Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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Morey notes that sediment transport was from the north and that
material was derived from Archean granite, gneiss and greenstone.
PALEOHELIKIAN
Siby Group
he Sibley Group is a red bed sequence, deposited 1298±33 million
years ago (Rb—Sr whole rock isochron, Franklin, 1970) extending from
the Sibley Peninsula north to Armstrong, Ontario, and east to Rossport.
The seven units which compose the Sibley Group are
(a) basal conglomerate
(b) sandstone
(c) sandy red muds tone
(d) chert—stromatolite
(e) limey red mudstone
(f) purple mudstone
(g) limestone
Polymictic basal conglomerate lentils are most common on the
western margin of Sibley outcrop. Locally derived Gunf lint taconite
boulders are found where the Sibley overlies the Gunf lint, but gran—
itic boulders prevail where the Sibley overlies Archean rock. Lentils
range to 15 feet in thickness, and occur in pre—Sibley valleys.
Cream, green, and pink sandstone forms the lowest semicontinuous
unit of the Sibley Group, and attains a thickness in the basin margins
of over 200 feet. Beds are poorly graded; ripple marks and cross
beds are present throughout, but are common only in the eastern margin
of sedimentation near Rossport. Beds are composed of 50 to 70 per
cent quartz, up to 8 per cent chert, 5 per cent feldspar, and 5 per
cent mica, cemented with calcite and minor barite. Syneresis cracks
are common near Edward Island. At the top of the unit, interbedded
sandstone and mudstone mark the beginning of the sandy red mudstone
unit.
The sandy red mudstone unit is composed of less than 50 per cent
quartz and feldspar clasts, in a red hematite—carbonate—clay—feldspar
matrix, and ranges to 300 feet thick near Rossport. Bedding is
moderately well developed. Brecciation and soft—sediment folding
are common in this unit; chaotic conglomerate lentils are exposed in
the western margin of outcrop near Dorion.
In the area south and east of Nipigon, the sandy red mudstone
is separated from the limey red mudstone by a thin, but laterally
continuous, chert unit. To the north and west of Nipigon a stro—
matolite unit may occupy the same position. The stromatolites exposed
1
using l.47x10U yr. Rb87 decay constant; using l.39x1011 yr.
constant, age is 1376±33 m.y. The latter may be compared with the
date of Faure and Kovach (1969).