Page 90 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY VOLCANIC ROCKS 77

random fashion (Fig. 6.12 & Table 6.9). Undifferentiated Jambor Baru Formation indicates an oceanic environment, while
Pre-Tertiary banded metavolcanics, slates and limestones, on the absence of plutonic fragments and presence of (altered) andesitic
northern margin of the Lubuksikaping Quadrangle (Rock et al. debris indicates that the volcanic source was nearby, perhaps
1983 geological map) are shown as part of the Woyla Group in within the accretionary complex but Wajzer (1986) suggested
the geological synthesis of Stephenson & Aspden (1982) and the source was a oceanic island arc in process of erosion. The
Rock et al. (1983, fig. 4). Aspden et al. (1982b) extended Jambor Baru Formation is bounded by strike-slip faults and a
subcrop of the Woyla Group up to the Sibolga Fault. These sliver of Parlumpangan-type volcanic rock is faulted within the
rocks are considered to belong to the Woyla Accretionary outcrop.
Assemblage, even though no ultrabasic rocks were described. The Simarobu Turbidite Formation is composed mostly of
Slivers of serpentinitized dunite and hartzburgites intruded volcaniclastic turbidites with minor calcareous siltstones, strongly
by thin basaltic dykes, are faulted within the Batu Nabontar Lime- deformed and metamorphosed in the greenschist facies. The
stone at the northeastern end of the Batang Natal section near calcareous siltstones may be recrystallized pelagic limestones,
Muarasoma (Wajzer 1986). These slices may be related to the while in the turbidites, the sparse quartz and K-feldspar and the
Pasaman Ultramafic Complex which crops out to the SE. This highly altered mafic volcanic clasts, suggest an intermediate
complex has a length of 75 km, an area in excess of 100 km 2 volcanic source and a trench or ocean-floor depositional environ-
(Rock et al. 1983), and is the largest ophiolite slice in Sumatra, ment (Wajzer 1986). The unit is affected by thrusts and later
although the thickness is not known. The complex is faulted strike-slip faults.
against an extensive limestone unit, the strike equivalent of the The Parlumpangan Volcanic Unit is of extrusive origin, prob-
Batu Nabontar Limestone and a mdlange unit, similar to the ably representing different levels of a volcanic pile constructed
Batang Natal Megabreccia in the Natal section. Wajzer et al. on the sea floor, which was emplaced and faulted within the
(1991) reported a Late Triassic foraminifer from a limestone accretionary complex. The Si Gala Gala Schist encloses, and
block in the Batang Natal Megabreccia, indicating that oceanic is strike-slip faulted against the Parlumpangan Volcanic Unit.
limestones, probably deposited on volcanic seamounts as old as Wajzer et al. (1991) interpret the Parlumpangan Volcanic Unit
Late Triassic, are incorporated in the accretion complex, either as fragments of a non-specific volcanic arc, but stress that the
as an olistrosomes or as mud diapirs. associated the Si Gala Gala Schists are derived from a con-
The depositional environment of the Muarasoma Turbidite For- tinentally based acid-intermediate volcanic arc. A volcanic
mation was probably in a small basin perched on the trench slope centre within the accretionary complex broken up by faulting is
of the accretion complex. The virtual absence of quartz in the a likely source of these two units.

THE BATANG NATAL ioma
BNL BNM
RIVER SECTION Jambor Baru Batu Nabontar
Formation
Limestone (BNL)
SOMA
0 1 2 3km \',,.
I I I I
BNL

Sandstone .:..
Si Gala Gala :.. .~..:.:.:.:.:.: ".:.:.:.:.:..
9 9 ..:.:.=.
9 .
Schists PV Muarasoma Turbidite " ' ' ' ' ' ' ' ' ' ' ' N
-.'.:.:.:,-
:.
: :: i i i i i ! i! i i! ) i! :~: " ~ ~ i i i . .egabrecciaBatang(BNM)Natal Formation (MTF)
"
M
,.
:.
~.TI
i
~
i
~
.
i
~
.~
~
;
,
Parlampungan
Volcanics (PV)
'~': :!iii!ii!ii!ii! iiii.~:i~STF .%..-....., ..., %, %, ..., 4/
-~L-L~z'J Manung al'x",,"
,." x"x ~ x Bath olitgh," x ~',, '
SimaroOu ,"x"x"x 87.0Ma J'x"-,'
Formation
(STF)
Panglong
Melange
Nab~a Volcanics 4 ~ Ranto Sore %,-,.'%..%.,
BNL -~": "L'." Formation
9
9 . . . .
:.:. .
-::: 9 Belok Gadang NATAL
~k : Siltstone
:
i b;> ~30 %" %" "" %" %" "" "" "" "" " "...
"V" %" "," %" %" %" "," %" V V %"
%" %" ",~ %" %" V "." "." %" %" %"
~" "4
Jgcan,c .......
--,." %.' %"
Tambak Baru 9 v v '..~ v v v %" %" %" v
%" --.-" v
Volcanics 9 ,,." %" %" %" "v" "-,-' "-,." ,,. ",,~ ~,-" %" "-.." "-.~ %" %" %" "-.-"
-.,.' %" %" %" -,.. ".,~ %" %,' %" %" %" v
9 %" .,... v
..,~5 ,.., 2Ma, v v %" %" v -~, %" ~- v v v %" %"
%-"
-
"" %'~ ..,..,.. " %" ~,~ f ~i:.iiiiiiiiiiiii:.--:~i!iiii Si'l<um'lou'!i!i!iiiiiiiiiiiiiiiiiii~
"
%
%
:Langsat ~!:!~::....:. ~:.--iiii::i::i::.T:ur:b!d:!!e.siiiiiii!i!i!i!i!i!!!!i!i!i!i:i::: S'"PANG GA,B,R Location of limestone bl ....- v -.., %" ~., ,.- %" v .4, 4, %" %" %" ,,. %"
9 ................ 9 ...... -..........-.-.....%....- .. .... 9 ~ with Late Triassic foraminife~ %'. . . . . .-~. v v v .~- . . . . 9 v
-.Volcanics..v %" 9 FAULT %" .... v %" .4....... v ..-
0 Locations for K/Ar dates } ~" ".29.7Ma" " %', v %"
-,- -," --," ,," v v v %" %" %" %" %" "'='=':.'=':';-:-='=-=-:'.':"-:':-:'":-=':':-='?:' ":"" ~5' %" ~," V ~," %" "I %" %" "~" V V "v'O ;~
-,.- -v- - 9 %" ~., %" %"
" ,~LANGSA1 0 ........, ~ %" %"
~..- ,,. %" %" %" 4... %" %" ,... i i ~ i AIR v v
Fig. 6.12. Simplified geological map of the Batang Natal river section. Adapted by Barber (2000) from Wajzer et al. (1991). S, serpentinite.