Page 60 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 47

Table 4.2. Lithology, environmental setting, structure, metamorphic grade and age constraints for units in the Batang Natal section (in order upstream from west to east,
see Fig. 4.4), from Wajzer et al. (1991
Unit* Lithology Environment Structure Metamorphism Age constraints

Langsat Volcanic Porphyritic basic Arc volcanics No ductile deformation Prehnite- Possibly intruded by Air
Unit volcanics pumpellyite Bangis Granites. K-Ar
28.2 Ma, 29.7 Ma
Si Kumbu Volcaniclastic debris Submarine fan--apron D2 large scale folds (F2) Prehnite- Intruded by andesite dykes
Turbidite flows, proximal and to volcanic arc on WNW-ESE axes pumpellyite K-Ar 40.1 • 4.6 Ma
Formation distal turbidites (NR45), 37.6 • 1.3 Ma
(NRI20)
Tambak Baru Andesitic volcanics Fragments of volcanic Di weak foliation (Si); D, Prehnite- Andesitic lava. K-Ar
Volcanic Unit arc and proximal pumpellyite/ 78.4 _+ 2.5 Ma (BN 133)
volcaniclastics greenschist
Simpang Gambit Volcanic breccia with Proximal sediments D i strong foliations ($1); Prehnite- ?Lovfenipora sp. In limestone
Megabreccia limestone megaclasts and derived from volcanic D? open folds and pumpellyite/ block (Late Jurassic-Early
Formation greywacke sandstones arc, with olistostromes crenulations (F:) greenschist Cretaceous)
Nabana Volcanic Basic volcanics (sometimes Ocean-floor basalts, No ductile deformation Prehnite-
Unit pillowed) amygdaloidal seamount pumpellyite/
to east keratophyres, greenschist
dolerite dykes
Panglong Breccias with chert, Mn M61ange D~ tight to isoclinal folds (F~); Slate grade Older than Belok Gadang
M61ange sedim, limestones and (olistostrome) D2 open to close folds (F2) siltstone
Formation volcanic clasts in chert of ocean-floor fold F~ on NW-SE axes
siltstone matrix materials and
pelagic sediments
Belok Gadang Volcaniclastic siltstones Unconformable on Dipping beds with no ductile Prehnite- Younger than Panglong
Siltstone with few fine sandstones Panglong M61ange; deformation pumpellyite M~lange Formation
Formation and rare conglomerates ?lower trench slope
basin fill
Ranto Sore Volcaniclastic Fluviatile intra-arc D2 open to close folds Unmetamorphosed ?Younger than adjacent units
Formation cross-bedded and deposits (F2) on NNW-SSE
channelled sandstones axes
and unsorted
conglomerates (lahars)
Parlumpangan Porphyritic andesites Fragments of No ductile deformation Prehnite-
Volcanic volcanic arc pumpellyite/
Unit greenschist
Si Gala Gala Banded quartz, Metasediments D~ schistosity (S~) and Greenschist
Schist Unit muscovite, derived from rodding (LI); D2 open
chlorite schists acid-intermediate to close folds (F2) on
volcanic arc NW-SE axes
province
Simarobu Volcaniclastic turbidites Ocean-floor or Foliation (S~); D2 open to Greenschist Cut by undeformed
Turbidite with minor calcareous trench deposit closed folds (F2); D I tight microdiorite dyke. K-At
Formation siltstones to isoclinal folds (F]) axial 49.5 +_ 2 Ma (NR 7)
plane on NNE-SSE axes
Batang Natal Large clasts of limestone, Melange formed as D~ tight to isoclinal folds (Fl); Slate grade Included limestone clasts
Megabreccia rare clastic sediments olistostrome or as D 2 open to closed folds contain Late Triassic
Formation and igneous rocks in mud diapirs in deform S i about foraminifer. Intruded by
slaty matrix accretionary NNE-SSW axes; D~ Batu Madingding Diorite.
complex tight to isoclinal folds (Fi) K-Ar 84.7 ___ 3.6 Ma
with axial plane foliation
(S j); D2 open to closed
folds (F2) detbrm Si on
NNW-SSE axes
Rantobi Thin bedded volcaniclastic Forearc basin Axial plane cleavage (S~); D~ Slate grade
Sandstone sandstones and deposits isoclinal folds (F~) with D2
Formation siltstone closed asymmetric folds
(F2) NW-SE axes
Jambor Baru Volcaniclastic conglomerate, Shallow marine and D I foliation (S~); Prehnite- Intruded by Batu
Formation sandstone, siltstone, deeper water D2 closed folds (F2) on pumpellyite/ Mandingding Diorite.
limestone and tuff forearc basin deposits NW-SE axes greenschist K-Ar 84.7 + 3.6 Ma
Muarasoma Thin bedded volcaniclastic Upper trench slope Di foliation (S0; D2 folds Prehnite-
Turbidite turbidites with a basin sediments (F2) on NW-SE axes pumpellyite/
Formation coarser-grained member greenschist
Batu Nabontar Massive recrystallized Open marine shelf Dl tight folds in Recrystallized Intruded by Batu Manunggal
Limestone limestone, rare fossils limestone interbedded tufts (F1), Batholith. K-Ar 87.0 Ma
Unit fossils show strain
*All units are cut by numerous faults and thrusts. Vertical faults often show horizontal slickensides indicating wrench fault movements. *K-At age of Manunggal
Batholith from Kanao et al. (1971). All other K-Ar ages from Wajzer et al. (1991).