Glacial-marine Deposits of Southern Maine

John Barker

Fall 2011 ES 767 Quaternary Geology
Dr. James S. Aber, Instructor

Table of Contents
Abstract
Introduction
Isostasy
Marine Deposits
Conclusion
References

Montegail Pond glaciomarine delta in eastern Maine, covered by blueberry fields (MGS, 2005b).


Abstract

During the Late Pleistocene there was a marine advancement in southern Maine. The advancement was initiated by a lithospheric depression, a relic of the overlying Laurentide Ice Sheet’s immense weight on the crust. The depression allowed low lying areas to be flooded. These areas allowed for the sea to interact with the retreating ice sheet and left behind an abundance of glaciomarine features characteristic of an ice-contact depositional environment. When the crust began to rebound the once subaqueous surface was exposed above sea-level. Glaciomarine features deposited throughout southern Maine include moraine belts, submarine fans, glaciomarine deltas, and the widespread Presumpscot Formation.


Introduction

The Pleistocene Epoch was approximately 2.5 million to 10,000 years ago. About 21,000 years ago the Laurentide Ice Sheet, covering northern North America, began retreating to the northwest from its maximum extent of the present day continental shelf of the Atlantic Ocean. The ice sheet reached the coast of Maine about 17,000 years ago (Borns et al., 2004). From the coast of Maine to the central part of the state a variety of glacial depositional and erosional features influenced by a retreating glacier in contact with an encroaching sea were left behind. The sea invaded the inner part of the continental region via a lithospheric depression, which was the result of the ice sheet’s immense weight, a concept known as isostasy (Aber, 2011). The ice-marginal environment led to the deposition of submarine fans, subaqueous moraine ridges, and deltas. The most distinguishable marine deposit is the widespread, fossiliferous marine silts and clays of the Presumpscot Formation (NFP, 2006). When the land finally rebounded above sea-level the sea-water was drained from the land to unveil the glacial deposits.

Maine’s landscape has been dramatically changed by the advance and retreat of great ice-sheets during the Pleistocene. These waning glaciers have left behind some of the world’s best examples of glacial-marine deposits. This report will describe the processes and the glacial features to the extent of sea encroachment, as the result of an isostatic depresssion, in the state of Maine (Figure 1).

Figure 1: Geologic map of Maine with illustrations of the ice retreat in association with a marine transgression.
Courtesy of the Maine Geological Survey, 2011.

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Isostasy

Accumulation of glacier ice on the lithospheric crust, attaining significant thickness and weight, can create a crustal depression several meters deep, a concept known as isostasy (Figure 2). These depressions are common with continental ice sheets inland and near the coast. When the glacier retreats a depression near the coast the depression is likely below sea-level and susceptible to sea invasion. These processes are responsible for the surficial geology of southern Maine. The crustal deformation in Maine during the Laurentide glaciation resulted in a large depression several hundred feet deep (MGS, 2005a). As a result of this depression a marine transgression ensued an was responsible for glacio-marine deposits in southern and central Maine. The sea reached up to the central part of Maine leaving Marine deposits 129 meters above sea-level (Figure 3).

Isostasy is a concept that describes the equilibrium between the lithosphere and the asthenosphere, on which the lithosphere floats. Equilibrium between the lithosphere and the asthenosphere refers to the mass and density of the crust and it’s buoyancy on the asthenosphere. Equilibrium is disrupted from imposed surficial disturbances such as volcanoes, river deltas, and ice-sheets, the crust then adjusts in a way to maintain an equilibrium state (Watts, 2001), by sinking into the asthenosphere. Based on seismic and gravity data these disturbances are shown to affect the outermost layers of the crust.

Isostatic equilibrium from ice-sheet loading is a slow process that takes several thousand years for the crust to completely respond to the loading (Aber, 2011). Based on the density of glacial ice, 0.9 grams per cubic meter (g/m3), and the density of asthenosphere rock, approximately 3.3 g/cm3, an ice sheet with a thickness of 1000 m could depress the crust as much as 275 m (Aber, 2011). As the crust is depressed the area around the depression may respond by rising, this is called the forebulge. Height of the forebulge depends on the on the amount of crustal loading. The forebulge may impact glacial melt-water drainage by creating proglacial lakes, seas, and restricting ice-marginal streams (Aber, 2011).

Unlike other types of loading, such as sediment loading from fluvial deltas or volcanoes, the retreat of ice sheets allows for the rebounding of the crust once the sheet is removed (Figure 4). Although crustal rebounding doesn’t take place for several thousand after glacial removal, the last major ice-sheet retreated several thousand years ago during the Pleistocene. Marine deposits are good indicators for establishing the amount of rebound that has taken place at coastal glaciated areas.

Figure 2: Illustration demonstrating isostacy.
Overbearing weight causes the crust to displace the mantle allowing the crust to depress (a).
Once the pressure of the weight is removed the mantle pushes the crust back into equilibrium (b) and (c) (Michael & Jones, 2010).

Figure 3: Map illustrating the marine transgression (dark blue) as the Laurentide Ice Sheet retreated.
Solid black isobars indicate approximate glacier positions during a certain time (MGS, 2010).

Figure 4: Isostatic rebounding in North Ameriac since 6,000 years ago.
This map shows that from the coast of Maine well into Canada the crust has rebounded 20 meters. Image adapted from Dutch, 2009.

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Marine Deposits

Moraines:

End moraines indicate the orientation of the glacier as it retreated from the area. The moraines in southern Maine are usually associated with marine deposit features. These moraines were deposited where the glacial ice, solid ground and sea were all in contact (Figure 5). Glacial sediment deposited to form subaqueous moraines was sourced from several processes: melting of the sediment-rich basal ice, sediment shoved up by the glacier, subamarine landslides, and sediment deposited by meltwater streams (MGS, 2005c). These processes created hundreds of moraines ranging from several feet to 50 feet in height, and hundreds of feet to a mile long, the largest of the moraines are in the eastern part of coastal Maine (Figure 6). These moraines are stratified and consist of glacial sand and gravel. The sand and gravel abundance serves as an important economic resource for the region (MGS, 2005c) (Figure 7).

Figure 5: Illustration showing the characterisitc ice-contact depositional environment.
The area within the "end moraine ridge" margins is where the glacial ice, solid ground, and open water
are in contact this is known as the grounding line (MGS, 2005c).

Figure 6: Sedgwick moraine ridges. Ridges like these act as previous position markers for glaciers (MGS, 2005c).

Figure 7: The Pond Ridge moraine from southeast Maine showing interfingering of coarse debris flows and marine mud.
Moraines composed of primarily of sand and gravel are important resources for the region (MGS, 2005c).

Submarine Fans:

Submarine fan deposits are the result of a subglacial meltwater tunnel discharging into standing water (Figure 8). Similar to deltas but they either lacked enough sediment to reach the water surface, or the glacier retreated to fast to accumulate sediment in time to reach the surface. Sediment make-up of submarine fans consists of thick sand and gravel accumulations that generally associated with low angled beds (Figure 9). Good examples of these submarine fans are the north trending, consecutive Sabattus Pond fan deposits in between the Pleasant Hill Delta and Marr Point Deltas(Figure 10) (MGS, 2011).

Figure 8: Illustration showing the relationship between deposits and depth as the glacier retreats from a standing water body.
Moraines and glacier to water relationship was previously discussed, fans indicate when the glacier was slightly above sea-level depositing sediment from the land (MGS,2005b).

Figure 9: Low-angled, inclined beds composed of well-bedded sand, gravel, boulder sediments (MGS, 2011).

Figure 10: The Sabattus Pond submarine fan deposits in between two glaciomarine deltas.
Orientation of the deposits indicate that glacier retreat was expediated in between the deltas, as the fans
did not have sufficient time to build up into deltas (MGS, 2011).

Glaciomarine Deltas:

Deltas appear as submarine fan deposits reach sea-level. Deltas in Maine are large are measurable in square miles (MGS, 2005a). There have been over a 100 glaciomarine delta deposits studied in southern Maine, with many more to be analyzed. Most of the delta deposits originate from either subglacial streams entering standing water, known as an ice-contact environment, or streams flowing through the river valley until reaching standing water, known as a glaciofluvial delta (Figure 11). As streams flowed through the glacier, eskers, sinuous ridges consisting of stratified sand and gravel, were deposited. Where these streams exit the glacier into static water the largest sediment, sand and gravels, are deposited at the margin of the glacier. These coarse deposits create a distinctive type of bedding referred to as Gilbert deposits. Gilbert type deposits are composed of a sequence of topset, foreset, and bottomset beds (Figure 12). The topsets consist of the coarsest sediment, dominated by gravels, deposited across the surface of the delta, although areas near the ice-margin consist of boulders. The foreset beds consist of the sediment, primarily sands with gravel mix, that was able to be carried to the front of the delta creating sloping beds reaching deeper water. Bottomset beds consist of the finer sediments, silts and clays. This sediment is carried to the front of the delta deposited at the foot of the foreset deposits creating a slightly inclined to horizontal bed. Gilbert bedding sequences, specifically the topset and foreset beds because of the sharp boundary between the two, are valuable for determining approximate sea-levels and predicting past shorelines (MGS, 2005a).

Figure 11: Illustration showing different glaciomarine depositional environments as the glacier retreats from standing water. Image adapted from Lønne, 1995.

Figure 12: Glaciomarine delat deposits demonstrating Gilbert type bedding (MGS, 2011)

Presumpscot Formation:

Soon after the sea encroached upon the depression and was in contact with the ice it created an environment that could easily deposit the clay rich Presumpscot Formation throughout the marine occupied area (Figure 13). This was accomplished via meltwater streams full of sediment from within the glacier. As the less dense freshwater streams entered the standing marine water they deposited the gravels and sands near the glacier margin and the fine grained silts and clays were able to be deposited farther distances until it was able to settle out. Water well drillers have documented that in some areas the Presumscot Formation is over 200 feet (MGS, 2005d). Known regionally as “blue clay” the Presumscot Formation color is usually gray (Figure 14) and the sediment size is usually dominated by silt-sized particles, tiny mineral fragments including quartz, feldspar, and mica are also incorporated. Massive bedding as well as layered bedding is present in the formation. Layering is the result of sediment plumes outwashed from the land during a discharge event such as a storm or surging meltwater stream and the sediment settling to the floor at different rates separating the silt from the clay. Sand layers can be found within the Presumscot and as the land emerged sand sized sediment becomes the dominant sediment type indicating a high energy depositional environment (MGS, 2005f). Invertebrate fossils found in the formation include clams, scallops, mussels, snails, and barnacles (Figure 15). Fish and mammal fossils have also been found (MGS, 2005e).

Figure 13: Distibution of the Presumpscot Formation (MGS, 2005d).

Figure 14: Presumpscot Formation and it's characteristic gray or "blue" color (MGS, 2005e).

Figure 15: Photograph of the Presumpscot Formation and the abundance of marine fossils.
Invertebrates found in the Presumpscot include clams and mussels, snails, and barnacles (MGS, 2005f).

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Conclusion

During the late Pleistocene Epoch a marine inundation of southern Maine took place. This transgression was the result of glacial isostasy which created a lithospheric depression produced by the immense weight of the Laurentide Ice Sheet. As the glacier retreated past the coast of Maine and from the depression, ocean water flooded the low lying areas and created an ice-contact environment that persisted for thousands of years. Sediment being washed from within the ice and land created marine deposits approximately 120 km inland. These deposits produced world class features such as moraine belts, submarine fans, glaciomarine deltas, and the widely dispersed Presumpscot Formation.

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References

Aber, J.S., 2011. Glacial Isostasy and Eustasy. ES331/767 Lecture 9. www.academic.emporia. edu/aberjame/ice/lec09/lec9.htm Retrieved Oct. 20, 2011.

Borns, H. W., Jr., Doner, L. A., Dorion, C. C., Jacobson, G. L., Jr., Kaplan, M. R., Kreutz, K. J., Lowell, T. V., Thompson, W. B., and Weddle, T. K., 2004, The deglaciation of Maine, U.S.A., in Ehlers, J., and Gibbard, P. L. (editors), Quaternary glaciations - extent and chronology, Part II: North America: Elsevier, Amsterdam, p. 89-109.

Dutch, S., 2009. Mountain Building and Crustal Deformation. University of Wisconsin-Green Bay. www.uwgb.edu/dutchs/EarthSC102Notes/102Orogeny.htm Retrieved Nov. 7, 2011.

Lønne, I., 1995. Sedimentary Facies and Depositional Architecture of Ice-contact Glaciomarine Systems. Sedimentary Geology 98, p. 13-43.

MGS, 2011. Evidence of Ice Retreat, East Shore of Sabattus Pond. Maine Geological Survey. www.maine.gov/doc/nrimc/mgs/explore/surficial/sites/nov02.htm. Retrieved Nov. 7, 2011.

MGS, 2010. Evidence for a Calving Embayment in the Penobscot River Valley, Bangor Maine. Maine Geological Survey. www.maine.gov/doc/nrimc/mgs/explore/surficial/sites/dec08.htm Retrieved Nov. 1, 2011

MGS, 2005a. Surficial Geologic History of Maine. Maine Geological Survey. Fact Sheet. www.maine.gov/doc/nrimc/mgs/explore/surficial/facts/surficial.htm. Retrieved Oct. 1, 2011.

MGS, 2005b. Maine’s Glacial Deltas. Maine Geological Survey. www.maine.gov/doc/nrimc/mgs/explore/surficial/facts/dec03.htm. Retrieved Nov. 7, 2011.

MGS, 2005c. Maine's Glacial Moraines: Living on the Edge. Maine Geological Survey. www.maine.gov/doc/nrimc/mgs/explore/surficial/facts/jan00.htm. Retrieved Nov. 15, 2011.

MGS, 2005d. A General introduction to the Presumpscot Formation-Maine’s “Blue Clay”. www.maine.gov/doc/nrimc/mgs/explore/surficial/facts/oct00.htm. Retrieved Nov. 15, 2011.

MGS, 2005e. Fossils Preserved in Marine Sediments. Maine Geological Survey. www.maine.gov/doc/nrimc/mgs/explore/fossils/sediment/fossil-sed.htm Retrieved Nov. 17, 2011.

MGS, 2005f. MGS, 2005f. Presumpscot Formation-The Rise and Fall of the Glacial Sea in Maine. Maine Geological Survey. www.maine.gov/doc/nrimc/mgs/explore/surficial/sites/aug01.htm. Retrieved Nov. 17, 2011.

NFP, 2006. Glacial and Archeollogical Features of the Penobscot Lowland, Central Maine: A guidebook prepared for the 69th annual field conference, of the Northeastern Friends of the Pleistocene, June 2-4, 2006, Orono, Maine, p. 43.

Pidwirny, M.P. and Jones, S., 2010. Sructure of the Earth. Chapter 10: Introduction to the Lithosphere. Fundamentals of Physical Geography, 2nd Edition.www.physicalgeography.net/fundamentals/10h.html. Retrieved Nov. 9, 2011.

Syverson, K. M. and Thompson, A.H., 2010. Evidence for a Calving Embayment in the Penobscot River Valley, Bangor, Maine. Maine Geological Survey. www.maine.gov/doc/nrimc/mgs/explore/surficial/sites/dec08-2.htm. Retrieved Oct. 1, 2011.

Watts, A.B., 2001. Isostasy and Flexure of the Lithosphere. Cambridge University Press. Cambridge, United Kingdom.

Weddle, T. K., Retelle M. J., 2001. Deglacial History and Relative Sea-level Changes, Northern New England and Adjacent Canada. Special Paper 351. Geological Society of America. Boulder, Colorado, 285 p.

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