Historical Bedrock Maps of Maine
Part IV: The 1985 Bedrock Geologic Map of Maine

Introduction

Since William Smith presented the first modern geologic map in 1815, of England and Wales with part of Scotland, geologic maps have become the standard way of portraying geologic information. There have been four significant maps showing Maine's bedrock geology, published in 1885, 1933, 1967, and 1985. Each map is a product of its time, reflecting the current status of knowledge and understanding of geologic principles by the author. So it is best to view each one as a progress report, representing an interpretation of myriad bits of information.

In using a geologic map, or any map for that matter, it is important for the reader to appreciate the purpose for which the map was made. Certain things are shown and many things are not shown. Coloration and labeling emphasize things of perceived importance. In preparing a geologic map, hundreds of decisions are made by the geologist, and scrutiny of the finished map gives insight into that thought process.

In other fact sheets we look at Charles Hitchcock's 1885 map, Arthur Keith's 1933 map, and the 1967 Preliminary Geologic Map of Maine (Hussey and others, 1967). This fact sheet presents the fourth (and current) State bedrock map, published in 1985. In comparing the maps, you will notice obvious differences in style and content. In each case we review the context in which the map was made (History), point out the way in which different rocks are portrayed (Lithology), discuss the geologic ages assigned to the various rocks of Maine (Age), and look at major geological relationships that can be deduced from reading the map (Regional Relationships). Selected close-up areas are presented as figures to illustrate these points.

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The 1985 Bedrock Geologic Map of Maine

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History

Though brief (only 18 years), the time between publication of the 1967 and 1985 Maine bedrock maps was unparalleled in the study of Maine geology. In the 1970's, a burst of mineral exploration activity generated requests from exploration firms for a way to locate occurrences of certain rock types which might host mineral deposits, such as basic volcanic rocks, regardless of their present metamorphic grade or stratigraphic position. Essentially, exploration geologists wanted more descriptive information about the rock units on the map. The Maine Geological Survey produced a preliminary lithologic map of Maine in 1973, but it never advanced past open-file status. State Geologist Walter Anderson viewed the 1985 map as an opportunity to bring this information forward.

In the mid-1970's, the academic community was energized by the revolutionary new principles of plate tectonics, which were being applied across the Appalachians to reinterpret decades of geologic work. Also in the 1970's and early 80's, there was great interest in whether modern earthquake activity might influence the siting of nuclear power plants. To address this issue, the Nuclear Regulatory Commission sponsored geologic field investigations of faults and brittle fractures in Maine.

Ultimately, the impetus for the 1985 Bedrock Geologic Map of Map came from the U.S. Department of Energy's (DOE) Crystalline Respository Program for storage of high-level nuclear waste. Western States identified as potential hosts for a high-level nuclear waste disposal site in either bedded salt deposits, basic volcanic rocks, or siliceous tuffs, exerted pressure on the DOE to consider other possible host rocks, including crystalline rocks, a broad term which includes igneous and high-grade metamorphic rocks. Some large granite bodies in Maine were identified as among the potential candidates. In order to collect as much information in as little time as possible, the DOE funded efforts in many eastern and midwestern States to compile updated geologic maps. The 1985 Bedrock Geologic Map of Maine (Osberg, Hussey, and Boone, 1985) and the 1985 Surficial Geologic Map of Maine (Thompson and Borns, 1985) were the result of these compilation efforts in Maine.

The highly charged political climate, significant infusion of federal funds, abundance of new geologic information, and revolutionary change in regional geologic concepts made the 1985 Bedrock Geologic Map of Maine a project unequaled in the history of Maine geology.

Figure 1

The wealth of increased information is reflected in the extensive list of references used to compile the map, including not only geologic mapping sources, but also sources of paleontological information and isotopic age information. In order to manage this large amount of information, the State was divided into 13 local areas, each of which was assigned to one or more of the 13 geologists designated as area compilers. The three editors and the compilation coordinator (Figure 1) brought all this work together into the final map, which then went through an extensive production and review process.

Lithology

Stratified rocks.

One major innovation of the 1985 map is the way rock units are portrayed (Osberg, 1984). On a traditional geologic map, each stratified rock unit (group, formation or member) is identified by a unique color-pattern combination. The lithologic description of that unit might then include information on metamorphic grade and other lithologic characteristics (bedding, grain size, etc.). This implies that the map unit has a consistent rock type or lithology throughout its area of distribution and spans a defined age range or period of geologic time. An example from the 1967 map is shown in Figure 2.

Figure 2

Figure 3

Figure 4

The 1985 map broke from the traditional practice of using of color and pattern together to designate a specific geologic group, formation or member, and instead uses color to designate the geologic age (Figure 3) and a light gray overprint pattern to designate the protolith (Figure 4) of each geologic unit. A protolith is the original sedimentary rock from which the existing metamorphic rock was derived.

In this way, color and pattern are used independently to give more information to the reader. On the 1967 map, a unique color and pattern combination is assigned to each map unit. On the 1985 map, color and protolith patterns are not unique - the same color and pattern may be used for different geologic formations or members. For example, the massive pelite member of the Devonian Carrabassett Formation (Dcm) and the Devonian Ironbound Mountain Formation (Dim) carry the same lavender color and the same protolith pattern of horizontal dashes.

Describing units by their protolith precursor rather than by their existing lithology emphasizes the continuity of stratigraphic units over large areas regardless of local names, and across metamorphic zones. For example, the Waterville Formation ranges from low metamorphic grade to the northeast (greenschist facies) to high metamorphic grade to the southwest (high-rank amphibolite facies). Identifying units by protolith also satisfies exploration geologists or others interested in easily finding all rocks of a particular origin or composition regardless of their subsequent metamorphic history or current mineralogy.

An important drawback of this scheme is that nowhere on the map is the actual lithology of the stratified rocks described explicitly. It is left to the reader to infer the lithology by considering the protolith information in combination with the metamorphic grade, indicated on the inset map of regional metamorphic zones. Details of other lithologic characteristics such as bedding style and grain size are also lost. The Sources of Geologic Information, listed at the right side of the map sheet, should be consulted for detailed rock descriptions in specific areas of the State.

Intrusive rocks: Blue plutons!?!?

Figure 5

Another departure from traditional geologic maps is maintaining the principal hue indicating geologic age even unto the igneous rock units (Figure 5). On most geologic maps, igneous rocks (especially plutonic rocks) are shown in contrasting warm (pink to red) colors, emphasizing their rock type. On the 1985 map, Devonian plutons are shown in shades of blue, similar to the color used for Devonian stratified rocks, to emphasize their age. To readily distinguish them from stratified rocks of similar age, colors of the plutonic rock units are embedded with a pattern of randomly oriented slash marks traditionally used to represent igneous rocks. The lithology of each plutonic rock unit is carried by letter symbols which refer to a diagram of intrusive rock compositions, and accessory mineralogy. The triangular diagram to the right of the main map is a modified version of the Streckeisen (1973) classification for plutonic rocks that gives rock names (Figure 5).

Comparison with the 1967 map

Figure 6

The greatest advances over the 1967 map are not so much in mapping new areas as in working out many relationships that had been left unresolved on the preliminary map. Most of the detail of the 1967 map is retained, yet the emphasis on age draws the reader's attention to groups of rocks that comprise sedimentary basins and volcanic sequences of regional importance. Many individual formations are now correlated over distances of tens or even hundreds of miles. Many local names used by previous workers are abandoned.

Only a relatively few formations cover large areas of the state. The predominant unit in northern Maine is the Seboomook Formation (Ds); in northwestern Maine the Hurricane Mountain (Ch) and Jim Pond (Cj) Formations; in central Maine the Carrabassett (Dc) and Madrid (DSm) Formations; in south-central Maine the Sangerville (Ss) and Vassalboro (SOv) Formations; in mid-coast to eastern Maine the Bucksport (DOb) and Flume Ridge (DOf) Formations; and in east coastal Maine the Penobscot (OCp) and Ellsworth (OZe) Formations. The Tectonic Map inset shows the distribution of generalized units of metasedimentary and volcanic rock.

Age

Portrayal

The ages of Maine rocks are portrayed on the map sheet in several ways. The label for each map unit begins with one or two capital letters that indicate the assigned age or age range for the unit, with corresponding color. The Stratigraphic Correlation Chart shows age relationships among stratified units by their position on the chart, older units toward the bottom and younger units toward the top, in relation to the geologic time scale along the sides of the chart. This chart makes it easy to find the oldest units, youngest units, or units of similar age across the State.

Justification

The basis for assigning ages to rocks comes from either fossil assemblages or isotopic age determinations in the laboratory. Although the map does not list the data that were used, it does refer the interested reader to the source information for age dates. For the stratified rocks, those units with fossil age control are marked with an asterisk in the List of Formations, and in the Stratigraphic Correlation Chart. Specific references to paleontologic information are indicated in the List of Formations by number which the reader can look up on the Source list. Eight volcanic rock units with published age dates are identified on the Tectonic Map, with reference to the original source literature by number. One hundred eight plutons with published age dates are identified on the Metamorphic Map, again with references by number to the accompanying Source list. Over two-thirds of the plutons have isotopic age data of some kind.

Comparison with the 1967 map

The bright green colors through central Maine highlight the through-going Silurian sedimentary basin, flanked by older rocks to the northwest and southeast in warmer colors. The greatest changes since 1967 are in the coastal areas. Rocks of the Cushing and Cape Elizabeth Formations are now shown as Ordovician or older rather than Devonian-Silurian. In western Penobscot Bay, some rocks previously shown as Silurian-Ordovician are now assigned to Cambrian and even Precambrian. Recent information had suggested that the Merrimack Group in the southwest should likewise be considered older than Silurian, but there was uncertainty about this change, as the wide-ranging SZ designation indicates. Rocks of the Chain Lakes massif are shown as Precambrian. As on the 1967 map, most plutonic rocks are still shown as Devonian, although a few plutons in southwestern Maine, notably the large Sebago pluton, are shown as Carboniferous.

Regional Relationships

Presentation

The inset maps and charts on the map sheet are intended largely to illustrate the regional relationships and interpretations implied on the map. The interpretive cross-sections show vertical sections through the map, depicting the interpreted structural relationships below ground. Folds and faults are prominently displayed. The Tectonic Map describes the geology in terms of plate tectonics, highlighting particular lithotectonic assemblages, genetically associated rock of a certain age and tectonic setting. The Stratigraphic Correlation Chart illustrates facies relationships between certain units, unconformities, and contacts whose original relationships are obscured by faulting (tectonic contacts). The distribution of Regional Metamorphic Zones is shown on a separate inset map.

Content

The influence of plate tectonic theory on the 1985 map is clear. This is the first bedrock map to present a unified concept of the regional bedrock elements. The Chain Lakes massif in northwestern Maine is described as a "terrane", likely exotic to other rocks of Maine. Volcanic rocks, presumably formed in subduction-related arcs, are shown through northern Maine for the Ordovician and in eastern Maine for the Devonian-Silurian. Mélange, commonly found in accretionary complexes near plate boundaries, is specifically mentioned on the Tectonic Map. Perhaps the boldest interpretation is that rocks of the Coastal Lithotectonic Block are in great sheets transported along thrust faults to their current positions (Gates and others, 1984). Though not explicit on the map, these would be considered "suspect terranes" in plate tectonic nomenclature. The Tectonic Map illustrates their integrity, with each sheet separated from other rocks on the map by a thrust fault. The Interpretive Cross Sections show these great sheets of coastal Maine have moved a minimum of tens of miles over the rocks they now rest on. Plate collisions are implied as having produced the fundamental structure of Maine's bedrock crust. Prominent unconformities are displayed in the Stratigraphic Correlation Chart in the Middle Ordovician (Taconic orogeny), latest Silurian (Salinic disturbance), and Middle Devonian (Acadian orogeny). The 1985 map is also the first to show the extent of the Norumbega fault zone as a regional feature which spans the state.

References

Gates, Olcott, Hussey, Arthur M., and Osberg, Philip H., 1984, Acadian thrust plates in coastal Maine (abstract): Geological Society of America, Abstracts with Programs, v. 16, no. 1, p. 17.

Hussey, Arthur M., II, Chapman, Carleton A., Doyle, Robert G., Osberg, Philip H., Pavlides, Louis, and Warner, Jeffrey (compilers), 1967, Preliminary geologic map of Maine: Maine Geological Survey, scale 1:500,000.

Osberg, Philip H., 1984, Strategy for the new geologic map of Maine (abstract): Geological Society of America, Abstracts with Programs, v. 16, no. 1, p. 55.

Osberg, Philip H., Hussey, Arthur M., II, and Boone, Gary M. (editors), 1985, Bedrock geologic map of Maine: Maine Geological Survey (Department of Conservation), scale 1:500,000.

Streckeisen, A.L., 1973, Classification and nomenclature recommended by the IUGS Subcommission on the Systematics of Igneous Rocks: Geotimes, v. 18, no. 10, p. 26-30.

Thompson, Woodrow B., and Borns, Harold W., Jr. (editors), 1985, Surficial geologic map of Maine: Maine Geological Survey (Department of Conservation), scale 1:500,000.

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Text by Marc C. Loiselle and Henry N. Berry IV

Graphics by Marc C. Loiselle

Originally published on the web as the February 2008 Site of the Month.

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Last updated on April 11, 2012

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