Succession and structure in the Llwyd Mawr Syncline,Caernarvonshire north Wales

A 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 south of the area.     

On the geology of the country around ivigtut,SW-Greenland

Based on recent survey work sponsored by the Geological Survey of Greenland, an attempt is made here to establish an appropriate background to the cryolite deposit at Ivigtut. The cryolite ore body occurs within the original top part of a stock of alkali granite which, during the younger time of the cratonic Gardar period, was emplaced into the older, folded basement rocks. Close to the granite stock a breccia pipe occurs. Both bodies were cut by tinguaitic dykes before the actual mise en place of the cryolite body. The various events which contributed to such a set-up are discussed from both local and regional angles. Certain older basement structures, such as e.g. a Ketilidian depression zone, a younger, partial mobilisation of the dyke-cut Ketilidian crust, followed by the ascent of fluorite-bearing granites, may well have influenced the super-imposed Gardar tectonics. During the Gardar period, a regional system of wrench faults was active — in interaction with Atlantic volcanism.

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Zusammenfassung Nach intensiver Kartierungstätigkeit während der letzten Jahre in SW-Grönland wird ein Versuch gemacht, die regionalgeologische Stellung des Kryolithvorkommens von Ivigtut zu erhellen. Mittels relativer Altersverhältnisse von Ganggesteinen, anderer Intrusiva und Verwerfungen wird gezeigt, daß die Kryolithbildung am Ende der kratogenen Gardar-Periode stattgefunden hat und die Platznahme des Kryoliths einem späteren Stadium als der Konsolidation des umgebenden Granits angehört. Strukturelle Merkmale noch älterer Ereignisse sind wahrscheinlich vorhanden. Eine alte ketilidische Depressionszone, eine jüngere (aber prä-Gardar) Wiederbewegung und partielle Mobilisation des alten gangdurchsetzten ketilidischen Sockels, welcher vom Vorstoß einiger flußspatführender Granite gefolgt wurde, scheinen die spätere kratogene Gardar-Tektonik beeinflußt zu haben. Während der Gardar-Periode ist in Interferenz mit atlantischem Vulkanismus ein regionales System von Blattverschiebungen aktiv gewesen.

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Résumé Les travaux de cartographie poursuivis activement durant les dernières années dans le S.W. du Groenland permettent à l'auteur de dégager la position des gisements de cryolite d'Ivigtut dans le cadre de la géologie régionale. En se basant sur les âges relatifs de filons, d'autres masses intrusives et de failles, il montre que la formation de la cryolithe a pris place à la fin de la période cratogénique de Gardar et que la mise en place de la cryolithe appartient à un stade postérieur à la consolidation du granite environnant. Il existe encore vraisemblablement des caractéristiques structurales liées à des phénomènes plus anciens. Une ancienne zone de dépression kétilidique, un remaniement plus jeune (mais prégardar) et une mobilisation partielle du vieux socle kétilidique traversé par des filons, suivie de la montée d'un granite à fluorine, semblent avoir influencé la tectonique cratogène Gardar survenue postérieurement. Pendant la période Gardar, un système régional de fractures a interféré avec un volcanisme de type atlantique.

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The geology of the Llanuwchllyn to Llanymawddwy area,south gwynedd,north wales

A geological map of the Caradoc rocks of the Llanuwchllyn to Llanymawddwy area is presented and the geology of the area described in detail. Hitherto unrecorded outcrops provide important stratigraphical evidence which is used to redraw the Caradoc-Ashgill Series boundary and many of the formational boundaries within the Caradoc Series. The stratigraphy of the area is revised to conform with the standard classification of the Caradoc Series in the Bala region. The only modifications are as follows: the Llaethnant Siltstone is proposed as a formational name for a group of beds which are broadly equivalent to the Glyn Gower Formation but which represent a distinct facies. The greater part of the Nod Glas Formation north of Dinas Mawddwy is shown to be laterally equivalent to the upper part of the Gelli-grin Formation; it is also subdivided into two members, the Dyfi Mudstone and the Corris Shale, differentiated on lithological and faunal grounds. The stratigraphy of the Dinas Mawddwy area is also amended. Faunal elements (genera) new to the Caradoc Series of this area, and to Wales, are noted.     

The structural geology of the North Roe area of Shetland

The metamorphic rocks forming the area are divided into four groups separated by easterly dipping shear zones. The two western groups are orthogneisses and the eastern two are metasediments. The structures indicate movements from east to west with the most westerly group of gneisses having been a stable block during the movements. The metamorphic and structural events in the area are considered to be part of the Caledonian Orogeny. A Devonian granophyre is intruded into the metamorphic rocks. Small areas of Devonian type sediments occur overlying the metamorphic rocks.     

Observations on the geology of the country around Chalfont St Giles,Buckinghamshire

This paper presents new field observations from the area around Chalfont St Giles and relates them to existing information from the wider Chiltern Hills region on lithostratigraphy, structural geology, geomorphological and karstic features, and landscape development. In this area the NW-SE aligned valley of the River Misbourne, an internationally rare chalk stream, and flanking interfluve plateaus form a transition zone on the chalk dipslope between the lowest elevations of outcropping chalk around Denham and the characteristic chalk valleys and clay-with-flints capped upland present from Amersham northwards. The area straddles the northernmost extent of both the Paleogene strata, represented by Lambeth Group outliers, and SW–NE aligned pre-Anglian palaeo-Thames River Terrace deposits, and contains an anomalous Lambeth Group, Quarrendon Gap, mid-slope platform. Ground investigations carried out for HS2 have helped substantiate other evidence for significant faulting across the area, with intermittent movement episodes dating from the Mid- to Late Cretaceous onwards, which have affected both the Chalk and Paleogene strata and had a major influence on shaping the valley network and wider landscape. Karstic features are widespread, including contemporary interfluve plateau level streams sinks around the Lambeth Group outliers forming active dolines, together with many concealed chalk dissolution related features. There are numerous, generally small scale, clay, chalk, sand and gravel workings most of which are long abandoned but still provide useful insights into the near surface geology.     

The Geology and Geochemistry of Porphyrite Iron Deposits in the Nanjing_Wuhu Area,Southeast China

For the iron deposits occurring in andesitic volcaic rocks of the Lower Yangtze Area.the genetic model for porphyrite iron deposits was proposed by chinese geologists more than ten years ago on the basis of their detailed studies in the Nanjing-Wuhu Basin.It comprises a set of deposits of different genetic types ranging from late magmatic segregation ,ore-magma injection,pneumato-hydatogenetic replacing and hydrothermal filling as well as sedimentary origin.The deposits are closely connected with the gabbro-diorite porphyrite subvolcanic intrusive bodies both in space and in genesis.Miner4alization and wall-rock alteration are consistent with the history of the magmatic evolution.Geochemical studies on trace elements and S,O,Sr isotopes have proved that the porphyrite iron deposits are of magmatic origin,The proposed model may be applied to iron ores associated with andesitic volcanites,for example,in Chile,Mexico,Pakistan,Turkey,etc.     

Proterozoic geology and regional correlation of the Ghanzi-Makunda area,western Botswana

The Ghanzi-Makunda area exposes three main Proterozoic assemblages. The oldest rocks belong to the Palaeoproterozoic (Eburnian) Okwa Basement Complex, which consists of porphyritic rhyolitic felsite and granitoids emplaced at 2055±4 Ma. A volcanic sequence named the Kgwebe volcanic complex consists of metarhyolites and metabasalts with interbedded tuff and agglomerate. These metavolcanic rocks represent a bimodal suite of continental tholeiites and high K rhyolites linked to the evolution of the Mesoproterozoic Kibaran orogenic system. Siliciclastic and carbonate rock successions of the Neoproterozoic to early Palaeozoic Ghanzi-Chobe Belt unconformably overlie the Mesoproterozoic Kgwebe volcanic complex. The Ghanzi-Chobe Supergroup comprises the Ghanzi Group and the Okwa Group. In Namibia, felsic lavas with UPb zircon ages of ca 750 Ma occur at the top of lithological units correlated to the Ghanzi Group. The deposition of the Ghanzi Group occured after 1020 Ma and before 750 Ma. In the Okwa Group, detrital zircons extracted from Neoproterozoic sedimentary rocks of the Takatswaane Formation yielded the following dates: 1887±14 Ma, 1246±4 Ma, 1054±5 Ma, 627±6 Ma and 579±12 Ma. The age of 579 ± 12 Ma is considered to represent the maximum depositional age of the Okwa Group. Based on the data in this paper, as well as lithological similarities, the Ghanzi Group is correlated with the Nosib Group of the Damara Belt, while the Okwa Group is correlated with the Nama Group in Namibia.     

Coastal geology and revised stratigraphy of the Brega area,northeastern Libya

While describing the geology of the Brega area, NE Libya, the Brega Sandstone Bed of Pliocene age was established as a new stratigraphic unit in this part of Libya (Kumar, 2014). New field data from the western regions of Brega especially along the Brega Area One beach provides some new significant geological information. New outcrops of the Brega Sandstone Bed and Ajdabiya Formation are described. A new hitherto unknown outcrop of beachrocks is also described. Based on these outcrops and their stratigraphic relationships, a new stratigraphy for the Brega area is proposed. The new outcrop of the Brega Sandstone Bed is much larger and is exposed along the sea coast. It is of grayish yellow to brown color and intensely bioturbated, and unlike the outcrops of this stratigraphic unit in the Brega Area Two, this outcrop shows distinct bedding planes that dip 15° to 20°E to ESE. A new outcrop of Pleistocene Ajdabiya Formation was observed in close vicinity of the newly observed outcrop of the Brega Sandstone Bed. There is an unconformable contact between the Ajdabiya Formation and underlying Brega Sandstone Bed. A new outcrop of beachrocks was observed on the southwestern end of the Brega Area One beach. These rocks are very hard, of gray to dark gray color, and are characterized by numerous circular to sub-circular holes of various diameter and depth; these are filled by floating crude oil that is derived from leakages and spills of crude oil pumped into ships for export from the Brega Port. These rocks are mostly unfossiliferous but at places, fossil corals and gastropods were noted. Presence of corals is problematic because there are no coral reefs offshore Braga or in the nearby waters of the Mediterranean Sea. It is possible that the fossil corals from Libya could have been eroded and transported by wind and water during the past and transported to the beaches of Brega. Ultimately, these corals along with calcareous beach sand were lithified by calcium carbonate and became parts of beachrocks. The gastropods found in the beachrocks are local shallow marine inhabitants, and they can be observed on the surface of the Brega Sandstone Bed as well as in the recent sediments.