Colluvial Units Research Articles

Continental rift evolution and drainage reorganization along the Dead Sea rift since the Miocene

The Dead Sea fault is a section of the Arabian-African plate boundary. Widespread field relations indicate that three major drainage systems (stages) occupied the landscape west of the Dead Sea fault since its initiation at ca. 20 Ma. Specifically, (1) an early to middle Miocene drainage system, only minorly reconfigured by the fault (all sediments of this system belong to the Hazeva Formation); (2) a late Miocene to early Pleistocene fault-parallel drainage system named Paran-Neqarot (all sediments of this system belong to the Arava and Zehiha Formations; and (3) the early Pleistocene to present drainage configuration. The temporal and spatial frameworks of drainage stage 1 are generally constrained by radiometric dating of interfingering volcanic units, and the onset and temporal and spatial frameworks of drainage stage 3 are well constrained by cosmogenic 10Be surface exposure ages. The timing and longevity of the rift-parallel drainage (stage 2) have until now been evasive to direct dating. The overall time gap between stages 1 and 3 is ∼12−13 million years. Thus, an early age (within this time gap) of stage 2 would imply an immediate response of drainage reorganization to rift tectonics, while a later age of this drainage system would imply a delayed response. We present 11 10Be-26Al cosmogenic burial ages of alluvial and colluvial units related to the fault-parallel drainage system (stage 2), which collectively constrain the time of deposition of the Arava Formation sediments in the central Negev to ca. 8 Ma. The general lack of stratigraphic order, together with the large dispersion of ages both across and within the sampling sites, attests to significant recycling of sediments from drainage stage 1 into the Arava Formation deposits. The termination of Arava and Zehiha Formation sediment deposition at ca. 1.8 Ma was determined previously using cosmogenic exposure ages of desert pavements that cover the formations. Combining the previously published data with our new data, we established the longevity and character of the Paran-Neqarot drainage system. This framework highlights the temporal aspect of drainage system build-up and collapse as it responded to transform and extensional plate boundary tectonics during the Neogene.

Paleoseismological evidence for historical ruptures along the Meduno Thrust (eastern Southern Alps, NE Italy).

We carried out new geological, morphotectonic, geophysical and paleoseismological investigations on the Meduno Thrust that belongs to the Pliocene-Quaternary front of the eastern Southern Alps in Friuli (NE Italy). The study area is located in the Carnic Prealps, where a series of alluvial terraces, linked to both climatic and tectonic pulses characterises the lower reach of the Meduna Valley. In correspondence of the oblique ramp of the Meduno Thrust, the Late Pleistocene Rivalunga terrace shows a set of scarps perpendicular to the Meduno valley, often modified by human activity. In order to reconstruct the tectonic setting of the area and identify the location for digging paleoseismological trenches, integrated geophysical investigations including electrical resistivity tomography, seismic refraction and reflection, ground penetrating radar and surface wave analyses (HVSR, ReMi and MASW), were carried out across the scarps of the Rivalunga terrace. Geophysical surveys pinpointed that in correspondence of the oblique ramp, stress is accommodated by a transpressive thrust system involving all the seismo-stratigraphic horizons apart from the ploughed soil. Trenching illustrated the Meduno Thrust movements during Late Pleistocene-Holocene. Trenches exhibited both shear planes and extrados fracturing, showing deformed alluvial and colluvial units. 14C datings of the colluvial units show that the most recent fault movements occurred after 1360 CE and 1670 CE. The age of the deformed stratigraphic units compared with the earthquakes listed in current catalogues, suggests that the 1776 earthquake (Mw 5.8, Io = 8–9 MCS) could represent the last seismic event linked to the Meduno thrust activity. This study provided new quantitative constraints improving seismic hazard assessment for Carnic prealpine area.

Holocene Paleoseismology of the Steamboat Mountain Site: Evidence for Full-Length Rupture of the Teton Fault, Wyoming

ABSTRACT The 72-km-long Teton fault in northwestern Wyoming is an ideal candidate for reconstructing the lateral extent of surface-rupturing earthquakes and testing models of normal-fault segmentation. To explore the history of earthquakes on the northern Teton fault, we hand-excavated two trenches at the Steamboat Mountain site, where the east-dipping Teton fault has vertically displaced west-sloping alluvial-fan surfaces. The trenches exposed glaciofluvial, alluvial-fan, and scarp-derived colluvial sediments and stratigraphic and structural evidence of two surface-rupturing earthquakes (SM1 and SM2). A Bayesian geochronologic model for the site includes three optically stimulated luminescence ages (∼12–17 ka) for the glaciofluvial units and 16 radiocarbon ages (∼1.2–8.6 ka) for the alluvial-fan and colluvial units and constrains SM1 and SM2 to 5.5±0.2 ka, 1σ (5.2–5.9 ka, 95%) and 9.7±0.9 ka, 1σ (8.5–11.5 ka, 95%), respectively. Structural, stratigraphic, and geomorphic relations yield vertical displacements for SM1 (2.0±0.6 m, 1σ) and SM2 (2.0±1.0 m, 1σ). The Steamboat Mountain paleoseismic chronology overlaps temporally with earthquakes interpreted from previous terrestrial and lacustrine paleoseismic data along the fault. Integrating these data, we infer that the youngest Teton fault rupture occurred at ∼5.3 ka, generated 1.7±1.0 m, 1σ of vertical displacement along 51–70 km of the fault, and had a moment magnitude (Mw) of ∼7.0–7.2. This rupture was apparently unimpeded by structural complexities along the Teton fault. The integrated chronology permits a previous full-length rupture at ∼10 ka and possible partial ruptures of the fault at ∼8–9 ka. To reconcile conflicting terrestrial and lacustrine paleoseismic data, we propose a hypothesis of alternating full- and partial-length ruptures of the Teton fault, including Mw∼6.5–7.2 earthquakes every ∼1.2 ky. Additional paleoseismic data for the northern and central sections of the fault would serve to test this bimodal rupture hypothesis.

Stratigraphy and chronology of Pleistocene coastal deposits in northern Aquitaine, France: a reinvestigation

Pleistocene deposits exposed along the coast of the Medoc area, south-west France, represent valuable palaeoenvironmental archives that have been the subject of extensive work in the past few decades. To further understand the palaeoenvironmental history and sedimentary dynamics of these deposits, a detailed lithostratigraphic study was performed on a series of new sections. Additionally, new chronological data were obtained from the combination of luminescence (OSL, IR-RF) and electron spin resonance (ESR) dating of quartz and feldspar grains (Kreutzer et al., 2018). The investigated sections comprise estuarine, lacustrine and peaty deposits (Negade Formation, Argiles du Gurp Formation), aeolian sands (Sables du Gurp Formation) and colluvial units (Gres de l'Amelie Formation). The Argiles du Gurp Formation, in which many remains of Palaeoloxodon antiquus were found, support the hypothesis of a progressive replacement of a tide-inluenced marsh by a peaty fresh-water pond during the Holsteinian Interglacial (MIS 11). IR-RF dating of green estuarine clays at L’Amelie, in the northernmost part of the investigated area, strongly suggests that they do not belong to the Argile du Gurp Formation, but were deposited during the MIS 9 sea level highstand. The estuarine deposits are overlain by sandsheets (Sables du Gurp Formation) dated to MIS 10 and MIS 8 at Pointe de la Negade in the southern part of the study area, and to MIS 8 at L’Amelie. Syngenetic frost cracks testify to a periglacial depositional environment. Regional studies show that the Sables du Gurp Formation is a local equivalent of the Sables des Landes Formation, and corresponds to the oldest aeolian phase recorded in south-west France so far. Colluvial material (Gres de l'Amelie Formation) dated to the Weichselian pleni-glacial (MIS 2) ill small palaeo-valleys incised in the Sables du Gurp Formation. Holocene coastal dunes cover the whole sequence.

Geoarchaeology of the Cobrinhos site (Vila Velha de Ródão, Portugal) - a record of the earliest Mousterian in western Iberia

Cobrinhos (Vila Velha do Ródão, central eastern Portugal) is a Mousterian site found during factory construction in 2014. This area is located in the Lower Tejo valley, which is characterized in terms of geomorphology by six river terraces, numbered downwards (T1 to T6), with Palaeolithic industries associated only with T4 to T6. Terrace T4 was recently dated as spanning ca. 340 ka to 155 ka, with Acheulean in the basal and middle levels and early Mousterian in the uppermost levels. The geological context at Cobrinhos is a colluvial unit that links to the top of T4. It has evidence for palaeoweathering with the same characteristics as seen in T1 to T4, considerably different from that seen in T5 and T6. Despite disturbance by ploughing, the site shows a uniform distribution of sizes and shapes of lithic artefacts, with thousands of implements <30 mm and a coherent Mousterian assemblage including Levallois and discoidal reduction pieces, Levallois flakes, blades and points, pseudo-Levallois points, notches, denticulates, sidescrapers, and an absence of Acheulean or Upper Palaeolithic tools. The available data suggest that the colluvial unit is coeval with the topmost T4 deposits and that the Cobrinhos industry is in its original geomorphological context. Although the colluvial unit cannot be dated directly, from the combination of site data and available published luminescence ages for T4, we suggest a probable age of ca. 165 to 155 ka for this industry. These results are of relevance in the investigation of the demise of archaic Pleistocene human populations and the proliferation of Neanderthal groups in Iberia.

First dating results for the Middle Pleistocene industries (Acheulean – Early Middle Palaeolithic) in the Pyrenees – Garonne region: a multi methods geochronological approach (TL, OSL and TT-OSL) of the Duclos and Roment

The recent development work of the A65 highway has given the opportunity to broaden the corpus of Lower and Middle Palaeolithic open-air sites known in the southern Aquitaine basin. The sites of Duclos (Auriac, Pyrenees-Orientales) and Romenteres (Le Vignau, Landes), discovered in this context, have yielded an abundant lithic record which has been attributed to the Acheulean of Iberian type for the earlier and, essentially, to the Early Middle Palaeolithic for the latter. The archaeological levels are intercalated in sequences that comprise aeolian silts and colluvial units separated by interglacial luvisols. The pedostratigraphic context of the two sites has provided a rare opportunity to propose a reliable chronostratigraphic framework in this area. To achieve this objective, a study combining several methods of luminescence dating was conducted to complete the geomorphological data. Optically Stimulated Luminescence (OSL) and Thermically Transferred OSL (TT-OSL) dating methods were applied in order to determine the ages of sedimentary quartz grains and thus, of the sampled deposits. In parallel, heated flint and quartzite samples were dated by thermoluminescence (TL). The Pyrenees-Garonne Acheulean industry of Duclos has been attributed to the boundary between Marine Isotopic Stages (MIS) 7 and 6 while the human occupations of Romenteres date from MIS 6 for the most recent series (Early Middle Palaeolithic) and from MIS 9 and 8 for the older.

Propriedades micromorfológicas de colúvios em encosta no médio vale do Rio Marrecas (Sudoeste do Estado do Paraná) - bases para distinção de formações superficiais alóctones e autóctones em substrato basáltico

Este artigo apresenta a organização microscópica de colúvios encontrados no médio vale do rio Marrecas (Sudoeste do Estado do Paraná), visando auxiliar na diferenciação genética de formações superficiais lateríticas alóctones daquelas autóctones no Planalto Basáltico. Os métodos incluem: i) o levantamento das propriedades sedimentológicas e a identificação de litofácies; e ii) a descrição micromorfológica de nove lâminas delgadas representantivas das unidades coluviais. A distribuição relativa dos constituintes e das microfeições por processos foram quantificadas nas lâminas, por meio da análise de imagem. As litofácies lama cascalhenta maciça com matriz suportada e cascalho lamoso maciço com matriz suportada foram identificadas. As propriedades microscópicas dos colúvios revelaram microfeições relacionadas a processos deposicionais, herdadas do material de origem e decorrentes de tranformações pós-deposicionais. As microfeições identificadas nas duas primeiras situações podem servir de marcadores da origem alóctone de formações superficiais ricas em oxihidróxidos de ferro, auxiliando na sua diferenciação genética.

Rare destructive earthquakes in Europe: The 1904 Bulgaria event case

Seismic hazard is difficult to assess in regions of low strain rates. A major limitation often relates to the absence of large instrumentally recorded events precluding any comparison between seismological data and paleoseismic or morphotectonic informations. We take advantage of the 1904 M s∼7.1 earthquake that struck the southern edge of stable Eurasia and investigate if morphotectonic and paleoseismic observations can provide a reliable estimate of the seismic potential of slow-slipping faults. We have conducted a paleoseismic study of the Krupnik normal fault thought to be responsible for the event. A section of the fault bearing remnants of a 2 m-high scarp has been selected at the base of triangular facets. The trenching site locates where the scarp cuts across colluviums washed from the bedrock facetted slopes. We excavated two neighbouring trenches, one across a well-preserved portion of the scarp, and one across a portion degraded by a landslide. The excavations reveal a set of coarse colluvial units faulted against bedrock and affected by secondary fissures. Faulting appears to have resulted from a single event with normal throw greater than 1.3 m that occurred before the emplacement of the landslide. Accelerator Mass Spectrometry (AMS) radiocarbon dates of charcoal samples are consistent with the interpretation that the Krupnik Fault slipped recently, most probably in 1904, after a long lasting (> 10 ka) period of quiescence. The morphotectonic and paleoseismic observations yield seismic moment estimates compatible with the instrumental magnitude of the event and indicate that destructive and infrequent earthquakes typify the regional seismic behaviour.

Active faulting in the western Pyrénées (France): Paleoseismic evidence for late Holocene ruptures

We have identified a 50-km-long active fault scarp, called herewith the Lourdes Fault, between the city of Lourdes and Arette village in the French Pyrénées. This region was affected by large and moderate earthquakes in 1660 (Io = VIII–IX, MSK 64,), in 1750 (Io = VIII, MSK 64) and in 1967 ( M d = 5.3, Io = VIII, MSK 64). Most earthquakes in this area are shallow and the few available focal mechanism solutions do not indicate a consistent pattern of active deformation. Field investigations in active tectonics indicate an East–West trending and up to 50-m-high fault scarp, in average, made of 3 contiguous linear fault sub-segments. To the north, the fault controls Quaternary basins and shows uplifted and tilted alluvial terraces. Deviated and abandoned stream channels of the southern block are likely due to the successive uplift of the northern block of the fault. Paleoseismic investigations coupled with geomorphic studies, georadar prospecting and trenching along the fault scarp illustrate the cumulative fault movements during the late Holocene. Trenches exhibit shear contacts with flexural slip faulting and thrust ruptures showing deformed alluvial units in buried channels. 14C dating of alluvial and colluvial units indicates a consistent age bracket from two different trenches and shows that the most recent fault movements occurred between 4221 BC and 2918 BC. Fault parameters and paleoseismic results imply that the Lourdes Fault and related sub-segments may produce a M W 6.5 to 7.1 earthquake. Fault parameters imply that the Lourdes Fault segment corresponds to a major seismic source in the western Pyrénées that may generate earthquakes possibly larger than the 1660 historical event.

Dating recent colluvial sequences with 210Pb and 137Cs along an active fault scarp, the Eliki Fault, Gulf of Corinth, Greece

Reliable dating is an essential element of palaeoseismological studies, yet whilst a suite of geochronological methods can now provide late Quaternary age control it remains very difficult to date modern events (i.e., those occurring within the last 150 years). This is significant because the starting point for many palaeoseismological investigations is a modern surface-rupturing event, whose geological effects need to be disentangled in trench stratigraphies from palaeoseismic ruptures. Two dating methods which, in combination, can provide robust dating control in recently deposited sediments are the 210Pb and 137Cs dating methods. Here, we test the applicability of using 210Pb and 137Cs to date colluvial sediments exposed in three trenches excavated across an earthquake fault—the Eliki fault, Gulf of Corinth, Greece—which ruptured in an earthquake in 1861. The 210Pb and 137Cs profiles observed in these colluvial sequences are relatively erratic due to the mixed nature of the sediments, i.e., their deposition in an environment where the supply of slope sediments is driven by seasonal rainfall, causing non-uniform sediment accretion and sediment reworking. In one trench, however, 210Pb dating, corroborated by 137Cs dating, indicates that a proposed post-1861 surface colluvial unit has been deposited over the period 1950 AD–present (at a rate of ca. 9 mm/year), and overlies a significantly older unit (>120 years old). The dating control provided here by 210Pb and 137Cs dating corroborates the published interpretation of the trench stratigraphy, and refines the 14C-based estimated dates for the upper unit. At two other trenches 210Pb and 137Cs dating only provided minimum ages (based on the presence or absence of 210Pb excess and 137Cs). Such approximate ages, however, may still useful in corroborating interpretations made using the trench stratigraphy, or, at sites which have long earthquake recurrence intervals, determining which earthquake event was responsible for a particular bed offset.

Calcrete development in mediterranean colluvial carbonate systems from SE Spain

Holocene colluvial deposits from the Prebetic Zone (Betic Cordillera, S Spain) contain calcretes separating two colluvial units. Calcrete profiles contain three horizons: cemented gravels, chalky and massive-laminar. Scanning electron microscopy study revealed the presence of calcified filaments, rounded peloids and coated grains which indicates that calcrete formation was controlled by the microbial activity related with plant roots. X-ray powder diffraction data showed the presence of smectite and hematite and the absence of sepiolite–palygorskite, suggesting a semi-arid climate. The main factors controlling the features of the different calcrete horizons are topography (lateral continuity), inputs of colluvial deposits (cemented gravels and chalky calcrete) and water availability (massive-laminar calcrete).

Late Pleistocene colluvial deposits in central Tanzania; erosional response to climatic change?

Two major colluvial deposits have been identified on the slopes of the Irangi Hills in north central Tanzania. The oldest of these occurs in shallow depressions on the pediment slopes. The time of deposition has been dated by the use of optical stimulated luminescence (OSL) dating techniques to Late Pleistocene. The OSL dates were 14,700 ± 1600, 14,200 ± 1500, and 11,400 ± 1300 years. In Late Pleistocene time the climate changed from dry to wet conditions throughout much of the tropics and this time period is frequently reported as a period characterised by enhanced soil erosion. It is likely that the older colluvial unit in the Irangi Hills was formed as a result of climatic change when rainfall increased ahead of a vegetation change and thereby provided conditions favourable for soil erosion. The younger colluvial unit yielded deposition ages of 400–600 years and is part of a number of successive erosion and deposition periods during the last millenium, probably as a result of anthropogenic impact on the landscape.

Geological evidence for strong historical earthquakes in an “aseismic” region: The Pollino case (Southern Italy)

The Pollino Range is the southernmost segment of the Southern Apennines at the boundary with the Calabrian Arc. While several strong earthquakes (magnitude 6.5–7.0) have occurred in nearby regions, the Pollino area has no known historical record of seismic events of magnitude > 5. We carried out an aerial photograph interpretation and a field survey of the Pollino fault (the major Quaternary normal fault of the area) in order to characterize geologically the seismic potential of this structure. We dug two sets of trenches across fault scarps within the apecies of latest Pleistocene to Holocene alluvial fans at the Masseria Quercia Marina (MQM) and Grotta Carbone (GC) sites, in the central segment of the southern Pollino Range front. At both sites we identified two surface faulting events affecting the alluvial fan deposits and two overlying colluvial units of historical age. The penultimate event produced a vertical offset of 80–90 cm at GC and 50–60 cm at MQM; while the last event produced a vertical offset of 40–50 cm at GC and few centimeters of offset at MQM. Detailed geomorphological field observations suggest that the two historical earthquakes reactivated the entire length of the Masseria Marzano-Civita segment of the Pollino fault (rupture length about 18 km). For events in this range of rupture length and vertical displacement, comparison with surface faulting earthquakes in the Apennines (and abroad) indicates a magnitude of 6.5–7.0. Therefore, the maximum potential earthquake and the seismic hazard of the Pollino area are significantly larger than that suggested by the available historical seismic catalogue.

Soils as a tool for estimating ages of Quaternary fault scarps in a hyperarid environment — the southern Arava valley, the Dead Sea Rift, Israel

On the alluvial fan of Nahal Shehoret in the extremely arid region of the Negev desert, Israel, Reg soils developed on stable alluvial surfaces in colluvium along terrace risers and in colluvial units on a complex fault scarp were compared. The degree of soil profile development provided an age estimate of the surface that was faulted, a maximum age estimate of the scarp or terrace riser, and an estimate of the maximum recurrence intervals of faulting events. IRSL dates of the tectonically displaced alluvial surfaces and colluvial units at the same site enabled us to re-evaluate the use of Reg soils for relative dating. The major faulting event started at 34.8 ± 4.3 ka. It displaced a late Pleistocene surface of the Nahal Shehoret alluvial fan dated at 56 ± 10.8 ka. Renewed fluvial activity and deposition of alluvium on the downfaulted block had almost terminated by 13.6 ± 2.3 ka. The scarp was formed during the latest Pleistocene and was followed by smaller faulting events which had only minor effects on the fault scarp morphology. The similarity of age estimates from IRSL dating and the degree of soil development in this environment indicates that soils can be used to date surfaces and colluvial units.

Soil catenas to estimate ages of movements on normal fault scarps, with an example from the Wasatch fault zone, Utah, USA

Trenches excavated across fault scarps on a 13 ka delta surface at Brigham City, Utah, USA, document four types of soil-catena phenomena. First, the relict summit soil is eroded at the scarp crest, and its components incorporated into the accumulating colluvial wedge at the scarp base. Second, the pre-faulting soil weakens laterally as it is traced scarpward beneath the colluvial wedge from a relict position beyond the scarp toe. Third, soils developed on individual colluvial wedges and on the underlying, pre-faulting surface merge downslope to form a cumulative soil profile at the scarp toeslope. Fourth, soil horizons thicken downslope, probably due to the combined affect of more available moisture, water infiltration, and sediment accumulation. Two soil catenas were used to evaluate the applicability of a continuity approach to paleoearthquake dating, by which percentages of total development time represented by each soil at a fault scarp footslope could be estimated. The continuity approach sets total soil development at the scarp footslope, including any buried pre-faulting soil plus the sum development of all colluvial wedge soils corrected by a slope factor ( F), equal to total soil development at the scarp summit. Soil development was characterized by Profile Development Index (PDI) and pedogenic clay parameters, using facies models to estimate parent material values for genetically-different colluvial units, and recognizing inherited soil components in colluvial wedges as parent material properties. The F slope factor, used to correct colluvial wedge soils for development attributable to slope position rather than age, was checked by using a nearby fluvial scarp as an independent control. Our soil data, when compared to independent age estimates, imply that rates of soil development have not been constant over the last 13 ka at Brigham City, but rather were considerably higher than average between 13 ka and 8.3 ka. Higher rates are probably due to a relatively high influx of eolian dust during the late Pleistocene to early Holocene following the lowering of Lake Bonneville, a large paleolake in the western USA. These variable rates of soil development preclude using the continuity approach for paleoearthquake dating at Brigham City. However, promising results showing that generally consistent F factors can be calculated for fault and fluvial scarps leave open the possibility that in areas where soil development rates have been relatively constant over the time period in question, the continuity approach may be a broadly applicable method for paleoearthquake dating.

Paleomagnetic evidence for late Cenozoic glaciations in the Mackenzie Mountains of the Northwest Territories, Canada

The Mackenzie Mountains were affected by montane valley glaciers during the Pleistocene and peripherally by the Laurentide Ice Sheet during the last glaciation. In this paper we report on magnetostratigraphic dating and correlation of three sections recording Late Pliocene to Late Pleistocene glaciations: Katherine Creek, Little Bear River, and Inlin Brook (located around 65°N, 127°W). Each section consists of a colluvial unit overlying a Pliocene pediment surface cut into Proterozoic or Paleozoic bedrock, or Tertiary gravel, which is in turn overlain by a stack of five, and in places six, montane tills, usually with soils developed at their surfaces, and capped by a Laurentide till. Normal and reversed magnetizations were recognized with single-domain magnetite as a dominant remanence carrier. The Katherine Creek section has a normally magnetized colluvium at its base, which is overlain by two reversed tills, succeeded by three normal tills. We interpret the top two tills to be of Brunhes age (&lt; 780 ka) but argue that the lowermost normal till is of probable Olduvai age (ca. 1.8 Ma). The two underlying tills are of Matuyama age (2.6 Ma to 780 ka), and the colluvial base is assigned to the Gauss (3.5–2.6 Ma). The Little Bear River section exposes a stratigraphic record similar to that found at Katherine Creek. Only four units could be assigned a paleomagnetic polarity, the others yielding incoherent results. Paleosols on the first and second till units were reversed and normal, respectively, and the top till was normal. Thus there is clear evidence of an older (reversed) Pleistocene glaciation and a magnetostratigraphic record compatible with that found at Katherine Creek. Magnetic measurements from Inlin Brook gave largely incoherent results, with the exception of the surface (Laurentide) till, which is normal. The glacial history recorded in the Mackenzie Mountains correlates well with other studies carried out in the Cordillera. The large-scale changes in climate revealed in these terrestrial records provide baseline data for paleoenvironmental reconstruction.

Ribbon Cliff landslide, Washington, and the earthquake of 14 December 1872

Abstract Estimates of the epicentral location and maximum intensity of the earthquake of 14 December 1872, the largest and oldest historic earthquake documented in the Pacific Northwest, are controversial largely because the estimates are based on ground effects. The Ribbon Cliff landslide is one of the more critical ground effects used to argue that the epicenter was in the vicinity of Lake Chelan in central Washington. Sketchy historical accounts link the Ribbon Cliff landslide to the 1872 earthquake, but a subsequent study disputed the historical accounts and, on the basis of dendrochronology, concluded that the landslide occurred more than 100 yr prior to the earthquake. However, Quaternary stratigraphic relations and the results of multiple dating techniques reported here indicate that the main Ribbon Cliff landslide probably occurred within a 14-yr period that includes the time of the 1872 earthquake. Although our study supports the historical accounts that link the landslide to the December 1872 earthquake, it does not prove that seismic shaking triggered the landslide. Geomorphic and stratigraphic relations show that Ribbon Cliff has a long and complex history and that the landslide was not primarily a rockfall-rockslide that originated from the cliff itself, as some believe, but rather that the main failure occurred in colluvium that had accumulated beneath the cliff. Four colluvial units and two volcanic ash deposits, the Mazama ash bed (erupted about 6.8 ka), and the Mount St. Helens set W (erupted about 1480 A.D.), underlie the slopes below Ribbon Cliff. Most of the colluvium involved in the landslide accumulated after deposition of the Mazama ash bed. Loss of support at the foot of the colluvial wedge because of undercutting by the Columbia River probably was an important contributing factor to slope failure.

Use of soils and colluvial deposits in analyzing tectonic events — The southern Arava Rift, Israel

Buried soils in colluvium units developed on downthrown fault blocks have been presumed to form during periods of tectonic quiescence. This assumes however, that all colluvial units on a fault scarp are tectonically induced, particularly for a multiple-event fault scarp. To test this hypothesis we compared the colluvial stratigraphy and pattern of soil development on two non-tectonic scarps (terrace risers) and a nearby multiple-event fault scarp on the same alluvial fan surface. Both slopes developed under extremely and climatic conditions. The colluvial stratigraphy and soil catenary relationship developed on terrace risers were found to be substantially different from those on the multiple-event fault scarp. The fault scarp colluvium contained several colluvial units separated by soils, whereas only two colluvial units without buried soils were found on the terrace risers. The colluvial units on the fault scarp were triangular in shape, while those on the terrace riser were concave and overlapped up the slope. Cumulic soils, indicating a continuous and low rate of deposition during pedogenesis, were formed along the terrace riser. In contrast, buried soils indicating episodic deposition occurred on the downthrown block of the multiple-event fault scarp. Accordingly, the colluvial stratigraphy developed on the fault scarp can be viewed as a record of distinct tectonic events with no climatically controlled deposition. The data indicate that soils and sedimentary stratigraphy can serve as useful indicators to distinguish terrace risers and single-event fault scarps from multiple-event fault scarps. These indicators can be helpful in distinguishing phases of tectonic activity and in evaluating recurrence intervals on a multiple-fault scarp.

Episodic late quaternary palaeogully erosion in northern KwaZulu-Natal, South Africa

Hillslope deposits in northern KwaZulu-Natal, South Africa are incised by gullies (dongas) which expose sequences of buried palaeosols developed within a succession of discontinuity-bounded colluvial sedimentary units. Infilled and buried palaeogully topography attests to at least four geomorphic cycles, comprising gully cut-and-fill and palaeosol formation, on hillslopes in the region during the past 135 ka. Past attempts at dating colluvial stratigraphic units were limited to radiocarbon dating of associated palaeosol organic matter and authigenic soil carbonates. This study presents a preliminary set of Infra Red Stimulated Luminescence (IRSL) age determinations pertaining to deposition of the succession of colluvial units infilling and burying palaeogullies at several sites of stratigraphic, geochronological and palaeoenvironmental importance in the region. Interpretation of the IRSL and 14C dates in the context of cyclical hillslope processes suggests that past episodes of palaeogully erosion and infill were not synchronous and individual palaeosol landsurfaces were diachronous during the late Pleistocene.

Use of Statistically Distinct Genesis-Lithology-Qualifier Map Units for Classifying Upland Soils at Yucca Mountain, Nevada, by Geomorphology and Physical Properties Affecting Infiltration

The purposes of this study were a) to statistically classify rocky, upland desert soils on a fault-block ridge at Yucca Mountain, Nevada, by geomorphology and physical properties affecting infiltration, and b) to evaluate the genesis-lithology-qualifier (GLQ) mapping method as a basis for this classification. Map-unit soils were sampled for grain-size distribution (texture), total-soil bulk density, fine-soil bulk density, rock-fragment bulk density and porosity, and slope, aspect, and position. Relations between geomorphic environments and physical property variables were evaluated using correlation and analysis-of-variance techniques, and statistically distinct map units were determined for GLQ mapping using multiple comparison technique. Hydraulic-conductivity values for each map unit were estimated based on mean silt and clay content. Statistical analyses indicated that fine-soil bulk density was related to slope position and that soil texture was more related to soil genesis than any other geomorphic variable. The bead-cone, irregular-hole, bulk-density-sampler method was used to provide the most accurate data for the steeply sloping conditions in the study area. The mapped soils were classified statistically as a residuum unit, a colluvial (creep) unit, a colluvial (creep-talus) unit, and a slide (flow) unit. Most often, textural variables of surficial soils were statistically distinct among the final map-unit soils. Estimates of saturated hydraulic conductivity of the four GLQ units ranged from 5.4 × 10 −5 cm/sec to 5.9 × 10 −6 cm/sec; these values may not be statistically different. Characteristic soil-horizon sequences were identified for each GLQ map unit, including a well-cemented caliche horizon in the slide (flow) unit, well-developed illuviation in the residuum unit, and statistically large mean surficial cobble counts in the colluvial (creep-talus) unit. These sequences, combined with the statistical distinction of textural variables among the GLQ map-unit soils, indicate that the GLQ map may be used as a valid system of hydrologic classification of soils and to estimate potential water infiltration.