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Structures in the Dog River Fault Zone Between Northfield and Montpelier, Vermont

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University of New Hampshire University of New Hampshire Scholars' Repository New England Intercollegiate Geological NEIGC Trips Excursion Collection 1-1-1987 Structures in the Dog River Fault Zone between Northfield and Montpelier, Vermont Westerman, David S. Follow this and additional works at: https://scholars.unh.edu/neigc_trips Recommended Citation Westerman, David S., "Structures in the Dog River Fault Zone between Northfield and Montpelier, Vermont" (1987). NEIGC Trips. 413. https://scholars.unh.edu/neigc_trips/413 This Text is brought to you for free and open access by the New England Intercollegiate Geological Excursion Collection at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in NEIGC Trips by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact [email protected]. A -6 STRUCTURES IN THE DOG RIVER FAULT ZONE BETWEEN NORTHFIELD AND MONTPELIER, VERMONT David S. Westerman Department of Earth Science Norwich University Northfield, Vermont 05663 INTRODUCTION The purposes of this trip are to examine the lithologic and structural character of rocks exposed in the Dog River fault zone (DRFZ) along a traverse between Northfield and Montpelier, Vermont, and in the process to better understand their tectonic significance. The rocks exposed within the DRFZ have long been recognized as significant from a stratigraphic standpoint, but recognition of their structural character is recent. This author has done detailed mapping along only a 10-mile section of the zone, but reconnaissance studies suggest that its structural character is similar north to the Quebec border and south as far as Randolph, Vermont, a distance of about 80 miles. Support for this work has come from the Vermont State Geologist (Dr. Charles A. Ratte) and from Norwich University. t» The location of the DRFZ (Figure 1) corresponds with a previously identified geologic entity to which several names have been applied. These include the # "Richardson Memorial Contact" (RMC), a pre-Silurian/ Silurian boundary, the Green Mountain Anticlinorium/Connecticut Valley-Gaspe Synclinorium transition, and the Taconian Line (TL). For a variety of reasons, none of these names is entirely appropriate for the regional structure to which they all refer. The "Richardson Memorial Contact (RMCl: Geologists working in Vermont during the past several decades have recognized that Richardson and his associates had identified a geologic boundary which could be traced the entire length of the State (and beyond). For purposes of informal communication, this boundary has been referred to as the RMC (Richardson Memorial Contact) with no implication as to the nature of the boundary. Rocks on the western side of the boundary were designated by Doll and others (1961) as the Missisquoi Formation of Ordovician age, but Richardson and Camp (1919) considered these rocks to be of Cambrian age. East of these "Cambrian" rocks, Richardson (1919) reported the identification by Dr. Rudolf Ruedemann, State Paleontologist, Albany, N.Y., of numerous graptolites from rocks of the Northfield and Waits River Formations. The ages of these fossils were reported to be Lower to Middle Ordovician. Richardson's mapping led him to conclude that this boundary was an erosional unconformity. Certainly, his identification of what he considered a basal conglomerate in the Shaw Mountain 109 A-6 20 Ml 0 L. 20 KM Figure 1. Tectonic map of northern Vermont. Western half after Stanley and Ratcliffe (1985). TK=Taconic klippe; Y=Lincoln Mountain and Green Mountain basement; RMC=Richardson Memorial Contact; DRFZ=Dog River Fault Zone; TL=Taconian Line (after Hatch, 1982); ML=Monroe Line 110 A-6 Formation of Currier and Jahns (1941) strongly influenced his conclusion that this boundary was an erosional unconformity. The primary problem with calling this boundary a "contact" is the implication of lithologic constancy along one side or the other, whereas formations on opposite sides of the RMC change frequently as a result of tectonic imbrication. Locally, conglomerates and crinoid-bearing, quartz-rich calcareous sandstones of the Shaw Mountain Formation are in fault contact with various rocks of the Missisquoi Formation, but most often the contact is between the dark gray slates and phyllites of the Northfield Formation and the wide variety of units of the Missisquoi Formation. Locally rocks of the Waits River Formation occur in contact with Missisquoi units. All of these contacts are also invariably fault contacts, but where the conglomerates are absent, the fine-grained rocks lack the lithologic basis for assuming that they rest on an erosional unconformity. Instead, rocks of the Northfield Formation appear to represent a distal facies of the Waits River Formation, often containing turbiditic, fine-grained graded beds. Intraformational conglomeratic horizons occur within rocks of the Northfield Formation, probably representing submarine slide conglomerates, but they do not appear to be localized along the western margin. Pre-Silurian/Silurian Boundary: Currier and Jahns (1941) reported the discovery of crinoidal limestone in the Shaw Mountain Formation near Northfield. They concluded at that time that the rocks were of Middle Ordovician age, but subsequent, more detailed studies have shown them to be most probably of Middle Silurian age (Boucot and Thompson, 1963; Doll, 1984). These determinations are supported by recent conodont studies (Denkler, K.E. and Harris, A.G., written comm., 1986). They identified 3 Pa element fragments of Ozarkodina sp. indet. of Silurian-Early Devonian morphotype and 1 P, 1 Sb, & 1 Sc element fragments of an oulodontiform apparatus from a 73.2 kg sample collected on Winch Hill Road off Route 12A just south of Lover's Lane (see Stop 10, this trip). The interpretation of Currier and Jahns (1941) that the eastern Vermont stratigraphic section started with the Shaw Mountain Formation above an unconformity and continued up to the east with the Northfield Formation and the Waits River Formation, was concurred with by others and was adopted on the Centennial Geologic Map of Vermont (Doll and others, 1961). The recognition of the Silurian age of the fossils from the Shaw Mountain Formation had been made by the time of publication of that map. By accepting the assumption regarding the stratigraphic section and by recognizing the age of the Shaw Mountain Formation, it became widely popular to refer to the RMC as a pre-Silurian/Silurian boundary. The assignment of an Ordovician age to rocks of the Missisquoi Formation (includes the Moretown Formation of Cady (1956)) on the western side of the RMC is very tenuously tied to a long distance correlation with fossiliferous rocks in Quebec. At this stage there is no reason to assume that an age assignment of 111 ► A -6 Ordovician is incorrect, but it is important to remember that it is an assumption, rather than an established fact. Regarding the age of the rocks of the Shaw Mountain Formation, all evidence suggests that the assignment of a Middle Silurian age is correct. The problem with referring to the name of the boundary in question using ages of rocks stems from newly reported graptolite data (Bothner and Berry, 1985; Bothner and Finney, 1986) which suggests that at least parts of the Northfield, Waits River and Gile Mountain Formations are of Ordovician age. If these age determinations hold up to further scrutiny, then the discontinuous lenses of the Middle Silurian Shaw Mountain Formation may be sandwiched between packages of older rocks and the RMC would only very locally be a pre-Silurian/ Silurian boundary. Green Mountain Anticlinorium/Connecticut Valley-Gaspe Synclinorium Transition: Cady (1960a, 1960b) and Doll and others (1961) popularized the notions that much of the western half of Vermont is anticlinorial in character (with the Precambrian basement of the Green Mountain massif having been uplifted and its previously continuous cover removed by erosion), while the eastern half of the State is synclinorial in character (a folded basin presumably resting on the same basement as is exposed to the west). Stanley and Ratcliffe (1985) have quite successfully replaced the simplified notion of a breached anticlinal structure with an elegant model in which the dominant structures are westward directed thrusts (Figure 1), but no such solution has been developed for the complex structures within the Connecticut Valley trough. The sedimentalogical and structural history of the Connecticut Valley rocks is still uncertain. Hatch (1986; this volume) has suggested that the pelitic rocks in the section (including the Northfield Formation and the Meetinghouse Slate) may be the youngest. This would create a situation in which the rocks in the core of the belt are oldest and those on the margins are youngest - an anticlinorial structure rather than synclinorial. Both the western margin (Westerman, 1985) and the eastern margin (Hatch, 1985; this volume) of the CV trough are recognized as fault zones, but the senses and extents of motion in these zones remain to be fully studied. A The Taconian Line: Cady (1960b) described the Connecticut Valley Syn­ clinorium as being bounded on its western margin by the Taconian unconformity which would correspond to what has been described above as the RMC, a pre-Sil/Sil boundary and the GMA/CV-GS transition. Cady concluded from his studies that the rocks west of the boundary had experienced the Taconic orogeny during Ordovician time while the rocks east of the line, being Silurian or younger in age, had not experienced that orogeny. Hatch (1982) used stratigraphic, structural and metamorphic bases to define this line which he named the Taconian Line. Hatch's work preceeded the recent reports of Ordovician graptolites in rocks from southeastern Quebec within the CV-GS (Bothner and Berry, 1985) and from the Montpelier area in the Northfield and Waits River Formations (Bothner and 112 A -6 Finney, 1986). It also preceeded recognition of the extent of strain which has occurred along that line in central Vermont.
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