Correlation between the Wonominta and the Broken Hill Blocks in western New South Wales: Evidence from Nd isotope studies of metasediments

Nd isotope studies of the oldest metasedimentary rocks from the Wonominta Block, western New South Wales reveal that these samples have a model age (T_DM) of 1780–2010 Ma, slightly younger than that of low‐grade Willyama Supergroup metasediments (1920–2160 Ma), and significantly younger than those ages previously reported from high‐grade rocks of the Broken Hill Block (2200–2300 Ma). These differences have important implications for tectonic reconstruction in this region and support a model of transitional tectonics from the Broken Hill to Wonominta Blocks, as suggested by earlier geochemical studies of mafic rocks. Those studies revealed that the mafic rocks from the basal sequence of the Wonominta Block may have formed in a back‐arc basin, developed from a propagating rifting, an environment contiguous to that in which Willyama Supergroup was deposited. These results also carry significant implications for tectonic reconstruction of eastern Australia.     

Stratigraphy,structure and tectonics of Lower Ordovician and Devonian strata of the central part of the Wonominta Block,western New South Wales

K‐Ar age determinations on granites from Central Queensland support the conclusions of field mapping, that the intrusives of the Anakie High are generally older than the rocks of the Drummond Basin. Results on granites from the Bowen‐Collinsville region indicate that they pre‐date most of the Bowen Basin sediments. Four ages are reported for basement rock from bore‐cores in the Great Artesian Basin. They confirm Precambrian basement to the south of the Mt. Isa‐Cloncurry area, and suggest possible Precambrian in the Long‐reach Spur. Two samples from the Nebine Ridge give Lower Carboniferous results. A previous report of Permian basement in the Surat Basin has been shown to be incorrect.     

Geochemistry of the Mt Wright Volcanics from the Wonominta Block,northwestern New South Wales

The Mt Wright Volcanics are located in the Wonominta Block of northwestern New South Wales. Detailed regional mapping has shown that the block is a composite tectonic unit and that the metavolcanic rocks described as the Mt Wright Volcanics may have been emplaced at different time from Late Proterozoic (northern section: Packsaddle, Nundora) to Early Cambrian (southern section: Mt Wright). Geochemical investigations, including major and trace elements, as well as analyses of relic clinopyroxene, show that the rocks have affinities with alkali basalt with light‐rare‐earth‐element‐enriched compositions. An intra‐plate extensional environment (such as rift‐ and/or plume‐related) is considered most likely for the formation of the rocks. Though metamorphosed to various degrees, the rocks apparently retain much of their primary Sr isotopic character (initial ⁸⁷Sr/⁸⁶Sr about 0.7032) and, apart from their age, resemble the Tertiary intraplate volcanism in eastern Australia. The Nd isotope analyses yield remarkably similar results between the two sections of the Mt Wright Volcanics, with ¹⁴³Nd/¹⁴⁴Nd between 0.51260 to 0.51271 and _εNd(T) 4.7 ±0.4 (calculated to 525 Ma). A kaersutite‐bearing xenolith found in the northern section of the volcanic sequence has a Nd isotope composition more depleted than its hosts with _εNd(T) of 7.7. The isotope results suggest that the Mt Wright Volcanics were derived from a depleted mantle source without significant crustal contribution. It is proposed that the Mt Wright Volcanics possibly represent the products of a rifting event that led to the breakup of the Proterozoic supercontinent during Early Cambrian in eastern Australia.     

Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah,USA)

Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments. Typically, they form in organic‐rich mudstones, where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments. As a result, carbonates that comprise concretions are usually characterized by broad ranges in δ¹³C and include values that are significantly depleted relative to seawater. This article reports results from a physical, petrographic and geochemical analysis of 238 concretions from the Wheeler Formation (Cambrian Series 3), Utah, USA, which are unusual in several respects. Most prominently, they formed in organic‐poor mudstones (total organic carbon = 0·1 to 0·5%) and are characterized by a narrow range of δ¹³C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ¹³C demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments, but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters. The large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix, which occurred via both displacive and replacive carbonate precipitation during early diagenesis. Stable isotope data from cogenetic pyrite (δ³⁴S) and silica (δ¹⁸O) phases provide insight into the evolution of biogeochemical processes during concretion growth, and suggest that concretions were formed almost entirely during sulphate reduction, with only minor modification thereafter. Concretions of the Wheeler Formation appear to represent an end‐member system of concretion formation in which rapid growth was promoted by ions supplied from sea‐water. As such, they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments.     

Halite cementation and carbonate diagenesis of intra-salt reservoirs from the Late Neoproterozoic to Early Cambrian Ara Group (South Oman Salt Basin)

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra‐salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self‐charging hydrocarbon system within partly highly overpressured carbonate bodies (‘stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post‐dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re‐precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack‐seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.     

华北板块南缘寒武系苗岭统碳酸盐岩硬底：缺乏生物扰动背景下的早期海底胶结作用

华北地台南缘寒武系苗岭统碳酸盐岩硬底发育在核形石灰岩和鲕粒灰岩之上,并明显截断下伏的碳酸盐沉积物。本文通过对硬底界面之下颗粒间的放射纤维状方解石胶结物和等厚环边的微晶胶结物分析,表明其形成于早期海底胶结作用,该时期早期胶结区靠近沉积物—水界面,易受潮汐和波浪冲刷而暴露海底,并在持续搅动的高能环境中经历磨蚀和平整,形成硬底。由于硬底形成后几乎没有受到强烈改造和持续生物侵蚀的影响,通常表现出简单、平坦的表面形态。研究区馒头组二段含硬底层段形成于低、高能交替的潮间—潮下水道环境,以微晶方解石为主要的胶结剂沉淀;张夏组含硬底层段形成于台内鲕粒滩高速建造期,以物理化学沉淀和早期海底胶结作用为主。研究区出现的硬底表明,该时期海水化学条件和海底生态环境利于碳酸盐沉积物的早期胶结。硬底作为早期海底胶结作用的突出证据,对于研究碳酸盐沉积物的早期成岩作用及岩化特征具有重要意义,其形成过程及成因也为古海洋化学条件和底栖生态系统的变化提供了主要依据。     

Late Quaternary core stratigraphy of the northern New South Wales continental shelf

On the southeast Australian continental margin, mixed siliciclastic and temperate carbonate sediments are presently forming along the narrow 20–35 km‐wide northern New South Wales shelf over an area of 4960 km². Here, year‐round, highly energetic waves rework inner and mid‐shelf clastic sediments by northward longshore currents or waning storm flows. The strong East Australian Current flows south, sweeping clastic and outer shelf biogenic sands and gravels. Quaternary siliciclastic inner shelf cores consist of fine to medium, lower shoreface sand and graded storm beds of fine to coarse sand. Physically abraded, disarticulated molluscs such as Donacidae and Glycymeridae form isolated gravel lags. Highstand inner shelf clastics accumulate at 0.53 m/10³ y in less than 50 m water depth. Clastic mid‐shelf cores contain well‐sorted, winnowed, medium shoreface sands, with a fine sand component. Fine sand and mud in this area is discharged mainly from New South Wales’ largest river, the Clarence. The seaward jutting of Byron Bay results in weakened East Australia Current flows through the mid‐shelf from Ballina to Yamba allowing the fine sediments to accumulate. Quaternary carbonate outer shelf cores have uniform and graded beds forming from the East Australian Current and are also influenced by less frequent storm energy. Modern clastic‐starved outer shelf hardgrounds are cemented by coralline algae and encrusting bryozoans. Clay‐sized particles are dominantly high‐Mg calcite with minor aragonite and smectite/kaolinite. Carbonate sands are rich in bryozoan fragments and sponge spicules. Distinctive (gravel‐sized) molluscs form isolated shells or shell lag deposits comprising Limopsidae and Pectinidae. The upper slope sediments are the only significant accumulation of surficial mud on the margin (18–36 wt%), filling the interstices of poorly sorted, biogenic gravels. Pectinid molluscs form a basal gravel lag. During highstand the outer shelf accumulates sediment at 0.40 m/10³ y, with the upper slope accumulating a lower 0.23 m/10³ y since transgression. Transgression produced a diachronous (14–10 ka) wave‐ravinement surface in all cores. Relict marine hardgrounds overlie the wave‐ravinement surface and are cemented by inorganic calcite from the shallow and warm East Australian Current. Transgressive estuarine deposits, oxygen isotope Stage 3–5 barriers or shallow bedrock underlie the wave‐ravinement surface on the inner and mid shelf. Northern New South Wales is an example of a low accommodation, wave‐ and oceanic current‐dominated margin that has produced mixed siliciclastic‐carbonate facies. Shelf ridge features that characterise many storm‐dominated margins are absent.     

Gisbornian (Caradoc) graptolites from New South Wales,Australia: systematics,biostratigraphy and evolution

The biodiversity record of graptolites from the Cheeseman's Creek Formation, considered herein as late Gisbornian (Caradoc) in age, has been substantially increased to fifteen taxa, including the new species Dicellograptus praemorrisi sp. nov. and Climacograptus vandenbergi sp. nov. Some of the records have global correlative significance enabling us to identify the wilsoni Biozone ( = calcaratus Biozone of eastern Australia). Several evolutionary lineages have been recognized:

• 1 Dicellograptus moffatensis (Carruthers, 1858) → D. praemorrisi sp. nov → D. morrisi Hopkinson, 1871
• 2 Glossograptus hincksi Hopkinson, 1872 → Glossograptus? sp.
• 3 Climacograptus bicornis (J. Hall, 1847) → C. vandenbergi sp. nov. → C. lanceolatus VandenBerg, 1990.

Copyright © 2001 John Wiley & Sons, Ltd.     

Ordovician Intrusive-related Gold-Copper Mineralization in West-Central New South Wales, Australia

Three major types of Ordovician intrusive-related gold-copper deposits are recognized in central-west New South Wales, Australia: porphyry, skarn and high sulphidation epithermal deposits. These deposits are mainly distributed within two Ordovician volcano-intrusive belts of the Lachlan Fold Belt: the Orange-Wellington Belt and the Parkes-Narromine Belt. Available isotopic age data suggest that mineralization of the three types of deposits is essentially coeval with the Ordovician intrusive rocks (480-430 Ma).Porphyry gold-copper deposits can be further divided into two groups. The first group is associated with monzonite showing shoshonitic features, represented by Cadia and Goonumbla. The second group is associated with diorite and dacite, including the Copper Hill and Cargo gold-copper deposits. Gold skarn is associated with Late Ordovician (430-439 Ma) monzonitic intrusive complexes in the Junction Reefs area (Sheahan-Grants, Frenchmans, and Cor-nishmens), Endeavour 6, 7 and 44, Big and Little Cadia     

The present-day stress field of New South Wales,Australia

The Australian continent displays the most complex pattern of present-day tectonic stress observed in any major continental area. Although plate boundary forces provide a well-established control on the large-scale (>500 km) orientation of maximum horizontal stress (S_Hmax), smaller-scale variations, caused by local forces, are poorly understood in Australia. Prior to this study, the World Stress Map database contained 101 S_Hmax orientation measurements for New South Wales (NSW), Australia, with the bulk of the data coming from shallow engineering tests in the Sydney Basin. In this study we interpret present-day stress indicators analysed from 58.6 km of borehole image logs in 135 coal-seam gas and petroleum wells in different sedimentary basins of NSW, including the Gunnedah, Clarence-Moreton, Sydney, Gloucester, Darling and Bowen–Surat basins. This study provides a refined stress map of NSW, with a total of 340 (A–E quality) S_Hmax orientations consisting of 186 stress indicators from borehole breakouts, 69 stress measurements from shallow engineering methods, 48 stress indicators from drilling-induced fractures, and 37 stress indicators from earthquake focal mechanism solutions. We define seven stress provinces throughout NSW and determine the mean orientation of the S_Hmax for each stress province. The results show that the S_Hmax is variable across the state, but broadly ranges from NE–SW to ESE–WNW. The S_Hmax is approximately E–W to ESE–WNW in the Darling Basin and Southeastern Seismogenic Zone that covers the west and south of NSW, respectively. However, the present-day S_Hmax rotates across the northeastern part of NSW, from approximately NE–SW in the South Sydney and Gloucester basins to ENE–WSW in the North Sydney, Clarence-Moreton and Gunnedah basins. Comparisons between the observed S_Hmax orientations and Australian stress models in the available literature reveal that previous numerical models were unable to satisfactorily predict the state of stress in NSW. Although clear regional present-day stress trends exist in NSW, there are also large perturbations observed locally within most stress provinces that demonstrate the significant control on local intraplate sources of stress. Local S_Hmax perturbations are interpreted to be due to basement topography, basin geometry, lithological contrasts, igneous intrusions, faults and fractures. Understanding and predicting local stress perturbations has major implications for determining the most productive fractures in petroleum systems, and for modelling the propagation direction and vertical height growth of induced hydraulic fractures in simulation of unconventional reservoirs.     

Structural fabric evidence for indentation tectonics in the Nambucca Block,southern New England Fold Belt,New South Wales

Structural studies of Lower Permian sequences exposed on wave‐cut platforms within the Nambucca Block, indicate that one to two ductile and two to three brittle — ductile/brittle events are recorded in the lower grade (sub‐greenschist facies) rocks; evidence for four, possibly five, ductile and at least three brittle — ductile/brittle events occurs in the higher grade (greenschist facies) rocks. Veins formed prior to the second ductile event are present in some outcrops. Further, the studies reveal a change in fold style from west‐southwest‐trending, open, south‐southeast‐verging, inclined folds (F₁ ⁰) at Grassy Head in the south, to east‐northeast‐trending, recumbent, isoclinal folds (F₁ ⁰; F₂ ⁰) at Nambucca Heads to the north, suggesting that strain increases towards the Coffs Harbour Block. A solution cleavage formed during D₁ in the lower grade rocks and cleavages defined by neocrystalline white mica developed during D₁ and D₂ in the higher grade rocks. South‐ to south‐southwest‐directed tectonic transport and north‐south shortening operated during these earlier events. Subsequently, north‐northeast‐trending, open, upright F₃ ² folds and inclined, northwest‐verging, northeast‐trending F₄ ² folds developed with poorly to moderately developed axial planar, crenulation cleavage (S₃ and S₄) formed by solution transfer processes. These folds formed heterogeneously in S₂ throughout the higher grade areas. Later northeast‐southwest shortening resulted in the formation of en échelon vein arrays and kink bands in both the lower and higher grade rocks. Shortening changed to east‐northeast‐west‐southwest during later north‐northeast to northeast, dextral, strike‐slip faulting and then to approximately northwest‐southeast during the formation of east‐southeast to southeast‐trending, strike‐slip faults. Cessation of faulting occurred prior to the emplacement of Triassic (229 Ma) granitoids. On a regional scale, S₁ trends east‐west and dips moderately to the north in areas unaffected by later events. S₂ has a similar trend to S₁ in less‐deformed areas, but is refolded about east‐west axes during D₃. S₃ is folded about east‐west axes in the highest grade, multiply deformed central part of the Nambucca Block. The deformation and regional metamorphism in the Nambucca Block is believed to be the result of indenter tectonics, whereby south‐directed movement of the Coffs Harbour Block during oroclinal bending, sequentially produced the east‐west‐trending structures. The effects of the Coffs Harbour Block were greatest during D₁ and D₂.     

Cenozoic tectonics and landform evolution of the coast and adjacent highlands of southeast New South Wales

The Upper Precambrian and Lower Palaeozoic Rocks in the Mt Lofty Ranges, South Australia, have been subjected to at least three phases of folding. The first involved the formation of inclined folds and less common reclined folds. These structures are overprinted by usually upright, moderately tight, second and third generation folds which may show a well developed axial plane crenulation cleavage.

The metamorphism commenced prior to the appearance of penetrative structures and continued in many areas until after the third phase of deformation. It appears to have had its greatest effect during the static period following the first phase of folding.

Mineral assemblages of the pelitic rocks indicate that the metamorphism is of the low pressure‐intermediate type and that there are at least four progressive zones of metamorphism, namely, chlorite, biotite, andalusite‐staurolite, and sillimanite. Cordierite occurs in the sillimanite zone and kyanite is sporadically distributed in the andalusite‐staurolite zone. In the Angaston‐Springton region separate andalusite and staurolite zone boundaries may be delineated which cross as they are traced towards Angaston. This relationship is considered to be due to higher pressures operating during metamorphism in the latter area.

The maximum pressure and temperature reached in the metamorphism of these rocks are discussed in the light of recent experimental data.     

New Maikhanellid Shells and Siphogonuchitid Spicule Bundles from the Cambrian Fortunian Stage of South China

Maikhanellids are a distinct group of Cambrian Fortunian small shelly fossils by the cap-shaped profiles and scaly shell ornamentation. According to the individual shell shape and ornamentation feature types of 50 maikhanellids fossils which were collected from Xixiang botia, we confirmed the family Maikhanellidae content of six genera, which are Maikhanella, Ramenta, Purella, Ramentoides, Yunnannopleura and Mediata. Unlike the halkieriid animals whose shells and spicules are articulated on different body parts, maikhanellids were presumably yielded by siphogonuchitid spicules penetrating the intermediate matrix (the "spicule shell" hypothesis), and this is the reason that some researchers treated maikhanellids and co-occurring siphogonuchitids as synonyms. The affinities of Maikhanellids and siphogonuchitids remain debatable. Here, we report three dimensionally phosphatized maikhanellid shells and siphogonuchitid spicule bundles from the Cambrian Fortunian small shell faunas of South China. They differ from all the previously reported maikhanellids and siphogonuchitids, and may represent new types. The intact and smooth surface underneath the partially preserved scales of the shells challenges the "spicule-shell" hypothesis. The present siphogonuchitid spicules differ from the maikhanellid scales in size, morphology, and .arrangement pattern, thus the spicules might not be derived from the shells. Whether the shells and spicule bundles were articulated on different body parts of the same animals remains unknown.     

New Bivalved Arthropods from the Cambrian (Series 3, Drumian Stage) of Western Hunan,South China

We report two new three-dimensionally phosphatized microfossils, Cambrolongispina reticulata gen. et sp. nov. and Cambrolongispina glabra gen. et sp. nov., from the middle Cambrian （Series 3, Drumian Stage） at Wangcun, Yongshun County, western Hunan, South China. They are bivalved arthropods, with thin, pliable, originally chitinous or chitin-calcareous shields （c. 350-517 I1 m long）. The shields are equipped with a pair of antero-dorsal spines. The spines are internally hollow, varying in length from 1/2 of to as long as the shield length in C. reticulata, and uniformly longer than the shield length in C. glabra. The spines of C. reticulata are ornamented with a longitudinal row of conical or blade-shaped denticles along the posterior edge. Cambrolongispina lacks marginal rims, valve lobation and sulci which are diagnostic of the Bradoriida sensu stricto. It also lacks a donblure/duplicature characteristic of phosphatocopids and some bradoriids. It could be related to the Monasteriidae （which may belong to Bradoriida sensu lato）, both characterized by a pair of antero-dorsal spines. Cambrolongispina may have been meiofaunai detritus feeders that lived on or within sediments. The antero-dorsal spines may have been sensory organs to detect nearby predators. The posterior denticles on these spines might have facilitated the sensory function of the spines.     

华北地台中、上寒武统露头层序地层学研究的新认识

在生物地层对比基础之上通过对河北顺平清醒剖面、山东张夏和河北赵各庄标准剖面以及北京西山剖面的层序地层研究,将华北地台张夏阶和上寒武统自下而上划分并命名了5个总体向上变浅、以间断加深面为界的层序:张夏层序、崮山层序、崮长层序、长山层序和风山层序,并将上寒武统的4个层序与美国的相应层序进行了对比;阐述了层序地层单位与原来的生物地层、岩石地层和年代地层单位之间的关系,并用层序界面对原有岩石地层和年代地层单位的界线进行了优化;同时探讨了在克拉通陆表海背景下层序地层的发育特征以及相应的地质填图方法。     

Coronate echinoderms from the Ordovician of the Llanddowror area,South Wales

A large sample of coronates (over 90 specimens) from a narrow horizon in the Upper Ordovician, Sholeshook Limestone, at Talfan Farm, Llanddowror, South Wales allows assessment of variation and growth in a population of these rare Palaeozoic echinoderms. Variation is extensive and we synonymize two previously described species. This occurrence extends the stratigraphic range of Cupulocorona rugosa Donovan and Paul down to Cautleyan, Zone 2. Internal moulds reveal new morphological features, especially of the coronal canals. The flattened blade‐like and kite‐shaped portions support the idea that the coronal canals functioned in respiratory gas exchange by increasing the surface area. We suggest that internal currents flowed up the inner (adoral) canal and down the outer (aboral) canal. Quantitative estimates of flow rate suggest that a volume of coelomic fluid equivalent to the entire body cavity could be flushed through the coronal canals in 10 seconds at most. These coronates probably lived attached by a relatively long stem with a simple, conical holdfast and bent over in gentle currents. They seem to have been characteristic of relatively high‐energy environments and are not generally distributed in the Sholeshook Limestone of South Wales. Copyright © 2014 John Wiley & Sons, Ltd.     

Evidence for relative sea-level falls during deposition of the Upper Carboniferous Millstone Grit,South Wales

Detailed sedimentological facies analysis of the fluvio-deltaic Millstone Grit succession (Upper Carboniferous) of South Wales reveals that a number of cyclothems bounded by marine flooding surfaces (marine bands) in these strata exhibit facies architectures that represent erosion, non-deposition and/or deposition during periods of falling relative sea-level. A major fluvial complex below the Subcrenatum Marine Band, the Farewell Rock, lies within an incised valley, with a regional unconformity (sequence boundary) at its base. This unconformity is marked by deep erosional relief, an identifiable time gap and an angular discordance in bedding. The Cumbriense Quartzite, a correlative unit containing several mature palaeosols, records a depositional hiatus on a terrace-like interfluve that lay beyond the margins of the coeval Farewell Rock valley. Cyclothems in the underlying Middle Shales contain additional surfaces and units of subtler character. Beneath the Cancellatum Marine Band, a thin (15 cm), calcareous siltstone bed (the ‘Amroth Granule Bed’) that directly overlies prodelta shales contains reworked bioclasts, bored phosphorite clasts and quartz granules. Quartz granules in this bed are interpreted to represent relict lowstand, fluviatile? deposits, which were reworked during later transgression. Three further cyclothems contain sharp-based, storm-reworked mouth bars that record an abrupt lowering of wave base, most probably during periods of falling relative sea-level. One of these cyclothems also contains a distributary channel complex, which records an abrupt influx of coarse-grained sediment of ambiguous origin. The significance of these subtle surfaces and units for intracyclothem stratigraphy has rarely been considered; their prevalence in the Middle Shales provides evidence for numerous, high-frequency relative sea-level falls, which were previously unrecognized. These relative sea-level falls appear to alternate coherently with the widespread sea-level rises recorded by the marine bands, suggesting that glacio-eustasy is their most likely driving mechanism. The notion of glacio-eustatic sea-level falls is supported by the correlation of the basal Farewell Rock sequence boundary with sequence boundaries documented in adjacent basins. The angular unconformity and a change in sediment provenance at the base of the Farewell Rock, however, suggest an additional tectonic control on stratigraphic architecture here, namely a short-lived phase of rifting or inversion prior to widespread fluvial incision. Copyright © 1998 John Wiley & Sons, Ltd.     

西藏冈底斯带始新世曲水岩基的岩浆混合作用:来自斜长石阴极发光特征和成分变化的证据

斜长石作为主要造岩矿物,是研究岩石成因、示踪岩浆演化和岩浆混合过程的有效工具.对冈底斯带曲水岩基始新世花岗闪长岩、二长花岗岩、闪长岩脉和暗色包体中的斜长石进行了阴极发光图像结构特征、电子探针主量元素和LA-ICP-MS微量元素成分的分析,揭示了斜长石复杂环带的成因和相关的岩浆过程.该区斜长石的阴极发光图像呈现出多种颜色且与其An值相对应,随着An值降低依次为绿色、蓝色和暗灰色或暗红色等,并发育补丁状环带、筛状环带、韵律环带等.花岗闪长岩、二长花岗岩中斜长石的An值具有相似的变化范围(20~55),而闪长岩脉和暗色包体中An值的变化范围较大(25~85),表明曲水岩基经历了复杂的开放过程.微量元素结果表明:花岗闪长岩与闪长岩脉和暗色微粒包体具有相同的Sr含量范围(600×10^-6~1 100×10^-6);而二长花岗岩的Sr含量(1 000×10^-6~2 400×10^-6)整体高于前者.以上研究表明,花岗闪长岩中阴极发光呈现绿色的核部或幔部是偏中性岩浆注入寄主岩岩浆混合的结果;具有高Sr含量的二长花岗岩认为是高Sr含量的岩浆结晶形成的;闪长岩脉和暗色微粒包体中的筛状结构斜长石为寄主岩捕掳晶.     

重庆南川三汇场寒武系碳酸盐岩中不同期次大气淡水作用的证据

多期构造叠加及多种成岩环境下的早古生代碳酸盐岩储集层形成机理十分复杂。重庆南川三汇场剖面寒武系碳酸盐岩出露完整,是研究沉积与成岩史的理想解剖点之一。研究表明,三汇场剖面寒武系白云岩分别经历了早期海水胶结、大气淡水淋滤、准同生白云岩化、中浅层埋藏、中期构造大气水作用、中晚期埋藏及热液作用和构造晚期表生大气水成岩作用,不同期次的大气淡水作用特征不同。在早成岩期,沿向上变浅的高频层序界面附近发育了鲕模孔、窗格溶孔等组构性溶孔,胶结物和充填物的δ¹⁸O、δ¹³C值均比同期海水负偏,⁸⁷Sr/⁸⁶Sr值与同期海水相近,不发光(CL),含有较小的液相包裹体;加里东晚期—海西期,发育了非组构扩溶孔洞缝,伴有氧化沥青,不发光充填物的δ¹⁸O、δ¹³C负偏、⁸⁷Sr/⁸⁶Sr值正偏,发育含烃的低温不混溶包裹体,并叠加后期的深埋藏及热液成岩作用;燕山期和喜马拉雅期, 发育孤立的非组构溶蚀孔洞、缝洞,多被巨晶方解石或黏土充填,方解石中δ¹⁸O、δ¹³C强烈负偏($\delta^{13}C_{PDB}$=-4.6‰~-23.4‰,$\delta^{18}C_{PDB}$=-8.6‰~-17.8‰)、含有低温不混溶包裹体(<28.5℃),是有机质或生物甲烷(细)菌参于下的表生大气水成岩作用产物。