BEDROCK GEOLOGIC MAP OF THE NUMBER NINE LAKE QUADRANGLE, NORTHERN MAINE

The bedrock geologic map of the Number Nine Lake 7.5' quadrangle of northern Maine is a product of the 3-year USGS Earth Mapping Resources Initiative (MRI) project entitled “Geologic mapping of manganese-bearing deposits in Aroostook County, Maine”. The project, launched in 2022, is aimed at understanding geologic setting and mineralization of the manganese deposits in the central and northern subdistricts of the Aroostook Manganese District that hosts the largest manganese reserve in USA. The project includes detailed 1:24,000-scale bedrock mapping and geochemical, geochronological, and bio-stratigraphical characterization of the manganese deposits and surrounding sedimentary and volcanic rocks of the Number Nine Lake, Mapleton, and Presque Isle quadrangles. Underlying the Number Nine Lake quadrangle and vicinity is a newly recognized Cambro (?)-Silurian lithotectonic terrane, referred to Number Nine Mountain terrane, with unique composition and tectonic history. The terrane is composed of several distinctive pre-Devonian sedimentary and volcanic rock formations that are separated by four unconformities (Taconic and Salinic). The terrane is fault bounded by the Silurian Aroostook-Matapédia Belt to the southeast and by the Cambrian-Ordovician peri-Gondwanan Weeksboro-Lunksoos Lake Belt to the southwest. The oldest formation is the newly discovered and tentatively named Three Brooks Conglomerate/Sandstone with a typical Laurentia detrital zircon spectrum and with the youngest zircon grain at 565 Ma, suggesting a peri-Laurentian affinity. The new Morehouse Brook Formation is an arc volcanic sequence dated 470 Ma. The redefined Nine Lake Formation is dominated by mélange; it is the first Salinic mélange ever recognized in Maine and New Brunswick. Several other Silurian formations are redefined and dated, including the conformable Spruce Top (basalt, 433 Ma), Dunn Brook (pyroclastic rocks, 433 Ma), and Maple Mountain (mudstone and siltstone that host the Maple-Harvey Fe-Mn deposit with hydrogenetic origin) formations and the Burnt Brook Formation (green phyllite, Homerian). Another newly recognized Devonian formation, named Scopan Lake Formation, is composed mainly of green (dominant) and red sandstone. It was deposited from Pragian to early Emsian based on palynological analyses.