共查询到20条相似文献，搜索用时 31 毫秒
1.
《Australian Journal of Earth Sciences》2013,60(6):931-954
Carboniferous (Visean to Westphalian) pyroclastics and lava flows in the Rocky Creek region, used to redefine the base of the Kiaman reversal, are formally defined or redefined and the status of the main formations clarified. These units include the Caroda Formation, containing the Kooringal Dacite, Boomi Rhyolite and Barney Springs Andesite Members; the Clifden Formation with the Wanganui Andesite, Glen Idle Rhyolite, Appleogue Dacite, Bexley Rhyolite, Pine Cliffs Rhyolite and Downs Rhyodacite Members; Rocky Creek Conglomerate with the Hazelvale Rhyodacite, Mt Hook Rhyolite, Darthula Rhyodacite and Pound Rock Rhyodacite Members; and Lark Hill Formation with the Eulowrie Pyroclastic, Tycannah Rhyodacite and The Tops Rhyolite Members; a number of informal units are also described. The restriction of most volcanic units to one of the three thrust blocks (Boomi, Kathrose and Darthula blocks) of the Rocky Creek region, suggests their current relationships reflect either shortening due to overthrusting or an original distribution affected by depositional or erosional processes. A westerly increase in the proportion of ignimbrites indicates nearness to sources in that direction. Intermediate volcanism, largely confined to southern and central parts of the Boomi block in the east, began in the Visean and ended in the early Namurian. Acid volcanism also began in the Visean in the northern Boomi block but, with the exception of the Peri Rhyolite Member of the Clifden Formation, did not become widespread until later in the Namurian and Westphalian. In contrast, only acid volcanism took place during the early Namurian to Westphalian in the Kathrose and Darthula blocks. Correlations based on AS3 and SL13 SHRIMP dates illustrate a discordance of about 3% when compared with the most likely location for the base of the Kiaman reversal. The bases of both the Rocky Creek Conglomerate and Lark Hill Formation appear to be slightly diachronous. 相似文献
2.
3.
P. W. Schmidt C. Aubourg P. G. Lennox J. Roberts 《Australian Journal of Earth Sciences》2013,60(6):547-560
The Hastings Terrane comprises two or three major fragments of the arc‐related Tamworth Belt of the southern New England Orogen, eastern Australia, and is now located in an apparently allochthonous position outboard of the subduction complex. A palaeomagnetic investigation of many rock units has been undertaken to shed light on this anomalous location and orientation of this terrane. Although many of the units have been overprinted, pre‐deformational magnetizations have been isolated in red beds of the Late Carboniferous Kullatine Formation from the northern part of the terrane. After restoring these directions to their palaeohorizontal (pre‐plunging and pre‐folding) orientations they appear to have been rotated 130° clockwise (or 230° anti‐clockwise) when compared with coeval magnetizations from regions to the west of the Hastings Terrane. Although these data are insensitive to translational displacements, a clockwise rotation is incompatible with models previously proposed on geological grounds. While an anti‐clockwise rotation is in the same sense as these models the magnitude appears to be too great by about 100°. Nevertheless, the palaeomagnetically determined rotation brings the palaeoslopes of the Tamworth Belt, facing east, and the Northern Hastings Terrane, facing west before rotation and facing southeast after rotation, into better agreement. A pole position of 14.4°N, 155.6°E (A95 = 6.9°) has been determined for the Kullatine Formation (after plunge and bedding correction but not corrected for the hypothetical rotation). Reversed magnetizations interpreted to have formed during original cooling are present in the Werrikimbe Volcanics. The pole position from the Werrikimbe Volcanics is at 31.6° S, 185.3° E (A95 = 26.6°). These rocks are the volcanic expression of widespread igneous activity during the Late Triassic (～ 226 Ma). While this activity is an obvious potential cause of the magnetic overprinting found in the older units, the magnetic directions from the volcanics and the overprints are not coincident. However, because only a few units could be sampled, the error in the mean direction from the volcanics makes it difficult to make a fair comparison with the directions of overprinted units. The overprint poles determined from normal polarity magnetizations of the Kullatine Formation is at 61.0°S, 155.6°E (A95 = 6.9°) and a basalt from Ellenborough is at 50.7° S, 148.8° E (A95 = 15.4°), and from reversed polarity magnetizations, also from the basalt at Ellenborough is at 49.4° S, 146.2° E (A95 = 20.4°). These are closer to either an Early Permian or a mid‐Cretaceous position, rather than a Late Triassic position, on the Australian apparent polar wandering path. Therefore, despite their mixed polarity, and global observations that the Permian and mid‐Cretaceous geomagnetic fields were of constant polarities, the age of these overprint magnetizations appears to be either Early Permian or mid‐Cretaceous. 相似文献
4.
A. L. Pickard 《Australian Journal of Earth Sciences》2013,60(3):491-507
Tuffaceous mudrocks are common in the banded iron‐formations (BIF) of the Brockman Iron Formation. These tuffaceous mudrocks are either stilpnomelane‐rich or siliceous. Their compositions reflect bimodal volcanic activity in the vicinity of the Hamersley BIF depositional site. They also contain complex zircon populations that record resedimentation, syndepositional volcanism and post‐depositional isotopic disturbance. The best estimates of depositional age are obtained from siliceous tuffaceous mudrocks in the Joffre Member that contain 2459 ± 3 Ma and 2454 ± 3 Ma zircon populations most likely derived from felsic volcanism coeval with BIF deposition. These dates constrain the sedimentation rates for the ～370 m‐thick Joffre Member BIF to >15 m per million years. Siliceous tuffaceous mudrocks are not present in the underlying ～120 m‐thick Dales Gorge Member and it is uncertain whether previously reported ages of ca 2479–2470 Ma for this unit reflect detrital/xenocrystic or syndepositional zircon populations in resedimented stilpnomelane‐rich tuffaceous mudrocks. The increased abundance of tuffaceous mudrocks in the Joffre Member suggests that a pulse of enhanced igneous and hydrothermal activity accompanied deposition of the bulk of the Brockman Iron Formation BIF after ca 2460 Ma. This preceded and culminated in the emplacement of the 2449 ± 3 Ma large igneous province represented by BIF and igneous rocks of the Weeli Wolli Formation and Woongarra Rhyolite. 相似文献
5.
The Carboniferous culm of the Pays-de-Sault is divided into two diachronous and synshortening series. These series are dated Late Visean (Pic d'Ourtiset series in a northern overthrust unit) and Early Namurian E2 (La Fajolle series in a southern underthrust unit) from an association of foraminifers, algae, and microproblematica identified in clasts of conglomerates. According to structural positions and facies criteria, these two series are interpreted as two turbiditic depocenters which were generated by southward thrust propagation during Late Visean and Early Namurian. At the scale of the Pyrenean Hercynian range, this evolution is consistent with a thrust and depocenter sequence propagating on the wedge-top depozone of a foreland basin system from the northeast (Mouthoumet subpyrenean massif) to the southwest (end of the High Primary Range) during Late Visean to Westphalian C time interval. 相似文献
6.
The stratigraphic succession of formations in the Myall district comprises in ascending order the Bunyah Beds, Wallanbah Formation, Kataway Mudstone, Boolambayte Formation (new names), Nerong Volcanics (E'ngel, 1962), Booti Booti Sandstone, Yagon Siltstone, Koolanock Sandstone, Muirs Creek Conglomerate (new names) and Alum Mountain Volcanics (Engel, 1962). The units range in age from possibly Devonian to possibly Permian, most being Carboniferous. The Mograni (new name), Tugrabakh (Voisey, 1940) and Mayers Flat Limestones (new name) are members of the Wallanbah Formation. The Violet Hill Volcanics (new name) is a member of the Yagon Siltstone. The Burdekins Gap Basalt Member and Lakes Road Rhyolite are members of the Alum Mountain Volcanics. Environments of deposition range from nonmarine (Nerong Volcanics, Alum Mountain Volcanics, Muirs Creek Conglomerate, upper part of Koolanock Sandstone) through shallow marine (Booti Booti Sandstone, lower part of Koolanock Sandstone, calcareous parts of Wallanbah Formation) to deep marine (most other units). Facies relationships indicate a progressive deepening of the sedimentary environment to the east throughout most of the Carboniferous sequence. The Tournaisian sequence is readily correlated with a similar sequence in the Rocky Creek and Belvue Synclines. Higher units are correlated with sequences at Gloucester (Campbell & McKelvey, 1972) and Booral (Campbell, 1962). 相似文献
7.
During the Mesozoic and Paleogene, the Precambrian rocks in the Pilbara, Western Australia, underwent erosion and deep weathering that produced an undulating landform now represented by the duricrusted and partly eroded Hamersley Surface. A reddened, ferruginous weathering zone occurs immediately beneath this duricrusted surface. Oriented block samples of ferruginised strata of the Neoarchean–early Paleoproterozoic Hamersley Group exposed within approximately 15 m below the duricrust were collected at 20 sites in roadcuts along the Great Northern Highway between Munjina and Newman and exposures along the adjoining Karijini Drive. Stepwise thermal demagnetisation of cored specimens revealed a stable, high-temperature (680°C) component carried by hematite, with a mean direction (n = 55 specimens) of declination D = 182.0°, inclination I = 52.9° (α95 = 3.6°), indicating a pole position at latitude λp = 77.6°S, longitude ?p = 113.2°E (A95 = 4.3°) and a paleolatitude λ = 33.5 +3.6/–3.3°S. Both normal and reversed polarities are present, indicating that the remanent magnetism was acquired over an interval of at least two polarity chrons (say 105–106 years). Chi-square tests on the determined pole position and three different sets of Cenozoic poles, namely those for dated volcanic rocks in eastern Australia supplemented by poles for Australian Cenozoic weathering horizons, and inferred poles from Pacific Ocean and Indian Ocean hotspot analyses and North American Cenozoic poles rotated to Australian coordinates, yielded a mean age of ca 24 ± 3 Ma, i.e. late Oligocene to early Miocene, interpreted as the time of formation of hematite in the sampled ferruginous zone. The ferruginous weathering occurred under globally warm conditions and was followed during the early to middle Miocene climatic optimum by the deposition of channel iron deposits, which incorporated detrital hematitic material derived from erosion of the ferruginous weathering zone beneath the Hamersley Surface. 相似文献
8.
C. Klootwijk 《Australian Journal of Earth Sciences》2016,63(5):513-549
Structural, magnetic and gravity trends of the southern New England Orogen (SNEO) indicate four oroclinal structures, none conclusively confirmed paleomagnetically. Curved structures of the Tamworth Belt (TB)—a continental forearc exposed across six tectono-stratigraphic blocks with interlinked Carboniferous stratigraphies and extensive ignimbritic rocks known to retain primary magnetisations despite prevalent overprinting—are prospective to oroclinal testing through comparison of Carboniferous pole paths for individual blocks. Pole paths (a) have been established for the Rocky Creek and Werrie blocks (northwestern/western TB), (b) are described herein for the Rouchel Block (southwestern TB), and (c) are forthcoming for the Gresford and Myall blocks (southern/southeastern TB). The Rouchel path derives from detailed paleomagnetic, rock magnetic and magnetic fabric studies. Thermal, alternating field and liquid nitrogen demagnetisations show a low-temperature overprint, attributed to late Oligocene weathering, and high-temperature (HT) primary and overprint components in both magnetite and hematite carriers, showing slight, systematic, directional differences with hematite providing the better cleaned site poles. Seven primary mean-site poles of Tournaisian and mainly Visean age and three overprint poles show six positive fold tests, five at 95% or higher confidence levels. Two dispersed groupings of intermediate (IT) and HT overprint site poles of Permian and Permo-Triassic age are attributed to early and late phases in oroclinal evolution of the SNEO. HT and IT/HT overprint site poles of mid-Carboniferous age are attributed to Variscan Australia–Asia convergence. Individual pole paths for the Rocky Creek, Werrie and Rouchel blocks show no noticeable rotation between them, indicating primary curvature for the southwestern TB. Their integrated SNEO pole path establishes a reference frame for determining rotations of the southern and southeastern TB. 相似文献
9.
S. Sheppard I. M. Tyler T. J. Griffin W. R. Taylor 《Australian Journal of Earth Sciences》2013,60(5):679-690
Palaeoproterozoic basaltic rocks in the Halls Creek Orogen form part of two stratigraphic sequences on either side of a major structure, the Angelo‐Halls Creek‐Osmond Fault System. The two sequences have contrasting geological histories and probably formed in different tectonostratigraphic terranes. To the east, basalts of the Biscay Formation, which are part of the Halls Creek Group, were erupted at ca 1880 Ma and deformed and metamorphosed first at low grade between ca 1845 and ca 1820 Ma. To the west, basalts of the Tickalara Metamorphics were deposited after 1865 Ma, and were metamorphosed at medium to high grade and intruded by tonalite and leucogranite sheets at 1850 to 1845 Ma. Two groups of metabasalts are identified in the Biscay Formation. Group 1 samples have compositions similar to enriched (E‐)MORB. Group 2 samples have lower TiO2, P2O5, Cr, Y, Nb and Zr contents, and trace‐element ratios (e.g. Ti/V and Zr/Nb), similar to low‐TiO2 continental flood basalts. Metabasalts from the Tickalara Metamorphics consist of depleted and enriched types. The depleted samples have high field strength element (HFSE) and rare‐earth element (REE) abundances similar to oceanic island arc/backarc basin tholeiites. The enriched samples have compositions similar to E‐MORB, and are similarto group 1 samples from the Biscay Formation. Basalts of the Biscay Formation were erupted on a passive continental margin, whereas those of the Tickalara Metamorphics formed in an oceanic island arc/backarc basin or ensialic marginal basin, the two terranes being brought together by ca 1820 Ma. This is consistent with the evolution of the Halls Creek Orogen during the Palaeoproterozoic by plate‐tectonic processes similar to those operating in the Phanerozoic. 相似文献
10.
Stratigraphy,distribution and geochemistry of widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics,South Australia 总被引：1，自引：0，他引：1
S. R. Allen C. J. Simpson J. McPhie S. J. Daly 《Australian Journal of Earth Sciences》2013,60(1):97-112
Three widespread felsic volcanic units, the Eucarro Rhyolite, Pondanna Dacite Member and Moonaree Dacite Member, have been distinguished in the Mesoproterozoic Gawler Range Volcanics. These three units are the largest in the Gawler Range Volcanics, each in excess of 500 km3. Each unit is ～300 m thick and includes a black, formerly glassy base, a granophyric columnar‐jointed interior, and an amygdaloidal outer margin. The units are very gently dipping and locally separated by thin (<20 m) lenses of either ignimbrite (Mt Double Ignimbrite), tuffaceous sandstone or faults. The youngest unit, the Moonaree Dacite Member, covers a central area with a diameter greater than 80 km. The southern two units have east‐west extents in the order of 180 km, but are much less extensive from south to north (5–60 km). All three units are dominated by euhedral phenocrysts and are relatively crystal rich. Both the Eucarro Rhyolite and Moonaree Dacite Member contain clasts of basement granitoid and other lithologies and are locally heterogeneous in texture and composition. Some granitoid clasts have disintegrated, liberating feldspar and quartz crystals into the surrounding host. These liberated crystals cause textural variations, but can be identified on the basis of shape (amoeboid or skeletal) and/or size (megacrysts). Textural and lithofacies characteristics are consistent with the interpretation that these units are lavas; the strongly elongate distribution and wide extent of the Eucarro Rhyolite and Pondanna Dacite Member could indicate that vents were aligned along an extensive east‐west‐trending fissure system. Stratigraphic nomenclature has been revised to better reflect the presence of the three emplacement units. The oldest unit, the Eucarro Rhyolite, is dominated by plagioclase‐phyric rhyolite that locally includes granitoid clasts and megacrysts. Along the northern margin, the rhyolite is amygdaloidal and has mingled with a quartz‐rich rhyolite (Paney Rhyolite Member). The Eucarro Rhyolite and Paney Rhyolite Member replace the formerly defined ‘Eucarro Dacite’, ‘Nonning Rhyodacite’, ‘Yannabie Rhyodacite’ and ‘Paney Rhyolite’. The two younger units, Pondanna Dacite Member and Moonaree Dacite Member, are compositionally and spatially distinct, newly defined members of the Yardea Dacite. 相似文献
11.
M. H. Adabi 《Australian Journal of Earth Sciences》2013,60(6):767-775
This study uses carbon isotope chemostratigraphy to propose an age for the Success Creek Group and Crimson Creek Formation in the absence of any direct radiometric dates, palaeomagnetic or reliable palaeontological data. The δ13C values were determined for the least‐altered dolomite samples. Suitable samples were selected on the basis of grainsize, cathodoluminescence petrography, most enriched δ18O values (> ‐2%o) low Mn/Sr ratios and low Fe and Mn concentrations. The average least‐altered, most 13C‐enriched dolomicrite samples in the youngest (No. 1) dolomite horizon are + 4.6%o. This is typical of Neoproterozoic (but not Cambrian) carbonates. The δ13C values of all dolomite samples in the succession are significantly positive (up to + 7.5%o) and the excursion characteristic of the Proterozoic/Cambrian boundary has not been observed. The lack of negative δ13C values in all dolomite samples studied also suggests an absence of correlatives of Sturtian and Varanger tillites in the dolomite successions. The δ13C values in all three dolomite horizons suggest a Neoproterozoic age between about 820 to 570 Ma (Cryogenian to Neoproterozoic III) on the current global compilation carbon isotope curves. This age for the Success Creek Group and Crimson Creek Formation, inferred from carbon isotope chemostratigraphy, can be substantiated by other evidence. The age of the Renison dolomites is constrained by K‐Ar dates of 708 ± 6 Ma from detrital muscovite in the underlying Oonah Formation and 588 ± 8 and 600 ± 8 Ma from doleritic rock in a lithostratigraphic equivalent of the Crimson Creek Formation from the Smithton Basin. Furthermore, acritarchs and the stromatolite Baicalia cf. B. burra also suggest a Neoproterozoic rather than Cambrian age. 相似文献
12.
The Arthur Lineament of northwestern Tasmania is a Cambrian (510 ± 10 Ma) high‐strain metamorphic belt. In the south it is composed of metasedimentary and mafic meta‐igneous lithologies of the ‘eastern’ Ahrberg Group, Bowry Formation and a high‐strain part of the Oonah Formation. Regionally, the lineament separates the Rocky Cape Group correlates and ‘western’ Ahrberg Group to its west from the relatively low‐strain parts of the Oonah Formation, and the correlated Burnie Formation, to its east. Early folding and thrusting caused emplacement of the allochthonous Bowry Formation, which is interpreted to occur as a fault‐bound slice, towards the eastern margin of the parautochthonous ‘eastern’ Ahrberg Group metasediments. The early stages of formation of the Arthur Lineament involved two folding events. The first deformation (CaD1) produced a schistose axial‐planar fabric and isoclinal folds synchronous with thrusting. The second deformation (CaD2) produced a coarser schistosity and tight to isoclinal folds. South‐plunging, north‐south stretching lineations, top to the south shear sense indicators, and south‐verging, downward‐facing folds in the Arthur Lineament suggest south‐directed transport. CaF1 and CaF2 were rotated to a north‐south trend in zones of high strain during the CaD2 event. CaD3, later in the Cambrian, folded the earlier foliations in the Arthur Lineament and produced west‐dipping steep thrusts, creating the linear expression of the structure. 相似文献
13.
The Timbarra Tablelands pluton is an extensive (～550 km2) complexly zoned intrusion forming one of many predominantly monzogranite I‐type plutons, which constitute the Moonbi Supersuite in northern New South Wales, Australia. It comprises an outer rim of Rocky River monzogranite (Zones 1–3), an intermediate zone of Sandy Creek syenogranite (Zones 4A–4C), surrounding a core of Surface Hill syenogranite (Zones 5–7). The suite is calc‐alkaline, high‐K, and varies from mildly metaluminous to weakly peraluminous with increasing fractionation. Average Rb/Sr ratios range from 0.4 in the least evolved very coarse‐grained monzogranite (Zone 3) to 46 in the most evolved very fine‐grained biotite microgranite (Zone 6). Trace‐element modelling indicates that the observed compositional variation could have been produced by crystal fractionation. New bulk rock major‐ and trace‐element data for 71 samples are presented, and indicate that a compositional continuum exists that varies between 63 and 78 wt% SiO2. Importantly, there is no systematic chemical variation with spatial distribution of samples from the core of the pluton to its margin, requiring multiple separate pulses of an evolving magma to explain compositional discontinuities. The pluton is interpreted to have been emplaced at mesozonal levels (～180 ± 60 MPa, 5–10 km depth) and crystallised at temperatures between 620 and 820°C under moderately oxidising conditions (log fO2 = ‐11.5 to ‐19). The association of gold‐molybdenite mineralisation at Timbarra with moderately oxidised I‐type magmas is consistent with fractionation‐redox controls on ore‐element behaviour in magmatic systems in other studies. 相似文献
14.
OLE BENNIKE KAREN LUISE KNUDSEN NIELS ABRAHAMSEN JENS BÖCHER HOLGER CREMER BERND WAGNER 《Boreas: An International Journal of Quaternary Research》2010,39(3):603-619
Bennike, O., Knudsen, K. L., Abrahamsen, N., Böcher, J., Cremer, H. & Wagner, B. 2010: Early Pleistocene sediments on Store Koldewey, northeast Greenland. Boreas, Vol. 39, pp. 603–619. 10.1111/j.1502‐3885.2010.00147.x. ISSN 0300‐9483. Marine Quaternary deposits, here named the Store Koldewey Formation, are found at ～120 m above sea level in northeast Greenland (76°N). The sequence is referred to the Olduvai normal polarity subchron at 1.95–1.78 Ma BP based on palaeomagnetic studies (palaeomagnetically reversed), amino acid epimerization ratios and evidence from marine and non‐marine fossils. The sediments and the fauna show that the sequence was deposited on a mid or inner shelf, and some elements of the marine mollusc and foraminiferal assemblages indicate water temperatures between ?1 and +1 °C and seasonal sea ice cover during deposition. Mean summer air temperatures were around 6 °C higher than at present, as demonstrated by the occurrence of southern extralimital terrestrial species. Well‐preserved remains of land plants indicate that the adjacent land area was dominated by sub‐arctic forest‐tundra with the trees Larix and Betula, shrubs, herbs and mosses. Most of the species represented as fossils have recent circumpolar geographical ranges. An extinct brachiopod species and an extinct gastropod species have been found, but the other macrofossils are referred to extant species. The brachiopod is erected as a new genus and species, Laugekochiana groenlandica. Correlation of the Koldewey Formation with the Île de France Formation farther to the north is suggested. Member A of the Kap København Formation in North Greenland is referred to the Late Pliocene, whereas Member B of the Kap København Formation is suggested to be slightly older than the Store Koldewey Formation. 相似文献
15.
M. Schätz T. Reischmann J. Tait V. Bachtadse H. Bahlburg U. Martin 《International Journal of Earth Sciences》2002,91(5):838-849
Early Palaeozoic volcanic and sedimentary rocks from the Saxothuringian Basin (Franconian Forest, northern Bavaria) have been subjected to detailed radiometric and palaeomagnetic studies in order to determine the tectonic environment and geographic setting in which they were deposited. Two hand samples were collected from the as yet undated pyroclastic flow deposits for 207Pb/206Pb age dating. Radiometric results for these samples, obtained by the single-zircon evaporation technique, are identical within error, and the mean age of all measured grains is 478.2ǃ.8 Ma (n=11). This age is considered to be primary and firmly constrains the eruption of the ignimbrites and formation of the subaqueous pyroclastic flows as having occurred in Early Ordovician (Arenig) times. Palaeomagnetic studies were carried out on these Early Ordovician volcanic rocks, and also on the biostratigraphically dated, Late Ordovician (Ashgillian) Döbra sandstones. The volcanic rocks carry up to three directions of magnetisation. The poorly defined, low and intermediate unblocking temperature directions are thought to represent secondary overprint directions of post-Ordovician age. The high temperature component, however, identified at temperatures of up to 580 °C, is of mixed polarity and passes the fold test with 99% confidence. The overall mean direction after bedding correction is 189°/76°, !95=11.6°, k=44.7 (25 samples, five sites), and is considered to be primary and Early Ordovician in origin. It yields a palaeo-south pole at 24°N and 007°E, which translates into palaeolatitudes of 63°+21.7°/-17.3° S for the Saxothuringian Terrane. Samples from the Late Ordovician Döbra sandstone are generally very weakly magnetised. A high temperature D component of magnetisation can be identified in some samples and yields a mean direction of 030°/-58°, !95=18.5°, k=25.7 (15 samples, four sites) after bedding correction. The Arenig palaeomagnetic results indicate high palaeolatitudes, but separation from northern Gondwana. This is in basic agreement with data from elsewhere in the Armorican Terrane Assemblage, all of which suggest high southerly palaeolatitudes in the Early Ordovician. The geochemical signatures of these rocks indicate emplacement in an extensional environment. These new data, therefore, are interpreted as marking the onset of rifting of Saxothuringia from the north African margin of Gondwana, and the start of the relative northward migration of the Saxothuringian Terrane. Although the Late Ordovician palaeomagnetic results presented here are only poorly constrained, they suggest an intermediate palaeolatitude for Saxothuringia in Ashgillian times, in good agreement with Late Ordovician palaeomagnetic data from the Barrandian. 相似文献
16.
H. G. Wilshire 《Australian Journal of Earth Sciences》2013,60(1):11-20
Late Permian (early Tatarian, 258 Ma) palaeosols exposed near Kiama and Dapto on the south coast of New South Wales can be recognised from their red colour, clayey and massive texture, and clay‐filled root traces. Harvey pedotype palaeosols are within the upper Jamberoo Sandstone Member of the Broughton Formation. Loveleigh pedotype palaeosols are within the basal Kiama Sandstone overlying columnar jointed flows of the Blow Hole Latite Member of the Broughton Formation. Both kinds of palaeosols are strongly ferruginised with little relict bedding, yet they are little weathered and have surprisingly high amounts of feldspar and pyroxene. Both also show deformation of subsurface layers comparable to the active layer of permafrosted soils. Root traces in these palaeosols are sparse and comparable to those of woody gymnosperms, not chambered like the known roots of Glossopteris. Plausible components of the taiga woodland represented by the palaeosols include Gangamopteris, cordaites, seed ferns, and equisetaleans. Frigid palaeoclimatic indications from the palaeosols are compatible with a palaeomagnetically determined palaeolatitude of 57–85°S, and thus indicate an equator‐pole climatic gradient in the Late Permian generally similar to that of today. 相似文献
17.
C. Klootwijk 《Australian Journal of Earth Sciences》2020,67(4):525-573
AbstractFour oroclinal structures have been identified from structural, magnetic and gravity trends across a Carboniferous continental arc, forearc basin [Tamworth Belt (TB)] and conjugate accretionary complex in the southern New England Orogen (SNEO) of eastern Australia. None of the structures has yet been confirmed conclusively by paleomagnetism as oroclinal. Ignimbrites are common within the forearc basin and have been demonstrated to retain primary magnetisations despite prevalent overprinting. They are well exposed across six major tectono-stratigraphic blocks with partly interlinked stratigraphies, making the forearc basin highly prospective to oroclinal testing by comparing pole path segments for individual blocks across curved structures. Paleomagnetic studies have shown no noticeable rotation across the western/southwestern TB (Rocky Creek, Werrie and Rouchel blocks), but documented herein is a minor counter-clockwise rotation of the Gresford Block of the southern TB. This study details paleomagnetic, rock magnetic and magnetic fabric results for 87 sites (969 samples) across the southern Gresford Block. Predominantly thermal, also alternating field and liquid nitrogen, demagnetisations show a widely present low-temperature overprint, attributed to regional late Oligocene weathering, and high-temperature primary and overprint components residing in both mainly magnetite and mainly hematite carriers. Subtle, but systematic, directional differences between magnetite and hematite subcomponents show the latter as the better cleaned, better-defined, preferred results, detailing nine primary poles of middle and late Carboniferous ages and Permian and Permo-Triassic overprints as observed elsewhere in the western/southwestern TB. The primary poles update a poorly defined mid-Carboniferous section of the SNEO pole path and demonstrate counter-clockwise rotation, quantified at about 15° ± 13° from comparison of mid-Carboniferous Martins Creek Ignimbrite Member poles for the Rouchel and Gresford blocks, that may not necessarily have been completed prior to the Hunter–Bowen phase of the Gondwanide Orogeny. This minor counter-clockwise rotation of the Gresford Block accentuates a primary curvature of the southwestern/southern TB and heralds further, more complex, rotations of the Myall Block of the southeastern TB. 相似文献
18.
RAN Bo ZHAO Xixi LIU Zhifei WANG Chengshan ZHU Lidong JIN Wei LI Yalin 《《地质学报》英文版》2016,90(3):858-869
Understanding the Cenozoic vertical-axis rotation in the Tibetan Plateau is crucial for continental dynamic evolution. Paleomagnetic and rock magnetic investigations were carried out for the Oligocene and Miocene continental rocks of the Hoh Xil basin in order to better understand the tectonic rotations of central Tibet. The study area was located in the Tongtianhe area located in the southern part of the Hoh Xil basin and northern margin of the Tanggula thrust system in central-northern Tibet. A total of 160 independently oriented paleomagnetic samples were drilled from the Tongtianhe section for this study. The magnetic properties of magnetite and hematite have been recognized by measurements of magnetic susceptibility vs. temperature curves and unblocking temperatures. The mean directions of the Oligocene Yaxicuo Group in stratigraphic coordinates(Declination/Inclination = 354.9°/29.3°, k = 33.0, α_(95) = 13.5°, N =5 Sites) and of the Miocene Wudaoliang Group in stratigraphic coordinates(Declination/Inclination = 3.6°/36.4°, k = 161.0, α_(95) = 9.7°, N =3 Sites) pass reversal tests, indicating the primary nature of the characteristic magnetizations. Our results suggested that the sampled areas in the Tuotuohe depression of the Hoh Xil basin have undergone no paleomagnetically detectable rotations under single thrusting from the Tanggula thrust system. Our findings, together with constraints from other tectonic characteristics reported by previous paleomagnetic studies, suggest tectonic rotations in the Cuoredejia and Wudaoliang depressions of the Hoh Xil basin were affected by strike-slip faulting of the Fenghuo Shan-Nangqian thrust systems. A closer examination of geological data and different vertical-axis rotation magnitudes suggest the tectonic history of the Hoh Xil basin may be controlled by thrust and strike-slip faulting since the Eocene. 相似文献
19.
It is concluded that there are three hydrocarbon generation and accumulation processes in northeastern Sichuan on the basis of the characteristics of solid bitumen, gas-light oils-heavy oils, homogenization temperature of fluid inclusions and diagenesis for beach- and reef-facies dolomite gas- bearing reservoirs in the Puguang Gas Field, northeastern Sichuan Basin, southern China. The first hydrocarbon generation and accumulation episode occurred in the Indosinian movement （late Middle Triassic）. The sapropelic source rocks of the O3w （Upper Ordovician Wufeng Formation）-S1l （Lower Silurian Longmaxi Formation） were buried at depths of 2500 m to 3000 m with the paleogeothermal temperature ranging from 70℃ to 95℃, which yielded heavy oil with lower maturity. At the same time, intercrystalline pores, framework pores and corrosion caused by organic acid were formed within the organic reef facies of P2ch （Upper Permian Changxing Formation）. And the first stage of hydrocarbon reservoir occurred, the level of surface porosity of residual solid bitumen {solid bitumen/ （solid bitumen ＋ residual porosity）} was higher than 60%. The second episode occurred during the Middle Yanshanian movement （late Middle Jurassic）. During that period, the mixed organic source rocks were deposited in an intra-platform sag during the Permian and sapropelic source rocks of O3w-S1l experienced a peak stage of crude oil or light oil and gas generation because they were buried at depths of 3500 m to 6800 m with paleogeothermal temperatures of 96-168℃. At that time, the level of surface porosity of residual solid bitumen of the T1f shoal facies reservoirs was between 25% and 35%, and the homogenization temperatures of the first and second stages of fluid inclusions varied from 100℃ to 150℃. The third episode occurred during the Late Yanshanian （Late Cretaceous） to the Himalayan movement. The hydrocarbon reservoirs formed during the T1f and P2ch had the deepest burial of 7700 m to 8700 m and paleogeotemperatu 相似文献
20.
M. Venkateshwarlu J. Mallikarjuna Rao 《Journal of the Geological Society of India》2013,81(3):330-336
Paleomagnetic investigations have been carried out on poorly determined radiometric age controls of Bhander sandstones within the vicinity of Bhopal Inlier of the Upper Vindhyan Supergroup. Available ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources and methods. Paleomagnetic data generated from the Bhander Group of Bhopal Inlier yielded a mean declination of 357° and mean inclination of 58° (k=17.69, α95 = 16.38) with a Virtual Geomagnetic Pole (VGP) at 74° N, 69.0° E. This pole position is falling close to the Malani Igneous Suite (MIS) mean palaeomagnetic pole of 67.8° N and 72.5° E (A95=8.8°) by Gregory et al. (2009). The results obtained from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone suggest that the Upper Vindhyan sequence may be older than is commonly thought earlier. 相似文献