Page 82 - Sumatra Geology, Resources and Tectonic Evolution
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PRE-TERTIARY VOLCANIC ROCKS 69
of Rosidi et al. (1976) is of Artinskian to Wordian (Murgabian) by Silitonga & Kastowo (1975). The results of the reappraisal of
age (Fontaine & Gafoer 1989) (see Fig. 4.11) and appears to be age of the limestone outliers of the Kuantan Formation (Fontaine
a facies of the Palepat Formation beneath the Tabir Formation. & Gafoer 1989) suggests that a similar reappraisal is required for
If so, the Tabir Formation could be younger than the Wordian. the volcanics in the main outcrop of the Phyllite and Shale Member.
In the Painan Quadrangle (Rosidi et al. 1976) the eastern part of To the NW the large strike-slip duplex structure within the
the Barisan Formation (obsolete term) includes discontinuous out- Sumatra Fault Zone in the Lubuksikaping Quadrangle (Rock
crops of the Palepat Formation which link up with the Silungkang et al. 1983) contains faulted outcrops of the Silungkang Formation
Formation (Table 6.4 & Fig. 6.5a,b). The Silungkang Formation is and the Panti Volcanic Formation, which are deformed lithologi-
Sakmarian-Wordian in age, although the upper age limit is cal correlatives, respectively, of the Volcanic and the Calcareous
not well controlled (Fontaine & Gafoer 1989). A K-Ar age of members of the Silungkang Formation in the type area.
248 _+ 10 Ma obtained from a volcanic rock from the Silungkang In the north of Sumatra, in the Takengon Quadrangle, the
Formation, reported by Nishimura et al. (1978), is in agreement Situtup Formation, contains metavolcanics and limestones.
with the stratigraphic age range. Cameron et al. (1983) suggest that the metavolcanics are mainly
Anomalous younger K-Ar ages are reported by Suwarna et al. of Late Permian age. Fossils from the associated limestones
(2000) from volcanic rocks from the Silungkang and Palepat are of Mid-Permian (Artinskian-Capitanian) and Mid-Late
formations, with an andesite from the Silungkang Formation Triassic (Ladinian-Norian) age (Fontaine & Gafoer 1989). The
dated at 140 _ 10 Ma, and an andesite from the Palepat Formation outcrops are allochthonous and much disrupted by Miocene
outcrop dated at 75 + 1 Ma. Evidently younger volcanic rocks thrusting. Hutchison (1994) following van Es (1919), included
have been mapped as part of the Palaeozoic outcrop, probably the Situtup volcanics within the West Sumatra Permian
because similar lithologies of different ages are intermixed in Plutonic-Volcanic Belt. If this correlation is correct and the
discontinuous exposures. The volcanic rock from the Silungkang Situtup Volcanic Formation extends into the Triassic, it is the
Formation which gave a 140 + 10 Ma age may be associated youngest component of this belt. However the presence of the
with the Lower Cretaceous andesites known to occur beneath Situtup Volcanic Formation in the belt may be a coincidence
the Tertiary sediments in the nearby Ombilin Basin, and dated at as Barber (2000) has suggested that this formation is an allochtho-
143 + 4Ma (Koning & Aulia 1985). As mentioned above, nous component from the Woyla Accretionary Complex.
Tobler (1922) also mapped Cretaceous volcanics as part of the
Palepat Formation.
Lithologies in the Silungkang Volcanic Member are similar Geochemistry of the Silungkang and Palepat Formations
to those in the Palepat Formation (Table 6.4). Outliers of the
Calcareous Member of the Silungkang Formation intercalated Chemical analyses of selected volcanics from the Silungkang and
with basalts 4kin NE of Tanjung Gadang, and diabase at Palepat Formations are presented by Suwarna et al. (2000) who
Lubukkarak (Gafoer et al. 1992a), were previously mapped as found that the compositions of the two formations are very
part of the Phyllite and Shale Member of the Kuantan Formation similar. The range of SiO2 in the Silungkang Formation is
I
(a) SILUNGKANG FORMATION l(b) 100o45'E
~ Limestones with
t= intercalations of
sandstone & slate
Basaltic extrusives
o Undifferentiated
Lower Permian
L.~,~,.~,.,:~-,:t Tufts & agglomerates
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