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Rock Properties


Density, Magnetic Susceptability, and Natural Remanent Magnetixation of Rocks in Minnesota

Val W. Chandler and Richard S. Lively



Every reasonable effort has been made to ensure the accuracy of the factual data on which this compilation is based; however, the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Minnesota Geological Survey in St. Paul. No claim is made that the data as shown are rigorously correct, however, and it should not be used to guide engineering-scale decisions without site-specific verification.


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(Compressed with WinZip. Uncompress to obtain DBF format, contains UTM, NAD83, zone 15 XYcoordinates, no base data)

Geologic interpretation of gravity and magnetic anomaly data in a given area is greatly enhanced if density, magnetic susceptibility and natural remanent magnetization (NRM) data are available for representative rock-types. Along with outcrop and drill-hole information, rock property data help relate geophysical anomaly signatures to probable rock types, and provide constraints on the use of anomaly data as a tool for mapping and for modeling geology at depth.

Most of the density and magnetization data contained in this database were acquired over the last two decades by the Minnesota Geological Survey (MGS) as part of an on-going program to collect rock properties. A group of Paleozoic samples were collected from Iowa and included in the database because they provide a representative suite of data for rocks present, but not widely exposed in Minnesota. Additional data were derived from studies by the U. S. Geological Survey (Bath, 1962; Beck, 1970; Beck and Lindsley, 1969; Books, 1972; Jahren, 1965), The University of Minnesota (Bleifuss, 1952, Mooney and Bleifuss, 1952), The University of Western Ontario (Palmer, 1970), and the Geological Survey of Canada (Dubois, 1962).

Values of rock density, magnetic susceptibility and natural remanent magnetization (NRM) are given in SI units (Systeme Internationale, meter kilogram seconds), as opposed to the older c.g.s. units (centimeter, gram, seconds). Missing or nodata values are listed as -999.

Conversions from SI units to cgs units can be made as follows:

Density-divide SI value by 1,000
Magnetic susceptibility-divide SI value by (4*pi)
NRM intensity-divide SI value by 1000

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The rock properties file was built as an ArcView (3.x) project and is made available through ArcIMS. It is likely that the first time the link is opened you will be asked to download the ArcIMS viewer and Java scripts. This is necessary to view this or other ArcIMS projects. The browser must be restarted after the installation. If you are logged into your local computer without Administrator permissions, the install will not proceed. Relog in, or get your IT staff to load the software.

If for some reason the Java installation fails, the data can be obtained from the MGS ftp site and added to ArcView as an event theme, using the X and Y coordinate fields for the point locations. The file contains over 4,000 records and has been compressed with WinZip. The coordinates are in NAD83, UTM, Zone 15.

The view contains several basemap themes that show at different scales and one theme of rock property data. The data are divided into two types, the largest is the sample-based results, the other contains site-based results. For the sample-based data, a given property in the table represents either a single measurement or an average of several measurements taken from a small volume of rock, such as a hand sample, a core segment, or an in-situ measurement at an outcrop (magnetic susceptibility only). For site-based data a property in the table represents an average of measurements from multiple samples (field = Num_sam) taken over a fairly large site, usually consisting of a quarry or large outcrop. Presently the site-based files consist mainly of NRM data collected for paleomagnetic studies; sample-based data from these studies are no longer available.

The basemap themes include Minnesota counties, 1:24,000 quad outlines, minor civil divisions, highways and geology polygons from the State Map S-20. Please note that the colors assigned to the geology polygons are randomly selected within the ArcIMS program and are not based upon rock type, or the S-20 colors.

Records may be selected by these location grids by drawing a box around groups of points or by geologic descriptions using the built-in query functions of ArcView. Additional ArcView shape fields may be added by the user as needed. Most locations are precise to within 200 meters or less of true position, although locational errors on the order of 400 meters may occur sporadically.

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STRUCTURE OF THE DATABASE (Missing or nodata values are listed as -999.):

Relate ID
This field gives the sample or field number as assigned by the original collector. This is commonly alpha-numeric and is not consistent for the entire database. Although the same sample number could exist for different samples, it is not common.

Geologic Descriptions- Major Division, Minor Division and Rock Type
Most of the geologic data in the theme tables are divided into three fields; major geologic division, minor geologic division and rock type. Major division refers to the geologic association of the sample on the scale of an orogen or subprovince (for example, Penokean orogen or Wawa subprovince). Minor division refers to a large map unit (for example, Ely Greenstone Belt or Giant's Range Batholith). Rock type is a very general description of the sample as given by the collector, and does not conform to any particular rock classification scheme. Thus, a rock that is described as a "gabbro" by one collector may be described as a "troctolite" by another. Additional geologic information may be given in the comment field (see below).

All density measurements are given in SI units (kilograms/cubic meter) and were made using a Jolly spring balance. Density measurements were made using small chips on the order of 2.5 cm. x 5 cm x1.0 cm., or small cores on the order of 2.5 cm in diameter and length. Measurements are precise to about 10 SI units. Every reasonable attempt was made to ensure that the samples were as unweathered as possible, and porous samples (such as sandstone) were soaked in water prior to measurement in order to remove as much air as possible. However unsaturated pore space may have persisted in some samples, and if a density appears to be unusually low, it should be used with caution. For the site-based data the number of samples used to compute the average density is provided in an adjacent field.

Magnetic Susceptibility
Magnetic susceptibilities are given in SI units, and are based on measurements from a variety of instruments. Wherever appropriate, corrections have been made for estimated air space (chip samples) and sample dimensions, in accordance with recommendations given by the manufacturers of a particular instrument. Most magnetic susceptibilities are precise to about 0.0001 SI units, although those acquired using the Scintrex device, as well as measurements by C. E. Jahren, G. D. Bath, and R. L. Bleifuss and H. M. Mooney (see citation field in database), can be as much as an order of magnitude less precise. For the site-based data the number of samples used to compute the average susceptibility is provided in an adjacent field.

This category provides supplemental information on a given sample. In some cases a formal or an informal rock unit name is given (for example, Bald Eagle Intrusion or Houghtailing Creek troctolite), and in other cases information regarding field relationships or the nature of the sample is given.

Magnetic Method
This category describes the methodology and instrumentation used for magnetic susceptibility measurements. Many of the magnetic susceptibility measurements by MGS staff were made with a Bison model 3101A magnetic susceptibility meter, using one the following modes: (1) in-situ (outcrop) measurements using a 6-inch coil; (2) crushed hand samples in plastic vials using an internal coil; and (3) small cores using an internal coil. Other magnetic susceptibility measurements by MGS were made using a hand-held EDA Instruments Model K-2 magnetic susceptibility meter on core, hand sample, or outcrops. Because the K-2 meter samples a very small volume of rock, presented values are commonly based on the average of 3 to 10 individual readings. Some MGS measurements were made using a Scintrex model SM-5 magnetic susceptibility meter, which operated very much like the EDA instrument. Laboratory measurements of magnetic susceptibility by C. E. Jahren, G. D. Bath, R. L. Bleifuss and H. M. Mooney (see citation field in database) were made using laboratory-built devices that were based on self-inductance in coils.

Where applicable, this field may also specify whether an astatic magnetometer or a spinner magnetometer was used for NRM measurements. For older studies these instruments were typically laboratory-built devices, such as those that were at the U. S. Geological Survey and the Geological Survey of Canada. Measurements of NRM without any specification in this field were made at MGS using a Schondstedt Model SSM-1 spinner magnetometer.

This category credits the individuals who acquired the data. If two names are given, the first designates who was responsible for field collection, and the second designates who did the laboratory measurements.

Intensity (J), declination (Dec) and Inclination (Inc) of Natural Remanent Magnetization (NRM)
The next three categories give information on the intensity (J_nrm), declination (Dec_nrm) and inclination of NRM (Inc_nrm), where available. Intensity is given in SI units. These sample-based NRM data represent initial (raw) readings with no "cleaning" by either thermal or alternating field (AF) demagnetization. Therefore, the user must be cautious of the effects of lightning strikes and other spurious NRM components. The site-based NRM data is commonly "cleaned" by either thermal or AF methods and information on this cleaning is provided in the comment field, where available (Num_sam). For the site-based data the number of samples used to compute the average NRM parameters is presented in an adjacent field. An additional field (Polarity) for site-based data gives the NRM polarity as N (normal or downward directed) or R (reversed or upward directed), and is given where no other NRM data are available. Two additional fields (Geo_stike) and (Geo_dip) refer to the strike and dip of the rock unit and if available, were used as a structural correction to the NRM direction (Dec_nrm and Inc_nrm). Unless noted, structural corrections (NRM struc cor) have not been applied to the NRM direction data.

Acquisition of these data was initially supported by the Aeromagnetic Survey Program of the Minnesota Geological Survey, which was funded by the Minnesota Legislature as recommended by the Legislative Commission on Minnesota Resources. Additional legislative support was provided through the Minerals Coordinating Committee and the State Special Appropriation for the Minnesota Geological Survey. Support was also provided through the U. S. Department of Energy (National Uranium Resource Evaluation; NURE) project), the U. S. Geological Survey (COGEOMAP project), and the MGS County Geologic Atlas Programs for Filmore, Pine, Stearns, and Wabasha Counties. Charles Jahren generously provided the data and samples that he and Gordon Bath acquired during the 1950's. Professors Myrl E. Beck, Jr. (Western Washington University) and H. Currie Palmer (The University of Western Ontario) generously provided field materials and other information from their own studies. Professor John C. Green (University of Minnesota, Duluth) provided unpublished paleomagnetic data from studies by Kenneth G. Books and others. In addition to those named in the citation category of the database, others who have assisted at various times with the MGS data include Matt Benson, Laura Beauchane, Douglas Bergstrom, Terri Haake, Lisa Kivaekas, Richard Lively, Angela McGowan, Peter McSwiggen, Sahu Sanghamitra, Bryan Schaap and Dale Setterholm.

References Cited

Bath, G. D., 1962, Magnetic anomalies and magnetizations of the Biwabik Iron Formation, Mesabi Area, Minnesota: Geophysics volume 27, number 5, p 627-650.
Beck, M. E., and Lindsley, N. C., 1969, Paleomagnetism of the Beaver Bay Complex, Minnesota: Journal of Geophysical Research, v. 74, p. 2002-2013.
Beck, M.E.,1970, Paleomagnetism of Keweenawan intrusive rocks, Minnesota, Journal of Geophysical Research, v. 75, p. 4985-4996.
Bleifuss, R. L., 1952, Correlation of magnetite content with magnetic susceptibility measurements of Minnesota Pre-cambrian rocks: University of Minnesota Master of Science thesis, 46p.
Books, K. G., 1972, Paleomagnetism of some Lake Superior Keweenawan rocks: U. S. Geological Survey Professional Paper 760, 42 p.
DuBois, P. M., 1962, Paleomagnetism and correlation of Keweenawan rocks: Geological Survey of Canada Bulletin 71, 75 p.
Jahren, C. E., 1965, Magnetization of Keweenawan rocks near Duluth, Minnesota, Geophysics, v. 30, p. 858-874.
Mooney H.M. and Bleifuss, R. L. 1953, Magnetic susceptibility measurements in Minnesota-Part 3, analysis of field results: Geophysics, v. 18, p. 383-393.
Palmer, H. C., 1970, Paleomagnetism and correlation of some middle Keweenawan rocks, Lake Superior: Canadian Journal of Earth Sciences, v. 7, p. 1410-1436.