OLD QUESTIONS, NEW INSIGHTS, AND SOME REFRACTORY PROBLEMS

Recent work in eastern and east-central Maine offers new insights about old issues, including:

• New U-Pb zircon ages of Maine Miramichi volcanic rocks combined with work by Wang in the Weeksboro-Lunksoos Lake and Munsungun-Winterville inliers to the north suggest a multiple-arc scenario for Ganderian accretion to Laurentia rather than the popular single arc model.
• Abrupt termination of the >300 km-long Miramichi terrane in east-central Maine results from eastward thrusting of Silurian cover rocks onto and locally over their Cambro-Ordovician Miramichi source, followed by Acadian upright folding and dissection by several generations of high angle faults.
• It is difficult to trace the southern Miramichi boundary fault beyond the terrane’s termination because it juxtaposes thus far indistinguishable sandstones. This fault possibly resolves long-standing debates about relationships in southwestern Maine between Central Maine (Berwick) and Fredericton trough (Kittery, Eliot) sandstones.
• Disappearance of Central Maine stratigraphic units in east-central Maine: Recent detailed mapping supports the 1985 Maine Bedrock map interpretation that these units are truncated by faults, but changes some of the map details.
• At one time, the “biggest remaining problem in Maine stratigraphy” was in eastern Maine between Houlton and Lincoln, and involved relationships between well-established stratigraphies in the Houlton and Waterville areas. Today, it has migrated southward to the Bangor-Orono area and concerns relationships among similar sand-rich turbidite units of the Central Maine basin.

Some longstanding problems remain and others have arisen as the result of continued study, including:

• Crust beneath the Central Maine/Aroostook-Matapedia (CMAM) basin: Recent studies show that the CMAM basin was a Late Ordovician – Early Devonian deep-water depocenter as much as 1,000 km wide, and hosted several emergent sediment sources. The nature of its crust remains uncertain as there is no evidence for subduction of an oceanic crust and no candidate for a modern analogue.
• ~430 Ma detrital zircons are widely distributed in rocks just a few million years younger, suggesting that they are not reworked from older strata but were deposited subaerially during supervolcano eruptions.