MAC. Endharto
Geological Research and Development Center
Abstract
This study attempts to analyse the depositional system of the Ngrayong Sandstone into the basin pattern with using the basin analysis approached, concerning the hydrocarbon explorations in Madura Island. The study area is divided into three main zones which initiated with the western segment of the island. There are some geological key traverses e.g., Gunung Geger, Gujug Laut, Waterfall, Gujug village, Tanah Merah, Torjun-Somber, Sapulu and Kombangan.
Along the Gunung Geger-Gujug Laut-Water Fall section (south margin-western portion) is remarked by the appearance of the depositional classic progradation which characterized by the sedimentary structures (i.e. lenticular bedding, flaser bedding etc), implicate that the Ngrayong Sandstone was deposited within the tidal sand flat (open coast) system subdivision, from supratidal-salt mars to the shallow portion of the subtidal. environment. At the open sand quarry outcrop near Gujug Laut Village, well exposed of the basal portion (2m thick) of the Ngrayong Sandstone sequence, dark grey silty sand and shale demonstrate ripple cross lamination and wavy bedding underlie bioclastic lithic arenite contains tabular cross bedding indicate the intertidal sand flat, lower portion of tidal flat under condition low current speed. Tabular cross bedding occurs in the bioclastic lithic arenite interpreted as the sand flat in the headward portion of the macrotidal estuaries. Shelf transgression of marine condition is shown by the occurrences of the sandy limestone, upper part of the sequence. The presence of the hexagonal gypsum crystal formed within the calystone indicates the evaporation process in subaquaceous of supratidal zone. Paleocurrents are measured from planar and trough cross beddings display southwest direction (200o- 190o). Based on the sedimentary structures and trace fossils found in the Ngrayong Sandstone unit, along the Torjun-Somber section.
Along the Sapulu traverses (northern part of the west segment), the sand unit of the Ngrayong Sandstone was precipitated and more influenced by fair-weather wave base to outer shelf-transition zones, the lithology is characterised by the presences of the coarsening upward of the sandy limestone and alternating mudstone and shalestone. Change gradually from low to high energy to the landward is also present in this area, which is evidenced by the occurrences of the packstone.
The Kombangan traverse exhibits subtidal type of sediment marked by trace fossil of Cruziana (Planolites.). Marine condition as shallow lower shoreface to deep upper shoreface usually associates with Skolithos and Cruziana
From : The 33rd Annual Convention & Exhibition 2004 Indonesian Association of Geologist