PALYNOLOGICAL DATING OF LOW-GRADE METAMORPHOSED ROCKS: APPLICATIONS TO EARLY PALEOZOIC ROCKS OF THE CENTRAL MAINE/AROOSTOOK-MATAPEDIA BASIN AND FREDERICTON TROUGH (NORTHERN APPALACHIANS) IN EASTERN AND EAST-CENTRAL MAINE, U.S.A.

摘要

Identification of spores, acritarchs, and chitinozoans in weakly metamorphosed (≥300°C) turbidites demonstrates the usefulness of palynological dating for hitherto unfossiliferous strata that underlie much of the Gander terrane in Maine. Combined with sparse macrofossil ages and geochronologic data from detrital zircons, volcanic horizons, and plutons, the fossils constrain the ages of strata in the Central Maine/Aroostook-Matapedia (CMAM) basin and Fredericton trough and help to resolve paleogeographic and tectonic problems.Ages for the provisional Brewer (Upper Ordovician to Llandovery) and Bangor (Wenlock) formations in the Bangor area fill the last remaining gap in CMAM basin stratigraphy and permit correlation across the entire basin, from western Maine to the New Brunswick border. Resulting lithofacies patterns document basinward facies changes from both eastern and western external sources, and also indicate internal sources within the basin. A progressive decrease in carbonate sediment in the CMAM basin toward the south suggests interaction between axial and basinward currents in which micrite transported by axial currents from the Matapedia platform in northern New Brunswick was progressively overwhelmed to the south by clastic sedimentation from flanking highland sources. Middle to late Silurian age ranges for the Flume Ridge Formation, the most extensive unit in the Fredericton trough, and the newly named County Road Formation permit correlation with rocks in similar structural positions in New Brunswick. Recent geologic mapping, the new age control, and improved understanding of facies relationships show that the CMAM basin was perhaps as wide as 800 to 1000 km prior to deformation; developed on continental crust after mid-Ordovician plate accretion; contained a body of deep water with anoxic bottom conditions and mostly pelagic organisms; and contains a series of overlapping submarine fans derived from multiple sources.