DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY

AEROMAGNETIC DATA PREPARED IN COOPERATION WITH THE MAINE GEOLOGICAL SU~VEY

GEOLOGIC AND AEROMAGNETIC MAP OF NORTHERN MAINE

By Arthur J. Boucot, Andrew Griscom, and John W. Allingham

GEOPHYSICAL INVESTIGATIONS MAP GP-312

PUBLISHED BY THE U. S. GEOLOGICAL SURVEY WASHINGTON. D.C. 1964 DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY TO-ACCOMPANY MAP GP-312

GEOLOGY

By

Arthur J. Boucot and Andrew Griscom

Introduction Middl~ Devonian age from the Mapleton Sandstone. Because it is· poorly indurated and relatively unde­ Information for the present compilation has been formed, it is believed to be of post-Acadian (Late derived from two main sources: Devonian) age, resting unconformably on both Upper (1) Geologists who have recently done or arenow Silurian and Lower Devonian strata. Lithologically doing geologic mapping in northern Maine {see index it is similar to the Upper Devonian Perry Formation map showing sources ef geologic information). This (Smith and White, 1905) in southeastern Maine_ (Boucot, information is reliable and reasonably detailed, al­ 1954, p. 148) and to the Horton Group of Mississippian though much of it is based on work that has not yet age (Bell, 1929) in Canada. been completed. Middle(?) Devonian (2) Data from reconnaissance by Boucot, largely Relatively fresh Middle(?) or Lower Devonian in 1954. In many areas for which no detailed informa­ plutonic rocks of granitic or mafic composition are tion is available, the geology has been inferred from common. Most of the granitic intrusives (Dg) are topographic and aeromagnetic trends, from a few col­ light-colored medium- to coarse-grained granites lections of fossils, and from an evaluation of Hitch­ and granodiorites; the mafic intrusives (Dd and Dga) cock's geologic descriptions (1861, 1862). The text include dark-colored diorite and gabbro in the 'south­ that follows summarizes the correlations. Although western part of the area. Conspicuous in northern the senior author has obtained information from the sources indicated (index map), he _is responsible for Maine are the mountains underlain by contact-meta­ morphosed stratified rocks (hornfels), shown as the interpretation herein. stippled areas on the geologic map, that rim the areas Post.Oevonian occupied by these Devonian plutonic rocks. Near Presque Isle, dikes of teschenite (t) intrude Lower Devonian sedimentary rocks of Late Silurian (early Ludlovian) All the stratified rocks older than the plutonic. age. There is no evidence for assigning a minimum rocks described above are tightly folded. The folding age to these dikes, but the relatively fresh condition of is of Early or Middle Devonian age associated with the the analcite in the teschenite suggests that the dikes Acadian orogeny, of Early Silurian or latest Ordovician postdate the Acadian orogeny. It is possible that the teschenites are similar in age to the "alkalic" rocks age associated with the Taconic orogeny. and also of pre-Ordovician age (Neuman, 1960). of the White Mountains plutonic-volcanic series, which Toulmin (1961, p. 775-779) considers to be about 185 Rocks of Schoharie age.-The youngest rocks million years old. Other occurrences of teschenite (Dssc) predating the Middle(?) Devonian plutonic rocks known in northern Maine (not shown on the map) are: in northern Maine include dark-colored sandstone and Spencer Stream, Somerset County, where it cuts slate of the Tomhegan Formation in the upper part of Seboomook Formation (Os) (Lower Devonian, Oris­ the Moose River Group (Boucot, 1961) in Piscataquis kany); Granny's Cap, Somerset County, where it in­ and Somerset Counties, and possibly also siltstone trudes Lower Devonian rhyolite (Ov); and Chain Lakes, and conglomerate northwest of Traveler Mountain in Franklin County, where it cuts Precambrian(?) gneiss Piscataquis County. The Tomhegan Formation contains (p~sg) (Boucot, Griffin, Denton, and Perry, 1959, p. marine fossils of Schoharie age (zone of Amphigenia) 16-17). (Boucot, 1961), whereas the siltstone and conglomerate have well-preserved plant remains of probable Early A diabase dike (d) has been mapped near the east Devonian age (Rankin, 1958), and a few ostracodes of edge of the map area by D. M. Larrabee·, U. S. Geo­ logical Survey, on the basis of aeromagnetic data and possible marine origin near the top of the section. a few outcrops. It is likely that the dike is of Triassic Beneath the strata of Schoharie age and the plant­ age. bearing beds correlated with them are discontinuous Middle Devonian bodies of extrusive rhyolite and pyroclastic rocks of rhyolitic to dacitic composition (Dv). Intrusive rocks The youngest known stratified formation in north­ (Di) in the strata underlying the extrusives are ...of ern Maine is the Mapleton Sandstone (Om) (Williams similar chemical composition and are concluded to be and Gregory, 1900) of the Presque Isle region, genetically associated with the extrusives. Datable Aroostook County. Fossiliferous cobbles of the Lower fossils have not been obtained from these volcanic Devonian Chapman Sandstone (Williams and Gregory, rocks, but their stratigraphic position either between 1900) have been found in the Mapleton Sandstone. strata of Oriskany and Schoharie age, or above strata Schopf (unpublished data) reports spores of probable of Oriskany age, places them all within the upper part of the Lower Devonian. Rocks. of Oriskany age. -Diabase and greenstone (Boucot and others, unpublished data). This unit (Dvg) of both extrusive and intrusive origin are associated is overlain conformably by the Edmunds Hill Andesite with stratified rocks of Oriskany age but have not of Gregory (1900) (Da), the Chapman Sandstone (Dsc), been observed associated with strata ofSchoha.rie age. and an unnamed slate (Dsl) (Boucot and others, un­ These mafic igneous rocks are not shown on the published data). The Edmunds Hill Andesite inter­ geologic map except for a large body of diabase, (Ddg), fingers with the Chapman Sandstone which in turn which is present north of Parlin Pond in Somerset appears to grade into the unnamed shale. The Chapman County. "Sandstone is of Helderberg age (Williams and Gregory, 1900) and th~ lower part of the unnamed slate has Dark-colored sandstone and slate (Dss) containing recently been shown to be of the same age (Boucot marine fossils of Oriskany age occur in four areas: and others, unpublished data). In the Presque Isle area in Somerset and Piscataquis Counties where they form units of Helderberg age overlie strata of early Lud­ the Tarratine Formation (Boucot, 1961) in the lower lovian age (Boucot and others, unpublished data) with part of the Moose River Group; near Big Spencer probable unconformity. Mountain in Piscataquis County; in the Telos Lake­ Red slate (Drs) interlayered with green slate, Soubunge Mountain area of Piscataquis County; and ~n volcanic rocks, and quartzite (Whisky Quartzite and the vicinity of Grand Lake in Penobscot County. The Capens Formation, Boucot, 1961 ). occurs in the sandstone and slate are interpreted as being laterally southern part of the Moosehead Lake region of Somer­ discontinuous and derived from separate source areas. set and Piscataquis Counties beneath the Seboomook Formation and above strata of Silurian or Devonian Equivalent to the dark-colored sandstone and slate age (DSls). Red slate also occurs near Ripogenus is the Seboomook Formation (Ds) (Boucot, 1961), the Dam in Piscataquis County beneath the Seboomook most widespread rock unit in northern Maine. The Formation and above greenstones of Silurian or Seboomook is dark-colored cyclically layered slate, Devonian age (DSv). Fossils of Helderberg age have siltstone, and fine-grained sandstone that has yielded been found in the southeast corner of the Pierce Pond fossils of Oriskany age in five widely separated areas, quadrangle in rocks similar to the Capens Formation. as well as fossils of Helderberg age (Boucot, Harper, Devonian or Silurian and Rhea, 1959) in its lower part in western Somerset County. Perkins (1925, p. 375) reported one occurrence .Extrusive and intrusive greenstone and diabase of Monograptus in it, but this remains unverified. The (DSv) near Ripogenus Dam are dated as Devonian or great rarity of fossils in this unit accounts in part Silurian. They are underlain by calcareous rocks for the difficulty in determining its age span. In (Sls) with Lower Silurian fossils (Willard, 1945). Somerset and Piscataquis Counties the Seboomook Sills(?) of greenstone (DSv) occur in the Maple and grades into fossiliferous sandstone (Dss) of Oriskany Hovey Mountains area of Aroostook County (Pavlides, age. Near Little Big Wood Pond in Somerset County in press). and at Lobster Lake in Piscataquis County it rests on Calcareous slate, calcareous sandstone, lime­ strata of Late Silurian (early Ludlow) age (Ssl). In stone, limestone conglomerate, and tuffaceous sand­ Penobscot and Aroostook Counties it rests on strata stone of uncertain age (DSls) are exposed in a number (DSls and Su) that in some places contain ·upper of areas in Somerset, Piscataquis, Penobscot, and Silurian· (early Ludlow) fossils, and in others, fossils Aroostook Counties. Late Silurian (Ludlow) and Hel­ of Helderberg age. derberg fossils have been obtained from these rocks The dark-colored sandstone and associated green­ in several places. stone (Dsg) of the Boundary Mountains in Somerset Silurian County belong to the Frontenac Formation ofprobable Early. Devonian age. In the Attean quadrangle the Rocks containing Late Silurian (Ludlow) fossils Frontenac overlies the Seboomook Formation with have been found in several areas. In Somerset County gradational contact (A. L. Albee, oralcommunication, such rocks include conglomerate, sandstone, and 1958; Marleau, 1957). However, in the Allagash Lake arkose (Scs) beneath strata of Oriskany age (Dss, Ds) area of Piscataquis County, the Frontenac underlies and above rocks of Precambrian(?) (pGqm) to Late the Seboomook Formation, suggesting an intertonguing Silurian (Ssl) age. In Somerset and Piscataquis relation between the two. Counties there are limestone, calcareous sandstone, and slate (Ssl). In the Perham-Stockholm area of Rocks of Helderberg age. -In one area of Somerset Aroostook County, conglomerate and sandstone (Scs) County the Beck Pond Limestone (Boucot, Harper, and ~ontain graptolites of early Ludlow age (Ruedemann, Rhea, 1959) (Dls), a unit of limestone and limestone 1947, p. 75). Near Presque Isle a unit of calcareous conglomerate containing fossils of Helderberg age, sandstone and slate (Ssl) contains fossils, including occurs beneath the Seboomook Formation. In another Conchidium qf Late Silurian (Ludlow) age. Pristio­ area of Somerset County felsite and limestone (Parker graptus and other graptolites from this unit (Ssl) have Bog Formation, Boucot, 1961) of Helderberg age are been dated as early Ludlovian (Berry, 1960). interbedded. The unit of undifferentiated Silurian rocks (Su) In Aroostook County, in the Presque Isle area, includes limestone, calcareous slate, and calcareO\JS White (1943) and Fletcher (oral communication, 1959) sandstone, locally with some tuff and conglomerate, found fossils of Helderberg age in tuff which is part in widely scattered occurrences. Fossils of Silurian of the Hedgehog Trachyte of Gregory (1900), a unit of age occur within this unit, but it has not been studied interlensing volcanic and sedimentary rocks (Dvg) in enough detail to allow subdivision. 2 In Aroostook County, Pavlides (in press) has Rhyolitic tuff (Ot) (the Kennebec Formation, Bou­ mapped a slate unit (Ssm) that encloses thin, lenticular cot, 1961) containing Middle Ordovician fossils occurs manganiferous strata Gontaining Lower Silurian fos­ a· few miles west of Moosehead Lake in Somerset sils. Rocks below this unit are tentatively assigned a County. ·A similar unit extends more than 20 miles Silurian(?) age, but confirmatory paleontologic evi­ northeast from Chamberlain Lake in Piscataquis dence is not available. Felsite and local volcanic County but fossils have not been discovered in it breccia (Svf) and green slate and graywacke (Ssg) with (B. A. Hall, oral communication, 1960). South of lenses of metaperlite (Svp) are included in this cate­ Grand Lake in the valley of the East Branch of the gory. Recent mapping in this area suggests that some Penobscot River an assemblage of greenstone flows qi these rocks may be older than Silurian and some and tuffs (Ovm), considered by Boucot to be of Early may be younger (L. Pavlides, oral communication, Ordovician age, occurs beneath Silurian and Ordovi­ 1!961). cian rocks (Sse and Osg). The tuffs contain brachio­ pods on nearby Sugarloaf Mountain (Neuman, 1960) .. At three localities fossiliferous calcareous units Middle Ordovician graptolites are in dark slate and have been mapped which contain a late Llandoverian bedded chert (Oss) that probably lie immediately or Wenlockian fauna. These localities are: east of above the greenstones along Wassataquoik Stream Kennebago Lake on the west side of Blanchard Pond, (Dodge, 1881; Ruedemann and Smith, 1935). Franklin County (R. J. Willard, oral communication, 1958); on Limestone Hill immediately west of Flag­ Ordovician or Cambrian staff Mountain, Somerset County; and at Ripogenus Beneath the Devonian and Silurian rocks in parts Dam, Piscataquis County (Willard, 1945). Conglomer­ of northern Maine is a complex of undifferentiated ates with late Llandovery fossils, and limestone with sedimentary rocks (O~u), felsic volcanic rocks (Ot:vf), probable Wenlock-age fossils are here included in a and mafic volcanic rocks (Ot:vm), which is thought to single unit (Sse), in a belt between the East Branch of be of Cambi"ian or Ordovician age. In general, these the Penobscot River and Marble Pond (Penobscot rocks are more phyllitic than the overlying Silurian County). strata, and where mapped in detail show field rela­ Silurian or Ordovician tions that suggest a major regional unconformity corresponding to the Taconic orogeny. No fossils A variety of felsic and mafic volcanic rocks (SOv) have been found in these rocks, but they are closely occur in several places in northern Maine between associated with the Middle Ordovician tuff (Ot) in rocks of Silurian and Devonian age and rocks of Somerset County and are lithologically similar to the probable pre-Silurian age. Fossils are almost un­ fossiliferous pre-Silurian rocks in Penobscot and known in these volcanic rocks, and little study has Aroostook Co.unties. been devoted to the volcanic rocks. It is probable that volcanics of both Silurian and Ordovician age Cambrian(?) have been lumped together in this unit. Interbedded quartzite and slate (Grand Falls For­ Ordovician marion of Ruedemann and Smith, 1935) (t:s) underlies with probable angular unconformity the Ordovician South. of Ripogenus Dam in Piscataquis County greenstone and tuff (Ovm) of the East Branch of the pre-Silurian gabbro (Oga) intrudes sedimentary rocks Penobscot River. This unit (t:s) may be of Cambrian (~s) that have been folded by a pre-Silurian orogeny. age because of its stratigraphic position and the Other mafic intrusive rocks of probable Ordovi­ presence of Oldhamia (Neuman, 1960). Similar rocks cian age include diorite (Od), gabbro (Oga), and ser­ are found south of Chesuncook Lake. Twelve miles pentine (Os) which intrude Cambrian or Ordovician east of Churchill Lake is a small area of dark-gray undifferentiated sedimentary rocks in Franklin and slates and phyllites which lie unconformably beneath Somerset Counties (Hurley and Thompson, 1950; fossiliferous Ordovician rocks (B. A. Hall, oral com­ Wing and Dawson, 1949). An altered granitic intrusive munication, 1960). This unit is also assigned a Cam­ (Og) is associated with Ordovician volcanic rocks brian(?) age. (Ovm) in northeastern Penobscot County and adjacent Cambrian, Ordovician, Silurian, or Devonian Aroostook County. Much of the area in the southern and eastern parts West of PreblJ.Ue Isle, Galcareous 'sa.ndstone and of the map is underlain by sedimentary rocks (Dt:us) siltstone (Osc) contain trilobites which H. B. Whitting­ and some volcanic rocks (Dt:uv) whose age and strati­ ton (oral communication, 1?60) considers to be of graphic relations are very poorly understood. The Ashgillian age (uppermost Ordovician). This unit is only known fosE1ilS from this map unit are favositid underlain by dark-colored sandstone, black shale, and corals and pelmatozoan debris from limestone at chert (Oss) which are of Middle Ordovician age Moro in Aroostook County. (Miller, 1947; Boucot and others, unpublished data). A single slate outcrop (Osg) of Ashgillian age is Precambrian(?) present just east of Pond Pitch on the East Branch of In western Somerset and Franklin Counties a the Penobscot River. Much of eastern Aroostook basement complex characterized chiefly by schist and County is underlain by platy limestone (Ols), the gneiss (p~sg) is overlain by Silurian and Lower . Aroostook Limestone (of earlier reports). White Devonian strata. At Jim Pond in Franklin County, ( 1943) mapped this unit as Silurian, but fossils now gneiss (pt:sg) is overlain to the southeast by Upper s}1ow that it may be of Middle Ordovician (Caradocian) Silurian strata (Ssl). The Silurian beds in turn abut age (Pavlides and others, 1961). pre-Silurian rocks (O~u) to the southeast, at a dis-

3 ranee of several hundred yards from the gneiss. The volcanics suggests that the gneiss is distinctly older close association of the pre-Silurian gneiss with the and may be Precambrian in age. less metamorphosed pre-Silurian slate, phyllite, and

AEROMAGNETIC DATA

By

John W.. Allingham

Introduction directly related to near-surface or exposed magnetic rock. Broad magnetic anomalies over downfolded About 12,000 square miles of northern Maine was aeromagnetically surveyed between 19SO and 19S9 by sedimentary . rocks are interpreted as being caused by the deeply buried crystalline floor of the basins. four organizations: U. S. Geological Survey; Bear The aeromagnetic patterns have a general north­ Creek Mining Company; Aero Service Corporation; easterly trend in conformity with strikes of the and the Maine Geological Survey (see index map). The Stratified rock, mafic intrusions,· and pyrrhotite- or U. S. Geological Survey and the Bear Creek Mining magnetite-bearing slate. Company used !4-mile and M-mile spacing between flight lines and an altitude of about SOO feet above the Anomalies associated with intrusive rocks ground. The Maine Geological Survey used one-mile Many intrusive rocks in northern Maine produce spacing and an altitude of 1,000 feet; and the Aero characteristic magnetic anomalies; for example, felsic Service Corporation used two-mile spacing and an rocks are typically associated with anomalies of low altitude of 1,000 feet. The wider spacing and higher gradient. Magnetically flat areas are associated with flight altitude produce a smoother magnetic pattern. rock (Dg), such as at Bald Mountain (S01N, All of the surveys measured total magnetic intensity. ~ranitic 4E.)l/. or_ with monzonite (Og), such as the pluton The present project was proposed by W. J. near Rockabema Lake (S11N., SSE). Magnetic lows are Dempsey in 19SS. The Maine Geolo6ical Survey pro­ associated with the centers of some intrusions having vided financial support for preparation of the magnetic small marginal diorite bodies, such as the quartz data. Aero Service Corporation and Bear Creek monzonite (pf:qm) west of Attean pond (SOsN., 40E.) Mining Company furnished unpublished aerq_magnetic (Albee, 1960, written communication). The low is pro­ maps for a part of the area; their cooperation is duced by the magnetic contrast between tl:te quartz grate{ully acknowledged. J. R. Kirby, B. L. Bennett, monzonite, its inclusions, and the adjacent _diorite and B. L. White of the U. S. Geological Survey com­ and granulite. Magnetic highs adjacent to other in­ piled the composite aeromagnetic map to an arbitrary trusions, such as the Onawa pluton (S03N., 47E.), datum. Aeromagnetic maps of larger scale have been are the result of marginal mafic rocks, mainly in­ published or placed in open file by the U. S. Geological trusive diorite or dioritic differentiates. Survey for most of the area. Absence of magnetic data Magnetic anomalies are commonly associated with over Mt. Katahdin and Turner Mountain is due to hornfels rims of intrusive bodies where the magnetite rugged topography. content of the intruded sedimentary rocks is in­ Magnetic anomalies are produced by a combina­ creased by metasomatic replacement or by oxidation tion of two magnetizations, one induced in the rocks of iron sulfide. The granite pluton at Meduxnekeag by the magnetic field of the earth, and the other Lake (S1oN., S7E.), for example, is surrounded by natural or remanent. The relationships of magnetic magnetic highs, which are related to magnetite and and geologic features, however, are reported without pyrrhotite-rich hornfels borders. regard to the type of magnetization. Most of the mag­ Some granitic stocks have internal magnetic zon­ netic anomalies in the area are caused by changes in ing that suggests a lithologic zoning. Tl).e ·quartz mon­ content of accessory magnetite disseminated through­ zonite intrusion (Dg) near Jackman (SOsN., 40E.) has out large volumes of bedrock. The presence of other an outer zone that is more magnetic than its core, minerals that are only slightly magnetic, such as suggesting a different border phase of the quartz pyrrhotite, ilmenite, and hematite, also contributes monzonite. Similarly the granitic stock (Dg) at Chand­ to the magnetic properties of the rock. The size and ler Mountain (S1SN., S2E.) also is magnetically zoned shape of a magnetic rock unit also affect the associated (Allingham, 1961); the anomaly resembles anomalies anomaly. Interbedded tabular volcanic rock units in observed over ring-dike structures. The intrusion at this area dip steeply enough to produce linear magnetic Arnold Pond (S03N., 36E.) has a magnetic coreand an anomalies. Small plutons are commonly elliptical or outer zone marked by magnetic lows. This pluton is rounded and produce anomalies of similar shape. rimmed by a few magnetic highs, which indicate mag- The aeromagnetic map is used as an aid in tracing magnetic rock units and defining their boundaries in Y Geographic locations are indicated by standard , areas where geologic information is sparse. In Army Map Service coordinates on the margin of the northern Maine, small, sharp .magnetic anomalies are map. 4 /netic hornfels. Other granitic intrusions (Dg) in this of this dike was inferred for an additional 35 miles to area show only the flat pattern that is characteristic diabase exposed southwest of Milo (S01N., 49-SOE.). of the regional magnetic gradient. Aflomalies associated with volcanic rocks Magnetic patterns delineate elongate diorite mas­ The aeromagnetic pattern over the slate belt north ses in the Katahain batholith. Aeromagnetic contours of Moosehead Lake, near Allagash Lake (513N., 46E.), outline one intrusion (Dd) near Holbrook Pond (SOaN .• for example, outlines a small anticlinorium and shows 49E.) and the northwest alinement of a similar mag­ its northeasterly trend. The anomalies over this netic pattern suggests the presence of another mafic anticlinal structure and at Canada Falls Deadwater body at Trout Mountain (507N., 51E.). This linear (508N., 41E.) are caused by altered basalt and gabbro feature continues as a fault at the northwest border associated with argillaceous sandstone (Dag). of th~ batholith. The subsurface contacts between the diorite and the granite . near Holbrook Pond were North of Mt. Katahdin, magnetic trends associated estimated from magnetic data to dip northeasterly and with mafic volcanic rocks outline a large southwest­ southwesterly about 55° (Allingham, 1960). ward-plunging anticlinorium having a core of _pre­ Silurian sedimentary rocks (SloN., 46E. to SZoN., Magnetic highs are commonly associated with 54 E.). These magnetic patterns are useful diorite or gabbro intrusives. Contrasting rock types distinct~ve for correlation of the volcanic units where they are such as diorite and quartz monzonite northwest of the concealed between areas of exposed rock. Moose River synclinorium can be distinguished by associated magnetic patterns. Some of the high­ A linear magnetic anomaly extending from Pres­ amplitude magnetic anomalies at the west border of que Isle Stream in Aroostook County (SfsN., 57E.) to the quartz monzonite (pGqm), for example the 1200- Otter Pond suggests the presence of a continuous unit, gamma anomaly near Whipple Pond (504N., 39E.), probably volcanic rocks or magnetite-bearing slate, are associated with diorite. Analysis of these mag­ in this little.-known area. South of this linear anomaly, netic anomalies Indicated steep or nearly vertical near Howe Brook, isolated volcanic rocks (DSv and contacts. DGuv), probably basaltic flows, produce complex magnetic patterns that ·resemble those over some Anomalies of intermediate amplitude are associ­ gabbro bodies. These small magnetic anomalies con­ ated with the gabbro (Dga) near the south end of tinue southwestward toward the anomalies over the Moosehead Lake (504N., 4SE.). The magneticpatterns Mt. Chase volcanic sequence (Sl!N., 54E.) and form outline the gabbro underlying the lake and show its a broad, open S-shaped pattern that may be correlated extension eastward toward Big Shanty Mountain ( 504N., with a stratigraphic unit and with a corresponding 4sE.). On the basis of magnetic information the gabbro S-shaped geologic structure. is believed to extend eastward, at depth, to Little Shanty Mountain, and from there southward to the Linear magnetic anomalies (SOON., 38E. toSOsN., gabbro at Katahdin Iron Works (503N., 48E.). where 46E.) that form a belt on the south flank of the Moose magnetic patterns show a more extensive subsurface River synclinorium are caused by elongate tabular mafic body. Southwest of Moosehead Lake, near Moxie bodies of volcanic rocks and near-surface mafic in­ Pond (502N., 43E.), the anomaly associated with the trusions striking about N. 60°E. and dipping southward gabbro is small but is paralleled by a linear magnetic from 15° to nearly 90°. feature of higher amplitude, believed to be caused by Anomalies associated with sedimentary rocks diabase, probably an offshoot of the gabbro, or by a magnetite-bearing slate. Interpretation of magnetic Some magnetic highs are directly related to sedi­ information indicates that this feature dips southward mentary rocks of appreciable susceptibility. A belt of about 60°. magnetic anomalies south of Moosehead Lake near Bingham (499N., 43E.) is.. associated with a sedimen­ A heterogeneous pattern of sharp magnetic highs tary sequence (OGu) that is mainly gray slate con­ and lows is commonly associated with mafic intru­ taining pyrrhotite, pyrite, or magnetite. These anom­ sions, such as observed over the gabbro (Oga) at alies not only show a structural trend but also suggest Bean Brook, west of rylt. K~tahdin (507N., 48E.). In a synclinal fold within a larger anticline, according addition, some gabbro b~dies have high-amplitude to dips interpreted from the magnetic data (Allingham, anomalies associated with their outer margins. These 1960). Near Smyrna Mills (511N., 57E.), magnetite­ magnetic highs are due to more magnetic zones with­ rich and pyrrhotite-rich slate produce broad arcuate in the gabbro and to marginal hornfels. This feature magnetic anomalies part-way around the riorth side is particularly noticeable over the gabbro at Dimmick of two granite plutons. These anomalies are partly Mountain (501N.,_ 43E.). parallel to the trend of anomalies associated with Southwest qf the Moose River synclinorium the volcanic rocks at Mt. Chase to the west and near area is magnetically complex, owing partly to the Howe Brook to the north. The anomalies associated closer spacing of flight lines and the lower altitude· with coalesced hornfels rims suggest several adjoin­ of the aircraft and partly to the complexity of the ing intrusions. bedrock geology. Many sedimentary rocks, such as those compris­ A diabase dike, ~00 to 200 feet wide, has beep ing manganiferous iron formations, although not of traced southwestward from an outcrop at East Grand ore grade, contain disseminated iron oxides in amounts Lake for more than 25 miles across the Danforth sufficient to produce substantial magnetic anomalies area (509N., 6oE.) by a distinctive linear aeromag­ where these rocks have been metamorphosed (Pav­ netic pattern. From the magnetic anomaly the presence lides, in press).

5 Anomalies associated with rocks underlying Boucot, A. J., Harper, C., andRhea,K., 1959, Geology sedimentary basins of the Beck Pond area, Township 3- Range 5,

A northeast-trending magnetic high outlines the Somerset County, Maine: Maine Geol. Survey, Moose River synclinorium (SO!N., 38E. to 508N., Spec. Geol. Studies Ser. No. 1, 33 p. 4sE.). Hurley and Thompson (1950) correlated the exposed igneous rocks in the synclinorium with aero­ Dodge, W. W., 1881, Lower Silurian fossils in northern magnetic anomalies. The magnetic contrast between Maine: Am. Jour. Sci., 3d ser., v. 22,p. 434-436. felsic volcanic and intrusive rocks (Dv, Di) near the Doyle, R. G., Young, R. S., and Wing, L. A., 1961, A /lynclinorial axis and the relatively nonmagnetic sand- detailed economic investigation of aeromagnetic / stone and slate (Dss, Ds) on the flanks of the syn­ anomalies in eastern Penobscot County, Maine: clinorium, however, is not large enough to contribute Maine Geol. Survey, Spec. Econ. Studies Ser. no. significantly to the broad observed anomaly. A mini­ 1, 69 p. mum depth of about 2 miles to magnetic rocks was determined from aeromagnetic profiles, using the Grantz, Arthur, and Zietz, Isidore, 1960, Possible method of Vacquier and others (1951). Geologic significance of broad magnetic highs over belts features indicate that this depth is too low. The broad of moderately deformed sedimentary rocks in anomaly is probably caused by mafic basement or Alaska and California: Art. 158 in U. S. Geol. Devonian gabbro intruding the synclinorium at depth. Survey Prof. Paper 400-B, p. B342-:-B347. From the depth of burial and from the amplitude and Hitchcock, C. H., 1861, General report upon the shape of the anomaly the rocks are estimated to have geology of Maine: Maine Board Agriculture, 6th a magnetic susceptibility of 1. 5 to 2.0 x lQ-3 cgs units. Ann. Rept. This susceptibility indicates a gabbroic or dioritic rock unit. The mass causing the anomaly is 5 miles 1862, Geology of Maine: Maine Board wide and about 3 1/2 miles thick, and is inclined 30° Agriculture, 7th Ann. Rept. to 40°N. The association of broad magnetic highs over Hurley, P. M., and Thompson, J. B., Jr., 1950, Air­ synclines has been observed elsewhere by Grantz and borne magnetometer and geological reconnais­ Zietz ( 1960). A similar high associated with a broad sance survey in northwestern Maine: Geol. Soc. synclinorium that trends northeast parallel to the St. America Bull., v. 61, p. 835-841. John and Allagash Rivers (SlsN., 43E. to 520N., 47E.) is shown by aeromagnetic profiles over relatively non­ .Keith, A., 1933, Preliminary geologic map of Maine: magnetic slate of Devonian age. Maine Geol. Survey. References cited Marleau, R., 1957, Preliminary report on Woburn area, Quebec: Quebec Dept. Mines Prelim. Rept. Albee, A. L., 1961, Boundary mountain anticlinorium, no. 336. west-central Maine and northern New Hampshire: Art. 168, !g U. S. Geol. Survey Prof. Paper 424-C, Miller, R. L., 1945, Geology oftheKatahdinpyrrhotite p. CS1-CS4. deposit and vicinity, Piscataquis County, Maine: Maine Geol. Survey Bull. 2, 21 p. Allingham, J. W., 1960, Use of aeromagnetic data to determine geologic structure in northern Maine: 194 7, Manganese deposits of Aroostook Art. 54, in U.S. Geol. Survey Prof. Paper 400-B, County, Maine: Maine Geol. Survey Bull. 4, 77 p. p. B117-B119. Neuman, R. B., 1960, Pre -Silurian stratigraphy in the 1961, Aeromagnetic interpretation of Shin Pond and Stacyville quadrangles, Maine:·Art. zoned intrusions in northern Maine: Art. 387, in 74 in U. S. Geol. Survey Prof. Paper 400-B, p. U. S. Geological Survey Prof. Paper 424-D, iJ." Bl66-B168. D265-D266. P a v lides, Louis, 1962, Geology and manganese deposits Bell, W. A., 1929, The Horton-Windsor District Nova of the Maple and Hovey Mountains area, Aroostook Scotia: Geol. Survey of Canada Mem. 155. ' County, Maine: U. S. Geol. Survey Prof. Paper 362 (in press). Balsley, J. R., Jr., and Kaiser, E. P., 1954, Aeromag­ netic· survey and geologic reconnaissance of part Pavlides, Louis, Neuman, R. B., and Berry, W. B. N., of Piscataquis County, Maine: U. S. Geol. Survey 1961, Age of the "ribbon rock" of Aroostook Geophys. Inv. Map GP-116. County, Maine: Art. 30 in U.S. Geol. Survey Prof. Paper 424-B, p. B6S-B67. Berry, W. B. N., 1960, Early Ludlow graptolites from the Ashland area, Maine: Jour. Paleontology, v. 34, Perkins, E. H., 1925, Contributions to the geology of p. 1158-1163. Maine, no. 2, pt. 1, The Moose River sandstone and its associated formations: Am. Jour. Sci., Boucot, A. J., 1954, Age of the Katahdin granite: Am. Sth ser., v. 10, p. 368-375. Jour. Sci., v. 252, p. 144-148. Philbrick, Shailer S~, 1936a, The contact metamor­ 1961, Stratigraphy of the Moose River phism of the Onawa pluton, Piscataquis County, synclinorium, Maine: U. S. Geol. Survey Bull. Maine: Am. Jour. Sci., Sth ser., v. 31, no. 181, 1111-E, p. 153-188. . p. 1-40. Boucot, A. J., Griffin, J. R., Denton, G. H., Perry, 1936b, The geology of the Appalachian P. S., 1959, The geology of a six mile section Trail in Maine, in Guide to the Appalachian Trail along Spencer Stream, Somerset County, Maine: in Maine: The Appalachian Trail Conference, Maine Geol. Survey, Spec. Geol. Studies Ser. No. Washington, D. C., p. 197-215. 2, 26 p. 6 Rankin, D. W., 1958, Lower Devoniannonmarinesedi­ White, W. S., Cloud, P. E., and Bridge, J., 1943, ments in the vicinity of Traveler Mountain, North­ Occurrences of manganese in eastern Aroostook Central Maine. (abs.): Geol. Soc. America Bull., County, Maine: U. S. Geol. Survey Bull. 940-E, v. 69, p. 1632. .p. 125-161. Ruedemann, Rudolf, 1947, Graptolites of North Amer­ Willard, Bradford, 1945, Silurian fossils from Ripe­ ica: Geol. Soc. America Mem. 19, 652 p. genus Dam, Maine: Jour. Paleontology, v. 19, p. 64-68. Ruedemann, Rudolf, and Smith, E. S. C., 1935, The Ordovician in Maine: Am. Jour. Sci., 5th ser., v. Williams, H. S., and Gregory, H., 1900, Contributions 30, no. 178, p. · 353-355. to the geology of Maine: U. S. Geol. Survey Bull. 165_, 212 p. Smith, G. 0., and White, C. D., 1905, The geology of the Perry Basin in southeastern Maine: U.S. Geol. Wing, L. A., and Dawson, A. S., 1949, Preliminary Survey Prof. Paper 35, 107 p. report on asbestos and associated rocks of north­ western Maine: in Report of the State Geologist, Toulmin, Friestley, 1961, Geological significance of 1947-1948, Augusta, Maine, p. 30-62. lead-alpha and isotopic age determinations of "alkalic" rocks of New England: Geol. Soc. Amer­ Wing, L. A., 1959, An aeromagnetic and geologic ica Bull., v. 72, p. 775-779. reconnaissance survey of portions of Penobscot, Piscataquis and Aroostook Counties, Maine: Maine Vacquier, Victor, Steen1and, N. C., Henderson, R. G., Geol. Survey GP. and G. Survey no. 4, 7 p. and Zietz, Isidore, 1951, Interpretation of aero­ magnetic maps: Geol. Soc. America Mem. 4 7, 151 p.

7