Beryllium-10 in Australasian tektites: Constraints on the location of the source crater

By using accelerator mass spectrometry we have measured the ¹⁰Be concentrations of 86 Australasian tektites. Corrected to the time of tektite production ∼0.8 My ago, the ¹⁰Be concentrations (10⁶ atom/g) range from 59 for a layered tektite from Huai Sai, Thailand, to 280 for an australite from New South Wales, Australia. The average value is 143 ± 50. When tektites are sorted by country, their average measured ¹⁰Be concentrations increase slowly with increasing distance from Southeast Asia, the probable location of the tektite producing event, from 59 ± 9 for 6 layered tektites from Laos to 136 ± 20 for 20 splash-form tektites from Australia. The lowest ¹⁰Be concentrations for tektites fall on or within a contour centered off the shore of Vietnam, south of the Gulf of Tonkin (107°E; 17°N), but also encompassing two other locations in the area of northeastern Thailand previously proposed for the site of a single tektite-producing impact. The ¹⁰Be concentrations of layered tektites show only a weak anticorrelation (R ∼ −0.3) with the numbers of relict crystalline inclusions.Loosely consolidated, fine-grained terrestrial sediments or recently consolidated sedimentary rocks are the most likely precursor materials. Dilution of sediments with other kinds of rock raises problems in mixing and is not supported by petrographic data. Sedimentary columns that have the right range of ¹⁰Be concentrations occur off the coasts of places where sedimentation rates are high relative to those in the deep sea. A single impact into such a region, 15 to 300 m thick, could have propelled near-surface, high-¹⁰Be material farthest—to Australia—while keeping the deeper-lying, low-¹⁰Be layers closer to home. We do not rule out, however, other proposed mechanisms for tektite formation.     

Soil moisture potential and water content in the unsaturated zone within the arid Ejina Oasis in Northwest China

Three soil profiles were selected in the Ejina Oasis, northwest China, to determine water content profiles and evolution of soil moisture potentials in the unsaturated zone within the arid area. The total soil moisture potentials have been monitored for about 3 months in 2001 at different depths in the soil profiles. The occurrence and movement of water in the unsaturated zone was analyzed using the zero flux plane (ZFP) method. It is shown that convergent ZFPs and divergent ZFPs may occur at depths between 0.5 and 3.0 m, and that the depth of the ZFPs was controlled by the root zone of plants growing on the land surface. Profiles of the total soil moisture potentials were observed to be coincident with those of the water contents at the three experimental sites. The total soil moisture potential showed a slight increasing trend and the ZFPs tend to vanish from summer to winter as the water extraction by roots decreased. Evapotranspiration through vegetation has an important bearing on the water content and the total potential in the unsaturated zone.     

Inferring the mass fraction of floc-deposited mud: application to fine-grained turbidites

Fine sediment deposition in the ocean is complicated by the cohesive nature of muds and their tendency to flocculate. The result is disaggregated inorganic grain size (DIGS) distributions of bottom sediment that are influenced by single‐grain and floc deposition. This study outlines a parametric model that characterizes bottom sediment DIGS distributions. Modelled parameters are then used to infer depositional conditions that account for the regional variation in the grain sizes deposited by turbidity currents on the Laurentian Fan–Sohm Abyssal Plain, offshore south‐eastern Canada. Results indicate that, on the channellized Laurentian Fan, the mass fraction of floc‐deposited mud increases only slightly downslope. The small evolution in this fraction arises because sediment concentration and turbulent energy are associated in turbidity currents. On the Sohm Abyssal Plain, however, the mass fraction of floc‐deposited mud decreases, probably as a result of lower sediment concentration at this source‐distal site. Estimates of the mass fraction of mud deposited as flocs suggest that floc deposition is the dominant mode by which sediment is lost from suspension, although single‐grain deposition contributes more to the depositional flux in proximal areas where high energy breaks flocs and in distal areas where low sediment concentration limits floc formation. It is concluded that, throughout the dispersal system, changes in the fraction of flocculated mud deposited from turbidity currents reflect changes in sediment concentration and energy downslope.     

Are isolated stable rigid clasts in shear zones equivalent to voids?

Particles in shear enclose important information about a rock's past and can potentially be used to decipher the kinematic history and mechanical behavior of a certain outcrop or region. Isolated rigid clasts in shear zones often exhibit systematic inclinations with respect to the shear-plane at small angles, tending towards the instantaneous stretching direction of the shear zone. This shape preferred orientation cannot be easily explained by any of the analytical theories used in geology. It was recently recognized that a weak mantle surrounding the clast or a slipping clast–matrix interface might be responsible for the development of the observed inclinations. Physical considerations lead us to conjecture that such mantled, rigid clasts can be effectively treated as voids that are not allowed to change their shape. The resulting equivalent void conjecture agrees well with numerical and field data and has the following important geological implications. (i) Clasts in shear zones can have stable positions in simple shear without the requirement of an additional pure shear component. (ii) The stable orientation can be approached either syn- or antithetically; hence, the clast can rotate against the applied shear sense. (iii) The strain needed to develop a strong shape preferred orientation is small (γ≈1) and therefore evaluations based on other theories may overestimate strain by orders of magnitude. (iv) The reconstruction of far-field shear flow conditions and kinematic vorticity analysis must be modified to incorporate these new findings.     

Middle Campanian–lowermost Maastrichtian nannofossil and foraminiferal biostratigraphy of the northwestern Australian margin

The middle–late Campanian was marked by an increase in the bioprovinciality of calcareous microfossil assemblages into distinct Tethyan, Transitional, and Austral Provinces that persisted to the end of the Maastrichtian. The northwestern Australian margin belonged to the Transitional Province and the absence of key Tethyan marker species such as Radotruncana calcarata and Gansserina gansseri has led petroleum companies operating in the area to use the locally developed KCCM integrated calcareous microfossil zonation scheme. The KCCM zonation is a composite scheme comprising calcareous nannofossil (KCN), planktonic foraminiferal (KPF) and benthonic foraminiferal (KBF) zones. This paper presents the definitions and revisions of Zones KCCM8–19, from the highest occurrence (HO) of Aspidolithus parcus constrictus to the lowest occurrence (LO) of Ceratolithoides aculeus, and builds on our previous early–late Maastrichtian study. The presence of a middle–upper Campanian disconformity is confirmed by microfossil evidence from the Vulcan Sub-basin, Exmouth and Wombat plateaus, and the Southern Carnarvon Platform. In the Vulcan Sub-basin and on the Exmouth Plateau (ODP Hole 762C) the hiatus extends from slightly above the LO of common Rugoglobigerina rugosa to above the LO of Quadrum gothicum. On the Wombat Plateau (ODP Hole 761B) it spans from above the LO of Heterohelix semicostata to above the LO of Quadrum gothicum; and in the Southern Carnarvon Platform the disconformity has its longest duration from above the HO of Heterohelix semicostata to above the LO of Quadrum sissinghii. A significant revision of the events which define Zones KCCM18 and 19 was necessary owing to the observation that the LO of Ceratolithoides aculeus occurs below the HOs of Archaeoglobigerina cretacea and Stensioeina granulata incondita and the LO of common Rugoglobigerina rugosa. In the original zonation these events were considered to be coincident.     

The Gondwanan Carboniferous-Permian Boundary Revisited: New Data from Australia and Argentina

The identification and correlation of the Carboniferous-Permian (Gzhelian-Asselian) boundary within the sedimentary sequences of Gondwana has always been a topic of debate. Type latest Carboniferous and earliest Permian marine sequences are characterised by warm tropical faunas and come from the Uralian Region of Russia and Kazakhstan. Faunas include conodonts and fusulinid foraminiferids which are prime tools for correlation. Such faunal groups are absent from most Gondwanan sequences where reliance for correlations must be placed primarily on brachiopods, bivalve molluscs and palynology. The Western Australian marine sequences, with their contained ammonoids, provide a pivotal link for the dating and correlating of Early Permian Gondwanan sequences with those of the type regions and their palynostratigraphical record is essential for trans-Australian correlations and correlations elsewhere throughout Gondwanaland.New data from the fully cored DM Tangorin DDHl bore hole, drilled in the Cranky Corner Basin, New South Wales, Australia, reveals a sequence of descending faunal zones. The stratigraphically highest zone with Eurydesrna cordaturn, encompasses the Late Sakmarian (Sterlitamakian). The middle zone with Torniopsis elongata, Sulciplica c r a m and Trigonotreta tangorini straddles the Sterlitamakian-Tastubian boundary, with the palynomorphs Pseudoreticulatispora pseudoreticulata high in the zone and Granulatisporites confluens low in the zone. An impoverished fauna with Trigonotreta nov., low in the Granulatisporites confluens Zone, is probably of latest Asselian or Tastubian age.Significant new data from Argentina has revealed marine faunas from below the occurrence of Granulatisporites confluens. These are considered to be of Asselian age. Outcrops of the Tupe Formation, with a marine fauna, at La Herradura Creek in the western Paganzo Basin, San Juan Province, are best regarded as being of mid to late Asselian age. The Tupe Fauna has been recognised as the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone. Previously, this fauna was considered to be of Late Carboniferous or Stephanian age. Three faunal associations are known from the Rio del Peii6n Formation, Rio Blanco Basin, La Rioja Province. The middle assemblage with Tivertonia, Costaturnulus, Kochiproductus and Trigonotreta, appears to correlate well with the Tupe Formation fauna. The lower assemblage, with Streptorhynchus, Etherilosia, Costaturnulus, Trigonotreta and a punctate spiriferid, as well as indeterminate productids, probably of Early Asselian age. The youngest assemblage includes a species ofRhynchopora that is close to Rhynchopora australasica from the latest Asselian-early Tastubian of Western Australia. The marine biostratigraphical data from Argentina has enabled a much greater understanding of the earliest Permian marine faunas to be achieved - a story that is apparently absent from the other cold and cool temperate regions of Gondwana.     

Fitting the Linear Model of Coregionalization by Generalized Least Squares

In geostatistical studies, the fitting of the linear model of coregionalization (LMC) to direct and cross experimental semivariograms is usually performed with a weighted least-squares (WLS) procedure based on the number of pairs of observations at each lag. So far, no study has investigated the efficiency of other least-squares procedures, such as ordinary least squares (OLS), generalized least squares (GLS), and WLS with other weighing functions, in the context of the LMC. In this article, we compare the statistical properties of the sill estimators obtained with eight least-squares procedures for fitting the LMC: OLS, four WLS, and three GLS. The WLS procedures are based on approximations of the variance of semivariogram estimates at each distance lag. The GLS procedures use a variance–covariance matrix of semivariogram estimates that is (i) estimated using the fourth-order moments with sill estimates (GLS₁), (ii) calculated using the fourth-order moments with the theoretical sills (GLS₂), and (iii) based on an approximation using the correlation between semivariogram estimates in the case of spatial independence of the observations (GLS₃). The current algorithm for fitting the LMC by WLS while ensuring the positive semidefiniteness of sill matrix estimates is modified to include any least-squares procedure. A Monte Carlo study is performed for 16 scenarios corresponding to different combinations of the number of variables, number of spatial structures, values of ranges, and scale dependence of the correlations among variables. Simulation results show that the mean square error is accounted for mostly by the variance of the sill estimators instead of their squared bias. Overall, the estimated GLS₁ and theoretical GLS₂ are the most efficient, followed by the WLS procedure that is based on the number of pairs of observations and the average distance at each lag. On that basis, GLS₁ can be recommended for future studies using the LMC.     

Ar–Ar age determinations of eclogitic clinopyroxene and garnet inclusions in diamonds from the Venetia and Orapa kimberlites

Ar–Ar age measurements are reported for selected eclogitic clinopyroxene and garnet inclusions in Orapa diamonds and clinopyroxene inclusions in Venetia diamonds. Laser drilling of encapsulated clinopyroxene inclusions within Venetia diamonds released a maximum of 3% of the total ⁴⁰Ar, indicating little diffusive transfer and storage of radiogenic ⁴⁰Ar at the diamond–inclusion boundary. Apparent ages obtained during stepped heating of three diamonds are consistent with diamond crystallisation occurring just prior to the kimberlite eruption 520 Ma ago. Stepped heating of three clinopyroxene-bearing Orapa diamonds gave ages of 906–1032 Ma, significantly above the eruption age, but consistent with previously determined isotopic ages. A few higher apparent ages hint at the presence an older generation of Orapa diamonds that formed >2500 Ma ago. Orapa garnets also contain measurable K contents, and record a range of ages between 1000 and 2500 Ma. The old apparent ages and lack of significant interface ⁴⁰Ar released by the laser probe, suggests that pre-eruption radiogenic ⁴⁰Ar and mantle-derived ⁴⁰Ar components are trapped in microinclusions within the pyroxene and garnet inclusions.     

Focal mechanism dependence of static stress triggering of earthquakes

We perform a global statistical investigation into the problem of Coulomb stress triggering of earthquakes by using the Harvard CMT catalogue of shallow earthquakes from 1976 to 2001. We consider ‘earthquake pairs’, i.e., pairs of successive earthquakes occurring near each other with similar focal mechanisms, and address the problem of whether or not the change of the Coulomb failure stress (CFS) produced by the first earthquake of the pair ‘encourages’ the occurrence of the second one. An interesting feature is that such a Coulomb stress-triggering phenomenon has an apparent focal mechanism dependence: for thrust earthquakes, a more evident triggering effect can be observed.