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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