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124

no proof that it does not occur at other points where the McMurray sandstone
is overlain by impervious strata. The Loon River formation of Peace river is
considered the equivalent of the MeMurray. .

Our knowledge of the structure of the Cretaceous formations .in the
Athabaska River district is very limited, but such information as we have
may be of use to those anticipating further operations. The Cretaceous
sediments of this district are nearly horizontal, and rest upon the Devonian
with an unconformity so slight that-it can be detected only by examination of
the contact for a distance of several miles. Between Grand rapids and Pelican
rapids, the dip of the Cretaceous strata is about 53 feet a mile south, and between
Pelican rapids and Athabaska the dip is about 10 feet a mile up the river. This
greater dip between Pelican rapids and Athabaska may be due, in part, to its
southwest direction and may indicate that the true dip of the Cretaceous strata
on the Athabaska is west of south. A low anticline crosses the river near Crooked
rapids; the dip on each side is only 3 or 4 feet per mile. Below McMurray the
strata are probably nearly horizontal and may have a slight north or northwest
dip.

To test the McMurray sandstone where it is overlain by impervious strata
it will, therefore, be necessary to conduct boring operations to the south or south-
west of its outcrop.

The oil struck in the shallow wells of the Athabaska Oils, Limited, opposite
the mouth of Dover river, occurs in a hollow of the Cretaceous-Devonian uncon-
formity. This depression is 12 miles long in the direction of the river and
opposite the mouth of Dover river has a depth of about 140 feet below the lime-
stone rim. Farther downstream there is a similar smaller depression.2

The problem of the origin of the hydrocarbons of the MeMurray sandstone
has not been solved. One theory is that they were derived from underlying
bituminous sediments of Devonian age; another, that they were forced in to the
porous sandstone from the sediments of later deposition.?

The structure of the Cretaceous on Peace river is described by F. H. McLearn
as follows:

“The salient features of the foothill structure in this district are the long,
low, east dips, of 10 degrees and less, and the steep west dips. The latter, affecting
only 2 miles or less of section, and separated by 10 to 12 miles of low east dip, are
probably related to incipient overthrusting. The structure as a whole represents
the dying out of the effects of the Rocky Mountain overthrust. The border of
the foothills is marked by an anticlinal structure pitching southward. This con-
sists of a single, large anticline on Portage mountain and in the canyon and of
two anticlines on Bull Head mountain.

“The transition from foothills to plains structure is very abrupt and takes
place where the Portage Mountain anticline is succeeded by an area of gentle
undulation and overthrust faulting extending as far east as the Gates. This
includes a broad, low anticline at Hudson Hope and a small broken anticline at
the Gates. From there to Cache creek there is a low east dip, under one-half
degree, with a local west dip equally low near the mouth of Cache creek. East-
ward to.several miles below St. John the structure seems to be almost flat. Near
the North Pine river and downstream the structure steepens with an east dip so
as to bring the Dunvegan sandstone almost to river-level a few miles below the

1 McLearn, F. H., Geol. Surv., Can., Sum. Rept., 1916.
2Huntley, L. G., Am. Inst., Min. Eng., Bull. 102, pp. 1345-1348,