS 11 interglacial.

Outburst flood origin of the Central Kalamazoo River Valley, Michigan, USA

Geomorphic evidence and stratigraphic information from boreholes suggest that the oversized Central Kalamazoo River Valley (CKRV) in southwest Michigan resulted from a catastrophic outburst flood emanating from subglacial channels under the Saginaw lobe of the Laurentide Ice Sheet. The CKRV occurs as a deeply incised trench over 2 km wide and in excess of 50 m deep situated in a reentrant formed by the Lake Michigan, Saginaw and Huron-Erie lobes. The course of the CKRV follows an irregular flow path that bisects the Kalamazoo Moraine of the Lake Michigan lobe. Erosional terraces near the mouth of the channel indicate that Lake Michigan lobe meltwater drained eastward prior to the westward Saginaw outburst. Prior to valley formation the Lake Michigan lobe had retreated westward to at least the Lake Border Moraine. With the Lake Michigan lobe absent to impede flow, drainage from the CKRV proceeded southwesterly until draining into glacial Lake Chicago near St. Joseph, Michigan. The outburst originated from a system of Saginaw tunnel channels that display convex-up flow profiles and contain eskers. Meltwater drainage transitioned from subglacial-to-ice marginal and proglacial environments. During the interval represented by the outburst, the Saginaw Lobe appears to have been in a relatively stationary position.

Distribution of Soil Microbial Biomass and Free-living Nematode Population in Terrace Chronosequences of Makhtesh-Ramon Crater

ABSTRACT In this study, we examined the effects of the age of erosional fluvial terraces of Makhtesh Ramon (Ramon crater) in the central Negev Desert on soil chemical and biological properties. There were significant effects of erosion age of these terraces on soil moisture, organic carbon, soil salinity, and electrical conductivity. It is known that soil biological activity in arid ecosystems is determined by well-known limiting factors such as soil moisture and organic matter. Significant (P < 0.002) differences in total nematode population and microbial biomass [(22.0–3.4 Cmic (µg C g−1 soil)] were observed between terraces. Biological activity of soils in lower and younger terraces was greater than in older and higher terraces. The ecophysiological status (qCO2) of the soil microbial community was found to decrease from a maximal value of 1.3 to 0.32 mg CO2‒C (gCmich)−1 along the terraces (from younger to older ones). This study illustrates the integrated effect of age, altitude, and the morphostratigraphic position of terraces on the biological activity of soils.

Semiannual patterns of erosion and deposition in upper Monterey Canyon from serial multibeam bathymetry

Recently acquired 3-m-resolution 244 kHz multibeam seafloor bathymetry (0.5 m depth precision) reveals geomorphology at sufficient detail to interpret small-scale features and short-term processes in the upper 4 km of Monterey Canyon, California. The study area includes the continental shelf and canyon features from 10 m to 250 m depth. The canyon floor contains an axial channel laterally bounded by elevated complex terrace surfaces. Sand waves with 2 m height and 35 m average wavelength dominate the active part of the canyon floor. The sand waves are strongly asymmetrical, indicating net down-canyon sediment transport in this reach. Terraces, including a broad 25-m-tall terrace complex near the head of the canyon, bear evidence of recent degradation of the canyon floor. Slump scars and gullies having a variety of sizes and relative ages shape the canyon walls. Serial georeferenced digital elevation models were analyzed to detect net changes in bathymetry or morphology occurring during both a six month period (September 2002 to March 2003) and a 24-h period (24 March to 25 March). Significant changes over the six month period include: (1) complete reorganization of the sand waves on the channel floor, (2) local channel degradation creating new 2-m-tall erosional terraces on the channel margins, (3) local channel widening that laterally eroded older channel margin terraces, and (4) 60 m extension of one minor gully head on a steep canyon wall. There were no discernable changes in morphology during the 24-h study period. Raster subtraction of serial bathymetric grids provides estimates of sediment erosion and deposition that occurred between the canyon head and a point 2 km down canyon during the six month study. Erosion of 320,000 m 3 (±80,000 m 3 ) of sediment occurred mainly in the tributaries, along the margins of the axial channel, and in the lowest 700 m of the analyzed reach. This eroded volume was approximately balanced by 260,000 m 3 (±70,000 m 3 ) of sediment deposition that was concentrated in the nearshore region along the rim of the canyon head. There was no measurable sediment gain or loss during the 24-h study period.

60 k.y. record of extension across the western boundary of the Basin and Range province: Estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath Lake Tahoe

Deformation across three major fault strands within the Lake Tahoe basin has been mapped by using a novel combination of high-resolution seismic chirp, airborne laser- and acoustic-multibeam-derived bathymetry, and deep- and shallow-water sediment cores. Submerged erosional terraces of late Pleistocene age (19.2 6 1.8 ka) record vertical de- formation across fault strands that ranges between 10 and 15 m; offset of 10 m is observed across the southern part of the West Tahoe fault. Avalanche deposits from the catastrophic McKinney Bay slide (ca. 60 ka) are offset across the Stateline fault by at least 21-25 m. The submerged shoreline terraces and debris avalanche provide marker beds with which to constrain the extensional history of the region for the past 60 k.y. This history is then used to assess the future seismic hazard of the region. Data on deformation across these two important marker beds, combined with chronological control from 14 C and optically stimulated luminescence measurements, yield an estimate of extension across the Lake Tahoe basin that is 0.4-0.5 mm/yr. On the basis of these measurements, there exists the potential for a large, seiche wave-generating M7 earthquake every ;3 k.y. Late Pleisto- cene and Holocene vertical deformation rates within the Tahoe basin are characteristic of Basin and Range faulting and place the Tahoe basin within the western limits of the extensional Basin and Range province.

The origin of the Sierra de Aracena Hollows in the Sierra Morena, Huelva, Andalucia, Spain

Hollows in the Sierra de Aracena, part of western sector of Sierra Morena region (Huelva, Spain), are geoecologically unusual macroforms. They are underlain by deeply weathered bedrock but have eutrophic soils with distinctive vegetation. Paleosols with very dark colours, a predominance of smectites and large amounts of total and free iron occur on the floors on the hollows. An evolutionary model is proposed for the hollows, involving differential weathering during the Mesozoic on plutonic and amphibolitic rocks, alpine tectonic activity followed by Quaternary erosion and exhumation leading to formation of erosional terraces.

Turbidite Variability and Architecture of Sand-Prone, Deep-Water Slopes: Eocene Clinoforms in the Central Basin, Spitsbergen

ABSTRACT The architecture and turbidite variability within six wedge-shaped (downslope-thinning), sand-prone slope accumulations are documented from Eocene shelf margins on Spitsbergen. The Central Basin formed as a small foreland or piggy-back basin, and the studied turbidites accumulated mainly on the slope portion of sand-prone clinoforms that developed during depocenter migration and infilling of the basin. The shelf-margin clinoforms have amplitudes (minimum water depths) from 100 to 350 meters, and their shelf, slope, and basin-floor segments are well imaged, and can be walked out along many of the mountainsides. Only a small percentage of the clinoforms are sand prone, and these developed when sea level occasionally fell to or below the shelf edge. Of the sand-prone clinoforms, some had their sand budget partitioned mainly out onto the basin floor (basin-floor fans), but most trapped the sand on the slope only. The latter are now visible as downslope-thinning wedges, some 2.5-3.5 km in downdip extent. The turbidites within this type of clinoform have been examined and classified. The lower-slope to base-of-slope segment of the studied clinoform complexes are dominated by lobes consisting of broad, shallow channels and sheet-like turbidites, becoming heterolithic and muddy out on the basin floor. Beds on the lower slope vary from thick (up to 4.5 meters), ungraded or laminated sandstones, to thinner ungraded sandstones with coarse cappings. The middle-slope segment of clinoform complexes is dominated by narrow channels (chutes) that feed downslope to progradational chute-mouth lobes. Chutes contain ungraded and laminated sandstone beds up to 3 m thick, whereas the chute-mouth lobes show alternations of thinner, ungraded to laminated or rippled sandstones. These lobes become more heterolithic and muddy downslope. The shelf-edge to upper-slope segment of clinoform complexes is dominated by upward-coarsening and -thickening sheetsands of steep-fronted shelf-edge deltas. The sandsheets of the delta front can be traced updip into mouth-bar and distributary-channel sandstones. It is argued that shelf-margin accretion, represented by the sand-prone slope wedges, was achieved mainly by sand-laden currents that flooded from the shelf edge as hyperpycnal flows. This hypothesis is supported by: (1) the direct connection between channel and mouth bar systems at the shelf edge, and the turbidites of the slope lobes, (2) the systematic progradational character of the slope lobes, (3) the absence of large-scale slump scars, gullies, or canyons on the slope, and (4) the dominance of a type of turbidite that implies deposition from sustained flow. Detailed examination of the architecture of one of the slope wedges shows that there are unconformities developed within the stratigraphy below the shelf edge and that these erosional terraces beheaded the deltas perched on the uppermost slope. The erosion surfaces indicate fall of sea level to this position. Despite the magnitude of this fall (up to 80 meters), the lack of canyons on the slope prevented the construction of basin-floor fans. Such falls of sea level, on non-canyoned slopes, simply promote sand-prone accretion of the shelf margin.