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151.
Origin and variability of a terminal Proterozoic primary silica precipitate,Athel Silicilyte,South Oman Salt Basin,Sultanate of Oman
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The Precambrian–Cambrian Athel Silicilyte is a 400 m thick, salt‐encased siliceous succession in the South Oman Salt Basin. It is a self‐sourcing hydrocarbon reservoir and comprises up to 95% microcrystalline quartz and exhibits wavy discontinuous lamination, comprising thin, alternating organic‐rich and silica‐rich layers. Textures and geochemical fingerprinting indicate that it is a primary precipitate formed by microbially mediated precipitation of silica from sea water, within the water column at the sulphidic/oxic interface. The unique occurrence of the Athel Silicilyte in the terminal Proterozoic implies that optimal conditions for this style of silica precipitation occurred only briefly. Basin anoxia, coupled with the growth of microbial mats, low pH and high silica pore water saturations, created optimal chemical conditions for silica precipitation. Volumes of microcrystalline quartz are highest within the transgressive and early highstand systems tract and towards the centre of the Athel Basin. At the basin margins, and within the late highstand systems tract, volumes of microcrystalline quartz decreased as the volume of detrital sediment increased. Mass‐balance calculations indicate that silica‐enriched sea water would have been supplied to the basin by infrequent marine incursions that replenished ambient sea water in the upper part of the water column. In conclusion, precipitation of the Athel Silicilyte was driven by the coincidence of basin restriction, limited clastic input, availability of organic matter and water column anoxia. The observation that there are few documented examples of chert deposits younger than ca 700 Ma, prior to the Cambrian explosion, suggests that although silica budgets within marine basins probably remained high prior to the evolution of silica‐secreting organisms, direct precipitation from sea water was restricted. This is tentatively related to the gradual increase in alkalinity of sea water through the Palaeo‐Proterozoic and Meso‐Proterozoic, such that silica precipitation could only occur through the coincidence of basin anoxia and low siliciclastic input. 相似文献
152.
Improved Vertical Streambed Flux Estimation Using Multiple Diurnal Temperature Methods in Series
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Analytical solutions that use diurnal temperature signals to estimate vertical fluxes between groundwater and surface water based on either amplitude ratios (Ar) or phase shifts (Δ?) produce results that rarely agree. Analytical solutions that simultaneously utilize Ar and Δ? within a single solution have more recently been derived, decreasing uncertainty in flux estimates in some applications. Benefits of combined (ArΔ?) methods also include that thermal diffusivity and sensor spacing can be calculated. However, poor identification of either Ar or Δ? from raw temperature signals can lead to erratic parameter estimates from ArΔ? methods. An add‐on program for VFLUX 2 is presented to address this issue. Using thermal diffusivity selected from an ArΔ? method during a reliable time period, fluxes are recalculated using an Ar method. This approach maximizes the benefits of the Ar and ArΔ? methods. Additionally, sensor spacing calculations can be used to identify periods with unreliable flux estimates, or to assess streambed scour. Using synthetic and field examples, the use of these solutions in series was particularly useful for gaining conditions where fluxes exceeded 1 m/d. 相似文献
153.
Transient storage zones (TSZs) are located at the interface of rivers and their abutting aquifers and play an important role in hydrological and biogeochemical functioning of rivers. The natural radioactive tracer 222Rn is a particularly well-suited tracer for studying TSZ water exchange and age. Although 222Rn measurement techniques have developed rapidly, there has been less progress in modeling 222Rn activities. Here, we combine field measurements with the numerical model HydroGeoSphere (HGS) to simulate 222Rn emanation, decay and transport during steady state (riffle-pool sequence) and transient (bank storage) conditions. Comparing the HGS mean water ages with the conventional 222Rn apparent ages during steady state showed a systemic underestimation of apparent age with increasing dispersion and especially where large concentration gradients exist within the subsurface. A large underestimation of apparent water age was also observed at the advective front during bank storage where regional high 222Rn groundwater mixes with newly infiltrated surface water. The explicit modeling of radiogenic tracers such as 222Rn offers a physical interpretation of this data as well as a useful way to test simplified apparent age models. 相似文献
154.
辐射处理金刚石的谱学研究及其意义 总被引:2,自引:0,他引:2
利用傅立叶显微红外光谱、紫外-可见-近红外吸收光谱、低温光致发光谱、拉曼光谱等4种谱学方法,对10颗辐照改色(R1-R10)的彩色天然金刚石进行了研究。利用可见吸收光谱和低温光致发光谱研究了金刚石石中的点缺陷产生系列的色心峰,指出在可见吸收光谱中除了有N3心吸收峰外,还可见595和637色心峰;在低温光致发光谱中发现有色心组合峰575,595和637等的出现,此组合峰与近红外区4929cm^-1处的H1b和5156cm^-1处的H1C色心峰共同存时,不仅是辐照处理绿色金刚石的鉴定证据,也是辐照处理各种彩色金刚石的重要的标志之一。本研究为辐射处理金刚石的鉴别和工艺流程提供了新的谱学依据和思路。 相似文献
155.
156.
Ian S. Buick Robert Frei Ian Cartwright 《Contributions to Mineralogy and Petrology》1999,135(2-3):244-254
Lower Calcsilicate Unit metasediments and underlying migmatitic Napperby Gneiss metagranite at Conical Hill in the Reynolds
Range, central Australia, underwent regional high-grade (∼680 to 720 °C), low-pressure/high-temperature metamorphism at 1594 ±
6 Ma. The Lower Calcsilicate Unit is extensively quartz veined and epidotised, and discordant grandite garnet + epidote quartz
veins may be traced over tens of metres depth into pegmatites that pooled at the Lower Calcsilicate Unit-Napperby Gneiss contact.
The quartz veins were probably precipitated by water-rich fluids that exsolved from partial melts derived from the Napperby
Gneiss during cooling from the peak of regional metamorphism to the wet granite solidus. Pb stepwise leaching (PbSL) on garnet
from three discordant quartz veins yielded comparable single mineral isochrons of 1566 ± 32 Ma, 1576 ± 3 Ma and 1577 ± 5 Ma,
which are interpreted as the age of garnet growth in the veins. These dates are in good agreement with previous Sensitive
High Resolution Ion Microprobe (SHRIMP) ages of zircon and monazite formed during high-temperature retrogression (1586 ± 5
to 1568 ± 4 Ma) elsewhere in the Reynolds Range. The relatively small age difference between peak metamorphism and retrograde
veining suggests that partial melting and melt crystallisation controlled fluid recycling in the high-grade rocks. However,
PbSL experiments on epidote intergrown with, and partially replacing, garnet in two of the veins yielded isochrons of 1454 ± 34
and 1469 ± 26 Ma. The ∼100–120 Ma age difference between intergrown garnet and late epidote from the same vein suggests that
the vein systems may have experienced multiple episodes of fluid flow.
Received: 24 April 1998 / Accepted: 17 December 1998 相似文献
157.
Controls on turbidite sand deposition during gravity-driven extension of a passive margin: examples from Miocene sediments in Block 4, Angola 总被引:1,自引:0,他引:1
In recent years, exploration of the Lower Congo Basin in Angola has focused on the Neogene turbidite sand play of the Malembo Formation. Gravity tectonics has played an important role during deposition of the Malembo Formation and has imparted a well-documented structural style to the post-rift sediments. An oceanward transition from thin-skinned extension through mobile salt and eventually to thin-skinned compressional structures characterises the post-rift sediments. There has been little discussion, however, regarding the influence of these structures on the deposition of the Malembo Formation turbidite sands. Block 4 lies at the southern margin of the Lower Congo Basin and is dominated by the thin-skinned extensional structural style. Using a multidisciplinary approach we trace the post-rift structural and stratigraphic evolution of this block to study the structural controls on Neogene turbidite sand deposition.In the Lower Congo Basin the transition from terrestrial rift basin to fully marine passive margin is recorded by late Aptian evaporites of the Loeme Formation. Extension of the overlying post-rift sequences has occurred where the Loeme Formation has been utilised as a detachment surface for extensional faults. Since the late Cretaceous, the passive margin sediments have moved down-slope on the Loeme detachment. This history of gravity-driven extension is recorded in the post-rift sediments of Block 4. Extension commenced in the Albian in the east of the block and migrated westwards with time. In the west, the extension occurred mainly in the Miocene and generated allochthonous fault blocks or “rafts”, separated by deep grabens. The Miocene extension occurred in two main phases with contrasting slip vectors; in the early Miocene the extension vector was to the west, switching to southwest-directed extension in the late Miocene. Early Miocene faults and half-grabens trend north–south whereas late Miocene structures trend northwest–southeast. The contrast in slip vectors between these two phases emphasises the differences in driving mechanisms: the early Miocene faulting was driven by basinward tilting of the passive margin, but gravity loading due to sedimentary progradation is considered the main driver for the late Miocene extension. The geological evolution of the late Miocene grabens is consistent with southwest-directed extension due to southwest progradation of the Congo fan.High-resolution biostratigraphic data identifies the turbidite sands in Block 4 as early Miocene (17.5–15.5 Ma) and late Miocene (10.5–5.5 Ma) in age. Deposition of these sands occurred during the two main phases of gravity-driven extension. Conditions of low sedimentation rates relative to high fault displacement rates were prevalent in the early Miocene. Seafloor depressions were generated in the hangingwalls of the main extensional faults, ultimately leading to capture of the turbidity currents. Lower Miocene turbidite sand bodies therefore trend north–south, parallel to the active faults. Cross-faults and relay ramps created local topographic highs capable of deflecting turbidite flows within the half grabens. Flow-stripping of turbidity currents across these features caused preferential deposition of sands across, and adjacent to, the highs. Turbidite sands deposited in the early part of the late Miocene were influenced by both the old north–south fault trends and by the new northwest–southeast fault trends. By latest Miocene times turbidite channels crosscut the active northwest–southeast-trending faults. These latest Miocene faults had limited potential to capture turbidity currents because the associated hangingwall grabens were rapidly filled as pro-delta sediments of the Congo fan prograded across the area from the northeast. 相似文献
158.
Large amounts of fluid, bound up in the hydrated upper layers of the ocean crust, are consumed at convergent margins and released in subduction zones through devolatilization. The liberated fluids may play an integral role in subduction zone processes, including the generation of arc-magmas. However, exhumed subduction zone rocks often record little evidence of large-scale fluid flow, especially at deeper levels within the subduction zone. Basaltic pillows from the high-pressure Corsican and Zermatt-Saas ophiolites show a range of δ18 O values that overall reflect seafloor alteration prior to subduction. However, comparison between the δ18 O values of the cores and rims of the pillows suggests that the δ18 O values of the pillow rims at least have been modified during subduction and high-pressure metamorphism. Pillows that have not undergone high-pressure metamorphism generally have rims with higher δ18 O values than their cores, whereas the converse is the case in pillows that have undergone high-pressure metamorphism. This reversal in the core to rim oxygen isotope relationship between unmetamorphosed and metamorphosed pillows is strong evidence for fluid–rock interaction occurring during subduction and high-pressure metamorphism. However, the preservation of different δ18 O values in the cores and rims of individual pillows and within and between different pillows suggests that fluid flow within the subduction zone was strongly channelled. Resetting of the δ18 O values in the pillow rims was probably due to fluid-hosted diffusion that occurred over relatively short time-scales (<1 Myr). 相似文献
159.
160.
Frederick J. Griffin Theresa DiMarco Karl L. Menard Joe A. Newman Edmund H. Smith Carol A. Vines Gary N. Cherr 《Estuaries and Coasts》2012,35(5):1229-1236
Pacific herring early life stages provide good model systems for studying effects of suspended sediments on estuarine organisms. To investigate effects on the herring larval stage, we used environmentally relevant particle concentrations for San Francisco Bay (200?C400?mg/L of particles <50???m in size) and exposure times of 16?h in a novel two-pump sediment suspension mesocosm. There were no mortalities during the 16-h suspended sediment incubation. Following sediment exposure, larvae were cultured in sediment-free water for up to 10?days during which survival and condition were measured. None were affected by previous sediment treatment. Four criteria for larval condition included growth, heart rate, prey capture, and critical swimming velocity. These results provide a framework for implementing regulatory decisions on anthropogenic activities such as dredging. 相似文献