DEPOSITIONAL FACIES AND STRUCTURAL ANALYSIS BASED ON FIELD OBSERVATION OF FRITU AREA, HALMAHERA ISLAND

摘要

Rocks exposed on the Halmahera Islands reflect sedimentation during periods of very complex tectonic. Much research and field work has been conducted by Apandi and Sudana (1980), Yasin (1980), Supriatno (1980), Hall and Nichols (1991), to determine the geology and hydrocarbon potential in the island. This field work has been conducted over a 100 km2 area of Fritu, Weda District, Central Halmahera. The focus of the study is along Parapara River, Talaga River, Babunyi River, Dodemangair River, Somdi River, Rica Hills, and Moro-moro. Data at more than 80 outcrops have been analyzed directly in the field and in laboratories. Samples were collected every 100 - 500 meters, but as a result of significant tectonic deformation, no section was measured in detail. Fracture analysis, topographic lineaments, strikes-dips and slickensides show that main compressive stress is oriented SW - NE. As a result, geological structures generally have a NW - SE orientation in accordance with the regional structure in the study area, influenced by Akelamo Fault. Interpretation is also based on the reverse bedding of Weda Formation on the surface. Analysis of lithostratigraphy, biostratigraphy, and petrography, yielded interpretation of depositional facies, paleogeograph, and shoreline displacement during deposition of these sediments, massive peridotite body (possibly representing part of an ophiolite complex) underlies the sedimentary sequence, and was emplaced due to tectonic activity of Late Cretaceous - Early Pliocene. Subsequently, in the Middle Miocene, this area experienced subsidence and reef (platform margin) - forereef packstone to wackestone facies were deposited in high environment; after a hiatus in the Eocene - Oligocene due to tectonic activity, a large basin developed with Early Miocene reef -foreslope wackestone in low - moderate energy environments; until these are overlain by shallow marine/upper slope shale facies sediments.