Page 41 - Sumatra Geology, Resources and Tectonic Evolution

28 CHAPTER 4

\ IoL%~'~ \A I as..~ - LAk , e , - - 9 9 I- ~..TobaTuffs........( I I

.
.
.
.
~
:
'
.
.
.
.
.
.
.
.
.
98 ~~%,1, Formation Toba,~: : : u9 10~ 101 ~,
L\-'.b.b,~.'~'-,_"IP" SlO/KAL~NG-'~'~}4~.". ~L...".". ". ". ". ? lc-~ '~ ~ Ma'or Faults
9 .'.'.'. ". ozoaBed. . ".'." '.'.',"
9 ~ Recent Volcanoes
/..,~ ~ ~ ~ ~ _ _ ~ " 9 9 ~ Pangunjungan ~N, (".,~ Permo-Triassic
-,,-',,...z ~ ",'~\'N~N"~E.~%8. i" i'-".N~ki~,Sibagandidg ~"" "~--dq:~.~li~_Member - k,~ ~ Imrusions
-t" Pal~ka ." ".'-Limestone MemlSer'.
"~~"~%~aF~ir'.'.\:.'.'.-.#/~i!~~IRANTAUPRAPAT ~ 4
~ue~-.'."~"A \\'.'. "%%"-'."-~~~;?" Z~ - ~'~'~ ~ ' ~
- 2'>N ~ -Formation_C~" ~ \%-v--:7 "~k 9 9 .'~i~q./'-,~./~/ ~Y.?N. N\~ " ~
e~aru 9 ." .., .'. 9 ". ". ". 2 ~
0 50 100km "~ ~'k~--~?~-~ \ ~ Bohorok Fm
........ ...' "':", _ encountered in
~'k ~'~%'L~,~DANG S[ DEN P UA a borehole
LATE PERMIAN-LATE TRIASSIC ~ ~ - ~ ' ~ ~
(Peusangan Group) \ ~'~Mbr ~ ........
. . . . . . . . \ ~, ~'#~.~'~a-'~ PAffARSIBUHAN
__ rkualu r-m, bllungKang ~-m, 'i ~ I'%Ui::lltli:l.ll _ ~. .
Telukkido Fro, Cubadak Fm ~, kFg~,~t~_,~..n.~:~ Lst~. 1 ~
Zuhur Formation \ ~ ~'~N~. LS~r~"-,~_ I PASIRPENGARAYAN
CARBONIFEROUS-?EARLY PERMIAN L, \ ~',~.'~'Q"~ "~,~,"'~ _
(Tapanuli Group) ......... "% ~ "~,,."~~ Pawan
, , [ ~4"2\\ ~ . ~ \ \ ~. "% Member
L~.~:~;~:;~I BohorokFormation ~ ' ~ a s i l ~ o n g i ~ " ~ \
[~i:~,~:~1 (Pebblymudstones) a ~ - o ~ . ~ ' - ~~ ",,>,..%,.x,,~,
~7,~.~ Alas Formation
(limestones)
Kluet/Kuantan Formati
Limestone Member (L Formation
_ 98 o uhur
Equator I I ~ - - - ~ ~t~-~ . . ~\~.~.'~'~'~/'~0rma!!..~ r
Fig. 4.4. Distribution of Carboniferous, Permian and Triassic stratigraphic units in north central Sumatra from GRDC map sheets, showing rock types and critical fossil
localities, as well as Late Permian to Triassic intrusives. Areas left blank are covered by Late Mesozoic to Quaternary sediments and volcanics.

Barisan Mountains to the north of Tapaktuan. Outcrops of the the central part of northern Sumatra metamorphic rocks include
Kluet Formation on the 1:250 000 map sheets are shown lying biotite-garnet-sillimanite schists, staurolite schists and biotite-
to the southwest of the outcrops of the Bohorok and Alas andalusite hornfels, chiastolite slate, quartzite, scapolite-bearing
formations and extend from Lake Tawar near Takengon in the calc-silicates, marbles and amphibolites. Some of these rocks,
north to Sibolga in the south (Figs 4.2 & 4.3). where they are associated with meta-limestones, are shown on
The formation consists predominantly of black slates, with the Takengon Quadrangle Sheet as part of the Alas Formation
phyllites, quartzose arenites and conglomeratic metagreywackes, (Cameron et al. 1983a) (Fig. 4.3).
the latter containing lithic clasts up to 40 cm in diameter. Poorly The surveyors attribute the metamorphism in the Kluet For-
sorted volcaniclastic wackes occur along the Sibolga to Tarutung mation to contact metamorphic effects (Cameron et al. 1982a).
road. The size and proportion of clasts in the conglomerates This is clearly the case for the hornfelses and chiastolite slates,
decreases across the outcrop from NE to SW. Locally there are but is less certain for garnet- and staurolite-bearing schists. An
calcareous horizons and detrital limestones. More massive obvious metamorphic aureole is developed around the Serbajadi
meta-limestones occur at Rerebe, south of Takengon (Fig. 4.3). Granite on the Langsa Sheet (Bennett et al. 1981c) where the
The sandstones are generally massive and commonly devoid of rocks are altered to musovite-biotite hornfels and wollastonite,
sedimentary structures, although in the type area of the Krueng diopside and phlogopite marbles and skarns. As the metamorphic
Kluet (Cameron et al. 1982b) and on the Sidikalang Sheet rocks in the Krueng Kluet are closely associated with concordant
(Aldiss et al. 1983), graded beds, mud clasts, slumped units, granitoids, and at Blangkejeren enclose concordant bodies of
load casts and dewatering structures, typical of deposition as garnetiferous gneiss, interpreted as intrusions, these were also
turbidites are reported. Rocks of the Kluet Formation have yet attributed to contact metamorphism.
to yield age-diagnostic fossils.
The rocks are metamorphosed, predominantly in the slate grade, Pangururan Bryozoan Bed. On the western shore of Lake Toba
but show varying degrees of metamorphism. An extensive area of at Pangururan in the Sidikalang Quadrangle, fossiliferous, calcar-
highly metamorphosed rocks of the Kluet Formation is shown eous, silty mudstones and limestones, with a rich shallow water
occupying the southwestern side of the outcrop on the Tapaktuan fauna are distinguished as the Pangururan Bryozoan Bed (Aldiss
Sheet, including the type area of Krueng Kluet (Cameron et al. et al. 1983) (Fig. 4.4). The limestones contain abundant shelly
1982b) (Fig. 4.3). The rocks are described as coarse muscovite- debris, including brachiopods, fenestellid bryozoa and crinoid frag-
biotite schists, sometimes garnetiferous, quartzo-feldspathic ments and some pelecypods. Decalcified, fan-shaped fenestellids up
gneisses and calc-silicate schists. In the Blangkejeren area in to 10 cm long are conspicuous on weathered bedding surfaces. The