Sedimentology Of The Lambir Formation (Late Miocene), Northern Sarawak, Malaysia

The Late Miocene Lambir Formation is extensively exposed along the road from Bekenu to Miri near the Bukit Lambir National Park. This formation is characterized by a succession of poorly consolidated sandstone and sand interbedded with soft clay in several exposures. Four sedimentary facies from three (3) outcrops of the Lambir Formation have been identified and interpreted. These sedimentary facies are categorized into three main units depending on their major lithology type: (1) mud-dominated unit: bioturbated heterolithic mudstone (F2); (2) sandstone-dominated facies: hummocky cross-bedded sandstone (F1) and trough cross-bedded sandstone (F2); (3) interbedded sandstone/mudstone unit: interbedded hummocky cross-bedded with bioturbated mudstone (F3). These four (4) facies are grouped into three (3) facies association which are (1) lower to middle shoreface, (2) upper shoreface and (3) offshore. The pattern of facies and presence of marker pollen species imply that these facies were deposited in the shallow marine setting with pronounced storm, wave and tidal influence along the paleo-margin. Acme of Zonocostites ramonae and high proportion back mangrove pollen suggest a former mangrove belt that was developed in conjunction with back mangrove swamp within coastal area. Pollen analysis suggests this sediment was deposited during Middle-to-Late Miocene period.