Provenance, environments, and tectonic signatures of Tanjong-Sandakan Formations, Northeast Sabah, Malaysia: Implications from sedimentology, palynology, and geochemistry

This study investigates marginal marine deposits found in the Tanjong and Sandakan formations from the Early to Late Miocene in Northeast Sabah. It examines sedimentology, trace elements (TE), rare earth elements (REE), hydrocarbon generation potential, and palynology to understand provenance, depositional environments, and paleoenvironmental conditions. Facies analysis identified four associations in the Tanjong and three in the Sandakan Formation, ranging from fluvial-deltaic to shallow marine environments. Mangrove-associated taxa, herbaceous and tree ferns typical of freshwater swamps, and coastal plants indicate a transition zone between terrestrial and marine environments. Siliciclastic deposits in the Northeast Sabah basin show varying redox conditions, ranging from dysoxic to oxic, as indicated by the Ni/Co, V/Cr, and U/Th ratios. These fluctuations may be linked to the tectonic evolution of the Sulu Sea and related sea-level lowstands. The low abundance of marine dinoflagellate cysts, along with high levels of rainforest and peat swamp pollen, suggests periods of subaerial exposure that contributed to the redox conditions. C-values and Sr/Cu ratios in the mudstone samples indicate semihumid to humid and warm conditions during the Miocene. Additionally, the distribution patterns of rare earth elements (REE) and the (La/Yb)N and Th/U ratios reveal consistent sedimentation rates in marginal marine environments. Palynofacies analysis indicates water column fluctuations, leading to oxidation during transport, resulting in low TOC values and kerogen types III and IV. Ternary diagrams of immobile TEs indicate that mudstone samples originate from active continental margins, linked to the rifting of the Sulu Sea during the Early to Middle Miocene. In contrast, sandstone samples exhibit characteristics of passive margins, likely due to recycled sediments from the early "Rajang Group Accretionary Complex." The plots for immobile TEs in the mudstones suggest intermediate and mafic sources, supporting the idea of uplift and erosion from deepwater sediments, as well as volcanic activity from the Cagayan and Sulu Arc systems. Sandstone samples may be sourced from acidic rocks associated with felsic intrusions in the Segama Valley, indicating a significant continental basement beneath the ophiolite. Keywords: Marginal marine, trace element, provenance, palynology, tectonic setting, Northeast Sabah.