Page 62 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTlARY STRATIGRAPHY 49

Golok Tuff C~slal luffs with sedimentary structures
Formation (water lain) and occasional fine to medium
(schematic)
interbeds

Massive limestone (biosparite) with shell
III I ~ I ~ I ~ I and algae
I i i i I i i
Pc+ ~r162 + ~+';
iul i i i i I I Pale coloured volcanics overlain by massive
limestone
+++~:+:C~++:~+o<)+:++~:+,+ Conglomerate with I(X)% carbonate clasts in
sandy shelly carbonate matrix
No exposure
Limestone conglomerate with basal scours
Massive limestone
Thinly-bedded limestone with dykes
I i i i i i i i i i,,, I Ill
i i i i i i'
i I i i I ! i Shelly oolite -heavily veined
i I i i i i i
Thinly interbedded with limestones and shelly
marls - boudinage~ marl flowage, veining
Thin pale tuff band in limestone conglomerate
Lubuk Peraku
Formation g
Dark marls containing blocks of dark volcanics
and limestone conglomerate (?tectonic)
Nearly t00% carbonate clasts
Conglomerate ? breccia. Poorly sorted, sub-
rounded to sub-angular clasts fi'om mm to
several m in size. Carbonate clasts include
bedded sandy limestone with bivalves, algal
fragments and solotary scleractinian corals
Minor, but significant volcanic clast component Fig. 4.17. Colunmar section through the Lubuk Peraku
Limestone and the Golok Tuff, measured in the Lubuk Peraku
river section, from McCarthy et al. (2001).

Saling Formation. The Saling Formation, which forms the northern sequence of ocean floor origin, together with fragments of a
part of the Gumai inlier, is composed of amygdaloidal and volcanic arc. Although the rocks are highly deformed and
porphyritic andesitic and basaltic lavas, breccias and tufts, associ- folded it is not clear from the descriptions whether they are imbri-
ated in the field with serpentinites and cherts. On the basis of cated to form an accretionary complex (Gafoer et al. 1992c). The
chemical analyses and discriminant plots the lavas have been strike of bedding and cleavage in the sediments is said to be
interpreted as tholeiites of oceanic affinity and have therefore north-south. The mapped east-west contact between the Saling
been interpreted as ocean floor basalts (Gafoer et al. 1992c). and the Lingsing formations is therefore presumably tectonic
However, the presence of andesites, the amygdaloidal and (Fig. 4.19).
porphyritic textures, suggests that the Saling Formation includes The Lingsing Formation has been interpreted as deposited in a
fragments of a volcanic arc. The lavas are cut by diorite bathyal environment (van Bemmelen 1949; Gafoer et al. 1992c).
dykes, regarded as contemporaneous with the lavas, and dated The presence of lavas interbedded with clastic deposits, suggests
by K-Ar analysis at 116 + 3 Ma (Early Cretaceous) (Gafoer that the Lingsing Formation represents more distal flows, volcani-
et al. 1992c). The description of the Saling Formation closely clastic sediments and clastic carbonates derived from a volcanic
resembles that of the Bentaro Volcanic Formation of Aceh arc, extending out into the ocean floor environment, represented
(Bennett et al. 198 la) and the Nabana Volcanic and Parlumpangan by the bedded cherts. These rocks resemble clastic units in
units of the Batang Natal (Wajzer et al. 1991). The Early the Lho'nga Formation of Aceh (Bennett et al. 1981a) and the
Cretaceous age shows that the Saling Volcanic Arc was active Belok Gadang Siltstone and Rantobi Sandstone formations of
contemporaneously with the Bentaro Arc of Aceh. Natal (Wajzer et al. 1991).

Lingsing Formation. The Lingsing Formation in the southern part Sepintiang Limestone Formation. In the Gumai inlier the Saling and
of the Gumai inlier (Fig. 4.19), contains igneous rocks similar Lingsing formations are overlain discordantly by the Sepingtiang
to those of the Saling Formation, interbedded with claystone, silt- Limestone Formation (Fig. 4.19). This is composed of massive,
stone, sandstone, calcilutite and chert. The Saling and Lingsing brecciated and bedded limestones, containing the coral Calamo-
formations are therefore considered to be contemporaneous. phylliopsis crassa (Late Jurassic), the foraminifers Pseudotextur-
Since tholeiitic basalts are associated with serpentinized ultrabasic ariella, small Cuneolina (Early Cretaceous) and Orbitolina sp.
pyroxenites and cherts, this assemblage is regarded as an ophiolitic (mid-Cretaceous). The contact between the Sepingtiang