The Siegas Formation of northwestern New Brunswick and northeastern Maine is composed mainly of sandstone and slate and has yielded fossils of early Llandovery age. It conformably overlies older rocks, indicating that there was continuous sedimentation in the Siegas area during the time of the Taconic orogeny. The formation consists of the three laterally equivalent facies, the lithic wacke, the quartz arenite, and the arkosic facies. Sandstones of the lithic wacke facies are made up mainly of mafic volcanic grains and their decomposition products: sodic plagioclase, angular quartz, and pyroxene. Sandstones of the quartz arenite facies consist mainly of medium-grained rounded quartz. Sandstones of the arkosic facies are composed mainly of potassium feldspar, quartz, and felsic plutonic fragments. The source rocks of the Siegas Formation included mafic volcanic rocks (probably andesite), felsic plutonic rocks (possibly granitic), and quartzose sandstones.Facies, paleocurrents, and regional paleogeographic evidence indicate that the source area of the Siegas Formation was in northwestern New Brunswick, a region now covered by Devonian sedimentary rocks. The source area was probably an isolated, relatively discrete uplift similar to others previously described in northeastern Maine. It probably consisted of Cambrian or Ordovician quartzose sandstones like those of the Quebec Group of the Temiscouata area, overlain by middle Ordovician andesites like those of northeastern Maine, and intruded by a "granitic" body like the Rockabema Quartz Diorite of the Weeksboro – Lunksoos Lake anticlinorium in eastern Maine. The site of active erosion of this uplift probably was shifted to the northwest in the late Llandovery, possibly accompanied by the development of local mafic volcanism.

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