Geology Reference
In-Depth Information
nored and the corresponding axial surface between domains 1 and 2 removed in order to
match the locations of the stratigraphic contacts. A new axial surface dip is determined
as the boundary between the two domains in contact (1 + 2 and 3) after the incorrect
axial surface is removed. The vertical dip selected for the forelimb provides a good match
to all the contacts except for the top of the Dc at location 2. A slight rounding of the con-
tact at this location will provide a match to the map geometry. The internal consistency
of the section based on constant thicknesses, planar domain dips and the mapped contact
locations and depths in the well is strong support for the interpretation.
Axial surfaces are shown as crossing in Fig. 6.22, an impossibility. Where two axial
surfaces intersect, the dip domain between them disappears and a new axial surface is
defined between the two remaining dip domains (Fig. 6.23). The final cross section
(Fig. 6.23) is an excellent overall fit to the dips and contact locations. Locations 1 and 2
are the only misfits. The misfits are quite small. At location 1, the base of the Mh does
not match the mapped outcrop location which could be caused by a second-order fold
at that point or by the mislocation of a poorly exposed contact. A very small domain
of thickened bedding is required at location 2 in order to keep the top of the Dc below
the surface of the ground and so that the contacts of the Dc and the Sm meet across the
axial surface. It is no surprise that bed thickness is not perfectly constant in such a
tight hinge. The surprise is that such a small region of thickening is required in the
hinge. The effect of the thickening of the Mtfp is to round the hinge, a feature that
might continue upward along the axial surface as well, but is shown as ending within
the Mtfp. Both the vertical domain and the thickened domain disappear at point 3
where a new axial surface bisects the angle between the remaining two domains
(2 and 4). The match of the top of the OCk across this axial surface is an additional
confirmation of the cross-section geometry because the location of the axial surface is
defined by intersection point 3, not by projection of the OCk contact.
Fig. 6.23.
Final constant-thickness, dip-domain cross section across the Sequatchie anticline. No verti-
cal exaggeration. The
numbered arrows
are explained in the text.
Small arrows
mark the contact loca-
tions. The
dashed line
is the level of the deepest horizon drilled
