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Morton A.,HM Research Associates | Morton A.,University of Cambridge | Fanning M.,Australian National University | Jones N.,British Geological Survey | Jones N.,Saudi Aramco
Geological Magazine | Year: 2010

The zircon age spectrum in a sample from the Canonbie Bridge Sandstone Formation (Asturian) of southern Scotland contains two main peaks. One is Early Carboniferous in age (348-318 Ma), and corresponds to the age of igneous activity during the Variscan Orogeny. The other is of late Neoproterozoic to early Cambrian age (693-523 Ma), corresponding to the Cadomian. Together, these two groups comprise 70 % of the zircon population. The presence of these two peaks shows unequivocally that a significant proportion of the sediment was derived from the Variscides of western or central Europe. The zircon population also contains a range of older Proterozoic zircons and a small Devonian component. These could have been derived from the Variscides, but it is possible that some were locally derived through recycling of northerly derived sandstones of Devonian-Carboniferous age. The zircon age data confirm previous suggestions of Variscide sourcing to the Canonbie area, made on the basis of petrographical, heavy mineral and palaeocurrent evidence, and extend the known northward distribution of Variscan-derived Westphalian sediment in the UK. Copyright © Cambridge University Press 2010.

Krippner A.,University of Gottingen | Meinhold G.,University of Gottingen | Morton A.C.,University of Cambridge | Morton A.C.,HM Research Associates | Von Eynatten H.,University of Gottingen
Sedimentary Geology | Year: 2014

This work is an attempt to evaluate six different garnet discrimination diagrams (one binary diagram and five ternary diagrams) commonly used by many researchers. The mineral chemistry of detrital garnet is a useful tool in sedimentary provenance studies, yet there is no clear-cut understanding of what garnet type originates from which host lithology. Several discrimination diagrams exist for garnet showing distinct compositional fields, separated by strict boundaries that are thought to reflect specific types of source rocks. For this study, a large dataset was compiled (N = 3532) encompassing major element compositions of garnets derived from various host lithologies, including metamorphic, igneous, and mantle-derived rocks, in order to test the applicability of the various discrimination schemes. The dataset contains mineral chemical data collected from the literature complemented with some new data (N = 530) from garnet-bearing metamorphic and ultramafic rocks in Austria and Norway. Discrimination of the tested diagrams only works for a small group of garnets derived from mantle rocks, granulite-facies metasedimentary rocks, and felsic igneous rocks. For other garnet types, the assignment to a certain type of host rock remains ambiguous. This is considered insufficient and therefore the evaluated diagrams should be used with great care. We further apply compositional biplot analysis to derive some hints towards future perspectives in detrital garnet discrimination. © 2014 Elsevier B.V.

Morton A.,HM Research Associates | Morton A.,University of Cambridge | Milne A.,British Petroleum
Petroleum Geoscience | Year: 2012

Hydrocarbons in the Clair Field, west of the Shetland Islands, are hosted by Devonian-Carboniferous clastic red beds deposited in a non-marine fluviolacustrine setting. The succession is almost entirely biostrati-graphically barren and, hence, alternative approaches to reservoir correlation are required. Heavy mineral analysis (HMA), which subdivides clastic successions on the basis of changes in provenance and sediment transport history, has proven successful in establishing a high-resolution correlation framework for the Clair Field. Since the technique offers a reliable and rapid method for monitoring the stratigraphy of the Clair reservoir succession, HMA has been undertaken on a real-time basis at well site for virtually all development wells during Phase 1 of the Clair Field development, and for all Phase 2 appraisal wells. Heavy mineral data can be acquired in less than 2 hours from receipt of sample. Consequently, owing to the relatively slow penetration rates frequently associated with Clair drilling, stratigraphic information is usually acquired ahead of logging while drilling. Heavy mineral data are used in the decision-making process in a variety of situations, including picking of casing and coring points, whether to maintain or alter well trajectory, and when to terminate drilling. In the Clair Field, formation tops can be subtle and, since HMA can establish trends and predict formation changes before they are encountered, they are critical in aiding geosteering decisions. HMA has also been used to monitor stratigraphy and to pick formation tops when logging tools have failed, allowing drilling to continue and avoiding tripping to change the bottom-hole assembly. The application of HMA to the Clair Field development is illustrated by reference to a number of wells drilled on the field since 2005. © 2012 EAGE/Geological Society of London.

Stoker M.S.,British Geological Survey | Kimbell G.S.,British Geological Survey | McInroy D.B.,British Geological Survey | Morton A.C.,HM Research Associates | Morton A.C.,University of Cambridge
Marine and Petroleum Geology | Year: 2012

A regional study of the Eocene succession in the UK sector of the Rockall Plateau has yielded new insights into the early opening history of the NE Atlantic continental margin. Data acquired from British Geological Survey borehole 94/3, on the Rockall High, provides a high-resolution record of post-rift, Early to Mid-Eocene, subaqueous fan-delta development and sporadic volcanic activity, represented by pillow lavas, tuffs and subaerial lavas. This sequence correlates with the East Rockall Wedge, which is one of several prograding sediment wedges identified across the Rockall Plateau whose development was largely terminated in the mid-Lutetian. Linking the biostratigraphical data with the magnetic anomaly pattern in the adjacent ocean basin indicates that this switch-off in fan-delta sedimentation and volcanism was coincident with the change from a segmented/transform margin to a continuously spreading margin during chron C21. However, late-stage easterly prograding sediment wedges developed on the Hatton High during late Mid- to Late Eocene times; these can only have been sourced from the Hatton High, which was developing as an anticline during this interval. This deformation occurred in response to Mid- to Late Eocene compression along the ocean margin, possibly associated with the reorganisation to oblique spreading in the Iceland Basin, which culminated at the end of the Eocene with the formation of the North Hatton Anticline, and the deformation (tilting) of these wedges. A series of intra-Eocene unconformities, of which the mid-Lutetian unconformity is the best example, has been traced from the Rockall Plateau to the Faroe-Shetland region and onto the Greenland conjugate margin bordering the early ocean basin. Whilst there appears to be some correlation with 3rd order changes in eustatic sea level, it is clear from this study that tectonomagmatic processes related to changes in spreading directions between Greenland and Eurasia, and/or mantle thermal perturbations cannot be discounted. © 2011.

Ando S.,University of Milan Bicocca | Morton A.,HM Research Associates | Morton A.,University of Cambridge | Garzanti E.,University of Milan Bicocca
Geological Society Special Publication | Year: 2015

Amphibole and garnet are among the most widespread heavy minerals in orogenic sediments. Their chemical composition and optical properties vary markedly and systematically with temperature and pressure conditions during growth, and thus provide important information on the metamorphic evolution of source areas that is crucial in palaeotectonic and palaeogeodynamic reconstructions. This study investigates the chemical composition of detrital amphiboles and garnets derived from parent rocks of progressively increasing metamorphic grade through a well-studied composite section across the Central and Southern Alps, including the granulitefacies core of the Late Palaeozoic orogen exposed in the Ivrea-Verbano Zone and the amphibolite- facies core of the Cenozoic orogen exposed in the Lepontine Dome. We specifically focus on metamorphic grade because it represents the best proxy for tectono-stratigraphic crustal level, and hence degree of unroofing of source areas. In river sands collected between metamorphic isograds corresponding to crystallization temperatures ranging from c. 500 °C to c. 850 °C, TiO2 gradually increases in detrital amphibole while its colour progressively changes from blue-green in the lower amphibolite-facies where actinolite, hornblende and tschermakite are most abundant, to brown in the granulite facies where pargasite is dominant. Detrital garnets display moderate gradual changes across the amphibolite-facies Lepontine Dome, where low-Mg 'type B' garnets predominate. Almandine-spessartine is spatially associated with abundance of pegmatites while entering the zone of anatexis (Southern Steep Belt), where grossular or grossular-andradite-spessartine are occasionally found. A sharp change occurs while reaching granulite-facies in the Ivrea-Verbano Zone, where high-Mn garnets disappear and 'type A' almandine-pyrope (from 'stronalite' metasediments) and 'type C' almandine-pyropegrossular (from metagabbros of the Mafic Complex) predominate. Also redefined in this article are a series of numerical indices based on amphibole colour and relative abundances of diverse key minerals (chloritoid, staurolite, andalusite, kyanite, fibrolitic and prismatic sillimanite), useful to accurately assess the average metamorphic grade of meta-igneous and metasedimentary source rocks. © The Geological Society of London 2014.

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