Daerah penelitian secara administrasi terletak di daerah Dayu dan sekitarnya, Kecamatan Gondangrejo, Kabupaten Karanganyar, Provinsi Jawa Tengah. Bagian Utara dari daerah penelitian dibatasi oleh Desa Bukuran, disebelah Timur dibatasi oleh Desa Jembangan, disebelah Barat dibatasi oleh Desa Krandowahono, dan disebelah Selatan dibatasi oleh Desa Rejosari. Stratigrafi daerah penelitian terdiri atas enam satuan litostratigrafi dengan urutan dari tua ke muda adalah Formasi Puren (Pliosen Awal-Akhir), Formasi Cemoro (Plistosen Awal), Formasi Bapang (Plistosen Awal-Tengah), Formasi Pohjajar (Plistosen Tengah), Mud Vulcano (Plistosen Akhir) mengintrusi satuan yang lebih tua sebelumnya, dan satuan Endapan Aluvial (Holosen) yang menumpang tidak selaras di atas batuan yang lebih tua. Struktur geologi yang berkembang pada daerah penelitian dipengaruhi oleh deformasi kedua Zona Kendeng yang berarah utara - selatan. Penelitian arah arus purba di daerah Dayu, menggunakan hasil pengukuran arah sumbu struktur sedimen cross-bedding yang tersebar di Formasi Bapang. Berdasarkan hasil pengukuran arah sumbu cross-bedding di delapan belas lokasi pengukuran, disimpulkan bahwa arah arus purba Formasi Bapang mengarah ke baratlaut dengan tinggian di sebelah tenggara. Arah arus purba Formasi Bapang berpola Unimodal (Tucker, 2003) yang mencirikan lingkungan pengendapan Fluvial, dengan material vulkanik yang diperkirakan bersumber dari sebelah tenggara lokasi penelitian.

This paper presents the characteristics of coastal and marine sediments of the northern part of Obi Island, Molucca. Grain size analyses were conducted on 24 coastal sediment and 32 marine sediment samples. Taphonomy of Homotrema rubrum was studied on three samples from coral reef that grows on the slope off Laiwui to determine current direction. The mean grain size ranges between 0.01 – 2.6 mm, very well sorted to very poorly sorted, and while skewness varies from very finely skewed to very coarsely skewed. While lithic fragments predominate coastal sediments, biogenic fragments form the majority of marine sediment grains. Textural parameters and sediment composition show three distinctive groups that represent: 1) Akeleletango in the west that is characterized by moderately sorted coarse to fine sand; 2) Tabuji – Laiwui that is characterized by poor to moderately sorted fine granules to fine sand that is interpreted as debris flow deposit; and 3) Anggai in the east that is characterized by poorly sorted coarse sand. Linear discriminate function analysis shows the dominance of shallow marine deposits in beach sediments with strong fluvial influence. The taphonomy of H. rubrum indicates that the prevailing current direction off Obi is westward, which is supported by current observation.

The Georadar (GPR) as a non-destructive technique has been used extensively for geological research. The study was conducted in order to identify coastal sediment characteristics of northern Obi coastal area by performing GPR radar facies analysis. The model of GPR used was Sirveyor 20 with MLF antenna frequency 40 Mhz, penetration depth was 10 meter, and Radan 5 as processing software which include time zero correction, spatial filtering, deconvolution, migration, adjustment amplitude and signal gain. Data interpretation describes georadar image/radargram based on radar facies methodology. The result showed three differences of radar facies unit. The radar facies unit can be divided into 2 facies units in which there are 2 other sub-units, namely unit 1A, 1B, 2A and 2B. Radar facies characterized by medium-high amplitude, continuous parallel-subparallel, medium-weak reflector. At the top part down to 3.75 m is Unit 1A that intersects with 1B. Below 3.75 – 7.75 m depth interval is Unit 2A, and unit 2B is from 7.75-10 meter depth interval. Unit 1 is interpreted as Quartenary alluvium (Qa) and unit B is Woi Formation (Tmpw). Radar facies characteristic indicated the differences of the geological unit.