Palaeoenvironmental Interpretation Of Late Miocene Outcrops (Miri And Seria Formations) Along Jalan Tutong In Brunei Darussalam

Abstract

Eleven Neogene outcrops (seven of the Miri Formation and four of the Seria Formation) from Brunei Darussalam are described. Detailed stratigraphic columns are presented with special emphasis on the sedimentary sequences, providing insights into the depositional environments. The rocks are comprised mostly of sandstone, claystone and a mix of both lithologies, and are piled up in cycles of coarsening upward successions, which are here interpreted as parasequences. Most sections are characterized by abundant and diverse sedimentary structures, reworked fossiliferous deposits and organic fragments such as ambers and wood remains. Certain beds are rich in fossils: among the macrofossils, the most common are bivalves, gastropods and followed by fish remains (teeth and otholiths), crustaceans, corals and echninoderms, which are abundant at specific locations. Among the microfossils, the most common are foraminifera, which are mostly dominated by either rotaliids or textulariids; the most common genera are Ammonia, Elphidium and Trochammina. Porcelaneous and planktonic foraminifera are rare. Ostracods are also found. Ichnofossils are quite abundant in all sections but are represented mostly by Ophiomorpha and Thalassinoides. In view of the observations, the sections are interpreted as evidence of shallow-marine conditions, in which wave, fluvial and tidal actions strongly influenced the depositional environment. In a few cases, deposition of sediment seems to reflect the transition from wave-dominated to tide-dominated conditions. Our findings indicate that in such shallow marine deposits, the interpretation of sediment type and sedimentary structures alone cannot indicate one specific sedimentary environment, but such observations should include palaeontological evidence to generate more accurate palaeoenvironmental reconstructions. Lastly, our results are discussed in relation to the sedimentary evolution of the region.

Current knowledge of the family Cardiliidae (Bivalvia, Mactroidea)

“…The paratype is from sample Tutong 15B slightly lower in the same formation. The Seria Formation was deposited during the Tortonian as was recently reconfirmed by microfossils from an outcrop of a younger and shallower part of the formation (Bukit Ambug, latest Tortonian; Roslim et al, 2016).…”

First fossil-leaf floras from Brunei Darussalam show dipterocarp dominance in Borneo by the Pliocene

“…At the times of deposition of these formations, what is now Borneo was a partially elevated area of eastern Sundaland, with overland connections to mainland Southeast Asia ( e.g ., Hall, 2017 ; Morley, 2018 ). The localities visited were primarily the same ones that Kocsis et al (2020) and Roslim et al (2021) sampled for amber and palynological content, respectively; those authors provided maps and updated stratigraphic relationships of the units (see also Roslim et al, 2020 ). Almost all outcrops visited had abundant but degraded or hashy compressed plant remains, primarily of twigs and small leaf fragments with little potential for identification, as noted in various geological studies ( Tate, 1976 ; Kocsis et al, 2018 ).…”

“…3 ), the late Miocene marine sediments of the Miri Formation are exposed and dip northwest at 30° ( e.g ., Wilford, 1961 ). The age of the Miocene sediment series ranges from 10–12 Ma (east-southeast of the site; Back et al, 2005 ) to 6–8 Ma (north-northeast of the site, near Tutong; Kocsis et al, 2018 ; Roslim et al, 2020 ).…”

“…Long ago, Geyler (1887) reported a small Miocene leaf flora from the shallow marine Belait Formation on the small Malaysian island of Labuan, located just north of Brunei near the mouth of Brunei Bay, from a site that is no longer accessible. From this lead, we decided to launch a field season of paleobotanical reconnaissance in the Belait Formation in Brunei, where it crops out at many locations ( Back et al, 2001 ; Lambiase & Tulot, 2013 ; Roslim et al, 2021 ), and in other Neogene strata with potential to hold plant remains ( Kocsis et al, 2020 ; Roslim et al, 2020 ; Roslim et al, 2021 ). This report details our discovery, preliminary descriptions, and interpretations of abundant fossils from two different compression-flora sites that are the first from Brunei, along with palynological samples from the same beds as the leaf fossils.…”

The Elasmobranch Fossil Record of the Indo-Australian Archipelago since the Miocene: A Literature Review and New Discoveries from Northern Borneo

“…The basal claystone layers are often rich in marine fossils. Both in abundance and diversity the fauna of the Seria Formation is richer compared with the Miri Formation and probably represent more open marine conditions, e.g., [34,41]. All of these lithostratigraphic units are covered by the Pliocene Liang Formation, which contains sediments of lagoonal and fluvial origin, often intercalated with thick conglomerate beds, and claystone-siltstone rich in plant remains [38].…”