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Journal of the Geological Society; 1991; v. 148; issue.6; p. 1079-1090;
DOI: 10.1144/gsjgs.148.6.1079
© 1991 Geological Society of London

Article

The lithostratigraphy, sedimentology and tectonic setting of the Monian Supergroup, western Anglesey, North Wales

EMRYS PHILLIPS

Department of Geology, University of Wales, PO Box 914 Cardiff

The Monian Supergroup of western Anglesey comprises three major lithostratigraphic units: the basal South Stack Group, the pelitic New Harbour Group, and the overlying Gwna Melange. The South Stack Group comprises a sequence of deformed turbiditic metasandstones, pelites and subordinate quartzites. A dominance of Tabcde and Tbcde, turbiditic units, high sandstone to mudstone ratio and the presence of coarsening and thickening upward sequences indicate that the South Stack Group was deposited in a sand-rich fan system. The quartzites form large-scale sand-bodies (10–25 m thick) and are interpreted as major channel-fill sequences that transported mature quartz-arenites from relatively shallow marine environments deeper into the basin, by-passing the inner to mid-fan environments. Whole-rock sedimentary geochemistry indicates that the South Stack Group was deposited in a continental margin setting. The New Harbour Group forms a monotonous sequence of highly deformed pelites, semipelites and subordinate volcaniclastic psammites which exhibit geochemical characteristics of sandstones deposited in a continental island arc setting. Petrographic data indicate that the New Harbour Group records the dissection and unroofing of a pre-existing andesitic arc complex. Limited palaeocurrent data obtained for the South Stack Group and facies changes within the New Harbour Group suggest that the Monian Supergroup was deposited within a NE–SW-trending basin. No evidence was found to suggest that the Monian Supergroup represents part of an accretionary prism, consequently the deposition of the Monian Supergroup within the trench of a Monian subduction system cannot be justified.





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