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

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Piermont Allochthon 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. 1539 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/11/1539/3381505/i0016-7606-104-11-1539.pdf by guest on 28 September 2021 ALTERNATIVE INTERPRETATION AND REPLY 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 sim- ilar 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 syn- cline 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 1540 Geological Society of America Bulletin, November 1992 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/11/1539/3381505/i0016-7606-104-11-1539.pdf by guest on 28 September 2021 ALTERNATIVE INTERPRETATION AND REPLY autochthon are in contact with the lower units of Bothner. I appreciate their comments.
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