|A Brief Summary of New England (Massachusetts) Geology
The state of Massachusetts and adjacent New York contain a nearly complete section across the northern Appalachians. Massachusetts appears to be broadly composed from rocks related to at least four tectonic plates that were active in the Paleozoic (from west to east: Laurentia, Medial New England, Avalon, Gondwana). Structural and metamorphic features are mainly the product of five recognized orogenic events, the Late Ordovician Taconic, the Late Silurian to Early Devonian Acadian (420-385 Ma), the Late Devonian to Early Mississippian Quaboagian (370-350 Ma), the Late Pennsylvanian Northfieldian (305-285 Ma), and the Permian Alleghenian (270-260 Ma) (Robinson et al., 1998). These rocks were further modified during Mesozoic extension associated with the opening of the modern Atlantic, involving faulting with vertical displacements as great as 8 km (i.e. Connecticut Valley Border Fault).
The western part of Massachusetts consists of rocks that formed on or near the margins of Laurentia in Proterozoic through Ordovician time. Evidence indicates that Laurentia occupied an equatorial position in the early Paleozoic when a thick sequence of marginal clastic rocks, carbonates, and deep-water shales were deposited. The Late Ordovician Taconic orogeny is generally thought to be associated with the closing of an ocean between Laurentia and “Medial New England”, and the collision of one or more arcs with Laurentia. The Devonian Acadian orogeny generally represents the closing of a second sedimentary and oceanic belt between the newly amalgamated Laurentia-Medial New England complex and the Avalon plate. Subsequent Late Paleozoic structural and metamorphic events are thought to be broadly related to collision of Gondwana with the previously amalgamated Laurentia-Medial New England-Avalon terranes.
The most traditional model for the Taconic orogeny involves east-dipping subduction (present-day coordinates) and the eventual collision between Laurentia and the Bronson Hill magmatic arc (Figure 1) (Stanley and Radcliff, 1985). Alternatively, Karabinos et al. (1998) suggested that the collision of an older arc, the Shelburne Falls arc, was responsible for the Taconic orogeny, and that after subduction reversal, the Andean-type Bronson Hill arc formed above a west-dipping subduction zone. For the present field trips, we will see rocks of the Rowe-Hawley zone (east to west: Hawley, Moretown, and Rowe Formations), believed to represent (in either model) the collision zone (suture?) of the Taconic orogeny. The Hawley Formation consists of arc-related volcanic and plutonic rocks (representing part of the Shelburne Falls arc?). Rocks of the Moretown Formation are believed to have formed adjacent to the Shelburne Falls (or Bronson Hill) arc at some distance from Laurentia. The Rowe Formation includes a variety of tectonites that may have formed within the trench.
The Connecticut Valley trough (east of the Rowe-Hawley zone) is composed of Silurian to Devonian metasedimentary rocks, deposited nonconformably on the Taconic arc rocks and suture, and deformed during the Acadian orogeny. Acadian deformation in this part of New England occurred between 390 and 370 Ma (Karabinos, 2003). According to the model of Karabinos et al. (1998), the trough may represent an extensional basin above the west-dipping subduction system. The Goshen Formation occupies the western part of the Connecticut Valley trough. Deformation recorded in this unit may represent the closing of the extensional basin during Acadian crustal shortening.
Rocks east of the Connecticut Valley trough consist of the Precambrian-Paleozoic Bronson Hill sequence deformed around the Pelham gneiss dome. The oldest rocks that core the gneiss dome include the ca. 600 Ma Poplar Mountain and Dry Hill gneisses (Fig. 5). These rocks are overlain by metamorphosed Ordovician to Devonian volcanic, sedimentary and plutonic rocks of the Bronson Hill sequence, including the Ordovician Monson gneiss and Littleton schist. The nature and timing of events that led to the development of the Pelham and other gneiss domes are issues of active research. Precambrian through Ordovician rocks certainly record Taconic tectonism, but all rocks are strongly overprinted by tectonism during the Acadian (420-385 Ma), Quaboagian (370-350 Ma), and Northfieldian (305-285 Ma) events (Robinson et al., 1998). These events involved early west-directed thrusting and nappe development, subsequent east-directed “back-folding”, and later dome formation. Together, these events probably represent convergence and progressive shearing between Gondwana and Laurentia preceding the final assembly of Pangaea. The culminating Alleghenian (270-260 Ma) orogeny does not seem to have strongly affected rocks of western Massachusetts (Robinson, 2003).
The Mesozoic Deerfield Basin lies north of the Hartford Basin, separated by the Amherst Inlier (see Wise, 1988). The most prominent marker in the Deerfield Basin is the lower Jurassic Deerfield Basalt, a flow separating the Triassic Sugarloaf Arkose from the Jurassic Turners Falls Sandstone. The main basin-controlling fault, the Eastern Border Fault, occurs approximately 6 km east of Turners Falls and traces the western limb of the Pelham gneiss dome. The present fill pf the Deerfield Basin probably represents the very bottom of a thick basin-fill sequence and a number of exposures of the basal unconformity can be seen in the area. The final events that shaped the fundamental landscape of western New England include ca.18 Ka glaciation and the subsequent filling and drainage of Glacial Lake Hitchcock (c.f. Brigham-Grette and Rittenour, 2003).
A Bit of History of the Turners Falls Area
(Mainly reprinted from Wise, 1988)
Prior to white occupation, this area was the home of the Pocumtuck Indians, a relatively peaceful group of the Algonquin peoples. The large falls of the Connecticut River constituted a ideal spot for fishing and the adjacent terraces provided good land for farming. Many artifacts have been found in the area.
Adrian Black first sailed up the lower Connecticut River in 1614 and by the 1630’s, William Pyncheon had established a navigation system along the river with towns beginning to be established at the expense of Indian lands. Farther south in Connecticut, some of the newly founded towns were attacked by Indians. The settlers retaliated by massacring 600 Pequot Indians near Mystic in 1637. However, in this area, the Pecumtucks had relatively good relations with the settlers and even sent canoes loaded with corn down river in 1638 to help some of the towns impoverished by the Pequot War. Later, the Pecumtucks had a misunderstanding with the Mohawks. The result was the destruction of much of the tribe in 1664.
Finally in 1675 an Indian uprising known as King Phillip’s War brought local feelings to a boil. In the late spring, 1676, a group of about 300 Pecumtucks were camped on the terrace at the north end of the present Turners Falls bridge. Subsequently, about 80 head of cattle disappeared from the area to the southwest. To avenge this, a band of 142 settlers led by Captain William Turner of Hatfield rode to the west side of the basalt ridge (in the field trip area). Shortly before dawn on May 17, 1676, this group left their horses just west of the ridge to sneak across the Falls River and over the ridge near the parking lot. The Indians were all asleep from a feast the night before and had no sentries posted. The attack killed most of the 300 inhabitants of the camp, a few trying to escape over the falls by canoe or swimming. Only one settler was lost.
The victorious party headed south very quickly in poor array, possibly because of a rumor of the approach of King Phillip and 1000 of his warriors. Captain Turner foolishly allowed the force to break into several small groups, which were attacked separately within a few miles by Indian survivors and by Indians from surrounding villages. About 40 of the settlers were killed in retreat, including Captain Turner as he was crossing the Green River in present day Greenfield. His second in commend, a Captain Samuel Holyoke of Springfield, finally established order and moved the survivors back to the safety of the stockade in Hatfield.
A tombstone-like monument at the north end of the bridge marks the site of the massacre and the events whereby Turners Falls got its present name.
Brigham-Grette, J. & Rittenour, T., 2003. Late Wisconsinan glacial history of the Connecticut River Valley and a new drainage history of Glacial Lake Hitchcock: varves, landforms, and stratigraphy. New Englan Intercollegiate Geological Conference Guidebook, 95, B6-1 - B6-34.
Goldstein, A. G., 1976. Brittle fracture history of the Montague Basin, north-central Massachusetts. Department of Geosciences, University of Massahchusetts Publication Series, 25.
Karabinos, P., Samson, S. D., Hepburn, J. C. & Stoll, H. M., 1998. Taconian orogeny in the New England Appalachians: Collision between Laurentia and Shelburne Falls arc. Geology, 26(3), 215-218.
Karabinos, P., Stoll, H. M. & Hepburn, J. C., 2003. The Shelburne Falls arc - Lost arc of the Taconian orogeny. New Englan Intercollegiate Geological Conference Guidebook, 95, B3-1 - B3-17.
Robinson, P., 2003. Tectonic-stratigraphic-metamorphic perspective of the New England Caledonides, west-central Massachusetts. New Englan Intercollegiate Geological Conference Guidebook, 95, A1-1 - A!-54.
Robinson, P., Tucker, R. D., Bradley, D., Berry IV, H. N. & Osberg, P. H., 1998. Paleozoic orogens in New England, USA.
Robinson, P., Tucker, R. D., Gromet, P. L., Ashenden, D. D., Williams, M. L., Reed, R. & Peterson, V. L., 1992. The Pelham dome, central Massachusetts: stratigraphy, geochronology, structure and metamorphism.
Stanley, R. S. & M., R. n., 1985. Tectonic synthesis of the Taconian orogeny in western New England. Geological Society of America Bulletin, 96(10), 1227-1250.
Suppe, J., 1985. Principles of Structural Geology. Prentice Hall, Inc., Englewood Cliffs, New Jersey.
Wise, D. U., 1988. Mesozoic stress history of the upper Connicut Valley at Turners Falls, Massachusetts. New Englan Intercollegiate Geological Conference Guidebook, 80, 351-372.
Wise, D. U. & Hubert, J. F., 2003. Evolving fault types, stress fields, and tectonics of the Early Mesozoic Deerfield Basin of Massachusetts. New Englan Intercollegiate Geological Conference Guidebook, 95, C5-1-C5-30.