ABSTRACT

Bedded chert comprises 5-10% of the volcanic (dacitic) and volcaniclastic Munsungun Lake Formation (Ordovician), of northwestern Maine. Three chert facies are recognized. These are 1) thick (10-42-m) sequences of medium- to thick-bedded chert; 2) thin-bedded (< 15 cm) chert interbedded with volcaniclastic sandstones; and 3) chert-pebble conglomerates and rip-up clasts within the basal portion of flows, breccias, agglomerates, tuffs, or sandstones.

The chert originated through combined biogenic sedimentation (radiolarians and siliceous sponges) and inorganic precipitation of silica derived from submarine weathering of the interbedded volcanics. The cherts are chemically identified as cherts that originated in a tectonically active setting similar to Tertiary and modern active margins or island arcs. They are not chemically similar to Mesozoic and Tertiary deep-sea cherts. Na₂O + K₂O/Al₂O₃ and TiO₂/Al₂O₃ variation diagrams show chemical dissimilarity between facies 1 and 2. The latter have higher alumina, alkali, and titania contents than the former. This is a reflection of relatively greater contributions of volcanogenically derived sediment for facies relative to facies 1. Facies 1 is interpreted as pelagic sediment deposited within small, starved basins. Facies 2 is interpreted as pelagic sediment deposited as Td and Te divisions of turbidites. Facies 3 is a resedimented variety of facies 1 and 2.

Petrographic study indicates that microfaunas comprise less than 10% of all chest facies. Surviving radiolarians appear to have undergone initial infilling with clays, microcrystalline quartz, and lengthfast chalcedony, followed by recrystallization to microquartz or replacement by chlorite. Sponge spicules have been recrystallized to microquartz or have been replaced by chlorite.