Sedimentology of the Permian Monodiexodina-bearing bed of the uppermost Kubang Pasu Formation, northwest Peninsular Malaysia: Interpretation as storm-generated, transgressive lag deposits

The Permian fusulinoid Monodiexodena commonly forms dense accumulations associated with siliciclastic marine strata. The Monodiexodina-bearing bed of the uppermost Kubang Pasu Formation of Perlis, northwest Peninsular Malaysia is 0.5 to 1.5 m thick and sharply overlies an approximately 15 m thick coarsening upward succession interpreted as a regressive, wave-influenced, coastal shallow marine parasequence. Petrographic analysis of thin sections indicates that it is a mixed siliciclastic-carbonate, sandy allochem limestone. The grains are predominantly composed of a mixture of Monodiexodina tests, bryozoan fragments (Rhombopora sp.), brachiopod shells and crinoid ossicles, but with a significant amount of fine-grained quartz grains (up to 27% rock volume). The long axes of the skeletal grains are generally oriented parallel to bedding. The Monodiexodina-bearing bed is also characterised by giant, round-crested, symmetrical ripples developed on its bedding plane, with wavelengths of up to 1.6 m and amplitudes of up to 10 cm. Smaller, sharp-crested symmetrical ripples are superimposed on the giant ripples. A cross-section of one giant symmetrical ripple indicates the presence of unidirectional cross-stratification. The giant symmetrical ripples are interpreted as storm-generated, coarse-grained, combined oscillatory and unidirectional current flow bedforms, based on the symmetrical profile, unidirectional cross-stratification and close vertical association with other wave- and storm-generated facies. Giant symmetrical ripples are characteristic of coarse-grained storm deposits (commonly referred to as coarse-grained ripples, or CGR), while hummocky cross-stratification is developed in beds of very fine- to fine-grained sand. Palaeocurrent analysis of the ripple marks on the top surface of the Monodiexodina-bearing bed indicates a roughly NW-SE strike orientation for the ripple crests of both the giant and smaller superimposed wave ripples. Modern-day CGRs display ripple crest orientations which are parallel to bathymetric contours, thus it is interpreted that the palaeoshoreline during deposition of the Monodiexodina-bearing bed was generally NW-SE, which is consistent with current palaegeographic reconstructions of Sibumasu during the Permian. The stratigraphic position of the Monodiexodina-bearing bed sharply overlying a regressive unit or parasequence, and successively being overlain by another parasequence indicates that it is a transgressive deposit overlying a flooding surface, with the predominance of wave- and storm-generated facies indicating wave ravinement