Facies Study of Lake Deposits Formation (Qpl) To Determine Deposition Environment of Ancient Limboto Lake: a Preliminary Result

Abstract

Limboto Lake located in the Gorontalo Province is a part of the places where the disclosure of various types of rocks as well as various tectonic deformations, the genesis of this lake is a low basin included in shallow lake type or type of flood plain. Development of Limboto Lake starts from the existence of the Neogene extension exactly on slab-rollback extension of the Banda embayment in Early Miocene, rapid uplift and subsidence near Gorontalo Basin in Late Miocene. Various problems that occur in the Limboto Lake both silting and susceptibility to lake water flows allegedly related to the tectonic activity that ever happened even today. This study reports new determination about the ancient environment of Limboto Lake. The purpose of the study is the facies study of the Lake Deposits Formation (Qpl) to determine the depositional environment of ancient Limboto Lake in Gorontalo Basin against its influence on the current Limboto Lake. The environment facies identified in the Gorontalo Basin are marine deltaic facies and shallow marine carbonate facies (carbonate rimmed shelf). The evidence found by stratigraphically features is: (a) Marine Deltaic Environment: (1) Lithologies: mainly sandstones (compositionally dominate immaturely) through muddy sandstones, sandy mudstone to mudstone; also the presence of lignite; compressed weakly and in the form unconsolidated, (2) Texture: not diagnostic, average sorting and rounding sand grains, (3) Structure: cross-lamination and wave-lamination of various types in the sandstones, flat-bedding, and channel common. Finer sediment show flasher, wavy bedding and are heterolithic. Some sediment containing rootlets; (b) Shallow-Marine Carbonate Facies: (1) Lithologies: Skeletal packstone-wackestone, and mudstone, (2) Texture: diverse, (3) Structure: cross-lamination, parallel-lamination, wave-lamination, desiccation crack; in the reef limestone is massive and unbedded, with intense bioturbation. Based on the ancient environment determine to encourage the high activity of silting Limboto Lake because the material around the lake easily detached.