The Many Expressions of a New England FormationLindley Hanson
Salem State College
Continent: North America
Country: United States of America
Region: New England Province, with a local focus on Central Maine
UTM coordinates and datum: none
Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy
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In undeformed or even moderately deformed regions geomorphologists are quick to recognize the distinctive topographic expression exhibited by a particular formation. Sandstone formations cap plateaus or form ridges, and predominantly shaly formations underlie gentle slopes and valleys. Formations composed largely of carbonate rock will stand tall in arid regions, but in more humid settings dissolve into lowlands or narrow valleys. Under such conditions, topography interpreted from contour maps, satellite imagery and DEMs can greatly facilitate bedrock mapping.
However, what happens in a more complexly deformed region, such as the New England Province, where formations are regionally metamorphosed, and in addition intruded by plutons of different temperatures? Under such conditions a single formation may create a variety of landscapes. Perhaps this is why the New England Province lacks the characteristic grain seen in its sister provinces in the Southern Appalachians. The Carrabassett Formation, which forms some of the highest and lowest topography in Central Maine, illustrates well how metamorphism can alter the topographic expression of a formation. Already we encounter a problem in describing the lithology. Although the Carrabassett Formation was originally muddy turbiditic shales it now ranges from slate to schist to gneiss and also hornfels depending on its metamorphic history. The term pelitic is therefore employed to describe any metamorphic rock derived from a shaly protolith.
The Carrabassett Formation is the lowermost formation of the Seboomook Group, which extends through Central Maine southeast into New Hampshire where it's known as the Littleton Formation. These predominantly pelitic (once clay-rich) strata were deposited in a deepwater basin by turbidity currents and submarine landslides (fig. 1) during the Late Silurian and Early Devonian. At this time the continental margin was flexed downward by the approaching Avalon microcontinent creating a foreland basin that migrated westward as convergence progressed. Thousands of feet of mud and muddy sand were discharged into the basin creating an internally complex deep-sea fan succession that spread along the basin axis. Lithofacies include distal pelite-rich turbidites, minor proximal quartz-rich channel sandstones, and olistostromes (intraformational debris flows)--each now displaying a slightly different, but subtle topographic expressions. Deposition ceased with the collision of the continents and termination of the Acadian Orogeny. The Carrabassett Formation was folded, regionally metamorphosed and intruded by plutons ranging from granite to gabbro. The resulting variations in texture (e.g. foliated vs. nonfoliated) and mineralogy attributed to metamorphic process and grade strongly influenced the formation's response to weathering and landscape development.
In north-central Maine, where regionally metamorphosed to greenschist facies slate the Carrabassett Formation creates rolling lowland interrupted by occasional ridges of more poorly cleaved olistostrome, and hard quartzite. On a human time scale the slate is unusually resistant to chemical weathering, which is attributed to its lack of carbonate and relatively stable mineral assemblage (principally sericite, quartz, chlorite, and minor biotite). Finely disseminated graphite (organic material) gives the slate its dark color. A roof of made of black Carrabassett slate from Monson, Maine is purported to have a lifetime of 700 years. Monuments over 150 years old still retain shallow the scratches used to guide the carver's script (fig. 2a). No doubt the durability of the slate explains why it was chosen for John F. Kennedy's monument and is used for sinks, lab and kitchen counter tops and foundation stone. Regardless of its chemical resistance, outcrops of vertically cleaved slate are highly susceptible to mechanical disintegration by freeze thaw processes when water is allowed to penetrate along cleavage planes (fig. 2b). This principal vulnerability is why the slate doesn't create a more dramatic topography.
The Carrabassett Formation also composes majestic hornfels mountains, often surrounding unspoiled crystalline lakes (fig. 3). With the exception of Mount Katahdin (discussed later) most of Maine's high peaks are hornfels formed by intrusion-related thermal metamorphism. Mountainous rims encircling irregular basins reveal where slates were contact metamorphosed by intruding Devonian plutons. The hotter the pluton, the greater its effect. So the largest hornfels mountains typically lie within the contact aeroles of gabbroic and dioritic plutons. Heat and fluids eminating from in intruding pluton recrystallized the country rock producing hornfels with a mechanically stronger interlocking texture and resistant mineral assemblage. In contrast, the more mafic mineralogy and weaker texture of the intruding pluton promoted its rapid decay and basin formation.
Examples of impressive hornfels highlands (figs. 4 and 5) include those around the south end of Moosehead Lake near Greenville, the mountains around Lake Onawa, northwest of Monson (fig. 5), and White Cap Range (partially seen on east side of fig. 4b), south of Mount Katahdin. These mountains and the lakes they surround comprise some of Maine's most spectacular landscapes, attracting both hikers and fisherman to the region.
Continuing southwest through Maine and into New Hampshire where both the intensity of regional metamorphism and depth of denudation are greater, Devonian strata were subject to high-grade regional metamorphism and intrusion by large sheet-like plutons producing complexly deformed granofels, porphryoblastic schists and gneisses, many of which are highly resistant. The sharp contrast in relief related to metamorphism becomes less pronounced. Nevertheless, many of New England's most notable peaks such as Bigelow (4,145 ft) and Sugarloaf (4.237 feet) mountains in Maine, and mounts Washington (6,288 ft), Sunapee (2,726 ft) and Monadnock (3.165 ft) in New Hampshire are composed of these Devonian turbidites.
Contrasting topographies may also be exhibited by a single pluton. In Maine, coarse plutonic rock typically underlies basins, often occupied by one or more lakes. In fact there are more lakes in Maine resting on intrusive rock than any other lithology. However, occasionally a pluton locally retains a finer, more resistant marginal facies that temporarily protects a weaker internal facies - Mount Katahdin (5,267 ft), the tallest mountain in Maine is a good example. The chilled upper margin of the the Katahdin laccolith is a resistant granophyre (the Summit facies) characterized by finely inter-grown quartz and feldspar. Once breached, the underlying coarse-grained biotite granite (Doubletop facies) is exposed and succumbs to granular disintegration. Grain boundaries in the Doubletop facies area sharp and non-interlocking resulting in a loss of cohesion once alteration progresses between mineral grains. The relative strength of these two igneous facies is responsible for the stunning 4,700 feet of relief exhibited by the Katahdin pluton.
- Bradley, D.C., and Hanson, L.S., 2002, Paleocurrent analysis of synorogenic clastic rocks within the Acadian orogen of Maine: Sedimentary Geology, v. 148, p. 425-447.
- Caldwell D.W., and Hanson L.S., 1987, Geomorphology and glacial geology of the Mount Katahdin region, north-central Maine. Centennial Field Guide Volume 5: Northeastern Section of the Geological Society of America: Vol. 5, No. 0 pp. 303-306
- Hanson, L.S., and Caldwell, D.W., 1989, The lithologic and structural controls on the geomorphology of the Mountainous Areas in north-central Maine, in R. Marvinney and R. Tucker, (Eds.), Studies in Maine Geology, V. 5, Maine Geological Survey, Augusta, Me., p. 1-21.
- Hanson, L.S., and Bradley, D. C., 1989, Sedimentary facies and tectonic interpretation of the Carrabassett Formation, north-central Maine, in R. Marvinney and R. Tucker, (Eds.), Studies in Maine Geology, V. 2, Maine Geological Survey, Augusta, Me., p. 101-125.
- Hanson, L.S., 1994b. Geology and geomorphology of the Borestone Mountain Audubon Sanctuary, Elliotsville TWP, Maine. In: Hanson, L.S. (Ed.), Guidebook to Field Trips in North-Central Maine, New England Intercollegiate Geological Conference 85th Annual Meeting, pp. 229-238.
- Hanson, L.S., 1983. Geology of rocks along the East Branch Pleasant River, Whitecap Range, central Maine. In: Caldwell, D. W. and Hanson, L.S. (Eds.), NEIGC Guidebook for the Greenville-Millinocket regions, north central Maine, New England Intercollegiate Geological Conference, 75th Annual Meeting, pp. 45-52.
- Railsback, B.L., 2007, Goldich's Weathering series explained in terms of bond strength in Fundamentals of Mineralogy and Geochemistry and e-book