COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

42 Bulletin de Ia Commission geologique de Finlande N: 0 212.
In the part of the outcrop between Paboasi and Sariehu these rocks are
undeformed and look like typical unmetamorphosed sandstones, etc., though
there is some evidence for the beginning of mechanical deformation in the
north of this sector. Bates (1939) re cords pebbles up to 10 inches in length
from some of the conglomerates. Pebbles are commonly quartz, pink »quartz
porphyry» (a metasomatic rock whose significance lS described later), greenstone,
phyllite, quartzite, and (1) porphyrite. The rock fragments are more
angular than the quartz and quartzite. These conglomerates are apparently
restricted to the western margin of the Group outcrop.
The sands tones range in colour from red and brown through purpie and
mauve (and rare green) to pale cream or buffo Isolated rounded pebbles are
often seen and one was identified as a coarse diorite. The less common siltstones
are similar to the sandstones though finer grained and usually darker.
Bedding is generally inconspicuous. The strike is parallel to the regional
trend of the Birrimian and dips range from 10° to 50° westwards, but eastward
dips of 26°, 29° and 45° have also been recorded. Beds, where visible,
are from one to five feet thick. Nice (1962) re cords dips, on a sandstone
cliff-face about 50 feet high, ranging from 45° at the base to 10° at the top.
On the other hand, he also mentions a twenty-five foot high exposure showing
no signs of bedding. A poorly developed graded bedding is the only
sedimentary structure that has been seen, and this rarely.
Upper Birrimian metavolcanics are exposed to the east of the Group
outcrop near Paboasi, but before Akoti is reached, going north, only Lower
Birrimian phyllite is found in this position. Within the Group outcrop
phyllite is exposed in its eastern half and diorites or metavolcanics in the
western half, giving an impression of interbedding that is later shown to be
false. The western margin of the Arenaceous Group is in contact with either
the diorites and other rocks of the Central Zone or with the metavolcanics
of the Border Zone. The boundary between the two Zones apparently winds
»into» and »out of» the Arenaceous Group outcrop. On its eastern margin
exposure is scanty in low-lying ground and there is no distinct boundary
between the Group and the phyllites and greywackes of the Lower Birrimian.
This indistinct eastern boundary is the only main feature of this southern
part of the Group outcrop which carries on into the northern part.
At the Mamnao latitude the signs of deformation that were noted in the
northern part of the southern sector become the most distinctive feature of
the rocks of the Group. Flattened pebbles in the conglomerates are in sharp
contrast to the angular fragments in the rocks near Paboasi. Dips from
Mamnao northward become high and many are vertical. At the Chine latitude
the Group ceases to be a continuous deposit and is represented simply by
undeformed sandstones occurring in the area of the Upper Birrimian outcrop.
The uncertain boundary between Upper and Lower Birrimian from