Vignettes > The many expressions of a New England formation
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The Many Expressions of a New England Formation

Lindley Hanson
Salem State College


Location

Continent: North America
Country: United States of America
Region: New England Province, with a local focus on Central Maine
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy


Click the images for a full-sized view.

The Carrabassett Formation is comprised of pelitic turbidites and olistostromes (mostly slump and debris flows) deposited in an actively migrating foredeep during the Acadian Orogeny. Image by Lindley Hanson.


A. Monument near Brownville Maine made of slate from the Carrabassett Formation. After 160 years the lettering and guidelines are still sharp attesting to this rock's resistance to chemical weathering. B. Slate ledge (approximately 9 feet high) a mile south of Greenville, Maine. The slate breaks along cleavages where it is vulnerable to free-thaw processes. This is the same formation that has been contact metamorphosed by the Moxie Pluton further north and forms the mountains around Moosehead Lake. Image by Lindley Hanson.


The gabbro is composed of unstable minerals (plagioclase, pyroxene, olivine and amphibole) and easily succumbs to granular disintegration. The interlocking texture and more stable mineral assemblage of the hornfels (quartz, K-feldpar, plagioclase, mica, cordierite and andalusite), makes it more resistant than both the plutonic rock that created it and the slate from which it was derived. Image by Lindley Hanson.


A. Digital elevation model (DEM) and geology of the Greenville area. The Carrabassett Formation is shown in gray and the Moxie pluton in purple. All of these mountains are hornfels, with the exception Big Squaw, which is largely injection hornfels mingling with a fine-grain border facies of the Moxie pluton. Geophysical work and lineations in the Moxie indicate that the intrusion dips southeast, explaining the asymmetry of the contact aerole. Beyond the contact aerole the lowlands are underlain by the Carrabasset slate (gray) and pre-Devonian metasedimentary rocks (yellow). Sites 1 (hornfels) and 2 (gabbro) locate the exposures pictured in the previous figure. B. Northeast view from the slope of Big Squaw Mountain showing the hornfels mountains on the other side of Moosehead Lake. The southern part of Moosehead lake is underlain by the Moxie pluton. Image by Lindley Hanson.


Hornfels rim encircling the Onawa pluton (purple) southeast of Greenville. The Carabassett Formation comprises the hornfels mountains and the lowlands beyond the contact areole. The Appalachian trail (not shown) treks northeast across Wilson Stream and along the hornfels ridge containing Barren, Fourth, Third and Columbus mountains. From there it moves north into the White Cap Range, also hornfels, and onto Mount Katahdin where it ends or begins. The near vertical cliffs around the basin coupled with the uniformity of the contact aerole suggest that the Onawa pluton is an elongate stock with steep margins. Image by Lindley Hanson.


Description

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.

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