Resistant Rocks Research Articles

Steep slopes in the Sudetes and their morphotectonic interpretation

The Sudetes is a block-faulted mountain range in Central Europe, at the NE margin of the Bohemian Massif. In the late Cenozoic it has been subject to differential uplift and subsidence and currently represents a horst-and-graben structure, superimposed on older relief due to rock-controlled denudation and erosion. In this paper, the distribution of steep slopes (>>gt; 15° and >>gt; 25°) is analyzed using a Digital Elevation Model (DEM) of 50 m spatial resolution, and their morphotectonic significance is discussed. Steep slopes occur in four major settings: heavily dissected and most elevated highlands, straight mountain fronts, narrow sinuous escarpments, and deeply incised river valleys. The former in particular may indicate areas subject to recent uplift, which is followed by efficient fluvial incision, so that little pre-uplift topography has survived. The image of many mountain fronts on the slope map is rather poor, which may be explained by the mechanical weakness of the rock building the footwall. At the same time, the association of the majority of tectonically-induced steep slopes with the most resistant rocks suggests that the intensity of recent uplift is generally low compared with the long-term rates of denudation and erosion.

O gnaisse facoidal: a mais carioca das rochas

The city of Rio de Janeiro is known by its natural beauties. The mountains and the sea make the city the postcard of Brazil. The sculpture of the carioca landscape is closely related to the augen gneiss, very resistant rock to the weathering and, for this reason, it stands out in the relief. It gives form for Sugar Loaf and Corcovado, for example. Augen gneiss was used in the construction of most of the historical buildings of the city, including museums and churches, many of them were built in the 19th century. It was used in the sculpture of ornaments, facades and frames of doors and windows. The exploitation of the augen gneiss was presented by Jean Baptiste Debret in his book "Voyage Pittoresque et Historique au Brésil". A picture of quarry is presented at Morro da Glória and describes the extraction method by slave labor. He informes that the augen gneiss is softer, less expensive and more easily exploited. It was destined, mainly, to the parts of the buildings that should be sculpted. This rock is still present in an important event of the history of the brazilian arts. Pedra do Sal, a stairway sculpted in the augen gneiss Downtown, was the place that African people met in the past to tell their histories, to do religious cults and to sing. In these meetings in Pedra do Sal samba was born

Resistivity structure of western Taupo Volcanic Zone, New Zealand

data from two deep‐penetration electrical resistivity surveys crossing the western margin of the Taupo Volcanic Zone (TVZ) near Tihoi, northwest of lake Taupo, North Island, New Zealand, have been used to identify the elements of the margin. In this vicinity the volcanologically defined boundary of the TVZ includes the andesitic cones of Pureora and Titiraupenga. The faulted margin of the TVZ lies east of these cones and is marked in the resistivity data by a dipping interface (faulted margin) between the resistive greywacke rocks to the west and the relatively conductive volcanic rocks that fill the upper few kilometres of the TVZ. The resistivity and gravity signatures of the margin are coincident in this vicinity. To the immediate east of the margin is a low‐resistivity zone c. 10 km wide and 2.5 km deep. The low‐resistivity material within this region is identified as old (>1 Ma) volcaniclastics that contain clays produced by low‐temperature alteration and devitrification. This material is reversely magnetised. We suggest that this zone was formed following the onset of active extension of the TVZ and has since remained relatively undisturbed while the focus of volcanism and deformation has moved to the east. The eastern boundary of this low‐resistivity zone coincides with the western edge of the whakamaru caldera (the boundary of the “young TVZ"). Although the resistivity data can be matched using an interface with a dip to either the east or west, an eastward dip would be consistent with the boundary having been formed during the collapse associated with the formation of the Whakamaru caldera. The resistivity signature within the Whakamaru caldera is consistent with >2 km thickness of younger, more resistive volcanic sequences, suggesting the low‐resistivity layers found to the west have been disrupted and downfaulted to be replaced with young, resistive caldera infill.

Hydraulic habitat composition and diversity in rural and urban stream reaches of the North Carolina Piedmont (USA)

AbstractThe differences between urban and rural streams regarding hydrological process, channel morphodynamics and ecosystem functioning have been highlighted by a number of studies in recent decades. The need to understand lotic ecosystem functioning in these environments at scales relevant to individual organisms has led to research focusing on hydraulic composition and structure over small areas of channel bed. In this study we map and analyze the hydraulic biotope composition of two urban and two rural stream reaches in the North Carolina (USA) Piedmont to determine if urban flow regimes and attendant channel modification processes might translate into important differences in hydraulic environment, and if so, what those differences are. Hydraulic biotope assemblages were found to vary only moderately in diversity per unit stream length between sites, but were distinctly different in composition. One important control on the differences between rural and urban streams was found to be the localized incision of urban channels into bedrock and saprolite. Resistant rock outcrop in the beds of urban streams creates rapid and riffle biotopes and long stretches of upstream pool habitat by impoundment. Urban reaches were found to be more homogeneous than rural reaches in hydraulic composition and were dominated by pools. Rural reaches, characterized by copious sandy alluvium in the bed, were dominated by runs or glides. Quantitative differentiation of biotopes based on four hydraulic indices generally yields coherent associations, although these may vary in content. Comparisons between hydraulic and biotic diversities suggest relationships favoring biotic functional group aggregation over species richness‐based indices of diversity. Because the majority of published hydraulic diversity analyses are based on coarse‐bed streams, further study of hydraulic diversity in streams with finer substrate is likely to be beneficial. Copyright © 2008 John Wiley & Sons, Ltd.

Tributary valley impoundment by trunk river floodplain development: a case study from the KwaZulu‐Natal Drakensberg foothills, eastern South Africa

AbstractThis paper investigates the origin and geomorphic evolution of Stillerust Vlei, a 189 ha wetland located approximately 150 km northwest of Durban in the temperate submontane foothills of the KwaZulu‐Natal Drakensberg Mountains. The investigation confirms the findings of previous research on the arid to semi‐arid South African interior, which established that many floodplain wetlands in eastern South Africa are located upstream of resistant rock barriers (dolerite intrusions) that cross river courses and form stable local base levels. Upstream of these barriers, rivers laterally plane less resistant Karoo sedimentary rocks (sandstones, mudstones), creating broad, low gradient valleys conducive to the formation of floodplain wetlands. In addition, the study examines how local levee and alluvial ridge accretion on the floodplain of Stillerust Vlei has impounded a small tributary valley, and drawing on observations of similar wetlands in the region, the paper explains the origin and geomorphic evolution of wetlands in floodplain‐abutting valleys, and associated streams that commonly become discontinuous toward their confluence with the trunk (floodplain) river. Controls on the origin and geomorphic evolution of Stillerust Vlei are placed within the context of slope‐channel decoupling and (dis)connectivity in sediment delivery, illustrating that wetlands are environments of deposition. As a result of dynamic trunk‐tributary relations, Stillerust Vlei holds a diversity of geomorphic features, and thus provides potential habitat for a diversity of biota. Copyright © 2008 John Wiley & Sons, Ltd.

Interpretation of the deep magnetotelluric soundings along the Austrian part of the CELEBRATION-007 profile

In the continuation of the CELEBRATION-007 deep seismic profile from Hungary to Austria a series of deep magnetotelluric soundings has been carried out, using the instruments (from GFZ MT instrument pool). In spite of the high noise level, the relatively good imaging of the structure of the sedimentary Graz Basin and possibly fluid filled conducting fractures in the resistive rock matrix of the Eastern Alps have been indicated. They might be potential source of geodynamics (earthquakes?).

Development of an automated desktop procedure for defining macro-reaches for river longitudinal profiles

This paper presents an automated desktop procedure for delineating river longitudinal profiles into macro-reaches for use in Ecological Reserve assessments and to aid freshwater ecosystem conservation planning. The procedure was developed for use where there are limited data and/or where a repeatable, statistically defensible regional or national assessment is required. The delineation of longitudinal profiles into macro-reaches between ‘controls' or ‘break points' such as exposed resistant rock formations, knick points, or significant changes in lithology provides the initial coarse filter for further assessment of lower levels of organisation, channel type for example. The division is necessary, as research has demonstrated that not all macro-reaches respond in the same way to disturbance or stress, nor do they have the same biotic assemblages. Four statistical methods (Von Neumann mean square error, CUSUM plots or unweighted values and the Worsley Likelihood Ratio Test (WLRT)) were used to define macro-reach breaks for four South African rivers (Crocodile, Olifants, Mhlathuze and Seekoei Rivers) and were compared to previously defined macro-reach delineations based on expert-driven approaches. Results indicate that the CUSUM and WLRT approaches most closely match the macro-reach breakspoints as defined by the expert-driven approach. An automated desktop procedure was developed for computing statistically defensible, multiple change points along profiles using an adaptation of the WLRT method. The adapted approach does not require an a priori knowledge of the break points, as is the case in other applications of the WLRT. It is concluded that the adapted WLRT approach can be used with a reasonable degree of certainty where there are insufficient data and/or where a regional or national assessment is required that is repeatable and statistically defensible. Where possible, however, there is no substitute for primary data collection, field work and a detailed expert-driven approach. Water SA Vol.32 (3) 2006: pp.395-402

Sediment transport and morphodynamics of the Douro River estuary

A combination of dredging data, hydrographic surveys and numerical modelling has been used to assess morphological change and sediment transport in the Douro River estuary. The system is dominated by sand- and gravel-sized sediments and confined by resistant rock types. The evolution of the bed in the last 20 years has been strongly influenced by the opening of a navigation channel. According to the data available to date, the average maintenance dredging volume has been of the order of 0.4 × 106 m3 year−1. Comparisons of hydrographic surveys reveal a rate of volume loss of the same magnitude. Apparently, maintenance dredging mainly involves local material, transported into the channel from shallower areas of the estuary. The results of numerical modelling indicate that the sediment transport capacity due to tidal currents is very limited. River flood events increase the transport capacity by several orders of magnitude, thus playing a critical role in sediment redistribution and supply to the coast. The average sediment transport capacity is estimated to be of the order of 0.1 × 106 m3 year−1 in most of the estuary and 0.5 × 106 m3 year−1 at the inlet, with a large uncertainty. It is concluded that, if morphological stability is set as an environmental objective, the dredged material should not be removed from the system but rather be used to nourish the estuarine beaches and the barrier spit.

Formation of amphitheater-headed valleys by waterfall erosion after large-scale slumping on Hawai'i

Amphitheater-headed valleys are common on the surfaces of Earth and Mars. The abrupt terminations of these valleys at their headwalls have been used extensively to argue for valley erosion from springs (i.e., seepage erosion or groundwater sapping) rather than surface runoff. This interpretation has significant implications for Martian hydrology and the associated prospects for life. A connection between channel form and the erosion processes induced by groundwater, however, has not been demonstrated in resistant rock. Perhaps the most widely cited terrestrial analogs for Martian amphitheater-headed valleys in basalt are the spectacular canyons of Kohala, Hawai‘i. Here we present new field observations and topographic analyses of the amphitheater-headed Kohala valleys. We found no evidence for intensively weathered rocks or alcoves around springs at valley headwalls. Instead, valley-head erosion appears to be dominated by waterfall plunge pools. Stream flow from peak annual precipitation events exceeds spring discharge by more than an order of magnitude, and such flow is responsible for evacuation of the coarse sediment that lines the streams. Bathymetric surveys along the Kohala coast have revealed a large submarine landslide, the Polol Slump, directly offshore of the Kohala valleys. We propose that the headscarp of this massive landslide is expressed as the present-day 400 m Kohala sea cliffs. As dominant streams poured over this headscarp as waterfalls, vertical plunge pool erosion and undercutting caused upstream propagation of knickpoints, eventually producing amphitheater-headed valleys. Island subsidence rates and the ages of volcanic eruptions and submarine terraces indicate that the average rate of valley headwall advance is as high as 60 mm/yr. We propose a simple expression for upslope headwall propagation by vertical waterfall erosion based on abrasion by impacting sediment particles in plunge pools. This model indicates that head-wall propagation depends nonlinearly on the sediment flux passing over the waterfall and linearly on the ratio of kinetic versus potential energy of sediment impacts. After the Polol Slump, many streams did not form upslope-propagating waterfalls because they had smaller discharges due to a radial drainage pattern and fault-bounded drainage divides, which prevented runoff from the wetter summit of the volcano. A threshold for headwall propagation due to sediment supply or sediment-transport capacity is consistent with the model. Island subsidence following valley formation has resulted in alluviation of the valley floors, which has created the observed U-shaped valley cross sections. Our interpretation implies that surface runoff can carve amphitheater-headed valleys and that seepage erosion cannot be inferred based solely on valley form on Earth, Mars, or other planets.

Geotechnical characteristics of the clayey soils and rocks of the North Lefkoşa, Nicosia, Cyprus

This paper outlines the engineering geological properties of the clayey soils of North Lefkosa (Nicosia, Cyprus). On the basis of their physical properties and their engineering characteristics, the Miocene, Pliocene and Quaternary deposits were divided into six units: unconsolidated man-made fill, unconsolidated surficial recent alluvium, unconsolidated surficial young alluvium, consolidated old alluvium, highly weathered soft rock units and slightly weathered relatively resistant rock units. Laboratory tests indicated a generally high swelling potential, which would account for the damage seen in many single and two-storey buildings as well as major roads.

Earth conductivity structures and their effects on geomagnetic induction in pipelines

Abstract. Anomalous, large pipe-to-soil potentials (PSP) have been observed along a natural gas pipeline in eastern Ontario, Canada, where there is a major geological contact between the highly resistive rocks of the Precambrian Shield to the west and the more conductive Paleozoic sediments to the east. This study tested the hypothesis that large variations of PSP are related to lateral changes of Earth conductivity under the pipeline. Concurrent and co-located PSP and magnetotelluric (MT) geophysical data were acquired in the study area. Results from the MT survey were used to model PSP variations based on distributed-source transmission line theory, using a spatially-variant surface geoelectric field. Different models were built to investigate the impact of different subsurface features. Good agreement between modelled and observed PSP was reached when impedance peaks related to major changes of subsurface geological conditions were included. The large PSP could therefore be attributed to the presence of resistive intrusive bodies in the upper crust and/or boundaries between tectonic terranes. This study demonstrated that combined PSP-MT investigations are a useful tool in the identification of potential hazards caused by geomagnetically induced currents in pipelines.

Differential weathering and erosion in an inselberg landscape in southern Zimbabwe: A morphometric study and some notes on factors influencing the long-term development of inselbergs

A morphometric analysis of inselberg systems in southern Zimbabwe indicated that there are two types of inselberg systems that differ in the mode of development. Inselberg systems characterized by backwearing tend to occur on granitoid rocks that are foliated, highly fractured, and prone to grusification. The inselbergs are mostly of the koppie-type and are covered to a great extent with regolith or debris. The second type of inselbergs consists of sparsely fractured granitoid rocks, and areas underlain by these rocks are characterized by clusters of closely spaced rock domes. In the case of these inselbergs, backwearing is not indicated. They appear to be the result of the exposure of structurally pre-designed, highly resistant rock compartments. The hypothesis that differences in lithology and structure may influence the rate and style of inselberg development was tested by means of a simulation model. The simulations are run under the same boundary conditions with differing assumptions concerning role of divergent weathering. The results show that both modes of inselberg development may occur alongside one another as long as a critical relief is maintained. The modelling provided also indicates that, although the inselberg systems must have integrated several sequences of environmental changes during their development, these changes were not able to divert the general trend of the process–response system away from its steady course. Lithological and structural factors, which are responsible for the differences in the mobility of material on the inselberg sideslopes, are thus considered to be the primary source of influence of the different modes of inselberg development. It is suggested that the compositional and structural diversity in granitoid rock terrains is what enables inselbergs to pass through different development modes and sequences as rocks of differing resistance become subject to weathering during the development of the landscape.

Can springs cut canyons into rock?

Amphitheater‐headed valleys on Earth and Mars are often assumed to result from erosion by emerging spring water (i.e., seepage erosion or groundwater sapping) rather than by surface runoff. The origin of such valleys has implications for landscape evolution on Earth and the hydrologic cycle and associated potential for life on other planets. In this paper we explore the evidence for seepage erosion in bedrock to address whether valley morphology can be used as a diagnostic indicator of seepage erosion. Seepage erosion is an important process in loose sediment where hydraulic forces cause grain detachment, often resulting in amphitheater‐headed valleys. However, the extension of these processes to resistant rock is uncertain. In sedimentary rocks, groundwater might control the shape and rate of valley formation. It is possible, however, that seepage plays only a secondary role to runoff processes. This seems likely in basaltic valleys on Earth, where little evidence exists for seepage erosion. Since the ability of seepage to erode bedrock valleys remains unclear and because many amphitheater‐headed valleys were probably carved by other processes, seepage erosion should not be inferred based solely on valley form.

The origin of fluoride in groundwater supply to Hermosillo City, Sonora, México

Anomalous high fluoride concentration up to 7.59 mg/dm3 is found in groundwater from “La Victoria” area. This water is used to supply drinking water to Hermosillo City, Sonora. Geochemistry of groundwater, relationship between physicochemical parameters, hydrogeology and geologic setting were correlated to define the origin and the geochemical mechanisms of groundwater fluorine enrichment. High fluoride concentration is associated with high bicarbonates, pH and temperature, and it decreases toward the west and south of the area. Fluoride is in negative correlation to calcium concentration. Sodium sulphate facies of regional deep water flow are related to high fluoride concentration. High electric resistivity rocks associated with granites from the Sierra Bachoco basement might be the deep source of fluoride. Outcropping of Sierra Bachoco in the west causes upward regional flow. Groundwater of longer residence time can be pumped there. The anomalous area is restricted to “La Victoria” because calcareous paleozoic rocks outcrop to the south.

Use of borehole radar reflection logging to monitor steam-enhanced remediation in fractured limestone—results of numerical modelling and a field experiment

Ground penetrating radar is an efficient geophysical method for the detection and location of fractures and fracture zones in electrically resistive rocks. In this study, the use of down-hole (borehole) radar reflection logs to monitor the injection of steam in fractured rocks was tested as part of a field-scale, steam-enhanced remediation pilot study conducted at a fractured limestone quarry contaminated with chlorinated hydrocarbons at the former Loring Air Force Base, Limestone, Maine, USA. In support of the pilot study, borehole radar reflection logs were collected three times (before, during, and near the end of steam injection) using broadband 100 MHz electric dipole antennas. Numerical modelling was performed to predict the effect of heating on radar-frequency electromagnetic (EM) wave velocity, attenuation, and fracture reflectivity. The modelling results indicate that EM wave velocity and attenuation change substantially if heating increases the electrical conductivity of the limestone matrix. Furthermore, the net effect of heat-induced variations in fracture-fluid dielectric properties on average medium velocity is insignificant because the expected total fracture porosity is low. In contrast, changes in fracture fluid electrical conductivity can have a significant effect on EM wave attenuation and fracture reflectivity. Total replacement of water by steam in a fracture decreases fracture reflectivity of a factor of 10 and induces a change in reflected wave polarity. Based on the numerical modelling results, a reflection amplitude analysis method was developed to delineate fractures where steam has displaced water. Radar reflection logs collected during the three acquisition periods were analysed in the frequency domain to determine if steam had replaced water in the fractures (after normalizing the logs to compensate for differences in antenna performance between logging runs). Analysis of the radar reflection logs from a borehole where the temperature increased substantially during the steam injection experiment shows an increase in attenuation and a decrease in reflectivity in the vicinity of the borehole. Results of applying the reflection amplitude analysis method developed for this study indicate that steam did not totally replace the water in most of the fractures. The observed decreases in reflectivity were consistent with an increase in fracture-water temperature, rather than the presence of steam. A limiting assumption of the reflection amplitude analysis method is the requirement for complete displacement of water in a fracture by steam.

Shore platform development around Lord Howe Island, southwest Pacific

This paper describes the morphology of near-horizontal basalt and calcarenite shore platforms around Lord Howe Island as well as the lithological and process environment in which they occur. The morphology of platforms around the island is highly varied. For instance, they occur at a wide range of elevations between low-tide level and several metres above the highest tides, and their width ranges between just a few metres to over 100 m. However, common to most platforms is that they have near-horizontal surfaces that terminate abruptly in a steep scarp at their seaward edge. Correlations indicate that the variability in platform elevation and width is attributable to variability in key parameters of erosion, such as rock resistance and shoreline water depth. Whereas wider platforms occur in rocks of lesser resistance, platform elevation is shown to increase both as rock resistance and the depth of water at the shoreline increases. Wave exposure has a significant control on platform width in some instances, but an over-arching relationship was not detected. Most platforms around the island appear to have developed over the Holocene and a conceptual model is proposed to account for the varied morphology of platforms that have developed over this period. Resistant plunging cliffs occur along about 18% of the rocky coastline of Lord Howe Island and represent the starting point for the model. Where cliffs have yielded to wave erosion platforms have been initiated at an elevation controlled predominantly by rock resistance and shoreline water depth. At Lord Howe Island narrow platforms up to about 20 m wide in exposed locations generally occur about 4 to 6 m above high tide level, and have apparently been subject to very infrequent erosion. Shore platforms about 20 to 40 m wide have generally formed in less resistant rocks, are somewhat lower at about 2 to 3 m above high tide level, and have been subject to more frequent erosion over the Holocene. Most basalt platforms around the island fall within these first two categories, but there are some instances of platforms about 50 m wide. These platforms have formed in rocks of comparatively low resistance and are relatively low in elevation occurring close to the level of high tide. An interesting feature of some of these wide platforms is that, unlike the morphology of narrower platforms, raised ramparts sometimes occur on their outer edge.

Emergence of the Canadian Rockies and adjacent plains: A comparison of physiography between end-of-Laramide time and the present day

The landscape of the Canadian Rockies in southern Alberta is not a direct result of constructional processes; that is, the ridges and peaks have not been pushed into the positions in which we see them today. Tectonic activity provided original elevation but not mountains: at the end of Laramide time, what are now the front ranges and foothills of the Rockies comprised a high-elevation upland of relatively low relief. The present mountain physiography is the result of 55–60 million years of post-orogenic differential erosion, in which more resistant rocks have been left at higher elevations than less-resistant rocks. The Canadian Rockies and the foothills are developed in a thin-skinned, thrust-and-fold belt created during the Laramide Orogeny; the adjacent Interior Plains cut across foreland basin sediments derived from the mountains. The mountains currently consist of large parts of ridges of well-indurated Paleozoic and, locally, Proterozoic rock alternating with valleys developed in soft Mesozoic clastic rock. In the foothills, where the soft Mesozoic rock is at the surface, relief is subdued, but ridges of more-resistant sandstone rise above shaley lowlands. The plains are relatively flat but also contain erosional outliers of higher paleo-plains-surfaces. Numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the Laramide Orogeny, while the western plains have lost at least 2 km, requiring that the local relief of the mountains and foothills that we see is erosional in origin. Local physiography is adjusted to lithology: the mountains have high relief because the exposed sub-Mesozoic rocks can hold up high, steep slopes, whereas the foothills have low relief because the underlying Cretaceous rocks cannot hold up high, steep slopes. The east-facing escarpment at the mountain front is a fault-line scarp along a low-angle thrust. Mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt, and physiography of the belt at the end of Laramide time (60–55 Ma) depended mainly on whether Mesozoic or Paleozoic/Proterozoic rocks were exposed at the surface at that time. A reconstruction using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges at the eastern edge of the Rocky Mountains were mostly or perhaps entirely covered with Mesozoic rocks and despite that high elevation had a hilly, not mountainous, character. The main ranges, in the central Rocky Mountains, were in part stripped of Mesozoic cover by then and more mountainous. Treeline was higher then, and the thrust belt may have been largely or entirely vegetated. Generation of modern relief in the front ranges, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-Laramide time. The Interior Plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments. Although some of the present low local relief of the plains results from weakness of underlying Cretaceous/Tertiary rocks, the low relief is probably largely related to the process of denudation.

The deep-seated slope deformation at Encampadana, Andorra: Representation of morphologic features by numerical modelling

The present study is intended to improve some aspects referring to the general understanding of deep-seated gravitational slope deformations (DSGSD) by the description of field data from the Encampadana slope in Andorra, and using a simplified numerical modelling. The site investigation of the Encampadana DSGSD revealed two major morphologic features: a prominent “graben” close to the summit and counterscarps at different altitudes of the slope. The geological data showed that the slope is affected by an antiform consisting of hard resistant rocks in its outer parts and a more ductile material in its inner part, subsequently called core. Currently available information does not allow the appraisal of the failure mechanism affecting the slope. In order to improve the knowledge of both the pattern of the slope deformation and the influence of the different geologic, structural and geomorphologic features, 2D finite element modelling has been carried out simulating the joint pattern of the rock slope by interface elements. The Encampadana slope is located at a river valley that was subjected to erosion by Pleistocene glaciers plus the subsequent decompression of the valley walls after their retreat. These stages were implemented into the model to account for the evolving stress history. During modelling, focus was set on the effect of three different factors: (1) geological discontinuities, (2) glacier erosion, and (3) a ductile core in the slope consisting of black carbonaceous shales. The calculations showed that a staged approach for simulating the stress history of the site was very appropriate, although the presence of tectonic stresses was not incorporated. The results indicated that the discontinuity set and the glacier erosion generate an opening of the joints, some toppling-effects and the development of counterscarps. Moreover, the incorporation of a ductile core produced a “graben”-like structure. All these features are consistent with the morphology observed in the field. In conclusion, the numerical modelling indicates that some realistic results representing the general deformation of the slope and the observed field features can be achieved even with very limited input data.

Crater gradation in Gusev crater and Meridiani Planum, Mars

The Mars Exploration Rovers investigated numerous craters in Gusev crater and Meridiani Planum during the first ∼400 sols of their missions. Craters vary in size and preservation state but are mostly due to secondary impacts at Gusev and primary impacts at Meridiani. Craters at both locations are modified primarily by eolian erosion and infilling and lack evidence for modification by aqueous processes. Effects of gradation on crater form are dependent on size, local lithology, slopes, and availability of mobile sediments. At Gusev, impacts into basaltic rubble create shallow craters and ejecta composed of resistant rocks. Ejecta initially experience eolian stripping, which becomes weathering‐limited as lags develop on ejecta surfaces and sediments are trapped within craters. Subsequent eolian gradation depends on the slow production of fines by weathering and impacts and is accompanied by minor mass wasting. At Meridiani the sulfate‐rich bedrock is more susceptible to eolian erosion, and exposed crater rims, walls, and ejecta are eroded, while lower interiors and low‐relief surfaces are increasingly infilled and buried by mostly basaltic sediments. Eolian processes outpace early mass wasting, often produce meters of erosion, and mantle some surfaces. Some small craters were likely completely eroded/buried. Craters >100 m in diameter on the Hesperian‐aged floor of Gusev are generally more pristine than on the Amazonian‐aged Meridiani plains. This conclusion contradicts interpretations from orbital views, which do not readily distinguish crater gradation state at Meridiani and reveal apparently subdued crater forms at Gusev that may suggest more gradation than has occurred.

Sub-Audio Magnetic Surveying for Shallow Occurrences of Conductive Manganese Ore, Woodie Woodie Area, Western Australia

Manganese (Mn) ore in the Woodie Woodie area occurs as podiform ore bodies in Archaean dolomite and chert breccia thought to be 2.5 billion years old. Electromagnetic conductivity mapping using time-domain helicopter electromagnetics (HoistEM) surveying successfully detected six new discoveries under regolith and host rock cover in the area. Sub-Audio Magnetic (SAM) surveys were carried out over known EM-responsive Mn deposits to test the ability of this technology to detect conductive, highgrade manganese deposits as equivalent magnetometric resistivity (EQMMR) anomalies. SAM survey data were compared to other geophysical methods over five EM-responsive manganese deposits, which varied in size and burial depth. The methods compared were high-resolution gravity, HoistEM, gradient-array induced polarization (GAIP), dipole-dipole induced polarization (DDIP), and ground time domain electromagnetic (TEM) surveying. The SAM technique demonstrates that this technology can be effective for identifying conductive, mineralised structures and regolith features at shallow depth, usually less than 40 m. Podiform ore zones below 40 m depth did not produce EQMMR anomalies. The SAM depth of penetration is limited by the ability for current to be channelled into conductive, podiform ore zones sitting in resistive host rocks, below a moderately conductive regolith cover.