Minggu, 18 November 2012

Geomorfologi Terapan |Firs Task

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Lembar 7

Fig.3.14 Vertical aerial photogragh ( 1:40,000) of the terraced cap of Muna Island, Indonesia, eith post-faulting.

Is best developed on the ridges.

The sub-recent reefcap has an estimated thickness of approximately 100 meters and unconformably overlies the older rocks(basalt,tuffs,serpertines) which emerge from it at certain localities. 14 C dating indicates a rate of uplift of the reef of apporoximately 0.7 mm/yr (Tjjiaet al, 1975). The 25 m level of the reef picture in the photograph has an approximate age of 36.370 B.P.

In cratogene areas of low relief, one might not have at firstsight,high expectations for movements occurring there, wich are of relatively small magnitude. Because of the low relief amplitude that characterizes most of these areas, important geomorphological consequences may nevertheless result and assist in the analysis of neotectonic. Major deviation of rivers, the  formation of lakes and swamps in depressed zones, and even vegetation patterns allowing for diferentation between dry and marshy land, may serve for indications. A good example is the Ob river basin in the USSR where anticlinal and synclinal axes were found in lowland areas reflecting recent movements along older structural ternds in the underlying rocks.comparable studies from the USSR are from Vilenkin(1968) and shumilow ( 1969).

Sri lanka, with its staircase of old plantion surfaces, is normally considered a tectonically stable zone, where Quaternary crustal movements cannot be expected. Nevertheless, when Herath(1962) studied the coastal development and protection near the town of negombo, situated along the west coast of the island, it became clear that the geomorphology of the coastal plain and the configuration of the coastline in the area were profoundly affected by differential Holocene tectonic movement along the

E-W  faultline or zone which was proven by airborne geophysical survey. As a consequence, a lagoon-barrier type of coast is found to the south of negombo, whereas to the north of the town, a recession of the coastline has occurred since the last few thousand years, as evidenced by the patterns of cut-off beach ridges and the present coastal forms and processes. (See alsosection 8 )

Lembar 8

Another clear example of how geomorphological features may reveal neotectonic movement in cratogene areas is theincipient south-eastward extension of the East African rift frist discovered on landsat images in northern Botswana. Discovered of the river water by these recent fault systems in a north-eastern and also in a south-western direction affects the whole hydrological situation in the Okavago Delta and adjacent areas of Botswana. Further to the east a complex and closed tectonic depression became the site a Quaternary lake, the size of which fluctuated under the influence of climatic varitation and the tectonic development mentioned above.Fig. 14,8. Illustrates the situation . ( Verstappen and cook ( Eds)1981)

Further to the north, other geomorphological  indication can be found which aid in clarifying the tectonical  history of the East African rift Zone. In Rwanda, for example, firting and accompanying tilting has resulted in drastic changes in the drainage pattern. The rivers, formerly draining towards the rift, have been diverted to from the head waters of the nile. The head waters have been largely converted into papyrus-covered lakes by way of tectonic damming.

Localized  neotectonic  movements are usually difficult to trace in low land areas, although  features such as aligned mud volcanoes  may be indication. The indications for regionalized neotectonic movements, however, are numerous and diversified. Zones of  by subsidence are often clearly marked by for example , broad flood plains with extensive inundations , swamps, and drowned forests.

 The map of fig. 3.16 pictures such a situation in Sumatra and the oblique airphoto of fig. 3.17 gives an example of the lowlands near the fly river, southern  New Guinea. Slightly higher grounds, with tropical  rain forests, indicate alternating zones of upheaval where flood plains tend to be some what narrower. The distributional pattern of finer and coarser sediments and their thickness are other good indicators in such low land areas. Geophysical data may  provide further evidence.

Slow, over all subsidence of a delta under the influence of its own weight or due to tectonic movement, is not always easily separted from a rise in sea or lake level. Young diferential crustal movement in the deltaic area, however can often be readily traced and have a great  impact on the development of numerous  deltas. Recent accretion near the mouth of the Digul River, New guinea , for example is of little importance, as shown by a comparative study of old and  new  maps and of recent air photo. The main, outlets of this huge river draining a large part of southern New Guinea have some unpronounced estuary characteristic. Drowned forests covering extensive areas further island, indicate that  recent subsidence is  common to considerable parts of the southern low land of the island. Most of the river load apparently is deposited there, which explain the stationary situation at the river mouth. It is astonishing that from a study of  the same  sources, it is astoningshing  that rapid from a study of the same sources, it is clear that rapid seaward displacement of the coastline  occurs several tens of  kilometers further to the south, near  the weastern side of  kelepom  island. The changes of the coast , evident from maps dating from 1903 and airphoto of 1945 are indicated in fig. 3.18. the growth there has  no direct relation to the sedimentation of the Digul River , but is due to the zone of recent uplift which passes through Cape Valsch and which can be traced from the air across Kolepom Island ( Frederik Hendrik, Dolak) at a short distance from the coast and further towards the southern low land of irian / new guinea. The larger part of  kolepom island is scarcely above sea level and is covered by reed swamps.

The digul river, farther to the east, is forced to alter its southern course and to take a western direction to the north of the well-known Merauke Ridge. The princess Marianne Straits which stretch in a south-south- west direction to the east of the vast kolepom Island is to be considered a lower course of the Digul River, which was in use when the river erosion was still capable of adequately counteracting the up warping of the latter ridge.the straits are still navigable due the scouring action of tidal currents.

Another example of a delta where tectonic movement enter the scene and counteract aggradational processes, is the Mamberamo Delta, located at the northerntip of Irian Barat, Western  New Guinea. The huge Memberamo River system drains the larger part of the island, north of the Central Range. Its main branches are the tuaral(rouffaer)Tariku (van der willingen) River and the taritatu(Idenburg) River, both of wich flow in the low lying and rather swampy belt, called Meervlakte and stretching in an east-west direction to the north of the Central Range. Downstream of their confluence, the Memberamo River Takes a northern direction and breaks through the east-westerly stretching Rouffaer- Van Rees Mountains to reach  the pacific Coast at cape d’Urville.

Recent accretion of the delta (fig.3.19) is unimportant which is easily explained by the fact that most of the river load is deposited further upstream , primarily in the Meervlakte zone, but to some extent also in an east-west zone of subsidence occurring in the deltaic area. This situation explains why the river quietly flows between its natural levees and has no connection with its former branches. It also clarifies the existence of an important saline intrusion as observed from the board mangrove belt stretching far island. A beach ridge.

 

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