Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea     

SANDRA PASSCHIER  CEES LABAN  CHRIS S. MESDAG  KENNETH F. RIJSDIJK 《Boreas: An International Journal of Quaternary Research》2010,39(3):633-647

Passchier, S., Laban, C., Mesdag, C.S. & Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483. Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions.  相似文献   

Editor's Report     

KENNETH J. HSÜ 《Sedimentology》1973,20(4):631-636

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Genesis of metapelitic migmatites in the Adirondack Mountains, New York     

P. J. LANCASTER  B. FU  F. Z. PAGE  N. T. KITA  M. E. BICKFORD  B. M. HILL  J. M. McLELLAND    J. W. VALLEY 《Journal of Metamorphic Geology》2009,27(1):41-54

Oxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. Values of δ¹⁸O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (～1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite facies NW Adirondacks and the granulite facies SE Adirondacks, δ¹⁸O_(Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is typically unzoned and δ¹⁸O_(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ¹⁸O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ¹⁸O_(Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ¹⁸O_(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ¹⁸O metasedimentary rocks.  相似文献   

WRITING FOR NONGEOGRAPHERS: LESSONS FROM THE J. B. JACKSON PRIZE     

KENNETH D. MADSEN 《Geographical review》2002,92(1):63-72

ABSTRACT. Recent presidents of the Association of American Geographers have made it clear that as a discipline we need to do more to reach outside the confines of academia. Since 1985, the J. B. Jackson Award has recognized individuals whose books provide examples of how to share professional geographical knowledge with the educated lay public. Through correspondence with these individuals, recommendations for improving geographers' outreach are presented.  相似文献   

Stream-dominated alluvial fan and lacustrine depositional systems in Cenozoic strike-slip basins, Denali fault system, Yukon Territory, Canada     

KENNETH D. RIDGWAY  PETER G. DECELLES† 《Sedimentology》1993,40(4):645-666

Ancient stream-dominated (‘wet’) alluvial fan deposits have received far less attention in the literature than their arid/semi-arid counterparts. The Cenozoic basin fills along the Denali fault system of the northwestern Canadian Cordillera provide excellent examples of stream-dominated alluvial fan deposits because they developed during the Eocene-Oligocene temperate climatic regime in an active strike-slip orogen. The Amphitheatre Formation filled several strike-slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike-slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream-dominated alluvial-fan systems. Proximal fan deposits in the Bates Lake Basin are characterized by disorganized, clast-supported, boulder conglomerate and minor matrix(mud)-supported conglomerate. Proximal facies are located along the faulted basin margins in areas where C:S = 80 to 100 and where the average MPS ranges from 30 to 60 cm. Proximal fan deposits grade into middle fan, channelized, well organized cobble conglomerates that form upward fining sequences, with an average thickness of 7 m. Middle fan deposits grade basinward into well-sorted, laterally continuous beds of normally graded sandstone interbedded with trough cross-stratified sandstone. These distal fan deposits are characteristic of areas where C:S = 20 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe deposits consist of lacustrine and axial fluvial facies. Palaeogeographic reconstruction of the Bates Lake Basin indicates that alluvial-fan sedimentation was concentrated in three parts of the basin. The largest alluvial-fan system abutted the strike-slip Duke River fault, and prograded westward across the axis of the basin. Two smaller, coarser grained fans prograded syntaxially northward from the normal-faulted southern basin margin. Facies analysis of the Burwash Basin indicates a similar transition from proximal to distal, stream-dominated alluvial fan environments, but with several key differences. Middle-fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m thick composed of fine-grained lithofacies and coal in the lower part, trough cross-stratified sandstone in the middle, and conglomerate in the upper part of the sequence. Upward-coarsening sequences, 90–140 m thick, also are common in the fan fringe lacustrine deposits. These sequences coarsen upward from mudstone, through fine grained, ripple-laminated sandstone, to coarse grained trough cross-stratified sandstone. The upward-coarsening sequences are basinwide, facies independent, and probably represent progradation of stream-dominated alluvial-fan depositional systems. Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream-dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal development in the intervening middle and distal fan areas was suppressed by the high frequency of unconfined flow events and lateral channel mobility.  相似文献   

Allogenic controls on the evolution of storm to tidal shelf sequences in the Early Proterozoic Uncompahgre Group, southwest Colorado, USA     

CHARLES W. HARRIS  KENNETH A. ERIKSSON 《Sedimentology》1990,37(2):189-213

Dominantly coarse-grained, shallow-marine, metasedimentary rocks of the Early Proterozoic Uncompahgre Group (UG) record periods of shoaling and drowning on different temporal scales that are attributed to episodic long-term oscillations in relative sea-level with superimposed shorter duration excursions in relative sea-level. Long-term events are probably tectonic whereas short-term events are eustatic. The 2–5 km thick Uncompahgre Group consists of 250–600 m thick, dominantly coarse-grained quartzite units (Q1–Q4) and 200–300 m thick mudstone/pelite units (P1–P5). Five depositional systems comprise the Uncompahgre Group. The outer shelf system (OSS) is composed of Bouma-type beds and intercalated mudstones that are transitional vertically to parallel-laminated to wave-rippled sandstones and hummocky cross-stratified sandstones of the inner shelf system (ISS). Trough cross-stratified sandstones comprise the shoreface system (SHS). The tidal inner shelf/shoreface system (TIS/SHS) consists of a complex interlayering of cross-bedded sandstones, thin-bedded conglomerates, mudstones and rippled sandstones. Trough cross-bedded pebbly sandstones and thin- to thick-bedded conglomerates represent the alluvial system (ALLS). Depositional systems in the UG are associated in transgressive and highstand-systems tracts that make up four sequences (1 to 4). Sequence boundaries do not correspond with lithostratigraphic boundaries but are defined by subtle unconformities. The basal Q1–P1 unit (lower sequence 1) consists of ALLS to TIS/ SHS to ISS comprising a transgressive systems tract. A maximum marine incursion is reflected by deposition of OSS facies in stratigraphic units P1–P2. Shoaling in the transition from P2 to the uppermedial portion of Q2 (OSS—ISS—SHS to a thick TIS/SHS—ALLS) records the highstand systems tract of upper sequence 1. A subtle disconformity/paraconformity delineates a type 2 sequence boundary at the top of the highstand systems tract. The drowning to shoaling pattern is replicated in sequence 2 (upper Q2 to P3 to upper medial Q3); sequence 3 (upper Q3 to P4 to upper-medial Q4); and an incomplete sequence 4 (upper Q4 through P5). Thinner shoaling intervals of OSS—ISS—SHS in P3 and in lower Q2, Q3 and Q4 represent parasequences. Sequences of 10⁷ years duration are attributed to periods of increasing and decreasing subsidence rates due to tectonism marginal to the sedimentary basin. Parasequences record shorter duration temporal controls of c. 10⁴ to 10⁵ years related to eustatic oscillations. As a consequence of shoaling and aggradation/ progradation in the highstand systems tract, TIS/SHS and ALLS overlie and are temporally separated from OSS to ISS to SHS. This transition records filling of the basin to sea-level leading to a shelf geometry that was conducive to tidal amplification. A composite relative sea-level curve integrating long-term pulsatory subsidence and short-term eustasy best explains the stratigraphic evolution of the Uncompahgre Group.  相似文献   

Thermal Anomalies In A Regional Metamorphic Terrane: An Isotopic Study of the Role of Fluids   总被引：1，自引：1，他引：1  

CHAMBERLAIN  C. PAGE; RUMBLE  DOUGLAS 《Journal of Petrology》1988,29(6):1215-1232

A series of ten localized, high-temperature, granulite faciesregions occurs within the regional metamorphic, sillimanitezone of New Hampshire, USA. These regions, or ‘hot spots’,measure 10–30km² in a sillimanite zone that extends 50kmeast-west and over 150km north-south. The hot spots are characterizedby an abrupt increase in temperature over a distance of a fewkilometers, from 550?C in the sillimanite zone to 700?C in thegranulite facies. Mineral assemblages in pelites change rapidlyover the same distance from sillimanite-staurolite-muscovite-garnet-biotite-quartzto sillimanite-K feldspar-cordierite-garnet-biotite-quartz inthe granulite facies. The hot spot located near Bristol, New Hampshire was chosenfor detailed study because it has a network of quartz-graphiteveins in its core. Oxygen isotope analyses of minerals and rocksfrom Bristol suggest a close approach to pervasive exchangeequilibration between disparate rock types throughout the sillimanitezone. A comparison between chlorite zone pelites and calc-silicategranofelses of equivalent stratigraphic age shows a range ofover 7 in ¹⁸O of whole rocks from lower grade, whereas sillimanitezone rocks from Bristol vary by no more than 3, only. Quartzseparated from the Bristol high-grade metasediments varies byno more than 2?3. Superimposed upon the regional, sillimanite-zone oxygen isotopehomogenization is a 1–2 km wide region in the core ofthe hot spot in which the ¹⁸O of metasediments has been furtherhomogenized. Both quartz from quartz-graphite veins and quartzfrom cordierite-bearing wall rocks have exchanged oxygen isotopes;all quartz has ¹⁸O=13?8?0?5 (SMOW). Radiometric ages of quartz-graphite veins and metasedimentarywall rocks show that the hydrothermal event and metamorphismin the hot spot were both of Acadian age. Hydrothermal overgrow-thson zircons from one of the veins give ages of 409 ? 6 Ma. Monazitefrom cordierite bearing pelites from the wall of the same veingives a U-Pb age of 392 ? 3 Ma, using conventional techniques. The coincidence of: (1) an isograd high; (2) an isotherm hotspot; (3) a network of quartz-graphite veins; and (4) an oxygenisotope alteration halo suggests a relationship between hydrothermalactivity and heating of metasediments. The geochronologicalevidence demonstrates that the hydrothermal event and hot spotmetamorphism were not greatly separated in time. It is proposedthat the hot spots represent loci where hot metamorphic fluidswere focused through a fracture system now recorded by quartz-graphiteveins.  相似文献   

Chronology of Murrumbidgee River palaeochannels on the Riverine Plain,southeastern Australia     

KENNETH PAGE  GERALD NANSON  DAVID PRICE 《第四纪科学杂志》1996,11(4):311-326

Four major periods of palaeochannel activity have been identified on the Murrumbidgee sector of the Riverine Plain of southeastern Australia. On the basis of stratigraphic information the channels reveal a picture of changing flow conditions during the last full glacial cycle. The ages of the periods were determined from nearly 40 thermoluminescence dates on surficial fluvial and aeolian sediments. These are named the Coleambally phase, which occurred from 105 to 80 ka (the mid- to latter part of Oxygen Isotope Stage 5), the Kerarbury phase from 55 to 35 ka (Stage 3), the Gum Creek phase from 35 to 25 ka (late Stage 3 to early Stage 2) and the Yanco phase from 20 to 13 ka (late Stage 2). The present flow regime was established by about 12 ka (Stage 1). The first two phases correlate with episodes of enhanced fluvial activity in northern and central Australia and with reduced dust activity globally. The phases in Stage 2 appear to be associated with seasonal snow melt and increased peak flows in periods flanking the Last Glacial Maximum. Source-bordering aeolian dunes associated with the Coleambally, Kerarbury and Yanco phases were found, however, the TL dates show that some have undergone aeolian reworking. Thermoluminescence dating and fluvial stratigraphy have revealed a detailed picture of Late Quaternary climate and flow regime changes that has the potential to extend to identified deposits stratigraphically older than those described here.  相似文献   

Particle size scales and classification of sediment types based on particle size distributions: Review and recommended procedures   总被引：1，自引：0，他引：1  

SIMON J. BLOTT  KENNETH PYE 《Sedimentology》2012,59(7):2071-2096

This paper provides a review of different particle size scales, size class terminology and particle size distribution (‘textural’) classification schemes which are widely used in sedimentology, geomorphology, soil science, aquatic ecology and civil engineering. It is concluded that a revised system of size class nomenclature, based on the Udden (1898) and Wentworth (1922) schemes, provides the most logical and consistent framework for use with sediments and a wide range of other particulate materials. A refined scheme is proposed which has five first‐order size classes (boulder, gravel, sand, silt and clay), each of which has five second‐order subdivisions with limits defined at one phi intervals. The scheme is simple and intuitively easy to understand. The paper also provides a review of previous schemes that have been proposed to describe and classify sediments on the basis of the proportions of gravel, sand and mud, or sand, silt and clay using trigons (also termed ternary diagrams). Many of these schemes do not have a logical basis and provide limited or uneven resolution. New gravel, sand and mud and sand, silt and clay classification systems are proposed that are both more logical and provide greater discriminatory power than previous schemes; they are therefore more suitable for use in environmental and forensic investigations. A new Microsoft Excel^® program, freely available to download from http://www.kpal.co.uk , allows rapid classification of sediments based on the proportions of gravel, sand and mud and sand, silt and clay proportions and graphical comparison of the data for different sample groups.  相似文献   

Integrated micro- and macro-scale analyses of Last Glacial Maximum Irish Sea Diamicts from Abermawr and Traeth y Mwnt, Wales, UK     

JOHN F. HIEMSTRA  KENNETH F. RIJSDIJK  DAVID J. A. EVANS  JAAP J. M. VAN DER MEER 《Boreas: An International Journal of Quaternary Research》2005,34(1):61-74

This study is part of a basin-wide re-evaluation of Irish Sea glacigenic deposits which aims to test whether diamicts, collectively known as Irish Sea Tills, represent in situ glacimarine sediments or sediments that have been reworked or deformed by the Last Glacial Maximum Irish Sea Glacier. New results are presented for two key localities at Abermawr and Traeth y Mwnt in Wales. Unlike previous studies in the Irish Sea region that have focused on macro-scale sedimentology and structural analyses, this study combines macro-scale and micro-scale sedimentary analyses. This approach reveals that the dominant diamict facies at Abermawr are subglacially deformed primary (glaci)marine deposits, emplaced by the Irish Sea Glacier. An inland glacial source is unlikely. The Traeth y Mwnt diamicts are likely to be subaqueous in origin, possibly formed in an ice-dammed lake in the Mwnt embayment. There are no indications of subglacial deformation or shearing at Mwnt; deformation structures are related to gravity-driven or density-driven mechanisms.  相似文献