Orographic controls on rain water isotope distribution in the Mount Lofty Ranges of South Australia

Hydrogen and oxygen isotopes of water are common environmental tracers used to investigate hydrological processes, such as evaporation, vegetation water use, surface water–groundwater interaction, and groundwater recharge. The water isotope signature in surface water and groundwater evolves from the initial rain signature. In mountain terrain, rain water stable isotope composition spatially varies due to complex orographic precipitation processes. Many studies have examined the isotope–elevation relationships, while few have quantitatively investigate the terrain aspect and slope effect on rain isotope distribution. In this paper, we examine the orographic effects more completely, including elevation, terrain slope and aspect, on stable isotope distribution in the Mount Lofty Ranges (MLR) of South Australia, using a multivariate regression model. The regression of precipitation isotope composition suggests that orographic effects are the dominant controls on isotope spatial variability. About 75% of spatial variability in δ¹⁸O and deuterium excess is represented by the regression using solely orography-related variables (elevation, terrain aspect and slope), with about 25% of δ¹⁸O spatial variability attributed to the terrain aspect and slope effect. The lapse rate is about −0.25‰ for every 100 m at both windward and leeward slopes. However, at the same elevation, δ¹⁸O at the leeward slope (eastern MLR) is 0.5‰ larger than that at the windward slope. The difference can be explained by different mechanisms – continuous rain-out processes on the windward side and sub-cloud evaporation on the leeward side. Both δ¹⁸O and deuterium excess maps (1 km resolution) are constructed based on the regression results for the MLR. Both maps are consistent with groundwater of local precipitation origin, and useful to examine groundwater recharge.     

Use of radiometric (Cs-137, Pb-210), geomorphic,and stratigraphic techniques to date recent oxbow sediments in the rio puerco drainage Grants uranium region,New Mexico

In the absence of historic geochemical baseline data for the Grants uranium region, environmental changes resulting from uranium mine-mill activities can be determined only by indirect methods. A methodology for determining the age of recent sediments in streams draining the region has been established based on combined geomorphic, stratigraphic, and radiometric dating techniques. Because clayrich sediments retain possible radionuclides and heavy metals derived from mineralization and mined sources, sample sites which contain fine-grained deposits that both predate and postdate mine-mill activity were located in abandoned-channel segments (oxbows) of major streams draining the eastern Grants uranium region. Aerial photographs (and derivative maps) taken between 1935 and 1971 provided the historical and geomorphic documentation of approximate dates of oxbow formation and ages of alluvial fills in the abandoned-channel segments. Pits were dug at these oxbow sites to determine stratigraphy and composition of the deposits. Samples collected from pit walls and auger holes below the pits were subjected to radiometric analysis by gamma ray spectrometry for the artificial radionuclide Cs-137 and the natural radionuclide Pb-210 as well as other U-238 and Th-232 daughters. Because of the dynamic nature of the system, absolute dating with Cs-137 was not possible but samples could be dated as either pre-or post-1950. The 1950 date is important because it marked the beginning of the uranium exploitation in the region. The Pb-210 dating was not possible because background Pb-210 was very high relative to fallout Pb-210.     

Orientation of framboidal pyrite in shale

Ordered framboids from Devonian Chattanooga Shale (U.S.A.) and Rammelsberg Banderz (Germany) show a common orientation in any one sample. This observation was made using a stereographic method of reconstruction in which the lineations within the framboids were measured on each of three planes cut mutually at right angles from the rock samples. The explanation for this preferred orientation is not yet known, but it may originate from crystal-growth during compaction of the shales.

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Zusammenfassung Pyrit-Himbeeren aus dem devonischen Chattanooga Shale (U.S.A.) und vom Rammelsberg Banderz zeigen bevorzugte Orientierungen ihrer inneren Struktur (Figure I) gegenüber der Sedimentschichtung. Diese Beobachtungen sind in stereographischen Projektionen aufgezeichnet. Der Vorgang der Orientierung mag auf die Kristallisation der Pyrit-Netzwerke während der Diagenese zurückzuführen sein.

     

Analysis of wax hydrocarbons in petroleum source rocks from the Damintun depression, eastern China, using high temperature gas chromatography

A detailed organic geochemical study; utilising petrography, biomarker hydrocarbon analysis and high temperature GC analysis of extractable wax hydrocarbon constituents was performed on four marginally oil window-mature source rocks from the Shahejie Formation (Eocene), Damintun depression in eastern China. The main maceral components in the source rocks were vitrinite, liptinite and exinite, with vitrinite being more abundant (>50 vol.%) in organic-lean samples whose TOC contents were between 1 and 2 wt.%. Large differences in pristane/phytane ratios suggested that the organic-rich samples were deposited in a less oxic depositional environment than that for the organic-lean rocks. The distribution of extractable wax hydrocarbons, determined by high temperature GC, showed a marked difference between these two sample types. The organic-rich samples contained high molecular weight hydrocarbons (HMWHCs) dominated by macrocrystalline n-alkanes (n-C₂₃–n-C₃₇, typically maximising at n-C₂₉), while the organic-lean samples contained lower amounts of extractable wax hydrocarbons but were relatively rich in microcrystalline components (> n-C₃₅). In all source rocks (Es3 and Es4), a noticeable odd-over-even predominance (OEP) of n-alkane chain lengths (up to n-C₆₅) was evident, consistent with a direct biological origin for the long n-alkyl chains. They were most probably formed during diagenesis from decarboxylation of predominantly even-carbon-numbered aliphatic acids originating from higher plant or lacustrine algal sources and/or were directly biosynthesised in hydrocarbon form. At least two other homologous series of branched/cyclic HMWHCs were observed, one of which was confirmed as a series of branched alkanes (probably methyl-branched). The carbon number distribution patterns of HMWHCs may be primarily controlled by thermal maturity and biogenic source input as well as being influenced by diagenetic reactions governed by depositional environmental conditions, as shown previously [Carlson, R.M.K., Teerman, S.C., Moldowan, J.M., Jacobson, S.R., Chan, E.I., Dorrough, K.S., Seetoo, W.C., Mertani, B., 1993. High temperature gas chromatography of high wax oils. In: Indonesian Petroleum Association, 22nd Annual Convention Proceedings. Jakarta, Indonesian, pp. 483–507. Carlson, R.M.K., Jacobsen, S.R., Moldowan, J.M., Chan E.I., 1994. Potential application of high temperature gas chromatography to Middle Eastern petroleum exploration and production. In: Al-Husseini, M.I. (Ed.), Geo'94, Vol 1., Selected Middle East Papers from The Middle East Petroleum Geoscience Conference, 1994; Gulf PetroLink. Manama, Bahrain, pp. 258–267]. Our study indicates for the first time that Es3 source rocks as well as Es4 facies contain HMWHCs. The distributions of extractable wax hydrocarbons suggest that both Es4 and Es3 members may potentially serve as important parent source rocks for generating waxy petroleum in this region.     

Palaeoproterozoic terrane assembly in the Lewisian Gneiss Complex on the Scottish mainland,south of Gruinard Bay: SHRIMP U–Pb zircon evidence

New SHRIMP U–Pb zircon geochronology and fieldwork integrated with reappraisal of earlier mapping demonstrates that the so-called ‘southern region’ of the mainland Lewisian Gneiss Complex comprises a package of distinct tectono-stratigraphic units. From south to north these are the Rona (3135–2889 Ma), Ialltaig (c. 2000 Ma) and Gairloch (ca. 2200 Ma) terranes. These terranes were metamorphosed and deformed separately until ca. 1670 Ma by which time they had been juxtaposed and were integral with terranes to the north. The northern boundary of the Palaeoproterozoic Gairloch terrane is a shear zone, north of which is the Archaean Gruinard terrane with 2860–2800 Ma protoliths and ca. 2730 Ma granulite facies metamorphism. In contrast, south of the Gairloch terrane, the Archaean gneisses of the Rona terrane have older protolith ages, underwent an anatectic event at ca. 2950 Ma and show no evidence of 2730 Ma granulite facies metamorphism. In current structural interpretations the Gruinard terrane forms a structural klippe over the intervening Gairloch terrane. However, the Rona and Gruinard terranes cannot be equivalent on age grounds, and are interpreted as unrelated different entities. Contained within the southern margin of the Gairloch terrane is the Ialltaig terrane, shown here to comprise an exotic slice of granulite facies Palaeoproterozoic crust, rather than Archaean basement as previously thought. The ca. 1877 Ma granulite facies metamorphism of the Ialltaig terrane is the youngest event that is unique to a single terrane in the mainland Complex, making it an upper estimate for the timing of amalgamation with surrounding tectonic units. U–Pb titanite ages of 1670 ± 12 Ma and ca. 1660 Ma for low-strain zones at Diabaig are interpreted to be cooling through the titanite closure temperature after the amphibolite facies reworking of these southern terranes and the southern margin of the Gruinard Terrane. These new data have implications for the tectonic setting of the mainland in relation to the Outer Hebrides and in the wider evolution of the basement in the North Atlantic.     

South Australian earthquakes, 1980–92

The seismicity of South Australia over the period 1980–92 is presented as a follow‐up to earlier studies. The South Australian seismic network has undergone a significant expansion in the last decade, and with it an increase in the number and precision of located earthquakes. The distribution of recent seismic activity is similar to the historical pattern of earthquakes and the previous instrumental seismicity maps, all of which show the three main areas as being the Flinders‐Mt Lofty Ranges, Eyre Peninsula, and the southeast. The one notable exception in the recent study is the presence of earthquake activity in the Musgrave Block, a previously aseismic region. Intensity characteristics are reported for earthquakes that were sufficiently widely felt. Fault plane solutions for three Flinders Ranges earthquakes (previously unpublished) are also presented; the focal mechanisms are consistent with predominant northeast‐southwest compression. The seismic moment method was used to estimate the seismic risk for the major population centres in terms of probability of exceedance of seismic intensity within a given period. These estimates are based on the recurrence parameters and intensity attenuation function for the region. The results place Adelaide close to the AS2121 ‐ 1979 Earthquake Code Zone I/Zone 2 boundary.     

Origin of petroleum in the Neoproterozoic–Cambrian South Oman Salt Basin

The South Oman Salt Basin (SOSB) is host to the world’s oldest known commercial deposits. Most of the South Oman oils have been proven to be associated with the source rocks of the Neoproterozoic to Cambrian Huqf Supergroup, but the assignment of oils to specific Huqf intervals or facies has been hampered by the geochemical similarity of the organic matter across the entire Huqf sequence, possibly as a consequence of limited change in the local palaeoenvironment and biota over the time of its deposition. This study was conducted to establish improved correlations between organic-rich rock units and reservoir fluids in the SOSB through detailed molecular and isotopic analysis of the Huqf Supergroup, with special emphasis directed towards understanding the Ara carbonate stringer play.Unusual biomarkers, tentatively identified as A-norsteranes, show distinctive patterns among carbonate stringer oils and rocks different from those observed in Nafun sediments and Ara rocks from the Athel basin. These putative A-norsteranes form the basis for new oil-source correlations in the SOSB and provide for the first time geochemical evidence of a self-charging mechanism for the carbonate stringer play. The paucity of markers specific to the Nafun Group (Shuram, Buah and Masirah Bay formations) confounds attempts to quantify their respective contributions to Huqf oil accumulations. Nafun inputs can only be determined on the basis of subtle differences between Nafun and Ara biomarker ratios. The most useful geochemical characteristics delineating Nafun Group organic matter from Ara Group intra-salt source rocks included: low relative abundance of mid-chain monomethyl alkanes (X-compounds); low relative abundance of gammacerane, 28,30-dinorhopane, 25,28,30-trinorhopane and 2-methylhopanes; low C₂₂T/C₂₁T and high C₂₃T/C₂₄T cheilanthanes ratio values. Based on these parameters, molecular evidence for major contributions of liquid hydrocarbons from Nafun Group sediments (Shuram, Buah and Masirah Bay formations) is lacking. Our results suggest that the majority of SOSB hydrocarbon accumulations originate from within the Ara group, either from the carbonate stringers or from the package of sediments that comprises the Thuleilat, Athel Silicilyte and U shale formations. Subtle aspects of the composition of some carbonate stringer and post-salt Huqf oils could suggest some degree of sourcing from the Nafun rocks but stronger evidence is needed to confirm this.     

Radiometric observations of sea temperature at 2.65 GHZ over the chesapeake bay

The present work describes the various corrections necessary in order to deduce ocean surface temperature fromS-band microwave radiometer measurements and applies these results to a series of data obtained with a high absolute accuracy radiometer. Measurements made with a 2.65 GHz radiometer from an aircraft flown over the Chesapeake Bay area are presented and compared in detail with accurately obtained sea truth data. For the calm sea, it was found that the observed brightness temperature agreed well with that calculated from the known sea surface and atmospheric properties over a fairly wide range of surface salinity values (0.2 per mille to 25 per mille). For cases where the surface wind speeds are of the order of 7 to 15 knots, an excess brightness temperature was observed which is attributable to surface roughness and microscale surface disturbances. The excess brightness temperature dependence on wind speed was found to correlate to a certain extent with the rms wave slope dependence on wind speed.     

A Shack–Hartmann wavefront sensor projected on to the sky with reduced focal anisoplanatism

A method for producing a laser guide star wavefront sensor for adaptive optics with reduced focal anisoplanatism is presented. A theoretical analysis and numerical simulations have been carried out and the results are presented. The technique, named Sky-Projected Laser Array Shack–Hartmann (SPLASH), is shown to suffer considerably less from focal anisoplanatism than a conventional laser guide star system. The method is potentially suitable for large telescope apertures (∼8 m), and possibly for extremely large telescopes.