The Decorah shale varies both vertically and laterally in its clay
mineral assemblages. It is principally an illitic shale in Minnesota,
but kaolinite is approximately of equal abundance in the basal part to
the southwest, reflecting nearness to the source area, most probably
the Transcontinental Arch. Lateral variations in clay mineral assemblages
of the Decorah are similar to that of the coextensive older Ordovician
Glenwood Formation, suggesting that the source was the
same and that both were deposited under similar conditions. The
vertical variation in clay mineral assemblages in the Decorah shale,
from kaolinite and illite toward the base to only illite in the upper part,
reflects the transgressive nature of the Decorah sea.
Certain engineering properties of the Decorah shale are related to
the orientation of clay minerals, The shale is relatively impermeable
perpendicular· to bedding where the clay minerals are arranged with
their shortest axis normal to the bedding surface, as in a deck-of-cards
arrangement, but is permeable parallel to bedding. In contrast, a
random clay mineral arrangement, as in the card-house structure, is
permeable in all directions. Both types of clay mineral arrangements
are present in the Decorah shale, Maintaining the natural moisture
content of the Decorah shale during and after construction aids in
stabilizing slopes and foundation bases.
The clay mineral data suggest that the Decorah shale may be satisfactory
for use in ceramic products such as face brick, sewer pipe,
lightweight aggregate, structural tile, and drain tile. Numerous areas
along the shale's outcrop belt in southeastern Minnesota would be
satisfactory for open pit mining.
Parham, Walter E.; Austin, George S..
RI-10 Clay Mineralogy, Fabric, and Industrial Uses of the Shale of the Decorah Formation, Southeastern Minnesota.
Minnesota Geological Survey.
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