Bedrock Geologic History of Maine
Robert G. Marvinney
Maine Geological Survey
Department of Conservation
99 State House Station
Augusta, Maine 04333
The geologic history that is recorded in Maine's bedrock covers more than half a billion years. Over this period of time a variety of geologic processes including erosion and sedimentation, mountain-building, deformation (folding and faulting), metamorphism, and igneous activity, have acted to produce the complex bedrock geology that we see today. Investigations ranging from basic geologic mapping to sophisticated deep-seismic reflection studies continue to improve our understanding of Maine's geology, so this summary can only be a description of our present interpretation of the state's geologic history, an interpretation which has already been significantly revised since an earlier version of this report (Loiselle and Thompson, 1987).
The large geologic wall map of the state (Osberg and others, 1985) shows hundreds of bedrock formations and igneous intrusions distinguished on the basis of age and rock type. For the purpose of this summary, these rocks have been grouped into nine major units shown in Figure 1. Eight groups of stratified rocks (layered rocks, including both sedimentary and volcanic rocks) are differentiated here. These are grouped on the basis of their age and possible place of origin. The ninth unit comprises all the major plutons in the state. Plutons are igneous rocks that form when magma (molten rock) cools and solidifies at depth. Maine plutons range in age from Ordovician through Cretaceous (500 to 65 million years old), with the preponderance being Late Silurian to Devonian (430 to 360 million years) in age. Each of the major rock groups will be discussed briefly in the following summary. Additionally, a condensed geologic history is presented in Table 1.
TABLE 1. SUMMARY OF GEOLOGIC HISTORY IN MAINE
|Age (in millions of years ago)*||Geologic Time||Event|
|present-10,000 years||Recent||Rebound of land surface to current level. Gradual sea level rise, continuing to present. Ongoing low-level seismic activity.|
|0.01-1.6||Pleistocene||Late Pleistocene: The last ice sheet depressed the crust up to 425 feet along coastal Maine. Major marine transgression followed glacial recession. Pre-existing surface water drainage and ground water flow patterns were significantly altered.
Early Pleistocene: Several periods of continental glaciation eroded the bedrock and deposited glacial sediments.
|1.6-66||Tertiary||Continued uplift and erosion of the Northern Appalachian Mountains. Stress release during uplift and erosion produces numerous fractures in the bedrock.|
|66-245||Late Mesozoic: Continued widening of the Atlantic Ocean. Limited igneous activity (southern Maine). Faulting and fracturing?
Early Mesozoic: Combined Europe and Africa rift apart from North America, opening the modern Atlantic Ocean. Faulting and fracturing of existing bedrock. Intrusion of mafic dikes (southwestern Maine).
|LATE PALEOZOIC ERA|
|245-286||Permian||Continued uplift and erosion of the Northern Appalachian Mountains.|
|286-360||Carboniferous||Intrusion of Sebago pluton. Last regional metamorphism and deformation (southwestern Maine). Transcurrent faulting (southern and eastern Maine).|
|MIDDLE PALEOZOIC ERA|
|360-417||Devonian||Middle and Late Devonian: Acadian orogeny Major episode of deformation and metamorphism caused by collision of Avalon microcontinent with North America. Most Maine rocks were affected. Burial of sediments in southwestern Maine to depths greater than 9 miles. Final development of ancestral Northern Appalachian Mountains. Major period of widespread igneous activity during and after mountain-building episode.
Early Devonian: Youngest sediments deposited prior to major mountain-building event.
|417-443||Silurian||Continued deposition in ancestral Atlantic Ocean (central Maine). Limited rifting apart of North American continental margin (northern Maine). Explosive volcanism and intrusion (east-coastal Maine).|
|EARLY PALEOZOIC ERA|
|443-495||Ordovician||Late Ordovician: Subsidence and re-initiation of sediment deposition following deformation and uplift.
Middle Ordovician: Taconic orogeny Deformation, uplift, and igneous activity related to the collision of several(?) offshore volcanic island arcs with North America.
Early Ordovician: Continued deposition of sediments and volcanic activity.
|495-545||Cambrian||Late Cambrian: Penobscottian orogeny Episode of deformation and metamorphism found in northwestern to north-central Maine attributed to microplate collision within ancestral Atlantic Ocean.
Sedimentation and limited volcanic activity in ancestral Atlantic Ocean.
|LATE PROTEROZOIC ERA|
|545-650(?)||Precambrian||Latest Precambrian: Deposition of limestone and other marine sediments eroded from an ancient microcontinent bordering the ancestral Atlantic Ocean (Penobscot Bay region).
Approximately 650 m.y.: Episode of metamorphism and pegmatite intrusion in unknown geologic setting (near Islesboro).
Events prior to approximately 650 million years ago are essentially unknown. Sedimentary and volcanic rocks of the Seven Hundred Acre Island Formation are more than 650 m.y. old. Rock and mineral fragments over a billion years old may be present in the Chain Lakes massif.
*The calibration (in years) of the geologic time scale is continually under revision. The ages listed in Table 1 are taken from Palmer (1983) and Tucker and McKerrow (1995).
This summary of Maine's geologic history requires some understanding of the theory of plate tectonics. According to this theory, the crust of the Earth is believed to consist of a small number of large, semi-rigid plates, all in motion relative to one another. The motions can be divided into three basic types although actual motions rarely can be simplified to this degree: convergent motion - two plates moving toward one another; divergent motion - two plates moving away from one another; and transcurrent motion - two plates sliding past one another. These plates may be composed of thick, relatively buoyant continental crust, or thinner and more dense oceanic crust, or parts of both. The interaction of these plates, especially through collisions resulting from convergent and transcurrent motions, is responsible for the most spectacular features of Maine's geology.
Figure 2 shows how new oceanic crust forms when magma rises to the surface and solidifies at a mid-ocean ridge, a place where two plates are diverging. The oceanic crust moves away from the ridge on the diverging plates and may eventually plunge beneath another plate at a subduction zone. Volcanic island arcs such as the Aleutian Islands, or mountain ranges such as the Andes, may form at such convergent boundaries, dependent on whether the crust of the overriding plate is oceanic as in Figure 2 (Aleutians) or continental (Andes). Continental crust is carried along on these plates, but is too buoyant to be subducted when involved in a collision. Instead, it is folded and forced upward to form spectacular mountain ranges like the Himalayas. Mountain-building events such as these are termed orogenies. Rifting of continental crust may produce a new ocean basin, such as is forming today in the Red Sea region of the Middle East. Continental margins which are also plate boundaries are termed active margins (North American Pacific Coast); continental margins which are within a plate are termed passive margins (modern North American Atlantic Coast).
A further refinement of the plate tectonics theory is the concept of microplates and "exotic" terranes. Largely through work along the Pacific Coast of North America, geologists have recognized that collisional continental margins are often composed of multiple, small plate fragments, both continental and oceanic in composition, which are distinctive and unrelated to one another in terms of origin. These are termed "exotic" because they have often formed in settings quite distant from one another (this interpretation often being based on differences between fossils of the same age) and have been brought together through the processes of subduction and collision, or transcurrent motion. This concept has been applied very successfully over the past decade to the ancient collisional margin of the Appalachians. While considerable debate continues concerning the nature and exact boundaries, it is generally accepted that the geology of Maine is composed of a mosaic of distinct "terranes"* (e.g. Osberg, 1978; Zen, 1983; Berry and Osberg, 1989; Robinson and others, 1998). These terranes were widely scattered microplates in an ancient ocean, the Iapetus Ocean (very broadly defined as the predecessor of the Atlantic, although many researchers apply different names to different portions of this ocean). While in the same relative position as the present-day Atlantic between the North American and Euro-African continents, the Iapetus contained many island archipelagos of oceanic and continental character, perhaps similar to the present-day Malay Archipelago between the Asian and Australian continents. [*The term "terrane" is applied to a piece of fault-bounded crust whose geologic history differs significantly from adjacent crust. Note the different connotation (and spelling) of "terrain."]
In Early Ordovician time, the "east" margin of ancestral North America ran through rocks of what now are western Massachusetts and Vermont. The rocks that would eventually form Maine had not yet been accreted to North America, but were still parts of island chains or microcontinents in the Iapetus Ocean. One recent reconstruction for Ordovician time (van der Pluijm and others, 1995) shows that ancestral North America was rotated more than 50 degrees clockwise from its present orientation (making the present "east" margin the south margin at that time), and the coast was located in tropical waters 20 degrees south of the equator. At the same time, some of the rocks that now comprise coastal Maine were 2500 miles away on the opposite shores of the Iapetus Ocean, at 60 degrees south latitude.
The following review, by time period, of Maine's geologic history draws heavily on the microplate refinement of plate tectonic theory. In addition to the reports cited above, others providing overviews of various aspects of Maine's bedrock geology include Hussey (1988), Guidotti (1989), Ludman (1986), and Osberg and others (1989).
Precambrian History (prior to 545 million years ago) Skip to Paleozoic Preserved in Central and Northern Maine.
There are two areas of the state in which Precambrian rocks are exposed: in the Chain Lakes massif of northwestern Maine and in the vicinity of Islesboro on Penobscot Bay (Figure 1). The Chain Lakes massif (relatively rigid rock body within a mountain belt) contains a complex array of metamorphosed sedimentary and volcanic rocks long thought to include the oldest rocks in Maine. One line of evidence suggests that some rocks may be as old as 1.5 billion years, significantly older than the Precambrian rocks of the closest North American crust to the west. Such an age contrast supports the hypothesis that the Chain Lakes rocks are the ancient crust of a microplate within Iapetus which was joined to North America at a later time. Another line of evidence has shown that individual mineral grains extracted from the rocks have a variety of ages, supporting the alternative hypothesis that the rocks themselves are on the order of 600 million years old, but contain a mixture of sedimentary fragments that inherited the age of the older rocks. They suggest that the Chain Lakes massif may represent rift deposits, generated as the Iapetus basin first opened, and which formed offshore from North America. See Boone and Boudette (1989) and Trzcienski and others (1992) for further discussion.
Some sedimentary and volcanic rocks on Seven Hundred Acre Island (Figure 1) were metamorphosed and cut by a pegmatite dated at 647 ± 4 million years old (Stewart and others, 1998). The rocks which are cut by the pegmatite must be older, but we don't know by how much. Presumably, these old rocks around Islesboro represent another crustal fragment in the Iapetus. Its relationship to the purported Chain Lakes fragment is unknown, but the rocks of the two Precambrian areas are quite different, so it is likely that they had entirely different origins. Faults bounding the Islesboro rocks show evidence of significant transcurrent motion, perhaps on the order of hundreds of miles.
Early Paleozoic Events Preserved in Central and Northern Maine (545 to 443 million years ago) Skip to Paleozoic Preserved in Coastal Maine.
During the earliest Paleozoic time, Iapetus may have been dotted with crustal fragments such as those discussed above. Sometime during the Cambrian, a subduction zone with its attendant island arc of volcanic and sedimentary rocks was initiated in Iapetus. During the latest Cambrian, this island arc collided with the Chain Lakes microplate in the first generally recognized orogenic event in Maine, the Penobscottian orogeny, proposed by Neuman (1967) on the basis of relationships he found east of Mt. Katahdin, and further described for a larger region by Boone and Boudette (1989). Deformation (folding and faulting) and low-grade metamorphism associated with this event are recorded in Precambrian through Upper Cambrian and lowest Ordovician rocks throughout the central portion of the state. Rocks which formed in a subduction zone and which show the effects of this event are restricted to a relatively narrow belt in north-central Maine (unit 4, (Figure 1). Their nature and restricted distribution are evidence for a microplate collision within the Iapetus, rather than a collision with North America proper.
Following rapidly (geologically speaking) on the heels of this event was the Taconian orogeny of Middle Ordovician time (~ 450 million years ago). As originally described by Zen (1972) and Rodgers (1971), during this event the various sedimentary rocks (sandstone, shale, limestone) of the continental shelf and slope were sliced and essentially stacked up on the continental margin. The classic faulting and folding associated with this orogenic event is found in western Vermont and eastern New York - the Taconic Mountains whence is derived the name of the event. In Maine, the Cambrian through Ordovician rocks of northernmost Maine, primarily, (unit 4, (Figure 1) show the effects of this event. Most geologists recognize this event as the collision of one or more island arc terranes with the eastern margin of North America (see Drake and others, 1989; Boone and Boudette, 1989). Limited igneous activity accompanied the Taconian orogeny and several significant Ordovician plutons are included in unit 9 (Figure 1).
Early Paleozoic Events Preserved in Coastal Maine (545 to 443 million years ago)) Skip to Middle Paleozoic.
While much is known of the history of Cambrian-Ordovician rocks of central and northern Maine, relatively little is certain of contemporary rocks in coastal Maine. This is due largely to more recent high grade metamorphism of the coastal rocks which has obscured much of the evidence for their early history. A general lack of age constraints in the form of fossils or datable rocks compounds the problem. In spite of this, a distinct geologic terrane has been identified through careful mapping (unit 2, (Figure 1). It is composed of highly metamorphosed volcanic and sedimentary rocks. The tectonic origin of these units is even more speculative than that of the northern Maine rocks partly because any rocks related to subduction processes which brought these terranes together either have not been recognized or were later destroyed. Differences among the few fossils that have been found in these rocks suggest that the Lower Paleozoic rocks of the mid-coastal area were parts of oceanic islands geographically isolated from rocks of similar age in northern Maine, and in eastern Maine and neighboring New Brunswick (Neuman, 1984; Berry and Osberg, 1989). A series of Paleozoic collisions eliminated this isolation.
Uncertainty as to place of origin and mode of emplacement also extends to the Silurian and Lower Devonian volcanic rocks (440-390 million years) of coastal Maine (unit 6, (Figure 1). The character of the volcanic rocks of the eastern part of this group indicates a rifting or divergence event that occurred elsewhere along a margin of the Iapetus Ocean (Gates and Moench, 1981). Likewise, the volcanic rocks of the central coastal portion of this group have some characteristics indicative of an island arc (subduction) setting. Continuing investigation is helping to unravel the complexities of this geology. In particular, several of the igneous plutons in eastern Maine are now known to be of Silurian rather than Devonian age (e.g. West and others, 1992), which indicates that significant tectonic events occurred during this time.
The orogenic events of the Early Paleozoic caused regional uplift which led to an unknown amount of erosion of the older rocks. In Late Ordovician time there was subsidence and renewed deposition along the eastern North American margin. In fact, geologists now can demonstrate evidence in Silurian rocks for rifting or divergence of plates, which is superimposed on the convergence structures of the older rocks (see Osberg and others, 1989). The Iapetus then consisted of a narrow basin which received sediment through Silurian and Devonian times from both the east and west. These rocks are grouped together as unit 5 on Figure 1.
The Silurian and Devonian rocks throughout central Maine are characterized by sandstone and slate which were originally sediments deposited in a deep-sea setting (see for example Hanson and Bradley, 1989). That much of these rocks have an eastern source means that in the east there must have been an uplifted, mountainous area which was shedding material through erosion. Initiation of an eastern source area is interpreted by many to herald the beginning of the next and most significant orogenic episode, the Acadian orogeny. This represented a collision in the Early Devonian between North America and a very significant land mass to the east, either the combined European/african continent, or a large intervening plate, or both. The dominant structural "grain" in Maine, the northeast-southwest trending belts that characterize the distribution of rock types, is due to the Acadian orogeny. This development is reviewed in detail by Osberg and others (1989). Another important geologic feature caused by this event is the high-grade metamorphism exhibited by the rocks in southwestern and coastal Maine. Original sandstones, shales, and volcanic rocks in these regions have been metamorphosed to high-grade gneisses and in places have even melted because they were up to 9 miles beneath the mountains hurled upward in this event. The vast majority of igneous plutons in the state owe their existence to the Acadian orogeny. Guidotti (1989) provides an excellent overview of this metamorphism.
Following the Acadian orogeny in the Early Devonian, limited deposition of post-orogenic sediments occurred in scattered locales, providing evidence of geologic conditions in Middle and Late Devonian time. These scattered deposits form the last major group of stratified rocks shown on the geologic map (unit 8) and represent erosion of the mountains built during the Acadian orogeny. These rocks consist mostly of sandstones and conglomerates deposited on land. The state fossil, Pertica quadrifaria, is found in one of these post-orogenic deposits.
Igneous activity continued for some time after the main collision of North America with Europe/Africa. The Sebago pluton in southern Maine is approximately 293 million years old (Late Carboniferous) (Tomascak and others, 1996), and its intrusion was accompanied by metamorphism and deformation of adjacent rocks (Lux and Guidotti, 1985). Most of the gemstone-rich pegmatites of Oxford County are related to this thermal episode, probably formed by melting of metasedimentary country rocks (Simmons and others, 1995). Some geologists suggest that this intrusive activity is related to the final collision of Africa with North America, the event responsible for the major fold and thrust belt of the southern Appalachians. A significant episode of transcurrent faulting in mid-coastal and eastern Maine occurred at about this time.
Shortly after emplacement of the Sebago pluton, there is evidence for the inception of the present day Atlantic Ocean in the intrusion of numerous dark igneous dikes throughout coastal Maine and inland (see Swanson, 1992; McHone, 1992). Sporadic igneous activity related to this rifting and opening of the Atlantic continued throughout the Mesozoic Era. Maine's youngest rocks are Cretaceous intrusions of the White Mountain series. These are small igneous complexes in the southern part of the state.
In summary, the geologic history recorded in Maine's bedrock spans over half a billion years. Several major cycles of deposition, deformation, and igneous activity related to plate tectonic subduction and collision are responsible for the complex bedrock that we observe today. Details of these events, particularly in the pre-Silurian rocks, are often obscure and difficult to uncover, but continued geologic investigation will slowly clarify and improve our understanding of the geology of the state.
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Stewart, D. B., Tucker, R. D., Ayuso, R. A., and Lux, D. R., 1998, Tectonic setting of the Islesboro terrane and a minimum age of the Seven Hundred Acre Island Formation, the oldest rocks in Maine: Geological Society of America, Abstracts with Program, v. 30, p. 76.
Swanson, M. T., 1992, Structural sequence and tectonic significance of Mesozoic dikes in southern coastal Maine, in Puffer, J. H., and Ragland, P. C. (editors), Eastern North American Mesozoic magmatism: Geological Society of America, Special Paper 268, p. 37-62.
Tomascak, P. B., Krogstad, E. J., and Walker, R. J., 1996, U-Pb monazite geochronology of granitic rocks from Maine: implications for late Paleozoic tectonics in the northern Appalachians: Journal of Geology, v. 104, p. 185-195.
Trzcienski, W. E., Jr., Rodgers, J., and Guidotti, C. V., 1992, Alternative hypothesis for the Chain Lakes massif, Maine and Quebec: American Journal of Science, v. 292, p. 508-532.
Tucker, R. D., and McKerrow, W. S., 1995, Early Paleozoic chronology: A review in light of new U-Pb zircon ages from Newfoundland and Britain: Canadian Journal of Earth Science, v. 32, p. 368-379.
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West, D. P., Jr., Ludman, A., and Lux, D. R., 1992, Silurian age for the Pocomoonshine gabbro-diorite, southeastern Maine, and its regional implications: American Journal of Science, v. 292, p. 253-273.
Zen, E-an, 1972, The Taconide zone and the Taconic orogeny in the western part of the northern Appalachian orogen: Geological Society of America, Special Paper 135, 72 p.
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Last updated on September 24, 2012