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Abstract Using a detailed petrographical procedure conceived for arenites rich in carbonate clasts, the influence of tectonism and eustacy on silicate/carbonate cycles of the Eocene Hecho Turbidite Complex has been tested, and the palaeogeography of the source/basin system outlined.
Both extrabasinal and intrabasinal sources of sediments were active during basin filling. The extrabasinal source terrains, located in the southern sector of the basin, were made of the Pyrenean crystalline basement (granites, gneisses and phyllites) overlain mainly by carbonate rocks (Cretaceous limestones and dolostones, minor chert and siltstones). The intrabasinal sources, represented by foramol shelf carbonate factories, provided penecontemporaneous carbonate bioclasts, intraclasts and peloidal grains.
Foreland thrusting in the South-Central Pyrenees has acted as the major control on the composition and architecture of the Hecho Turbidite Complex. Strong uplift of old silicate and carbonate source terrains during southward thrust propagation was responsible for erosion, swamping and/or reduction of shelfal areas, and gave rise to siliciclastic and carbonate basinal sequences (silicate arenites and calclithites) during lowstand stages. Conversely, hybrid arenites (mixture of extrabasinal and intrabasinal grains) originated from resedimentation of marginal shelf sediments produced in carbonate factories active during the initial phase of sea-level rise. Hybrid arenites with minor intrabasinal content also formed during one stage of relative sea-level fall from the erosion of previously accumulated highstand complexes.
During resedimentation processes, hybrid sands underwent marked hydraulic selection documented by deposits depleted in carbonate grains in the channel area, and by thin-bedded turbidites rich in platy-skeletal fragments, low-density peloids and void-rich bioclasts down-basin. 相似文献
Both extrabasinal and intrabasinal sources of sediments were active during basin filling. The extrabasinal source terrains, located in the southern sector of the basin, were made of the Pyrenean crystalline basement (granites, gneisses and phyllites) overlain mainly by carbonate rocks (Cretaceous limestones and dolostones, minor chert and siltstones). The intrabasinal sources, represented by foramol shelf carbonate factories, provided penecontemporaneous carbonate bioclasts, intraclasts and peloidal grains.
Foreland thrusting in the South-Central Pyrenees has acted as the major control on the composition and architecture of the Hecho Turbidite Complex. Strong uplift of old silicate and carbonate source terrains during southward thrust propagation was responsible for erosion, swamping and/or reduction of shelfal areas, and gave rise to siliciclastic and carbonate basinal sequences (silicate arenites and calclithites) during lowstand stages. Conversely, hybrid arenites (mixture of extrabasinal and intrabasinal grains) originated from resedimentation of marginal shelf sediments produced in carbonate factories active during the initial phase of sea-level rise. Hybrid arenites with minor intrabasinal content also formed during one stage of relative sea-level fall from the erosion of previously accumulated highstand complexes.
During resedimentation processes, hybrid sands underwent marked hydraulic selection documented by deposits depleted in carbonate grains in the channel area, and by thin-bedded turbidites rich in platy-skeletal fragments, low-density peloids and void-rich bioclasts down-basin. 相似文献
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Rhodolith-bearing limestones as transgressive marker beds: fossil and modern examples from North Island, New Zealand 总被引：2，自引：0，他引：2
Rhodoliths are nodular structures composed mainly of the superimposed thalli of calcareous red algae. Because their development is controlled by an array of ecological parameters, rhodoliths are a valuable source of palaeoenvironmental information. However, despite their common use in palaeoecological reconstructions, the stratigraphic significance of rhodolith accumulations seldom has been addressed in detail. In a study of Cenozoic rhodolith‐bearing deposits from the North Island of New Zealand, rhodolithic units, usually of limited lateral extent, typically occur above major unconformities at the base of deepening upwards successions. Two types of transgressive rhodolith‐bearing deposits may be distinguished on the basis of texture and rhodolith internal structure: (i) type A deposits are clast‐supported rhodolithic rudstones containing abundant pebbles and cobbles reworked from the substrate, and are characterized by rhodoliths with a compact concentric to columnar internal structure and a high nucleus to algal cover ratio; (ii) type B deposits are rhodolithic floatstones with a matrix usually consisting of bryozoan fragments, benthic foraminifera and echinoid fragments or terrigenous silty fine sand. The rhodoliths of type B units usually have a loose internal framework with irregular to branched crusts. The two contrasting rhodolith‐bearing units are interpreted as characteristic facies of transgressive systems tract deposits, analogous to shell concentrations formed under conditions of low net sedimentation. Type A deposits are correlated with relatively high‐energy settings and/or narrow submerged palaeotopographic lows, whereas type B deposits are interpreted as forming in lower‐energy settings. The association between transgression and development of rhodolithic facies is confirmed by observations of a modern rhodolith production site at Whangaparaoa Peninsula in North Island, where algal nodules grow above a ravinement surface cut during the Holocene sea‐level rise, and also by a review of published fossil examples, many of which show stratigraphic and compositional attributes analogous to those of the New Zealand occurrences. The review indicates that transgressive rhodolith accumulations develop more commonly in, but are not restricted to, non‐tropical settings. It is suggested that a combination of factors, such as low net sedimentary input, nature of the substrate, sea‐level rise and inherited physiography contribute to determine the relationship between rhodolith‐bearing deposits and transgressive settings. 相似文献
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DANIELA FONTANA 《Sedimentology》1991,38(6):1085-1095
The Upper Cretaceous Pietraforte Formation, an allochthonous unit of the Ligurian domain in the northern Apennines, provides a case study of the importance of detrital carbonate grains for provenance determination in sandstones. The Pietraforte Formation is composed of turbidite sandstones with subordinate conglomerate, deposited in an external sector of the Ligurian ocean, close to the Adriatic margin. The sandstones have a lithic composition, characterized by abundant sedimentary and metasedimentary rock fragments (35–56% of the terrigenous framework), little feldspar (<7%) that is almost exclusively plagioclase, and a high ratio of fine- to coarse-grained polycrystalline quartzose grains to total quartzose grains (average Qp/Qt=0.37). Carbonate rock fragments dominate the lithic association of both sandstones and conglomerates and provide the most detailed information for provenance determination. They are composed primarily of dolostones and a wide variety of limestones containing identifiable age-diagnostic microfossils. Fossils and rock textures of carbonate clasts document the erosion of Upper Triassic to Lower Cretaceous shelf and pelagic carbonate units which can be matched with Mesozoic rock types present in the Tuscan domain of the northern Apennines. Compositional results constrain the source of the Pietraforte Formation sandstones to the western margin of the Adriatic plate, from uplifted sedimentary and metasedimentary rocks of the Tuscan domain and its low-grade metamorphic basement. Coeval intrabasinal sources provided additional supplies to the depositional basin of the Pietraforte Formation; this intrabasinal supply consists of shelf carbonate allochems, planktonic foraminifera and argillaceous rip-up clasts. The presence of carbonate grains from shallow-water environments may indicate the existence during deposition of marginal shelf areas favourable for carbonate allochem production. 相似文献
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Temperature and Bulk Composition Control on the Growth of Monazite and Zircon During Low-pressure Anatexis (Mount Stafford, Central Australia) 总被引：5，自引：0，他引：5
The formation, age and trace element composition of zircon andmonazite were investigated across the prograde, low-pressuremetamorphic sequence at Mount Stafford (central Australia).Three pairs of inter-layered metapelites and metapsammites weresampled in migmatites from amphibolite-facies (T 600°C)to granulite-facies conditions (T 800°C). Sensitive high-resolutionion microprobe U–Pb dating on metamorphic zircon rimsand on monazite indicates that granulite-facies metamorphismoccurred between 1795 and 1805 Ma. The intrusion of an associatedgranite was coeval with metamorphism at 1802 ± 3 Ma andis unlikely to be the heat source for the prograde metamorphism.Metamorphic growth of zircon started at T 750°C, well abovethe pelite solidus. Zircon is more abundant in the metapelites,which experienced higher degrees of partial melting comparedwith the associated metapsammites. In contrast, monazite growthinitiated under sub-solidus prograde conditions. At granulite-faciesconditions two distinct metamorphic domains were observed inmonazite. Textural observations, petrology and the trace elementcomposition of monazite and garnet provide evidence that thefirst metamorphic monazite domain grew prior to garnet duringprograde conditions and the second in equilibrium with garnetand zircon close to the metamorphic peak. Ages from sub-solidus,prograde and peak metamorphic monazite and zircon are not distinguishablewithin error, indicating that heating took place in less than20 Myr. KEY WORDS: accessory phases; anatexis; trace element partitioning; U–Pb dating 相似文献
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DANIELA A. VALLINI BIRGER RASMUSSEN BRYAN KRAPE&#; IAN R. FLETCHER NEAL J. MCNAUGHTON 《Sedimentology》2005,52(1):101-122
An ≈ 26 m thick unit of phosphatic sandstone and black shale (the Phosphatic Unit) in the Palaeoproterozoic Mount Barren Group of south-western Australia contains abundant authigenic xenotime crystals showing well-preserved diagenetic textures. Despite extensive regional deformation and thermal metamorphism, the peak of which occurred at ≈ 1205 Ma, the Phosphatic Unit was preserved as a low-strain envelope because of its pre-compaction carbonate and phosphate cementation. In situ U–Pb geochronology of xenotime reveals four discrete age populations at 1693 ± 4, 1645 ± 3, 1578 ± 10 and 1481 ± 21 Ma. When integrated with petrography, the age data place a timeframe on: (i) sediment deposition; (ii) phosphogenesis; (iii) diagenetic cement infilling; (iv) diagenetic pyrite formation; (v) secondary porosity generation; (vi) hydrocarbon migration; (vii) burial compaction; and (viii) hydrothermal alteration, up until peak thermal metamorphism. Xenotime growth at ≈ 1693 Ma occurred prior to compaction, whereas xenotime growth at ≈ 1645 Ma occurred during burial. Xenotime growth at ≈ 1580 Ma and at ≈ 1480 Ma appears to be the far-field record of thermotectonic events associated with intracontinental extension and magmatism recorded elsewhere in Australia. Geochemical analysis, integrated with geochronology, shows a systematic increase in MREE/HREE in xenotime crystals with decreasing age and with increasing stratigraphic depth. Coupled with a decrease in xenotime abundance and age with depth, it suggests that: (i) the main focus of porosity infilling was at the top of the Phosphatic Unit and progressed downwards over the > 200 Myr period of porosity infilling, and (ii) the changes in xenotime REE chemistry may be due to an influx of MREE from increasing amounts of dissolved apatite or changes, with respect to REE solubility, in the physiochemical nature of the fluids with burial depth. 相似文献
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ROBERTO SULPIZIO DANIELA MELE PIERFRANCESCO DELLINO LUIGI LA VOLPE 《Sedimentology》2007,54(3):607-635
Small-scale pyroclastic density currents (PDCs) associated with the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy, were generated by both magmatic and phreatomagmatic explosive fragmentation. The resulting deposits were emplaced under flow boundary conditions dominated by varying combinations of grain interaction, fluid escape and traction processes. Stratigraphic and lithofacies analysis of these PDCs offers a new perspective on the en masse versus progressive aggradation debate for PDC deposition. In particular, the analyses indicate that PDCs were density stratified with a basal underflow dominated by grain interactions. The underflows comprised trains of self-organized granular pulses of variable thickness and magnitude, depending on the overall particle concentration and fluid turbulence. A change in gradient between the upper and lower slopes of the volcano promoted deposition and the different pulses aggraded sequentially (stepwise). In this model each pulse stops en masse and the whole deposit aggrades progressively. Particle concentration, density, mean velocity, and flow height were assessed for the studied PDCs using differaent methods for massive and stratified deposits. The calculated mobility of the flows was 0·2 to 0·3, in the expected range for small-scale PDCs. 相似文献
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