Wave-tide depositional setting on the Middle to Late Miocene outcrops in Miri, Sarawak and Brunei Darussalam, Northwest Borneo

The Northwest Borneo basin has recently garnered significant attention from researchers across various fields, with a primary focus on both onshore and offshore geology. The Miri, Lambir, and Belait Formations are the principal study areas, with specific emphasis on aspects related to oil and gas. In this study, sedimentological analysis has identified two predominant groups of lithofacies: wave-generated lithofacies and tide-generated lithofacies. The wave-generated lithofacies include occurrences of lithofacies such as planar, swaley, and hummocky cross-stratified sandstone, primarily associated with wave-dominated shoreline deposits. These have been grouped into five lithofacies associations (LA): foreshore (LA 1), lower shoreface (LA 2), middle shoreface (LA 3), upper shoreface (LA 4), and upper offshore (LA 5). On the other hand, tide-generated lithofacies are represented by numerous heterolithic structures and herringbone cross-stratified formations, further categorized into lagoon (LA 6), tidal sandbar sandstone (LA 7), tidal to sub-tidal sand flat (LA 8), and sub-tidal mud flat (LA 9). The percentage of wave-tide activity suggests that both events are nearly equivalent, with the outcrop predominantly displaying either wave or tide deposits based on the observed sedimentary structures. The paleocurrent analysis suggests that the regional paleocurrent direction was either northeast or southwest. Consequently, the siliciclastic detritus was sourced from the Rajang Group. Tidal activity is believed to have occurred during mean sea level and is preserved in the wave sediment. Therefore, the study suggests that the deposition within the study areas occurred within the range of the outer part of the estuary (i.e., open-mouth area) to a shallow sandy sea.