f Regional Controls on Carbonate Developments in Central Luconia, Offshore Sarawak

The Central Luconia province in the offshore NW Sarawak is a stable platform, flanked by two areas of active deltaic sedimentations. It is characterized by extensive development of carbonate which started in the Early Miocene through Late Miocene. Up to present, more than 200 Miocene carbonate build-ups have been mapped and some 70 have been tested (Hutchison, 2005; Vahrenkamp et al, 2004, Epting, 1980). The Central Luconia is part of the present-day Sunda platform, and forms its NE shelf edge. It is<br>located in an intermediate position between areas of subsidence and faulting in the north and zones of pronounced compressional tectonic in the south. It is separated from the Baram delta province by a shear zone associated with the West Baram Line in the NE, and to the SW is the Penian High. Together with other structural provinces of Sarawak, the Central Luconia is believed to be an integral part of the Sundaland and is interpreted to be underlain by the continental basement (Hutchison, 2005). The Sarawak basin as a whole was formed as a result of NW-SE trending right-lateral fault movement during the late Oligocene-Miocene times (Ismail, 1997). This dextral movement is thought to be responsible for creating the NW-SE trending paleo-highs and lows and had also positioned the paleo-coastline in the same trend. However, due to fluctuating nature of the sea levels and tectonic tilting, these coastlines appeared curvy during most of the lowstand sea level phases. Structurally, the Central Luconia appears to have been rotated in counter-clockwise direction. The block is bounded by shear zones, associated with a dextral-wrench movement along the West Balingian line. Beside this, a few dextral-wrench faults propagated in the same trend were described by Ismail (1997),<br>namely, the Mukah Line, Igan-Oya line and others. It is likely that the Central Luconia formed the central part of the rotated blocks. Recent study on regional seismic lines indicates that the Central Luconia has undergone extension during Middle Miocene and was followed by continuous compressional phase for most of the Middle to Late Miocene. During the extensional and the subsequent isostatic readjustments, the Central Luconia was a depression bounded by the uplifted regions which formed the basin edges in the SW, south and SE. Areas in proximity to the main uplifted region in the east were dominated by clastics, whereas, the carbonate occurred on the rifted margin in the west associated with the rising of sea levels. The subsequent compressional tectonic had resulted in inversion and folding of the basin-fill strata. This phase was responsible for elevating the Central Luconia region and characterizing it with widespread presences of tight anticlinal folds. These folds formed the ideal sites for widespread carbonate growth during high sea level phases of the latest Middle Miocene to Late Miocene. The fluctuating low sea level phase of the Late Miocene had punctuated the buildups by karstified surfaces that indicate subaerial exposures and had demised some of it. With exception of a few buildups, most were drowned during the major sea level rise at the base of Pliocene.