Geology Reference
In-Depth Information
Fig. 11.22.
Area-depth interpretation of the Tip Top anticline, Wyoming thrust belt, western U.S.
a
Cross
section, areas below horizon 1 match the best-fit area-depth line (Groshong and Epard 1994, after Webel
1987).
b
Best-fit area-depth line (after Epard and Groshong 1993)
The area-depth curves for both grabens are straight lines with a modest amount
of scatter (Fig. 11.21b). This amount of scatter is normal and so the section is vali-
dated. The best-fit lines go to zero area at depths below the reference level of -199 and
-77 m, representing the predicted depths of the detachment. The agreement between
the depths to detachment, calculated independently for two different grabens of dif-
ferent structural styles, is additional evidence of a valid result. The displacement on
the lower detachment is 39 m for the Franklin Hill half graben and 101 m for the Strip
Mine full graben.
The Tip Top anticline provides a field example of a compressional fold formed by
shortening at the front of the Wyoming thrust belt (Fig. 11.22a). The upper part of the
structure is constrained by surface geology and the wells but the deeper structure is
open to question. Is the interpretation valid? The presence or absence of layer-paral-
lel strain is unknown, as is the position of the lower detachment.
The data points from nine measurable horizons in the anticline fall reasonably
close to the least-squares best-fit straight line
h
= (1 / 0.51)
S
- 1.10, with correlation
coefficient 0.961 (Fig. 11.22b). This result validates the interpretation and indicates
that the structure could be a locally balanced detachment fold (Sect. 11.4.3). The
displacement on the lower detachment is 0.51 km. The depth of the basal detachment
is 1.10 km below the reference level at the base of the Triassic. This places the lower
detachment at the stratigraphic level of the top of the crystalline basement. The ex-
trapolation of the section to the lower detachment in Fig. 11.22a is based on the area-
depth line and shows the size of the structural closures predicted from the area-depth
line for horizons 10-12 below the base of the Triassic.
