FEET

5000

SEA LEVEL

Mural Fm, _ limestone

Midas Fm, | siltite

Yanks Peak Fm,

Yankee Belle Fm, quartzite

METRES

SEA LEVEL

Z S27
a
VA
YEE.
LA
si

quartzite Cunningham Fm, _ limestone

Isaac Fm, phyllite

Figure 33. Structural cross-section E-E’ from near Black Stuart Mountain palinspastically restored
to before the Columbian Orogeny. Palinspastic section ‘A’ assumes the unconformity at the base
of the Black Stuart Group is horizontal. Palinspastic section ‘B’ assumes the base of the Mural
Formation is horizontal.

In terms of Columbian tectonics the palinspastic sec-
tion demonstrates approximately 70% shortening in the
area between Kimball Mountain and the hill northeast
of Little River. This is the minimum amount of shorten-
ing possible with the structures of cross-sections E-E’ and
D-D’. The majority of this shortening is accommodated
by thrusting.

The section in Figure 33A assumes pre-Black Stuart
tectonism and peneplanation. It shows that pre-Black
Stuart deformation must have been mild in the area of
cross-sections E-E’ and D-D’. The possible deformation
is minor enough to be confused with the disconformable
situation depicted in Figure 33B.

It is possible that the stratigraphic relationships at the
unconformity are due to block faulting. Areas where the

46

highest level of stratigraphy is preserved would represent
downfaulted blocks. Such a block faulted regime would
indicate a pre-Black Stuart tensional event. Then, three
possible explanations for the mode of formation of the
pre-Black Stuart unconformity configuration are: 1) ten-
sion, causing a block faulted regime, uplift and subse-
quent peneplanation, 2) uplift and differential erosion
causing disconformable conditions, and 3) compression,

causing broad-scale warping, uplift and subsequent
peneplanation.

Northwestward from the palinspastic section the
Black Stuart is known to cut downsection from Mural
Formation to Yankee Belle Formation. This is not a grad-
ual northwestward bevelling, but one of low topography.

The unconformable relationships thus far described
are at odds with the conclusions of Johnston and Uglow