Geochemistry, Geophysics, Geosystems

Quantitative bedrock geology of the conterminous United States of America

Authors


Abstract

[1] We quantitatively analyze the area-age distribution of bedrock based on data from the most recent geologic map of the conterminous United States of America [King and Beikman, 1974a, 1974b], made available in digital form by the United States Geologic Survey. The area-age distribution agrees surprisingly well with older data [Higgs, 1949] but provides much higher temporal resolution. The mean stratigraphic age of all sedimentary bedrock is ∼134 Myr; that of Tertiary-Cambrian sediments is ∼104 Myr. The analysis also reveals area coverage of some minor lithologies, such as ultramafic rocks that cover ∼0.15% of the conterminous United States. Area coverage of 162 lithostratigraphic units is made available as an Excel data sheet.

1. Introduction

[2] The United States Geological Survey (USGS) has made the geologic map of the United States [King and Beikman, 1974a, 1974b] available in digital format http://minerals.usgs.gov/kb/). This map shows bedrock geology of the conterminous United States at a scale of 1:2,500,000. The digitized version of this map consists of 12,934 polygons (∼600 km2 per polygon, on average), grouped by age and lithology into 162 lithostratigraphic units. According to United States and Canadian mapping tradition, abundant Quaternary glacial deposits along the northern border of the United States are not included in this map. Strictly speaking it is therefore a subcrop map.

2. Methods

[3] In order to quantitatively investigate the area-age relationship of bedrock the USGS geologic tapestry data set was downloaded as an ArcInfo coverage in decimal degree units cast on the Clarke 1866 datum. The data were reprojected using ArcInfo 8.0 to the Albers equal-area conic projection based on the same datum. The following projection parameters were chosen: central meridian, −96; reference latitude, 37.5; standard parallel 1, 29.5; standard parallel 2, 45.5; false easting, 0; and false northing, 0. ArcInfo was then used to build polygon topology and assign area values to each unit polygon. The unit area values (in km2) were summarized using ArcView 3.2, exported to a dBASE file and imported and reformatted in MS Excel. The results are shown in the auxiliary material (found at http://www.g-cubed.org) together with an abbreviated rock description. Abbreviations for “order” and “unit” are from King and Beikman [1974a, 1974b]. For sedimentary units (green italics in the auxiliary material) upper and lower age boundaries [Harland et al., 1982], duration, and percent surface area of the conterminous United States are also listed. We choose the Harland et al. [1982] timescale to minimize shifts in time-stratigraphic classification since the late 1960s and early 1970s, when the most recent geologic map of the conterminous United States was compiled [King and Beikman, 1974a, 1974b]. Total calculated surface area not covered by water is 7,782,952 km2, of which 6,476,730 km2, or 83.2%, is sedimentary bedrock, slightly larger than the 75.7% (79% with metasediments) estimated based on an older bedrock map [Gilluly et al., 1970].

[4] Data for sedimentary bedrock (91 units) were used to plot normalized cumulative surface area for each time period mapped (Figure 1, red squares). Area values of undifferentiated units such as “Triassic and Permian” were divided according to duration (km2 Myr−1) among the subunits. Uncertainties introduced by this procedure are small because less then 0.6% of the total surface area analyzed is comprised of such composite units. For systems encompassing more than one epoch, upper and lower stratigraphic age boundaries were calculated by taking the midpoint between nearby epoch boundaries. For example, the Chesterian (upper Mississippian) encompasses the Serpukhovian and upper Visean epochs. The age of the upper bound of the Serpukhovian (320 Ma) was therefore taken as the age of the upper bound of the Chesterian, whereas the midpoint (342.5 Ma) of the upper and lower age boundaries of the Visean, 333 Ma and 352 Ma, respectively, was taken as the lower bound for the Chesterian. Systems that encompass less than a full epoch were assigned upper and lower bounds of that epoch. This “broadening” of the actual age range has negligible effects on the results. For some units not listed by Harland et al. [1982], information on time-correlative systems were used to define upper and lower ages [Bally and Palmer, 1989]. For Precambrian systems data by Reed et al. [1993] and Harland et al. [1982] were used to define ages of upper and lower bounds.

Figure 1.

Cumulative area-age distribution of sedimentary bedrock of Tertiary-Cambrian age. Red squares: bedrock in the conterminous United States (this study), based on data by King and Beikman, [1974a, 1974b]. Gray squares indicate sedimentary bedrock in the conterminous United States [Higgs, 1994] based on data by Stose and Ljungstedt [1932]. Black symbols show global sedimentary bedrock [Blatt and Jones, 1975] (open squares, mean; inverted triangles, upper limit; upright triangles, lower limit). Black open circles show sedimentary bedrock in North America [Gilluly, 1969]. All stratigraphic ages are based on Harland et al. [1982]. Hatched horizontal line marks 50% cumulative area. Dotted vertical lines mark the mean stratigraphic age of Tertiary-Cambrian sedimentary bedrock (red, this study; gray, Higgs [1949]).

3. Results and Discussion

[5] Similar data by Higgs [1949] that were based on cutting and weighing mapped units of an older version of the geologic map of the United States [Stose and Ljungstedt, 1932] deviate only slightly from our results (Figure 1, black squares). However, our data more than double the temporal resolution. An older analysis at coarser spatial resolution for the entire North American continent (Gilluly [1969] based on the 1:5,000,000 map of North America [North American Geologic Map Committee, 1965]) gives an area-age relationship that is similar in shape but shifted to older ages. This offset is mainly caused by extensive areas of Middle-Lower Phanerozoic sedimentary bedrock in eastern Canada, south of the Hudson Bay, and in the Canadian Arctic, that are not included in our and Higgs [1949] analyses. The area-age distribution of the conterminous United States is also reasonably similar to the global distribution of continental sedimentary rocks (Figure 1, black lines with open squares, upright, and inverted triangles that mark the mean as well as lower and upper limits, respectively) as estimated by Blatt and Jones [1975]. Notable differences between the area-age distribution of the conterminous United States and global estimates are the overabundance of Middle Triassic-Carboniferous sedimentary rocks and the scarcity of Early Cretaceous-Late Triassic sedimentary rocks in the conterminous United States. The scarcity of Jurassic sedimentary bedrock was first noted by Higgs [1949]. He suspected that the distribution of U.S. bedrock is not representative of the global land surface. However, Gilluly [1969], Blatt and Jones [1975], and Bluth and Kump [1991] also noted a scarcity of Jurassic bedrock in their compilations of North American [Gillully, 1969] and global [Blatt and Jones, 1975; Bluth and Kump, 1991] age distributions of sedimentary rocks and suggested that this is a real feature. Gilluly [1969] invoked excessive erosion of young sediments during the Cretaceous and/or deficient accumulation during the Jurassic. In contrast, Blatt and Jones [1975] suggested that this feature reflects efficient burial of Jurassic sedimentary rocks by the unusually intense transgressions during the Cretaceous.

[6] According to our analysis the mean stratigraphic age of Tertiary-Cambrian sedimentary bedrock (91 units) in the conterminous United States is ∼104 Myr (Figure 1). This value is within the range of global sediments of Tertiary-Cambrian age (100–130 Myr [Blatt and Jones, 1975]) but younger than that calculated based on older data from Higgs [1949] (∼160 Myr) and Gilluly [1969] (∼190 Myr). If we include Quaternary and Precambrian sediments in our analysis (101 units), the mean stratigraphic age of sedimentary bedrock (subcrop) increases to ∼134 Myr.

[7] The data reflect the lithologic composition of the eroding continental crust in the United States at an unprecedented resolution. Such information can be used to estimate the chemical composition of the eroding continental crust, an important input parameter in models of global geochemical cycles [e.g., Bluth and Kump, 1991]. For example, the crustal budget of Os and Ir may be strongly influenced by the area coverage of ultramafic rocks (auxiliary material: order, 76; unit, um) that typically have 2 orders of magnitude higher Os and Ir concentrations than average upper continental crust [Peucker-Ehrenbrink and Jahn, 2001]. On the basis of our analysis (11,791 km2, equivalent to ∼0.15 area%), ultramafic rocks account for ∼15% of the upper crustal Os and Ir inventories. A refined understanding of area-age distributions of continental rocks is also useful for improving models of sedimentary recycling rates though time [e.g., Blatt and Jones, 1975].

Acknowledgments

[8] We very much appreciate the advice of the USGS “tapestry” team, notably Kate Barton, David Howell, and Joe Vigil. Comments by two anonymous reviews helped in preparing the final version of this manuscript. Financial support was provided through the U.S. National Science Foundation (NSF-EAR-0125873). This is WHOI contribution 10,749.

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