Low-latitude “dusty events” vs. high-latitude “icy Heinrich events”

It has been proposed that tropical events could have participated in the triggering of the classic, high-latitude, iceberg-discharge Heinrich events (HE). We explore low-latitude Heinrich events equivalents at high resolution, in a piston core recovered from the tropical north-western African margin. They are characterized by an increase of total dust, lacustrine diatoms and fibrous lacustrine clay minerals. Thus, low-latitude events clearly reflect severe aridity events that occurred over Africa at the Saharan latitudes, probably induced by southward shifts of the Inter Tropical Convergence Zone. At a first approximation, it seems that there is more likely synchronicity between the high-latitude Heinrich Events (HEs) and low-latitude events (LLE), rather than asynchronous behaviours.     

Compositional classification of “kimberlitic” and “non-kimberlitic” ilmenite

Ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration, and its distinction from non-kimberlitic rock types is important. This is particularly true for regions where these minerals are present in relatively low abundance, and they are the dominant kimberlitic indicator mineral recovered. Difficulty in visually differentiating kimberlitic from non-kimberlitic ilmenite in exploration concentrates is also an issue, and distinguishing kimberlitic ilmenite from those derive from other similar rocks, such as ultramafic lamprophyres, is practically impossible. Ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources.

Various published data sets from selected kimberlitic (including kimberlites, lamproites, and various ultramafic lamprophyres) and non-kimberlitic rock types have been compiled and evaluated in terms of their major element compositions. Compositional fields and bounding reference lines for ilmenites derived from kimberlites (sensu stricto), ultramafic lamprophyres, and other non-kimberlitic rock types have been defined primarily on MgO–TiO₂ graphs as well as MgO–Cr₂O₃ relationships.     

Platform subsidence mechanisms and “eustatic” sea-level changes

The boundaries between major world-wide sequences of sediments on continental platforms are due primarily to either eustatic changes or to systematic uplift of the continental interior. If thermal contraction of the lithosphere controls basin subsidence, basins would continue to subside during times of low eustatic sea level. Calculations indicate that significant gaps in the geological record could be produced by modest eustatic sealevel changes even in rapidly subsiding basins. For example, a 95 m withdrawal could have produced the sub-Devonian unconformity in the Michigan Basin. The vertical amplitude of eustatic variations can be determined directly from the depth of paleotopographic valleys (135m for sub-Pennsylvanian of Illinois Basin) and from the present elevation of sediments deposited during highstands (300 m above present sea level). The Illinois Basin aparently continued to subside during the sub-Pennsylvanian regression.     

“Rust” contamination of formation waters from producing wells

In the Alberta Basin there is a significant difference in the content of Fe between formation waters from drillstem tests and formation waters from producing wells. This was demonstrated using a data set comprising 525 formation waters from drillstem tests and 107 formation waters from producing wells. Both a cross-plot of Mn and Fe in the two sets of formation waters and box plots of the same data sets showed that formation waters from producing wells have dominantly Fe>Mn, compared with those from drillstem tests which are characterized by Mn>Fe. Suspecting that “rust” contamination from well casing and ancillary equipment was the cause, the pH values of the samples were compared to see if the two data sets also differed in pH. It was demonstrated that not only are formation waters from drillstem tests less acidic than those from producing wells, but there is a systematic statistical trend of increasing acidity with age of the strata (temperature, depth, and increased amounts of the acid gases — H₂S and CO₂). The difference between the pH of formation waters from drillstem tests and producing wells is attributed to the partial scrubbing of the acid gases from the fluids produced during the drillstem test; this results in less acidic formation waters. Vanadium may also be enhanced in formation waters from producing wells. This note reports these differences and cautions too much reliance on values for Fe in waters from producing wells.     

“Traditional” women, “modern” water: Linking gender and commodification in Rajasthan, India

In this paper, I analyze the connections made between women and water in a Rajasthani drinking water supply project as a significant part of drinking water’s commodification. For development policy makers, water progressing from something free to something valued by price is inevitable when moving economies toward modernity and development. My findings indicate that water is not commodified simply by charging money for it, but through a series of discourses and acts that link it to other “modern” objects and give it value. One of these objects is “women”. I argue that through women’s participation activities that link gender and modernity to new responsibilities and increased mobility for village women involving the clean water supply, a “traditional” Rajasthani woman becomes “modern”. Water, in parallel, becomes “new”, “improved” and worth paying for. Women and water resources are further connected through project staff’s efforts to promote latrines by targeting women as their primary users. The research shows that villagers applied their own meanings to latrines, some of which precluded women using them. This paper fills a gap in feminist political ecology, which often overlooks how gender is created through natural resource interventions, by concerning itself with how new meanings of “water” and “women” are mutually constructed through struggles over water use and its commodification. It contributes to critical development geography literatures by demonstrating that women’s participation approaches to natural resource development act as both constraints and opportunities for village constituents. It examines an under-explored area of gender and water research by tracing village-level struggles over meanings of latrines.     

The “Red Rock Fault”, Cheshire: A critical review

The credentials of the so-called Red Rock Fault, along the eastern border of the Cheshire Plain, and at the boundary between the Trias or Permo-Trias on the west, and the Carboniferous rocks on the east, are examined from the published records. The conclusion reached is that the boundary probably represents an unconformity, not a fault. It may be taken to be a test case as to the authenticity of other well known and long established ‘faults’ in the structure of Britain.     

The origin of garnet and clinopyroxene in “depleted” Kaapvaal peridotites

A detailed petrographic, major and trace element and isotope (Re–Os) study is presented on 18 xenoliths from Northern Lesotho kimberlites. The samples represent typical coarse, low-temperature garnet and spinel peridotites and span a P–T range from 60 to 150 km depth. With the exception of one sample (that belongs to the ilmenite–rutile–phlogopite–sulphide suite (IRPS) suite first described by [B. Harte, P.A. Winterburn, J.J. Gurney, Metasomatic and enrichment phenomena in garnet peridotite facies mantle xenoliths from the Matsoku kimberlite pipe, Lesotho. In: Menzies, M. (Ed.), Mantle metsasomatism. Academic Press, London 1987, 145–220.]), all samples considered here have high Mg# and show strong depletion in CaO and Al₂O₃. They have bulk rock Re depletion ages (T_RD) >2.5 Ga and are therefore interpreted as residua from large volume melting in the Archaean. A characteristic of Kaapvaal xenoliths, however, is their high SiO₂ concentrations, and hence, modal orthopyroxene contents that are inconsistent with a simple residual origin of these samples. Moreover, trace element signatures show strong overall incompatible element enrichment and REE disequilibrium between garnet and clinopyroxene. Textural and subtle major element disequilibria were also observed. We therefore conclude that garnet and clinopyroxene are not co-genetic and suggest that (most) clinopyroxene in the Archaean Kaapvaal peridotite xenoliths is of metasomatic origin and crystallized relatively recently, possibly from a melt precursory to the kimberlite.

Possible explanations for the origin of garnet are exsolution from a high-temperature, Al- and Ca-rich orthopyroxene (indicating primary melt extraction at shallow levels) or a majorite phase (primary melting at >6 GPa). Mass balance calculations, however, show that not all garnet observed in the samples today is of a simple exsolution origin. The extreme LREE enrichment (sigmoidal REE pattern in all garnet cores) is also inconsistent with exsolution from a residual orthopyroxene. Therefore, extensive metasomatism and probably re-crystallization of the lithosphere after melt-depletion and garnet exsolution is required to obtain the present textural and compositional features of the xenoliths. The metasomatic agent that modified or perhaps even precipitated garnet was a highly fractionated melt or fluid that might have been derived from the asthenosphere or from recycled oceanic crust. Since, to date, partitioning of trace elements between orthopyroxene and garnet/clinopyroxene is poorly constrained, it was impossible to assess if orthopyroxene is in chemical equilibrium with garnet or clinopyroxene. Therefore, further trace element and isotopic studies are required to constrain the timing of garnet introduction/modification and its possible link with the SiO₂ enrichment of the Kaapvaal lithosphere.     

A “T-Z” approach for cyclic axial loading analysis of single piles

A simple and efficient numerical approach is presented for the cyclic axial loading analysis of vertical single piles embedded in a layered soil profile. The soil medium along the embedded pile is represented by simple “t–z” curves which define the shear stres-vertical displacement response of the soil at each particular depth. A hyperbolic “t-z” representation of the soil medium presented by Chin and Poulos, which caters for the case of a two-layered and “Gibson” soil profiles, is utilised. Under cyclic loading conditions, the well-known Masing's criteria governing the unloading and reloading responses are incorporated into the “t-z” curves. The cyclic loading effects of pile capacity degradation and accumulation of pile displacement are catered for in an approximate manner. Some numerical results are presented to show the important parameters affecting the cyclic response of single piles embedded in a layered soil. Finally, a comparison with field measurements of a cyclic pile load test shows general agreement between numerical and field results.     

The physics of “falling curtain” dry magnetic separation

The separation process in a “falling curtain” of particles is investigated in detail. Each particle experiences gravity, air drag and magnetic forces, as well as collisions with other particles. A simple kinetic theory model is used to model these processes and the separation distance between two different minerals is calculated. The maximum throughput of a superconducting magnet can be as high as 10 tons/hour per meter of effective magnet length, with a recovery in excess of 90%.     

The indigenous origin of Witwatersrand “carbon”

In the Witwatersrand approximately 40% of the gold is intimately associated with so-called “carbon” in “carbon seam reefs”, which occur in over a dozen paleoplacers, many of them concentrated at two stratigraphic levels in the 7000-m-thick succession of Archean siliciclastic sedimentary rocks. This is reduced carbon, present as kerogen admixed in various proportions with derivative (now solid) bitumen(s). Oil generation and migration were active geological processes during Early Earth history. Numerous possible source rocks for oil generation, including the carbon seams themselves, occur within the Witwatersrand basin. In the Witwatersrand ore, oil-bearing fluid inclusions are also present, derived like the bitumen, by thermal maturation of the kerogen. The presence of kerogen and bitumen in the Witwatersrand sedimentary rocks, together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover, as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested. Paleochannels, which truncated auriferous carbon seams early in the depositional history, are of widespread occurrence, and micro-synsedimentary faults offset carbon seams. The carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development. The various features highlighting the nature and spatial distribution of Witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes.     

Mesozoic “Red Beds” and its Evolution in the Hefei Basin

The Hefei Basin is the largest basin in the North China landmass with complete and well-preserved Mesozoic and Cenozoic strata. In the basin there developed a suite of extremely thick “red beds” in the Mesozoic. Owing to complex evolution processes and a lack of paleontological traces, there have been controversies regarding the division and correlation of this suite of red beds. Based on results obtained in recent years in drilling, seismic and surface geological investigations and in consideration of relationships between seismic sequences and regional tectonic events, as well as evidence in paleontology, petrology and isotopic dating, this paper preliminarily puts forward the following ideas about the sequence stratigraphic framework of the continental “red beds” in the Hefei Basin. (1) The Zhougongshan Formation and the Yuantongshan Formation have similar lithologic, geophysical and paleontological characteristics, so we incorporate them into a single formation, called the Yuantongshan Formation, and the original Zhougongshan and Yuantongshan Formations are regarded as the upper and the lower parts of the newly defined Yuantongshan Formation. Its age is the Middle Jurassic; (2) the Zhuxiang Formation belongs to the Upper Jurassic Series and (3) the age of the Xiangdaopu Formation is the Lower Cretaceous. Furthermore, signatures of depositional evolution are analyzed in the paper based on features of seismic reflection, outcrops and drilling data. The Early and Middle Jurassic is characterized by a foreland basin, which is influenced mainly by uplift and longitudinal compression of the Dabieshan Mountains; the Lower Jurassic System has a relatively small depositional area; the Middle Jurassic strata are distributed extensively over the whole basin, marking the summit of basin development; a flexure basin is characteristic of the Late Jurassic, manifesting a joint effect of the Dabieshan and Zhangbaling Mountains with the former being more significant. In the Early Cretaceous, the Xiangdaopu Formation was distributed in the Daqiao depression, evidently affected by extension of the Tanlu fault; in the Late Cretaceous, the Hefei Basin was subjected to dismembering and the Zhangqiao Formation was distributed in the east-west direction along the downthrown side of the fault.     

More about Diring Yuriakh: Unsolved geoarchaeological problems at a “lower” paleolithic site in central Siberia

The geoarchaeology of Diring Yuriakh, perhaps the oldest firmly documented archaeological site in Siberia, at more than 260,000 years old, is important for modeling the peopling of Northern Asia and North America. This article focuses on some important details about the stratigraphy of the Diring site, which have previously only been published in Russian. There are two major geoarchaeological problems at Diring. First, the stratigraphic position of the layer containing the supposed pebble tools (relative to Middle Pleistocene sediments at the site) is not well defined. Second, there remains doubt as to the nature of the association between the majority of the Diring artifacts and the sediments dated to between ca. 267,000 and 366,000 years ago. Until these problems are resolved, the proposed Lower Paleolithic dating for the Diring site must be considered as provisional. © 1999 John Wiley & Sons, Inc.     

The geoarchaeology of Iron Age “moated” sites of the Upper Mae Nam Mun Valley,N.E. Thailand. I: Palaeodrainage,site–landscape relationships and the origins of the “moats”

The study focuses on the “moated” Iron Age sites of N.E. Thailand, first identified as significant prehistoric settlement sites in the 1940s from aerial photography. Two more recent photograph sets are used to map the surficial geology and prehistoric site distribution for a study area west of Phimai, N.E. Thailand, with a focus on site–landscape relationships and, in particular, relationships between site location and form and patterns of palaeodrainage. The derived record of the surficial geology reflects several phases of palaeodrainage, characterized by differing locations and types of former river channels. Of note is the differentiation between a recent period in which river channels, including those presently active, are single-string meandering channels, and an older period of broad belts of meandering multistring channels. The prehistoric site distribution correlates closely with the older drainage, and for many, the encircling channels (the “moats”) are closely associated with former river channels. These relationships provide a critical and novel model for site distribution; several implications arise, supported by emerging field evidence, and introducing issues for archaeological debate: (i) there is no need, as has been done in the past, to invoke prehistoric artificial forms of drainage associated with the sites; (ii) the definition of the encircling channels as “moats” is seriously called into question; and (iii) the inferred geomorphological evolution of the floodplain implies past changes in environmental parameters such as run-off, climate or biophysical environments. Since the sites are all located in or beside ancient meander belts, these parameters should now be introduced into archaeological discussions regarding the establishment, history, evolution, and abandonment of the Iron Age sites. Methodologically, this article illustrates the need to be aware of the complexity of aerial photograph interpretation in archaeological survey, showing that careful analysis of aerial photograph information may have a significant impact upon the modeling of prehistoric interpretations. Further stratigraphical studies will be reported subsequently, and will refine the models presented here. © 1999 John Wiley & Sons, Inc.     

“Normal” and “inverted” metamorphic isograds and their relation to syntectonic proterozoic batholiths in the Wopmay Orogen, Northwest Territories, Canada

Three progressive metamorphic suites are developed in pelitic rocks of the northern Wopmay Orogen. Two suites are related to the Hepburn Batholith and one to the Wentzel Batholith. All three suites are cut by post-metamorphic wrench faults, some of which have significant vertical displacement. The structural relief so provided reveals that medium-and high-grade isograds associated with the Hepburn Batholith dip inward towards the batholith and are thus “hot-side-up”. Isograds associated with the Wentzel Batholith dip away from the batholith and are thus “hot-side-down”. It is concluded that Hepburn Batholith has the form of the flattened funnel fed from depth, and that Wentzel Batholith is the arched roof of an intrusive complex of unknown shape at depth.     

Response to Thrift's “The future of geography”

Reviews and observations about the status of the discipline of geography, no matter how positive, invariably raise programmatic concerns. These concerns have a long history that arise from geography's struggles to find an identity that embraces its many parts and yet are consistent with the logic by which the academy partitions knowledge. Pedagogy and research historically claimed by geography is currently being reinvented and relabeled under such headings as “integrated environmental science” and “spatial science”, and these developments have the potential to change the breadth of the “geographic imagination”. Several observations about dominant explanatory perspectives and substantive domains of geographic enquiry are also provided.     

Biogeochemical controls on authigenic carbonate formation at the Chapopote “asphalt volcano”, Bay of Campeche

The Campeche Knolls in the Bay of Campeche, southern Gulf of Mexico, were investigated through detailed seafloor mapping, ROV surveys, and sediment and pore water sampling. The knolls are elongated, submarine hills created by salt tectonics with a positive relief of up to 800 m above the surrounding seafloor. Several of the knolls are associated with sea-surface oil slicks identified from satellite data, indicating the presence of hydrocarbon seeps on the seafloor. One of the knolls, named “Chapopote”, was studied in detail during two international research expeditions (SO174/2 and M67/2) and is characterized by extensive hydrocarbon seepage including large asphalt flows, oil and gas seeps, and seafloor gas hydrate deposits. Chemosymbiotic biological communities and authigenic carbonate deposits are associated with the seeps and are the result of both biogeochemical turnover and the interaction between downward-diffusing seawater and hydrocarbon-rich pore fluids at shallow sediment depth. Authigenic carbonates are characterized by aragonite, exhibit a porous texture, and are cemented by a matrix of microsparitic to sparitic aragonite. Macropores of the carbonates were completely filled with liquid oil. Carbonate microfabrics include peloidal or clotted fabrics that may indicate the existence of microenvironments resulting from microbial metabolism. Banded/botryoidal aragonite cements line the intra- and bioclasts and incompletely fill the pore spaces. The stable carbon isotopic composition of authigenic aragonite varies between − 28.6‰ and − 17.9‰ (PDB), identifying oil oxidation as the primary source of carbon to the DIC pool, while lipid biomarker data demonstrate the concurrent existence of microbial communities responsible for anaerobic oxidation of methane (AOM). These observations indicate the presence of additional, AOM-independent reactions responsible for carbon sequestration at hydrocarbon seeps and demonstrate the complexity of biogeochemical processes at seep sites in the Gulf of Mexico basin. Oxygen isotope data of authigenic aragonite vary from + 2.5‰ to + 3.8‰ (PDB), indicating carbonate precipitation in slight disequilibrium with the surrounding pore fluids.     

Paragenesis of “box-work geodes”, Tampa Bay, Florida

An unusual suite of silicified rocks was excavated during a recent harbour-deepening project in Tampa Bay, Florida. These rocks, which we have termed “box-work geodes”, are composed of convoluted, intersecting silica walls enclosing cavities which are either voids or filled with relatively pure monoclinic palygorskite. The “box-work geodes” are interpreted as having formed in shallow lagoonal environments, similar to the Coorong Lagoon of South Australia. Synaeresis of syngenetic palygorskite was followed by opal deposition and case hardening of the material. Subsequent chemical deposition of chalcedony, megacrystalline quartz, barite, and calcite on the void facing walls indicates an open chemical system.

The existence of opal saturated lagoons, as inferred from the “box-work geodes”, suggests that much of the replacement chert, porcelanite, and silicified fossils in the Tertiary deposits of peninsular Florida formed in the shallow subsurface. Subsequent weathering of carbonates and clays not encapsulated in the box works has resulted in formation of a green montmorillonite residual clay bed.     

THE SURF “BREAK”: KEY TO PALEOGEOGRAPHY?

Ancient geosynclinal coastal deposits, preserved and exposed for present-day examination, probably represent interior—and hence protected—shores, and were therefore characterized by low-to-moderate breaker energy levels, as is the modern Gulf of Mexico. Modern low-energy beach sand size distributions are distinguished by an inflection, here called the surf “break”, formed by a winnowing or statistical filtering process, by the breakers, in the finer sizes. The result is a two-segment curve having an intersection in many cases not far from 1.5 phi. If the energy level is very low, a tail of fines may be added, producing a three-segment curve having two inflection points. Along high energy beaches, the surf “break” occurs at such large grain sizes that it cannot be preserved in sand; whether or not it can be recognized in gravel is not known. Ancient sandstones which represent the breaker zone, along a low-to-moderate energy coast, should also exhibit the surf “break”. The Lyons Sandstone, a known Permian beach deposit, provides an example. Other examples are cited. Any useful indicator of the shoreline position, in lithified rocks, should have three characteristics: ease of recognition, high reliability, and strong probability of preservation. The surf “break” scores reasonably well on all three points. Hence it may be an important key to paleogeography. Nevertheless, there still remains a question inasmuch as certain river sands have similar inflections. Whether these kinks were inherited from near-by, but older, beach deposits, or were formed by stream action, is not known. The reliability of the surf “break” is estimated at 80% or better, except for the case where near-shore sands, of whatever age, are reworked briefly by running water.     

“Lag” — a geochemical sampling medium for arid regions

Desert land surfaces are commonly characterized by a veneer or pavement of siliceous and/or ferruginous stony material. This material can be shown in most cases to be predominantly bedrock-derived, despite often severe modification by prolonged weathering, and can therefore be selectively sampled and analyzed as an indicator of bedrock geochemistry in geochemical surveys.Most pavements probably owe their existence to the interaction of several dispersion mechanisms. However, a common factor in all cases is the concentration of coarse particles at the surface as a result of the selective removal by erosion of fine dilutant material. Hence, the preferred name when used in the geochemical context is “lag”.In lag sampling, particles in the range 2.0–6.0 mm are screened on site from the unconsolidated surface material. Material in this size range has been found to be quite uniformly distributed over a wide range of arid region environments, including areas where residual soils are severely diluted by transported alluvial and aeolian materials.Data from exploration programmes for Au, Cu-Pb-Zn-Ba, and Ni allow comparison of results for lag sampling with those for alternative sample media in a variety of arid region environments. Analysis of lag samples for Au, Cu and As clearly indicates the presence of bedrock Au mineralization in the Paterson and Eastern Goldfields Provinces of Western Australia. In these areas both lags and soils exhibit good anomaly contrast, but lags show more extensive lateral dispersion, leading to advantages in reconnaissance exploration.Strong anomalies for Ni and Cu are developed in lags, compared with subdued response in fine-fraction soils over a Ni sulphide occurrence in the Eastern Goldfields which has been subjected to deep lateritic weathering.Lag geochemistry also clearly reflects sub-economic base metal and barite occurrences in the McArthur Basin, N.T., in spite of the dilution of surface soils by sands probably related to a Mesozoic marine incursion. Orientation sampling over a Pb-Zn prospect in the Pine Creek Geosyncline has demonstrated optimum response in lag samples compared with various size fractions of the associated lithosols.Variable dilution of lag samples by coarse quartz sand can be a problem in areas with substantial transported overburden. A simple procedure to ‘correct’ trace-element values using regression analysis based on the Fe content of samples is described as a means of reducing ‘noise’ resulting from such matrix variations.