THE NEWLY DISCOVERED, GIANT FISH RIVER POST-ACADIAN RIFT SYSTEM IN THE NORTHERN APPALACHIANS OF MAINE

An extensive post-Acadian rift system was discovered in the Fish River chain of lakes in northern Maine in spring 2020. The system consists of several half-graben-style rift basins of varying sizes. The Red River redbed basin, with an estimated area of 117 km², occurs along Red River, St. Froid Lake, and Eagle Lake and consists predominantly of red sandstone and mudstone. The Portage Lake greenbed basin, with an estimated total area of 95 km², occurs between Portage Lake and Pennington Mountain and is composed predominantly of green sandstone, mudstone, and shale. A NW-striking left-lateral fault has considerably displaced this basin. The greenbeds observed between Square Lake and Cross Lake are likely a displaced fault sliver of the Portage Lake basin. The Burpee Brook valley redbed/greenbed basin, with an estimated area of 45 km², occurs along the Burpee Brook valley between Portage Lake and Ashland and comprises red and green sandstone, mudstone, and shale. Another but much smaller set of red sandstone occurs along a regional fault to the west of Big Machias Lake. The redbeds and greenbeds are rich in fossils of Devonian-style plants that are dominated by Psilophyton and Pertica among several less common genera (e.g., Protolepidodendrid and Taeniocrada). The lithology and the fossils correlate to the post-Acadian, early Middle Devonian Mapleton Formation (19 km²) and Trout Valley Formation (28 km²), which represent the only two known locations with the plant genera of the Middle Devonian in the United States. The terrestrial redbeds/greenbeds unconformably overlay the Ordovician Winterville Formation volcanics/subvolcanics and the lower Devonian Seboomook Group basal conglomerate, fossiliferous limestone/limey sandstone, and turbidites. The latter ones were either locally or thoroughly foliated during Acadian orogeny. The NE-striking, orogen-parallel rift system was likely caused by post-orogenic isostatic adjustment, collapse, and extension after a thickened crust was formed by Acadian orogeny along the Appalachian Mountains. The rift basins were significantly displaced by several regional faults, probably during Appalachian Orogeny. The Fish River chain of lakes region owes its spectacular ridge and valley topography to the underlying NE-striking horst-and-graben structures of the rift system.