Clastic Intrusions in Deep-Sea Fan Deposits of the Rosroe Formation, Lower Ordovician, Western Ireland

Abstract

ABSTRACT The Rosroe Formation contains discordant elastic intrusions and essentially stratiform clastic intrusions (up to 40 cm thick) that display little evidence of injection. Features of the elastic intrusions, such as upper-surface irregularities and tapered lateral margins, also occur in wedged clastic layers (up to 30 cm thick) in the Rosroe Formation and suggest they may be injected sheets. A few of the mostly granule-grade injected sheets in the Rosroe Formation contain banding that is remarkably similar to the banding in the formation's stratified, pebbly sandstone. Together with the existence of essentially stratiform-injected sheets, the banding suggests that some elastic sills in deep-sea fan deposits may be indistinguishable from normally deposited beds. Contactual features (sharp upper contacts, upper-surface irregularities, truncation of superjacent strata, upper-surface inclusions, nesting, and blunt, steep, and tapered lateral margins), and internal characters (lenticular breccia and trains of aligned laminite clasts) of the elastic intrusions in the Rosroe Formation provide tentative criteria for the identification of clastic sills that simulate normally dep sited beds in deep-sea fan deposits.