COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

40 Bulletin de la Commission geologique de Finlande N: 0 212.
of its origin in the late stages of the Birrimide orogeny.l) This deposit, near
Bibiani in the Western Region of Ghana, has previously been described as
either Tarkwanian (Bates 1939) or latest Lower Birrimian in age (Hirst and
Junner 1946).
Table 1. Stratigraphical succession.
Assumed
(Hirst and Junner 1946)
Proposed
(this paper)
TARKWAIAN
. unconformity
UPPER BIRRIMIAN
ARENACEOUS GROUP
LowER BIRRIMIAN
ARENACEOUS ?
GROUP .
TARKWAIAN
. unconformity
unconformity
UPPER BIRRIMIAN
LowER BIRRIMIAX
GENERAL DESCRIPTION
The Birrimian of the Bibiani area has a regional NNE-SSW strike and
the most prominent feature is the hill range underlain by the metavolcanics
and other rocks of the Upper Birrimian. This range is flanked on the east
by low-lying ground underlain by phyllites, greywackes, and other minor
rocks of the Lower Birrimian (Lobjoit 1963). The Upper Birrimian outcrop
has been divided into a Central Zone of metasomatised metayolcanics and
two flanking Border Zones of unmetasomatised metavolcanics. The latter
are mainly metabasites, though andesitic and more acid rocks are represented,
and they have been orogenically metamorphosed (Misch 1949) in the green
schist facies. The metasomatites are quartz diorites, grading to adamellites,
and are representative of the amphibolite facies (Lobjoit 1963 and manuscript
in preparation). This account is concerned only with the eastern
flank of the Bibiani hills.
The deposit to be described occurs as a strip from a half to one mile
wide on the lower slopes of the hills, where it occupies a position astride the
Lower-Upper Birrimian boundary. The rocks have been called the Arenaceous
Group by reason of the volumetrie superiority of the sandstones over
the conglomerates and siltstones.
1) The term •• Birrimide» has been introduced by the writer as a name for the orogeny because
the term ).Birrimia11» has hitherto been used for both the system of geosynclinal rocks and the subsequent
orogeny and its accompanying infracrustal rocks. 'rhus ).Birrimian granitesl\ is a distinctly
mislcnding expression.