Consideration of stratigraphic sections from five areas of northeastern Maine suggests that marked lateral lithofacies changes occur within the Ordovician and Silurian systems over very short distances. Three regionally extensive Ordovician (Caradocian-Ashgillian) lithofacies are present in the area of this study: (1) a western volcanic-graywacke-slate facies (Winterville Formation), (2) a medial slate-graywacke facies (Madawaska Lake Formation), and (3) an eastern slate-limestone-graywacke facies (lower Carys Mills Formation). The zones of gradual transition between these Ordovician lithofacies are subparallel to the trends of major structural elements. All of the lithofacies appear to have formed in relatively deep water and relatively far from cratonal source areas.
The transition from Ordovician to Silurian is conformable and gradual in the eastern part of the area in contrast to abrupt and unconformable transitions in the central and western portions. Mild Taconian deformation during the latest Ordovician and earliest Silurian created western emergent areas that shed clastic debris into a continuing Silurian sedimentary basin in the east.
In the eastern part of the area, continuous and fossiliferous stratigraphic successions representing almost the entire Silurian are present. The Early Silurian (Llandoverian-early Wenlockian) is represented by three principal lithofacies: (1) a western sandstone-conglomerate-slate facies (Frenchville Formation), (2) a central phyllitic slate-limestone-ironsione facies (New Sweden Formation), and (3) an eastern calcareous mudstone-limestone facies (Spragueville Formation). The Frenchville and New Sweden Formations contain material eroded from exposed Winterville and Madawaska Lake rocks in the Taconian land area(s) to the west. The Lower Silurian units are overlain conformably by thin-bedded flysch (Jemtland Formation) of late Wenlockian-early Ludlovian age that was also derived for the most part from western source areas. The youngest eastern Silurian unit is the Fogelin Hill Formation which overlies the Jemtland Formation conformably and consists of thinly interlayered red slate, green slate, and fine-grained, laminated, calcareous graywacke.
In the western part of the region, the Lower and Middle Silurian are not present, and Upper Silurian sandstone, conglomerate, limestone, shale, and volcanic rocks unconformably overlie the Winterville Formation. The Upper Silurian sedimentary rocks, derived largely from the Winterville Formation, formed along irregular and possibly narrow shelves around emergent areas of the western Taconian land mass that had become largely submerged by Late Silurian time.
No melange units have been found in the Ordovician and Silurian rocks of northeastern Maine to suggest the presence of a subduction zone as proposed by Bird and Dewey (1970). The Winterville volcanic rocks may be interpreted to be part of a complex island arc with a trench located outside the area to the southeast or northwest.