13 November 2015
STRATIGRAPHY
The
stratigraphy of geological map East Wetar Sheet follows
Qal ALLUVIUM; consists of
pebbles, gravels, sands and silts. As fluvial and coastal deposits. They
distribute near mouth-river of Meta Sakir and Meta Arwala in the eastern part
of the sheet.
Ql CORALLINE LIMESTONE;
consists of coralline limestone and breccia. The age of the unit is Holocene.
The unit unconformably ovelies the older volcanic rocks and exposes along the
coast of Wetar island. In the Nila cape and Eden cape it forms terrace, which
indicate that unit has been uplifted about 500 meter above sea level.
Qtv OLD VOLCANIC ROCKS; consists
of lavas, breccia agglomerate and pumiceous tuff. Lavas are pale grey-dark
grey, porphyritic, jointed and andesitic. The unit distributes around south
coast of the western part of the sheet. Thickness of the unit about 350 m (Noya
et al. 1986). This unit unconformably covers the Alor formation. Base on the
stratigraphic the age of unit is presumed to be Plio-Pleistocene.
Tmpa ALOR FORMATION; consists of
andesitic and basaltic lavas and breccias, intercalated by sandstone and tuff.
The age of the unit is presumed to be Late Miocene – Early Pliocene. The lower
part interfingers with the Naumatang formation. Type locally of the formation
is on the Alor island (Noya, drr, 1986).
Tmn NAUMATANG FORMATION;
consists of dacite lavas, breccias and dacite agglomerates intercalated by
tuffaceous sandstones, tuff and sandy limestone. The calcareous sandstone
contains Globigerinoides sp., Pulleniatina sp., and Textularia sp. indicating the age of the
formation is not older than Late Miocene (Budiman, 1978). This formation is
exposed in the middle part of the sheet, trending from west to east. This unit
unconformably overlies the Tihu formation, the rhyolitic volcanic rocks and
intrusive rocks and interfingers with lower part of the alor formation, and its
good exposure at Meta Naumatang.
Tm (di,gd,gr,da) INTRUSIVE
ROCKS; consists of diorite (di), granodiorite (gd), granite (gr) and dacite
(da). These rocks intrude the Tihu formation and the rhyolitic volcanic rocks.
Radiometric dating the diorite shows a 12,6 + 1,3 million years or
Middle Miocene age (Abbott & Chamalaun, 1981).
Tmvs SAKIR RHYOLITIC VOLCANIC ROCKS;
consists of rhyolite lavas. This unit distributes in the western part of the
Lalaitu mountains and the head waters of meta vanula. This unit is intruded by the
Middle Miocene dacitic dykes and conformably covers the Tihu formation. The age
of this unit I presumed to be early Middle Miocene or late Early Miocene. The
unit is well cropped out at the head waters of Sakir river.
Tmt TIHU FORMATION; consists of
andesite and basal breccias, lavas and tuff. This unit distribute in the
southern part of the sheet and in the Lalaitu mountain. The unit is named based
on good out crop near lake Tihu and it can be correlated with the Kiro
formation in the Ende sheet which 19 + 2 million years or Early Miocene age
(Nishimura et al., 1981).
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| Figure : Correlation Unit of East Wetar Geological Map |
STRUCTURES AND TECTONICS
The area
belongs to volcanic banda arc but there is no active volcanic. Geology
structures found in the East Wetar are weak faults and folds. Moreover, from
airphotographs and ERTS imageries show lineament which its possibly the faults.
There are two main fault direction, northwest – southeast and almost east –
west or parallel to the island elongation. The faults are sinistral and normal.
The strike slip faults trending east – west cut the pre-Late Miocene the Tihu
formation and rhyolitic volcanic rocks, while the strike slip faults trending
northeast – southeast until Early Pliocene. The normal faults trending
northwest – southeast are cut the Late Miocene rocks.
Especially,
in Tihu lake there are two normal faults which are held the lake. These faults
are presumed as a graben system occurring in the post Late Miocene and the
forming the Tihu lake was formed. Volcanic activity occur in Early Miocene –
Middle Miocene resulting the Tihu formation and rhyolitic volcanic rocks; then
it is followed by uplifting together with dioritic intrusions in a Middle
Miocene age. In the early Late Miocene, the volcanic activity occurred again
yielding the dacitic rocks and form the Naumatang formation, together with
andesitic to basaltic rocks of the Alor formation. In the Plio-Pleistocene,
uplifting occurs the volcanic activity turned up in the south coast of the
sheet forming the old volcanic rocks. At that time Wetar island becomes a land.
The last uplifting is in the Late Pleistocene and then coralline limestone was
deposited. The uplifting is continuing until now.
MINERAL AND ENERGY RESOURCES
Andesitic
breccia distributing widely in this area can be used for building construction,
but in this time is un-useful. Clay is as tuff weathering products, around
granite in the Lalaitu mountains, it can be used for ceramic materials. The mineralization
in this area is indicated by pyrite in the dacitic rocks.
Geological map of east wetar can be downloaded at the following link : 2508 East Wetar
Geological map of east wetar can be downloaded at the following link : 2508 East Wetar
Reference
G. Burhan, S.
Koesoemadinata, D. Kadarisman, S.A. Mangga and Y. Noya. Geological Map of the
East Wetar Sheet, Nusa Tenggara. 1997.