82

Section on Gravel River

The following description of the rocks, exposed on Gravel river is taken from
Joseph Keele’s “Reconnaissance across the Mackenzie mountains on the Pelly, -
Ross, and Gravel rivers, Yukon, and Northwest Territories.”

“The mountains of the watershed at the head of the Ross river are formed
of alternating beds of dark, compact quartzite and grey shale and slate. .

“About ten miles east of the divide some yellowish, crystalline limestones
occur in low isolated cliffs along the embryo Gravel river, but the principal
rocks are dark, sandy shales, striped grey slates, and micaceous sandstone or
quartzose schists. Rocks of this character extend eastward to mount Sekwi,
about fifty miles from the divide, and then end GMEUDUY tees. «sce ea ee

“A radically different geological province begins at mount Sekwi, and lime-
stones, dolomites, sandstones, and conglomerates, etc., of various bright colours,
replace the sombre rocks to the westward.”

The eastern part of mount Sekwi is composed of dove-coloured limestones.
These pass into grey argillites and are interbedded with sandstone or quartzite.
Farther east there is a section of over 2,000 feet of rather pure limestone. These
rocks are of Silurian age and vary in attitude from nearly horizontal to vertical.

Purple and greenish argillites, followed upward by dolomite, calcareous
sandstone, and limestone occur at the junction of the Natla and Gravel rivers.
These beds, which are of Cambrian age, have a total thickness of 4,000 feet and
are inclined at a low angle to the west. Dolomite forms a large part of this
section.

Below the mouth of the Natla the mountains are composed of rocks quite
different from the foregoing, beneath which they appear to dip. ‘They are
probably of Cambrian age and consist of

Feet.
Brown,:micacequs,ssandy. platess 6.9), 30). 99) EF SO | eee 1,100
Conplomeraiter ss ies ae eae) eet ee ae Peper ae Cred Gey ano 2,000
Coarsely laminated, hematite and siliceous SIQCGr sg eg per oi 100
Dolomite) and:arsillitest eee SES ee. SIOAOEE eG 1,0:010

The conglomerates form the wall rock of Shezal canyon.

Below Twitya river the rocks composing the Tigonankweine range consist
of 4,000 feet of alternating beds of argillite, dolomite, and limestone, succeeded
upward by a sill of diabase, 100 feet thick, and 1,500 feet of sandstone. The
diabase sill has an horizontal extent of several miles. This series, which is
referred to the Ordovician system, lies nearly horizontal. The sandstones, whose
prevailing colour is reddish, grow thicker towards the east and opposite the
mouth of Nainlin brook are about 4,500 feet thick, forming the entire mountain
mass. They extend down the river as far as Inlin brook where they are replaced
by limestones. ;

These limestones are probably of Devonian age. They are more or less
massive, are broken into several fault blocks, dip .to the southwest, have low
escarpments facing northeast, and constitute the eastern foothills of Mackenzie
mountains. With decrease in the altitude of the foothills to the eastward the
limestones become less tilted and broken, the bedding is thinner, and shaly layers
appear. In some of these, fossils of Devonian age were found.

“About twenty-five miles from the Mackenzie some soft sandstones and
conglomerates are exposed for a few miles along the north bank of the Gravel

1 Geol. Surv., Can., Pub. No. 1097,