DEPARTMENT OF AGRICULTURE, CONSERVATION AND FORESTRY Maine Geological Survey Robert G. Marvinney, State Geologist OPEN-FILE NO. 20-13 Title: Bedrock geology of the The Horseback quadrangle, Maine Chunzeng Wang Date: September 2020 Contents: 25 p. report and color map Recommended Citation: Wang, Chunzeng, 2020, Bedrock geology of the The Horseback quadrangle, Maine: Maine Geological Survey, Open-File Report 20-13, 25 p. report and color map, scale 1:24,000. Bedrock geology of the The Horseback quadrangle, Maine TABLE OF CONTENTS Introduction .............................................................................................................................................. 1 Geologic Setting ....................................................................................................................................... 1 Stratigraphy .............................................................................................................................................. 2 County Road Formation (Scr) ........................................................................................................... 3 Flume Ridge Formation (Sf) ............................................................................................................. 3 Flume Ridge Formation – Hall Hill Member (Sfh)........................................................................... 5 Bucksport Formation (Sb) ................................................................................................................. 6 Great Pond Formation (Mgp)(new name) ......................................................................................... 9 Igneous Rocks ........................................................................................................................................ 11 Lucerne Granite (Dl) ....................................................................................................................... 11 Turner Mountain Syenite (Dtm) ..................................................................................................... 12 Saddleback Brook Leucogranite (Dsb) ........................................................................................... 13 Structural Geology ................................................................................................................................. 21 Deformation structures within metasedimentary formations .......................................................... 21 The Norumbega fault system .......................................................................................................... 22 Geologic History .................................................................................................................................... 22 Acknowledgments .................................................................................................................................. 23 References Cited .................................................................................................................................... 23 Bedrock Geology of the The Horseback 7.5′ Quadrangle, Maine Chunzeng Wang, PhD College of Arts and Sciences University of Maine at Presque Isle Presque Isle, Maine 04769 Field bedrock mapping in the Horseback quadran- INTRODUCTION gle by the author started in summer 2004 with mapping The Horseback 7.5′ quadrangle lies mostly in the neighboring Great Pond quadrangle (Wang, 2012). Hancock County of eastern-central Maine between 44° In summer 2011, with financial assistance of a Universi- 52′30″ and 45°00′ north latitude and between 68°22′30″ ty of Maine Trustee Professorship, the author did and 68°30′ west longitude. The topography in the additional mapping in the Horseback quadrangle. A quadrangle is directly related to differential weathering USGS STATEMAP Program grant allowed completion and erosion of the bedrock. For example, igneous of fieldwork in 2013. A Zillman Family Professorship intrusions stand in high relief to form rounded hills and funded geochemical and geochronological analyses in prominent ridges in the southeast, whereas metasedi- 2014. mentary and sedimentary rocks stand low in relief and GEOLOGIC SETTING form low, rolling hills and lowlands with large swamps in the central and northern parts of the quadrangle. Due The region around the Horseback quadrangle is to considerable glaciation during the Pleistocene Epoch, underlain by three slightly metamorphosed Paleozoic the area is characterized by prominent glacial land- lithostratigraphic terranes (Osberg et al., 1985) and the forms, in particular by a well-developed esker, named Norumbega fault system (Plate 1). From the northwest The Horseback esker. The esker begins at Simmons Hill to the southeast, the terranes include: (1) the eastern in the northwest corner of the quadrangle and extends lobe of the post-Taconic Aroostook-Matapedia/ southeastward for nearly 12 kilometers (km) to the Waterville belt located to the west and northwest of the southwest side of Turner Mountain in the neighboring quadrangle; (2) the Ganderian Miramichi terrane which Great Pond quadrangle. is a Cambrian-Ordovician sedimentary and volcanic The Horseback quadrangle is sparsely populated assemblage exposed in the neighboring Greenfield with only several houses and seasonal camps scattered quadrangle in the northwest; and (3) the post-Taconic, mostly along Myra Road of Greenfield Township in the Silurian Fredericton Trough turbidite suite. The Horse- north. Most of the map area is covered by timber back quadrangle is entirely underlain by the Fredericton forests, and bedrock exposures comprise less than 2% of Trough. In the previously published maps and reports, the total area. Lumber roads such as Stud Mill Road in the metasedimentary rocks on the northwest side of the the north provide access to most of the map area. Norumbega fault system were called Vassalboro Bedrock geologic mapping, mostly reconnaissance Formation (Perkins and Smith, 1925; Osberg, 1968; mapping, was performed in the Horseback 7.5′ quadran- Osberg et al., 1985) which was later re-named Hutchins gle and neighboring quadrangles in the 1960s and 1970s Corner Formation (Osberg, 1988) or Vassalboro Group by geologists McGregor (1963), Stoeser (1966), Gilman - Brewer Formation (Pollock, 2011); and the metasedi- (1974), Griffin (1976a, 1976b), Wones (1977, 1980), mentary rocks on the southeast side of the fault zone and Wones and Ayuso (1993). Among them, the most were mapped as Bucksport Formation (Trefethen, 1950; valuable reconnaissance maps were made by Griffin Wing, 1957; Osberg et al., 1985). The metasedimentary (1976a) and Wones and Ayuso (1993). Early workers rocks on both sides of the fault zone are composed identified and described major rock types within the predominantly of thick piles of deep-water, sandy quadrangle such as metasedimentary rocks, intrusive turbidites that were tightly folded and metamorphosed igneous rocks, and unmetamorphosed redbeds. More to lower greenschist facies conditions (chlorite zone) recent workers focused on the Norumbega fault system during the Late Silurian to Early Devonian Acadian that is the most prominent geologic feature of the orogeny. quadrangle (Wang and Ludman, 2003, 2004, 2012, The geology of the Horseback area and vicinity is 2013; Wang et al., 2014) and the Lucerne Granite that is also characterized by emplacement of large granitic exposed in the southeast of the quadrangle (Wones, plutons that are members of the Coastal Maine Magmat- 1980; Wones and Ayuso, 1993). ic Province, for example, the Lucerne and Deblois Editor’s note: Technically, when using the name of the quadrangle the text should read ‘the The Horseback’. For readability the second ‘the’ is removed in the report. Chunzeng Wang plutons. The Lucerne Granite is elongated in the redbeds. In the past, the metasedimentary package direction of 210°; it extends from Great Pond area northwest of the Norumbega fault system in the quad- southwestward to the town of Orland in the Penobscot rangle and vicinity was mapped as “Sangerville For- Bay area for at least 85 km with an exposed area of 625 mation” and “Vassalboro Formation” by Griffin (1976a) km2. The Deblois, with its exposed area of 1670 km2, is or “Vassalboro Formation” by Osberg et al. (1985), the largest pluton in the Coastal Maine Magmatic Wones and Ayuso (1993), and Wang (2012); the Province. Based on Wang (2012), both Lucerne and metasedimentary package on the southeast side of the Deblois plutons connect on the east side of Great Pond fault zone as “Bucksport Formation” by Osberg et al. and indeed occur as a single batholith. (1985), Wones and Ayuso (1993), and Wang (2012). The dominant structural feature in the region is the Field mapping and observations suggest that both Norumbega fault system. The system is one of the packages of the metamorphosed sedimentary rocks are largest and longest transcurrent fault zones in the very similar in lithology and in sedimentary features and northern Appalachian Mountains (Hubbard et al., 1995; that both could be correlated to the Flume Ridge Ludman et al., 1999). It extends for at least 450 km, Formation (Osberg et al., 1985; Ludman, 1991) that from the Casco Bay region in southwestern Maine to crops out in the area north and northeast of the Horse- central New Brunswick in Maritime Canada. The fault back quadrangle. Based on correlation with the Flume zone was initiated as an orogen-scale ductile shear Ridge Formation in the neighboring Greenfield quad-