Basin Across the Northern Appalachians ofMaine and Adjacent Areas

U.S. Department of the InteriorU.S. Geological Survey

OF THE ACADIAN OROGEN AND FORELAND BASIN ACROSS THE NORTHERN APPALACHIANS—U.S. Geological Survey Professional Paper 1624

Professional Paper 1624
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Migration of the Acadian Orogen and Foreland

Basin Across the Northern Appalachians

of Maine and Adjacent Areas

By Dwight C. Bradley, Robert D. Tucker, Daniel R. Lux,

Anita G. Harris, and D. Colin McGregor

Professional Paper 1624
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or fi rm names in this publication

is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

United States Government Printing Offi ce: 2000

For additional copies please contact:

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Text edited by George A. Havach

Layout and design by Stephen L. Scott

Manuscript approved for publication, November 8, 1999

 Library of Congress Cataloging-in-Publication Data

Migration of the Acadian Orogen and Foreland Basin across the Northern Appalachians

of Maine and adjacent areas / Dwight C. Bradley... [et al.].

 p. cm. -- (U.S. Geological Survey professional paper ; 1624)

 Includes bibliographical references.

 1. Orogeny--Maine. 2. Geology, Stratigraphic--Silurian. 3. Geology,

 Stratigraphic--Devonian. 4. Geology, Structural--Maine. I. Bradley, Dwight Culver. II.

 

 QE75 .P9 v. 1624

 [QE621.5.U]

 557.3 s--dc21

 [551.7’32’09741]

 00-057835
CONTENTS

 Abstract---------------------------------------------------------------------------------------------- 1

 Introduction----------------------------------------------------------------------------------------- 1

 Regional Geology---------------------------------------------------------------------------------- 2

 Determining the Timing of Collisional Orogeny from the Rock Record------------------- 5

 Paleontologic and Stratigraphic Evidence for the Date of the Acadian Orogeny--------- 6

 Passamaquoddy Bay, Maine and New Brunswick------------------------------------- 6

 Fredericton Basin, Maine and New Brunswick---------------------------------------- 6

 Canterbury Inlier, New Brunswick------------------------------------------------------- 6

 Carlisle Area, New Brunswick------------------------------------------------------------ 7

 Presque Isle Quadrangle, Maine---------------------------------------------------------- 7

 Central Maine Basin----------------------------------------------------------------------- 7

 Littleton Area, New Hampshire----------------------------------------------------------- 11

 Southwestern Part of the Moose River Synclinorium, Maine------------------------- 11

 Northeastern Part of the Moose River Synclinorium, Maine------------------------- 12

 Traveler-Chesuncook Area, Maine------------------------------------------------------- 12

 Munsungun Anticlinorium, Maine------------------------------------------------------- 16

 Pennington Anticlinorium, Maine-------------------------------------------------------- 17

 Madawaska Area, New Brunswick------------------------------------------------------- 17

 LaPatrie and Ste.-Cécile de Whitton, Eastern Townships, Québec------------------- 17

 Northwestern Margin of the Connecticut Valley-Gaspé Basin, Québec------------- 18

 Montreal, Québec--------------------------------------------------------------------------- 18

 Isotopic Ages of Acadian Plutons--------------------------------------------------------------- 18

 U-Pb Analytical Methods------------------------------------------------------------------ 19

 40Ar/39Ar Analytical Methods------------------------------------------------------------- 19

 Ludlovian, Pridolian, and Lochkovian Plutons---------------------------------------- - 19

 Early Emsian Plutons---------------------------------------------------------------------- 20

 Previous Results---------------------------------------------------------------------- 20

 Russell Mountain Pluton------------------------------------------------------------ 23

 Sebec Lake Pluton------------------------------------------------------------------- 23

 Bald Mountain Pluton--------------------------------------------------------------- 23

 Onawa Pluton------------------------------------------------------------------------- 23

 Shirley-Blanchard Pluton----------------------------------------------------------- 23

 Mattamiscontis Pluton--------------------------------------------------------------- 23

 Ebeemee Pluton---------------------------------------------------------------------- 27

 Seboeis Pluton------------------------------------------------------------------------ 27

 Moxie Pluton------------------------------------------------------------------------- 29

 Katahdin Pluton---------------------------------------------------------------------- 29

 Harrington Pluton-------------------------------------------------------------------- 29

 Late Emsian to Eifelian Plutons----------------------------------------------------------- 29

 Givetian to Early Tournaisian Plutons---------------------------------------------------- 29

 Previous Results---------------------------------------------------------------------- 29

 Center Pond Pluton------------------------------------------------------------------ 34

 Hammond Ridge Sill---------------------------------------------------------------- 34

 Hog Island Pluton-------------------------------------------------------------------- 34

 Beaver Cove Pluton------------------------------------------------------------------ 34

 Deboullie Pluton--------------------------------------------------------------------- 34

III
CONTENTS

Isotopic Ages of Acadian Plutons—Continued

 Givetian to Early Tournaisian Plutons—Continued 

 Priestly Pluton----------------------------------------------------------------------- 34 

 Chandler Pluton---------------------------------------------------------------------- 35

 Seboeis Pluton----------------------------------------------------------------------- 35

 The Silurian-Devonian Time Scale: Basis for Correlations Between 

 Paleontologically and Isotopically Dated Events---------------------------------------- 35

 Location Through Time of the Acadian Deformation Front and Foreland Basin--------- 35

 Palinspastic Considerations--------------------------------------------------------------- 36

 Early Ludlow-------------------------------------------------------------------------------- 39

 Early Lochkovian--------------------------------------------------------------------------- 40

 Pragian--------------------------------------------------------------------------------------- 41

 Early Emsian-------------------------------------------------------------------------------- 41

 Late Emsian to Early Eifelian------------------------------------------------------------- 41

 Late Eifelian to Early Givetian------------------------------------------------------------ 41

 Earliest Frasnian---------------------------------------------------------------------------- 42

 Implications----------------------------------------------------------------------------------------- 42

 Age of the Acadian Orogeny-------------------------------------------------------------- 42

 Rate and Trajectory of Deformation-Front Migration--------------------------------- 42

 Physical Setting of Magmatism----------------------------------------------------------- 43

 Coastal Volcanic Belt and the Ludlovian to Lochkovian Plutonic Belt------ 43

 Piscataquis Volcanic Belt and the Emsian Plutonic Belt----------------------- 43

 Syncollisional Versus Precollisional Plate Geometry---------------------------------- 43

 Hinterland Deformational Regimes and Regional Partitioning of Acadian 

 Deformation------------------------------------------------------------------------------ 43

 Foreland Deformational Regimes-------------------------------------------------------- 44

 Suggestions for Further Work-------------------------------------------------------------------- 44

 References Cited----------------------------------------------------------------------------------- 45

PLATES

[Plates follow references]

 

 1. Photomicrographs of Late Silurian to early Middle Devonian conodonts from Maine and southern Québec, 

 Canada

 2. Photomicrographs of Late Silurian to earliest Devonian conodonts from Maine

FIGURES

 1. Geologic map of Maine and adjacent areas------------------------------------------------------------------------------------- 3

 2. Map of the study area, showing major paleogeographic elements and structures----------------------------------------- 4

 3. Cross-sectional diagram showing a conceptual model of two-plate collision---------------------------------------------- 5

 4. Stratigraphic sections bearing on the age of Acadian deformation in the study area-------------------------------------- 8

 5. Correlation chart showing conodont and palynomorph age ranges for samples dated in this study-------------------- 12

 6. Map of Maine and adjacent areas, showing distribution of Silurian and Devonian plutons------------------------------ 20

 7. U-Pb concordia diagrams of igneous rocks------------------------------------------------------------------------------------- 21

 8. 40Ar/39Ar age spectra of igneous rocks------------------------------------------------------------------------------------------- 22

 9. Thin-section photomicrographs of contact-metamorphosed rocks adjacent to plutons----------------------------------- 28

 10. Map of the study area, showing successive positions of the Acadian deformation front--------------------------------- 36

IV
 11. Map of the study area, showing successive positions of the leading edge of the Acadian foreland basin---------- 37

 12. Schematic, nonpalinspastic paleogeographic maps of the study area, showing migration of the orogen and 

 foreland basin-------------------------------------------------------------------------------------------------------------------- 38

 13. Map of the study area, showing local estimates of Acadian and younger shortening---------------------------------- 40

TABLES 

 1. Palynologic collections from Devonian rocks, Maine--------------------------------------------------------------------- - 10

 2. Conodont collections from Silurian and Devonian rocks, Maine and Québec------------------------------------------ 13

 3. Emplacement ages of plutons mapped in fi gure 6-------------------------------------------------------------------------- 24

 4. U-Pb isotope-dilution analyses of zircon from plutonic rocks, Maine--------------------------------------------------- 26

 5. 40Ar/39Ar data for plutonic rocks, Maine------------------------------------------------------------------------------------- 30

 6. Comparison of time scales----------------------------------------------------------------------------------------------------- 35

 7. Data used to estimate Acadian and younger shortening in fi gure 13----------------------------------------------------- 39

V
1

Abstract

We reconstruct seven sequential positions of the Acadian 

deformation front and foreland basin to illustrate the northwestward migration of the orogenic belt across Maine and 

adjacent areas from Late Silurian to Middle Devonian time. 

The reconstructions are based on (1) U-Pb and 40Ar/39Ar 

ages of pretectonic, syntectonic, and posttectonic plutons; (2) 

conodont and palynomorph ages of key strata that predate 

or postdate the local age of deformation; and (3) interpretations of the depositional settings of these strata: far foreland, 

foreland basin, inner trench slope, and orogenic hinterland. 

Tight correlations between isotopically and paleontologically 

dated events are made possible by recent improvements in the 

Silurian-Devonian time scale.

During early Ludlow time (~423 Ma), the deformation 

front must have lain near the present midline of the Fredericton Basin, between the posttectonic Pocomoonshine pluton 

(423 Ma) to the southeast and the coeval graptolitic Smalls 

Falls Formation, 70 km across strike to the northwest. During 

early Lochkovian time (~417 Ma), the deformation front was 

near the present midline of the Central Maine Basin, as 

recorded by the northwestward advance of slope deposits 

of the Carrabassett Formation across axially transported turbidites of the Madrid Formation. The early Emsian (407–406 

Ma) deformation front lay along or near the Lobster anticlinorium; its position is bracketed between the posttectonic Russell Mountain pluton (406 Ma) to the southeast and the coeval 

brachiopod-bearing Tomhegan molasse, 50 km across strike to 

the northwest. At the Emsian-Eifelian boundary (~394 Ma), 

the deformation front was located along the Pennington-Munsungun anticlinorium, northwest of the posttectonic Mapleton 

and Trout Valley Formations but southeast of the youngest 

nonmarine clastic rocks in the pretectonic Fish River Lake 

Formation. At the Eifelian-Givetian boundary (~387.5 Ma), 

the deformation front was probably somewhere near the 

midline of the Connecticut Valley-Gaspé Basin, northwest 

of a belt of posttectonic plutons in Québec, including the 

384-Ma Scotstown pluton, but southeast of three occurrences 

of deformed Eifelian carbonates along the basin’s northwestern margin. At the Givetian-Frasnian boundary (~382.5 Ma), 

the deformation front was somewhere to the northwest of 

these Eifelian carbonate outcrops. The orogen thus migrated 

northwestward about 240 km across strike (present distance) 

in about 40.5 m.y. Meanwhile, on the outboard (southeasterly) 

side of the orogen, a boundary between deformed and undeformed rocks advanced southeastward at least 50 km, probably during the Early and (or) Middle Devonian.

The migration pattern of the orogen and foreland basin 

suggests that during collision, a southeasterly plate that 

included the Acadian orogenic wedge and its Avalonian backstop overrode the Taconic-modifi ed margin of North America. 

The implied minimum plate-convergence rate is about 6 

mm/yr. The actual rate must have been considerably faster 

because this calculation was done on a nonpalinspastic base 

map; a more accurate estimate will have to await a careful 

assessment of Acadian shortening. If shortening reduced 

Maine to half its pre-Acadian width (a conservative estimate 

in light of the regional-scale tight to isoclinal folding), this 

situation would imply a plate-convergence rate of 12 mm/yr, 

at the slow end of the normal range of modern plate motions. 

The reconstructed positions of the orogen and foreland basin 

also constrain the setting of several Silurian and Devonian 

episodes of volcanism and plutonism, certain post-Acadian 

deformations in the Acadian orogenic hinterland, and certain 

pre-Acadian deformations in what was then the Acadian foreland.

Introduction

More than three decades after Wilson’s (1966) fi rst platetectonic interpretation of the Appalachians, a consensus on 

the plate geometry that led to the Acadian orogeny still has 

not emerged, though not for lack of trying. In this report, 

we present new results and review old evidence bearing on 

orogenic timing on a regional scale. By focusing on the 

syncollisional history, we can make new headway on the Acadian problem without getting bogged down in controversies 

regarding the precollisional plate geometry.

Donahoe and Pajari (1973) presented evidence that the 

Acadian orogeny was diachronous across strike, beginning in 

Migration of the Acadian Orogen and Foreland 

Basin Across the Northern Appalachians 

of Maine and Adjacent Areas

By Dwight C. Bradley, Robert D. Tucker, Daniel R. Lux, 

Anita G. Harris, and D. Colin McGregor1 

1

Geological Survey of Canada Contribution No. 1998178.
2 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

the Early Devonian along the Maine-New Brunswick coast 

and ending during the Middle Devonian in Québec, near the 

Gulf of the St. Lawrence. This fi nding was based on the 

age and distribution of fossiliferous strata known to predate 

and postdate deformation and on isotopic ages of posttectonic 

plutons. Five developments make it worthwhile to revisit this 

topic. First, the Silurian-Devonian time scale has undergone 

major revisions (Tucker and McKerrow, 1995; Tucker, and 

others, 1998). The new time scale is calibrated by U-Pb zircon 

ages of ash beds with good biostratigraphic control; most of 

the series and stage boundaries are 10 to 20 m.y. older than on 

the time scale available to Donahoe and Pajari (1973), and the 

durations of individual series and stages differ widely as well. 

Second, many Acadian plutons have now been reliably dated 

by U-Pb and 40Ar/39Ar methods. The new ages, with errors of 

1 to 3 m.y., reveal that most of the Rb-Sr and conventional 

K-Ar ages used in Donohoe and Pajari’s (1973) analysis were 

about as far off as the time scale. Third, certain key stratigraphic units that bear on the position of the deformation 

front over time have now been dated by using conodonts 

or spores with much greater precision and accuracy than 

was ever possible by using brachiopods, corals, and plants. 

Fourth, paleocurrent directions and depositional environments 

are now known for several key stratigraphic units, such as the 

Madrid and Carrabassett Formations (Hanson and Bradley, 

1989, 1993; Bradley and Hanson, 1989); these data have 

helped us to locate the foreland basin during two time intervals—and, by inference, to locate the deformation front at the 

same times. Finally, as we will show, current understanding of 

orogenic wedges, fl exural foreland basins, and orogenic backstops makes it possible to glean more information from conventional stratigraphic evidence about orogenic timing than 

was possible in the 1970's.

New U-Pb, 40Ar/39Ar, conodont, and palynologic data, 

which were the basis for the regional tectonic conclusions by 

Bradley and others (1998) and Robinson and others (1998), 

are documented here. These new results, as well as previously 

published data, enable us to track seven sequential positions 

of the deformation front as the fi rst wave of Acadian deformation migrated cratonward across the northern Appalachians 

during Late Silurian to Middle Devonian time. This information has implications for (1) the rates, trajectories, and 

amounts of plate convergence; (2) the syncollisional plate 

geometry; (3) strike-slip and thrust partitioning during collision; and (4) the relation between various suites of igneous 

rocks and the orogenic belt as it existed at the time of magmatism.

Acknowledgments.—Bradley and Harris were funded by 

the Gilbert Fellowship program of the U.S. Geological 

Survey; Tucker was funded by U.S. National Science Foundation grants EAR–9506693 and EAR–9304142. Reviews by 

Tom Armstrong and Allan Ludman substantially improved the 

manuscript. We thank Zhan Peng (Washington University), 

Dan Griffi n (University of Maine, Orono), and Tim Plucinsky 

(U.S. Geological Survey) for help in their respective geochronology and mineral-separation labs, and Kevin Moore for help 

in the fi eld. Many colleagues generously shared their knowledge and time, including Lindley Hanson (Salem State College), Dykstra Eusden (Bates College), Jim Hibbard (North 

Carolina State University), Allan Ludman (Queens College), 

Philip Osberg (University of Maine at Orono), Steve Pollock 

(University of Southern Maine), Peter Robinson (University 

of Massachusetts), David Roy (Boston College), Wally Bothner (University of New Hampshire), Charles ver Straeten 

(New York State Geological Survey), Tom Uyeno and Cees 

van Stall (Geological Survey of Canada), Kim Hannula 

(Middlebury College), Sandra Barr (Acadia University), and 

Michael Foose, Tom Armstrong, D.B. Stewart, Douglas 

Rankin, and Bob Moench (U.S. Geological Survey). Alison 

Till (U.S. Geological Survey) helped with interpretations of 

thin sections. None of this research would have been possible 

without the stratigraphic framework that largely rests on fossil 

identifi cations by Art Boucot and William Berry. Logistic 

support was provided by Dee Caldwell (Boston University), 

Bob Marvinney (Maine Geological Survey), Darrell Cowing 

(U.S. Geological Survey, Maine Water District offi ce), and the 

Bradley family of Randolph, N.H.

Regional Geology

The study area is the Acadian-deformed belt of Maine, 

plus adjacent parts of Vermont, New Hampshire, the Eastern 

Townships of Québec, and New Brunswick. The regional 

geology was reviewed by Osberg and others (1989) and Robinson and others (1998). The area of fi gure 1 is subdivided 

into 10 map units, 7 of which are relevant to the timing of 

Acadian deformation. Middle Ordovician and older rocks are 

grouped together in fi gure 1 because they provide no useful 

constraints on the age of Acadian tectonism.

Deep-water Silurian strata defi ne four basins: the Connecticut Valley-Gaspé Basin, the Aroostook-Matapedia Basin, 

the Fredericton Basin, and the Central Maine Basin (fi g. 2), 

the last two of which have been identifi ed by some workers 

(Bradley, 1983; Ludman and others, 1993) as the former 

site of an ocean whose closure resulted in the Acadian 

orogeny. Although this interpretation remains controversial, 

the nature of the depositional basement of these deep-water 

basins makes little difference for the present study of syncollisional tectonics.

Flanking the deep-water basins are four tracts where preSilurian rocks are now exposed. These belts—Avalonia, the 

Bronson Hill-Pennington anticlinorial belt, Miramichi, and 

the Taconic orogen (fi g. 2)—either were undergoing active 

erosion during at least part of the Silurian or were the sites of 

shallow-water deposition. The shallow-water deposits include 

carbonate and locally derived siliciclastic rocks that range in 

age from Llandovery to Eifelian. One goal of our study was 

to obtain precise conodont ages from the youngest carbonate 

rocks in various key stratigraphic sections.

From Late Silurian to Middle Devonian time, much of 

the area of fi gure 1 was inundated by a thick succession 

of clastic rocks, which we regard as the fi ll of a migrating 

Acadian foreland basin (Bradley, 1983, 1987, 1997; Hanson 

and Bradley, 1989). The foreland-basin succession includes 

all of the Devonian fl ysch and Devonian molasse and part of 
3

the deep-water Silurian sequence. For want of sedimentologic 

studies, the Silurian deep-water deposits cannot yet be confi -

dently subdivided into foreland-basin and pre-foreland-basin 

successions. A second focus of the present study was to 

obtain more tightly constrained fossil ages from key forelandbasin deposits. Palynology has proved to be most useful in 

this regard; several calcareous concretions from siliciclastic 

turbidites were processed for conodonts but were found to be 

barren.

Silurian and Devonian volcanic rocks occur in two broad 

belts (fi gs. 1, 2). Rocks of the Coastal volcanic belt were 

erupted into basement of the Avalonian terrane along the coast 

of Maine and New Brunswick. Farther north lies a second belt 

of Silurian-Devonian volcanic rocks that crop out along parts 

of the Bronson Hill-Pennington anticlinorial belt, the Aroostook-Matapedia Basin, the Miramichi anticlinorium, and the 

Connecticut Valley-Gaspé Basin. The northerly belt has been 

called the Piscataquis volcanic belt (or magmatic belt) in New 

Figure 1.—Geologic map of Maine and adjacent parts of New Brunswick, Québec, New Hampshire, and Vermont, showing distribution of strata 

in study area that help constrain date of Acadian foreland-basin sedimentation and (or) Acadian deformation. Numbered localities are keyed to 

fi gure 4. LBF, Lubec-Belleisle Fault; SPF, Sennebec Pond Fault; THF, Turtle Head Fault.

Regional Geology
4 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

England (Bradley, 1983) and the Tobique volcanic belt in 

Canada (Keppie and Dostal, 1994). The Piscataquis volcanic 

rocks were erupted in a belt that had been accreted to North 

America during the Ordovician.

In all but the northernmost part of the study area (fi g. 

1), the rocks were deformed during a Silurian to Devonian 

orogeny that can be termed “Acadian” in the loose sense 

favored here. Where deformation was polyphase, the date of 

the fi rst deformation event (D1) is of greatest interest for the 

present purposes. The style and intensity of Acadian deformation vary considerably across the study area; the common 

thread is that the deformation was dominantly contractional. 

In the low-grade rocks of northern and central Maine, Acadian 

structures are mostly upright, tight to isoclinal folds. At higher 

metamorphic grades in central New Hampshire, east-directed 

fold nappes predominate (Eusden and others, 1996); in southern New Hampshire, nappes have westerly vergence (Thompson and others, 1968; Robinson and others, 1998). Some 

strike-slip faulting also occurred during Late Silurian and 

Devonian times, as discussed below.

Silurian and Devonian (Acadian) plutons, ranging in 

composition from gabbro to granite, are widespread in the 

study area (fi g. 1); many are demonstrably syntectonic or 

posttectonic. A major goal of our study was to date key 

plutons across a broad swath from the Maine coast to the 

Maine-Québec border, using U-Pb and 40Ar/39Ar methods. As 

we will show, the locus of plutonism shifted over time.

Of the various post-Acadian rocks in the study area (fi g. 

1), only Middle and Upper Devonian sedimentary and volcanic rocks that overlie Acadian-deformed rocks are relevant to 

Figure 2.—Maine and adjacent parts of New Brunswick, Québec, New Hampshire, and Vermont, showing distribution 

of major Silurian-Devonian paleogeographic elements in study area. BHA, Bronson Hill anticlinorium; BMA, Boundary 

Mountains anticlinorium; GMA, Green Mountain-Sutton Mountain anticlinorium; LMA, Lobster Mountain anticlinorium; 

MIA, Miramichi anticlinorium; MRS, Moose River synclinorium; MUA, Munsungun anticlinorium; PA, Pennington 

anticlinorium.
5

orogenic timing. In this report, we present new palynologic 

ages for two key post-Acadian units, ages that previously had 

been documented only in unpublished paleontologic reports 

of the Geological Survey of Canada. Other post-Acadian 

rocks in the study area include Carboniferous to Cretaceous 

plutons and Carboniferous to Jurassic sedimentary rocks.

Determining the Timing

of Collisional Orogeny

from the Rock Record

Traditionally, orogenesis in a given place has been judged 

to be younger than the youngest deformed strata but older 

than the oldest unit that unconformably overlies the deformed 

rocks. For most formations in the study area (fi g. 1), we know 

more than whether they participated in Acadian deformation: 

interpretations of the tectonic setting of the dated rocks can 

supplement more traditional evidence for orogenic timing.

Typical contractional orogenic systems have three components: (1) an underriding plate, (2) an overriding plate, 

and, between them, (3) an actively deforming orogenic wedge

(fi g. 3). Two broad depositional regimes exist side by side 

on the underriding plate. The foreland basin is a marine 

or nonmarine sedimentary basin that fl anks the orogen and 

is typically fi lled with orogenically derived sediment. (The 

terms “foreland basin” and “foredeep” are sometimes used 

interchangeably, but a foredeep, more properly, is an underfi lled foreland basin characterized by deep-marine fl ysch sedimentation.) The term “far foreland” is coined here for the 

Figure 3.—Conceptual model of a two-plate collision. A, Cross section showing positions of far foreland, foreland basin, 

orogenic wedge, and hinterland, as discussed in text. B, Plate convergence causes each of these paleogeographic elements to 

migrate to left over time at a rate similar to plate-convergence rate, giving rise to succession of depositional and deformational 

events in a given rock sequence.

Determining the Timing of Collisional Orogeny from the Rock Record
6 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

region beyond the limit of orogen-derived sedimentation. A 

forebulge—a broad, gentle uplift formed by lithospheric fl exure—may or may not be present in the distal part of a foreland 

basin or in the far foreland.

The deformation front is the surface expression of the 

gently dipping boundary between the underriding plate and 

orogenic wedge; it may be a sharp boundary at an emergent 

thrust fault or a broad zone of folding. Much, but not all, 

of the convergence between the underriding and overriding 

plates typically occurs at and just to the rear of the deformation front and is manifested as the D1 event that is of greatest 

interest here. Some additional convergence may be taken up 

farther to the rear, within the orogenic wedge, accounting for 

some D2 and later events. The boundary between the actively 

deforming orogenic wedge and the relatively rigid—or even 

extensional—hinterland on the overriding plate is called the 

backstop.

The model just outlined is suffi ciently generalized that it 

applies to several plate-tectonic regimes, such as arc-passive 

margin collision, Andean-style foreland contraction, and arcarc collision. Figure 3 might be thought of as a single frame of 

a moving picture; as time goes on, new rocks of the underriding plate are fed into the orogenic system. 

Most of the relevant Silurian-Devonian strata in the Acadian orogen can be placed with at least fair confi dence in 

fi gure 3. In the descriptions of stratigraphic sections that 

follow, we focus not only on the conventional stratigraphic 

evidence (preorogenic versus postorogenic) but also on observations that enable us to place each section in the far foreland, 

foreland basin, orogenic wedge, or hinterland at a particular 

time. We use only those age constraints provided by fossils 

where they actually occur; the New England tradition of 

extrapolating fossil ages across strike hinders the recognition 

of diachronous facies, which are characteristic of all collisional orogens in the world.

Paleontologic and Stratigraphic 

Evidence for the Date of the 

Acadian Orogeny

Passamaquoddy Bay, Maine and New 

Brunswick

The Coastal volcanic belt section of southeastern Maine 

(loc. 38, fi gs. 1, 4) and southwestern New Brunswick consists 

of 8 km of interbedded bimodal volcanic and fossiliferous 

sedimentary rocks of late Llandovery to Lochkovian age. The 

following highlights (from Gates and Moench, 1981) are key 

to Acadian tectonic problems. The late Llandovery Quoddy 

Formation, mostly graptolitic shale, contains a few tuffaceous 

horizons that record the fi rst known pulse of Silurian volcanism in the belt (loc. 38, fi g. 4). Mixed volcanism and 

ever-shallowing marine sedimentation continued throughout 

Wenlock, Ludlow, and Pridoli time and the fi rst part of the 

Lochkovian (Dennys, Edmunds, Leighton, Hershey, and lower 

Eastport Formations). In New Brunswick, the upper part of 

the Eastport Formation consists of fl uvial sandstone, siltstone, 

and conglomerate (Pickerill and others, 1978). The Eastport 

Formation, which is the youngest pre-Acadian stratifi ed unit, 

has been assigned a Lochkovian age on the basis of a 

restricted fauna of lingulids, gastropods, pelecypods, and 

ostracodes (Berdan, 1971; Pickerill and Pajari, 1976). We 

note that the Castine Volcanics in the Penobscot Bay region, 

which were once regarded as Silurian and thus part of the 

Coastal volcanic belt, have recently yielded a U-Pb zircon 

age of 505 Ma (R.D. Tucker, unpub. data., 1997) and so are 

irrelevant to the Acadian orogeny. All the volcanic units were 

involved in Acadian folding and development of a regional 

east-northeast-striking cleavage (Pickerill and others, 1978, 

p. 42). The Red Beach pluton has yielded two confl icting 

isotopic ages, both suspect: a Givetian Rb-Sr isochron age 

of 385±6 Ma (Spooner and Fairbairn, 1970; age recalculated 

herein) and a Lochkovian U-Pb age of 415±6 Ma (Jurinski, 

1990). The nominal U-Pb age is likely too old because the 

pluton truncates regional-scale Acadian folds that deform 

the Lochkovian Eastport Formation (Abbott, 1991); however, 

Rb-Sr ages from coastal Maine plutons have consistently 

proved to be too young. Accordingly, the age of folding 

cannot be determined with confi dence. The Perry Formation 

unconformably overlies both the Eastport Formation and the 

Red Beach pluton. The basal conglomerate of the Perry Formation contains boulders of unmistakable Red Beach granite. 

Plant fossils suggest a general Late Devonian age for the 

Perry Formation (Kasper and others, 1988, p. 127); an ash 

bed in the Perry Formation was processed for datable zircons 

but was barren.

Fredericton Basin, Maine and New Brunswick

In easternmost Maine, the thick, isoclinally folded turbidites of the Fredericton Basin (or “Trough” of some workers) 

are devoid of fossils (loc. 37, fi gs. 1, 4; Ludman and others, 

1993). Nonetheless, some useful information regarding the 

timing of Acadian deformation can be gleaned. The youngest 

turbidite unit, the Flume Ridge Formation of presumed 

Silurian age (Ludman and others, 1993), was penetratively 

deformed before being intruded by the 423±2-Ma Pocomoonshine pluton (see below). In New Brunswick, however, 

graptolites from turbidites (loc. 35, fi gs. 1, 4) reveal that 

sedimentation was still taking place in the northwestern part 

of the basin at this time. Specifi cally, the Burtts Corners Beds 

have yielded graptolites ranging in age from Cyrtograptus 

linnarssoni Zone (Wenlock) to Monograptus nilsonni Zone 

(early Ludlow) (Fyffe, 1995, p. 352–353).

Canterbury Inlier, New Brunswick

The Canterbury inlier (loc. 34, fi gs. 1, 4) lies along the 

southeast fl ank of the Miramichi anticlinorium. The inlier 

is separated by the Pokiok batholith from the main part of 

the Fredericton Basin, and the stratigraphy differs markedly. 

The Silurian-Devonian section is poorly fossiliferous, but 
7

the youngest stratifi ed unit (Hartin Formation), which is of 

greatest interest here, is dated reasonably well. The Hartin 

Formation is composed of sandstone and slate, plus minor 

limestone, conglomerate, and felsic volcanic rocks. A brachiopod fauna indicates a Helderbergian age (that is, Lochkovian; 

A.J. Boucot, in Venugopal, 1979, p. 17). The Hartin Formation has been contact-metamorphosed by the Harkshaw and 

Skiff Lake phases of the Pokiok batholith (Venugopal, 1979, 

p. 28), which have yielded U-Pb ages of 411±2 Ma (sphene) 

and 409±2 Ma (zircon), respectively (Bevier and Whalen, 

1990a). Along strike in easternmost Maine, the Skiff Lake 

phase truncates Acadian structures that deform Silurian-Devonian strata (Ludman, 1990), and so appears to be broadly 

posttectonic. Acadian deformation in this area must therefore 

have occurred during the late Lochkovian or the Pragian. 

Given that Acadian deformation must have followed closely 

upon deposition of the Hartin Formation, a foreland-basin 

depositional setting seems likely.

Carlisle Area, New Brunswick

Acadian deformation is anomalously young in the Aroostook-Matapedia Basin in western New Brunswick (loc. 33, 

fi gs. 1, 4). The Late Ordovician and Silurian were times 

of deep-water sedimentation (Carys Mills and Smyrna Mills 

Formations; St. Peter, 1982). The Smyrna Mills Formation 

is overlain by the Costigan Mountain Formation of predominantly felsic volcanic rocks; it is undated but regarded as 

probably Lower Devonian on the basis of correlations with 

fossiliferous rocks to the northeast (St. Peter, 1982, p. 35). 

The youngest pre-Acadian unit is the Wapske Formation, 

estimated to be 4 km thick. It consists of slate, siltstone, 

sandstone, and conglomerate, with isolated mafi c volcanic 

lenses (St. Peter, 1982). Clasts in conglomerate of the Wapske 

Formation can be traced to the Smyrna Mills, Costigan Mountain, and other older units (St. Peter, 1982, p. 40). The 

lower part of the Wapske Formation has yielded brachiopods 

from fi ve localities that, according to A.J. Boucot (in St. 

Peter, 1982, p. 45), indicate a late Helderbergian (Becraft-Port 

Ewen) age. This interval corresponds approximately to that 

of the delta and pesavis conodont zones of the Lochkovian. 

Spores of late Emsian or early Eifelian age have been recovered from the upper, nonmarine part of the formation (D.C. 

McGregor, in St. Peter, 1982, p. 478). We suggest that the 

Wapske Formation was deposited in a foreland-basin setting 

and that it was derived, at least in part, from an Acadian 

orogenic source to the south, which must have existed by 

about 411 Ma.

Presque Isle Quadrangle, Maine

The Presque Isle-Mapleton section (loc. 31, fi gs. 1, 4) 

provides age constraints on the timing of Acadian foreland-basin sedimentation, the main Acadian folding, and 

postorogenic hinterland sedimentation. Deep-water sedimentation (slate, deep-water limestone, siliciclastic turbidites) 

was already underway by the Caradoc and continued into 

the Ludlow (Carys Mills and Spragueville Formations and 

Perham Group; Roy and Mencher, 1976). A gap in the 

megafossil record, spanning the late Ludlow, Pridoli, and 

early Lochkovian, corresponds to the “Salinic Disturbance”

of Boucot and others (1964). Whatever its cause, this hiatus 

did not last as long as once suspected—no more than 6 m.y., 

judging from the new time scale (Tucker and others, 1998). 

The next rocks to be deposited were volcanic rocks (Dockendorff Group), deltaic sandstone (Chapman Sandstone), 

and prodeltaic turbidites (Swanback Formation). Boucot and 

others (1964) showed these three units to be lateral equivalents of Lochkovian (“New Scotland”) age. Roy (1980) interpreted the Swanback and Chapman Formations as easterly 

derived fl ysch and molasse, respectively, which we assign to 

the Acadian foreland basin. Regional Acadian folding took 

place some time after deposition of the Lochkovian strata. 

The folded rocks are unconformably overlain by the Mapleton Formation, a local accumulation of nonmarine conglomerate and sandstone (Boucot and others, 1964). Conglomerate 

clasts in the Mapleton include fossiliferous clasts from the 

Chapman Sandstone and various older units (Boucot and 

others, 1964, p. 61–62; Roy and White, 1987). On the basis of 

plant fossils, the Mapleton was previously assigned a Middle 

Devonian, possibly Givetian age (Kasper and others, 1988). 

We now assign the Mapleton a slightly older age, early or 

middle Eifelian (Grandispora velatus-Rhabdosporites langii

Zone), on the basis of spores from material submitted by Eli 

Mencher to D.C. McGregor during the early 1960’s (table 1; 

McGregor, 1962; new slides examined, 1989). The following 

forms were identifi ed: Apiculiretusispora gaspiensis, Ancyrospora sp., Calamospora cf. C. atava (Naumova) McGregor, 

Calyptosporites sp., Emphanisporites annulatus, E. rotatus, 

Grandispora velata, Stenozonotriletes sp., Dibolisporites 

echinaceus, and Acinosporites macrospinosus?. The Mapleton 

is folded into a gentle syncline of middle Eifelian or younger 

age, which is nearly coaxial with an older, tighter syncline of 

the main phase of Acadian deformation (Boucot and others, 

1964, p. 73).

Central Maine Basin

The stratigraphy of the Central Maine Basin is best 

known from its northwest fl ank in the Rangeley and Phillips 

quadrangles (loc. 26, fi gs. 1, 4; Moench, 1971), where the 

Silurian-Devonian deep-water section is about 8 km thick and 

most of the formations have their type localities (Moench 

and Pankiwskyj, 1988). Key sedimentary-facies interpretations and paleocurrent data for these units, however, are from 

150 km to the east in the Jo Mary Mountain area (loc. 30, fi g. 

1; Hanson and Bradley, 1989), where the rocks are at much 

lower metamorphic grade and sedimentary features are better 

preserved. Complicating matters still further, the fossil control 

that is critical for present purposes is from yet a third area, 

Kingsbury-Guilford (locs. 28, 29, fi gs. 1, 4), which before 

Acadian shortening lay many tens of kilometers from the 

basin’s northwestern margin. The following discussion draws 

on information from all of these places.

Paleontologic and Stratigraphic Evidence for the Date of the Acadian Orogeny
8 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

suggest that the Smalls Falls is the youngest unit of northwest 

derivation (Hanson and Bradley, 1993). It consists of rustyweathering turbiditic sandstone; the type Smalls Falls Formation is assigned a Ludlow age on the basis of fossils in 

broadly similar strata from a more southeasterly strike belt, 

as discussed below. The upper sequence, which was derived 

from outboard sources, consists of two widespread units—the 

Madrid and Carrabassett Formations—with poorly understood younger strata of local distribution. The Madrid 

Formation, in central Maine, is a thick succession of sandstone-dominated, siliciclastic turbidites; it becomes increasFigure 4.—Stratigraphic sections bearing on date of Acadian deformation and (or) foreland-basin sedimentation in study area (fi g. 1). 

Numerical ages of various stage boundaries from time scale of Tucker and others (1998); spore zones from Richardson and McGregor (1986). 

Units: Dbk, Beck Pond Limestone; Dbr, Bear Pond Limestone Member; Dc, Carrabassett Formation; Dch, Chapman Sandstone; Dco, Compton 

Formation; De, Eastport Formation; Deh, Edmunds Hill Andesite; Df, Famine Limestone; Dfp, Frost Pond Formation; Dh, Hildreths Formation; Dha, 

Hartin Formation; Dhr, Hersey Formation; Dhv, Hedgehog Volcanics; Dk, Kineo Rhyolite; Dl, Littleton Formation; Dm, Matagamon Sandstone; 

Dmc, McKenney Ponds Limestone Member; Dmi, Millimagassett Lake Formation; Dmp, Mapleton Formation; Dnd, Nadeau Thoroughfare 

beds (informal); Dpe, Perry Formation; Ds, Seboomook Group (formerly Formation); DSac, Ayres Cliff Formation; DScm, Costigan Mountain 

Formation; DSfl , lower part of the Fish River Lake Formation; DShl, Hersey and Leighton Formations; Dsl, Square Lake Limestone; DSm, 

Madrid Formation; DSpb, Parker Bog Formation; DSsp, Spider Lake Formation; Dsw, Swanback Formation; DSwb, West Branch Volcanics; 

Dta, Tarratine Formation; Dte, Temiscouata Formation; Dtf, The Forks Formation; Dto, Tomhegan Formation; Dtr, Traveler Rhyolite; Dtu, 

Touladi Limestone; Dtv, Trout Valley Formation; Du, unnamed Devonian strata; Dw, Wapske Formation; Sb, Burtts Corners beds; Scc, Cross 

Creek beds; Scl, Canterbury Limestone; Scq, Clough Quartzite; Sd, Dennys Formation; Se, Edmunds Formation; Sf, Fitch Formation; Sfr, 

Along the northwest fl ank of the basin, the SilurianDevonian section is divisible into lower and upper parts on 

the basis of provenance and paleocurrents (Hanson and Bradley, 1993). Rocks assigned to the lower sequence (Greenvale 

Cove, Rangeley, Perry Mountain, and Smalls Falls Formations; see fi g. 4) are known or inferred to have been derived 

from the northwest. A northwesterly provenance is well established for the Llandovery Rangeley Formation (at loc. 26, fi gs. 

1, 4), which contains conglomerate clasts from the Attean 

pluton some 50 km to the northwest (near loc. 12, fi g. 1; 

Moench and Pankiwskyj, 1988). Meager paleocurrent data 

Square Lake

St. Froid

Lake-Fish

River

21, 22

Eastern

Townships

3, 4

Montreal

1

Ste.-

Georges

5

Lac

Temiscouata

7

Madawaska

24

Munsungun

18 16, 17

Northern

Moosehead

and Brassua

Lakes

15

Littleton

9, 10

Grand

Lake

Seboeis

20

TravelerChesuncook

23

B

B

C

B

C

S

S

B

C S

S

B

S

B B

B

B

K

B

B

C

B B, K

B

B,K

B,K G

G

B

B

P

Sf Dsl

Dnd

Dnd

Dtu

Spe

Dte

Dl

Dto

Dta

Dtr

Dk

Dfp

DSwb

Srp

Ds

Df

SOc

DSfl

Ds

DSsp

Dmi

Sgls Su

Scq

Ds

Dm

Dtv

DSac

Dco

SOm

Du

SERIES/

 STAGE

SYSTEM

CONODONT

 ZONE

SELECTED

 SPORE

 ZONE

382.5

409

424

376.5

387.5

394

413.5

418

419

362

micrornatus

breconensis

polygonalis

annulatus

velata

douglastown.

devonicus

bohemica

staurogn.

celloni

rhenana

sagitta

amorphogn.

pireneae

dehiscens

delta

pesavis

sulcatus

kindlei

eurekaensis

siluricus

eostein.

ploeckensis

crispa

varcus

australis

kockelianus

ensensis

gronbergi

inversus /

laticostatus

serotinus

partitus

patulus

costatus

hesp.-wosch.

snajdri

praesulcata

hassi

jamieae

rhenana

linguiformis

triangularis

crepida

rhomboidea

marginifera

trachyfera

postera

expansa

disparilis

falsiovalis

transitans punctata

herm.- crist.

Famennian Frasnian

CARB.

Tour.

(part)

(part)

Givet. Eifelian Emsian

DEVONIAN

Loch.

Prid.Lud.

SILURIAN (part)

Prag. Wen. Lland. (part)

Nonmarine cgl. and ss.

Deltaic sandstone

Siliciclastic turbidites

Deep-water carbonates

Shallow-water carbonates

Volcanic rocks

Shale, siltstone

Volcaniclastics

Key pluton

Acadian deformation

Quartzite

Euxinic turbditites
9

ingly calcareous to the southwest in western Maine and New 

Hampshire. Paleocurrents clearly show a southwesterly direction of fl ow, along the basin axis (Bradley and Hanson, 1989). 

The gradationally overlying Carrabassett Formation consists 

of another kilometer or so of chaotic, pelitic olistostrome 

with subordinate, coherent sand and silt turbidites. Paleocurrents show overall northerly fl ow (Hanson and Bradley, 1993). 

Hanson and Bradley (1989) interpreted the Madrid and Carrabassett Formations as having been deposited along a forelandbasin axis and on a north-facing slope, respectively; however, 

neither the Madrid nor the Carrabassett Formation has yielded 

useful fossils. The Madrid Formation is early Ludlow or 

Flume Ridge Formation; Sgls, Grand Lake Seboeis Formation; Sh, Hardwood Mountain Formation; Shb, Hayes Brook beds; Sl, Leighton

Formation; SOc, Cabano Formation; SOm, Matapedia Group; Spe, Perham Formation; Spm, Perry Mountain Formation; Spw, Pocowogamis 

Conglomerate; Sq, Quoddy Formation; Sr, Rangeley Formation; Srp, Ripogenus Formation; Ssa, Sangerville Formation; Ssf, Smalls Falls 

Formation; Ssp, Spragueville Formation; Sss, Scott Siding Slate; Stx, Taxis River beds; Su, unnamed Silurian strata. References (numbers at 

top of columns): 1, Boucot and others (1986); 3, 4, Hueber and others (1990); 5, Uyeno and Lespérance (1997); 7, Uyeno and Lespérance (1997); 

9, 10, Boucot and Arndt (1960); 11, Albee and Boudette (1972) and this report; 12, Boucot and others (1959) and this report; 13, Boucot and 

Heath (1969) and this report; 14, Boucot and Heath (1969) and this report; 15, Boucot and Heath (1969); 16, 17, Griscom (1976), Rankin 

(1968), and this report; 18, Hall (1970); 20, Hibbard (1993); 21, 22, Boone (1970) and this report; 23, this report; 24, St. Peter and Boucot 

(1981); 26, Moench and Pankiwksyj (1988); 27, Marvinney (1984) and this report; 28, 29, Pankiwskyj and others (1976), Ludman (1978) 

and this report; 31, Boucot and others (1964) and this report; 33, St. Peter (1982); 34, Venugopal (1979); 35, Fyffe (1995); 37, West and 

others (1992); 38, Gates and Moench (1981).

younger. The Carrabassett Formation has yielded nondiagnostic brachiopods and plant fragments consistent with an Early 

Devonian age (Espenshade and Boudette, 1967, p. F12–F13), 

and is cut by the earliest Emsian Sebec Lake pluton.

The depositional history of the central part of the Central 

Maine Basin is not as well understood as that of the northwest 

fl ank because we lack detailed sedimentologic studies and 

deformation and metamorphism in the basin are more intense. 

A few observations bear on the timing of Acadian deformation. Most of the area is underlain by the Sangerville Formation, an isoclinally folded succession of siliciclastic and 

calcareous turbidites many kilometers thick. The Sangerville 

Paleontologic and Stratigraphic Evidence for the Date of the Acadian Orogeny

McKenney

Ponds

13

Little

Big Wood

Pond

11

Beck and

 Bear Ponds

12

Upper

Kennebec

27

Spencer

Gore

14

Carlisle

33

Presque

Isle

31

RangeleyStratton

26

Canterbury

inlier

34

Burtts

Corners

35

KingsburyGuilford

28, 29

Eastport

Fredericton

Basin in

Maine

37 38

B

O

G,

B

B

G G

C

B

C

B B

B B

C

C

B B

C

S

S

G

Pl

B

Dta

Dmc

Ds Ds

Dbk Sh Dbr

Dch Dsw Deh

Dhv

Spe

Dmp

Dw

DSpb

Dc

Dtf Ssf

Spm

DSm

Dc Dh

Ds

Ssf

Ssa

DSm

Dc

Sb

Stx Shb

De Dhr

Sl

Se

Sq

Dpe

Ssp

DScm

Sr

Scc

Sd

Sfr

Ds Dha

Spw Scl

Sss

382.5

409

424

376.5

387.5

394

413.5

418

419

362

SYSTEM

CONODONT

 ZONE

SPORE

 ZONE

flexuosa

pusillites

nitidus 

micrornatus

breconensis

polygonalis

annulatus

devonicus

lemurata

optivus

ovalis

torquata

SERIES/ STAGE

velata

douglastown.

bohemica

staurogn.

celloni

rhenana

sagitta

amorphogn.

pireneae

dehiscens

delta

pesavis

sulcatus

kindlei

eurekaensis

siluricus

eostein.

ploeckensis

crispa

varcus

australis

kockelianus

ensensis

gronbergi

inversus /

laticostatus

serotinus

partitus

patulus

costatus

hesp.-wosch.

snajdri

praesulcata

hassi

jamieae

rhenana

linguiformis

triangularis

crepida

rhomboidea

marginifera

trachyfera

postera

expansa

disparilis

falsiovalis

transitans punctata

herm.- crist.

Famennian Frasnian

CARB.

Tour.

(part)

(part)

Givet. Eifelian Emsian

DEVONIAN

Loch.

Pri.Lud. Prag. Wen.

SILURIAN (part)

Lland(part)

 Bars in columns show age constraints by fossil groups:

B, brachiopod C, conodont G, graptolite 

K, coral O, ostracode P, pelecypod

Pl, plant S, palynomorph 
10 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians Table 1. Palynologic data from Devonian rocks, Maine [GSC, Geological Survey of Canada. Do., ditto]
11

has yielded graptolites from 13 localities (Pankiwskyj and 

others, 1976); the most diagnostic collection (loc. 28, fi gs. 1, 

4) is middle Wenlock. The overlying Smalls Falls Formation 

has also yielded graptolites from several localities; the most 

diagnostic collection (loc. 29, fi gs. 1, 4) is early Ludlow 

(Pankiwskyj and others, 1976). The overlying Madrid and 

Carrabassett Formations are the youngest units along the 

central part of the basin. The Carrabassett Formation is 

absent in the most southerly strike belt of Madrid Formation 

either because it has since been eroded away (Moench 

and Pankiwskyj, 1988) or because it was never deposited 

(Hanson and Bradley, 1989).

Littleton Area, New Hampshire

The classic Silurian-Devonian section near Littleton, 

New Hampshire (loc. 9, fi gs. 1, 4; Billings, 1937), constrains the timing of both Acadian foreland-basin sedimentation and deformation. The Lower Silurian Clough Quartzite 

unconformably overlies Ordovician rocks and is overlain by 

the Fitch Formation, which consists of calcareous metasiltstone, quartzite, granofels, and some limestone. Harris and 

others (1983, p. 731) reported an age of late Ludlow to 

mid-Pridoli for the Fitch Formation in the Littleton area, on 

the basis of a conodont fauna representing the Ozarkodina 

remscheidensis eosteinhornensis Zone; along strike in Massachusetts, strata assigned to the Fitch Formation are as 

young as the woschmidti to eurekaensis Zones of the Lochkovian (Elbert and others, 1988). The Clough and Fitch are 

relatively thin, shallow-marine deposits; the Fitch has been 

interpreted as a far-foreland deposit (Bradley, 1983). The 

overlying Littleton Formation is an approximately 1.6-kmthick fl ysch succession (a minimum thickness, because the 

top is not preserved) deposited in a foreland-basin setting 

(Bradley, 1983). It has yielded Emsian brachiopods at two 

localities in the Littleton area (loc. 9, fi gs. 1, 4) and at a third 

locality in Whitefi eld (loc. 10, fi gs. 1, 4), where the fossils 

occur only a few tens of meters above the base (Boucot and 

Arndt, 1960, p. 41–43). At Gale River (loc. 39, fi g. 1) in the 

next strike belt to the east, a tuff from low in the Littleton 

Formation has yielded an Emsian U-Pb zircon age of 407±2 

Ma (R.D. Tucker and D. Rankin, unpub. data, 1998). Still 

farther east, at Beaver Brook (loc. 8, fi g. 1), a brachiopod 

fauna suggests a slightly older age (“Oriskany, possibly 

Esopus”; Boucot and Rumble, 1980, p. 192–194) than elsewhere (locs. 9, 10, fi g. 1). The Littleton thus appears to be 

diachronous across strike at the latitude of its type area.

Southwestern Part of the Moose River 

Synclinorium, Maine

In the southwestern part of the Moose River synclinorium, western interior Maine, local carbonate-bearing units 

overlie Ordovician and older rocks, providing good age constraints on the onset of Acadian foreland-basin sedimentation. 

At Beck Pond (loc. 12, fi gs. 1, 4), the post-Ordovician section 

begins with the Beck Pond Limestone (Boucot and others, 

1959) and includes reef limestone intercalated with boulder 

conglomerate derived from the nearby Attean pluton of Ordovician age. A rich brachiopod fauna initially suggested an 

“upper Helderberg, New Scotland” age (Boucot and Heath, 

1969); however, conodonts from the upper part of the Beck 

Pond (member 5 of Boucot and others, 1959) include Belodella sp., Decoriconus sp., Dvorakia sp., Icriodus sp. indet., 

and Ozarkodina remscheidensis eosteinhornensis, indicating, 

instead, a very latest Pridoli to early Lochkovian age (pl. 1, 

fi gs. 1–13; table 2). The Beck Pond Limestone is overlain 

by a considerable thickness, probably 2 to 3 km, of Lower 

Devonian fl ysch assigned to the Seboomook Group (formerly 

Formation; Pollock, 1987). Boucot and others (1959) interpreted the contact between Member 5 of the Beck Pond and 

overlying slate of the Seboomook as an angular unconformity. 

Upon digging out this contact (at loc. 10 of Boucot and others, 

1959), we observed, instead, that reefal limestone debris interfi ngers with black silty shale. The conodonts thus date the 

top of the Beck Pond Limestone, the base of the Seboomook 

fl ysch sequence, and the onset of Acadian foreland-basin sedimentation.

About 1 km to the north (loc. 12, fi gs. 1, 4), the Bear 

Pond Limestone (Boucot and others, 1959) is a carbonate lens 

or buildup not far above the base of the Seboomook Group. It 

is known from a single outcrop that exposes about 10 m of 

section. A rich brachiopod fauna again indicates an “upper 

Helderberg, New Scotland” age (Boucot and Heath, 1969) for 

the buildup and, by implication, for the immediately overlying 

and underlying siltstone and sandstone of the Seboomook 

Group. A large conodont sample yielded a meager conodont 

fauna of Belodella devonica, Ozarkodina remscheidensis 

eosteinhornensis, and fragments of Icriodus sp. indet. or 

Pedavis sp. indet., allowing a possible age range from late 

Ludlow to earliest Pragian (table 2). Nearby, Boucot and 

Heath (1969, p. 36) reported several brachiopod occurrences 

that suggest a Pragian age for all but the lowest beds of 

the Seboomook Group in the Moose River synclinorium. The 

Seboomook Group is the youngest pre-Acadian stratifi ed unit 

in the area; it is not known to extend upward beyond the 

Pragian.

About 15 km to the northeast, the post-Ordovician section (loc. 13, fi gs. 1, 4) begins with the McKenney Ponds 

Limestone Member of the Tarratine Formation. Boucot and 

Heath (1969) reported a “Becraft-Oriskany” (late Lochkovian to Pragian) age based on brachiopods. This assignment 

can be somewhat refi ned on the basis of our conodont collections (table 2). The most diagnostic of three samples 

(USGS colln. 12516–SD, table 2), located 2 m stratigraphically below siltstone of the Tarratine Formation, yielded 

Belodella devonica, Icriodus sp. indet., Ozarkodina remscheidensis, and Pseudooneotodus beckmanni (pl. 1, fi gs. 

19–26). A Lochkovian, but not earliest Lochkovian, age is 

indicated by the icriodids, which are of a post-I. woschmidtii 

morphotype. The limestone member is gradationally overlain 

by sandstone of the main body of the Tarratine Formation, 

which both interfi ngers with and overlies the Seboomook 

Group and likewise is interpreted as part of the forelandbasin fi ll. The Tarratine Formation has yielded Oriskany-age 

(Pragian) brachiopods from several nearby localities (Boucot 

and Heath, 1969).

Paleontologic and Stratigraphic Evidence for the Date of the Acadian Orogeny
12 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Figure 5.—Correlation chart showing conodont age ranges for samples dated in this report, and palynomorph age ranges for 

samples dated in this report and for samples from northern Maine that were previously discussed in internal paleontologic reports 

of the Geological Survey of Canada (McGregor, 1962, 1963, 1968, 1992).

Northeastern Part of the Moose River 

Synclinorium, Maine

 The northeastern part of the Moose River synclinorium 

in Maine provides crucial stratigraphic constraints on the age 

of the youngest foreland-basin deposits. In the area of northern 

Moosehead and Brassua Lakes (loc. 15, fi gs. 1, 4), Ordovician 

and older basement rocks are overlain by poorly dated Silurian 

calcareous rocks and Devonian(?) red shale (Boucot and Heath, 

1969). The Acadian foreland-basin succession begins with the 

Tarratine Formation, which is about 2 km thick in this area 

(Boucot and Heath, 1969, pl. 19). The Tarratine Formation 

has yielded abundant Oriskany-age—that is, Pragian—brachiopods (Boucot and Heath, 1969, p. 27). The Kineo Rhyolite, 

sandwiched between the Tarratine Formation, below, and the 

Tomhegan Formation, above, represents a pulse of silicic magmatism in the foreland basin. The Kineo Rhyolite has yielded 

only discordant zircon ages, marked by inheritance. It is comparable in stratigraphic position, composition, and eruptive 

environment to the 407- to 406-Ma Traveler Rhyolite (Rankin 

and Tucker, 1995) 65 km to the northeast (loc. 17, fi gs. 1, 

4); the Kineo and Traveler are presumably coeval. The Kineo 

ranges in thickness from 0 m at its north limit to about 1,200 

m just a few kilometers to the south. It is overlain by the main 

body of the Tomhegan Formation, consisting of crossbedded, 

probably deltaic sandstone (L. Hanson and D. Bradley, unpub. 

data). Boucot and Heath (1969, p. 17) estimated the thickness 

at about 1,800 m—a minimum, because the top is not preserved. Brachiopods in the Tomhegan indicate a “Schoharie”

(late Emsian; Boucot and Heath, 1969, p. 20) age. The Tomhegan is the youngest pre-Acadian unit in this part of Maine, 

indicating that the Acadian deformation here was no earlier 

than the late Emsian.

Traveler-Chesuncook Area, Maine

Along the west side of the Katahdin batholith (loc. 

16, fi gs. 1, 4), Silurian sedimentation began with deposition 

of shallow marine conglomerate, limestone, and siltstone 

assigned by Griscom (1976) to the Ripogenus Formation. Various localities within the Ripogenus Formation have yielded 

brachiopods of Llandovery (C3–C5), late Wenlock, and possible Ludlow age (Boucot and Heath, 1969, p. 53). A conodont sample (95MDW130) from Ripogenus Dam (loc. 16, 

fi gs. 1, 4; table 2) yielded Decoriconus sp. indet., Dvorakia

sp. indet., Oulodus sp. indet., Ozarkodina excavata, and Panderodus sp. indet. (pl. 2, fi gs. 1–5); this collection permits 

an age range of late Ludlow to early Lochkovian. The Ripogemicrornatus

breconensis

polygonalis

annulatus

velata

douglastown.

devonicus

bohemica

staurogn.

celloni

rhenana

sagitta

amorphogn.

pireneae

dehiscens

delta

pesavis

sulcatus

kindlei

eurekaensis

siluricus

eostein.

ploeckensis

crispa

varcus

australis

kockelianus

ensensis

gronbergi

inversus /

laticostatus

serotinus

partitus

patulus

costatus

hesp.-wosch.

snajdri

hassi

jamieae

rhenana

linguiformis

disparilis

falsiovalis

transitans

punctata

herm.- crist.

Frasnian Givetian Eifelian

DEVONIAN (part)

Emsian Loch. Prid.Ludlow

SILURIAN (part)

Pragian Wenlock Llandovery (part)

SERIES/

 STAGE

SYSTEM

CONODONT

ZONE

SPORE

ZONE

Unit DSus of

Osberg and

others, 1985

(loc. 42)

Unit Ds of

Osberg and

others, 1985

(loc. 41)

Fish River Lake Fm.

of Boone, 1970

(locs. 22 & 40)

Fish River

Lake Fm.

of Boone, 1970

(loc. 22)

Trout

Valley

Fm.

(loc. 17)

Mapleton

Fm.

(loc. 31)

Unit Dus of

Osberg and

others, 1985

(loc. 25)

95MDw125

95MDw120

EM-856

95MDw116A

95MDw117

95MDw118

MK-62-8

MK-62-9

MK-62-10

CL-186

0-0105776

DR-1

95MDw210

95MDw208B

The Forks Fm.

(loc. 27)95MDw155 93MDw18

Square

Lake Ls.

(loc. 23)

McKenney Ponds

Ls. Member

(loc. 13)

Bear Pond

Ls. Member

(loc. 12)

Parker

Bog Fm.

(loc. 14)

Hardwood

Mtn. Fm.

(loc. 11)

Mountain

House

Wharf Ls.

(loc. 2)

Fish River

Lake Fm.

(loc. 21)

 Ripogenus

Fm.

(loc. 16)

Beck

Pond Ls.

(loc. 12)

95MDw123A

95MDw123C

95MDw129

95MDw211

95MDw212B

95MDw130

95MDw216B

95MDw217A

95MDw217B

95MDw220E

95MDw220A, 220B

95MDw207A

95MDw207C

95MDw207D

Conodont age range

Palynomorph age range
13 Paleontologic and Stratigraphic Evidence for the Date of the Acadian Orogeny

Table 2. Conodont collections from Silurian and Devonian rocks, Maine and Québec.[See plates 1 and 2 for illustrations of key conodonts; see fi gure 5 for the Silurian and Devonian conodont zonation and a graphical display of the age range of each sample. CAI, conodont-alterati
14 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians Table 2. Conodont collections from Silurian and Devonian rocks, Maine and Québec

—Continued.
15 Paleontologic and Stratigraphic Evidence for the Date of the Acadian Orogeny

Table 2. Conodont collections from Silurian and Devonian rocks, Maine and Québec—Continued.
16 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians Table 2. Conodont collections from Silurian and Devonian rocks, Maine and Québec

—Continued.

nus is gradationally overlain by siltstone and andesite (West 

Branch Formation), followed by red shale (Frost Pond Shale); 

these units are barren of fossils but are probably Lochkovian. 

The overlying Seboomook Group, which begins the Acadianderived clastic succession, consists of thin-bedded turbidites 

that have yielded Oriskany-age (Pragian) brachiopods from 

a locality at Chesuncook Lake (R.B. Neumann, in Griscom, 

1976, p. 147–148). The Seboomook Group is overlain progradationally by the Matamagon Sandstone, which has also 

yielded Pragian brachiopods (for example, R.B. Neumann, in 

Griscom, 1976, p. 151–152). North of the Katahdin pluton 

(loc. 17, fi gs. 1, 4), the Seboomook and Matagamon together 

compose a prodelta and delta deposited by west-directed 

paleocurrents (Hall and others, 1976; Pollock and others, 

1988). The Matagamon Sandstone is overlain conformably by 

the Traveler Rhyolite. There appears to have been no hiatus 

between the two units because clastic dikes of Matagamon 

Sandstone intrude upward into the basal rhyolite (Rankin, 

1968). Concordant U-Pb zircon ages of 406 Ma from the 

Black Cat Member and 407 Ma from the Pogy Member 

(Rankin and Tucker, 1995) indicate an early Emsian age for 

the Traveler Rhyolite and, most likely as well, for the uppermost beds of the Matagamon Sandstone. The Silurian through 

upper Emsian strata were involved in the regional Acadian 

folding, which produced a regional northeast-striking cleavage.

The Traveler Rhyolite is overlain by essentially undeformed nonmarine sandstone and conglomerate of the 

Trout Valley Formation. Although an angular unconformity cannot be demonstrated at outcrop, 1:62,500-scale mapping shows that the Trout Valley Formation truncates major 

structures in the Traveler (Rankin, 1968). Kasper and 

others (1988) reviewed the history of age assignments for 

the Trout Valley on the basis of plant fossils. Palynomorphs from the Trout Valley (McGregor, 1992) indicate a 

latest Emsian or earliest Eifelian age, corresponding to the 

douglastownense-eurypterota spore assemblage zone or the 

patulus and partitus conodont zones (table 1). The Trout 

Valley is, therefore, signifi cantly younger than the Traveler 

Rhyolite.

Munsungun Anticlinorium, Maine

The Munsungun anticlinorium is cored by Ordovician 

and older rocks, which are unconformably overlain by a complex assortment of locally derived clastic, volcanic, and carbonate rocks (loc. 18, fi gs. 1, 4). Hall (1970) assigned these 

rocks to the East Branch Group and to the laterally equivalent 

Spider Lake Formation. Corals and brachiopods from several 

fossil occurrences give ages of Ludlow, Pridoli, and “middle 

or upper Helderberg” (Hall, 1970, p. 35). Next follows the 

Seboomook Group, the youngest unit in the area, which has 

yielded brachiopods of Oriskany-age (Pragian) age from three 

places (loc. 18, fi gs. 1, 4; Hall, 1970, p. 41). Thus, Acadian 

deformation must be Emsian or younger along the Munsungun anticlinorium.

A similar succession has been reported from the Millimagassett Lake area, at the south end of the Munsungun anticlinorium (loc. 20, fi gs. 1, 4). Here, the Grand Lake Seboeis 
Paleontologic and Stratigraphic Evidence for the Date of the Acadian Orogeny 17

Group consists of slate, sandstone, conglomerate, calcareous 

slate, and volcanic rocks. Hibbard (1994, p. 216) cited nine 

fossil collections that give a range of “Late Silurian to earliest 

Devonian.” (No details were published). The Grand Lake 

Seboeis Group grades up into, but may interfi nger with, the 

Millimagassett Lake Formation, which has yielded Late Silurian, most likely Pridoli, brachiopods and corals near its base 

(Hibbard, 1993); presumably, its upper part extends at least 

into the Lochkovian, if not the Pragian. According to Hibbard 

(1994), the Millimagassett Lake Formation is a fl ysch succession resembling the Seboomook Group. It appears to be the 

oldest such fl ysch yet identifi ed along the anticlinorial belt, 

possibly 2 to 3 m.y. older than the Seboomook Group at 

Beck Pond (loc. 12, fi gs. 1, 4) and 5 to 10 m.y. older than 

along the Munsungun anticlinorium (loc. 18, fi gs. 1, 4). In the 

Millimagassett Lake area, Hibbard (1994) reported evidence 

for an early Acadian sinistral shear deformation that was followed by the main, regional Acadian folding on northeasttrending axes. Both phases of deformation are Lochkovian or 

younger.

Pennington Anticlinorium, Maine

To determine the timing of Acadian foreland-basin sedimentation and deformation along the northwestern limb of 

the Pennington anticlinorium (locs. 21–23, fi gs. 1, 4), we 

must fi rst unravel some stratigraphic problems. The Silurian 

to Devonian Fish River Lake Formation of Boone (1970) consists of sandstone, siltstone, conglomerate, mafi c and silicic 

volcanic rocks, and impure limestone. Unnamed unit DSus, 

as shown on the bedrock geologic map of Maine (Osberg 

and others, 1985), occurs along strike from the Fish River 

Lake Formation at the same broad stratigraphic level. New 

information reported here suggests that both the Fish River 

Lake and unit DSus are made up of two unrelated successions: an older one that includes volcanic and calcareous 

rocks, and a younger one composed of plant-bearing siltstone, 

sandstone, and conglomerate.

Carbonate rocks from the Fish River Lake Formation 

have yielded brachiopods of Late Silurian (“Cobleskill,” Pridoli) and Early Devonian (“New Scotland,” Lochkovian) age 

(A.J. Boucot, in Boone, 1970, p. 33–34). We processed a 

shaly limestone from one of Boucot’s Cobleskill-age locations 

(loc. 21, fi gs. 1, 4) for conodonts, but the sample yielded only 

nondiagnostic Silurian-Devonian forms (table 2). Within unit 

DSus, the informally named Square Lake Limestone occurs in 

a single lakeshore exposure (loc. 23, fi gs. 1, 4). A.J. Boucot 

(in Boone, 1970) placed the brachiopod and coral fauna “defi -

nitely within the lower Helderberg series” (that is, woschmidti

Zone to at least eurekaensis Zone of the Lochkovian). Conodonts from the most diagnostic of two samples of reef limestone give an age range of late Ludlow to early Lochkovian 

(pl. 2, fi gs. 22–32, 36–38; loc. 23, fi gs. 1, 4; table 2).

Plant-bearing sandstone and siltstone from the Fish River 

Lake Formation of Boone (1970) and unit DSus have yielded 

spores, some of which are considerably younger than the brachchiopods (table 1; McGregor, 1962, 1963, 1968, 1996) This 

conclusion is based on material submitted (without lithologic 

descriptions) to D.C. McGregor by Eli Mencher in the 1960's 

and by D.C. Bradley in 1995. In unit DSus, the most closely 

dated of two siltstone samples is clearly Early Devonian, probably Lochkovian or early Pragian (table 1). Bedding and a 

strong cleavage (presumably Acadian) in these rocks both are 

subvertical. In the Fish River Lake Formation, two samples 

of dark, thin-bedded siltstone are of late Pragian-early Emsian 

age (polygonalis-emsiensis Zone). These rocks, which resemble 

the Seboomook Group, are also folded and cleaved. Three 

sandstone samples from 1995 and two samples of unknown 

lithology from the 1960’s are of late Emsian or early Eifelian 

age (douglastownense-eurypterota Zone). The 1995 samples of 

this age are from a succession of fl uvial or marginal-marine 

plant-bearing sandstone and conglomerate at Nadeau Thoroughfare (loc. 22, fi gs. 1, 4), which dip moderately (50°) but 

lack cleavage. Finally, one sample of unknown lithology submitted to McGregor by Mencher is early or middle Eifelian 

(velata-langii Zone).

Piecing together these various facts, we suggest that farforeland carbonate sedimentation was underway during the 

Late Silurian and at least part of the Lochkovian. An infl ux 

of orogenically derived siliciclastic materials began during late 

Lochkovian or early Pragian time. Deposition of the plant-bearing sandstone probably lasted from about late Emsian into 

the early or middle Eifelian. Taken together, the palyniferous 

strata can be interpreted as components of a prograding foreland-basin sequence, akin to the somewhat older SeboomookMatagamon succession farther south.

Madawaska Area, New Brunswick

Two useful constraints on the date of the Acadian orogeny 

are available from northwesternmost New Brunswick, where 

the Aroostook-Matapedia and Connecticut Valley-Gaspé Basins 

come together (loc. 24, fi gs. 1, 4). The Upper Ordovician 

and Silurian are represented by various deep-water facies (St. 

Peter, 1977). The youngest Silurian rocks are mostly green slate 

and calcareous siltstone of the Perham Formation, which has 

yielded graptolites as young as middle to late Ludlow (St. 

Peter, 1977, p. 30). There follows a gap in the fossil record 

spanning the Pridoli, Lochkovian, and Pragian. The overlying 

Temiscouata Formation consists of several kilometers of siliciclastic turbidites characterized by abundant slump folds; Hesse 

and Dalton (1995) interpreted the Temiscouata as a submarine 

foreland-basin succession. It has yielded brachiopods now 

assigned an Emsian age (St. Peter and Boucot, 1981, p. 91). 

The Temiscouata Formation has been penetratively deformed; 

Acadian deformation must be Emsian or younger in age.

LaPatrie and Ste.-Cécile de Whitton, Eastern 

Townships, Québec

In the Eastern Townships of Québec, the axial region of 

the Connecticut Valley-Gaspé Basin is occupied by a thick 

succession of Acadian-deformed sandstone and slate assigned 

to the Compton Formation (Slivitski and St.-Julien, 1987). 

Sandstone from the Compton Formation has yielded Emsian 
18 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

plant fossils (from locs. 3, 4, fi gs. 1, 4; Hueber and others, 

1990). Although the age of onset of foreland-basin sedimentation in this area is unknown, the presence of Emsian forelandbasin deposits suggests that the orogen was not far off at 

the time. Acadian deformation in the Eastern Townships must 

have taken place between 409–394 Ma (possible age range 

of the Compton Formation) and the 384±2-Ma Scotstown 

pluton, which is the oldest postfolding pluton that intrudes the 

Compton Formation (Simonetti and Doig, 1990).

Northwestern Margin of the Connecticut 

Valley-Gaspé Basin, Québec

Three localities along the northwestern margin of the 

Connecticut Valley-Gaspé Basin in Québec provide important 

constraints on the age of Acadian foreland-basin sedimentation and deformation. The most informative strata are at 

St.-Georges (loc. 5, fi gs. 1, 4), where the Eifelian Famine Formation unconformably overlies the Ordovician Magog Group 

(Uyeno and Lespérance, 1997). The Famine begins with a 

basal conglomerate, overlain by limestone, which is coral 

bearing at the bottom but becomes progressively shalier 

upsection. The maximum preserved thickness is 215 m. 

Uyeno and Lesperance (1997) documented a conodont fauna 

of costatus Zone age, a refi nement on the previously quoted 

Eifelian age (Oliver, 1971), which was based on corals and 

brachiopods. The Famine Formation is the youngest unit at 

St.-Georges; it was involved in regional Acadian deformation, 

which thus must postdate the costatus Zone.

In the Lac Temiscouata area (loc. 7, fi gs. 1, 4), the situation is similar to that at St.-Georges. The key unit, comparable 

to the Famine, is the Touladi Limestone, which rests with 

slight angular unconformity on various older units ranging in 

age from possibly Ordovician to Late Silurian (Uyeno and 

Lespérance, 1997). The Touladi consists of mostly bioclastic 

limestone, with a maximum preserved thickness of 105 m. 

Recent conodont studies by Uyeno and Lespérance (1997) 

indicate a position in either the australis or kockelianus Zone 

of the Eifelian, an assignment that refi nes but does not signifi -

cantly alter the Eifelian age quoted by Boucot and Drapeau 

(1968).

Devonian limestone at Lac Memphremagog (loc. 2, fi g. 

1) suggests a similar interpretation, although biostratigraphic 

control is not so tight. There, the Mountain House Wharf 

Limestone is a dark, slaty limestone surrounded by, and probably faulted against, Ordovician rocks (Boucot and Drapeau, 

1968). It has yielded a brachiopod and coral fauna of Eifelian 

age (Boucot and Drapeau, 1968, p. 9). Three of four large 

conodont samples yielded only poorly preserved forms that 

could be assigned only a middle Emsian to Givetian age (table 

2).

The coral-bearing shallow-water limestone at St.-

Georges, Lac Temiscouata, and Lac Memphremagog was 

probably deposited in a far-foreland setting. The upwarddeepening trend at St.-Georges most likely records the beginnings of foreland-basin subsidence. Reckoning that an interval 

of foreland-basin sedimentation would have preceded deformation, the Acadian orogeny most likely took place at St.-

Georges during the Givetian or, conceivably, the late Eifelian. 

Deformation at Lac Temiscouata probably took place a few 

million years later than at St.-Georges.

Montreal, Québec

Devonian strata do not crop out anywhere north of the 

Mohawk River in central New York but were once present 

in Montreal, 330 km to the north (loc. 1, fi gs. 1, 4). The 

Ste. Hélène Island breccia (Boucot and others, 1986) is a 

Cretaceous diatreme that was emplaced into a region of gentle 

foreland deformation about 20 km cratonward of the Paleozoic thrust front. The diatreme contains an assortment of 

xenoliths that reveal the former existence of a Devonian succession similar to that in eastern and central New York State. 

This diatreme, in the words of J.B. Thompson, is a “drill 

hole in the sky.” The inferred sequence is Lochkovian (Helderbergian) limestone, Pragian (Oriskany-age) sandstone and 

limestone, Emsian (Bois Blanc or Schoharie age) limestone, 

Eifelian (early and late Onondagan) limestone, and Givetian 

(Hamilton-age) micritic limestone, siltstone, and sandstone. 

The latter siltstone and sandstone resemble the Hamilton, 

which in the Catskills of eastern New York State constitutes 

the base of the main foreland-basin sequence. By implication, 

the leading edge of the foreland basin was near Montreal 

during Givetian time.

Isotopic Ages of Acadian Plutons

When the present study was fi rst conceived, only a few 

high-precision U-Pb and 40Ar/39Ar ages were available for 

Silurian and Devonian plutons in the study area (fi g. 1)—not 

enough to reveal a clear regional age pattern for the plutons. 

Accordingly, we set out to date all the signifi cant Acadian 

plutons in a swath from the Maine coast to the MaineQuébec border. New U-Pb and (or) 40Ar/39Ar ages are 

reported here for 17 plutons. Other new ages from the geochronology laboratory of Washington University, St. Louis, 

were reported by Tucker and others (in press) for plutons 

near and on the Maine coast, by Rankin and Tucker (1995) 

for the Katahdin area, by Solar and others (1998) for western 

interior Maine, and by Eusden and others (in press) for 

northern New Hampshire. Figure 8 and table 3, which summarize the best available emplacement ages, clearly reveal 

plutonic belts of about 420 Ma near the coast and 407–406 

Ma farther inland, which are essential in positioning the 

deformation front. Other plutons are signifi cantly younger 

than about 406 Ma, some of which also occur in discrete 

belts, but they are not so useful in tracking the Acadian 

front.

Acadian plutons fall into three broad and somewhat overlapping categories, pretectonic, syntectonic, and posttectonic, 

with respect to the local fi rst phase (D1) of Acadian deformation. A pluton might be classifi ed as posttectonic, even though 

the Acadian orogeny was still underway elsewhere during the 

pluton’s emplacement. We considered three types of anecdotal 

evidence in assessing the age of plutonism relative to Acadian 
19

deformation: (1) map-scale relations to Acadian folds and 

faults, (2) plutonic-rock fabrics, and (3) metamorphic textures 

in the contact aureole.

U-Pb Analytical Methods

U-Pb analyses (fi g. 7; table 4) were performed at the 

geochronology laboratory of Washington University, St. Louis. 

Zircon was extracted from 5- to 10-kg samples, using standard 

techniques of density and magnetic separation. Zircon grains 

were selected for analysis on the basis of size, color, clarity, 

and morphology. Sample sizes varied, depending on U content, 

grain size, and age, but most analyses were performed on 

fractions of 5 to 20 crystals, generally with less than 200 

ng of radiogenic Pb. All analyses were air abraded (Krogh, 

1982) and cleaned sequentially in warm 4N HNO3, water, and 

distilled acetone to remove components carrying common Pb. 

Washed and weighed zircon fractions were then loaded in 

Tefl on bombs, spiked with a mixed 205Pb-235U tracer solution, 

and digested in 48 percent HF and 7N HNO3 for 72 hours 

at 210°C. After digestion and conversion to chloride form, 

Pb and U were purifi ed by using ion-exchange techniques, 

as described in Krogh (1973). Total-procedure blanks for Pb 

and U, measured during the period of analysis, were 1–8 and 

less than 1 pg, respectively; total common-Pb abundances are 

reported for each analysis in table 4. Initial-Pb compositions 

result in insignifi cant changes to the calculated ages.

Isotopic-ratio measurements of Pb and U were made in 

a VG Sector 54 thermal-ionization mass spectrometer with 

enhanced pumping capacity, seven movable collectors, and 

a Daly-type detector with ion-counting capability. Lead and 

uranium were loaded together on outgassed single-Re fi laments with silica gel and phosphoric acid, and all measurements were made by the method of peak hopping 

in ion-counting mode. Ion-beam intensities ranged from 

0.5×10−13 to 1.5×10−13 A for 206Pb+

 and from 0.5×10−13 to 

2.5×10−13 A for 235U (measured as UO2

+

). Daly bias and 

nonlinearity were periodically monitored with U.S. National 

Institute of Standards & Technology and Central Bureau for 

Nuclear Measurements isotopic reference materials, and correction factors and errors for Daly gain were used in data 

reduction.

Errors for the 238U/206Pb, 235U/207Pb, and 207Pb/206Pb ages 

were estimated by using the method of Ludwig (1980); all age 

uncertainties are quoted at the 95-percent-confi dence level. 

Cited ages are the mean 207Pb/206Pb age of concordant or 

slightly discordant analyses weighted according to the inverse 

variance of each analysis (Ludwig, 1992); the quoted age 

error is the standard error of the average value calculated by 

using the assigned error for each analysis. The reliability of all 

cited ages may be evaluated by the mean square of weighted 

deviates (MSWD), which is a measure of the observed scatter 

to that predicted by the assigned errors to each analysis. For 

all samples, the MSWD is much less than 1, indicating that 

assigned errors may be overestimated.

40Ar/39Ar Analytical Methods

40Ar/39Ar analyses (fi g. 8; table 5) were performed at the 

geochronology laboratory at the University of Maine, Orono. 

Samples were separated by using standard magnetic and density separation techniques. The purity of the samples was 

estimated to be greater than 99.5 percent. Samples, fl ux monitors (interlaboratory standard SBG–7), and K and Ca salts 

were encapsulated in Al foil and sealed in silica glass vials. 

These vials were irradiated in the L67 position of the Ford 

Nuclear Reactor at the University of Michigan. Micas and 

fl ux monitors weighed approximately 35 mg. Samples were 

heated in a molybdenum crucible within the ultra-high-vacuum system on line to the mass spectrometer using radio-frequency induction. Temperatures have an estimated uncertainty 

of ±50°C. Inert gases were purifi ed by using standard gettering 

techniques. The Ar-isotopic composition was measured digitally, using a Nuclide 6–60–SGA 1.25 mass spectrometer. All 

data were corrected for mass discrimination and interfering Ar 

isotopes produced during irradiation (Dalrymple and others, 

1981). The decay constants recommended by Steiger and Jäger 

(1977) were used to calculate the ages. Error calculations, 

which included both the uncertainty in the analytical measurement and the uncertainty in the J value, are reported at the 2σ

level. Each sample was analyzed in a minimum of 8 and no 

more than 15 increments. A plateau age represents the mean of 

ages in consecutive increments that do not differ on the basis 

of 2σ analytical uncertainties.

Ludlow, Pridoli, and Lochkovian Plutons

Until a few years ago, the numerous gabbroic to granitic 

plutons along the Maine coast were all assigned to the Devonian, but in light of modern 40Ar/39Ar and U-Pb dating and the 

new time scale, the oldest group of plutons now is known to 

range from Ludlow to Lochkovian. The fi rst pluton in Maine 

to yield an undoubted Silurian age was the Pocomoonshine 

gabbrodiorite (PO, fi g. 6), which yielded an 40Ar/39Ar amphibole plateau age of 422.7±3 Ma (West and others, 1992). The 

pluton is posttectonic: it intruded already-deformed turbidites 

of the presumably Lower Silurian Flume Ridge Formation 

and “stitches” the fault contact between the Fredericton Basin 

and the St. Croix belt to the south (West and others, 1992). 

New zircon ages from the Utopia (423.0±1.0 Ma; UT, fi g. 

6) and Welsford (422.0±1.0 Ma; WL, fi g. 6) plutons (M.L. 

Bevier, written commun., 1996) extend the belt of known 

Ludlow plutons into southern New Brunswick. Several plutons in the Penobscot Bay area have also yielded concordant 

zircon ages of 424 to 417 Ma—that is, Late Silurian to earliest 

Devonian—including the Somesville (424.0±2.0 Ma; SV, fi g. 

6), Blinn Hill (424.0±2.0 Ma; BH, fi g. 6), Lake St. George 

(422.0±2.0 Ma; LS, fi g. 6), North Union (422.0±2.0 Ma; NU, 

fi g. 6), Spruce Head (421.0±1.0 Ma; SH, fi g. 6), Youngtown 

(420.0±2.0 Ma; YT, fi g. 6), Sedgwick (419.5±1.0 Ma; SE, 

fi g. 6), South Penobscot (419.2±2.2 Ma; SP, fi g. 6), Cadillac 

(419.0±2.0 Ma; CD, fi g. 6), Northport (419.0±2.0 Ma; NP, fi g. 

6), and Lincoln (417.7±1.0 Ma; LI, fi g. 6) (table 3; Seaman 

Isotopic Ages of Acadian Plutons
Figure 6.—Maine and adjacent parts of New Brunswick, Québec, New Hampshire, and Vermont, showing distribution of Late Silurian, 

Devonian, and earliest Carboniferous plutons. Boxes show best estimate of intrusive age (in Ma; concordant U-Pb or 40Ar/39Ar plateau) 

and two-letter abbreviation, keyed to table 3.

20 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

and others, 1995; Stewart and others, 1995b; Tucker and 

others, in press) plutons. The North Union and Lake St. 

George plutons are composed of granitic gneiss inferred to 

have been emplaced syntectonically (Tucker and others, in 

press). West and others (1995) reported widespread Late Silurian to earliest Devonian 40Ar/39Ar ages from metamorphic 

hornblende outboard of the Sennebec Pond Fault, essentially 

coeval with the plutons listed above.

Early Emsian Plutons

Previous Results

Nearly two dozen closely dated plutons of Emsian age 

defi ne a northeast-southwest belt that can be traced across 

the entire strike length of fi gure 6 and beyond, into Massachusetts. Previously, there was some indication of plutonic 

activity at about 400 to 410 Ma along this trend, but little 
Isotopic Ages of Acadian Plutons 21

Figure 7.—U-Pb concordia diagrams of intrusive rocks dated in this report. 206Pb/238

U

207Pb/235U

206Pb/238

U

206Pb/238

U

206Pb/238

U

206Pb/238

U

206Pb/238

U

206Pb/238

U

206Pb/238

U

206Pb/238

U

206Pb/238

U

206Pb/238

U

207Pb/ 207Pb/235U 235U

207Pb/235U 207Pb/ 207Pb/235U 235U

207Pb/235U 207Pb/235U 207Pb/235U

207Pb/235U 207Pb/235U

EMSIAN INTRUSIONS

FRASNIAN AND FAMENNIAN INTRUSIONS

Harrington

Onawa

Granodiorite

Ebeemee

Mattamiscontis Russell Mtn.

Bald Mtn.

Sebec Lake Seboeis

Granodiorite Shirley

404.9±4.4 Ma 

Blanchard

407.2±1.5 Ma 

Hammond

Ridge Sill

Priestly

360.5±3.3 Ma

Hog Island

371±3Ma

Center Pond

 377±3Ma

406.9±1Ma

407.8±2.5Ma

405.1±2.9Ma

405.7±2.6Ma

405.8±2.5Ma

406.9±3.6Ma 406.0±1.3Ma

407.9±2.6Ma

377.2±1.5Ma

10 11

MSWD=0.065

405

410

18, 19, 20

10

11

35

28

27

26

23

24 25

32

30, 31

4—Blanchard

5—Blanchard

41—Shirley

34

36

37

6—Blanchard

40—Shirley

12

13

15 16

14

17

7—Center Pd.

22—Hog Island

29—Priestly

8—Center Pd.

42

2

404

410

406

408

396

398

402

404

406

408

400

406

410

402

414

402

404

406

408

400

408

402

404

406

400

406

408

402

404

404

406

408

398

400

402

408

404

406

410

395

400

405

415

410

372

378

374

376

360 370

365

380

375

370

.0635

.0640

.0645

.0650

.0655

.0660

.480 .485 .490 .495 .500 .475 .480 .485 .490 .495

.0630

.0635

.0640

.0645

.0650

.0655

.0630

.0635

.0640

.0645

.0650

.0655

.475 .480 .485 .490 .495 .486 .490 .494 .498 .480 .484 .488 .492 .496

.0645

.0650

.0655

.0660

.0635

.0640

.0645

.0650

.0655

.0635

.0640

.0645

.0650

.0655

.480 .484 .488 .492 .496 .484 .486 .488 .490 .492 .494 .496

.0638

.0642

.0646

.0650

.0654

.438 .442 .446 .450

.0585

.0590

.0595

.0600

.0605

.42 .43 .44 .45 .46

.056

.057

.058

.059

.060

.061

.470 .475 .480 .485 .490 .495 .500 .505

.063

.064

.065

.066

.062

.486 .488 .492 .494 .496 .498 .490 .500

.0654

.0658

.0650

.0646

.0642
22 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Figure 8.—40Ar/39Ar age spectra of intrusive rocks dated in this report. Shaded rectangles correspond to data used to calculate plateau ages.

evidence to suggest that most plutons along the belt would 

turn out to fall within the same narrow age range. In western 

New Brunswick, Bevier and Whalen (1990a) reported U-Pb 

ages of 409±2 Ma for the Skiff Lake phase of the Pokiok 

batholith and 402±1 Ma for the Allandale phase (SK and 

AL, respectively, fi g. 6). In eastern Maine, Hubacher and Lux 

(1987) obtained 40Ar/39Ar ages, which are now known to be 

Emsian, from three plutons that truncate regional Acadian 

structures: the Pleasant Lake (400.5±4.5 Ma; PL, fi g. 6), 

Hunt Ridge (401.5±4 Ma; HU, fi g. 6), and Cochrane Lake 

(404.3±3.7 Ma; CL, fi g. 6) plutons. In western Maine, Solar 

and others (1998) recently reported Emsian U-Pb ages from 

the South Roxbury granite (408.2±2.5 Ma; SR, fi g. 6), 

Swift River granite (407.9±1.9 Ma; SW, fi g. 6), Redington 

pluton (407.6±4.6 Ma; RD, fi g. 6), North Roxbury granite 

(404.3±1.9 Ma; NR, fi g. 6), Lexington pluton (404.3±1.8 

Ma; LX, fi g. 6), and various phases of the Phillips pluton 

(405.3±1.8, 403.6±2.2, and 403.5±1.6 Ma; PH, fi g. 6). In the 

Presidential Range of northern New Hampshire, the Wamsutta 

pluton (WM, fi g. 6), which was emplaced during F1 Acadian 

folding, has yielded a U-Pb zircon age of 408.4±1.9 Ma 

(Eusden and others, in press). In New Hampshire, outside the 

area of fi gure 6, Emsian U-Pb ages have been obtained for 

the Spaulding pluton (408.4±1.9 Ma on zircon; Robinson and 

Tucker, 1996), the Ashuelot pluton (403±3 Ma on monazite; 

Robinson and Tucker, 1996), and the Cardigan pluton (404±2 

Ma on zircon; R.D. Tucker, unpub. data). Finally, the Prescott 

gabbro, which postdates the nappe phase of regional Acadian 

Priestly

Moxie Central

Sebec Lake

Sebec Lake

Moxie East

Mattamiscontis

Priestly

Shirley Seboeis Granodiorite

Seboeis Granite Seboeis Diorite

Onawa gabbro

Russell Mtn. Russell Mtn.

Ebeemee

Chandler Deboullie granodiorite Deboullie monzonite

Hog Island

Bald Mtn. Bald Mtn.

Beaver Cove

Deboullie monzonite

Center Pond

95MDw169 biotite

Total gas age = 399.0 ± 4.1 Ma

Plateau age = 407.8 ± 2.4 Ma

95MDw187

biotite

Total gas age = 406.4 ± 3.9 Ma

Plateau age = 406.3 ± 3.8 Ma

95MDw164B biotite

 Total gas age = 397.3 ± 3.8 Ma

Plateau age = 402.1 ± 2.8 Ma

95MDw158

biotite

Total gas age = 366.2 ± 3.6 Ma

Plateau age = 368.7 ± 2.9 Ma

95MDw106

biotite

Total gas age = 366.5 ± 3.7 Ma

No plateau

95MDw114 biotite

Total gas age = 363.0 ± 3.5 Ma

Plateau age = 363.2 ± 3.0 Ma

95MDw115

biotite

Total gas age = 363.9 ± 4.6 Ma

Plateau age = 364.2 ± 3.2 Ma

95MDw112 biotite

Total gas age = 365.3 ± 3.5 Ma

Plateau age = 365.4 ± 3.2 Ma

95MDw107 biotite

Total gas age = 359.3 ± 3.4 Ma

Plateau age = 360.3 ± 2.7 Ma

95MDw108 biotite

Total gas age = 362.3 ± 4.0 Ma

Plateau age = 363.2 ± 2.3 Ma

95MDw198 biotite

Total gas age = 403.6 ± 3.9 Ma

Plateau age = 404.4 ± 3.4 Ma

95MDw71

muscovite

Total gas age = 374.4 ± 4.1 Ma

Plateau age = 377.2±4.4 Ma

95MDw205 biotite

Total gas age = 366.9±6.7 Ma

Plateau age = 368.6±3.1 Ma

95MDw143

biotite

Total gas age = 392.4 ± 3.9 Ma

Plateau age = 394.3 ± 3.7 Ma

95MDw146

biotite

Total gas age = 388.3 ± 3.7 Ma

Plateau age = 390.9 ± 3.0 Ma

95MDw68B biotite

Total gas age = 396.2 ± 3.8 Ma

Plateau age = 400.1 ± 3.7 Ma

95MDw80

biotite

Total gas age = 389.7 ± 3.6 Ma

Plateau age = 393.4 ± 3.0 Ma

95MDw98

muscovite

Total gas age = 393.4 ± 3.9 Ma

Plateau age = 393.6 ± 2.7 Ma

95MDw72

biotite Total gas age = 362.0 ± 5.1 Ma

Plateau age = 392.5±2.5 Ma

95MDw195 biotite

Total gas age = 368.8 ± 3.6 Ma

Plateau age = 372.4 ± 2.7 Ma

95MDw90B biotite

Total gas age = 389.5 ± 3.7 Ma

Plateau age = 390.3 ± 3.0 Ma

95MDw139B

biotite

Total gas age = 363.4 ± 3.9 Ma

Plateau age = 364.7 ± 4.3 Ma

95MDw135B

muscovite

Total gas age = 400.2 ± 6.7 Ma

Plateau age = 401.3 ± 2.8 Ma

95MDw135A biotite

Total gas age = 396.3±3.8 Ma

No plateau

Apparent Age (Ma) 300

400

0 100

Apparent Age (Ma) 300

400

0 100

Apparent Age (Ma) 300

400

0 100

Apparent Age (Ma) 300

400

0 100

Apparent Age (Ma) 300

400

0 100

Apparent Age (Ma) 300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

0 100

300

400

100 Cumulative percent 39Ar released 0

Cumulative percent 39Ar released

Cumulative percent 39Ar released

Cumulative percent 39Ar released

Cumulative percent 39Ar released

Cumulative percent 39Ar released

Cumulative percent 39Ar released

Cumulative percent 39Ar released

Cumulative percent 39Ar released Cumulative percent 39Ar released Cumulative percent 39Ar released

Cumulative percent 39Ar released Cumulative percent 39Ar released Cumulative percent 39Ar released

Cumulative percent 39Ar released Cumulative percent 39Ar released Cumulative percent 39Ar released

Cumulative percent 39Ar released Cumulative percent 39Ar released Cumulative percent 39Ar released Cumulative percent 39Ar released

Cumulative percent 39Ar released

Cumulative percent 39Ar released

Cumulative percent 39Ar released
Isotopic Ages of Acadian Plutons 23

deformation in Massachusetts, has a U-Pb age of 407±3/2 Ma 

(Tucker and Robinson, 1990). Far outboard of the main belt of 

Emsian magmatism are two outliers: the Haskell Hill pluton 

(U-Pb age, 408±5 Ma; HH, fi g. 6; Tucker and others, in press) 

and the Berry Brook gabbro-diorite (40Ar/39Ar age, 410 Ma; 

BB, fi g. 6; Ludman and Idleman, 1998).

Russell Mountain Pluton

The Russell Mountain pluton (RU, fi g. 6) is a unfoliated 

two-mica granodiorite. Mapping by Ludman (1978) showed 

that the pluton truncates regional folds in the Madrid and Carrabassett Formations, and so, on this basis, it must postdate at 

least some Acadian deformation. Contact-metamorphic rocks 

on the northwestern margin, however, have a schistose foliation and aligned chiastolite with biotite strain shadows (fi g. 

9A). Because metamorphic biotite is incipient or absent outside the aureole (Ludman, 1978), some deformation must 

have taken place while the pluton was being emplaced. 

Whether deformation in the contact aureole was caused by 

pluton emplacement or was a regional event is unknown. Two 

concordant zircon fractions yield a mean 207Pb/206Pb age of 

406.0±1.3 Ma (fi g. 7). Two biotite fractions yielded 40Ar/39Ar 

plateau ages of 393.6±2.7 and 393.4±3.0 Ma (fi g. 8).

Sebec Lake Pluton

The Sebec Lake pluton (SL, fi g. 6) is a unfoliated biotite 

granodiorite that, like the Russell Mountain pluton, truncates 

regional folds involving the Madrid and Carrabassett Formations (Griffi n, 1971). On the basis of its map-scale relations, 

it must postdate at least some Acadian deformation. Contact-metamorphic textures, however, are synkinematic. Metamorphosed Carrabassett Formation in the aureole contains 

large andalusite porphyroblasts with asymmetric strain shadows made up of biotite and chlorite aligned in the foliation 

(fi g. 9B). Outside the contact aureole, the rocks are at subbiotite grade; thus, the biotite and andalusite are both related 

to plutonism. Whether deformation in the contact aureole was 

caused by pluton emplacement or was a regional event is 

unknown. We obtained two concordant zircon fractions that 

yielded a mean 207Pb/206Pb age of 407.8±2.5 Ma (fi g. 7) and 40Ar/39Ar plateau ages of 392.5±2.5 Ma from biotite and 

377.2±4.4 Ma from muscovite (fi g. 8).

Bald Mountain Pluton

The Bald Mountain pluton (BM, fi g. 6) is a unfoliated 

two-mica granodiorite; in map view, it has the shape of an 

ellipse with its long axis normal to regional strike. Mapping 

by Espenshade and Boudette (1967) and Ludman (1978) 

showed that the pluton truncates regional folds in the Madrid 

and Carrabassett Formations and on this basis would appear 

to postdate at least some Acadian deformation. Unoriented 

chiastolite and biotite porphyroblasts in steeply dipping contact-metamorphosed strata also suggest posttectonic emplacement (fi g. 9C). A single concordant zircon fraction indicates an 

emplacement age near 408 Ma (table 4). 40Ar/39Ar ages were 

obtained from three biotite separates, two of which yielded 

plateaus of 390.9±3.0 and 394.3±3.7 Ma, giving a mean age 

of 392.6±2 Ma for cooling through the closure temperature of 

argon in biotite (fi g. 8).

Onawa Pluton

The Onawa pluton (ON, fi g. 6) is a zoned body with a 

granodiorite core and a gabbroic rim. It truncates and defl ects 

the regional structural grain defi ned by steeply dipping bedding in the Carrabassett Formation. Thin-section observations 

reported by Van Heteren and Kusky (1994) suggest that 

andalusite in the contact aureole overgrew a preexisting cleavage; thus, the pluton is posttectonic, at least in part. We 

obtained three concordant zircon fractions that yielded a mean 

207Pb/206Pb age of 405.1±2.9 Ma for the granodiorite (fi g. 9), 

and an 40Ar/39Ar plateau age of 400.1±3.7 Ma on biotite from 

the gabbro (fi g. 8).

Shirley-Blanchard Pluton

The Shirley-Blanchard pluton (SB, fi g. 6) is a composite 

pluton that includes a more mafi c phase of pyroxene-hornblende granodiorite (Blanchard body) and a more felsic phase 

of granodiorite (Shirley body; Espenshade and Boudette, 

1967). The pluton truncates regional-scale tight to isoclinal 

folds involving the Madrid and Carrabassett Formations. From 

the Shirley body, we obtained two concordant zircon analyses 

(fi g. 7) with a mean 207Pb/206Pb age of 404.9±4.4 Ma and 

an 40Ar/39Ar plateau age of 390.3±3.0 Ma on biotite (fi g. 

10). From the Blanchard body, we obtained two concordant 

zircon fractions (fi g. 7) that yielded a mean 207Pb/206Pb age of 

407.2±1.5 Ma. The weighted mean 207Pb/206Pb age of all zircon 

analyses from the Shirley-Blanchard body is 406.9±1.4 Ma 

(fi g. 7), our preferred age for this composite pluton.

Mattamiscontis Pluton

The Mattamiscontis pluton (MA, fi g. 6) is an unfoliated 

biotite granite; fi eldwork for this study was limited to geochronologic sampling of the pluton and brief examination of 

outcrops in the contact aureole. Reconnaissance mapping compiled by Osberg and others (1985) showed the pluton cutting 

a syncline cored by Carrabassett Formation; similarly, at one 

outcrop in the contact aureole, tightly folded metapelite displays a weak axial-planar cleavage that is overgrown by cordierite. A thin section from another outcrop, however, shows 

oriented cordierite surrounded by biotite strain shadows. (The 

regional metamorphic grade is chlorite.) These observations 

together suggest that the pluton postdates Acadian folding, 

but that some deformation—either regional or pluton related—

accompanied pluton emplacement. A granite sample yielded 

three concordant zircon analyses with a mean 207Pb/206Pb age 

of 406.9±3.6 Ma (fi g. 7) and an 40Ar/39Ar biotite plateau age 

of 402.1±2.8 Ma (fi g. 8).
24 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Table 3. Emplacement ages of plutons mapped in fi gure 6.

[State or province: ME, Maine; NB, New Brunswick; NH, New Hampshire; QU, Québec; ]
Isotopic Ages of Acadian Plutons 25

Table 3. Emplacement ages of plutons mapped in fi gure 6—Continued.
26 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Table 4. U-Pb isotope-dilution analyses of zircon from plutonic rocks, Maine.

[Brackets, fi eld No. (U.S. Geological Survey colln. No.)]
Isotopic Ages of Acadian Plutons 27

Table 4. U-Pb isotope-dilution analyses of zircon from plutonic rocks, Maine—Continued.

Ebeemee Pluton

The Ebeemee pluton (EB, fi g. 6) is a small, unfoliated 

stock of biotite granite; fi eldwork for this study was limited 

to sampling in the pluton and a fruitless search for informative outcrops in the contact aureole. Reconnaissance mapping 

compiled by Osberg and others (1985) showed the pluton cutting a doubly plunging anticline cored by the Madrid Formation and fl anked by the Carrabassett Formation; on this 

basis, the pluton would appear to postdate Acadian folding. 

Two concordant zircon analyses yielded a 207Pb/206Pb age of 

405.7±2.6 Ma, and a biotite separate yielded an 40Ar/39Ar 

plateau age of 407.8±2.4 Ma (fi g. 10).

Seboeis Pluton

The Seboeis pluton (SS, fi g. 6) is shown on the bedrock 

geologic map of Maine (Osberg and others, 1985) as a large 

(5 by 30 km) body that cuts the Madrid and Smalls Falls 

Formations and intervening faults. Because the pluton has not 

been mapped in detail and was sampled only in reconnaissance fashion, the following observations are necessarily preliminary. We dated four phases. Weakly foliated granodiorite 

at Schoodic Lake yielded two concordant zircon fractions that 

give a mean 207Pb/206Pb age of 405.8±2.5 Ma (fi g. 7). Near 

Dudley Rips, an unfoliated granite that yielded an 40Ar/39Ar 

muscovite plateau age of 401.3±2.8 Ma (fi g. 8) intrudes a tec-
Figure 9.—Thin-section photomicrographs of contact-metamorphosed rocks adjacent to Acadian plutons. All sections are cut normal to foliation and 

are viewed in plane-polarized light. Field of view in each photomicrograph is about 4 mm across. A, Syntectonic metamorphic textures in aureole of the 

Russell Mountain pluton. Large, randomly oriented chiastolite phenocrysts are fl anked by strain shadows containing aligned fl akes of biotite, implying 

some deformation during contact metamorphism. B, Syntectonic metamorphic textures in aureole of the Sebec Lake pluton. Chiastolite phenocrysts 

are fl anked by strain shadows containing aligned chlorite and biotite, implying some deformation during contact metamorphism. C, Static metamorphic 

textures in aureole of the Bald Mountain pluton, showing randomly oriented, retrograded chiastolite phenocrysts and randomly oriented biotite. Rock 

shows no vestige of pervasive regional foliation found outside contact aureole and so may have been metamorphosed before regional deformation. 

Alternatively, contact metamorphism might have been intense enough to have obliterated all preexisting fabrics. D, Syntectonic metamorphic textures 

in aureole of the Hog Island pluton. Large cordierite phenocrysts overgrow a preexisting foliation defi ned by aligned biotite and white mica; this fi rst 

foliation now survives only within cordierite grains. A second, postcordierite foliation is defi ned by biotite in pressure shadows. Although metamorphic 

textures in this rock are broadly syntectonic, on the basis of regional relations, the fi rst Acadian deformation probably preceded emplacement of the Hog 

Island pluton by 15 to 20 m.y. E, Posttectonic metamorphic textures in aureole of the Priestly pluton. Cordierite phenocrysts overgrow a weak foliation 

that lies at a high angle to bedding; biotite in groundmass is randomly oriented. 

28 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians
Isotopic Ages of Acadian Plutons 29

tonically foliated diorite that yielded a slightly hump shaped 

40Ar/39Ar age spectrum with a total-gas age of 396.3±3.8 

Ma (fi g. 8). Evidently, the diorite was intruded before 401 

Ma and before at least some of the regional deformation. A 

much younger biotite age from rocks that had been previously 

mapped as part of the Seboeis pluton at East Branch Lake is 

discussed below in the subsection entitled “Givetian to Early 

Tournaisian Plutons.”

Moxie Pluton

The northwestern (inboard) part of the Emsian plutonic 

belt is dominated by two large intrusions: the Moxie gabbro and 

the Katahdin quartz monzonite. The intrusive relations of the 

Moxie pluton (MX, fi g. 6) differ considerably from one end to 

the other. In the Jo Mary Mountain area at the pluton’s east 

end (loc. 30, fi g. 1), the contact aureole contains remarkably 

pristine, gently dipping strata that display perfectly preserved 

crosslaminae (Hanson and Bradley, 1989). We observed no evidence of contact-metamorphic minerals overgrowing a regional 

slaty cleavage, which is visible everywhere outside the aureole. 

Midlength along the pluton (at loc. 44, fi g. 1), however, contact-metamorphic andalusite overgrows an axial-planar cleavage associated with nearly recumbent mesoscale folds. At the 

pluton’s west end, rocks of the contact aureole show a strong 

foliation defi ned by biotite and white mica and by the preferred 

orientation of andalusite with asymmetric strain shadows of 

biotite. Thus, at three places where we have anecdotal observations, the pluton shows local evidence of predating regional 

Acadian folding, of postdating Acadian folding, and of synkinematic emplacement. We dated the Moxie pluton in its eastern 

and central sections and obtained 40Ar/39Ar biotite plateau ages 

of 404.4±3.4 and 406.3±3.8 Ma, respectively (fi g. 8).

Katahdin Pluton

The Katahdin pluton (KA, fi g. 6) is a large (60 by 35 km) 

body of unfoliated quartz monzonite. It has long had an enigmatic age relationship to the Acadian deformation, but this 

problem can now be resolved—or, at least, narrowed—with 

a better time scale and precise U-Pb ages (Bradley and 

others, 1996; Rankin and Tucker, 1995). The Katahdin pluton 

intrudes rocks as young as the Seboomook Group (Pragian), 

the Matagamon Sandstone (Pragian and earliest Emsian), and 

the pluton's own volcanic carapace, the Traveler Rhyolite 

(406 Ma; Rankin and Tucker, 1995). Near Harrington Lake 

(HA, fi g. 6), the Seboomook Group near the pluton consists 

of gently dipping hornfelsed siltstone and sandstone that preserve pristine crossbedding. These rocks show no sign of ever 

having acquired the regional, northeast-striking, axial-planar 

cleavage that is pervasive just outside the contact aureole 

and that typically obscures or obliterates delicate sedimentary 

structures. In this area, the rocks in the contact aureole were 

baked and never cleaved, suggesting that the pluton is locally 

pretectonic. Along the pluton’s northeastern border, in contrast, Neumann (1967) mapped a breccia zone consisting of 

plastically deformed metasedimentary-rock fragments, set in 

a gneissic granitic matrix. These rocks appear to have formed 

along a syntectonic intrusive contact. The Katahdin pluton 

resisted many early attempts to date it accurately, but Denning 

and Lux (1989) reported an 40Ar/39Ar biotite plateau age 

of 400.1±1 Ma. More recently, Rankin and Tucker (1995) 

reported a 207Pb/206Pb age of 406.9±0.4 Ma that is the 

weighted average of fi ve concordant zircon fractions.

Harrington Pluton

The Harrington pluton (HA, fi g. 6) is a laccolith-like satellite 

to the Katahdin pluton (Griscom, 1976). Its map pattern (Griscom, 

1976) implies that it may have intruded a folding sedimentary 

section because its basal contact is planar and concordant with 

homoclinally dipping beds in the subjacent Seboomook Group, 

whereas its upper contact is concordant with regionally folded 

bedding in the overlying part of the Seboomook Group. The 

Harrington pluton yielded three concordant zircon analyses, with 

a mean 207Pb/206Pb age of 406.9±1 Ma (fi g. 7), which suggests an 

Emsian age for the folding.

Late Emsian to Eifelian Plutons

A few plutons in the age range 400–387.5 Ma are scattered across the study area (fi g. 6) but form no obvious pattern. The Magagauadavic pluton in southern New Brunswick 

has a U-Pb age of 396±1 Ma (MG, fi g. 6; M.L. Bevier, written 

commun., 1996). A biotite granite phase of the Mooselookmeguntic pluton (MO, fi g. 6) yielded a U-Pb age of 388.9±1.6 

Ma (Solar and others, 1998). The North Searsmont granitic 

gneiss (NS, fi g. 6) yielded a U-Pb age of 389.0±2.0 Ma 

(Tucker and others, in press). The Horserace quartz diorite 

(HR, fi g. 6), a small body within the Katahdin batholith, 

yielded a concordant U-Pb zircon age of 392±3.1 Ma 

(R.D. Tucker, unpub. data); an 40Ar/39Ar hornblende age of 

374.9±3.2 Ma was previously reported for this pluton (Denning and Lux, 1989).

Givetian to Early Tournasian Plutons2 

Previous Results

The best-defi ned group of intrusions of this age consists 

of Frasnian plutons in central Maine. Tucker and others (in 

press) recently obtained U-Pb ages for four of these plutons: 

the Hartland (379±1 Ma; HD, fi g. 6), Old Point (379±3 Ma; 

OP, fi g. 6), Rome (378±1 Ma; RO, fi g. 6), and Togus (378±1 

Ma; TO, fi g. 6). New ages are reported below for the Center 

Pond pluton and a sill at Hammond Ridge, which clearly 

belong in this group. Two other plutons that may belong in 

the same group crop out near the Maine coast: the Lucerne 

(380±4 Ma; LU, fi g. 6; R.E. Zartman and M.D. Gallego, in 

Marvin and Dobson, 1979, p. 18) and the Deblois (384±5 Ma; 

2

Plutons in this age range are widespread in the area of fi gure 6. In 

hindsight, they are not particularly relevant to the main point of this report, but 

they are important to late Acadian events, and their place in the scheme of things 

could only have been learned by obtaining reliable ages.
30 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Table 5. 40Ar/39Ar data for plutonic rocks, Maine.
Isotopic Ages of Acadian Plutons 31

Table 5. 40Ar/39Ar data for plutonic rocks, Maine—Continued.
32 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Table 5. 40Ar/39Ar data for plutonic rocks, Maine—Continued.
Isotopic Ages of Acadian Plutons 33

Table 5. 40Ar/39Ar data for plutonic rocks, Maine—Continued.
34 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

DE, fi g. 6; A. Ludman, written commun., 1998). Both of these 

plutons have been dated by the U-Pb zircon method, but both 

ages have fairly large errors.

A cluster of seven previously dated plutons in the Eastern 

Townships of Québec and adjacent Maine fall within the 

Givetian to Famennian age range. Those plutons dated by the 

U-Pb method include the Scotstown (384±2 Ma; SC, fi g. 6), 

Lac aux Araignées (in Québec, or Spider Lake, in Maine; 

383±3 Ma; LA, fi g. 6), Winslow (377±6 Ma; WI, fi g. 6), 

Aylmer (375±3 Ma; AY, fi g. 6), and Ste.-Cécile (374±1 

Ma; ST, fi g. 6; Simonetti and Doig, 1990). Plutons in this 

group dated by the 40Ar/39Ar method include the Chain of 

Ponds (373.3±2 Ma; CH, fi g. 6) and Big Island Pond plutons 

(367.7±1.3 Ma; BI, fi g. 6; Heitzler and others, 1988). The 

newly dated Hog Island and Beaver Cove, discussed below, 

appear to belong in this group. Three other new ages from 

the Priestly, Deboullie, and Chandler plutons, also discussed 

below, are slightly younger but occur within the same general 

strike belt. Five previously dated plutons of Famennian age 

form a belt near the coast. In Maine, these plutons include the 

Waldoboro pluton (368±2 Ma; WD, fi g. 6), the Mount Waldo 

pluton (371±2 Ma; MW, fi g. 6), and two small pegmatite 

bodies dated at 367±1 and 371±1 Ma (GA, GR, fi g. 6; Tucker 

and others, in press). (All ages are U-Pb zircon ages.) Similarly, M.L. Bevier (written commun., 1996) reported a U-Pb 

age of 366.5±1 Ma from the Mount Douglas pluton in southern New Brunswick (MD, fi g. 6).

Center Pond Pluton

The Center Pond pluton (CP, fi g. 6) is a hornblendebearing granodiorite that intrudes steeply dipping, calcareous 

siltstone of Silurian age (Scambos, 1980). Two concordant 

zircon analyses yielded a mean 207Pb/206Pb age of 377±3 Ma 

(fi g. 7). The pluton is cut in half and offset dextrally about 

2 km by the Center Pond fault (Scambos, 1980). A biotite 

separate from a penetratively dextrally sheared granitoid near 

the fault yielded an 40Ar/39Ar age of 368.7±2.9 Ma (fi g. 8), 

which may date the strike-slip event.

Hammond Ridge Sill

At Hammond Ridge in the aureole of the Katahdin pluton 

(HM, fi g. 6), a 50-cm-thick, tabular, concordant body of 

felsite interlayered with hornfels of the Carrabassett Formation yielded fi ve concordant zircon fractions with a mean 207Pb/206Pb age of 377.2±1.5 Ma (fi g. 7). This sill was at 

fi rst mistaken for an ash bed that would have dated the Carrabassett Formation (the reason why we analyzed so many 

fractions), but its young age requires that it be a sill.

Hog Island Pluton

The Hog Island pluton (HI, fi g. 6) intrudes Ordovician 

granodiorite, Silurian calcareous rocks, and Devonian fl ysch 

of the Seboomook Group at the north end of the Boundary 

Mountains anticlinorium. The pluton was mapped by Albee 

and Boudette (1972), who interpreted the contact metamorphism as either synregional or postregional metamorphism. 

A metapelite that contains biotite and cordierite, both formed 

during the contact-metamorphic event (the regional metamorphic grade is chlorite), is shown in fi gure 9D. Cordierite 

porphyroblasts overgrew a weak foliation, which, we suggest, 

formed during an earlier Acadian deformation; a second foliation is defi ned by aligned biotite, some of which occurs in 

strain shadows around cordierite. Although the Hog Island 

pluton is probably much younger than the fi rst Acadian deformation, its emplacement was nonetheless synkinematic. A 

single zircon fraction yielded a concordant 207Pb/206Pb age of 

371±3 (fi g. 7); biotite yielded an 40Ar/39Ar plateau age of 

368.6±3.1 Ma (fi g. 8).

Beaver Cove Pluton

The Beaver Cove pluton (BC, fi g. 6) is a small, unfoliated stock of leucocratic biotite granodiorite surrounded by 

the Moxie pluton. We obtained an 40Ar/39Ar plateau age of 

372.4±2.7 Ma from a biotite separate (fi g. 8), suggesting that 

the Beaver Cove pluton is as much as 34 m.y. younger than 

the Moxie pluton and unrelated to it (see Gabis and others, 

1994).

Deboullie Pluton

The Deboullie pluton (DB, fi g. 6) is a composite body 

that includes syenite, granodiorite, and monzonite phases 

(Boone, 1958). Boone’s mapping showed that the pluton truncates steeply dipping bedding and cleavage in the Seboomook 

Group and so postdates the main regional cleavage. Textures 

from the contact aureole, however, indicate that plutonism 

was followed by some additional shortening: Contact-metamorphic biotite is cut by a strong pressure-solution cleavage 

that parallels the regional fabric. Two samples of pink 

monzonite yielded 40Ar/39Ar plateau ages of 365.4±3.2 and 

363.2±3.0 Ma on biotite. Similarly, biotite from granodiorite 

yielded an 40Ar/39Ar age of 364.2±3.2 (fi g. 8).

Priestly Pluton

The Priestly pluton (PR, fi g. 6), a stock of unfoliated 

biotite-hornblende granodiorite, intrudes fl ysch of the 

Seboomook Group (Boudette and others, 1976). Textures in 

the contact aureole suggest that the pluton was emplaced into 

already-foliated slate. At the north end of the pluton, cordierite 

porphyroblasts contain inclusion trails of an older slaty cleavage (fi g. 9E). Biotite is randomly oriented and indicates static 

conditions of contact metamorphism; cordierite porphyroblasts grew preferentially in the direction of the older foliation, apparently also under static metamorphic conditions. A 

single zircon fraction yielded a concordant 207Pb/206Pb age of 

360.5±3.3 (fi g. 7). 40Ar/39Ar plateau ages of 363.2±2.3 and 

360.3±2.7 Ma were obtained on biotite (fi g. 8).
Location Through Time of the Acadian Deformation Front and Foreland Basin 35

Chandler Pluton

The Chandler pluton (CH, fi g. 6) intrudes fl ysch of the 

Seboomook Group and Ordovician mafi c igneous rocks along 

the eastern limb of the Munsungun anticlinorium. As far as 

we are aware, the pluton and surrounding rocks have never 

been mapped in detail, and outcrop is not plentiful. A total-gas 

40Ar/39Ar age of 366.5±3.7 Ma was obtained on biotite (fi g. 8); 

the age spectrum is slightly hump shaped.

Seboeis Pluton

As mentioned above, although the Seboeis pluton is 

shown on the “Geologic Map of Maine” (Osberg and others, 

1985) as a single body, it appears to include granodiorite, 

granite, and diorite of Emsian age, as well as a much younger 

phase. At East Branch Lake (SE, fi g. 6), an unfoliated biotite 

granodiorite yielded an 40Ar/39Ar plateau age of 364.7±4.3Ma 

(fi g. 8).

The Silurian-Devonian Time Scale:

Basis for Correlations Between

Paleontologically and Isotopically

Dated Events

We are able to make precise—and, in some cases, unexpected—correlations between isotopically and paleontologically dated events because of a parallel effort to improve 

the Silurian-Devonian time scale (Tucker and others, 1998). 

New tiepoints require signifi cant changes to previous time 

scales; each tiepoint is based on replicate, concordant, isotopedilution U-Pb zircon analyses of ashes or volcanic strata, dated 

to within one or two conodont zones. The key dates are as 

follows: (1) a concordant U-Pb zircon age of 417.6±1.0 Ma 

on the Bald Hill ash from the early Lochkovian Kalkberg 

Formation of eastern New York; (2) a concordant U-Pb zircon 

age of 408.3±1.9 Ma on the Sprout Brook ash from the early 

Emsian Esopus Formation of eastern York; (3) a concordant 

U-Pb zircon age of 391.4±1.8 Ma on the Eifelian Tioga ash of 

Wytheville, Va., confi rming the concordant U-Pb monazite age 

of 390.0±0.5 Ma reported for the Tioga ash of Pennsylvania 

by Roden and others (1990); (4) a concordant U-Pb zircon age 

of 381.1±1.3 Ma on the Little War Gap ash from the Frasnian 

part of the Chattanooga Shale of eastern Tennessee; and (5 and 

6) concordant U-Pb zircon ages of 363.8±2.2 and 363.4±1.8 

Ma on rhyolite that bracket a closely dated palyniferous horizon in the Famennian Piskahegan Group of New Brunswick. 

The vertical axis in fi gure 4 is numerically calibrated according to the new time scale of Tucker and others (1998), which is 

contrasted with that of Harland and others (1990) in table 6.

These revisions are critical to the present study. Using the 

new time scale, most of the closely dated plutons listed in table 

3 are now correlated with a younger stage than they would be 

on the time scale of Harland and others (1990). Among the 

plutons that are critical to positioning the deformation front, 

the new time scale shows that (1) the 424- to 417-Ma plutons 

in southern Maine are Ludlow to Lochkovian rather than 

Ludlow only, (2) the 404-408 plutons of interior Maine are 

Emsian rather than Lochkovian, and (3) the Scotstown pluton 

of the Eastern Townships is Givetian rather than Eifelian. 

Looking at it another way, the new time scale implies that the 

Sebec Lake, Onawa, Russell Mountain, and Blanchard plutons 

are the same age as the Tomhegan Formation, and not the Beck 

Pond Limestone, as implied by the Harland and others (1990) 

time scale. Paleogeographic maps based on the old time scale 

would have been fraught with such errors.

Location Through Time of the Acadian 

Deformation Front and Foreland Basin

In this section, we synthesize the foregoing results with 

the aid of two maps (fi gs. 10, 11) that track the migration of 

the leading edge of the foreland basin and of the deformation 

front. Seven time slices are depicted, each spanning a few 

million years. The gross paleogeography and key stratigraphic 

localities are shown in fi gure 12 for the four best-constrained 

time slices.

In constructing these fi gures, we interpolated the leading 

edge of the foreland basin at a given time between the positions of foreland-basin and far-foreland deposits of that age. 

Likewise, the deformation front was interpolated between fossiliferous foreland-basin deposits laid down before regional 

deformation and dated plutons that cut Acadian structures. In 

some places, as discussed below, fi gures 10 and 11 were used 

interdependently to help fi ll in gaps in knowledge; mostly, 

however, these two maps are based on independent data that 

nonetheless lead to the same general conclusions. For the 

purposes of fi gure 10, the details of locally complex Acadian 

deformation histories are not crucial because the point is 

merely to locate a boundary between rocks that had already 

undergone at least some contractional deformation from those 

that still had not. Likewise, unresolved aspects of the pretectonic, syntectonic, or posttectonic nature of various plutons 

Table 6.—Comparison of time scales.

[All values in millions of years before present (Ma)]
36 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

are not crucial for present purposes, so long as evidence 

exists that some regional deformation occurred in a particular 

plutonic belt before pluton emplacement.

Palinspastic Considerations

The key localities are plotted on a present-day base map; 

no attempt has been made to remove the effects of Acadian 

folding and thrust faulting, post-Acadian shortening, or dextral displacement across the Norumbega or other strike-slip 

faults. We originally had intended to use a palinspastic base, 

but we found it impossible to construct such a map on the 

basis of published maps and cross sections. There are simply 

too few good marker units to unravel the structure, and the 

exposure in Maine is too poor.

We can, however, make three comments in support of our 

use of the present-day base. First, none of the rocks in the 

study area (fi g. 1) has been displaced enormous, immeasurable distances across terrane-bounding faults since the beginning of the Ludlow. By that time, the Pocomoonshine pluton 

had linked the Fredericton Basin with the St. Croix belt (West 

Figure 10.—Maine and adjacent parts of New Brunswick, Québec, New Hampshire, and Vermont, showing successive positions of Acadian 

deformation front as it migrated across the northern Appalachians.

71°

70°

69°

44°

45°

73°

68° 44° 45°

46°

47°

68°

69°

73° 46° 72° 71° 70 °

72°

67° 66°

66°

67°

0 60 KM

0 40 MILES

Not deformed until

early Eifelian or later

Limit of Paleozoic deformation

Outboard limit of deformation, Ludlow-Lochkovian

Early Ludlow (~423 Ma)

Base of Lochkovian (~418 Ma)

Early Pragian

(~412 Ma)

Early Emsian

(406-407 Ma)

Base of Eifelian (394 Ma)

Base of Givetian (387.5 Ma)

Outboard limit of deformation by mid-Devonian

GR

O

WTH OF THE OROGEN

Base of Frasnian (382.5 Ma)

Toward NW

Toward SE

Position of the Acadian

Deformation Front

Key dated pluton

Key stratigraphic locality
Location Through Time of the Acadian Deformation Front and Foreland Basin

37

and others, 1992), which, in turn, was linked by clasts in the 

Silurian Oak Bay conglomerate to the Coastal volcanic belt 

(loc. 43, fi g. 1; Ludman, 1981). The Fredericton Basin and 

Miramichi anticlinorium were linked by conglomerate clasts 

in the Taxis River beds by the Early Silurian (loc. 36, fi g. 

1). The Aroostook-Matapedia Basin was two sided because 

it has provenance linkages to anticlinoria to both the west and 

east (Berry and Osberg, 1989; Hopeck, 1991). Specifi cally, 

the Llandovery Frenchville Formation near the basin’s western margin was derived from the Pennington anticlinorium 

just to the west (loc. 25, fi g. 1; Roy and Mencher, 1976), 

whereas the conglomeratic Daggett Ridge Formation (probably Late Ordovician to middle Silurian; Ludman, 1990) on 

the eastern margin was derived from the Miramichi anticlinorium (loc. 36, fi g. 1). The Central Maine Basin is linked 

stratigraphically to older rocks on the northwest; specifi cally, 

conglomerate in the Llandovery Rangeley Formation (loc. 26, 

fi g. 1) includes clasts traced to the Attean pluton along the 

Boundary Mountains anticlinorium (Moench and Pankiwskyj, 

1988). Finally, the Connecticut Valley-Gaspé Basin can be 

Figure 11.—Maine and adjacent parts of New Brunswick, Québec, New Hampshire, and Vermont, showing successive positions of leading edge of 

the Acadian foreland basin, constructed by following approach used by Bradley (1989) for the Taconic foreland basin.

71°

70°

69°

44°

45°

73°

68° 44° 45°

46°

47°

68°

69°

73° 46° 72° 71° 70 °

72°

67° 66°

66°

67°

0 60 KM

0 40 MILES

Base of Givetian (387.5 Ma)

Base of Eifelian (394 Ma)

Early Emsian (407-406 Ma)

Early Ludlow (~423 Ma)

Base of Lochkovian

(~418 Ma)

Early Pragian (~412 Ma)

Key dated pluton

Key stratigraphic locality

Position of the Leading Edge

of the Acadian Foreland Basin

FORELAND

BA

SIN MIGRATION

Location Through Time of the Acadian Deformation Front and Foreland Basin
38 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Figure 12.—Schematic, nonpalinspastic paleogeographic maps of study area (see fi g. 1), showing migration of orogen and foreland basin 

during four time slices. Circles, locations of fossils in stratifi ed rocks of appropriate age range, except for the Traveler Rhyolite, which is 

dated isotopically; black squares, locations of plutons in appropriate age range. Boxes show age and two-letter abbreviation for a few 

selected plutons, keyed to fi gure 6 and table 3. Toothed line denotes Acadian deformation front, which probably was a blind thrust rather 

than an emergent thrust during much of Late Silurian to Middle Devonian time. Paleogeography during the Eifelian and Emsian is unknown 

for area south of orogen (present coordinates).

seen as a foreshortened, two-sided basin because (1) Late 

Ordovician to Early Silurian conglomerate of the Cabano and 

Depot Mountain Formations along its northwestern margin in 

Québec and northernmost Maine was derived from Taconic 

highlands that lay to the northwest (loc. 6, fi g. 1; Roy, 1989) 

and (2) along the opposite basin margin in northern Maine, 

the Fish River Lake Formation includes sandstone and conglomerate that can be traced to sources in the Pennington 

anticlinorium, just to the east (loc. 21, fi g. 1; Boone, 1970). 

Although the contact between the Central Maine and Aroostook-Matapedia Basins is probably a major thrust fault (A. 

Ludman, written commun., 1998), the two basins shared a 

common northwesterly source region during the Silurian and 

so are not exotic relative to each other. Similarly, the Central 

Maine and Fredericton Basins are in fault contact and may 

not have been continuous with each other, as some workers 

0 60 KM

0 40 MILES

0 60 KM

0 40 MILES

0 60 KM

0 40 MILES

0 60 KM

0 40 MILES

73° 46° 72 ° 71° 70°

69° 68° 44° 45°

47°

68°

69°

67° 66°

66°

71°

70°

44°

45°

73°

46°

72°

67°

Temiscouata

turbidites

FORELAND

BASIN

FAR FORELAND

OROGEN

Smalls Falls

turbidites

Burtts Corners

turbidites

Ripogenus

carbonates

Edmunds volcanics

Hardwood Mtn.

carbonates

Fitch

carbonates

Fish River Lake

carbonates &

volcanics

Final limit of Paleozoic shortening

EARLY

LUDLOVIAN

423-422 Ma

423, PO

FAR FORELAND

OROGEN

Square Lake

Limestone

FORELAND

BASIN

Hartin Fm.

turbidites,

volcanics

Eastport Volcanics

Carrabassett

& Madrid

turbidites Chapman Ss.,

Dockendorff

Volcanics

Final limit of Paleozoic shortening

EARLY

LOCHKOVIAN

418-415 Ma

Limestone clasts in

Ste. Helenes diatreme LUDLOW

424-419 Ma

Beck Pd. &

McKenney Pd.

Limestones

418, LI

normal

faulting

sinistral

crossfaulting

FAR FORELAND

OROGEN

FORELAND BASIN

Final limit of Paleozoic shortening

EARLY

EMSIAN

408-406 Ma

Fish River Lk.

sandstone

Compton

turbidites

Littleton

turbidites Tomhegan

sandstone

Traveler

Rhyolite

406, RU

407, KA

FAR FORELAND

FORELAND

Final limit of Paleozoic shortening

EIFELIAN

~394-387.5 Ma

Fish River Lk.

ss., cgl.

Mtn. House

Wharf

limestone

Famine

limestone

Touladi

limestone

Limestone clasts in

Ste. Hélène breccia

OROGEN

Mapleton

ss., cgl.

Trout Valley

ss., cgl.

Wapske

ss., cgl.

BASIN

392, HR
Location Through Time of the Acadian Deformation Front and Foreland Basin 39

have suggested (McKerrow and Ziegler, 1971; Bradley, 1983). 

Nonetheless, they are linked by the chain of provenance relationships discussed above.

Second, evidence for the amount of fold-related Acadian 

shortening can be gleaned from some 1:62,500-scale quadrangle maps. We estimated shortening on 15 cross sections 

that were selected because they show at least one readily 

mapped contact, such as a carbonate-fl ysch contact. Using the 

sinuous-bed-length method, the ratio of the palinspastic crosssectional length to length in the deformed state was found to 

range from 1.30 to 2.78 (fi g. 13), with a mean of 1.83. These 

results, if taken as being representative of the entire orogen, 

suggest that the pre-Acadian across-strike width of fi gure 1 

north of the Sennebec Pond fault was about twice the present 

width. The actual initial width was most likely even greater 

because we were able to estimate shortening only for those 

exceptional places with simple, traceable structure. In the 

southeasterly part of the Central Maine Basin in Maine 

(Osberg, 1988) and in the northwestern part of the same 

basin in New Hampshire (Eusden and others, 1996), early 

recumbent folds imply much greater amounts of shortening. 

Our estimate of shortening also does not account for the 

effects of penetrative strain.

Third, the only potentially problematic post-Acadian 

strike-slip fault in the study area (fi g. 1) is the Devonian to 

Carboniferous Norumbega Fault System. The displacement on 

this fault system may be large, but not so large that the rocks 

on either side are unrelated. Near the Maine-New Brunswick 

border, turbidites assigned to the Lower to middle Silurian 

Flume Ridge Formation occur at the same chlorite grade 

both northwest and southeast of the fault (Ludman, 1981). 

Map relations reviewed by Ludman and West (1999) suggest 

125 to 140 km of cumulative dextral displacement across the 

Norumbega Fault System.

In summary, the successive deformation fronts mapped in 

fi gure 10 are probably more than twice as close together as 

they would be on an accurate palinspastic base. Also, localities outboard of the Norumbega Fault System were probably 

about 125 to 140 km farther northeast during the Acadian 

orogeny. Because the Norumbega Fault System approximately 

parallels Late Silurian paleogeographic belts, however, it 

does not signifi cantly affect any of the conclusions presented 

below.

Early Ludlow

The Acadian orogeny was underway by the beginning of 

the Ludlow. The deformation front in fi gure 10 was placed 

between the posttectonic Pocomoonshine pluton at locality 37 

(fi g. 1) and the graptolite-bearing Smalls Falls Formation and 

Table 7.—Data used to estimate Acadian and younger shortening in fi gure 13.
40 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Figure 13.—Maine and adjacent parts of New Brunswick, Québec, New Hampshire, and Vermont, 

showing local estimates of Acadian and younger shortening, determined by sinuous-bed-length method, 

across 12 published cross sections. Boxed numbers are ratios of initial to fi nal cross-sectional lengths; 

for example, a value of 2.63 corresponds to a cross section that is now 5.4 km long but is inferred to have 

been 14.1 km long, by unfolding one or more marker horizons. Data are listed in table 7.

Burtts Corners beds at localities 29 and 35. The boundary 

between the foreland basin and far foreland was placed northwest of localities 29 and 35 but southeast of such shallowmarine carbonate units as the Hardwood Mountain, Spider 

Lake, Ripogenus, Grand Lake Seboeis, and Fish River Lake 

Formations (at locs. 11, 18, 16, 20, and 21, respectively).

Although our main emphasis in this report has been on 

tracking the northwestward migration of the Acadian deformation front, we also note evidence that a second, outboard 

boundary between deformed and undeformed rocks migrated 

a much shorter distance to the southeast. In the Coastal volcanic belt, volcanism and sedimentation continued without 

interruption through most of the Silurian and into the 

Lochkovian. The southeast boundary between deformed and 

undeformed rocks must have lain somewhere between the 

Pocomoonshine pluton and locality 38 (fi g. 1) by early 

Ludlow time. It does not appear to have moved southeast 

until after Lochkovian time—that is, after deposition of the 

youngest Acadian-deformed strata at Eastport (loc. 38).

Early Lochkovian

During early Lochkovian time, the Acadian deformation 

front was located along the northwest fl ank of the Central 

Maine Basin and near the eastern margin of the AroostookMatapedia Basin. Its position is bracketed by foreland-basin 

and far-foreland strata to the northwest and syntectonic and 

posttectonic plutons to the southeast. Plutonic rocks within 

the range 418.5 to 415 Ma delimit the zone of active deformation; these rocks include the syntectonic Lincoln sill (417 Ma) 

and the Hartfi eld pluton (415 Ma).

A Lochkovian foreland basin fl anked the plutonic belt on 

the northwest. Strata assigned to this stage of the migrating 

foreland basin include the Hartin Formation (loc. 34, fi g. 1), 

the Chapman Sandstone and Swanback Formation (loc. 31), 

a part of the Millimigassett Lake Formation (loc. 20), and 

the Madrid and Carrabassett Formations in the Central Maine 

Basin (for example, at locs. 26 and 30). The early Lochkovian 

position of the leading edge of the foreland basin (fi g. 11) is 

Paleozoic thrust front

71°

44°

73° 46° 72° 71°

72°

70°

69° 68° 44° 45°

46°

47°

68°

69°

70 °

67° 66°

66°

67°

45°

73°

0 60 KM

0 40 MILES

Line of cross section

Local estimates of Acadian shortening

1.38 Ratio of initial to final length

Map letter in table 7

1.33

1.30

2.94

1.96

1.82

1.30

1.47

2.63

2.13

1.72

1.41

1.19

A

B

C D

E

F

G

H

L

K

J

I

F

Norumbega fault (125-140 km separation)
41

accurately placed at localities 12 and 13, where the Beck Pond 

and McKenney Ponds Limestones grade into or interfi nger 

with the Seboomook Group and Tarratine Formation, respectively. Carbonate rocks of Lochkovian age include parts of 

the lower Fish River Lake Formation (loc. 21), Spider Lake 

Formation (loc. 18), and Square Lake Limestone (loc. 23). 

These rocks presumably were deposited beyond the limits of 

the Lochkovian foreland basin.

Pragian

Two lines of evidence show that the Pragian foreland 

basin may have migrated 15 to 20 km (present distance) cratonward from its early Lochkovian position (fi g. 11). Along 

the Munsungun anticlinorium (loc. 16, fi g. 1), the Pragian 

Seboomook fl ysch encroached over Lochkovian far-foreland 

carbonates. Similarly, in the Pennington anticlinorium (loc. 

23), Pragian and younger fl yschlike siltstone of unit Dus 

overlies the Square Lake Limestone. Within the foreland 

basin itself, the deltaic Matagamon Sandstone prograded over 

prodelta turbidites of the Seboomook Group. Although we 

have no direct evidence for migration of the deformation 

front from its Lochkovian position, in constructing fi gure 

10, we have assumed that the deformational front migrated 

the same distance as the foreland-basin margin did from its 

previous position (fi g. 11). This assumption seems reasonable 

because the deformation front and foreland-basin front did

migrate in tandem during every interval, such as Ludlow to 

Lochkovian, for which have adequate data.

Early Emsian

The clearest picture of Acadian paleogeography in Maine 

is in the early Emsian, at 407 to 406 Ma (Bradley and others, 

1996). We can identify a foreland basin and orogenic wedge. 

Superimposed on both basin and wedge and straddling the 

deformation front was a volcanic-plutonic belt. Many Emsian 

plutons truncate Acadian map-scale structures, including the 

newly dated Sebec Lake, Russell Mountain, and ShirleyBlanchard (SL, RU, and SB, respectively, fi g. 6). The deformation front was plotted to the northwest of such plutons. 

The early Emsian foreland basin includes the younger strata 

of the vast “Devonian slate belt” of Boucot (1970). Forelandbasin fl ysch units that have yielded Emsian fossils include the 

Temiscouata Formation (loc. 24, fi g. 1), the Compton Formation (locs. 3, 4), and the Littleton Formation (locs. 9, 10). It 

seems likely, though unproved, that parts of the Seboomook 

Group in the Connecticut Valley-Gaspé Basin in northwestern 

Maine and the Gile Mountain Formation of Vermont are also 

Emsian. Early Emsian molasse deposits in the study area 

(fi g. 1) include the Tomhegan Formation (loc. 15) and, as 

suggested above, the uppermost part of the Matagamon Sandstone (loc. 17). These molasse facies must have been laid 

down near the proximal, southeastern margin of the foreland 

basin. The cratonward boundary of the early Emsian foreland 

basin can be positioned by using negative evidence. As discussed below, the three occurrences of Eifelian limestone 

along the northwestern margin of the Connecticut ValleyGaspé Basin rest everywhere on pre-Emsian rocks, and so 

Emsian foreland-basin deposits—which are kilometers thick 

where present—probably were never deposited that far northwest.

The deformation front must have corresponded to the 

Lobster Mountains anticlinorium (LMA, fi g. 2), between the 

site of Tomhegan molasse deposition and the site of the 

plutons that were emplaced into the deforming or recently 

deformed orogenic wedge. The Moxie and Katahdin plutons 

(KA and MX, respectively, fi g. 6) were emplaced in the general vicinity of the deformation front (Bradley and others, 

1996).

Late Emsian to Early Eifelian

By the time of the Emsian-Eifelian boundary (~394 Ma), 

the Acadian deformation front had migrated some distance 

cratonward, to the approximate position of the Pennington 

anticlinorium (PA, fi g. 2). Southeast of the deformation front 

(fi g. 10), late Emsian and younger strata postdate Acadian 

deformation. To the northwest, foreland-basin and far-foreland deposits of this age range predate Acadian deformation. 

At locality 22 (fi g. 1), the upper siliciclastic part of Boone’s 

(1970) Fish River Lake Formation (“Nadeau Thoroughfare 

beds”) were deposited in a foreland-basin setting. At localities 

2, 5, and 7, Eifelian limestone was deposited in a far-foreland 

setting. The Mapleton Formation and Trout Valley Formation 

are similar in age to the upper part of the Fish River Lake 

Formation. Both the Mapleton and Trout Valley Formations, 

however, postdate the main phase of Acadian deformation, 

and so we place them southeast of the deformation front.

The stratigraphy of the Carlisle area (loc. 33, fi g. 1) complicates an otherwise-straightforward paleogeographic picture. 

Though coeval with the Mapleton Formation, the Wapske Formation was involved in the same regional “Acadian” folding 

as the rest of the Silurian to Devonian section. One possible 

explanation is that the Carlisle area escaped Acadian deformation until the late Eifelian or later (fi gs. 10, 12). A second possible explanation is that the fi rst phase of Acadian deformation, 

which in the Presque Isle area created the unconformity below 

the Mapleton Formation, bypassed the Carlisle area entirely. 

In this scenario, the folding at Carlisle would be related to 

the same, later event that caused gentle, late-stage or postAcadian folding of the Mapleton Formation at Presque Isle. 

A third possible explanation is that the Wapske Formation 

was deposited farther to the northwest and was thrust into its 

present position during a late-stage Acadian event.

Late Eifelian to Early Givetian

By the time of the Eifelian-Givetian boundary (~387.5 Ma), 

the Acadian deformation front had again migrated cratonward 

from its previous position, especially on the basis of a belt 

of plutons in the Eastern Townships of Québec. These plutons 

intrude already-deformed Silurian and Devonian strata of the 

Location Through Time of the Acadian Deformation Front and Foreland Basin
42 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Connecticut Valley-Gaspé Basin; the oldest and, thus, most signifi cant pluton is the Scotstown pluton (384±2 Ma; SC, fi g. 6; 

Simonetti and Doig, 1990). The deformation front must have 

been situated somewhere to the northwest by about 384 Ma.

Just a few million years before emplacement of the 

Scotstown pluton, Eifelian carbonates were still being deposited at locality 5 (fi g. 1). These strata are penetratively 

deformed, indicating that the fi nal cratonward limit of Acadian deformation front was at least that far northwest. (The 

Paleozoic deformation front shown on fi gure 1 could be as 

old as Ordovician.) Exactly when the deformation took place 

is unknown, but it was likely at least a few million years 

after deposition of the last carbonates (the youngest being 

the Famine Limestone, loc. 5), long enough to allow for an 

interval of foreland-basin sedimentation before deformation.

The late Eifelian to early Givetian stratigraphic record 

in the study area (fi g. 1) is confi ned to a few clasts in the 

Ste. Hélène Island breccia, near Montreal (loc. 1). The clasts 

include calcareous siltstone, sandstone, and some limestone 

of late Eifelian to early Givetian age (Boucot and others, 

1986). By comparison of these rock types with the Catskill 

clastic wedge in New York, Montreal was probably near the 

cratonward edge of the Acadian foreland basin at about the 

Eifelian-Givetian boundary (fi g. 11).

Earliest Frasnian

The deformation front at the beginning of Frasnian time 

has been plotted in fi gure 10 on the basis of slender evidence. 

Deformed Eifelian carbonates at localities 2, 5, and 7 (fi g. 1) 

show that the Acadian orogeny was felt at least that far to 

the northwest. The deformation front was extrapolated from its 

position at the beginning of the Givetian, assuming a nearly 

constant convergence rate. The fi nal northwestward limit of 

Acadian shortening is unknown; it might correspond to the 

Paleozoic thrust front (fi g. 10) or, alternatively, might lie 

somewhere between the Frasian deformation front and the 

Paleozoic thrust front. Along the Paleozoic deformation front 

near Albany, N.Y., strata as young as Eifelian were involved in 

thrust faulting, probably Acadian but conceivably Alleghanian.

Implications

Age of the Acadian orogeny

A zone of active deformation migrated across Maine 

and adjacent parts of New England and Canada from early 

Ludlow through Givetian time. Initially, the orogen was a 

narrow belt, but it widened over time, eventually encompassing the entire State of Maine (fi g. 10). The overall pattern 

of cratonward migration of the orogen is similar to that suggested by Donahoe and Pajari (1973) but is much changed 

in light of our new data from Maine and our improved time 

scale.

The Acadian orogeny has traditionally been regarded as 

a Devonian event. During the 1990’s, however, new pluton 

ages were interpreted to show that the supposedly Devonian 

orogeny in parts of the northern Appalachians was actually 

Silurian (Bevier and Whalen, 1990b; West and others, 1992, 

1995; Stewart and others, 1995a). Now, in light of revisions 

to the time scale that fi x the Silurian-Devonian boundary at 

about 418 Ma, some of the purportedly Silurian plutons are 

back once again in the Early Devonian (for example, the 

North Pole Stream and Mount Elizabeth plutons of Bevier and 

Whalen, 1990b). The rest fall within the fi nal 5 m.y. of the 

Silurian.

The northwestward migration of the deformation front 

(fi g. 10) might be interpreted in two ways. One interpretation 

is that the orogeny occurred in pulses, each pulse separated 

by tectonic stability and each with some different cause. Some 

workers (for example, Boucot and others, 1964; Stewart and 

others, 1995a) have applied the term “Salinic” to Late Silurian 

orogenic events in Maine—events that, in their view, are 

distinct from the Devonian Acadian orogeny. Similarly, other 

names for orogenies might be coined for other positions of the 

deformation front (fi g. 10). We prefer to explain the pattern 

in fi gure 10 as the result of a single plate-convergent event 

which swept the region over a long period of time. Existing 

data cannot resolve this question.

Rate and Trajectory of Deformation-Front 

Migration

In fi gure 10, the deformation front is shown to have 

migrated about 240 km across strike in about 40.5 m.y. 

Assuming continuous rather than episodic convergence, this 

implies a rate of about 6 km/m.y. (0.6 cm/yr). The actual rate 

must have been considerably faster because, as noted above, 

the base map is nonpalinspastic. Assuming, for the sake of 

discussion, that Acadian shortening reduced Maine to half its 

original across-strike width, the rate of thrust-front migration 

would be about 12 km/m.y. (1.2 cm/yr).

It is likely, but not assured, that the rate of thrust-front 

migration approximately equaled the rate of plate convergence (Bradley and Kusky, 1986; Bradley, 1989). This is 

generally the case at oceanic subduction zones. Similarly, 

in the snowplow analogy for orogenic wedges (wherein a 

snowplow pushes a wedge of snow ahead of the plowblade), 

the deformation front in the snow migrates at about the same 

rate as the plow advances. A less likely alternative is that the 

advance of Acadian thrusts was directly linked not to plate 

convergence but to extensional orogenic collapse, as in the 

Carpathians (Royden and Burchfi eld, 1989). Although little 

is yet known about Acadian hinterland tectonics between 

about 420 and 380 Ma (see section below entitled “Hinterland 

Deformational Regimes...”), there is no evidence for wholesale regional extension of the magnitude required by this 

mechanism.

On the basis of the along-strike diachronism of forelandbasin deposits in the northern Appalachians (younger toward 

the southwest), Bradley (1987, 1997) suggested that Acadian 

plate convergence had a dextral component. The migration of 

black-shale depocenters across the Acadian foreland basin in 
43

the central Appalachians led Ettensohn (1987) to the same 

conclusion. The present study area does not extend far enough 

along strike to provide clear evidence either for or against 

along-strike diachronism.

Physical Setting of Magmatism

One by-product of this study has been to show that many 

of the Acadian plutons of Maine occur in distinct age belts 

(fi g. 6). Moreover, fi gure 12 shows how the various plutons 

and volcanic rocks were spatially related to the orogen as it 

existed at the time of magmatism.

Coastal Volcanic Belt and the Ludlow to 

Lochkovian Plutonic Belt

On geochemical grounds, Gates and Moench (1981), 

Hogan and Sinha (1989), and Seamans and others (1999) have 

suggested that the Ludlow to Lochkovian volcanic rocks and 

plutons of coastal Maine formed in an extensional setting. 

The migration pattern of the deformation front (fi gs. 10, 12), 

however, implies that magmatism took place in an overall setting of plate convergence. The same conclusion is suggested 

on a more local scale by the fact that the Lochkovian Lincoln 

sill was emplaced synkinematically (Tucker and others, in 

press). Judging from fi gure 12, magmatism took place in 

the upper plate of a two-plate system, presumably above an 

A-type subduction zone.

Piscataquis Volcanic Belt and the Emsian Plutonic 

Belt

The cause or causes of Ludlow to Emsian volcanism 

along the Piscataquis volcanic belt have long been debated, 

and although a resolution is beyond the scope of this report, 

our fi ndings do provide an important new constraint: all of 

the volcanic rocks are broadly syncollisional and were erupted 

in a foreland setting. Most of the Ludlow to Lochkovian 

volcanic rocks (East Branch Group, West Branch Volcanics, 

and Fish River Lake, Spider Lake, Grand Lake Seboeis, and 

Allagash Lake Formations) were erupted in a far-foreland 

position. However, several volcanic units of Lochkovian and 

Emsian age are interbedded with or grade laterally into foreland-basin deposits and thus formed in a proximal-foreland 

position (Hedgehog Volcanics, Edmunds Hill Andesite, Hartin 

Formation, Traveler Rhyolite, and Kineo Rhyolite). Three 

alternative interpretations have been advanced for part or 

all of the Piscataquis volcanic belt. (1) On the basis 

of basalt discriminant plots, Hon and others (1992) and 

Keppie and Dostal (1994) interpreted the Spider Lake 

and West Branch volcanic rocks as having formed in an 

intracontinental-rift setting, related to so-called “transpressive 

rifting.” In light of our fi ndings, any extension would necessarily have taken place in a collisional foreland setting, such 

as might have been caused by lithospheric fl exure (Bradley 

and Kidd, 1991) or by Baikal-style cracking of the lower 

plate (Sengör and others, 1978). We note that the geochemical 

signature of volcanic rocks in foreland settings has not been 

adequately characterized and that discriminant plots used by 

the above-cited authors do not even identify fi elds for volcanic 

rocks formed in this tectonic setting. (2) Tucker and others (in 

press) have suggested that the Emsian foreland-basin rhyolites and the Emsian plutons just to the southeast were a 

product of lower-lithospheric delamination of the downgoing 

(North American) plate, which was being subducted toward 

the southeast. (3) Bradley (1983) interpreted these units as 

arc volcanic rocks formed over a northwest-dipping subduction zone. This model would explain magmatism in all 

three settings—far foreland, proximal foreland, and orogenic 

wedge—as the consequence of an orogenic wedge above a 

southeast-dipping subduction zone colliding with (and overriding) a magmatic arc formed over a northwest-dipping subduction zone. (An updated version of this tectonic model was 

illustrated by Hanson and Bradley, 1989.)

Syncollisional Versus Precollisional Plate 

Geometry

Tectonic interpretations of the Acadian orogeny have long 

been debated. Part of the controversy regarding Acadian plate 

geometry may be sidestepped for present purposes by separately considering the plate geometry during two intervals—

before initial impact of the converging plates and during the 

actual collision. The paired migration of the orogen and foreland basin shown in fi gures 10 through 12 suggests that, 

during collision, an upper plate consisting of the growing orogenic wedge and its Avalonian backstop overrode the Taconicmodifi ed margin of North America. The implied A-type 

subduction zone that operated during collision presumably 

evolved naturally out of a B-type subduction zone of the 

same southeast-dipping polarity, where ocean fl oor that once 

existed between Avalonia and North America was consumed 

during the late Llandovery and Wenlock.

What remains to be settled is the nature of the northwestern margin of the Central Maine Basin before collision. This 

is a much-debated topic, and we, the coauthors, are ourselves 

divided on the subject. In the two main tectonic models, both 

of which have variants, it was either (1) a “passive” or rifted 

margin on the backside of an arc that had collided with North 

America during Ordovician time (for example, Robinson and 

others, 1998; Tucker and others, in press), or (2) a convergent 

margin that took up some of the motion between Avalonia and 

North America during Silurian time (Bradley, 1983; Eusden 

and others, 1996). In the second model, the northwestern 

subduction complex and arc were overridden by, and downfl exed beneath, the southeastern one described in the preceding paragraph (Bradley, 1983). In the one-subduction-zone 

model (Robinson and others, 1998), the northwestern passive 

margin was overridden by, and downfl exed beneath, a 

southeastern subduction-collision complex. Although the two 

models differ signifi cantly up until the time of collision, they 

are much the same afterward.

Implications
44 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Hinterland Deformational Regimes

and Regional Partitioning

of Acadian Deformation

The present study has focused on tracking the fi rst Acadian contractional deformation. In most places, however, 

Acadian deformation was polyphase. Figure 10 provides 

a framework for understanding those post-D1 events that 

occurred from about 423 to 382.5 Ma—that is, the ages of the 

oldest and youngest deformation fronts. Three examples will 

be discussed here.

Stewart and others (1995a) described a complex history 

of motion on a network of northeast-striking dextral faults 

in Penobscot Bay. The oldest of these, the Penobscot BaySmith Cove-North Blue Hill Fault, displaces isograds around 

the Segwick pluton (419.5±1 Ma) but is cut by the South 

Penobscot Intrusive Suite (419.2±2.2 Ma). Later, the South 

Penobscot Intrusive Suite was cut by the dextral Turtle Head 

Fault, which was intruded, in turn, by the Lucerne pluton (380±4 

Ma; Wones, 1991). Where plate convergence has an oblique 

component, deformation can be partitioned into thrusting in 

the orogenic wedge and strike-slip faulting farther to the rear 

(Dewey, 1980). If this explanation is applicable to the latest 

Silurian episode of dextral strike-slip faulting on the Maine coast, 

subduction of Central Maine Basin crust beneath the Coastal 

volcanic belt would have had a dextral-oblique component.

Near Waterville (WA, fi g. 6), a 399-Ma intermediate dike 

cuts the fi rst-phase Acadian folds but is deformed by a second 

generation of folds that must be older than a static metamorphic event dated at 380 Ma (Osberg, 1988; Tucker and others, 

in press). The second folding must therefore have taken place 

when the Acadian deformation front was somewhere cratonward of its early Emsian position (fi g. 10), perhaps 100 km 

to the northwest of Waterville. Accordingly, this contractional 

deformation was an out-of-sequence, hinterland event.

The Horserace pluton is a small body with a U-Pb age 

of 392 Ma (R.D. Tucker, unpub. data) within the 407-Ma 

Katahdin batholith. It was emplaced, probably synkinematically (Hon, 1980, p. 73), along a northwest-striking highangle fault (the West Branch fault) that sinistrally offsets the 

western margin of the Katahdin pluton. This is one of many 

crossfaults that disrupt the bedrock map pattern in northern 

and central Maine (Osberg and others, 1985). Referring to 

fi gures 10 through 12, at about 392 Ma, the deformation front 

lay perhaps 70 km to the northwest. Like the second folding 

at Waterville, sinistral motion on the West Branch Fault took 

place in the Acadian hinterland, while the orogen was still 

advancing toward North America.

Foreland Deformational Regimes

Figures 10 through 12 also provide insights into preAcadian events that took place in what was then the orogenic 

foreland. Two northeast-striking high-angle faults at Spencer 

Mountain (loc. 12, fi g. 1) cut the Hardwood Mountain Formation (Burroughs, 1979), to which we assign a late Ludlow 

to Pridoli age (table 2). The map pattern indicates that one 

of these faults is overlain by the Hobbstown Conglomerate, 

which ranges from possibly as old as Pridoli to defi nitely as 

young as Pragian (Boucot and Heath, 1969). The sense of 

motion is not well established. On the basis of the relative 

age of faulted strata, Burroughs (1979) showed both faults as 

having down-to-the-northwest motion. Whatever the exact age 

of faulting and the sense of motion, it is clear that faulting 

took place in a foreland setting and that the high-angle faults 

paralleled the approaching orogenic front. On this basis, they 

would appear to resemble normal faults of the Taconic foreland of New York, which formed in response to lithospheric 

fl exure (Bradley and Kidd, 1991).

Suggestions for Further Work

Since an infl uential paper by Naylor (1970), the Acadian 

has commonly been described as “an abrupt and brief event.”

This description appears to be only half-true. Although the 

fi rst phase of deformation was, indeed, abrupt, lasting but a 

few million years at any given place, it took some 40 m.y. 

for the deformation front to make its way across Maine. 

Our study thus confi rms the fi ndings of Donahoe and Pajari 

(1973), in general if not in particular.

Figures 10 through 12 could be improved with better 

age control for several key units. The Flume Ridge, Madrid, 

and Carrabassett Formations each have a particular tectonic 

signifi cance but are so loosely dated by fossils as to be barely 

useful for the present purposes. A dedicated search for fossils, 

especially palynomorphs, and (or) datable pyroclastic horizons in these units would be worth the effort; tuff horizons, 

1 to 2 cm thick, commonly yield zircons, and many of these 

horizons have probably been overlooked. Tighter paleontologic control is also needed, though less urgently, for the 

Hartin, Perry, and Swanback Formations, the Chapman Sandstone, and the Seboomook Group in northernmost Maine. 

Likewise, modern geochronologic studies are needed for the 

West Branch, Spider Lake, and Hedgehog Volcanics, the Red 

Beach and Lucerne plutons in Maine, and the many Devonian 

plutons in the Northeast Kingdom of Vermont.

The Devonian time scale still has signifi cant holes to be 

fi lled in the Pragian, late Emsian, and Givetian; until these 

intervals are better calibrated, correlations between isotopically and paleontologically dated rocks in these age ranges 

will remain equivocal.

The depositional environments, paleocurrents, and provenance of unnamed unit Dsus and the Sangerville, Smalls Falls, 

Hartin, Wapske, Fish River Lake, and Swanback Formations 

warrant detailed study; we have assigned these units to a foreland-basin setting on the basis of lithology and regional relations alone. The Wapske is of special interest because it was 

evidently deformed about 15 m.y. later than rocks along strike 

in Maine (fi g. 10). Accordingly, the Wapske Formation may 

have been deposited in a “piggyback” basin, a suggestion that 

might be evaluated by using detailed sedimentologic studies.

Our assessments of the pretectonic, syntectonic, or posttectonic age of plutons are admittedly cursory, based as they 

are on anecdotal observations of a few thin sections and 
45

one or two traverses from country rock into pluton. Detailed 

investigations would be especially useful for those plutons, 

such as the Moxie and Katahdin, that appear to have been 

emplaced very close to the deformation front.

The present study has emphasized evidence bearing on 

the northwestward migration of the Acadian orogen, but we 

have barely touched on its migration to the southeast. Did 

it migrate in a stepwise fashion, or gradually? Although the 

sedimentary record from the outboard side of the orogen 

is comparatively sparse, the Eastport Formation of coastal 

Maine and New Brunswick may preserve a record of synorogenic sedimentation off the backside of the growing Acadian 

mountains.

A palinspastic base will be essential for discerning the 

geographic relations between correlative rocks at the time 

they were formed and for quantifying the rate of plate convergence during collision.

Finally, one of the most intriguing recent developments in 

geodynamics involves previously unappreciated links between 

climate and tectonics. Judging from studies of the Southern 

Alps of New Zealand (for example, Beaumont and others, 

1992), synorogenic rainfall and consequent erosion can profoundly affect the evolution of the underlying deforming 

rocks. Hoffman and Grotzinger (1993) suggested that an 

asymmetric orographic climate can infl uence the metamorphic fi eld gradient, the width of the thrust belt, the extent 

of footwall involvement in deformation, and the thickness 

and compositional maturity of the foreland-basin fi ll. Acadian 

synorogenic climate and its possible infl uences on structure, 

metamorphism, and stratigraphy have not been investigated, 

but there is good reason to do so. Early Devonian paleogeographic reconstructions imply that the Acadian mountains had 

an asymmetric orographic climate, with the west side wetter 

(Witzke and Heckel, 1988). Such a climatic asymmetry, if 

borne out by paleoclimatologic studies of Devonian strata in 

the study area, might be explored as a possible control on the 

migration of the main Acadian deformation front.

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West, D.P., Guidotti, C.V., and Lux, D.R., 1995, Silurian orogenesis in 

western Penobscot Bay region, Maine: Canadian Journal of 

Earth Sciences, v. 32, p. 1845–1858.

West, D.P., Ludman, A., and Lux, D.R., 1992, Silurian age for the Pocomoonshine gabbro-diorite, southeastern Maine and its regional 

tectonic implications: American Journal of Science, v. 292, p. 

253–273.

Wilson, J.T., 1966, Did the Atlantic close and then re-open again?: 

Nature, v. 211, p. 676–686.

Witzke, B.J., and Heckel, P.H., 1988, Paleoclimatic indicators and 

inferred Devonian paleolatitudes of Euramerica, in McMillan, 

N.J., Embry, A.F., and Glass, D.J., eds., Devonian of the world: 

International Symposium on the Devonian System, 2d, Proceedings, v. 1, p. 49–63.

Wones, D.R., 1991, Bedrock geologic map of the Orland quadrangle, 

Hancock and Penobscot Counties, Maine: U.S. Geological 

Survey Geologic Quadrangle Map GQ–1692, scale 1:62,500.

References Cited
50 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians
References Cited 51

PLATES 1 AND 2

[Contact photographs of the plates in this report are available, at cost, from the U.S. Geological Survey Library, Federal Center,

Denver, Colorado 80225]
52 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Plate 1.—Late Silurian to early Middle Devonian (late Ludlow to Eifelian) conodonts from Maine and southern Quebec, Canada. 

Scanning-electron photomicrographs. Illustrated specimens are reposited in the U.S. National Museum (USNM), Washington, D.C. 

See text fi gure 1 for geographic location (locality no.), text fi gure 4 for stratigraphic position, and table 2 for faunal assemblage, age 

assignment, biofacies, and conodont-alteration index.

Figures 1–13. Beck Pond Limestone, locality 12. USGS colln. 12518–SD. Magnifi cation, 50×.

 1–6. Ozarkodina remscheidensis eosteinhornensis (Walliser), Pa, inner (figs. 1, 3) and outer (fig. 2) lateral 

 views, Pb, M, and Sc (lateral views) elements. USNM nos. 508893–508897.

 7, 8. Dvorakia sp., Sb element, posterior and anterior views. USNM no. 508898.

 9, 10. Icriodus sp. indet., coniform Sc element, outer and inner lateral views. USNM no. 508899.

 11, 12. Decoriconus fragilis (Branson and Mehl), inner and outer lateral views. USNM no. 508900.

 13. Belodella sp., Sa element, lateral view. USNM no. 508901.

 14–18. McKenney Ponds Limestone, locality 13. USGS colln. 12515–SD.

 14, 15. Pseudooneotodus beckmanni (Bischoff and Sannemann), upper and lateral views. Magnification, 

 75×. USNM no. 508902.

 16–18. Icriodus sp. indet., I elements (incomplete), upper views. Magnifications, 40× (figs. 16, 17) and 35×

 (fi g. 18). USNM nos. 508903–508905.

 19–26. McKenney Ponds Limestone, locality 13. USGS colln. 12516–SD.

 19–24. Icriodus sp. indet., coniform elements. Magnifi cation, 40×.

 19, 20. Sa element, opposite lateral views. USNM no. 508906.

 21, 22. M? element, inner and outer lateral views. USNM no. 508907.

 23, 24. Sd element, opposite lateral views, USNM no. 508908.

 25, 26. Ozarkodina remscheidensis (Ziegler), Pa elements, inner lateral views. Magnification, 50×. USNM nos. 

 508909–508910.

 27, 28. Ozarkodina remscheidensis (Ziegler), Pa elements, inner lateral and upper views. Magnification, 50×. USNM nos. 

 508911–508912, McKenney Ponds Limestone. USGS colln. 12517–SD.

 29–31. Mountain House Wharf Limestone, locality 2. Icriodus sp. indet., I elements, upper views. Magnifi cation, 45×. 

 29. USNM no. 508913, USGS colln. 12527–SD.

 30, 31. USNM nos. 508914–508915, USGS colln. 12526–SD. 
References Cited 53
54 Migration of the Acadian Orogen and Foreland Basin Across the Northern Appalachians

Plate 2.—Late Silurian to earliest Devonian (late Ludlow to early Lochkovian) conodonts from Maine. Scanning-electron photomicrographs; 

illustrated specimens are reposited in the U.S. National Museum (USNM), Washington, D.C. See text fi gure 1 for geographic location (locality 

no.), text fi gure 4 for stratigraphic position, and table 2 for faunal assemblage, age assignment, biofacies, and conodont-alteration index.

Figures 1-5. Ripogenus Formation, locality 16. USGS colln. 12513-SD.

 1, 2. Dvorakia sp. indet., inner and outer lateral views of Sb element. Magnifi cation, 50×. USNM no. 508869.

 3, 4. Ozarkodina excavata (Branson and Mehl), inner lateral views of Sb and Sc elements. Magnification, 50×. 

 USNM nos. 508870-508871.

 5. Digyrate apparatus, inner lateral view of Sb element. Magnifi cation, 60×. USNM no. 508872.

 6-9. Parker Bog Formation, locality 14. Oulodus elegans, inner lateral views of Pa, Pb, M, and Sb elements. Magnifi cation, 

 60×. USNM nos. 508873-508876, USGS colln. 12521-SD. 

 10, 11. Parker Bog Formation, locality 14. Pedavis sp. indet., posterior and anterior views of coniform S element. Magnifi cation, 

 75×. USNM no. 508877, USGS colln. 12521-SD.

 12-20. Hardwood Mountain Formation, locality 11. USGS colln. 12523-SD.

 12. Ozarkodina excavata excavata (Branson and Mehl), Pa element, outer lateral view. Magnification, 50×. 

 USNM no. 508878.

 13-16. Corryssognathus dubius (Rhodes), Sb and Sc elements, outer and inner lateral and posterior and anterior 

 views. Magnifi cation, 50×. USNM nos. 508879-508880.

 17-20. Ozarkodina remscheidensis remscheidensis (Ziegler), Pa elements, inner and outer lateral views. Magnifi cations, 

 45× (fi gs. 17, 18) and 50× (fi gs. 19, 20). USNM nos. 508881-508882.

 21. Hardwood Mountain Formation, locality 11. Ozarkodina confl uens Branson and Mehl, Pa element, inner lateral view. 

 Magnifi cation, 35×. USNM no. 508883, USGS colln. 12524-SD.

 22-38. Square Lake Limestone, locality 23. USGS colln. 12511-SD.

 22-26. Decoriconus fragilis (Branson and Mehl). Magnifi cation, 75×.

 22, 23. Sc elements (23 lost after photography), inner lateral views, USNM no. 508884.

 24-26. Sb elements, inner and two outer lateral views of two specimens, USNM nos. 508885 and 508886.

 27-35. Belodella cf. B. resima (Philip). Magnifi cations, 60× (fi gs. 27-34) and 110× (fi g. 35).

 27, 28. M element, outer and inner lateral views, USNM no. 508887.

 29, 30, 35 Sb1 element, inner and outer lateral and posterior views. USNM no. 508888.

 31, 32. Sa element, opposite lateral views. USNM no. 508889.

 33, 34. Sc element, outer and inner lateral views. USNM no. 508890.

 36, 37. Dvorakia sp., Sc element, inner and outer lateral views. Magnifi cation, 50×. USNM no. 508891.

 38. Pseudooneotodus beckmanni (Bischoff and Sannemann), oblique lower posterior view. Magnification, 75×. USNM 

 no. 508892.
55
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Basin Across the Northern Appalachians ofMaine and Adjacent Areas

U.S. Department of the InteriorU.S. Geological Survey

OF THE ACADIAN OROGEN AND FORELAND BASIN ACROSS THE NORTHERN APPALACHIANS—U.S. Geological Survey Professional Paper 1624

Professional Paper 1624