Impact of lake‐level changes on the formation of thermogene travertine in continental rifts: Evidence from Lake Bogoria,Kenya Rift Valley     

ROBIN W. RENAUT  R. BERNHART OWEN  BRIAN JONES  JEAN‐JACQUES TIERCELIN  CORINNE TARITS  JOHN K. EGO  KURT O. KONHAUSER 《Sedimentology》2013,60(2):428-468

Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO₃ waters containing little Ca (<2 mg l^?1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO₂ degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca²⁺ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.  相似文献   

Performance evaluation and work-load estimation for geographic information systems     

MICHAEL F. GOODGHILD  BRIAN R. RIZZO 《International journal of geographical information science》2013,27(1):67-76

Abstract

Agencies acquiring GIS hardware and software are faced with uncertainty at two levels: over the degree to which the proposed system will perform the functions required, and over the degree to which it is capable of doing so within proposed production schedules. As the field matures the second concern is becoming more significant. A formal model of the process of acquiring a GIS is presented, based on the conceptual level of defining GIS sub-tasks. The appropriateness of the approach is illustrated using performance data from the Canada Land Data System. It is possible to construct reasonably accurate models of system resource utilization using simple predictors and least squares techniques, and a combination of inductive and deductive reasoning. The model has been implemented in an interactive package for MS-DOS systems.  相似文献   

Sedimentology and Chronology of Paleogene Coarse Clastic Rocks in East-Central Tibet and Their Relationship to Early Tectonic Uplift     

ZHOU Jiangyu WANG Jianghai K. H. BRIAN A. YIN M. S. MATTHEW 《《地质学报》英文版》2007,81(3):398-408

A systematic sedimentological and chronological study of typical Paleogene basins in eastcentral Tibet suggests that the depositional characteristics of extensively developed huge-bedded, purplish-red coarse clastic rocks formed in a tectonic setting of regional thrusting and strike-slipping represent a typical dry and hot subaerial alluvial fan environment formed in a proximal and rapidaccumulating sediment body in debris flows and a fan-surface braided river. Combining results from basin-fill sequences, sequences of coarse clastic rocks, fauna and sporo-pollen associations and thermochronological data, it is conduded that the coarse clastic rocks formed in the period of 54.2- 24.1 Ma, nearly coeval with the formation of Paleogene basins in the northern （Nangqen-Yushu thrust belt）, middle （Batang-Lijiang fault belt）, and disintegration of large basins in the southern （LanpingSimao fold belt） segments of Tibet. The widespread massive-bedded coarse clastic rocks, fold thrusting and strike-slip, thrust shortening, and igneous activities in the Paleogene basins of eastcentral Tibet indicate that an early diachronous tectonic uplift might have occurred in the Tibetan Plateau from Middle Eocene to Oligocene, related to the initial stage of collision of the Indian and Asian plates.  相似文献   

Tectono-sedimentary analysis of alternate-polarity half-graben basin-fill successions: Late Devonian-Early Carboniferous Horton Group, Cape Breton Island, Nova Scotia     

ANTHONY P. Hamblin  BRIAN R. Rust † 《Basin Research》1989,2(4):239-255

Abstract The Devono-Carboniferous Horton Group of Cape Breton Island was mostly deposited in two fault-bounded asymmetric sub-basins which were part of a large intracontinental rift system. This system lay at a palaeolatitude of about 10–15^o S–a warm, semi-arid climate. The half-graben sub-basins had opposed polarity, were approximately 100 times 50 km in size and were separated by a narrow zone of elevated Acadian basement. These features are common to the adjacent structural segments of known rifts, and are unlike those of transtensive pull-apart systems. Sedimentation occurred in four successive depositional systems which reflect a tectonic evolution of increased and then decreased extensional subsidence through the 8–12 Myr interval represented. Post-Acadian sedimentation began with System 1 bimodal volcanics and grey distal braided fluvial sediments deposited in a slowly subsiding broad linear sag basin. System 2 consists of reddened braidplain sediments near fault-bounded margins and mudflat/playa sediments in sub-basin centres, deposited in two discrete asymmetric sub-basins with a general upward-fining trend. Gradual expansion of the mudflat setting and confinement of coarse marginal fades is interpreted as a response to increasingly rapid and deep fault-bounded subsidence. Depositional System 3, is a complex of grey lacustrine offshore, shoreline and fan delta facies deposited in two adjacent half-graben segments with opposed polarity of asymmetry. An increased rate of tectonic subsidence allowed a large standing body of water to accumulate lacustrine sediments along the axis of each sub-basin during this phase of maximum subsidence. System 4 consists of reddened proximal alluvial fan, medial fluvial and distal grey meandering fluvial/floodplain sediments which accumulated in sub-basins with fault-bounded margins and asymmetry identical to those of earlier systems, indicating a continuation of tectonic style. However, an overall coarsening-upward trend indicates waning of active fault-related subsidence and consequent progradation of marginal coarse wedges to fill the sub-basins. Rapid marine transgression and deposition of Windsor Group carbonates, evaporites and elastics continued within a more extensive rift basin during renewed fault-bounded subsidence.  相似文献   

Relative dating of moraines using moraine morphometric and boulder weathering criteria, Kigluaik Mountains, Alaska     

BRIAN J. PECK  DARRELL S. KAUFMAN  PARKER E. CALKIN 《Boreas: An International Journal of Quaternary Research》1990,19(3):226-239

Six measurements of surface-boulder weathering and seven measures of moraine morphometrics were taken at 57 sites in the Kigluaik Mountains, Alaska, to test the morphostratigraphic division of five Quaternary glacial units, to test a threefold subdivision of the late Pleistocene glacial unit, and to estimate the timing of glacial advances. Group means from 70% of the relative-age measures exhibit a positive relationship with relative age of the glacial units. The measures most effective at differentiating between moraine groups were: boulder frequency, tall-boulder frequency, boulder height, distal slope, and boulder angularity. Results of dixriminant analysis indicate that moraine-morphometric measures provided slightly better classification results than those of boulder weathering. Discriminant analysis correctly classified 89% of the a priori grouped sites. Multivariate analysis supports the attempted threefold subdivision of the latest Pleistocene Mount Osborn moraines at the 0.056 level of significance. Estimated ages of 60,000 BP for the Salmon Lake and 165,000 BP for the Stewart River glacial units were interpolated by using the relative-age data and ages of 18,000 BP for the Mount Osborn and 810,000 BP for the Nome River units.  相似文献   

The Origin of Taranaki Bay' in Early New Zealand Maps     

BRIAN HOOKER 《New Zealand geographer》1990,46(2):92-94

  相似文献   

A Critical Note on ‘Eco‐Civic Regionalisation’ as the Basis for Local Government Boundaries in Australia     

BRIAN DOLLERY  LIN CRASE 《The Australian geographer》2004,35(3):289-300

The notion of ‘eco‐civic regionalisation’ has been applied recently to New South Wales by Brunckhorst, Coop and Reeve (2004 Brunckhorst D Coop P Reeve I 2004 An eco‐civic regionalisation for rural New South Wales: final report to the NSW government Institute for Rural Futures and Centre for Bioregional Resource Management, University of New England, Armidale  [Google Scholar]) in order to identify the appropriate administrative boundaries for ‘socio‐civic’ regions and ‘biophysical’ regions. On the basis of this analysis, they recommended inter alia that 49 future non‐metropolitan ‘local government areas’ be established. This proposal was adopted with alacrity by advocates of the NSW government's program of compulsory council amalgamation, including official ‘Facilitators’ appointed by the State government to draft formal consolidation proposals. This paper disputes the applicability of ‘eco‐civic regionalisation’ as the foundation for local government boundaries in Australia.  相似文献   

THE DISTRIBUTION AND ECOLOGICAL-BIOLOGICAL FEATURES OF CREPIDULA ONYX IN HONG KONG     

BRIAN MORTON  叶颖薇  黄宗国 《海洋学报(英文版)》1984,3(2):263-275

C. onyx, originally distributed in California, was found in great quantities in Victoria Harbour, Hong Kong in 1982 and was identified according to the study of its morphology and the radula teeth. Lots of the individuals of the species lived in overlapping colony, with the female at the lower, and the male in the upper while the sex-transforming individual at the middle. They attached on the shells of Perna viridis in a high density of 715 ind./m2. Studies have been carried out on its population structure, individual size, age and sex composition and on reproductive biology, such as sex transformation, change of the length of penis and fecundity.  相似文献   

An intercomparison of hydrological network-design technologies     

MARSHALL E. MOSS  GARY D. TASKER 《水文科学杂志》2013,58(3):209-221

Abstract

Two network-design technologies are compared by random sub-sampling of actual streamflow data. The technologies, Network Analysis for Regional Information (NARI) and Network Analysis Using Generalized Least Squares (NAUGLS), have a common objective, viz. to maximize regional information within a limited budget and time horizon. The data used for intercomparison are from a network of 146 streamgauges in the central part of the United States. In general, the results for the illustrative example indicate that the NAUGLS method conveys more information than the NARI method to the network designer interested in maximizing regional information about mean annual flows with a limited budget.  相似文献   

Towards a PV-θ view of the general circulation   总被引：2，自引：0，他引：2  

BRIAN J. HOSKINS 《Tellus. Series B, Chemical and physical meteorology》1991,43(4):27-35

  相似文献