Page 58 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 45

TH E BATA N G NATAL -.soma
BNL BNM •"'"'•---
RIVER SECTION Jambor Baru :.~ Batu Nabontar
Formation
...
~, Limestone (BNL)
d~OMA
0 1 2 3km
I I l ........ I . .,~: : :'.
BNL
Rantobi
Sandstone . . : \ ....
Si Gala Gala ... :. :. :. ::: 9 :.:.. ..:.:.:.:..
.
Schists ,~,~::: : :: : : :: : ::~ PV Muarasoma Turbidite "" :" :" :" :
"'" ! ~ ~ "" Natal Formation (MTF)
:i: ;fi : i : ~~;~~'m :.N." Batang
Parlampungan
Volcanics (PV) ~.'...... .~:.~ Megabreccia
.:::::::::: . (BNM)
,'5"iiiiiiiii:: ~ : STF
' 'i:i:i:i:!: ",,',~',, 87.0Ma z,
,
~q:iiii ! Simarobu
Turbidite
~ ~ i Formation MUARASOMA"
Panglong 9 (STF) 44.8
Melange
Nabana Volcanics .~ ~ Ranto Sore
~,, BNL .,<,,^,, Formation
9 . .
...,..
~!! Betok Gadang NATAL
Siltstone v v v v ,r "r -, ".:::~ -... -,,~
!
.:::::::GAMBIR~ %,* %~ %g %g %', %g %~- %,'%,, ~ ~ ' " "-:-:
vvvv,~,vvvv"H""
::::::::::::::::::::::::::::::::::: ~ %a %', ",~ o~*%p %e %'* %', %,, %', V
.................. lfi }~Langj sat Volcanic' "~ ~
Tambak Baru
::::::::::::::::::::::::::::::::::::::::::::::::: %*~ ",P ".P ,.,~^r ~ .........
~ %" -r -r o %" %," ",d' V %" %P V
to" "o,r %." "v" %" "v" v "-,," ",~ %" %" %'* ~ %'. %'.
"~' "o,P %" %," %" ~g' ",/ %~ ",d' %," %-" Nr ",r
~}~
%,- -,,e %g %,- %,- v ;4::::::~!:i:Turbidites::::::::::::::::::::::::::::::: SIMPANG GAMBIR ~& Location of limestone block ' %" ~" %" ",-" %" 'N e %*" V %" %" ~'r %"
v, Langsat ,#,- with Late Triassic foraminifera %" %" %" %," %" ",e" %r %4 %y %p %r %r
v, Volcanics v v _,~,O Locations for K/Ar dates %'29.7Ma-" v v v v ",r v v v v
"e" %'* %" %" -,,e '~.#" %," %,,
0 10 20km
I I I I
v
v ....... v.+ : "S"vv : iiii!i!i!i, i .
:
, v
,,,,
Fig. 4.15. Geological map of the Batang Natal river section, North Sumatra. Inset shows isotopic dates, from Wajzer et al. (1991). S is serpentinite.
A minimum age for the Woyla Group is provided by the feature of the clastic units in the Woyla Group of the Natal
Manunggal Batholith, dated at 87.0Ma (Late Cretaceous) area is that they are ahnost completely devoid of quartz,
(Kanao et al. 1971, quoted in Rock et al. 1983), which intrudes suggesting that they have an entirely oceanic, rather than a conti-
limestones and serpentinites at the NW end of the Batang Natal nental origin (Wajzer et al. 1991).
section. The study established several additional age constraints for the
Woyla Group, using fossil evidence and radiometric dating. A
Study by Wajzer et al. (1991). The Batang Natal section was further specimen of Lovfenipora was obtained from a limestone
mapped in detail by Marek Wajzer from the University of block in the Simpang Gambir Megabreccia near the southwestern
London, in a follow-up study to the Northern Sumatra Survey, end of the Batang Natal section, and a Late Triassic foraminifer
in collaboration with BGS and with the assistance of Syarif was found in a limestone clast in the Batang Natal Megabreccia
Hidayat and Suharsono of GRDC (Wajzer et al. 1991). The in the central part of the section. Diorite intruded into the
mapping was supported by petrographic, geochemical and radio- Jambor Baru and Batang Natal Megabreccia Formations at Batu
metric studies. Wajzer et al. (1991) found that each of the units Madingding gave a K-At age of 84.7 4- 3.6 Ma and an andesite
recognized by Rock et al. (1983) in the Woyla Group, was com- in the Tambak Baru Volcanic unit, interpreted as a fragment of
posite, with the same lithologies repeated many times throughout a volcanic arc, gave 78.4 4-2.5 Ma. Both these lavas and the
the section, apparently in a random fashion (Fig. 4.15). Wajzer intrusions are of Late Cretaceous age. Andesite dykes intruded
et al. (1991) distinguished 16 lithostratigraphical units in the into the Si Kumbu Turbidite Formation (i.e. Sikubu Formation
Natal section. Correlation of these units with the mapping with of Rock et al. 1983), and regarded as contemporaneous with sedi-
those recognized by Rock et al. (1983) is shown in Table 4.1. mentation of this unit, gave K-Ar ages of 40.1 4- 4.6 Ma and
Detailed accounts of these lithological units are given in 37.6 _+ 1.3 Ma (Late Eocene) (Wajzer et al. 1991). Samples
Table 4.2. Many of the lithologies are similar to rock types collected from the Air Bangis granites and analysed by
described from the Woyla Group in Aceh, and by Rock et al. Wajzer gave K-Ar ages of 29.7_ 1.6 and 28.2 4-1.2Ma
(1983), with the addition of several outcrops of m61ange, (Late Oligocene) (Wajzer et al. 1991) showing that the Cretaceous
composed of blocks in a fine grained matrix, decribed as age for these granites suggested by Rock et al. (1983) was
'megabreccia' in Table 4.2 and Figure 4.15. One important incorrect.