Perubahan fenomena geomorfik daerah kepesisiran di sekeliling Gunungapi Muria Jawa Tengah :: Kajian Paleogeomorfologi

SUNARTO, Promotor Prof.Dr. Sutanto

2004 | Disertasi | S3 Geografi

Penelitian ini dilakukan di Provinsi Jawa Tengah pada empat kabupaten, yaitu Demak, Jepara, Pati, dan Kudus. Ada tiga tujuan dalam penelitian ini yaitu (1) mengetahui perubahan spasiotemporal fenomena morfologi Delta Wulan dari waktu ke waktu dan pengaruh perubahan tersebut terhadap pantai di sekitarnya, (2) mengetahui sebab-sebab perbedaan perkembangan beting gisik di daerah kepesisiran sebelah barat dan sebelah timur Gunungapi Muria, serta (3) mengetahui perkembangan spasiotemporal paleogeomorfologi daerah kepesisiran di sekeliling Gunungapi Muria. Metode yang digunakan adalah menggunakan metode deskriptif-eksplanatori yang berusaha menginterpretasikan genesis dan evolusi sekuensial daerah kepesisiran di sekeliling Gunungapi Muria berdasarkan data morfologi pantai, sedimen pantai, kelautan, dan pentarikhan radiokarbon. Cara menganalisis data mendasarkan penalaran sebabakibat (causal-effect) dan analogi melalui tingkatan eksplanasi, yaitu (a) tingkat deskriptif, (b) tingkat komparatif, (c) tingkat asosiatif, dan (d) tingkat kausal. Untuk menguji hipotesis dalam penelitian ini dilakukan dengan cara logika induktif. Hasil penelitian dapat digeneralisasi, bahwa daerah kepesisiran di sekeliling Gunungapi Muria telah terjadi perubahan fenomena geomorfik. Perubahan tersebut diakibatkan oleh dinamika iklim dan dinamika laut. Macam-macam temuan perubahan fenomena geomorfik adalah perkembangan Delta Wulan, erosi marin, pembentukan chenier, pembentukan dataran aluvial di Selat Muria purba, pembentukan sekuen beting gisik, dan terbentuknya gua marin. Delta Wulan dalam selang waktu 70 tahun telah berubah bentuk dari arcuate menjadi digitate, luas delta bertambah 0,393 km2/tahun, panjang garis pantai bertambah 338,57 km/tahun, dan panjang sungai utamanya bertambah 72,86 m/tahun. Perkembangan Delta Wulan menyebabkan laut lebih dinamis dan mengakibatkan erosi marin di daerah perdesaan sepanjang pantai di Kabupaten Jepara dan pembentukan chenier di sepanjang pantai di Kabupaten Demak. Perkembangan beting gisik di daerah kepesisiran barat dan timur Gunungapi Muria berbeda, karena pengaruh dari gelombang, proses marin, evolusi garis pantai, dan arus laut. Paleogeomorfologi daerah kepesisiran selatan Gunungapi Muria semula merupakan selat yang berkembang menjadi rataan pasut dengan banyak antasan dan ditumbuhi mangrove. Rataan pasut itu sekarang telah menjadi dataran aluvial. Daerah kepesisiran barat Gunungapi Muria pada 6.000 tahun BP masih tergenang laut terbukti dijumpai terumbu karang berumur 6.530 tahun BP. Beting gisik yang ada sekarang ini berumur 4.600 tahun BP semula berupa gosong penghalang yang ada di bawah muka laut. Daerah kepesisiran sebelah utara Gunungapi Muria relatif lebih sempit dan banyak dijumpai perbukitan vulkanik. Di lereng perbukitan vulkanik pada ketinggian sekitar 15 m dijumpai gua marin dan batuan terabrasi yang mengindikasikan muka laut pada 9.000 tahun BP. Di daerah kepesisiran timur Gunungapi Muria terbentuk empat lajur beting gisik sekuensial dengan rata-rata selama 1.055 tahun terbentuk satu lajur beting gisik. Namun sejak 1.760 tahun BP hingga kini tidak terbentuk beting gisik baru, karena pasokan material vulkanik dari hasil letusan Gunungapi Muria telah terhenti, sehingga berkembang rataan pasut.

This research was carried out in four regencies i.e. Demak, Jepara, Pati and Kudus, Central Java Province. There are three objectives in this research, i.e. (1) to analyse the spatiotemporal change of morphological phenomena of the Wulan Delta from time to time and the influence of the change to the shore next to it, (2) to analyse the difference causes of beach ridge development in the western and the eastern coastal area of the Muria Volcano, and (3) to synthesize the spatiotemporal development of the coastal palaeogeomorphology in the Muria Volcano and its surroundings. This research adopted a descriptive-explanatory method that to interpret the genesis and the sequential evolution of the coastal area in the Muria Volcano and its surroundings. Data analysis technique based on causal-effect reasoning and analogy by explanation stage: (a) descriptive stage, (b) comparative stage, (c) associative stage and (d) causal stage. For hypothesis test in this research was be done by inductive logic method. The results of this research can be generalized that the coastal area in the surrounding Muria Volcano has been occurred the change of geomorphic phenomena. This change has been effected by climate change and coastal dynamics. The finding types of the change of geomorphic phenomena are development of the Wulan Delta, marine erosion, formation of chenier, formation of alluvial plains in the ancient Muria Strait, formation of sequential beach ridges and formation of marine notches on the slope of the volcanic hills. The Wulan Delta during 70 years changed from arcuate form to digitate form, the delta width increased 0.393km2/year, the long of delta shoreline increased 338.57 m/year, the long of main stream increased 72.86 m/year. The development of the Wulan Delta affect the sea water more dynamic. The marine dynamics caused marine erosion in the rural area at the longshore of Jepara Regency and the chenier formation in the rural area at the longshore of Demak Regency. Development of beach ridges in the western and the eastern coastal area of the Muria Volcano is differ, because there are some influences of sea wave, marine processes, evolution of shoreline and sea current. Palaeogeomorphology of the southern coastal area of the Muria Volcano originally is strait which developed to become the tidal flat with many creeks and was grew mangrove. Now, that tidal flat was became the alluvial plain. The western coastal area of the Muria Volcano in 6,000 years BP was still inundated by sea water. That condition can be confirmed by coral reef which have an age of 6,530 years BP. The present beach ridge have an age of 4,600 years BP, originally is a barrier bar. The northern coastal area of the Muria Volcano relatively narrow and were occupied some volcanic hills. On the slope of the volcanic hills at elevation of + 15 m were exposed marine notches and abraded rocks. Those phenomena showed the sea level in 9,000 years BP. In the eastern coastal area of the Muria Volcano was formed four sequential stripes of beach ridges. One stripe beach ridge was formed in every 1,055 years. However, since 1,760 years BP until now there is no formation of beach ridge, because there was no volcanic materials supply from explosive products of the Muria Volcano anymore. Therefore, the tidal flat has been developed.

Kata Kunci : Geografi Fisik,Perubahan Geomorfik,Daerah Kepesisiran Gunung