THE MARS HILL CONGLOMERATE, NORTHEASTERN MAINE: A PRE-ACADIAN SUBMARINE DEBRIS-FLOW COMPLEX?

Recent reconnaissance mapping following the construction of the $55-million Mars Hill wind farm at Mars Hill Mountain in Aroostook County, northeastern Maine, in 2006, has significantly helped our understanding of the nature and origin of the “Mars Hill Conglomerate” that underlies the mountain. The “conglomerate” is historically classified as a member of the upper Ordovician - lower Silurian Carys Mills Formation which is a package of dominantly argillaceous carbonate and thin-bedded siltstone and shale. The mapping shows that the “conglomerate” is actually a complex of primarily multiple successions of conglomerate of widely varied thickness with several intercalated thinner layers of laminated siltstone. The complex is presented as the core of an anticline, and conformably overlain by the typical Carys Mills argillaceous carbonate. The complex was metamorphosed during Acadian orogeny at low or sub-greenschist facies and characterized by a set of pervasive anastomosing and spaced cleavage striking at around 030 - 040 degrees. Each conglomerate unit is matrix-supported, poorly-sorted, massive and structureless, with erosional sole markings such as scours and mud/siltstone-chip breccia observed over the top of each underlying laminated siltstone, indicating an origin of submarine violent, high-density debris flow. The clasts are composed dominantly of sedimentary rocks such as sandstone, siltstone, and black shale, and metasedimentary equivalents such as quartzite with minor volcanics. Comparison of the clasts to the Miramichi Terrane/Arc formations located regionally in the southeast, suggests that the clasts might have been sourced from the Miramichi. The observation of alternating successions of massive conglomerate and laminated siltstone demonstrates not only a dramatic change between violent, high energy environment and quiet, low energy environment, but also multiple pulses/cycles of submarine mass movement during probably the Late Ordovician Period. Each pulse/cycle began with massive debris/gravel flow and ended by laminated siltstone/mud deposits. It is suggested that the complex was deposited in a shelf-slope environment and most likely within a deep submarine canyon.