GEOLOGICAL OF ALOR AND WEST WETAR SHEET, NUSA TENGGARA

STRATIGRAPHY

The stratigraphy of geological map Alor and West Wetar Sheet follows

Qal ALLUVIAL AND COASTAL DEPOSITS; consists of gravels, pebbles, sand, silt and muds. Deposited in fluvial and around Kalabahi and Maritaing gulfs in Alor Island.

Qalk LAKE DEPOSITS; consists of conglomerate and sandstone. Conglomerate, polymict; composed of andesite, dacite and basalt; matrix is coarse sandstone. Sandstone, grey, coarse grained, poorly sorted. Generally this unit is unconsolidated, horizontal bedding, well exposed in the eastern part of Alor island, as a rework of volcanic rocks transported from rivers in the Lantoka area.

Ql CORALLINE LIMESTONE; consists of coralline limestone and coral breccia. Coralline limestone, yellowish white; compact, reaches an elevation of approximately 350 m above sea level; in north Kebola cape, it forms terrace.

Qtv OLD VOLCANIC PRODUCT; consists of lavas, breccia and pumiceous sandy tuff. Lavas are pyroxene and hornblende andesite. Locally, they show columnar jointing. Breccia clasts are pyroxene andesite and hornblende andesite. Matrix is tuffaceous sandstone, easily loose. Pumiceous sandy tuffs, generally dacitic composition. Locally, there are found lapilli tuff intercalations. The upper part is more pumiceous friable. In some places it shows crossbedding. In Alor, this unit is found in the Boguibui and Pemang areas. In west Wetar, it can be found in south coast and Kambing island. The thickness of the unit is estimated 350 meters, and it overlies unconformably the Alor formation. The absolute age of andesite in Kambing island is Pliocene – Plistocene (Abbott and Chamalaun, 1981).

Tpdi SIDONGMANG QUARTZ DIORITE; consists of quartz diorite, grey; compact; composed of andesine, hornblende and quartz. The hornblende has been altered to chlorite. This rock is exposed a dykes intruding the Alor formation; wide about 20 m. The quartz diorite is thought to be a Late Pliocene age. It exposed in Sidongmang and Lamataga.

Tmpl LAKA FORMATION; consists of calcareous tuff and sandy tuff intercalated with fine breccia and conglomerate; locally intercalated with marl and limestone. The calcareous tuff contains Globigerinoide sp., Pulleniatina sp., Textullaria sp., Sphaeroidinella dehiscens BLOW, Pulleniatina primalis BANNER & BLOW, Globorotalia truncatulinoides D’ORBIGNY, and Globorotalia acostaensis BLOW. This fossils content indicated a Late Miocene – Pliocene age and neritic depositional environment. Dips of the unit are 30 – 35⁰, with strike trending west – east. Thickness is approximately 1500m. This unit I presumed to interfinger with the Alor formation and is unconformably overlain by the coralline limestone.

Tmpa ALOR FORMATION; consists of lavas and breccia, intercalated with tuff. Lavas are hornblende andesite, biotite hornblende andesite, hyalo andesite and pyroxene basalt. Locally, showing strong alteration, mineralized and sheeting joint structure. In Pido, central Alor the dioritic dyke intrudes this unit. Tuff intercalations are silicified with dips 10 – 45⁰, with strikes of northwest – southeast and northeast – southwest. Thickness, estimated of this unit formation is 1000 m. Age of this unit presumed to be a Late Miocene – Early Pliocene and interfingers with the Laka formation distributes widely and almost covers the whole Alor and West Wetar.

Tmt TANAHAU FORMATION; consists of dacite lava, tuff and breccia. Lava, light grey – dark grey, dacitic, silicified and contain sulphide mineralizations of copper and lead (Goenadi, 1971). Breccia is light to dark grey; compact; clasts of dacitic composition; matrix sandy tuff, silicified. Tuff has been silicified. This unit is found in south of Pido, southwest Alor and Lisa in West Wetar. The formation is a Middle Miocene age (Suwarna, et al., 1983) intruded by quartz diorite.

Tmda DACITE; light grey in colour; texture is fine to medium – grained. This unit intrudes the Tamaneng granodiorite about 2 meters thick. Good exposed in the Tamaneng area.

Ttgd TAMENANG GRANODIORITE; greenish grey; porphyritic texture, composed of andesine – oligoclase and pyroxene, partly altered to chlorite. It exposed in Tamenang area, central Alor. Based on the similarity to lithology and stratigraphic position with the Wolowaru granite the unit is assigned an Early Miocene age (Suwarna, et al., 1983).

Figure : Correlation of Geological Map Unit of Alor and West Wetar, Nusa Tenggara

STRUCTURES AND TECTONICS

Geological structures found in the Alor and West Wetar sheet are folds, faults and lineaments. Generally, the structures are trending northeast – southwest, but some are northwest – southeast. The strong folds are found the rocks of Late Miocene – Early Pliocene age, dips of 35⁰, these folds are found in the tuff of the Laka formation trending east – west. Based on these evidence, the process of folding presumably took place in Plio-Plistocene time. Faults found in this area are normal faults, trending west – east and northeast – southwest. They are found in rocks of Late Miocene – Early Pliocene age up to the youngest rocks, therefore, probably most faulting took places persists until present lime. Lineaments that could be observed clearly on SLAR and landsat imageries, in general, are nearly all are trending northwest – southeast and northeast – southwest direction. Those lineaments cut Late Miocene – Early Pliocene rocks. Most probably, some of the lineaments represent faults, but convicing evidences were not clear in the field. A graben trending northwest – southwest divides cape Kebola from Alor land.

Geological history of this sheet is commenced by granodioritic intrusion of the Early Miocene, and it is uplifted to the surface. In the early Middle Miocene, the area experienced volcanic activity which produces dacitic rocks (Tanahau formation). There are some dacitic dykes intruding the granodiorite. At that time the sheet area is still a terrestrial environment. The sedimentation in the Alor island occurred in the Late Miocene – Early Pliocene. During that time the area is a basin form where the Laka formation is deposited meanwhile a part of the Alor island is a land. In the end of Late Miocene time the area is uplifted and deformed to all pre-existing rocks. The deformation is followed by a magmatic activity forming dykes of quartz diorite penetrating the Alor formation. In Late Pliocene – Early Pleistocene, the volcanic activity still continued until the present day, producing volcanic rocks of the andesitic rocks. In the Late Pleistocene, the whole area is uplifted and become a land; and faulting takes place in the whole rocks. The uplifting movement continues until present time. Therefore, terrace of limestone and lake deposited area formed in some places.

MINERAL AND ENERGY RESOURCES

Mineral resources found in the Alor and West Wetar sheet are lead, pyrite, chalcopyrite, malachite, gypsum, limestone, igneous rock, gravel and sands. Indication of the lead, pyrite, chalcopyrite and malachite are found in quartz veins in the Alor formation at the Kalaisi, Kuneman, central Alor District. In the east Alor district, the mineralizations are found in Tamenang, Sesemana Mazmur and Pido (Goenadi, 1971), resulted the dioritic and dacitic dykes. Gypsum fragments are found in the Laka formation, Sesemana Mazmur area, East Alor district. Construction materials are limestone, igneous rocks, gravels and sands. The limestone in the coralline limestone present in the northwestern part Alor, sand and gravels are in the alluvium and the coastal deposits. Igneous rocks as the volcanics and intrusion rocks are found abundantly in the area. Energy resources are hot springs found in the fault; same contain Sulphur.

Geological map of Alor and West Wetar can be downloaded at the following link : 2407-2408 Alor and West Wetar

Reference

Y. Noya, G. Burhan, S. Koesoemadinata and S.A. Mangga. Geological Map of the Alor and West Wetar Sheets, Nusa Tenggara. 1997.