Relict Features Research Articles

Evolution of Cheekye fan, Squamish, British Columbia: Holocene sedimentation and implications for hazard assessment

Cheekye fan, a large (~25 km2) fan located at the head of Howe Sound, southwestern British Columbia, has its origins in the collapse of the western flank of Mount Garibaldi onto a waning Late Pleistocene glacier, followed by post-glacial redistribution of colluvial-glacial sediments. Reconstruction of internal fan architecture allowed characterization of the rapid decline in sediment delivery to the lower fan through the paraglacial period, a result holding important implications for hazard assessment. A ground penetrating radar profile on lower Cheekye fan has reflectors with a steep (25°), westerly dip lying between 48 and 26 m above sea level, which are interpreted as foreset beds. Radiocarbon ages from marine deltaic deposits at the head of Howe Sound indicate that the sea stood at about 45 m above sea level at 10 200 BP. Together, the data indicate that the head of Howe Sound was deglaciated by 10 200 BP, that the fan had prograded 2.5 km into the fjord by this time, and continued to prograde as relative sea level fell. Reflectors at -10 m above sea level with a steep southerly dip along the southern edge of the lower fan, and radiocarbon ages in this vicinity, indicate it had reached close to its modern extent by 8000-9000 BP. Calculation of the volume of sediment stored in the lower fan indicates that at least 90% of the material was deposited before 6000 BP. This indicates that the Cheekye fan is largely a relict feature, putting its large size into context for future hazard assessments.

High-resolution swath sonar investigation of sand ridge, dune and ribbon morphology in the offshore environment of the New Jersey margin

Sand ridges on the northeast US Atlantic shelf form in the near-shore environment, most likely in response to storm-driven flows. As the Holocene transgression has continued, the ridges have been transferred to an offshore hydrodynamic regime, where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly. Here, we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller-scale features in an effort to place constraints on bedform development in these deeper waters. A recent high-resolution swath sonar survey mapped a portion of the New Jersey shelf from ∼20 m water depth to the shelf break (∼120 m), imaging both sand ridges and smaller-scaled dunes and ribbons in far greater detail than has been previously possible. Using a robust statistical analysis, we find that the gross morphology of ridges (height, width, length) does not change with depth beyond ∼20 m water depth, and changes in ridge orientation generally mirror changes in regional contour orientation. Hence, ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime. However, on the inner shelf (∼20 m water depth to the Mid-Shelf shore), we do find evidence in the ridge shape, which has an asymmetry opposite to that seen near shore, and in the complex backscatter response that some important modifications to ridges are taking place at these water depths. In contrast, on the mid-shelf (from the Mid-Shelf shore to the Franklin shore), ridges tend to have higher backscatter at the crests, implying that these are largely winnowed, relict features. Lineated, smaller-scale (∼100–500 m wavelength, <1 m rms height) features are also present in the seafloor morphology. On the inner shelf these are ∼North-trending, and are evidently transverse to an onshore-directed flow (dunes), whereas on the mid shelf these are ∼NE/SW-trending and are parallel to a SW-directed flow (ribbons). The inner shelf dunes frequently exist on the seaward flanks of ridges, whereas the mid-shelf ribbons only inhabit large NE/SW-oriented swales between clusters of ∼ENE-trending ridges. Morphologic and seismic evidence suggests that the ribbon-floored swales represent erosional intrusions into the otherwise relict ridge morphology. Also present on the mid-shelf are asymmetric blow-out pits and transverse dune packets, both of which are consistent with a SW-directed flow. Relict iceberg scours are present on the outer shelf Hudson Apron, evidently preserved by outcropping stiff clays, and are overlain in some places by ∼ENE-trending ridges.

Lithofacies distribution in relation to the geomorphic provinces of Prydz Bay, East Antarctica

Over the past 15 years, Japanese, Australian and Russian expeditions to Prydz Bay have collected about 30 000 km of bathymetric data, 6000 km of sidescan sonar data and more than 250 sediment grab and core samples. These data were used in the present study to compile surficial sediment, bathymetric, and geomorphological maps of the Prydz Bay region. Lithofacies distribution was determined by surficial sediment data analysis using sample matrix (Q-mode) and cluster analysis techniques based on data from 206 sites. Data included percentage biogenic silica (opal), calcium carbonate, gravel, mud, and relative abundance of two diatom species (Fragilariopsis curta and F. kerguelensis). Five lithofacies are identified from the available data: (1) slightly gravelly sandy mud (g)sM lithofacies, (2) siliceous mud and diatom ooze (SMO) lithofacies, (3) F. kerguelensis pelagic ooze lithofacies, (4) F. curta gravelly muddy sand gmS lithofacies and (5) calcareous gravel lithofacies. In many areas the lithofacies correlate to geomorphological provinces as defined by previous investigators using 3.5 kHz and sidescan sonar data. In some cases, Holocene SMO sediments are seen to drape over iceberg plough marks, implying that these are relict features. These five lithofacies are likely to dominate most of the East Antarctic shelf region and may be helpful in defining sedimentary successions resulting from ice-sheet advance and retreat over glacial-interglacial cycles.

Middle-Holocene timberline fuctuation: influence on the genesis of Podzols (Spodosols), Norra Storfjället Massif, northern Sweden

Fluctuations of the altitudinal position and composition of the timberline in northern Sweden have occurred in response to climatic changes following deglaciation ( c. 8800 yrs BP). In this study a soil sequence of predominantly Podzols (Spodosols) was analysed to determine if the soil profiles located above the present timberline were actively forming in the low-alpine environment, or whether they represent relict features of a once stronger pedogenic regime. The results of the soil analysis indicate a relatively uniform parent material with increasing fines near the surface reflecting aeolian influxed sediment. The soil profiles located above the timberline are generally characterized by darker colours, although with a lower accumulation of organic carbon and either similar or higher iron and aluminium in the spodic horizon than are found in the profiles located below the timberline. The alpine Podzols, generally located in protected swales, are quite distinct from Brunisols (Inceptisols) and Regosols (Entisols) found in slopes and swells on the predominantly undulating morainic and glaciofluvial landscape. This appears to indicate that Spodosols located above the present timberline are not actively being modified under the present low-alpine environmental conditions, but rather were initiated during the Atlantic Chronozone when a dwarf or stunted forest extended to approximately 900 m above sea level.

The nature of three ancient woodland soils in southern England

Abstract. Despite a wealth of published research on the nature of woodland soils, little is known about the nature of soils on sites that have supported woodland for many hundreds of years, namely ancient woodland. The properties and variability of soils in three ancient woods; one in the New Forest, Hampshire and two in Berkshire, were compared with those under recent woods. The acidity of ancient and recent woodland soils was high and remarkably similar. Only where cultivation of soils had preceded woodland establishment was soil acidity lower. The quantity of carbon in the soils studied was inversely related to soil acidity and the ancient woods had accumulated larger quantities of carbon than their recent counterparts. The quantities of Ca2+, Mg2+ and K+ were larger in the ancient woods except where prior cultivation had taken place. Total and organic phosphate contents of the ancient woodland soils were also consistently larger. The nature and pattern of soil variability in ancient woodland soils was quite distinct from that found in recent woods. Overall, the variability of soil acidity, carbon content and organic phosphate was larger in the ancient woodland soils but the pattern of variability differed between the soil properties. No clear association existed between the pattern of soil acidity and individual trees. At the surface of some of the woodland soils, however, carbon distribution appeared to be associated with individual trees. At depth in the ancient woodland soils, the association with the existing vegetation cover was not so clear. It is probable that the ancient woodland soils retained relict features of previous vegetation cover. Organic phosphate distribution was very strongly associated with the present vegetation cover. The pattern of distribution of organic phosphate appeared to be stronger than that of soil acidity and carbon content.

Pedogenesis in Alpine Ecosystems of the Eastern Uinta Mountains, Utah, U.S.A.

We characterized 12 pedons from alpine and upper subalpine areas (>3300 m) along the south slope of the eastern Uinta Mountains in northeast Utah. Although the soils are derived mostly from quartzitic residuum and till, half of them have received calcareous eolian sediment, probably from the Uinta Basin. This eolian sediment is trapped effectively by Geum/sedge communities and has changed the soil chemistry to where more than half of the soils have a base saturation in excess of 50%. One of the pedons (U90-5) above treeline has a distinct argillic horizon, suggesting that clay movement is either a contemporary process in alpine soils of the central Rocky Mountains or that the argillans are relict features from a previous milder soil climate. Although some soils above 3400 m have a mean annual temperature of −2°C or lower, they do not contain permafrost in the upper 2 m. Alpine soils of the Uintas described in this study are Aquic Cryorthents; Pergelic, Dystric, and Typic Cryochrepts; Typic, Lithic, and Pergelic Cryumbrepts; Pergelic Cryoborolls; and Pergelic and Typic Cryoboralfs.

Dolomite chimneys as possible evidence of coastal fluid expulsion, uppermost Otago continental slope, southern New Zealand

A suite of carbonate chimneys, slabs and ridges occur on a 1 km2, 220 m-deep terrace on the uppermost continental slope, east of the Otago Peninsula, New Zealand. Carbonate content and petrographic observations suggest cementation took place just below the sediment-water interface. Encrustation, borings and the abundance of chimneys and carbonate debris observed today on the uppermost slope suggest that the chimneys have been exhumed and are relict features, with a maximum radiocarbon age of 33,000 ± 550 yr B.P. Negative δ13C values of −4.9 to −12.8%. PDB indicate that the carbon contained within the cement was derived from the biogenic degradation of organic matter. A δ18O composition of +4.9 to +5.4%. PDB suggests a marine origin. Palaeotempertures of precipitation are consistent with modern bottom water temperatures. Cement in the chimneys is considered to be derived from aquifer-forced ground water, marine in character, and sourced from the exposed inner-middle shelf during glacially lowered sea level. This contrasts with the tectonically induced fluid flow proposed for vent sites on convergent margins.

Constraints on Macquarie Ridge tectonics provided by Harvard focal mechanisms and teleseismic earthquake locations

In this study we evaluate teleseismically determined focal mechanisms and epicenters for earthquakes along the Macquarie Ridge Complex (MRC) from 45°S to 61°S and 155°E to 168°E, a region characterized by some previous investigators as undergoing subduction initiation. From 65 centroid moment tensors reported by Harvard, we develop statistical guidelines for choosing 26 which represent better determined, more reliable focal mechanisms for tectonic analysis. Although thrust mechanisms occur in the north, near Fiordland, elsewhere along the MRC the better determined mechanisms virtually all indicate that present‐day motion along most of the MRC is strike‐slip. This is consistent with sidescan sonar and multichannel reflection data collected between 50°S and 57°S on 1994 and 1996 cruises; the active plate boundary zone appears to be quite narrow (&lt;5 km wide), and no active compressional features can be observed on the seafloor. If we determine a rotation pole for plate boundary motion using only slip vectors from better determined Harvard mechanisms along the MRC, the best fitting “instantaneous” pole is at 57.4°S, 179.4°E, about 2.5° north of the NUVEL‐I Australian‐Pacific pole, which averages motion over the last 3.0 my. If the MRC pole was formerly farther south than at present, this could explain the existence of relict features associated with crustal shortening, such as bathymetric highs and troughs; yet, the absence of active features such as thrust faults, etc., suggests ongoing compression or subduction initiation. We also carefully read arrival times for P phases for 53 earthquakes at 16 teleseismic stations, selected to represent a range of azimuths surrounding the earthquakes; we relocated these earthquakes using standard joint epicentral determination (JED) methods. While most of the better quality relocations lie on or very close to the Australia‐Pacific boundary as determined on the 1994 cruise, a few epicenters occur well away from the boundary, apparently on Cenozoic fracture zones. Thus, on the Macquarie Ridge Complex and other major strike‐slip boundaries (e.g., in California), it appears that the very largest earthquakes occur on the principal plate boundary fault but that other earthquakes, some quite large, may occur away from the boundary along zones of preexisting weakness.

THE FORMATION AND SIGNIFICANCE OF CORALLINE SILICA SPELEOTHEMS IN THE SYDNEY BASIN, SOUTHEASTERN AUSTRALIA

The vast majority of speleothems are composed of calcium carbonate, but this paper focuses on a suite of less-commonly reported speleothems, those formed almost entirely of silica, and on a quartzose rather than carbonate base rock. On the quartz sandstones of the Sydney region of southeastern Australia, small branched coralline silica stalactites, up to 10 mm in diameter and 80 mm in length, frequently are found. These stalactites have not been described in detail before and are clear evidence of the solution of silica (including quartz) from within the sandstones, and its transport and redeposition at the rock surface. Detailed analysis using SEM, XRD, and thin-section techniques revealed layered composition of both amorphous opal-A and crypto-crystalline chalcedony. Calcite forms no part of these stalactites. The internal structure also shows that they do not form in a manner similar to calcite stalactites. While the growth rate of these silica stalactites remains unknown, it is believed to be quite slow. But they clearly are not relict features and are forming under present-day temperate-climatic conditions, not the tropical conditions demanded by some authors for silica stalactites elsewhere. Further detailed study of these and other silica speleothems is continuing, as they have the potential to be used as important records of palaeoclimatic and land-use change. [Key words: silica karst, speleothem, coralline silica, silica stalactite, opal-A, chalcedony, quartz, silica solution.]

The rock varnish in the Messak Settafet (Fezzan, Libyan Sahara), age, archaeological context, and paleo-environmental implication

The manganese-rich rock varnish from the Messak Settafet plateau is a relict feature that developed at the end of the Holocene humid cycle, before the onset of the present-day hyperarid conditions. The Messak Settafet varnish consists of three microlayers of different composition and micromorphological features. The innermost microlayer is composed of illuvial clay. The second, middle microlayer is the classic manganese-rich rock varnish. The outermost microlayer is composed of unaltered aeolian dust. Each microlayer formed under different climatic conditions, and the microstratigraphy records a climatic evolution from a wet environment towards progressive desertification. The relationship between desert varnish, rock engravings, and radiocarbon-dated archaeological evidence and the comparison with the Holocene climatic evolution as from cave deposits suggest that most of the varnish developed since the end of the VI millennium B.P. under a semiarid climate characterized by steppe-type vegetation, up to the onset of desert conditions, during the IV millennium B.P. © 1996 John Wiley & Sons, Inc.

Geomorphology and sediments of sand ramps in the Mojave desert

Sand ramps are amalgamated accumulations of aeolian, fluvial, and talus deposits. They have developed as a result of the interactions of wind-blown sand and desert piedmont processes adjacent to mountain ranges in the Mojave Desert and occur next to regional and local sand transport corridors. Sand ramps are dominated by aeolian sand that was deposited as sand sheets against the mountain front, but also include significant fluvial units. Periods of geomorphic stability are represented by paleosols and talus accumulations. The sand ramps provide a record of the response of aeolian processes to climatic change in the Mojave Desert. Most are now relict features and accumulated in periods of higher sediment supply from fluctuating and/or desiccating paleolakes during the late Pleistocene and early Holocene.

Suitability of habitat for spawning lake trout

We provide further insight into the reproductive ecology and spawning requirements of lake trout. New comparative information about substrate characteristics, sediment transport, quality of interstitial water at spawning substrates, and the role of temperature in site selection and time of spawning is given for lakes Simcoe and Manitou (Ontario) and Seneca Lake (New York). Spawning lake trout commonly use stable lag deposits derived from glacial sediments, or relict features such as fans, bars or submerged talus slopes. Artificial breakwaters of broken material may also provide suitable substrates. Optimal particle sizes range from 4 to 10 cm diameter but larger materials to 30 cm are also successfully utilized for spawning. The transport of finer particulates by wind generated water movements may limit the suitability of some substrates and successful spawning sites are usually remote from depositional effects. Successful embryo development is associated with low nutrient conditions, with high dissolved oxygen (>7 mg L-1) and with low un-ionized ammonia (<12.5 μg L-1) in the interstitial water of spawning substrates. Shallow-water spawning appears to be the common strategy of colonizing lake trout. Some deepwater spawning in the Great Lakes may reflect initial colonization in shallow-water and adaptation to later increases in water level, but some may also reflect unique behavioural and physiological adaptations. Temperature is an important cue, and many wild and hatchery stocks spawn at 8 to 13 °C with latitudinal shifts in the actual time of spawning. These requirements are summarized as a dichotomous key for evaluation of approaches to restoration of lost or damaged lake trout stocks.

Morphological characteristics and interpretation of some polygenetic soils in loess in southern South Island, New Zealand

Abstract Soils with morphological features resulting from more than one cycle of soil formation occur widely in the loess terrains of New Zealand. Fragiochrepts (Fragic Pallic Soils) developed in loess on seasonally dry terraces and downlands in the southern South Island have long been considered to be polygenetic in that the fragipan is a relict feature. It follows, therefore, that at least some of the Dystrochrepts (Firm Brown Soils) developed in loess in more humid adjacent regions may also be polygenetic, as the two great groups form a continuum. Field evidence shows that some Dystrochrepts in the humid areas have fragipan-like features in advanced stages of destruction suggesting polygenesis and implying climatic change. Other features that suggest polygenesis include pedosphere stripping whereby the solum of a soil is removed by erosion down to the upper surface of a consolidated layer, such as a fragipan, prior to the deposition of a subsequent loess layer. Several Fragiochrepts in the region have a very sharp upper surface to the fragipan; the morphological features of the horizon itself are in marked contrast to those of the overlying solum; and polygonal fissure patterns in the respective horizons do not match vertically. Offset fissure patterns have also been observed in some Dystrochrepts, though soil development tends to obscure any morphological breaks. Dystrochrepts developed in loess over gravels show generally similar features: the gravels are compacted; are in a position equivalent to that of a fragipan; have a stone line or lag at the surface indicative of deflation; and overlying loess forms a solum comparable in thickness to sola in Fragiochrepts (ca. 50 cm) in the region. These three similar features suggest varying climate as well as polygenesis.

10Be and 26Al Evidence for Exceptionally Low Rates of Australian Bedrock Erosion and the Likely Existence of Pre-Pleistocene Landscapes

Analysis of in situ-produced 10Be and 26Al demonstrate that some Australian inselbergs (granitic domes) are among the most stable geomorphic surfaces in the world, eroding at rates as low as 0.6 m/myr. Twenty isotopic analyses of ten samples suggest that the bare bedrock tops of these isolated granitic domes have been exposed to cosmic radiation for at least 0.5 myr and are eroding at a mean model rate less than 0.7 ± 0.1 m/myr (n = 6). Isotopic analyses of samples collected from the top of Mt. Wudinna, Little Wudinna, and Yarwondutta Rock are most consistent with continuous exposure during erosion. On the other hand, samples collected from the flanks of some domes and the tops of other domes have isotopic abundances suggestive of complex exposure histories including burial and reexposure. These unique isotopic data provide the first quantitative information for evaluating longstanding, but previously untested, hypotheses regarding the antiquity and development of these enigmatic landforms and indicate that some Eyre Peninsula inselbergs are probably relict features of pre-Pleistocene landscapes.

Weathering and soil development on fan surfaces as a function of height above modern drainageways, Roan Mountain, North Carolina

A relative-age chronosequence has been established for fan surfaces on the west and south piedmont slopes of Roan Mountain, North Carolina, based on measurements of weathering and soil-development indices at 20 sites. In addition to standard soil properties, measurements were made of amphibolite clast weathering-rind thickness, maximum surface boulder size, and mean size and sorting of a 10 kg whole-sediment sample from the C horizon. Soils lacking Bt horizons occur only at high altitudes at or near the heads of young fans. Excepting these locations, the youngest fan surfaces have soils with Bt horizons generally less than 60 cm thick, hues of 10-7.5YR, maximum clay contents less than 35 percent, rind thicknesses less than 1–2 mm, and maximum boulder diameters of 1–2 m. Soils of the oldest fan surfaces have Bt horizons 75–200 cm thick, hues of 5-2.5YR, rind thicknesses of 6–14 mm, maximum clay contents of 40–60 percent, and maximum boulder diameters less than 0.75 m. Although the youngest fan surfaces are close to the level of modern drainageways, the oldest stand as much as 20 m above these drainageways and are obviously relict features. An index of fan height above modern drainageways was devised that combines height above lateral drainageways with height above the main stream of the valley into which the fan debouches. Many weathering and soil-development properties correlate significantly with this height index, including weathering-rind thickness ( r=0.86), Munsell hue (−0.65), maximum percent clay (0.59), graphic mean (phi units) of whole-sediment particle-size analysis (0.54); maximum boulder diameter (−0.52); maximum percent free iron in B horizon (0.50), and thickness of Bt horizon (0.49). A soil weathering/soil development index using these parameters shows a 0.82 correlation coefficient with the height index. A cluster analysis shows that the 20 sites can be placed into three distinct groups, suggesting that the formation of fan surfaces may be episodic on long; time scales. A comparison of the fan map with a county soils map shows that the oldest fan surface is fairly consistently mapped as Dillsboro series, but that the younger fan surfaces show no consistent relationship to soils mapping.

The fluvial geochemistry and denudation rate of the Guayana Shield in Venezuela, Colombia, and Brazil

The Guayana Shield is composed of Early to Mid-Precambrian igneous and metamorphic basement rocks with a quartzitic platform cover. The complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment. Total erosion rates are extremely slow, ∼10 m/m.y., with equal contributions from the dissolved and suspended loads. However, the former is largely silica with ratios of Si to total cation equivalents [Si:TZ +] ranging to in excess of three. Weathering is “complete” to kaolinite and gibbsite, i.e., the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate. In basins of relatively homogeneous lithology, Rb/Sr isochrons constructed from the river data agree with the whole-rock ages from the drainages; thus, all the common, refractory, Rb-containing minerals (K-feldspar, mica) are completely dissolved. The thick, lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other Southern Hemisphere Shields. In the absence of active tectonics or greatly accelerated mechanical erosion, the weathering rates of these basement rocks must be quite insensitive to environmental change.

Geomorphological signatures of varying climate

A LONG-STANDING question in geomorphology1,2 is whether the topography of a particular landscape is in balance with current climate-driven processes, or contains relict signatures of past climates. For the glaciated landscapes of the Northern Hemisphere, the latter obviously applies but the situation is far from clear in regions where climate-driven processes have changed only in intensity, rather than character. We have addressed this question using a mathematical model of landscape evolution3,4 and find that both cases—contemporary balance and relict features—are possible. For a sinusoidal climate fluctuation, we find that all climate states (wet and dry) leave geomorphological signatures only when there is no active uplift. With active uplift and the associated increase in erosion, the topography tracks the current climate and any relict features are likely to reflect only the wettest conditions previously experienced by the landscape. In both cases, the temporal evolution of the landscape in response to cyclic climate forcing is complex, and leads to the unexpected result that valley density is largest during periods dominated by slow downslope movement of sediment, rather than during times of strong fluvial incision, as would be anticipated from steady-state models5–7.

Models for landform development on passive margins. Some implications for relief development in glaciated areas

Models for pre-glacial relief along passive continental margins must reflect the style and rate of uplift, and the rates of weathering and erosion, during at least the last 100–150 Ma. Passive margins have varied elevation, patterns of rifting, uplift and flexuring. A distinction is recognised between low-lying and steeply rising margins, the latter associated with major crustal upwarps. Continuous flexuring of the crust will maintain disequilibrium in weathering systems and known or possible rates of weathering and erosion appear compatible with weathering penetration during uplift. Under hot-humid conditions a rate of weathering of 50–100 mm ka t1¯ is possible and could produce 15–30 m of saprolite in 1 Ma (with 70% surface loss), while long-term denudation rates seldom exceed 40 mm ka −1. Deep, sandy and corestone profiles in zones of marginal dissection indicate effectiveness of weathering during uplift. Examples from southern Africa, southern Australia, South America and western Africa illustrate 4 morpho-tectonic settings: low elevation with minimum tilt, low domal uplift with coastal tilt, high elevation steeply arched, and high elevation faulted margin with back tilting. These examples are compared with higher latitude marginal landscapes in southeastern USA, Fennoscandia and Scotland. It is concluded that pre-glacial survival implies more than small relict features and includes: exhumed surfaces from beneath sedimentary covers; basins and troughs following zones of deeper weathering, and local planations, now warped and elevated to varying heights, above which inselbergs are locally common. While relict saprolites are found beneath duricrusts in the tropics, and strata-bound in high latitudes, most saprolite is better understood as having a dynamic history throughout the Neogene, including the Quaternary, whether in continuously warm or periodically cold climates.

Stable isotopic and trace element evidence for restricted fluid migration in 2 GPa eclogites

ABSTRACTMineral stable isotopic and trace element studies in 2 GPa banded eclogites of the Tauern Window, eastern Alps, record mm‐ to cm‐scale heterogeneities that reflect compositional variations in the accompanying metamorphic fluids. A close correlation between dolomite mode and dolomite δ18O is consistent with equilibrium partitioning among coexisting minerals and fluids. Small variations in dolomite δ13C values correspond with δ18O variations, but an overall decrease in dolomite δ13C by c. 1%o across a 12‐cm sample is a relict feature that pre‐dates eclogite equilibration. Garnet, omphacite, and clinozoisite rims show little systematic mineral‐mineral partitioning behaviour for Ti, V, Cr, Y, Sr, or Zr; major elements, however, are well equilibrated among these same minerals. Despite the apparent lack of mineral‐mineral trace element equilibration, most of the trace elements vary systematically with water activity calculated in each layer. Trace element behaviour during the eclogite metamorphism thus appears to have been controlled largely by mineral‐fluid interactions along grain boundaries.Shallow structural levels in other subduction complexes (c. 10‐45 km) typically exhibit fracture‐controlled permeability and extensive metasomatism, but there is no field or geochemical evidence for extensive fluid advection during high‐pressure metamorphism in the Tauern eclogites. Because most dewatering and devolatilization during tectonic burial occurs prior to eclogite conditions, the volumetric fluid/rock ratio in eclogites should generally be low. Low fluid/rock ratios, coupled with the possible non‐wetting nature of the fluids, permits the production and preservation of fine‐scale chemical heterogeneities in deeply subducted eclogites and associated fluids. However, the eventual breakdown at greater depth of volatile‐bearing dolomite, phengite, clinozoisite, zoisite, or amphibole could lead to renewed fracture‐controlled fluid release from the subducted rocks to regions appropriate for arc magma generation.

Origin of the sparry magnesite deposits around Bauri, Almora district, Uttar Pradesh, India

Seven pockets of variable dimensions of strata-bound sparry magnesite within the Middle Proterozoic Gangolihat Formation around Bauri in the Almora district, Kumaun, Lesser Himalaya, have been investigated petrographically and geochemically. The lenses and pockets of megacrystalline, bladed, occasionally stellate, magnesite aggregates invariably enclosed by stromatolitic or massive dolostones, often exhibit a concordant relationship with the latter. Besides the sharp contrast in crystal-linity of magnesite and dolostones and the patches of the latter in the former, relict features such as layers of chert, cryptocrystalline silica veins and stromatolitic structures are discernible in the magnesite. There is a gradual increase in MgO and FeO with a corresponding decrease in CaO, and a striking depletion in Sr from dolostone to magnesite but no noteworthy variation in other major or minor elements nor in insoluble contents. Both the dolostones and magnesites are characterised by the same range of oxygen isotope ratios. However, a marked enrichment of lighter carbon isotopes in magnesites is noted. Based on these observations, it is inferred that the magnesite around Bauri is a product of diagenetic magnesitisation of penecontemporaneous dolomite in a restricted biohermal tidal flat environment.