Makalahnya pak Tjia HD, geolog yang mendapatkan gelar doktornya dengan meneliti bebatuan unik di Karangsambung (Kebumen) dan kini bermukim di Malaysia. Tak disangka, Pulau Langkawi yang populer itu ternyata merupakan sisa tumbukan benda langit setidaknya dalam kurun waktu 10 juta tahun silam. Pada seminar 28 Oktober 2008 silam di ITB, mbak Dewi Pramesti Kusumaningrum
telah mempresentasikan rencana penelitiannya dengan judul
Initiation Study in Search of Impact Crater in Indonesia dalam sesi Earth and Space Sciences yang diketuai pak Dr. Moedji Raharto. So, perburuan kawah meteor di Indonesia telah dimulai..
Ayo pak Praboto, terus berburu via Google Earth
HC Potential of Meteorite Impact Structures, Focus on Sundaland
H. D. Tjia
Jakarta, January 21, 2004
Craters abound on the surfaces of the Moon, solid planets and their satellites. Prior to the manned Apollo missions, views that the craters of the Moon were products of volcanism or of meteorite impacts were shared about equally. The collected Moon rocks show definitive features of high pressure but relatively low-temperature metamorphism that overwhelmingly favour impact origin. On Earth, the suspected impact depression in the Southwestern United States known as Meteor crater, was found associated with high-pressure quartz, or coesite. Currently some 300 terrestrial structures are considered products of impacts by extraterrestrial objects. Almost two hundred of these have been proven as such by combinations of the following: (1) arcuate to circular surface morphology, (2) circular gravity anomaly patterns, (3) shatter cones, (4) poly-megabreccias containing
cleaved quartz, (5) quartz and feldspars with mosaicism (patchy extinction), (6) high-pressure quartz polymorphs of coesite and stishovite, (7) anomalously high Iridium, (8) diaplectic glass, and (9) microdiamonds. The comparatively low density of terrestrial impact craters on the Earth's surface is attributable to reworking by exogenous processes of weathering, erosion, organic activity, burial by younger deposits, and the 70 per cent surface cover by water. Impact craters should be as common on Earth as on the solid extraterrestrial bodies. Calculations suggest a mean probability of over 15,000 significant impact craters having hit land since the Archaean. On land the average depth to diameter ratio of an impact crater is 1 : 0.2 , while rim height is about 4 per cent of the total diameter. Also on land, the diameter of simple, bowl-shaped impact craters probably do not exceed 4 km in igneous rocks and about 2 km in sedimentary rock. Beyond these
limits, complex crater morphologies develop as result of flattening through gravity.
The petroleum significance of meteorite impact structures is -so far- very minor, but may be more important for several reasons. Renewed attention to impact structure plays is relatively recent and was fueled by the 1991 single-strike discovery (25 MMBO, 15 BCFG recoverable reserves) in the vicinity of Ames, Oklahoma, U.S.A. About twenty years earlier other significant discoveries were made at Red Wing Creek, North Dakota (20 MMBO, 25 BCFG), and at the world-famous Chicxulub region, Yucatan Peninsula, Mexico (total recoverable reserves of 30 BBO, 15 TCFG). However, the structures of these earlier finds were not identified as astroblemes and at the time of their discoveries the respective reservoirs were considered ordinary fractured carbonates and fractured granite-and- carbonates. The petroleum occurrences in the vicinity of Ames, a small town in Oklahoma,
U.S.A., were the main topic of a special meeting in the early 1990s. The reservoir rocks of the Ames structure are lithified "basement" granodiorite and Cambro-Ordovician Arbuckle dolomite breccia on the crater rim; 2D seismic indicates an erratic, pocket-like distribution of its porosity and permeability. Trapping is by subsurface closures with one known structural closure. A dolomite caprock and overlying Middle Ordovician Simpson Shale provide the cap and seal. The source consists of the Arbuckle sediments or rocks that were exposed to the "cracking" environment of the meteor impact, and the shales that were deposited in the crater. Flow is c. 250 to 500 BOPD; with some of the wells producing in excess of 250,000 bbl oil, while one particular well has yielded 3 BCFG. Recovery has been in the 10% to 60% range.
Continental Southeast Asia includes the tectonic platforms of Indosinia and Sundaland . By virtue of its longer exposure, its thinly
covered pre-Tertiary "basement" areas can be expected to host more impact structures than regions containing thick Cenozoic sediments. Good reservoir is provided by brecciated and fractured basement rocks, while traps may comprise rim anticlines, irregularly distributed breccia pockets within craters, and central rebound peaks (in the larger impact structures). Internal seals may be provided by impact-melt cloaks and a top seal by the blanketing sediments. For the Southeast Asian impact structures, the hydrocarbon charge is likely to originate in the Tertiary basins. Several impact structures have been recently discovered in pre-Tertiary Peninsular Malaysia. In the Langkawi islands three of four arcuate ridges are associated with cleaved quartz that crops out as a sill and dyke complex. Other shock-metamorphic features include ribbon quartz and mosaicism.
The two major craters, named Mahsuri Rings, partially overlap and each is about 2.4 km
across. In 280o - 100o direction their centres are 600 metres apart. Bouguer gravity cross sections prove their crater form, one being 45 m deep, the other attaining 107 m depth into the target rocks of Carbo-Permian Singa Formation. Both depressions are filled, the top surface consisting of Quaternary alluvium. The partially encircling hills of Singa Formation crest at less than 150 m elevation and are open to the west. Towards SW are two other circular/arcuate topographic features: Temoyong Ring and horseshoe-shaped Tepor Island. The diameters decrease in the same direction: approximately 800 m and 500 m, respectively. These four ring-like structures have been interpreted as products of serial impacts by a flight of extraterrestrial projectiles arriving from the Southwest. The impact age is not yet determined and field relations only indicate a post-granite (Triassic-Jurassic) event. In the case that the morphologically young Mahsuri Rings are not
exhumed features, impact could have occurred within the last 10 million years.
Another proven impact structure is the Paloh Ring that straddles the state boundary between Terengganu and Pahang. The proof consists of planar deformation features (PDFs) and mosaicism in vein quartz intruded into undivided Carboniferous metasedimentary strata that compose the lower eastern slope of the 623m high Paloh peak. PDFs are also seen in thin sections of quartz phenoclasts of polymict-breccia boulders in Sungai Mengkuang that drains the east side of the hill. Bukit Paloh is the peak on the high, circular topography surrounding a deep depression, now referred to as Paloh Ring-1. This ring-like topography is 3.5 km across and has local relief of the order of 150 to 200 metres. A small -about 0.5 km across-circular depression is located on the Paloh Ring-1. The south half of the larger ring consists of felsic igneous rock whose K/Ar age is 243 Ma, while the
north half is composed of Carboniferous metasedimentary rocks. The youthful morphology of Paloh Ring-1 points to a geologically young impact event, estimated to be not later than Late Miocene. More than a decade earlier, PDFs in quartz were discovered in partly weathered granite underlying the Quaternary basalt at Gebeng, some 40 km SE of Bukit Paloh. The granite at Bukit Ubi quarry, located in the same area but nearer to Kuantan town, also contains quartz with PDFs. These two findings are the initial proofs for impact products in the whole of Malaysia. It is unresolved if the Gebeng-Bukit Ubi impact products and those at Bukit Paloh originated from the same event.
The third proven impact products are at Bukit Bunuh near the world-famous palaeolithic site of Kota Tampan along the Perak River. Recently Mokhtar Saidin, geo-archaeologist of Universiti Sains Malaysia, verbally reported a 1.74 Ma fission-track age for a volcanic agglomerate composed
of unsorted small to huge fragments of quartz, quartzite, schist, felsic igneous rocks, and other as yet unspecified rock types. The megabreccia is set in light-coloured groundmass, superficially resembling volcanic tuff. Thin-section examination of the quartz turned up parallel planar fractures, mosaicismal extinction and also several cleavage sets. Detailed studies are in progress, but there is little doubt that this "agglomerate" is in fact suevite , an impact breccia. In spite of alteration by oxidation, hydrolisis and local secondary mineralisation of fractures and other voids, porosity estimates by visual inspection are of the order of 10 per cent.
Satellite and synthetic-aperture radar images complemented by topographic maps and aerial photographs indicate over 60 prominent arcuate and circular features throughout Malaysia. Craters, geological and geophysical field studies, presence of suevite, and PDFs in quartz prove the impact origin
for the Mahsuri Rings in Langkawi, the Paloh Ring-1 on the Terengganu-Pahang border, and the megabreccia at Bukit Bunuh (Kota Tampan) in central Perak. Thirty seven other arcuate/circular features could be impact products and will need confirmatory studies; eight other structures appear to represent domal intrusions or exhumed volcanic complexes, while the remaining sixteen are meander scrolls and structural basins. Contributing factors of extraterrestrial impacts to petroleum systems comprise depressions favouring source-rock development, enhancement of reservoir quality in most of the target rocks, providing a trapping environment for fluids, and perhaps accelerate maturation. Given that prominent impact structures are not rare in Malaysia and that during the Cretaceous-Palaeoge ne vast regions of continental Southeast Asia were subaerially exposed, impact-structures probably occur in significant numbers. The initial steps of testing impact-structure
plays include expansion and verification of the regional database of impact structures and re-examination of seemingly anomalous subsurface structures that are already known.