摘要:

AbstractAlternating field and thermal demagnetization of igneous rocks of the Malvern Hills identifies a number of magnetite‐held components which are characterized by a high blocking temperature (M2) componentD= 283°,I= 47°, and lower blocking temperature (M3) componentD= 269°,I= −43° which is of complex origin or more than one age. Two subordinate components are (M1)D= 7°,I= 56° and (M4)D= 174°,I= 51° in later dolerites. A pervasive hematite‐held remanence with a meanD= 186°,I= −5° is linked to Hercynian palaeofield directions and the uplift/folding of the Malvernian axis. The similarity of the magnetization directions in the Stanner–Hanter (702 Ma) and Malvernian (681 Ma) rocks suggests that folding of the Palaeozoic rocks in the Malvern Hills was achieved by upthrust of the basement and involved little folding of the latter. The Old Radnor sediments possess a post‐folding remanenceD= 117°,I= −13° of probable Cambrian age and a subordinate remanence which may be Hercynian in age.The late Precambrian–Cambrian palaeomagnetic record (ca.700–500 Ma) of England and Wales is compared with data from the Armorican Massif. Although the apparent polar wander (a.p.w.) paths are widely dissimilar prior to 550 Ma, the two regions had similar latitudes and went through similar palaeolatitudinal movements throughout this interval. The palaeomagnetic data support models involving tectonic rotations but little closure across th

ISSN:0072-1050    

DOI:10.1002/gj.3350170202

出版商:John Wiley&Sons Ltd    

年代:1982

数据来源: WILEY

摘要:

AbstractThe history of research on the Ordovician rocks of Portugal is briefly outlined and the biostratigraphy of different regions is reviewed. Ten stratigraphic columns are presented for selected areas and the lithological and faunal characteristics are described. The trilobites belong to theSelenopeltisprovince and evidence is given for the recognition of two faunal subprovinces (termed associations by Henry 1980) in north and central Portugal during the Arenig to Llandeilo. The current state of knowledge of Ordovician palaeogeography in Portugal is discussed and maps are presented for the Arenig and Llandeilo Series.

ISSN:0072-1050    

DOI:10.1002/gj.3350170203

出版商:John Wiley&Sons Ltd    

年代:1982

数据来源: WILEY

摘要:

AbstractEvidence for 35 former cwm glaciers has been mapped in Snowdonia, N. Wales. Glacial limits are based on end moraines, boulder and drift limits, and the down‐valley extent of hummocky moraine. Protalus ramparts indicate that at least 16 semi‐permanent snow beds down to altitudes of 150 m also existed. Radiocarbon dates and pollen stratigraphy indicate that the glaciers and snow beds belong to the Loch Lomond (Younger Dryas) Stadial. The existence of a large number of ‘Older Series Moraines’ is questionned. Glacier reconstruction and contouring allows calculation of fan line altitudes, and trend surface analysis indicates that they rise from 450 m in the southwest to 700 m in the northeast. It is inferred that mean July temperature at sea‐level was around 8·5°C. Two differences between north and south Britain are recognized. Firstly in Snowdonia, as in the Lake District and Southern Uplands, the dominance of southerly air streams appears less than in the Scottish Highlands. Secondly, whereas in the Scottish Highlands, the glaciers appear to have stagnatedin situfollowing initial active retreat, some of those farther south were active during much of the time their margin

ISSN:0072-1050    

DOI:10.1002/gj.3350170204

出版商:John Wiley&Sons Ltd    

年代:1982

数据来源: WILEY

摘要:

AbstractMiddle Ordovician sediments of the St. Lawrence Lowland, eastern Canada, and its northeastward extension to St‐Siméon, are subdivided into the numerous formations of the Chazy, Black River and Trenton Groups. Details of each formation and interpretation of environments of deposition are presented and a coherent model for the development of the upper Middle Ordovician Trenton Group throughout the region is presented. In the southwest, around Montreal, a complete and continuous Middle Ordovician sequence is present and Trenton Group sediments overlie well‐developed tidal flat and lagoonal (Black River Group) and mixed shallow subtidal (Chazy Group) sediments. This sequence was deposited on a slowly subsiding, essentially flat, broad shelf environment. Northeastward from Montreal, toward the Montmorency Promontory of the Quebec City area, basal Middle Ordovician sediments become younger and the extent of the shelf area narrowed significantly. The latter resulted in skeletal shoal sediments (lower Trenton Group) developing closer to shore and concomitant less well‐developed clastic‐rich lagoonal sediments (Black River Group and basal Trenton Group). At Montmorency Promontory the shoal sediments (basal Trenton Group) accumulated along an irregular and rugged coastline. Northeast of the Promontory a steep onshore to offshore profile and rapidly deposited basal inshore clastics (Black River Group) precluded the deposition of skeletal shoals and rapid submergence promoted the early development of deeper shelf (middle and upper Trenton Group) and slope and basin (top Trenton Group, Saint Irénée Formation) sediments. In contrast, corresponding offshore sediments (middle and upper Trenton Group) in the southwest reflect a lower depositional gradient and more gradual subsidence. These patterns of deposition were determined by the interaction of the changing nature of the Ordovician coastline southwest, at, and northeast of the Montmorency Promontory and the variable subsidence rates influenced by the eastward evolving Taco

ISSN:0072-1050    

DOI:10.1002/gj.3350170205

出版商:John Wiley&Sons Ltd    

年代:1982

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350170206

出版商:John Wiley&Sons Ltd    

年代:1982

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350170207

出版商:John Wiley&Sons Ltd    

年代:1982

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350170209

出版商:John Wiley&Sons Ltd    

年代:1982

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350170201

出版商:John Wiley&Sons Ltd    

年代:1982

数据来源: WILEY