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Figure 8. Cross section showing the changes in vein composition around some sediment-hosted disseminated gold deposits. Silica-rich (jasperoid) veins occur near the center of the gold deposit, while carbonate-rich (calcite) veins occur peripheral to the main mineralized area.
Digital Geologic Data Sets: The Example of the Quebec-MaineGulf of Maine Global Geoscience Transect
This data set...will
stimulate novel interpretations of the processes that affected the
crust of this region.
split by continental rifting in the Mesozoic (165 million years ago) to form the Atlantic Ocean, and reaches the ocean basin south of Georges Bank. The data set for this large area consists of samples of data for the United States and Canada that have different reference bases and projections. It includes parts of several different geologic maps and combines terrestrial and marine data of numerous types. Therefore, the data present the opportunity to develop and apply a generic digital format to display and analyze a complex regional problem. Because this transect has been so successful, it will be a useful prototype for other large regional geologic syntheses underway within the USGS, in other governmental agencies, and around the world in the Global Geoscience Transect project.
The Digital Line Graph (DLG) format developed by the USGS for planimetric data was extended to be a prototype for a standard DLG to describe the geologic data for this transect. The prototype standard DLG coding scheme that was developed can be readily edited or extended, can be used to prepare colored geologic maps, cross sections, and three-dimensional models for publication, and may be widely applicable to other regional geologic problems or transects.
To create the digital data, geologic maps of Maine and Quebec on stable base materials were machine-scanned to capture geographic boundaries, bedrock geology, and major hydrographic features such as large lakes and streams, shorelines, and offshore islands. A draft map of the Gulf of Maine was digitized by hand. The standard planimetric DLG codes were applied to the hydrographic and geographic data.
Because the standard scale for Global Geoscience Transects is 1:1,000,000, only linear geologic features such as contacts and faults were shown in addition to the areas (polygons) of mapped units. A major and minor attribute code was assigned to each linear feature; different kinds of faults were assigned different codes on the basis of the node-to-node direction of the line (arc) so that symbols identifying the type of fault are shown only on the appropriate side of each fault
The coding scheme provides a mechanism for describing geological data such as age, composition, rock type, tectonic setting, the formal or informal name of the rocks, and map color and pattern, by attaching numeric attributes to the digital data. The three-digit major codes and four-digit minor codes used are compatible with the USGS planimetric DLG and are based on widely used conventions for naming rock units.
Figure 9. Index map showing the location of the Quebec-Maine-Gulf of Maine transect and the pilot study area.
After being digitized, the various geologic maps were merged, which required substantial amounts of editing to match up the geologic contacts. Nonetheless, more than 300 formations and hundreds of plutonic rock masses have been successfully represented and can be readily displayed at any scale. They can also be selectively sorted and edited by using widely available software and hardware. Because this data set exists in a generic DLG format, it can be sampled, subdivided, and transferred electronically to networked computers that combine geologic, geographic, and geophysical data in new ways to show unprecedented details about the Earth's crust, which will stimulate novel interpretations of the processes that affected the crust of this region.
An example of a computergenerated three-dimensional model of the Earth's crust is shown in figure 10. It was created from digital data sets. The digital geological data were generalized and are shown draped on a digital elevation model of the topography of the region. The shapes of granitic and gabbro plutons (massive bodies of intruded rock), as determined by gravity models for the regional gravity data and measured densities of field samples, are shown in the model as cavities with depth contours at 0.6-mile (1-kilometer) intervals. The modelled shapes of individual plutons such as the Lexington batholith (seen in the center of fig. 10) can be displayed in considerable detail, as shown in the top part of figure 10. This figure shows the shape of the Lexington batholith as it would appear if seen from the southwest and from below the Earth's surface. The model in figure 10 also shows the trace of a seismic reflection profile in which the conspicuous saddle in the middle of the pluton can be seen as weak reflectors. Another combination of filtered gravity and magnetic data with
intermediate wavelengths was used to model the shape of this pluton, and the results were consistent with the model shown in figure 10. The availability of all these digitized data on a common base and in compatible formats enables ready comparisons to be made between diverse kinds of geological and geophysical data. Figure 10 also shows the approximate shape of the Kearsarge-Central Maine (Merrimack) synclinorium, the base of Cambrian to Ordovician (approximately 540 to 485 million years old) ophiolite and melange assemblages and the underlying Chain Lakes massif of Precambrian age (more than 570 million years old), the southeastern edge of Grenville-type crust (more than 880 million years old), and the base of the Earth's crust (Moho), all inferred from, and constrained by, digital seismic reflection and refraction data. Because the surfaces shown in this model are all digitized, a geological cross section through the model can be drawn quickly in any direction.
Work on the Quebec-Maine-Gulf of Maine transect has demonstrated the ability to make compatible and transferable many large geographic, geologic, and geophysical data sets. A key component of this effort was the development of a widely applicable DLG code for geologic maps to describe rock names, ages, lithologies, compositions, structures, tectonic setting of deposition, and linear map features, such as faults and contacts. The DLG codes, which are compatible with the standard planimetric DLG in use by the USGS, can be considered for use as a prototype for a geologic standard. Unquestionably, further development of this code for geologic point data (such as strike and dip measurements) and for surficial deposits is required, but that is believed to be readily achievable. Thus, progress toward the development of a standard digital code that could result in mapping uniformity and compatibility in geoscience mapping has begun.
Figure 10. Three-dimensional model of region near Lexington batholith (bottom) and of Lexington batholith seen from below. Route of seismic reflection profile shown by heavy black line crossing batholith in top part of figure.