Abstract— After the impact that formed Haughton crater, 22.4 ± 1.4 Ma ago (early Miocene), the cavity filled with water and began to accumulate lacustrine sediments. These preserve detailed evidence of pre-impact stratigraphy and post-impact morphology and development of the crater, as well as of the climatic and biotic regime in which it lay. In this report we formally designate these sediments as the Haughton Formation, of which only a 48 m thick remnant covering approximately 7 km2 still exists. Dolomite-rich, poorly-sorted silt, fine sand, and mud are the principal lithologies. The formation unconformably overlies a blanket of allochthonous impact breccia forming the floor of the original crater. Presence of a debris-flow deposit in the base of the sequence indicates that lacustine deposition began very shortly after crater formation. The Haughton Formation contains a moderately diverse and highly endemic vertebrate fauna as well as palynomorphs and plant macrofossils that indicate a cool-temperate climatic regime. A small percentage of reworked Late Cretaceous and early Tertiary palynomorphs point to the former existence of the Eureka Sound Formation in the drainage area of the crater. In addition, the distribution of the lake beds indicates the absence of an inner ring on the west side of the crater, and the 3° to 3.5° inward dip of Haughton strata implies that the central mass has subsided approximately 300 to 350 m since deposition began. 相似文献
Hydrothermal sulfide–oxide–gold mineral assemblages in gold deposits in the Archaean St. Ives gold camp in Western Australia
indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time. Reduced
alteration assemblages (pyrrhotite–pyrite) occur in deposits in the southwest of the camp (e.g., Argo, Junction deposits)
and moderately to strongly oxidized assemblages (magnetite–pyrite, hematite–pyrite) occur in deposits in the Central Corridor
in the northeast (e.g., North Orchin, Revenge deposits). Reduced mineral assemblages flank the Central Corridor of oxidized
deposits and, locally, cut across it along E–W trending faults. Oxidized mineral assemblages in the Central Corridor are focused
on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1,000 m below present surface.
Hydrothermal magnetite predates and is synchronous with early phases of gold-associated albite–carbonate–pyrite–biotite–chlorite
hydrothermal alteration. Later-stage, gold-associated pyrite is in equilibrium with hematite. The spatial distribution and
temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly
synchronous hydrothermal fluids with contrasting redox states. Sulfur isotope constraints support the argument that the different
mineral assemblages reflect differences in redox conditions. The δ34S values for pyrite for the St. Ives gold camp range between −8.4‰ and +5.1‰ with the negative values occurring in oxidized
magnetite-rich domains and slightly negative or positive values occurring in reduced, pyrrhotitic domains. Preliminary spatial
and paragenetic analysis of the distribution of iron sulfides and oxides in the St. Ives camp suggests that gold grades are
highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite–pyrite
to relatively oxidized magnetite–pyrite and hematite–pyrite both in space and time. Gold deposition is inferred to have occurred
where fluids of contrasting redox state mixed. 相似文献
Habitat Suitability (HS) models have been extensively used by conservation planners to estimate the spatial distribution of threatened species and of species of commercial interest. In this work we compare three HS models for the estimation of commercial yield potential and the identification of suitable sites for Tapes philippinarum rearing in the Sacca di Goro lagoon (Italy) on the basis of six environmental factors. The habitat suitability index (HSI) is based on expert opinion while the habitat suitability conditional (HSC) is calibrated on observational data. The habitat suitability mixed (HSM) model is a two-part model combining expert knowledge and regression analysis: the first component of the model uses logistic regression to identify the areas in which clams are likely to be present; the second part applies the same parameter-specific suitability functions of the HSI model only in the areas previously identified as productive by the logistic component.The HS models were validated on an independent data set and estimates of potential yield of the Goro lagoon were compared. The effectiveness of the three approaches is then discussed in terms of predicted yield and identification of suitable sites for farming. 相似文献
We have carried out stratigraphic mapping in the Upper Miocene basalt lava pile around the fjords Mjoifjördur and Seydisfjördur, eastern Iceland. The mapping is based on conventional methods including the use of interbasaltic clastic horizons and petrographically distinct lava groups. These units are also used to provide correlations with the Nordfjördur area south of Mjoifjördur. We present a 3-km composite stratigraphic column for the area between Mjoifjördur and Seydisfjördur. The geology of this area shows some differences from the classical model of Walker for the structure of eastern Iceland partly due to the fact that most of Mjoifjördur is not in the vicinity of central volcanoes. Detailed laboratory measurements of remanent magnetization were carried out on oriented core samples from 363 lavas in 10 selected profiles. The local paleomagnetic polarity stratigraphy supports correlations made on the basis of other criteria. Over 20 geomagnetic reversals are recorded in the eastern Iceland lava pile in a period approximately 13-10 Ma ago. The geomagnetic field during this period averages to a central axial dipole field, and its overall statistical properties resemble those obtained in earlier surveys in Iceland. 相似文献
The best information on which to base estimates of future flood frequencies is records of past flood events. Where there is a substantial record at the location for which estimates are desired the estimation process is generally straighforward, although a variety of methods is used and there is major uncertainty in the estimates. In general, the frequency of future events is assumed to be indicated by the observed frequency of past events under constant controlling watershed conditions.Techniques are available for using information on historical (pre-record) flood data to improve the reliability of flood frequency estimates. There are methods for detecting and managing extremely unusual actual events (outliers) and for improving the reliability of short-record estimates based on long-record data at related locations. Regional correlation analysis is usable for establishing flood frequency estimates for locations where records are not available.Detailed hydrologic analysis, usually involving rainfall-runoff studies, is required for establishing flood frequency relationships for modified conditions of the watershed or, in many cases, for establishing flood frequency estimates for newly formed drainage systems such as in urban areas and airports.The principal use of flood frequency functions is to compare expected changes in flood damages (due to a contemplated action) with the economic and social costs or benefits of the contemplated action. 相似文献
The Nxau Nxau kimberlites in northwest Botswana belong to the Xaudum kimberlite province that also includes the Sikereti, Kaudom and Gura kimberlite clusters in north-east Namibia. The Nxau Nxau kimberlites lie on the southernmost extension of the Congo Craton, which incorporates part of the Damara Orogenic Belt on its margin. The Xaudum kimberlite province is geographically isolated from other known clusters but occurs within the limits of the NW-SE oriented, Karoo-aged Okavango Dyke Swarm and near NE-SW faults interpreted as the early stages of the East African Rift System. Petrographic, geochronological and isotopic studies were undertaken to characterise the nature of these kimberlites and the timing of their emplacement. The Nxau Nxau kimberlites exhibit groundmass textures, mineral phases and Sr-isotope compositions (87Sr/86Sri of 0.7036 ± 0.0002; 2σ) that are characteristic of archetypal (Group I) kimberlites. U-Pb perovskite, 40Ar/39Ar phlogopite and Rb-Sr phlogopite ages indicate that the kimberlites were emplaced in the Cretaceous, with perovskite from four samples yielding a preferred weighted average U-Pb age of 84 ± 4 Ma (2σ). This age is typical of many kimberlites in southern Africa, indicating that the Xaudum occurrences form part of this widespread Late Cretaceous kimberlite magmatic province. This time marks a significant period of tectonic stress reorganisation that could have provided the trigger for kimberlite magmatism. In this regard, the Nxau Nxau kimberlites may form part of a NE-SW oriented trend such as the Lucapa corridor, with implications for further undiscovered kimberlites along this corridor.
Granitoids from the central Mawson Escarpment (southern Prince Charles Mountains, East Antarctica) range in age from Archaean
to Early Ordovician. U–Pb dating of zircon from these rocks indicates that they were emplaced in three distinct pulses: at
3,519 ± 20, 2,123 ± 12 Ma and between 530 and 490 Ma. The Archaean rocks form a layer-parallel sheet of limited extent observed
in the vicinity of Harbour Bluff. This granitoid is of tonalitic-trondhjemitic composition and has a Sr-undepleted, Y-depleted
character typical of Archaean TTG suites. εNd and TDM values for these rocks are −2.1 and 3.8 Ga, respectively. Subsequent Palaeoproterozoic intrusions are of granitic composition
(senso stricto) with pronounced negative Sr anomalies. These rocks have εNd and TDM values of −4.8 and 2.87 Ga, indicating that these rocks were probably melted from an appreciably younger source than that
tapped by the Early Archaean orthogneiss. The remaining intrusions are of Early Cambrian to Ordovician age and were emplaced
coincident with the major orogenic event observed in this region. Cambro–Ordovician intrusive activity included the emplacement
of layer-parallel pre-deformational granite sheets at approximately 530 Ma, and the intrusion of cross cutting post-tectonic
granitic and pegmatitic dykes at ca. 490 Ma. These intrusive events bracket middle- to upper-amphibolite facies deformation
and metamorphism, the age of which is constrained to ca. 510 Ma—the age obtained from a syn-tectonic leucogneiss. Nd–Sr isotope
data from the more felsic Cambro–Ordovican intrusions (SiO2 > 70 wt%), represented by the post-tectonic granite and pegmatite dykes, suggest these rocks were derived from Late Archaean
or Palaeoproterozoic continental crust (TDM ∼ 3.5–2.3 Ga, εNd ∼ −21.8 to −25.9) not dissimilar to that tapped by the Early Proterozoic intrusions. In contrast, the compositionally more
intermediate rocks (SiO2 < 65 wt%), represented by the metaluminous pre-tectonic Turk orthogneiss, appear to have melted from a notably younger lithospheric
or depleted mantle source (TDM = 1.91 Ga, εNd ∼ −14.5). The Turk orthogneiss additionally shows isotopic (low 143Nd/144Nd and low 87Sr/86Sr) and geochemical (high Sr/Y) similarities to magmas generated at modern plate boundaries—the first time such a signature
has been identified for Cambrian intrusive rocks in this sector of East Antarctica. These data demonstrate that: (1) the intrusive
history of the Lambert Complex differs from that observed in the adjacent tectonic provinces exposed to the north and the
south and (2) the geochemical characteristics of the most mafic of the known Cambrian intrusions are supportive of the notion
that Cambrian orogenesis occurred at a plate boundary. This leads to the conclusion that the discrete tectonic provinces observed
in the southern Prince Charles Mountains were likely juxtaposed as a result of Early Cambrian tectonism. 相似文献
The Devonian Lake Boga Granite in northern Victoria, while almost entirely under thin Murray Basin cover, is one of the largest plutons in the western Lachlan Fold Belt. Its only exposure is a quarry penetrating the Cenozoic sediments. In the quarry, prominent pod pegmatites and miarolitic cavities suggest a high level of emplacement. The granite, a non-magnetic, fractionated S-type, contains a large range of accessory minerals, including primary uranium- and REE-bearing phosphates and oxides, and primary copper sulfides. Monazite-series minerals show an exceptionally wide range of compositions, from normal monazite-(Ce) through cheralite (Ca – Th-rich) to rare huttonitic monazite (Th-rich) and brabantite; U contents in monazite also vary widely (0 – 7.9 wt%). Primary low-Ca uraninites are well preserved and are unusual in having low Th/U and LREE. Late-stage cavity fluorapatite crystals up to several centimetres across show intricate elemental zoning patterns with extreme U gradients (<10 – 6900 ppm) in some crystals. New 40Ar – 39Ar ages for magmatic biotite, muscovite and K-rich feldspar average 365 ± 3 Ma, which approximates the emplacement age of the granite. This is supported by a 377 ± 12 Ma U – Th – Pb (CHIME) age for primary uraninite. New whole-rock geochemical data support earlier observations: the granite is strongly fractionated (SiO2 70.7 – 76.0 wt%; 4.2 – 0.6 wt% FeOt) and peraluminous (ASI = 1.23 – 1.45), and has slightly elevated Na2O and P2O5 (0.30 wt%) contents compared with other fractionated S-type granites from the Lachlan Fold Belt. Trace-element abundances are typical of fractionated granites, although U and Cu concentrations vary strongly and reach >60 and ≈1400 ppm, respectively. REE patterns also vary strongly, from LREE-enriched with moderate Eu depletion, to flat with strong Eu depletion. The flattest of the REE patterns, in samples with FeOtotal < 1%, are characterised by M-type tetrad effects. These and other samples also show low (subcrustal average) and variable Zr/Hf (35 – 16) and Nb/Ta (8 – 4) ratios; these and other unusual elemental fractionations are related to changes in elemental partitioning during the late magmatic stage, when felsic peraluminous magma and high-temperature magmatic fluid coexisted. 相似文献