Typology and mechanisms of coastal erosion in siliciclastic rocks of the NW Borneo coastline (Sarawak, Malaysia)

Abstract

The northwest coast of Sarawak State, to the south of Miri city, consists of tilted siliciclastic rocks of the Miocene age. The region is rapidly developing, and tourism is increasing along this stretch of coast where cliff-backed sandy beaches alternate with rocky headlands. To date, no systematic study of coastal erosion features has been undertaken. In this research, five sites were studied and their erosion features were documented. A typology has been established in which the features are classified according to three size categories (macro-, meso- and microscale). Macroscale erosion features, exceeding 10 m size, include numerous examples of mass wasting, rock falls, sea caves, and arches. These were examined and mapped in detail at the two most popular geotourism sites. Mesoscale erosion features, from a few meters to less than 1 m in size, include widened joints and bedding planes, pockets, gullies, and runnels, as well as tafoni and notches. Microscale erosion features, less than 1 cm in size, include isopod perforations in soft sandstone. The methodology is a field approach with a number of processes inferred and related to lithology and geological structure, including mechanical action by fresh and sea water and bioerosion. Cave formation is examined at different sites, leading to identification of three contrasting cave types: tabular caves with lithological control and domed caves with collapse control, both in moderately tilted strata, as well as prismatic caves with joint control in steeply dipping strata. The study provides evidence of the magnitude of recent coastal recession, landslides, and arch widening and collapse features. It emphasizes the dynamic nature of coastal processes in the region, including sources of sediment supply and mobilization. The likelihood of future rapid mass erosion events is discussed.