THE NATURE OF SEDIMENTATION AND VOLCANISM OF THE MIDDLE TO LATE ORDOVICIAN ARC, NORTHERN MAINE WITH COMMENTARY ON THE CHEMISTRY AND ORIGIN OF ASSOCIATED CHERT

The nature of the volcanic arc in northern Maine is preserved in volcaniclastic sedimentary rocks together with mafic and felsic volcanic rocks. As such they record the history of the Ordovician arc. Four important formational units are present. The Bluffer Pond and Ingalls Brook Road formations are comprised of mafic pillow flows and associated mafic agglomerate. The Munsungun Lake Formation is a heterogeneous assemblage of felsic tuffs, lapilli tuffs, tuff breccias and pyroclastic flows together with volcaniclastic, quartz poor turbidite sandstones, chert and miscellaneous siltstone and siliceous mudstone. Quartz –Feldspar – Lithic (QFL) plots of the sandstones show the sand was derived from an arc undergoing early stages of dissection. Conglomerate of the Rowe Lake Formation consists of thick beds of disorganized pebble to cobble conglomerate. Clasts are almost universally a variety of aphanitic volcanics. Pebble to cobble sized grains of chert in the conglomerate were derived from the Munsungun Lake Formation. The Blind Brook Formation was deposited in a restricted, localized basin. Sand was also arc derived. Chronology of the arc is still incomplete, but as currently understood, active volcanism appears confined to the Dapingian and Darriwilian stages while dissection of the arc occurred from Sandbian to upper Katian and possibly Hirnantian stages.

Chert in the Munsungun Lake Formation is a compact ultrafine grained sedimentary rock with conchoidal fracture and waxy luster. There are multiple color variations including black, grays, greenish grays and grayish reds. Parallel lamination is ubiquitous and burrow structures are common. It is chemically heterogeneous with major rock forming oxides (exclusive of SiO₂) ranging from approximately 4% to 23%. This chemistry is consistent with chert in the Normanskill Formation of eastern New York and elsewhere. Thin beds in meter to decimeter thick sequences are locally common. It is also found as rip-up clasts near the base of thicker sandstone beds and within pyroclastic flows. Munsungun chert is a product of biogenic activity combined with the deposition of terrigenous sediment. Alternating biogenic and terrigenous lamina produce varve – like layering. Terrigenous lamina originated from eolian dust or hemipelagic clay sized grains.