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Geophysical Investigations, Tri-State District

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Geological Basis for Geophysical Correlations

Magnetic Anomalies

Any factors which cause local variations in the vertical magnetic intensity of the earth's magnetic field will cause the vertical magnetometer to record local anomalies. Such local magnetic variations generally are caused either by local variations in the magnetite content of a given rock or by variations in the distance from the surface (structural effects) of a rock having uniform magnetite content. In this connection, certain generalizations may be made: As a general rule, igneous rocks have a greater magnetic intensity than sedimentary rocks, and basic igneous rocks have a greater magnetic intensity than acidic rocks. Clastic sediments generally exhibit greater magnetic intensity than chemically precipitated sediments, and sandstones are generally greater in magnetic intensity than shales.

Following are some of the factors which might possibly account for magnetic anomalies in the Tri-State district:

  1. Causes for increase in vertical component of magnetic intensity.
    1. Topographic highs on the pre-Cambrian igneous surface.
    2. Basic intrusives or differentiates in the pre-Cambrian granite.
    3. Igneous intrusives cutting the lower portion of. the Paleozoic section.
    4. Arching of the sediments which would bring nearer to the surface the probably more magnetic pre-Mississippian clastics.
    5. Pennsylvanian sandstones (probably more magnetic than the Pennsylvanian shales) found locally nearer to, or at, the surface as a result of:
      1. The normal regional dip.
      2. Local folding.
      3. Local slumping into caverns in the underlying Mississippian limestone.
  2. Causes for decrease in vertical component of magnetic intensity.
    1. Topographic lows on the pre-Cambrian surface.
    2. Downwarping of the sediments which would remove farther from the surface the probably more magnetic pre-Mississippian clastics.
    3. Pennsylvanian sandstones (probably more magnetic than the Pennsylvanian shales) found locally farther from the surface than the shales as a result of:
      1. Normal regional dip.
      2. Local folding.
      3. Local slumping into caverns in the underlying Mississippian limestone.

Gravity Anomalies

Any factors which cause local variations in the total mass of a given section of the earth's crust will result in an increase or a decrease in the local value of the earth's gravitational field, Such resultant local variations in the earth's gravitational field can be measured directly by the gravity meter. In this connection, certain generalizations can be made: as a general rule, igneous rocks have a greater specific gravity than sedimentary rocks, and basic igneous rocks have greater specific gravity than acidic rocks. Whenever rocks of higher specific gravities are, by structural deformation, brought nearer than normal to the surface, the structure responsible for this condition may cause gravity maxima on the earth's surface. Any factors which create voids, moreover, tend to reduce the mass of a given section of the earth's crust. Therefore, such features as caverns, brecciation, and fracturing may cause gravity minima.

Following are some of the factors which might possibly account for gravity anomalies in the Tri-State district:

  1. Causes for gravity maxima.
    1. Topographic highs on the pre-Cambrian igneous surface.
    2. Basic intrusives or differentiates in the pre-Cambrian granite.
    3. Igneous intrusives cutting the lower portion of the Paleo,zoic section.
    4. Dolomitization (dolomite S. G. 2.8-2.9) of limestone (calcite S. G. 2.71).
    5. Filling of voids normally present due to secondary cementation-especially chertification.
    6. Local lithologic variations in the Mississippian and Pennsylvanian sediments.
  2. Causes for gravity minima.
    1. Topographic lows on the pre-Cambrian igneous surface.
    2. Silicification (chert S. G. 2.64) of limestone (calcite S. G. 2.71).
    3. Development of secondary voids as a result of fracturing, brecciation, or solution (including development of caverns).
    4. Local lithologic variations in the Mississippian and Pennsylvanian sediments.

Natural Potential Anomalies

Natural potential anomalies are the result of differences in electrical potential between two points in the earth's crust. In this connection, certain generalizations may be made: Any factors which serve to create water-soluble, metallic salts in a region of active groundwater movement will cause the natural development of potentials (galvanic action). Since most ground-waters are weak electrolytes, a marked uni-directional ground-water flowage will generally produce measurable potentials. Sulphide oxidation is one means of developing electrolytes.

Following are some of the factors which might cause the development of natural potentials in the Tri-State district:

  1. Relatively rapid, constant, uni-directional flow of groundwater. Such flow may indicate that the rock strata at the horizons being studied are porous and permeable, brecciated, fractured, or cavernous.
  2. Oxidation of sulphides producing electrolytes. Because the rate of oxidation of pyrite (or marcasite) is more rapid than that of galena or sphalerite, pyrite or marcasite concentrations may be related to greater natural potentials than galena or sphalerite concentrations. Therefore, a negative potential center mayor may not indicate the presence of sulphide concentration. If such a sulphide concentration is present, it may be economically valuable sulphide (here galena or sphalerite) or it may be sulphide without economic value in this district (pyrite or marcasite) .

Resistivity Anomalies

Resistivity anomalies are the result of differences in resistance to the passage of electric current exhibited by rocks at diverse points within the earth's crust. Since, for this work, no attempt was made to secure-an effective current flow to a depth greater than 400 feet, the anomalies were due chiefly to resistivity variations within this distance from the surface. In this connection, certain generalizations may be made: The apparent resistivity of a selected portion of the subsurface is a function of the separate resistivities of its several components and of the size, shape, and structural configuration of the components. Fresh waters are relatively poor conductors. The best natural conductors generally are electrolytic solutions and connate waters. The ground resistivity, therefore, will decrease with an increase in the soluble, metallic salt content of the contained ground-water. Most sulphides, with the notable exception of sphalerite, have relatively low resistivity. Such low resistivity is characteristic of galena, pyrite, and marcasite. The resistivity of sphalerite, however, is often very little different from that of accompanying gangue minerals.

The depth to an anomalous condition was calculated by a modification of the Roman method (Jakosky, 1940, pp. 292-299).

Following are some of the factors which might cause the development of resistivity anomalies within a general range of 400 feet from the surface in the Tri-State district:

  1. Causes for high resistivity anomalies.
    1. Presence of dry caverns, dry voids, or dry fractures (above thc water table).
    2. Local arching bringing the often dense Mississippian limestones closer to the surface than the porous, generally water-saturated Pennsylvanian clastics.
    3. Filling of normally water-filled voids in the Mississippian limestone due to secondary cementation; viz., chertification or dolomitization.
  2. Causes for low resistivity anomalies.
    1. Presence of concentrations of galena, pyrite, or marcasite.
    2. Sphalerite might or might not cause such anomalies.
    3. Presence of zones of water-saturated rock-especially where the water has a high electrolyte content.
    4. Presence of voids filled by electrolytic solutions. Voids may be pores, fractures, or caverns.
    5. Local down-warping or slumping, creating a thicker section of the porous, generally water-saturated Pennsylvanian clastics.

Geothermal Anomalies

Temperature variations within the earth's crust, independent of diurnal atmospheric variations, constitute geothermal anomalies. Any factors which upset the thermal equilibrium within the earth's crust produce geothermal anomalies. Since all geothermal measurements on this project were taken in shallow holes, any factors serving to change the thermal equilibrium must be of a type the effects of which would be apparent near the surface.

Following are some of the factors which might cause geothermal anomalies under the conditions of study in the Tri-State district:

  1. Causes for high geothermal anomalies.
    1. Oxidation of sulphides-especially pyrite and marcasite.
    2. Arching of the sediments, or decreased depth to the basement rocks.
    3. Local variations in surface drainage resulting from topographic irregularities or variations in lithologic permeability.
  2. Causes for low geothermal anomalies.
    1. Marked and rapid flow of ground-water, usually descending.
    2. Down-warping or slumping of sediments.
    3. Local variations in surface drainage resulting from local topographic irregularities or variations in lithologic permeability.

Geochemical Anomalies

Geochemical anomalies, as the term is here used, refer to variations in spectrographically determinable concentrations of metals in soil or rock samples. Any factors which would serve to produce a variation in the content of one or more metals in a soil or rock sample would, therefore, produce geochemical anomalies.

Following are some of the factors which might cause geochemical anomalies in the Tri-State district:

  1. Precipitation of minor amounts of metals from mineralizing solutions which rose above the main ore zone during the period of mineralization.
  2. Local chemical or lithologic variations in the rock or soil.
  3. Local variations in surface drainage which might produce minor differential leaching or precipitation of metals in the soil.
  4. Local variations in vegetative growth which might subtract from, or add to, the normal metallic assemblage within the soil.

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Kansas Geological Survey, Geology
Placed on web Oct. 29, 2018; originally published December, 1942.
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The URL for this page is http://www.kgs.ku.edu/Publications/Bulletins/44/06_basis.html