The Merrions Tuff is an Early Devonian volcanogenic formation occurring within a thick, regionally extensive flysch-like sequence of Siluro-Devonian age. The formation contains both lavas and volcaniclastics of dacite-andesite and rhyodacite affinity. The volcaniclastics are dominantly grain-supported, felspathic crystal aggregates or arenites organized into very thick (metre to tens of metres) sedimentation units that are regionally extensive. Sedimentation units are massive and show continuous size grading only where sedimentation units are relatively thin (<2 m). More commonly sedimentation units are not graded or graded only in the upper parts. Sedimentation units are often juxtaposed without intervening pelitic horizons. The characteristics of the sedimentation units suggest emplacement by "cold state," large scale, subaqueous mass-flows. Normal or continuous size grading suggests that turbulent suspension was the dominant grain-support/flow mechanism. Lack of continuous size grading implies that dispersive pressure (intertial), involving a high degree of grain collisions, and the buoyancy effect of the interstitial fluid may also have been important grain-support/flow mechanisms. Regional facies relationships suggest emplacement within a deep marine basin. Although the high sand-to-shale ratio and the non-graded to partly graded aspect of the thick Merrions Tuff sedimentation units may suggest emplacement in inner-fan valley or mid-fan channel subenvironments of submarine fan systems, the laterally continuous and homogenous character of the formation regionally, mitigates against this. At present, a relatively even basin-floor surface is preferred.