Erosion Of Rocks Research Articles

Thermal and mechanical aspects of magma emplacement in giant dike swarms

We consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts. For driving pressure gradients inferred for giant dike swarms, thicknesses of <10 m would enable dikes to transport magma laterally over the distances observed in the field (up to thousands of kilometers) without suffering thermal lock‐up. Using time‐dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions, we investigate the possibility that the observed dike thicknesses (of the order of 100 m) result from thermal erosion of the country rocks during dike emplacement. This implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure. It is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts. The presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement. In this case, measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion. Shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width, which implies that thicker dikes may have less tendency to produce consistent fabric alignment. Our results suggest that if the dike was propagating downslope off a plume‐related topographic swell, the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse (implosion) of the shallow magma plumbing system feeding the intrusion. Radial dikes that erupted at the periphery of the topographic uplift might have increased (rather than decreased) extensional stresses in the crust within the topographic uplift upon their solidification.

Dynamics of the stream‐power river incision model: Implications for height limits of mountain ranges, landscape response timescales, and research needs

The longitudinal profiles of bedrock channels are a major component of the relief structure of mountainous drainage basins and therefore limit the elevation of peaks and ridges. Further, bedrock channels communicate tectonic and climatic signals across the landscape, thus dictating, to first order, the dynamic response of mountainous landscapes to external forcings. We review and explore the stream‐power erosion model in an effort to (1) elucidate its consequences in terms of large‐scale topographic (fluvial) relief and its sensitivity to tectonic and climatic forcing, (2) derive a relationship for system response time to tectonic perturbations, (3) determine the sensitivity of model behavior to various model parameters, and (4) integrate the above to suggest useful guidelines for further study of bedrock channel systems and for future refinement of the streampower erosion law. Dimensional analysis reveals that the dynamic behavior of the stream‐power erosion model is governed by a single nondimensional group that we term the uplift‐erosion number, greatly reducing the number of variables that need to be considered in the sensitivity analysis. The degree of nonlinearity in the relationship between stream incision rate and channel gradient (slope exponent n) emerges as a fundamental unknown. The physics of the active erosion processes directly influence this nonlinearity, which is shown to dictate the relationship between the uplift‐erosion number, the equilibrium stream channel gradient, and the total fluvial relief of mountain ranges. Similarly, the predicted response time to changes in rock uplift rate is shown to depend on climate, rock strength, and the magnitude of tectonic perturbation, with the slope exponent n controlling the degree of dependence on these various factors. For typical drainage basin geometries the response time is relatively insensitive to the size of the system. Work on the physics of bedrock erosion processes, their sensitivity to extreme floods, their transient responses to sudden changes in climate or uplift rate, and the scaling of local rock erosion studies to reach‐scale modeling studies are most sorely needed.

Energy Concentrated and Self-Resonating Mini-Extended Jet Nozzle Used for Jet Drilling

Theoretical, experimental, and field test studies of the self-resonating pulsating jet have been carried out. The interaction mechanism of vortices and outlet wall is described. On the basis of theoretical calculations and experimental data, a new mini-extended and energy concentrated self-resonating pulsating jet nozzle can be recommended. Hydrodynamic experiments, rock erosion tests, and field tests in a drilling engineering environment were carried out using the new nozzle. The results show that the new nozzle has stronger erosion capability than the cone-type or conventional organ-pipe self-resonating nozzle jet. The field test results in drilling engineering show that the outlets of the newly designed nozzles have strong anti-scouring ability. The new exit design of the nozzle can increase its life span.

Flux and fate of fluvial sediments leaving large islands in the East Indies

Because of their generally small drainage basin areas, high topographic relief, relatively young and erodible rocks, and heavy rainfall, rivers draining the high-standing islands of the East Indies transport a disproportionately large amount of sediment to the ocean. Rivers on the islands of Sumatera (Sumatra), Jawa (Java), Borneo, Sulawesi (Celebes), Timor and New Guinea are calculated to discharge about 4.2×10 9 t of sediment annually. Although these six islands only account for about 2% of the land area draining into the global ocean, they may be responsible for as much as 20 to 25% of the sediment export. Fluvial sediment leaving these islands is discharged into several distinctly different provinces: shallow epicontinental seas such as the Sunda Shelf, Gulf of Papua and Sea of Arafura; and narrow-shelf, active margins along the western and southern sides of Sumatra and Java, and the north coast of New Guinea. High-resolution seismic profiles in the Gulf of Papua (New Guinea) show a clinoform sequence of Holocene sediments pinching out on the mid- to outer shelf, with sediment thickness locally greater than 40 m near the coast; some — but perhaps not much — sediment escapes to the outer shelf and the deeper Papua Trough beyond. In contrast, seismic profiles off northern New Guinea show river-derived sediment prograding over and by-passing a narrow shelf that locally has buried a relict barrier reef. A small fraction of the sediment escaping the northern shelf may be transported to the eastern equatorial Pacific by way of the Equatorial Counter Current, where it may help fertilize equatorial upwelling.

Geomorphic aspects of groundwater flow

The many roles that groundwater plays in landscape evolution are becoming more widely appreciated. In this overview, three major categories of groundwater processes and resulting landforms are considered: (1) Dissolution creates various karst geometries, mainly in carbonate rocks, in response to conditions of recharge, geologic setting, lithology, and groundwater circulation. Denudation and cave formation rates can be estimated from kinetic and hydraulic parameters. (2) Groundwater weathering generates regoliths of residual alteration products at weathering fronts, and subsequent exhumation exposes corestones, flared slopes, balanced rocks, domed inselbergs, and etchplains of regional importance. Groundwater relocation of dissolved salts creates duricrusts of various compositions, which become landforms. (3) Soil and rock erosion by groundwater processes include piping, seepage erosion, and sapping, important agents in slope retreat and headward gully migration. Thresholds and limits are important in many chemical and mechanical groundwater actions. A quantitative, morphometric approach to groundwater landforms and processes is exemplified by selected studies in carbonate and clastic terrains of ancient and recent origins.

Provenance of Cretaceous synorogenic sandstones in the Eastern Alps: constraints from framework petrography, heavy mineral analysis and mineral chemistry

The detrital components of Cretaceous sedimentary rocks in the Northern Calcareous Alps reflect the early Alpine geodynamic evolution of the Austroalpine microplate. Two contrasting source areas are distinguished on the base of light and heavy mineral analysis. The first source area is located at the southeastern margin of the Austroalpine and is composed of Palaeozoic sediments and metamorphic rocks, Mesozoic carbonate rocks, and ultrabasic rocks derived from the suture zone of the Vardar/Meliata Ocean. The second source area is located in the northwest in a Lower Austroalpine position near the transpressive plate margin that juxtaposes Austroalpine and Penninic units. This source area comprises Palaeozoic low-grade metamorphic rocks including high-pressure (HP) rocks, late Palaeozoic (meta)sediments, Mesozoic carbonate rocks, and ultrabasic rocks from obducted slices of Penninic oceanic crust. Chemical analyses of detrital white mica, amphibole and garnet support the discrimination between the two source areas. Tourmaline chemistry calls for a significant amount of metasedimentary rocks in the source area. Granitoid rocks and high-grade metamorphic rocks are rare. Blue amphibole, phengite, and chloritoid composition suggest the erosion of lower blueschist facies rocks in the northwestern source area. We suggest a modified model of Austroalpine Valanginian to Coniacian tectono-sedimentary evolution which is based on (1) an onset of subduction of the Penninic Ocean no earlier than Late Cretaceous, (2) deposition of the analyzed sedimentary rocks in piggyback basins, and (3) a reconstruction of provenance as proposed in this study.

Surface processes in the Venus highlands: Results from analysis of Magellan and Arecibo data

The highlands of Venus are characterized by an altitude‐dependent change in radar backscatter and microwave emissivity, likely produced by surface‐atmosphere weathering reactions. We analyzed Magellan and Arecibo data for these regions to study the roughness of the surface, lower radar‐backscatter areas at the highest elevations, and possible causes for areas of anomalous behavior in Maxwell Montes. Arecibo data show that circular and linear radar polarization ratios rise with decreasing emissivity and increasing Fresnel reflectivity, supporting the hypothesis that surface scattering dominates the return from the highlands. The maximum values of these polarization ratios are consistent with a significant component of multiple‐bounce scattering. We calibrated the Arecibo backscatter values using areas of overlap with Magellan coverage, and found that the echo at high incidence angles (up to 70°) from the highlands is lower than expected for a predominantly diffuse scattering regime. This behavior may be due to geometric effects in multiple scattering from surface rocks, but further modeling is required. Areas of lower radar backscatter above an upper critical elevation are found to be generally consistent across the equatorial highlands, with the shift in microwave properties occurring over as little as 500 m of elevation. These surfaces are not simply characterized, however, by the absence of a highly reflective component. Surface morphology and radar‐scattering properties suggest that a mantling deposit forms at the highest elevations, most likely by in situ erosion of the original rock. In Ovda Regio, this process mantles or has removed surface festoon structure at the 1‐ to 10‐m scale, implying a significant depth for the weathered layer. Similar radar‐dark areas occur in Maxwell Montes but are apparently unrelated to the current topography of the region. Possible reasons for these observations include mass wasting from areas of steep slopes, compositional or age differences within the montes, vertical tectonic shifts of relict contacts, local topographic effects on surface temperature, or errors in the Magellan topography data in the rugged terrain. While there is evidence for some of these effects in the existing data, no single model at present appears to satisfy all occurrences of high‐altitude, radar‐dark terrain. New measurements of the surface and lower atmosphere chemistry of Venus are needed to further refine these conclusions.

Thermal and mechanical models for the structural and metamorphic evolution of the Zanskar High Himalaya

Abstract The regional Barrovian facies metamorphic rocks of the High Himalayan Slab in Zanskar are bounded along the base by the southwest vergent Main Central Thrust (MCT) with its characteristic zone of inverted isograds, and along the top of the slab by the northeast dipping Zanskar Shear Zone (ZSZ), part of the South Tibetan Detachment (STD) System of normal faults. Summarized here are the results of systematic mapping combined with detailed P-T-t data from the Zanskar Himalaya, and models for the thermal and mechanical evolution of the middle and deep crustal rocks of the Himalaya are discussed. Temperatures and pressures increase dramatically up-structural section across the MCT Zone from biotite through garnet, staurolite and kyanite grade to sillimanite + muscovite, and decrease along the top of the stab beneath the ZSZ normal faults. In Zanskar, peak P-T conditions of the M1 kyanite-grade rocks are 550–680°C and 9.5–10.5kbar, and M2 sillimanite-grade gneisses were formed at 650–770°C and 4.5–7 kbar. The core of the High Himalayan Zone is a 30 km wide zone of sillimanite + K-feldspar-grade gneisses, migmatites and anatectic leucogranites with a right way-up isograd sequence above and an inverted sequence below. Thermal models of thrusting a hot slab over a cold slab and frictional heating along the MCT are not supported by the structural and thermobarometric data. Mechanical models of post-metamorphic structural disruption by folding and thrusting of a pre-existing, right way-up metamorphic sequence are compatible with structural and P-T data. Ductile shearing along the MCT Zone has structurally condensed the isograds, and final motion along the ZSZ postdated leucogranite crystallization (21.5-19.5 Ma; U-Pb monazite ages) and emplacement in the footwall. Crustal shortening and thickening, resulting in prograde metamorphism, lasted from the time of India-Asia collision at 54-50Ma until at least 30-25 Ma. Crustal melting is constrained at 20.8-19.5 Ma along the core of the stab. Late stage out-of-sequence thrusting within the High Himalayan Slab effectively maintained the crustal thickening process enabling P-T conditions to remain high for 10-5 Ma. Rapid exhumation between 21-18.5 Ma was accompanied by removal of 18–25 km of overburden by erosion at exhumation rates of 6–10 mm yr −1 . This early Miocene period of high exhumation rates, rapid erosion and exhumation of rocks buried at τ25 km depth is interpreted to indicate the uplift of high mountains and erosion of deep valleys, probably with high precipitation during that time.

Abstract: Interplay of Uplift, Erosion, Sedimentation and Preservation of Middle Jurassic Rocks, Big Horn Basin, Wyoming 

Abstract: Interplay of Uplift, Erosion, Sedimentation and Preservation of Middle Jurassic Rocks, Big Horn Basin, Wyoming 

Safer sips: removing arsenic from drinking water.

U.S. researchers are developing technologies that may someday protect millions of people worldwide whose drinking water is tainted with arsenic. Arsenic is released into water from soil and rock erosion, and is also a by-product of industrial processes including semiconductor manufacturing, petroleum refining, and mining and smelting operations.

Table Nunatak: a key outcrop of Upper Cretaceous shallow-marine strata in the southern Larsen Basin, Antarctic Peninsula

The northern, James Ross Island region of the Larsen Basin, on the eastern, back-arc margin of the Antarctic Peninsula magmatic arc, includes one of the thickest and most complete Upper Cretaceous sedimentary successions exposed in the Southern Hemisphere. However, the southern part of the basin remains poorly known, mainly owing to inaccessibility and lack of exposure. Table Nunatak, an isolated, 1-km-long, 400-m-wide outcrop at the tip of Kenyon Peninsula, is the only known exposure of Upper Cretaceous or younger strata in this region. The 62-m-thick succession exposed there is assigned to the newly defined Table Nunatak Formation. It consists mainly of sharp-based, amalgamated beds of fine-grained sandstone up to 2.8 m thick, with subordinate intervals of intensely bioturbated mudstone. Wave ripples are present at some levels, and locally developed swaley cross-stratification provides evidence for storm-generated combined-flow deposition. However, most sandstone beds appear to be internally structureless apart from normal grading, and are interpreted as the direct suspension deposits of highly sediment-charged storm- and/or flood-related flows. The succession represents relatively nearshore deposition, probably at the mouth of a river or deltaic distributary channel. Charcoalified plant debris, abundant at the tops of some sandstone beds, suggests a periodically wildfire-swept hinterland forested largely by coniferous trees. Dinoflagellate cyst assemblages indicate a late Santonian age, and suggest correlation with the basal part of the Lachman Crags Member of the Santa Marta Formation (Marambio Group) on James Ross Island. Palaeocurrents, sandstone petrography and the high sediment supply rate proposed for the Table Nunatak Formation, suggest a relatively high-relief source area to the west, with large-scale erosion of granitoid plutons and metamorphic rocks, possibly related to arc uplift during a mid-Cretaceous compressional episode. The formation is evidence of a major southward extension of the Upper Cretaceous strata exposed in the northern Larsen Basin, and suggests lateral continuity of shallow-marine deposition for at least 500–600 km along the Weddell Sea margin of the Antarctic Peninsula in Santonian times.

Implications of late Eocene to early Miocene clay mineral assemblages in McMurdo Sound (Ross Sea, Antarctica) on paleoclimate and ice dynamics

The clay mineral assemblages of upper Eocene to lower Miocene sediments recovered at the CIROS-1 and MSSTS-1 drill sites on the McMurdo Sound shelf, Antarctica, were analyzed in order to reconstruct the Cenozoic Antarctic paleoclimate and ice dynamics. The assemblages are dominated by smectite and illite, with minor amounts of chlorite and kaolinite. The highest smectite amounts and best smectite crystallinities occur in the upper Eocene part of CIROS-1, below 425–445 mbsf. They indicate that during their deposition, chemical weathering conditions prevailed on the nearby continent. Large parts of East Antarctica were probably ice-free at that time, but some glaciers reached the sea and contributed to the glaciomarine sedimentation. In contrast, only minor total amounts of smectite are present in Oligocene and younger sediments due to the shift to mainly physical weathering on an ice-covered Antarctic continent. However, relative smectite percentages rise to more than 60% during two late Oligocene intervals (ca. 27.5–26.2 and 25.0–24.5 Ma) and during one early Miocene interval starting at ca. 23.3 Ma. These intervals are characterized by ice masses coming probably from the south, where volcanic rocks acted as a source, as also indicated by the composition of the sand and gravel fractions. During the other intervals, the ice came from the west, where the physical erosion of basement rocks and sedimentary rocks of the Beacon Supergroup in the Transantarctic Mountains provided high illite concentrations. Because the two drill sites are only 4 km apart, their clay mineral records can be correlated. This led to a new interpretation of the Oligocene paleomagnetic data of the MSSTS-1 site and to a more detailed lithostratigraphic correlation of the Miocene parts of the cores.

Thermodynamics of lateral dike propagation: Implications for crustal accretion at slow spreading mid‐ocean ridges

We consider solidification of hot fluid flowing through a rigid‐wall channel of infinite extent. The calculated “thermal arrest” lengths are used to investigate the role of magma freezing in limiting the propagation distance of lateral dike intrusions. Our results demonstrate that for reasonable parameters the propagation distances of meter‐wide dikes do not exceed the wavelength of crustal thickness variations or transform fault spacing along slow spreading ridges. This suggests that thermal controls on the crustal melt delivery system could be an important factor in modulating these variations. Unlike published results for a finite channel, which predict unlimited meltback of the channel walls if the prefreezing fluid velocity exceeds some critical value, any flow into an infinite channel will eventually freeze, provided that shear heating in the magma is negligible. The thermal arrest distances depend strongly on the average dike thickness h (∝h4 for dikes driven by an along‐strike topographic slope and ∝h2 for dikes driven by an excess source pressure). Thermal erosion of the country rocks associated with lateral dike intrusions is likely to be confined to a very small region near the magma source. Substantial correlations between the along‐strike bathymetry and geochemistry of the erupted lavas along individual ridge segments may be consistent with high‐level basalt fractionation in the laterally propagating dikes.

Type, distribution, and origin of sediments of the Gandoca-Manzanillo National Wildlife Refuge, Limón, Costa Rica

Sediments and the marine current system at the Gandoca-Manzanilio National Wildlife Refuge. Cari­bbean coast of Costa Rica, define two sedimentary environments. One between Punta Uva and Manzanillo. where sediments were derived from the coral reefs and local geological fonnations. The Other lies between Punta Mona and Rio Sixaola. where sediments arrive primarily from outside tile an:a. fine-grained sediments were collected off rivers and creeks mouths. and adjacent beaches. The highest percentage of carbonates (60 to 80 %) was found off Punta Uva, and consisted mainly of mollusk, algal and coral fragments, and Foraminifera. Magnetite found in large quantities off Manzanillo, and also observed on the beach. was derived locally from the Rio Banano Formation. Clay and silt were found in high concentrations from Punta Mona to Rio Sixaola. Other minerals. pyroxene and hy­robiotile, probably originated from the erosion of intrusive rock s cropping out in the central section of the Tala­manea Range. The sediment plume of Rio Sixaola extends several kilometers off shore, flowing northwest and af­fecting coral reefs off Punta Mona. Other an:as of the Refuge an: not exposed to high concentrations of terrigenous sediments. However, as a result of deforestation, new constructions and removal of riparian and coastal forests, more coral reefs of the Refuge will be affected by sediments.

Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal

Sedimentological and provenance data from the lower Miocene–Pliocene Dumri Formation and Siwalik Group in western Nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the Himalaya and the Cenozoic 87 Sr/ 86 Sr record of the ocean. In western Nepal, the Dumri Formation is an ∼750–1300-m-thick fluvial sandstone and overbank mudstone unit. The Siwalik Group is >4200 m thick and consists of a lower member (>850 m) of 2–12-m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m) of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that middle Miocene–Pliocene rivers in western Nepal flowed southward, transverse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the paleo-Ganges River) in Siwalik Group sandstones. A major increase in fluvial channel size is recorded by the transition from lower to middle Siwalik members at ∼10.8 Ma, probably in response to an increase in seasonal discharge. Modal petrographic data from sandstones in the Dumri Formation and the Siwalik Group manifest an upsection enrichment in potassium feldspar, carbonate lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern river sands provide some control on potential source terranes for the Miocene–Pliocene sandstones. The Dumri Formation was most likely derived from erosion of sedimentary and low-grade metasedimentary rocks in the Tibetan (Tethyan) Himalayan zone during early Miocene emplacement of the Main Central thrust. The presence in Dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the Greater Himalayan zone, perhaps in response to tectonic unroofing by extensional detachment faults of the South Tibetan detachment system. During deposition of the lower Siwalik Group (∼15–11 Ma), emplacement of the Dadeldhura thrust sheet (one of the synformal crystalline thrust sheets of the southern Himalaya) on top of the Dumri Formation supplied abundant metasedimentary lithic fragments to the foreland basin. A steady supply of plagioclase grains and high-grade minerals was maintained by deeper erosion into the Main Central thrust sheet. From ∼11 Ma to the present, K-feldspar sand increased steadily, suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Himalayan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group (∼4–5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern rivers and Siwalik Group sandstones that cluster at 460–530 Ma, ∼850–1200 Ma, ∼1.8–2.0 Ga, and ∼2.5 Ga. An abundance of Cambrian–Ordovician grains in the Siwalik Group suggests sources of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are consistent with sources in the Greater and Lesser Himalayan zones. An overall upsection increase in zircons older than 1.7 Ga suggests increasing aerial exposure of Lesser Himalayan rocks. None of the detrital zircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogranites, but this may be attributable to the inheritance problems that characterize the U-Pb geochronology of the leucogranites. When compared with recent studies of the 87 Sr/ 86 Sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the Lesser Himalayan zone and Cambrian–Ordovician granitic rocks of the crystalline thrust sheets in central and eastern Nepal may have played a significant role in elevating the 87 Sr/ 86 Sr ratio of middle Miocene synorogenic sediments in the Indo-Gangetic foreland basin and the Bengal fan, as well as global seawater.

L'érosion hydrique dans les terrains peu résistants des Préalpes françaises du sud. Mesures et variables explicatives sur parcelles et bassins-versants

Abstract : Measurements of the current erosion of certain soft rocks were conducted in a sector of the French Alpine foreland. A combination of tried and tested standard methods was used with a view to obtaining the most precise results possible. The results confirmed that a layer of between 0.5 and 1.0 cm is eroded annually in the Upper Bathonian and Callovo-Oxfordian black marls, the thickness (more than 1 500 metres) and extent of which make them the most common rock formation in this region. The other formations are also subject to considerable denudation, details of which are also given in the article. Observations reveal that, on bare ground, denudation is much greater on north-facing slopes (1.2 cm) than on south-facing slopes (0.5 cm), that the degree of slope has virtually no influence on erosion, and that, on the other hand, the angle formed by the slope and the dip of the layers causes marked differences between slopes, in a similar way to the effect of orientation. Finally, during the main pluvio-erosive events responsible for this denudation, it is the kinetic energy of the precipitation which plays the main role. On the other hand, the amount of erosion for this period is not found to be correlated with the prior state of the ground any more than it is with the runoff.

Geochemical Contribution to Study of Chemical and Mechanical Weathering of a Mediterranean Watershed: The Hérault Basin, S France

Rock and soil erosion by rainwater and rivers are the major geological processes which modify the earth's surface morphology 1. One way to estimate the current erosion rates of watersheds is to study of the loads transported by rivers to the oceans 2. The transported load can be subdivided into the dissolved load ( 0.2~tm) which

Jurassic epithermal Au–Ag deposits of Patagonia, Argentina

Important precious metal deposits have been discovered during the last 10 years in the Deseado Massif region of Patagonia, Argentina. This region is a plateau consisting of Middle to Upper Jurassic volcanic rocks that host fracture-controlled epithermal Au–Ag mineralization. These mineral deposits represent low sulfidation type hydrothermal systems and formed following the main period of volcanism, probably during the Late Jurassic period. The presence of silica sinters and veins interpreted to be feeder structures indicates that the tops of some deposits have been preserved, with erosion of younger sedimentary and volcanic rocks now exposing these systems. These deposits represent diverse erosional levels ranging from sinter formed at the paleosurface, to intermediate Au–Ag-rich quartz veins, to base metal-bearing Au–Ag veins that represent deeper levels of the epithermal systems. The best known deposits are in the Cerro Vanguardia district where vein systems occur over a 350 km 2 area and veins can be traced for more than 14 km. Geological mapping of the district has resulted in the definition of over 140 km of veins. Current reserves are over 3.2 million ounces Au equivalent with grades of 10 g/t Au and 120 g/t Ag. The geologic characteristics of the region and the presence of major Au–Ag deposits provide targets for continuing exploration in the Deseado Massif.

Evolution of the early devonian bindook volcanic complex, wollondilly basin, eastern lachlan fold belt

The Early Devonian Bindook Volcanic Complex consists of a thick silicic volcanic and associated sedimentary succession filling the extensional Wollondilly Basin in the northeastern Lachlan Fold Belt. The basal part of the succession (Tangerang Formation) is exposed in the central and southeastern Wollondilly Basin where it unconformably overlies Ordovician rocks or conformably overlies the Late Silurian to Early Devonian Bungonia Limestone. Six volcanic members, including three new members, are now recognised in the Tangerang Formation and three major facies have been delineated in the associated sedimentary sequence. The oldest part of the sequence near Windellama consists of a quartz turbidite facies deposited at moderate water depths together with the shallow‐marine shelf Windellama Limestone and Brooklyn Conglomerate Members deposited close to the eastern margin of the basin. Farther north the shelf facies consists of marine shale and sandstone which become progressively more tuffaceous northwards towards Marulan. The Devils Pulpit Member (new unit) is a shallow‐marine volcaniclastic unit marking the first major volcanic eruptions in the region. The overlying shallow‐marine sedimentary facies is tuffaceous in the north, contains a central Ordovician‐derived quartzose (?deltaic) facies and a predominantly mixed facies farther south. The initial volcanism occurred in an undefined area north of Marulan. A period of non‐marine exposure, erosion and later deposition of quartzose rocks marked a considerable break in volcanic activity. Volcanism recommenced with the widespread emplacement of the Kerillon Tuff Member (new unit), a thick, non‐welded rhyolitic ignimbrite followed by dacitic welded ignimbrite and air‐fall tuff produced by a large magnitude eruption leading to caldera collapse in the central part of the Bindook Volcanic Complex, together with an additional small eruptive centre near Lumley Park. The overlying Kerrawarra Dacite Member (new unit) is lava‐like in character but it also has the dimensions of an ignimbrite and covers a large part of the central Bindook Volcanic Complex. The Carne Dacite Member is interpreted as a series of subvolcanic intrusions including laccoliths, cryptodomes and sills. The Tangerang Formation is overlain by the extensive crystal‐rich Joaramin Ignimbrite (new unit) that was erupted from an undefined centre in the central or northern Bindook Volcanic Complex. The volcanic units at Wombeyan and the Kowmung Volcaniclastics in the northwestern part of the complex are probably lateral time‐equivalents of the Tangerang Formation and Joaramin Ignimbrite. All three successions pre‐date the major subaerial volcanic plateau‐forming eruptions represented by the Barrallier Ignimbrite (new unit). The latter post‐dated folding and an extensive erosional phase, and unconformably overlies many of the older units in the Bindook Volcanic Complex. This ignimbrite was probably erupted from a large caldera in the northern part of the complex and probably represents surface expressions of part of the intruding Marulan Batholith. The final volcanic episode is represented by the volcanic units at Yerranderie which formed around a crater at the northern end of the exposed Bindook Volcanic Complex.

Sulfur Isotopes in Komatiite-Hosted Ni-Cu Sulfide Deposits from the Morro do Ferro Greenstone Belt, Southeastern Brazil

In the Morro do Ferro greenstone belt in southeastern Brazil, Ni-Cu massive sulfides hosted by komatiites occur in the vicinity of the town of Fortaleza de Minas. Sulfur-isotope abundances were analyzed in sulfides from the main orebody in order to determine the source of the sulfur. The main sulfide minerals in the ore are pyrrhotite (65%), pentlandite (30%), and chalcopyrite (5%). The δ34S values of the ores fall within a relatively narrow range, but are slightly higher than values for similar occurrences in other greenstone belts. As there does not seem to be evidence of thermal erosion of country rocks and sulfur assimilation, the heavier sulfur possibly is the result of crustal contamination of the ascending komatiite magma and separation of an immiscible sulfide liquid already at this stage.