摘要:

AbstractThe relationship between depositional grain orientation and current direction is discussed, and a method to assist in distinguishing between depositional and deformational magnetic fabrics is suggested. Magnetic fabric measurements from Middle and Upper Cambrian strata at St. Tudwal's Peninsula are presented. In the Maentwrog Beds (Upper Cambrian) a correspondence is found between the trend of the maximum magnetic susceptibility axes and the trend of the palaeocurrent as deduced from sole markings. In the Caered Mudstones and Flags (Middle Cambrian) the directions of maximum magnetic susceptibility are used to indicate palaeocurrent trends.

ISSN:0072-1050    

DOI:10.1002/gj.3350050201

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

摘要:

AbstractThe Whin Sill and its related dykes are intruded into Carboniferous strata in the North of England. Together they form a distinct petrographical province in which basic magmatism appears to have been closely related to a single phase in the structural history of the area. The relevant stratigraphical, structural and isotopic evidence suggests to the authors that the Whin Sill suite was intruded during the Stephanian, around 295 ± 6 m.y. This conclusion is supported by twenty‐one new potassium‐argon age determinations accompanied by a petrographical study of the factors which cause discrepancy in isotopic results from Whin Sill r

ISSN:0072-1050    

DOI:10.1002/gj.3350050202

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

摘要:

AbstractThe paper presents a one mile to the inch geological map of the southern part of the Mainland of Shetland (excluding areas underlain by Old Red rocks) and an account of the metamorphic rocks occurring there.The metasediments mostly belong to the East Mainland Succession which is about 14 km thick. They dip vertically and strike north‐south. The metasediments in the west were originally laminated sandstones devoid of current bedding and grading and composed of poorly weathered material. They include several limestones up to half a kilometer thick. In the east the metasediments were originally gritty quartzites and sandstones together with shales which are usually finer grained and composed of more weathered material than those to the west. On two occasions the deposition of these rocks was interrupted by outpourings of spilites.The rocks show signs of four metamorphisms. The first metamorphism was accompanied by a penetrative deformation causing elongation of the rocks in a north‐south direction, shortening in an east‐west direction and elongation and shortening in different areas in a vertical direction. As a result of this metamorphism the pelitic rocks over the whole area became schists containing such minerals as biotite, chlorite, muscovite, quartz and feldspar, and possibly garnet. In a later metamorphism a belt of these rocks a kilometer or so wide became gneissified, probably due to the passage of high temperature fluids. In various parts of the area such minerals as biotite, chlorite, garnet, staurolite, chloritoid, and kyanite grew as porphyroblasts in a series of episodes referred to collectively as the porphyroblast metamorphism. After these metamorphisms the area was intruded, 400 m.y. ago, by a number of granites. In places these have produced extensive thermal aureoles containing such minerals as staurolite, chloritoid, andalusite, kyanite, sillimanite, garnet, muscovite, biotite and chlorite in pelitic rocks. The emplacement of one of the granites was accompanied by large and small scale folding.Several major faults are described including the Nesting tear fault with a 16 km dextral displacement and amélangezone beneath a

ISSN:0072-1050    

DOI:10.1002/gj.3350050203

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

摘要:

AbstractA thick series of Caradocian volcanics is described. The volcanics are basic‐intermediate and it is own that they have a number of characteristics which are peculiar to emplacement in a subaqueous environment. A new interpretation of the stratigraphy results from a consideration of the folding of the successio

ISSN:0072-1050    

DOI:10.1002/gj.3350050204

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350050205

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

摘要:

AbstractAn early and a late calcite cernent are distinguished in limestones of Carboniferous age in N. Wales and Silurian age in Shropshire, England. Solution of aragonite, and preferential pressure solution of carbonate grains in day‐rich beds are considered to be the sources of the early and late cements, respectively. An association between the distribution of ferroan calcite and clay minerals indicates that clays may be an important contributor of iron during the precipitation of the late cernen

ISSN:0072-1050    

DOI:10.1002/gj.3350050206

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

摘要:

AbstractLarge pressure gradients and unrecognizable variations of pressure occur when solid substances are compressee by cold anvils (pistons) and heated by internal heaters. To improve the reliability of the results the miniaturization of samples and preliminary firing of pyrophyllite (which transmits pressure) are suggested. The equilibrium coesite—stishovite may be empirically expressed as:Pkb= 85 + 0·364 T (°C)Stishovite must be playing an important role in the mantle at depths greater than 500 km. Quartz and stishovite do not coexist in nature whatever PT‐condition

ISSN:0072-1050    

DOI:10.1002/gj.3350050207

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

摘要:

AbstractThe view thatGryphaeabecame uncoiled as it evolved, recently presented by T. P. Burnaby (1965), is shown to be based on the incorrect use of a size dependent parameter. His result is largely the outcome of the larger mean size of the higher subzone specimens in Hallam's collections. When this size dependence is allowed for, then the results of Burnaby's method are similar to those given by Philip (1962). An additional treatment of Hallam's data is given and it is shown that the slopes of the reduced major axes fitted to Hallam's samples by Philip (1962) are an adequate index of the tightness of coiling (estimated by s/r) of the larger specimens in the samples. Limitations of Burnaby's method are briefly discussed.

ISSN:0072-1050    

DOI:10.1002/gj.3350050208

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

摘要:

AbstractA revised stratigraphy of the Malvern, Abberley and Ledbury Hills is given. The Silurian succession is considered in detail and shows a series of five major sedimentary rhythms or cycles between an initial Llandovery transgression and regression at the close of the Silurian period. The succession exhibits an alternation of shale with either limestones and/or terrigenous clastics, and the rhythmic nature of the sequence suggests a periodic change in base level. A number of different lithological facies can be distinguished in the limestone formations, particularly in the Wenlock limestone. A detailed faunal list is given for most of the Silurian sequence. From the evidence of the stratigraphic succession, an attempt has been made to decipher the complex geological history of the area.

ISSN:0072-1050    

DOI:10.1002/gj.3350050209

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY

摘要:

AbstractA succession of strata ranging in age from Upper Cambrian to Valentian is described. They occupy ground in, and adjacent to, the Llwyd Mawr syncline. Most of the rocks are marine but during mid‐Caradocian (Soudleyan) times a rhyolitic ignimbrite sheet was emplaced and rests disconformably on Llanvirn slates. It has a remanant thickness of 2300 ft and is a single cooling unit. A second volcanic episode (Lower Longvillian) produced 600 ft of bedded pyroclastic rocks with only subordinate ignimbrites. Mid‐Caradocian minor intrusions of rhyolite occur as domes and sills in association with the volcanic rocks. Minor intrusions of dolerite and micro‐tonalite are probably of Caledonian age.Three phases of movement are recognized within the main end‐Silurian Caledonian movements. The first movement phase, F1, produced the main fold architecture and the dominant associated axial‐planar cleavage, S1. The folds, which possess a Caledonoid trend, usually stand vertically, but rarely they verge slightly to the southeast. They are periclinal in style. The second movement phase, F2, was weak and produced a sporadically distributed set of open folds and associated axial‐planar crenulation cleavage, S2, of Caledonoid strike. The axial‐planes dip gently to the southeast quadrant. Folds of the third movement phase, F3, plunge northwest and the associated axial‐planar crenulation cleavage, S3, dips steeply to the northeast. The F3 folds produce a pronounced arcuation in the trace of the Fl structures from an azimuth of 60° in the northwest to 10° in the

ISSN:0072-1050    

DOI:10.1002/gj.3350050210

出版商:John Wiley&Sons Ltd    

年代:1967

数据来源: WILEY