High Resolution Biostratigraphy snd Correlation Accross Paleogene Boundaries In The Ombilin Basin, West Sumatra

Abstrak

The Ombilin basin is located within Barisan Mountain Range of West Sumatera Province. The basin is one of few intermontane basins in Indonesia which expose Paleogene lacustrine sediment, braided stream deposits and marginal alluvial debris fans. The Ombilin basin is well known due to its large coal reserves in Sawahlunto Formation. Many authors proposed different stratigraphic frame work of the Ombilin basin mainly built upon surface geological survey. Only limited publication discussed the stratigraphy of this basin based on detailed subsurface data. The main goal of this research is to understand the Paleogene biostratigraphy of the Ombilin basin, based on palynological, foraminiferal and nannofossil records which are obtained from cutting samples of exploration well. The methodology includes laboratory preparation and microscopic analysis. Biostratigraphic analyses apply the quantitative method, which require logging and counting of microfossils in every analyzed sample. The number of foraminifers resulted from the observation of 50 grams of washed samples, the number of nannoplankton is obtained from 400 fields of view within sample slide, whereas the number of palynomorph is derived from 250 recovered specimens.
The study results in that the age of the analyzed sediments range from Early Miocene to Eocene. The boundary of Early Miocene/Oligocene is defined by the occurrence of last occurrence of palynomorph taxa Meyeripollis naharkotensis which is supported by occurrence of foraminiferal species Globigerinoides primordius and Ammonia umbonata and nannoplankton index Cyclicargolithus floridanus (small). Meanwhile, the Oligocene/ Eocene boundary are remarked by the first occurrence of palynomorph index Cicatricosisporites dorogensis. These biomarkers can be correlated in each well for better understanding of depositional history in studied area.