Page 48 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 35

mudstones of the Bohorok Formation represent deposits from a epidotized basaltic breccia and agglomerate, schistose locally
melting floating ice-shelf or icebergs, which are interbedded where they have been involved in thrust zones. From this descrip-
with turbiditic sands and shales, passing into distal turbidites tion it is possible that these volcanics belong to the Woyla Group
and deep water shales further offshore in the Kluet Formation. and have been intercalated with the limestones by thrusting.
The limestones of the Alas Formation, with oolites and current Fossils have been recovered from the limestones of the
bedding, as described in the foregoing account, represent Situtup Formation. They include the foraminifers, Agathammina/
shallow water carbonates deposited on a 'high' in the continental Agathaminoides sp., Planinvolutina cf. mesotriassica, Involutina
shelf environment. sp. ?sinuosa, Parafusulina sp., Pseudodoliolina sp., Neoschwa-
Fontaine & Gafoer (1989) relate the fauna and algal flora of gerina sp. and a coral Thecosmilia sp. (Cameron et al. 1983).
the Visdan Alas limestones to those found elsewhere in the Some of these fossils are of mid-Permian age (Parafusulina, Pseu-
Sibumasu Block, in western Peninsular Malaya, Thailand and dodoliolina and Neoschwagerina), while others are of Mid-Late
Burma. On the other hand, they relate the fauna and algal flora Triassic age (lnvolutina, Planinvolutina cf. mesotriassica and
of the limestones in the Visdan Kuantan Formation to those Thecosmilia) (Fontaine & Gafoer 1989). From this fossil evidence
of the eastern Peninsular Malaya and the Indochina Block in it is possible that the limestone constitutes a continuous deposi-
Thailand, Laos and Vietnam. tional sequence extending from the mid-Permian to Late Triassic,
While the Alas limestones could have been deposited in a and that the absence of Late Permian and Early Triassic fossils
cool environment, the fauna and flora of the Kuantan limestones is due to the accident of collection. More probably, as elsewhere
clearly indicate a tropical environment of deposition. Since the in Sumatra, there is an important unconformity within the
Alas and Kuantan formations are contemporaneous, they must outcrop, in which Upper Permian and Lower Triassic rocks
have been deposited in different environments on separate are absent. Unfortunately the relationship between Permian and
plates, and were only been brought together in Sumatra by post- Triassic components of these outcrop are unknown. These
Carboniferous movements. This relationship is indicated on the relationships should be the subject of future investigation.
Fontaine & Gafoer's (1989) Carboniferous palaeogeographic
reconstruction of Sumatra (Fig. 4.9) by an arbitrary WNW-ESE Ujeuen Limestone Formation (Fig. 4.3). The Ujeuen Limestone
boundary, separating the Kuantan Formation from the outcrops Formation outcrops as massive limestones to the south of
of the Kluet, Alas and Bohorok formations to the north. This Lhokseumawe where they are relatively innaccessible and
line has no present structural expression. poorly known. No fossils have been reported from these outcrops
(Cameron et al. 1983).

Tawar Formation (Fig. 4.3). Bedded to massive limestones with
Peusangan Group (Permo-Triassic)
minor phyllites cropping out on either side of Lake Tawar near
Takengon are designated the Tawar Formation (Cameron et al.
During the North Sumatra Survey, Pre-Tertiary rock units lying to 1983). Massive limestones, identified on the Takengon Quadran-
the NW of the Sumatran Fault System, which were apparently gle Sheet as a Reefal Member, occur along the northern side of
less deformed than the Tapanuli Group, were classified in the the lake. Phyllites and massive volcanics to the south of the lake
Peusangan Group, named from the Peusangan River which flows are identified as the Toweren Member. No fossils have been
northwards from Lake Tawar to the Andaman Sea. Fossil evidence found in any of these units. On the map they occur as thrust
showed that some of these units are of Permian and Triassic age slices imbricated with the slates and phyllites of the Carbonifer-
(Cameron et al. 1980). This terminology was subsequently ous-Permian Kluet Formation, the Jurassic-Cretaceous Woyla
extended to all Permo-Triassic units throughout Sumatra Group and Tertiary sediments. Again, it is possible that the
(McCourt et al. 1993). Because the outcrops of the Permo-Triassic phyllites and volcanies of the Toweren Member belong to the
units are so scattered and correlations uncertain, each occurrence
has been given a separate formation name (Fig. 4.10). Many of Woyla Group.
the units include limestones, some of which are fossiliferous Sembuang Formation (Fig. 4.3). Fifty kilometres to the east of Lake
so that the age may be precisely determined, but others are so
recrystallized that fossils are unrecognizable. These units, with Tawar is the outcrop of the Sembuang Formation composed of
massive recrystallized limestones overlying metamorphosed
discussion of the evidence for their ages, will be described in quartz sandstones (Cameron et al. 1983). No fossils have been
order from north to south.
reported.
Uneun Unit (Fig. 4.3). The Uneun Unit composed of slates, Kaloi Limestone Formation (Fig. 4.3). The Kaloi Limestone
metamorphosed limestones and epidotized basic volcanics is Formation crops out 40 km to the SSW of Langsa, where it is
named from the Kreung Uneun in the Takengon Quadrangle described as massive reddish tuffaceous limestone and dolomite,
(Cameron et al. 1983), and extends northwards onto the adjacent pock-marked by sink holes and flanked by fossiliferous shales,
Lhokseumawe Quadrangle (Keats et al. 1981). No fossils have limestones and sandstones (Bennett et al. 1981c). The massive
been found in this unit. The Unuen Unit probably incorporates limestones have yielded the trilobite Phillipsia aft. sumatraensis
rock units which should more appropriately have been included of Permian age (Tesch 1916). Forltaine (in Fontaine & Gafoer
in the Carboniferous Kluet Formation (slates) or the Jurassic- 1989) reports Halobia, and the shales have yielded Neoproetus
Cretaceous Woyla Group (epidotized basalts). indicus and Fenestella retiformis indicating a Late Triassic age.
in confirmation of the age, Metcalfe (1989a) obtained a specimen
Situtup Limestone Formation (Fig. 4.3). Bedded or massive fossili- of a Triassic conodont, Epigondondolella postera Kozer and
ferous limestones and intermediate volcanics cropping out in Mostler, from limestones and mudstones of the Kaloi Formation
Gle Situtup, a mountain 40 km to the NW of Takengon, have in the Sungai Kaloi, 5 km upstream from Kaloi. The relationship
been designated the Situtup Limestone Formation ('Sitotop between the Permian and Triassic components of this unit is
Limestone Formation' on the Takengon Quadrangle Sheet) unknown.
(Cameron et al. 1983). Other limestone outcrops are shown
resting on thrust planes above Tertiary sediments, or on units of Batumilmil Limestone Formation (Fig. 4.3). Fossiliferous 'reefal'
the Jurassic-Cretaceous Woyla Group, which crops out exten- limestones and grey calcilutites with chert lenses of the
sively to the west. On the map the volcanic rocks are shown Batumilmil Limestone Formation outcrop in the eastern foothills
cropping out within the main limestone, and are described as of the Barisan Mountains to the SW of Medan. Fossils include