Piermont Allochthon" in the Littleton-Moosilauke Area of West-Central New Hampshire: Alternative Interpretation and Reply

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The "Piermont Allochthon" in the Littleton-Moosilauke area of west-central New Hampshire: Alternative interpretation and reply

Alternative interpretation

MARLAND P. BILLINGS Professor Emeritus, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138

ABSTRACT BACKGROUND INFORMATION The name was recorded with the Committee Several recent papers have accepted the Figure 1, a simplified version of the map by on Stratigraphic Names of the U.S. Geological idea that there is a large overthrust, the Moench (1989), may be used as a basis of this Survey (Wilmarth, 1938). Other obvious names "Piermont allochthon," in west-central New presentation. The "allochthon" of Moench and for the formation, such as Gardner, Lyman, Lis- Hampshire. In my opinion, such an over- co-workers consists largely of rocks assigned by bon, Monroe, and Hunt were preoccupied, and thrust does not exist. For the reasons given me to the Albee Formation and interpreted by Albee was therefore selected. below, I believe that stratigraphic and struc- me as lying at the base of the stratigraphic se- To conserve space, the descriptions of the tural proof of this hypothesis are lacking and quence in that area. The rocks in the "autoch- type localities for the Littleton-Moosilauke area that other interpretations can fit the known thon" of Moench and others, on the other hand, (Billings, 1935) were not repeated in my more facts as well or better. correspond in general to the upper part of the comprehensive paper (Billings, 1937, footnote stratigraphic sequence deduced by me (Billings, p. 472). Mafic dikes and sills, both pre- and INTRODUCTION 1937, Fig. 2). This sequence consists, beginning post-metamorphic, are common in the Littleton at the top, of the Lower Devonian Littleton area (Billings, 1937, PI. 1 and p. 512-517). Pre- Several recent papers (Moench and Aleini- Formation and the Silurian Fitch and Clough metamorphic soda-rhyolite dikes and sills were koff, 1987; Moench and others, 1987; Moench, formations. Unconformably beneath these is a believed to be contemporaneous with the Lower 1989 and 1990) on the Littleton, New Hamp- Middle Ordovician volcanic-slate complex. I Devonian volcanic rocks of the Littleton Forma- shire, area have described the "Piermont alloch- mapped what I considered to be the uppermost tion. The Albee Formation has also been corre- thon," a hypothetical thrust sheet in northwest- slate as the Partridge Formation and the rocks lated with the Middle Ordovician(?) Moretown ern New Hampshire and adjacent portions of beneath it as the Ammonoosuc Volcanics (Bill- Formation of Vermont (Billings, 1935, p. 98; see Vermont, Maine, and Quebec (Fig. 1). Accord- ings, 1935, 1937). I also proposed the name also Doll and others, 1961; and Hatch, 1991). ing to the authors, this thrust sheet roots 15-20 "Albee Formation" for interbedded slate and East of the Littleton area, in eastern New miles to the southeast of its present location and quartzite that I believed to underlie the Ammo- Hampshire and western Maine, a thick sequence was emplaced -400 m.y. ago during the Aca- noosuc Volcanics. Nearly all of the rocks of my of clastic sedimentary rocks lies between the Si- dian orogeny. In the most recent paper Albee Formation, however, are regarded by lurian Madrid (=Fitch) Formation and the Or- (Moench, 1990), the thrust sheet is extended Moench and co-workers as allochthonous and dovician (Hatch and others, 1983). This se- south to Orford, New Hampshire, and north- are correlated by them with Silurian units found quence, of Lower and Middle Silurian age, is eastward for many miles into the Boundary farther east in New Hampshire and in north- thousands of feet thick. It has been divided, from Mountain region on the Maine-Quebec border. western Maine. top to bottom, into the Smalls Falls, Perry Through the courtesy of John Lyons, I have had Considerable misunderstanding has arisen Mountain, and Rangeley Formations (Moench access to a Dartmouth College thesis describing concerning the type area of the Albee Forma- and Boudette, 1987, and references therein). In the thrust in the Woodsville area (Hafner- tion, which was first described (Billings, 1935, the past (Billings, 1941, 1956), I assigned var- Douglass, 1986). Robert Moench kindly sent p. 9-10) as follows. "The name Albee Formation ious ages to the corresponding pre-Littleton me a manuscript, which was prepared for the is for a group of slates and quartzites typically formations in eastern New Hampshire, but I 28th International Geological Congress in 1989, exposed on Gardner Mountain. The type local- now accept their Silurian age following Hatch that has since appeared in print (Moench, 1989). ity is that part of Gardner Mountain that lies and others (1983). Moench and his associates between Hunt Mountain and Albee Hill. . .. (Moench and others, 1987) believe that rocks of One of the best exposures is on the southeast these units, as well as certain variants of the The interpretation for which this Alternative Inter- side of Hunt Mountain, where the Lyman- Madrid and Littleton Formations, are correla- pretation is proposed may be found (see References tive with the rocks (my Albee Formation) that Cited) in Moench and Aleinikoff, 1987; Moench and Monroe Road crosses the crest of Gardner others, 1987; and Moench, 1989, 1990. Mountain." constitute the "Piermont allochthon."

Geological Society of America Bulletin, v. 104, p. 1539-1545, 3 figs., November 1992.

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tain Formation by Moench and his associates. I am not familiar with the Perry Mountain For- mation in western Maine, but Norman Hatch (1990, personal commun.) told me that it is similar to my type Albee Formation on Gardner Mountain. This, however, does not necessarily mean that they occupy the same stratigraphic position. The rocks on Gardner Mountain are also lithologically similar to those of the Ordo- vician Moretown Formation of eastern Vermont (Doll and others, 1961), a lithologic correlation that has recently been reaffirmed (Hatch, 1991). Farther north in New Hampshire and in northwestern Maine, as well as in small areas southeast of the Ammonoosuc fault in Figure 1, a slate-quartzite unit, lithologically very similar to the rocks on Gardner Mountain, has been called "Albee" in the past (Moench and Pan- kiwskyj, 1988; Green, 1964; Billings, 1937), but this unit has been reassigned to the Dead River Formation on several recent maps (Osberg and others, 1985, Lyons and others, 1986, and Moench, 1989, 1990). These "Dead River" rocks are accepted by all as pre-Silurian, and I believe they are the true correlatives of the type Albee Formation. Moench and associates regard certain areas of rock on Gardner Mountain as volcanic members of the Perry Mountain and Rangeley Forma- tions, but I regard these as intrusive rocks (Bill- ings, 1936). Moench and associates show some occurrences of the Rangeley Formation on Gardner Mountain, but Hadley (1942) assigned a Rangeley-type rock to his Piermont member of the Albee Formation. In summary, lithologic similarity is not suffi- cient evidence to correlate the rocks on Gardner Mountain with the western Maine sequence. To me they are stratigraphically beneath the Am- monoosuc Volcanics and of Ordovician age. Figure 1. Simplified geological sketch map of a part of the Littleton-Moosilauke area, based on a map by Moench (1989, Fig. 1). Solid heavy lines are faults; the Ammonoosuc fault is Structural Evidence marked "D/U." The solid dashed line marks the location of the inferred "Foster Hill fault" of Moench and others (1989), regarded by them as the sole thrust of their "Piermont allochthon." The Silurian and Devonian (SD) rocks The symbol "all" designates the supposed Silurian and Devonian rocks in the "allochthon" of northwest of the Ammonoosuc Fault near Lit- Moench and others. This is also the Albee Formation of Billings (1935, 1937). The symbols tleton, New Hampshire, are in the core of a "SD," "Oam," "OCd" and "Oh" designate the "autochthon" of Moench and others (1989). syncline. The Ammonoosuc Volcanics and Par- SD includes the Silurian (Gough and Fitch Formations) and Devonian (Littleton Formation); tridge Formation (Oam) are also in this syn- Oam includes their Quimby, Partridge, and Ammonoosuc Formations (the Partridge and cline. The Ammonoosuc-Albee contact, regard- Ammonoosuc Formations of Billings); Ocd is their Dead River Formation (regarded by Bill- ed by Moench, in this area, as the "sole thrust" ings as part of the Albee Formation); and the areas marked "Oh" are Ordovician plutonic of the Piermont allochthon, wraps around the rocks. Devonian plutonic rocks, "Dnh," postdate the emplacement of the inferred allochthon. axis of this syncline near Woodsville. The syncline here plunges northeast, implying that the Albee-Ammonoosuc boundary, their sole thrust, ALLOCHTHON? It must be integrated with structure, stratigraphic must also plunge northeast beneath the au- sequence, and paleontology (if fossils are pres- tochthon! Can the "autochthon" lie above the Lithologie Evidence ent). Rocks and even sequences may be repeated "allochthon"? in the stratigraphy. Typically the youngest units of an autochtho- Correlation by lithologie similarity is a basic Many of the rocks on Gardner Mountain nous series are the ones that lie directly beneath concept in geology, but it is insufficient by itself. have been assigned to the Silurian Perry Moun- an allochthon, so that the highest units of the

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autochthon are in contact with the lower units of Bothner. I appreciate their comments. Professor Hatch, N. L., Jr., Moench, R. H., and Lyons, J. B., 1983, Silurian-Lower Devonian stratigraphy of eastern and south-central New Hampshire— the allochthon. In the Piermont allochthon, as Lyons kindly loaned me his copy of Hafner- Extensions from western Maine: American Journal of Science, v. 283, shown by Moench, the reverse is true. Moench's p. 739-761. Douglass' thesis. Robert Moench gave me a pre- Henderson, D. M., Billings, M. P., Creasy, J., and Wood, A., J 977, Geology of cross sections are not convincing in showing liminary copy of his guidebook article for the the Crawford Notch Quadrangle, New Hampshire: New Hampshire Department of Resources and Economic Development. how this anomaly could arise. In conclusion, I International Geological Congress and gave me Lyons, J. B., Bothner, W. A., Moench, R. H., and Thompson, J. B., Jr., 1986, believe that the "allochthon" structurally under- Interim Geologic Map of New Hampshire: Durham, New Hampshire, petrographic modes of the Perry Mountain Office of State Geologist, New Hampshire, Open-File Map 86-1, scale lies the "autochthon,'* that the rocks in it are Formation. James B. Thompson, Jr., provided 1:250,000. Moench, R. H., 1989, Day 4—Metamorphic stratigraphy and structure of the stratigraphically beneath the Ammonoosuc Vol- editorial assistance during the final stages. Connecticut Valley area, Littleton to Piermont, New Hampshire, in canics, and that they can appropriately be as- Lyons, J. B., and Bothner, W. A., eds., A transect through the New England Appalachians: Washington, D.C., American Geophysical signed to an Albee Formation of probable REFERENCES CITED Union, 28th International Geological Congress, Field Trip Guidebook T162, p. 45-53. Ordovician age. Billings, M. P., 1935, Geology of the Littleton and Moosilauke quadrangles. 1990, The Piermont allochthon, northern Connecticut Valley area, New New Hampshire: New Hampshire Planning and Development England—Preliminary description and resource implications, in Slack, Commission. J. F., ed., Summary results of the Glens Falls CUSMAP project, New SUMMARY 1937, Regional metamorphism of the Littleton-Moosilauke area, New York, Vermont, and New Hampshire: U.S. Geological Survey Bul- Hampshire: Geological Society of America Bulletin, v. 48, p. 463-566. letin 1887, p. J1-J23. 1941, Structure and metamorphism in the Mt. Washington area, New Moench, R. H., and Aleinikoff, J. N., 1987, The Piermont allochthon of Hampshire: Geological Society of America Bulletin, v. 52, p. 863-936. northwestern New Hampshire: Stratigraphic and Isotopic evidence: I conclude from structural evidence that the 1956, The Geology of New Hampshire, Pan II: Bedrock Geology New Geological Society of America Abstracts with Programs, v. 19, no. 1, Hampshire Planning and Development Commission, 200 p. p. 30. "allochthon" of Moench and associates is be- Billings, M. P., and Fowler-Billings, K., 1975, Geology of the Gotham Quad- Moench, R. H., and Boudette, E. L., 1987, Stratigraphy of the Rangeley area, neath the "autochthon," and that the "alloch- rangle, New Hampshire, Maine: New Hampshire Department of Re- western Maine, in Centennial Field Guide, Northeastern Section: sources and Economic Development Bulletin, v. 6, 120 p. Boulder, Colorado, Geological Society of America, p. 273-278. thon" is stratigraphically beneath the Ammonoo- Billings, M. P., Fowler-Billings, K., Chapman, C. A., Chapman, R. W., and Moench, R. H., and Pankiwskyj, K., 1988, Geologic map of western interior Goldthwait, R. P., 1946, The Geology of the Mt. Washington Quad- Maine: U.S. Geological Survey Map 1-1692. suc Volcanics. The Albee Formation is therefore rangle, New Hampshire: New Hampshire Department of Resources and Moench, R. H„ Hafner-Douglass, K., Jährling, C., and Pyke, A., 1987, Meta- probably Middle Ordovician and correlative Economic Development, 44 p. morphic stratigraphy of the classic Littleton area, New Hampshire: Geo- Doll, C. G, Cady, W. M„ Thompson, J. B„ Jr., and Billings, M. P., 1961, logical Society of America Centennial Field Guide, Northeastern with the Dead River Formation, rather than Centennial Geologic Map of Vermont: Montpelier, Vermont, Vermont Section, v. 5, p. 247-255. correlative with the Perry Mountain and Range- Geological Survey, scale 1:250,000. Osberg, P. H., Hussey, A. M„ H, and Boone, G. M„ 1985, Bedrock Geologic Green, J. C„ 1964, Stratigraphy and structure of the Boundary Mountain Map of Maine: Augusta, Maine, Maine Geological Survey. State of ley Formations. Radiometric ages of intrusive anticline in the Errol quadrangle, New Hampshire-Maine: Geological Maine Department of Conservation, scale 1:500,000. Society of America Special Paper 77,78 p. Wilmarth, M. G., 1938, Lexicon of the geologic names of the United States: rhyolitic rocks do not date the formations they Hadley, J. B., 1942, Stratigraphy, structure, and petrology of the Mt. Cube, New U.S. Geological Survey Bulletin 896, Pt. 1, 1244 p. Hampshire: Geological Society of America Bulletin, v. 53, p. 113-176. intrude. I agree with Hatch (1991) that much, if Hafner-Douglass, K., 1986, Stratigraphy, structure, and chemical analyses of not all, of the original Albee Formation is better bedrock geology, Woodsville quadrangle, New Hampshire-Vermont (M.S. thesis]: Hanover, New Hampshire, Dartmouth College. correlated with the Moretown Formation of Hatch, N. L„ Jr.. 1991, The Moretown Formation of eastern Vermont and possible correlatives east of the Connecticut Valley trough: Geological Vermont, than with units in the Rangeley, Society of America Abstracts with Programs, v. 23, p. A42. Maine, sequence of Moench. Hatch, N. L., Jr., and Moench, R. H., 1984, Bedrock geologic map of the wilderness and roadless areas of the White Mountain National Forest, Coos, Carroll, and Grafton Counties, New Hampshire: U.S. Geological Survey Miscellaneous Field Studies Map MF-1584A. ACKNOWLEDGMENTS Hatch, N. L.. Jr., and Wall, E. R., 1986, Stratigraphy and metamorphism of the Silurian and Lower Devonian rocks of the western part of the Merri- mack Synclinorium, Pinkham Notch area, east-central New Hampshire: Drafts of this paper have been read by John Guide Book for Field Trips in south-western Maine: Lewiston, Maine, New England intercollegiate Geological Conference, Bates College, MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 5,1991 B. Lyons, Norman L. Hatch, Jr., and Wallace A. p. 138-163. MANUSCRIPT ACCEPTED JANUARY 24,1992

ROBERT H. MOENCH U.S. Geological Survey, Branch of Central Mineral Resources, M.S. 930, Federal Center, Denver, Colorado 80225

ABSTRACT formation previously mapped in the Central that the Piermont allochthon has traveled Maine trough. In the Littleton area, the al- several to many tens of kilometers relative to Careful mapping combined with routine lochthon is bounded on the west by the Mon- the southeast adjacent Bronson Hill-Boun- observation of primary geopetal features and roe fault, and on the east by the Foster Hill dary Mountains anticlinorium. For reasons supported by U-Pb zircon age determinations sole fault, which coincides with the contact given elsewhere, however, my previous in- prove the existence of the Piermont alloch- that Billings mapped between his Albee For- terpretation that the allochthon is an Acadian thon in a 200-km-long belt that includes the mation and the overlying Ordovician Ammo- thrust sheet is probably not tenable. type area of the Ordovician Albee Formation noosuc Volcanics. This contact must be a of M. P. Billings near Littleton, New Hamp- fault, because stratigraphic sequences on op- INTRODUCTION AND shire. The Albee Formation of Billings is di- posite sides face (young) toward, and are BACKGROUND vided into seven stratigraphic units that are truncated against, one another. Where ex- correlated with one Upper Ordovician(?) posed, the Foster Hill fault is a sharp, pre- I welcome Professor Billings' Alternative In- formation, one Lower Silurian(?) formation, metamorphic boundary with only local evi- terpretation of revisions that I have made of his three Silurian formations, one Upper Silu- dence of shearing. classic mapping of six decades ago in the rian(?) formation, and one Lower Devonian Regional geologic relationships indicate Littleton-Moosilauke area (Billings, 1935,

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1937). His original mapping stimulated a vast amount of research in New England that will continue, I trust, for many decades to come. The focus of this Reply is on Billings' impor- tant points. Figure 1 of Billings' Alternative In- terpretation is oversimplified from Figure 1 of Moench (1989), which, together with Figure 4 of Moench (1989) and the description of Stop 8 of Moench and others (1987), contains evidence that the contact between his Albee Formation (my units of the Piermont allochthon) and the Ammonoosuc Volcanics is a fault. I have since refined this mapping on the basis of several months of field work. The results of this work are incorporated on Figure 1 of this paper, which is simplified from part of a detailed map at a scale of 1:48,000, supported by U-Pb zircon dating (Moench and Aleinikoff, 1991b).

LITHOLOGIC EVIDENCE AND AGE

Figure 2 illustrates the stratigraphic sequence that Billings described and the revisions that I have proposed. My studies were focused on the unfossiliferous Albee Formation, Ammonoosuc Volcanics, and Partridge Formation (Billings, 1935, 1937), all of which he considers to be Ordovician in age. Billings' interpretation of the well-dated Lower Silurian Clough Quartzite, Upper Silurian Fitch Formation, and Lower Devonian Littleton Formation remains intact, except that available paleontologic data suggest that the contact between the Clough Quartzite and Fitch Formation is a disconformity. I correlate six of the seven mapped formations of the Piermont allochthon with the Upper Or- dovician(?) Quimby Formation, the Lower Silu- rian^) Greenvale Cove Formation, the Lower Silurian Rangeley Formation, the Silurian Perry Mountain and Smalls Falls Formations, and the Upper Silurian(?) Madrid Formation. The type localities of these units are near Rangeley, Maine, near the northwestern side of the Central Maine trough (Fig. 1, inset). I correlate the uppermost unit of the allochthon with the Lower L Devonian Littleton Formation, whose type lo- 3km cality is near Littleton, New Hampshire, in the area of Figure 1. Sequential identity is the essen- tial basis for these correlations; lithologic similarity is implied, of course, as are facies differences were recognized by economic geologists (Marge- Descriptions of all of the stratigraphic units of that one must expect to find. The main differ- son, 1982) before I started work in the area. the allochthon and western Maine are in ences are the thinness and relative landward These rocks host several small volcanogenic Moench and others (1987), Moench (1989, facies of the Rangeley Formation (about Vi km massive-sulfide deposits. I determined sequence 1990), Moench and Boudette (1987), and in the allochthon versus 3 km in Maine) and the by routine observation of primary geopetal fea- Moench and Pankiwskyj (1988, and references presence of metavolcanic rocks in the Perry tures mainly, but not entirely, graded bedding at therein). One example will suffice to show how Mountain and younger formations of the alloch- or across the mapped contacts. Such features I subdivided the Albee Formation in its type thon (but sparse in Maine). According to Bill- were not commonly utilized by mappers in New area on Gardner Mountain. Billings (1937, ings, all of the igneous rocks in his Albee England before the 1960s, but where fossils are p. 472) listed, but did not separate, black and Formation on Gardner Mountain are intrusive. absent, I know of no other way to do detailed green slate and quartzite of various character in Metavolcanic rocks do occur, however, and stratigraphy in complex metamorphic terranes. his description of the type formation. I found,

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quartzite at the base of the Perry Mountain Figure 1. Simplified geologic map and interpretive structure sections of a part of the Formation containing rip-up clots of black slate Littleton-Moosilauke area; modified from 1:48,000 map (Moench and Aleinikoff (1991b). derived from the underlying Rangeley Forma- Autochthonous Bronson Hill sequence: DSb, Lower Silurian dough Quartzite, Upper Silu- tion. These correlations are reinforced by the rian Fitch Formation, and Lower Devonian Littleton Formation, undivided; Oqa, Upper presence of weakly calcareous, fine-grained Ordovician unnamed upper member of Ammonoosuc Volcanics and Upper Ordovician(?) metasedimentary rocks and lead-gray slate akin, Quimby Formation undivided; Opa, Upper and Middle Ordovician main body of Ammonoosuc respectively, to the Madrid and Littleton Forma- Volcanics and Ordovician Partridge Formation, undivided. tions, in the proper sequence above the Smalls Piermont allochthon: DSp, Silurian Smalls Falls Formation, Upper Silurian(?) Madrid Falls Formation. Formation, and Lower Devonian Littleton Formation, undivided. Spr, Lower Silurian Isotopic dating of volcanic or metavolcanic Rangeley Formation and Silurian Perry Mountain Formation, undivided. rocks, where present, should be added to Bill- Intrusive rocks: Dnh, Devonian New Hampshire Plutonic Suite; Oh, Late Ordovician High- ings' statement that correlation by lithology landcroft Plutonic Suite; Oo, Late Ordovician Oliverian Plutonic Suite; Ojt, Middle Ordovi- "must be integrated with structure, stratigraphic cian Joslin Turn pluton. sequence, and paleontology if fossils are pres- Faults: MNF, Monroe; BLF, Bill Little; AF, Ammonoosuc; FHS, Foster Hill sole fault. ent." Of three U-Pb zircon ages determined by Synclines: WMS, Walker Mountain; SHBS, Salmon Hole Brook; GHS, Garnet Hill. J. N. Aleinikoff (cited in Moench, 1990, p. J2; Metamorphic facies (on A-A', Pre-Triassic Structure): GS, greenschist; E, epidote- and Moench and Aleinikoff, 1991a, 1991b) amphibolite; AA, lower amphibolite. from weakly metamorphosed felsic igneous Solid square, site of U-Pb zircon age determination (data from Aleinikoff and Moench, 1987, rocks exposed on Gardner Mountain, two repre- 1992; Lyons and others, 1986). sent slightly discordant sills, and one represents a Inset: CVT, Connecticut Valley trough; PA, Piermont allochthon; SLR, Second Lake rift of bed. The dated bed was sampled where shown in Moench and others (1992; modified from Harwood, 1969); BBA, Bronson Hill-Boundary Figure 1; it is about 4 m thick and is composed Mountains anticlinorium; CMT, Central Maine trough; STH, Silurian tectonic hinge (arrow of internally stratified, fine-grained rhyolite por- points to thicker Silurian deposits). phyry. The bed is part of an assemblage of Equal-Area Stereodiagram (lower hemisphere): dots, 48 poles to main foliation; contours weakly metamorphosed, well-stratified felsic ash are 3%, 10%, and 20% on plunge of 30 minor late folds and 3 bedding-foliation intersections and lapilli tuff; rhyolite flows; and basaltic tuff (Hafner-Douglass, 1986, PI. II; R. H. Moench unpub. observations). and flows that are interstratified with green slate and quartzite of the Perry Mountain Formation

MOENCH BILLINGS CORRE- (1989, FIG. 2, MODIFIED) (1935, 1937) LATION PIERMONT BRONSON HILL Figure 2. Stratigraphie columns and map ALLOCHTHON SEQUENCE symbols for the Littleton-Moosilauke area for LITTLETON / LITTLETON LITTLETON Figure 1 of the Alternative Interpretation of FORMATION FORMATION FORMATION Billings and Figure 1 of this Reply (modified MADRID FITCH FITCH FORMATION DSp FORMATION from Moench and others, 1987, Fig. 2). Al- FORMATION lochthonous formations in parentheses are SMALLS FALLS FORMATION mapped south of area. SD Hiatus DSb PERRY MOUNTAIN FORMATION Spr RANGELEY CLOUGH FORMATION CLOUGH QUARTZITE QUARTZITE however, that the black and green slate (with interbedded quartzite) are separable into distinc- (GREENVALE Hiatus COVE tive, mappable formations. These units eventu- FORMATION) ally resolved into a lower unit of rusty-weather- Hiatus (QUIMBY QUIMBY ing, gray-black slate with interbedded quartzite FORMATION) FORMATION (now the Rangeley Formation), a medial unit of UNNÄMED Oqa UPPER MEMBER less rusty to nonrusty green slate with inter- OF bedded quartzite and locally abundant fine- AMMONOOSUC VOLCANICS grained metavolcanic rocks (now the volcanic PARTRIDGE and sedimentary facies of the Perry Mountain PARTRIDGE FORMATION FORMATION Formation), and an upper unit of very rusty- Oam MAIN BODY OF Opa weathering black slate with minor amounts of AMMONOOSUC AMMONOOSUC VOLCANICS VOLCANICS quartzite and metavolcanic rocks (now the > Smalls Falls Formation). This is mainly a graded ALBEE all bedding sequence, but I also found thick beds of FORMATION

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on Gardner Mountain. The sample yielded an syncline deforms these folds; its axis plunges var- (Fig. 2) consist of the following stratigraphic se- upper intercept age of 414 ±4 Ma, somewhat iably from northeast to southwest, as controlled quence (Fig. 2): (1) main body of Ammonoosuc younger than I would expect for the Perry by the angles between the axial surface cleavage Volcanics and overlying black schist of Partridge Mountain Formation, but clearly Late Silurian and the older structural attitudes. Where the Formation (unit Opa); (2) unnamed upper on available recent time scales, which place the syncline crosses the bent Spr-Oqa and Oqa-Opa member of Ammonoosuc Volcanics and overly- Silurian-Devonian boundary at ca. 408 or 409 contacts northeast of Woodsville (Fig. 1), its ing black schist of Quimby Formation (unit Ma. Ages obtained from the two sills are within axial surface cleavage intersects and deforms Oqa). Where the lower contact of the unnamed error of the age obtained from the bed. Older earlier foliation and bedding. In this area, small upper member is exposed east of Swiftwater Silurian ages have been obtained from metavol- folds and lineations produced by this intersec- (Fig. 1), graded bedding clearly indicates that it canic rocks that I assign to the Perry Mountain tion plunge steeply at widely variable angles, but is conformably underlain by the Partridge For- Formation 130-150 km farther north in New mainly about 75°S35°W, as shown on the mation. At Bath, the same contact is sharp, but Hampshire (Moench, 1990, p. J13; Moench and equal-area stereodiagram (Fig. 1). This diagram the basal felsic metatuff bed of the upper Aleinikoff, 1991a). is nearly identical to Figure 44 of Hafner- member contains rip-up slivers of black slate On the sole basis of lithologic similarity, Bill- Douglass (1986) for approximately the same derived from the underlying Partridge Forma- ings strongly favors a correlation of the rocks on area. The steep measured southwesterly plunge tion. About 1 km south of Foster Hill (Fig. 1), Gardner Mountain with the Moretown Member differs from Billings' statement that the Walker the basal metatuff contains rip-up blocks of of the Missisquoi Formation, of presumed Ordo- Mountain syncline plunges northeast near black slate derived from the Partridge Forma- vician age, exposed just west of the western con- Woodsville, but it accords with data of Billings tion. Near Foster Hill, the Partridge Formation tact of the Connecticut Valley trough in (1937, p. 522, fig. 5) for the Salmon Hole Brook is absent, probably because of erosion prior to Vermont. This correlation, but not the age, is syncline, southeast of the Ammonoosuc fault, deposition of the upper member. supported by a U-Pb zircon age of 418 ± 1 Ma which I interpret as a Triassic normal fault. Ac- This subdivision of the Ammonoosuc Volcan- reported by Aleinikoff and Karabinos (1990, cording to my mapping, the Walker Mountain ics is based on mapping and is reported in the p. D2-D5) from felsic gneiss in the Barnard and Salmon Hole Brook synclines are the same, papers that Billings criticizes; it is supported by Member of the Missisquoi Formation at Bridge- as shown in sections A-A' (Fig. 1). In my opin- two new U-Pb zircon ages reported by Aleini- water, Vermont. The fact that this Silurian age is ion, therefore, Billings' conclusion that his Albee koff and Moench (1992) and by previously pub- within error of the Gardner Mountain determi- Formation lies structurally below the Ammo- lished data, which are shown on Figure 1. To nation opens the possibility that rocks now noosuc Volcanics is not valid. Structure sections summarize, whereas the upper member of the mapped in the Piermont allochthon east of the of Moench (1990, fig. 3) depict the allochthon Ammonoosuc Volcanics is no older than 444 ± Connecticut Valley trough are also west of the in the Piermont area lying above pre-Silurian 4 Ma (new determination obtained from the trough. rocks. basal metatuff at Bath), the main body is no The crucial question is whether or not Bill- younger than the 450 ± 5 Ma and 456 + 3 Ma STRUCTURAL EVIDENCE ings' Albee Formation at Gardner Mountain is ages previously reported for plutons of the High- stratigraphically overlain by the Ammonoosuc landcroft and Oliverian Plutonic Suites; these The most compelling evidence in favor of Bill- Volcanics. According to my mapping, the plutons intrude the main body but not the upper ings' interpretation is the fact that the fossilifer- Albee-Ammonoosuc contact is a fault, because member. The oldest metavolcanic rocks of the ous Silurian and Lower Devonian rocks occupy different units that I recognize in both forma- main body are probably coeval with the tona- the core of the Walker Mountain syncline, tions are truncated at the contact. Specifically, litic Joslin Turn pluton, dated at 469 ± 1.5 Ma which plunges about 45° northeast where the units Opa and Oqa of my Figure 1 define a (new determination). lower contact of unit DS (Fig. 1) is folded 5 km west-facing sequence that abuts against units Spr Although I have not found outcrops of the north of Bath (Billings, 1937, p. 520-521). If and DSp of the allochthon (Fig. 1). I have called Foster Hill fault in the area of Figure 1, the fault one is unaware of critical stratigraphic and this surface of truncation the Foster Hill sole (where exposed farther south and northeast) is structural details, shown on Figure 1 and fault and have mapped it from near Piermont to a sharp, premetamorphic boundary between the described below, it is logical to assume from the Magalloway Mountain, New Hampshire (Fig. 1, juxtaposed formations. Evidence of shearing is simple map pattern of Billings' Figure 1 that the inset). The newly recognized pre-Silurian stratig- only locally convincing, despite the necessity Devonian and Silurian rocks of the synclinal raphy of the Littleton area is basic to my of at least several tens of kilometers of core are flanked to the west and southwest by interpretation. displacement. belts of successively older rocks. I should mention that I no longer recognize According to my mapping, the Walker the Dead River Formation, shown on Billings' Mountain syncline is superimposed on a pre- Figure 1, in the Littleton-Moosilauke area. COMMENT existing, complex structural configuration. The Rocks that I previously mapped as Dead River present configuration is the result of at least two of the autochthon (Moench, 1989, Fig. 1), west Billings' inferences that the rocks of the Pier- deformations in areas underlain by Silurian and of Bath, New Hampshire, I now assign to the mont allochthon are Ordovician in age and that Lower Devonian rocks, and three deformations Rangeley and Perry Mountain Formations of they lie stratigraphically below the Ordovician in areas underlain by Ordovician rocks. The the allochthon, and I place the Foster Hill sole Ammonosuc Volcanics are the simplest and best lower contact of unit DSb (Figs. 1,2) is a major fault at the Rangeley-Ammonoosuc boundary. interpretations of the evidence that was available unconformity that truncates folds that deform According to my mapping, pre-Silurian rocks to him when he completed his mapping (Bill- the Ordovician rocks. The Walker Mountain of the autochthonous Bronson Hill sequence ings, 1935). The rocks lack fossils, and precise

1540 Geological Society of America Bulletin, November 1992

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isotopic dating of the kind that has greatly sup- ACKNOWLEDGMENTS Hafner-Douglass, Katrin, 1986, Stratigraphic, structural, and geochemical analyses of bedrock geology, Woodsville quadrangle. New Hampshire- ported my work did not become available until Vermont [M.S. thesis]: Hanover, New Hampshire, Dartmouth College, 117 p. very recently. Moreover, primary geopetal fea- Many thanks to Katrin Hafner-Douglass, Harwood, D. S., 1969, The Second Lake anticline—A major structure on the tures in metamorphic rocks were not routinely Christian E. Jahrling II, Anne R. Pyke (now northwest limb of the Boundary Mountains anticlinorium, northern New Hampshire, west-central Maine, and adjacent Quebec: U.S. Geo- observed and utilized by mappers in New Eng- Peel), and Jon Marc Staude for their able assist- logical Survey Professional Paper 650-D, p. D100-D115. Lyons, J. B., Aleinikoff, J. N., and Zartman, R. E., 1986, Uranium-lead- land much before the 1960s. Further, new ance during various stages of the mapping and to thorium-ages of the Highlandcroft Plutonic Suite, northern New Eng- topographic quadrangle maps at 1:24,000 have Wallace A. Bothner, John B. Lyons, Sarah S. land: American Journal of Science, v. 286, p. 489-509. Margeson, G. B., 1982, Iron-rich rocks of Gardner Mountain, New Hampshire, greatly facilitated stratigraphic subdivisions that Moench and Arthur G. Sylvester for their help- and their significance to base metal distribution [M.S. thesis]: London, Ontario, University of Western Ontario, 184 p. would have been tedious and perhaps impossi- ful advice and critical comments concerning Moench, R. H., 1989, Metamorphic stratigraphy and structure of the Piermont ble on the old 1:62,500 maps. Finally, because this Reply. I take full responsibility, however, allochthon, Connecticut Valley area, Littleton to Piermont, New Hamp- shire, Day 4 in Lyons, J. B„ and Bothner, W. A., IGC Field Trip the western Maine sequence was unknown until for the whole Reply. From 1983 to 1988, my TI62—Transect across the New England Appalachians: American Geophysical Union, 28th International Geological Congress, Washing- three decades after Billings completed his work, work in the Littleton-Moosilauke-Piermont area ton, D.C., Guidebook, p. 45-53. he had no reason to suspect that his Albee For- 1990, The Piermont allochthon, northern Connecticut Valley area. New was done as part of the Sherbrooke-Lewiston England—Preliminary description and resource implications: US. Geo- mation was anything other than Ordovician or and Glens Falls projects of the Conterminous logical Survey Bulletin 1887, p. J-l-J-23. Moench, R. H., and AleinikofT, J. N., 1991a, The Piermont allochthon of older. Despite my own long experience in United States Mineral Assessment Program of northern New England -A displaced remnant of a post-Taconian ex- Maine, I worked several years in New Hamp- tensional sub-basin marginal to the Central Maine trough: Geological the U.S. Geological Survey. Since then, my field Society of America Abstracts with Programs, v. 23, no. 1, p. 106. shire before I began to recognize the western expenses have been partly defrayed by the U.S. 1991b, Geologic map of the Littleton-Moosiluake-Piermont area, NH- Maine sequence in the strike belt of Gardner VT—Type area of the Piermont allochthon: Geological Society of Geological Survey and the New Hampshire America Abstracts with Programs, v. 23, no. 1, p. 106. Mountain. Moench, R. H., and Boudette, E. L., 1987, Stratigraphy of the Rangeley area, Geological Survey. western Maine, in Centennial Field Guide, Northeastern Section: Evidence given in my previous papers indi- Boulder, Colorado, Geological Society of America, p. 273-278, Moench, R. H., and Pankiwskyj, K. A., 1988, Geologic map of western interior cates that the Piermont allochthon has traveled REFERENCES CITED Maine; with contributions by G. M. Boone, E. L. Boudette, Allan Lud- man, W. R. Newell, and T. L Vehrs: U.S. Geological Survey Miscel- several to many tens of kilometers relative to the Aleinikoff. J. N„ and Karabinos, Paul, 1990, Zircon U-Pb data for the More- laneous Investigations Series Map 1-1692, scale 1:250,000 (includes southeast-adjacent Bronson Hill-Boundary town and Barnard Volcanic Members of the Missisquoi Formation and 21 p. pamphlet). a dike cutting the Standing Pond Volcanics, southeastern Vermont: U.S. Moench, R. H„ Hafner-Douglass, Katrin, Jahrling, C. E. II, and Pyke, A. R., Mountains anticlinorium (Fig. 1, inset). For rea- Geological Survey Bulletin 1887, p. D-l-D-10. 1987, Metamorphic stratigraphy of the classic Littleton area. New Aleinikoff, J. N„ and Moench, R. H„ 1987, U-Pb geochronoiogy and Pb Hampshire, in Centennial Field Guide, Northeastern Section: Boulder, sons given in Moench and others (1992), how- isotopic systematics of plutonic rocks in northern New Hampshire: Colorado, Geological Society of America, p. 247-256. ever, the allochthon may not be greatly alloch- Geological Society of America Abstracts with Programs, v. 19, no. 1, Moench, R. H., Bothner, W. A., Marvinney, R. G., and Pollock, S. G„ 1992, p. 1. The Second Lake rift, northern New England—Possible resolution of thonous relative to the northwest-adjacent 1992, U-Pb zircon ages of the Ordovician Ammonoosuc Volcanics and the Piermont allochthon-Frontenac Formation problem: Geological related plutons near Littleton and Milan, New Hampshire: Geological Society of America Abstracts with Programs, v. 24, no. 3, p. 63. Connecticut Valley trough, and my previous in- Society of America Abstracts with Programs, v. 24, no. 3, p. 2. terpretation that the allochthon is an Acadian Billings, M. P., 1935, Geology of the Littleton and Moosilauke quadrangles, New Hampshire: Concord, New Hampshire Planning and Develop- thrust sheet that is bounded on all sides by major ment Commission, 51 p. 1937, Regional metamorphism of the Littleton-Moosilauke area, New MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 15,1992 faults is probably not tenable. Hampshire: Geological Society of America Bulletin, v. 48, p. 463-566. MANUSCRIPT ACCEITFD JANUARY 24, 1992

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1540 Geological Society of America Bulletin, November 1992

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