The genesis of Archean pyroclastic rocks of the Manigotagan River Formation, southeastern Manitoba
Seneshen, David Michael
The 50 to 500+ m thick, Manigotagan River Formation is a subvertically dipping, overturned, homoclinal sequence that is a transition zone between the underlying volcanic Narrows Formation and overlying sedimentary Edmunds Lake Formation. The formation consists of seven members that, in ascending order, are: felsic ignimbrites, mafic to intermediate tephra-fall deposits, felsic to intermediate mass-flow deposits and minor mafic lava flows, intermediate peperite, mafic lava flows, felsic turbidites, and felsic ignimbrites. This study focuses on the genesis of the ignimbrites and tephra-fall deposits within a 1.2 km long segment of the formation. The ignimbrites range in thickness from 6 to 50 m and extend beyond the study area. They are composed of variable proportions of recognizable juvenile and cognate particles in a fine-grained granoblastic aggregate inferred to be recrystallized vitric ash. The ignimbrites are composite sheets composed of 5 to at least 12 flow units, most of which comprise a lower, non-stratified, graded or upgraded lapilli-tuff division and upper bedded or massive, normally graded or apparently ungraded, upper tuff division; less commonly they consist of only a lapilli-tuff divison. The ignimbrites were apparently emplaced subaqueously following subaerial eruptions. The lapilli-tuff divisions were emplaced by high density turbidity flows possibly transformed from pyroclastic flows that entered the sea. Fine ash was probably produced by a combination of processes including (1) separation of fines above a collapsing eruption column, (2) elutriation of fines from fluidized pyroclastic flows, (3) littoral explosions, and (4) wave winnowing of the proximal parts of the lapilli-tuff division. Low density turbidity flows, triggered by earthquakes and/or wave activity, transported and deposited the fine ash over the preceding lapilli-tuff divisions to form the upper tuff divisions. A unique feature of the igimbrites is the coexistence of plagioclase-phyric pumice and pyrogenic quartz crystals in flow units. The source of the pumice remains speculative, but it may have been eroded from an earlier plinian air-fall deposit by pyroclastic flows. Tephra-fall deposits range in thickness from 17 to 91 m and also extend beyond the study area. The tephra-fall member consists of three, compositionally distinct, laterally continuous, blanket-like units that, in ascending order, are: (1) lower mafic to intermediate lapilli-tuff and tuff, (2) mafic lapilli-tuff and tuff, and (3) upper mafic to intermediate lapilli-tuff and tuff. The units are composed of lapilli- to block-sized, vesicular to non-vesicular, felsic and mafic fragments, rare crystal particles, and 35 to 97% fine-grained matrix. The units consist of relatively thin, alternating, ungraded, poorly sorted, matrix-supported, fragment-rich and fragment-poor beds with gradational bed contacts. In the central mafic lapilli-tuff and tuff unit, fragments show variable degrees of post-depositional, heat-induced alteration indiating that the fragments were still hot when they were incorporated in the deposit. The lapilli-tuff and tuff comprising Member 2 were apparently emplaced subaqueously as tephra-fall following shallow water, or possibly, subaerial, flank fissure-type, phreatomagmatic eruptions.