An early Ordovician turbidite system on the Gondwana margin: the southeastern Manx Group, Isle of Man

Abstract The southeastern part of the Manx Group comprises sandstone-rich deep-marine turbidites of Arenig (early Ordovician) age, deposited on the Avalonian segment of the Gondwana margin. The main area comprises a 2 km thick succession from thin-bedded wacke sandstone and mudstone of the Lonan Formation up into medium-bedded wacke and arenite of the Santon Formation. This succession is interpreted as the distal part of a mixed mud-sand ramp overlain by a major distributary channel and lobe system. The Keristal Member is a unit of thick-bedded quartz arenite within the Lonan Formation recording a short-lived incised channel system with small terminal sand lobes. Flutes and scours in the Lonan and Santon Formations show west-southwest directed transport, contrasting with north-northwest directed estimates from ripple cross-lamination. Constraint by margin topography may have caused the higher concentration components of the flows to run along-margin, whilst flow-stripping of the lower concentration flow-tops allowed them to collapse down the regional north-northwest facing palaeoslope. The southwestern and northeastern areas contain fault-bounded successions that can be correlated only tentatively with the main area. In the southwest, thin-bedded wackes and mudstones of the Port Erin Formation pass up into the quartzose Mull Hill Formation, which shows the thickening-up motif of a fan lobe. In the northeast, the Creg Agneash Formation resembles the Mull Hill Formation, and probably overlies the quartzose wackes of the Ny Garvain Formation. The contrast of arenite and wacke sandstones in the Manx Group may have resulted partly from the intrabasinal separation of sand and mud, e.g. by flow stripping. However, the major factor was the availability on the basin margin of both clean and muddy sediment. The quartz arenite sands were probably sourced from the widespread Armorican quartzite facies of Gondwana, constraining the rifting of Avalonia from its parent continent to mid-Arenig time or later.

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