No 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.

The present study provides new data on the diversity, abundance and depth distribution of larger foraminifera (LBF) from a number of sites offshore BruneiDarussalam. This region, indeed very close to the modern LBF hotspot diversity, has always been overlooked and no data are available to date. The investigated sites are called Pelong rocks, Abana Reef, Oil RigWreck, AmericanWreck, AustralianWreck and BluewaterWreck. Altogether, 16 taxa ofmodern LBF have been identified and the most common families are the Amphistiginidae, Calcarinidae and Nummulitidae. The purpose of this study is to estimate the depth distribution of each recovered LBF species and to compare it with that known from other localities in the Indo-Pacific realm to check for similarities and differences in environmental and ecological conditions. Such comparisons are very important as LBF depth distributions are strongly influenced by light penetration in the water column and sea floor substrate. Bruneiwaters are generally rich in suspendedmaterial due to the large rivers discharging in the Brunei Bay, and the sea floor is mostly covered by fine mud where LBF have never been reported. The data come from recently sampled sandy patches where LBF flourish and they depict a relatively high biodiversity, which seems to be comparable with the values available for the nearby located diversity hotspot.

The study aims to get some additional knowledge on the modern seafloor composition offshore Brunei Darussalam by looking at the recent stratigraphic succession of the deposited sediments and their distribution patterns. For this reason, 10 shallow cores (22 to 46 cm thick) have been collected by scuba diving along two depth transects spanning from water depth of 20 to 60 m. One of the transects has been sampled north-northwest of the Muara village, just in front of the Brunei Bay and the other one off the coast near Tutong town, away from major sedimentary inputs. The results obtained portray two different sea bottom compositions and two different depth-related sediment distributions. The Muara transect is highly rich in mud and yielded abundant biogenic component at all investigated depths. The Tutong transect has a higher sand content but display constant changes along with depth. The sediment is mostly composed by biogenic grains such as rests of sponges, foraminifera, molluscs and echinoderms; the not biogenic grains are for the vast majority made of quartz. The sandy fractions of both transects have been tested for cyclicity and all cores can be described by functions with comparable periods, thus indicating that an oscillatory environmental event such as the alternation of the monsoon seasons, has similar influence on the seafloor of both transects.

<p>The geology of a depositional system can mostly be described by looking at sedimentary structures and sedimentary composition. However, in areas of complex shoreline mixed-process system (influenced by fluvial-tides-marine processes), many factors should be put into consideration. In Brunei, the mixed-sediment types occur extensively. The geology is mainly characterized here by thick Neogene siliciclastic facies ranging from fluvial, tidal and marine sediments deposited during periods of deltaic to shelfal setting, affected also by tectonic events. Due to this, differentiating tide and wave dominated facies is often a major challenge in the region.</p><p>In this study, it is emphasized that in order to support interpretations on these transitional facies, specific factors such as ichnofacies and microfossil content can be considered. Pollen and spores are more expected and useful in rather terrestrial systems, whilst in marine environments dinoflagellates, foraminifera and nannofossils could be convenient if preserved. Foraminifera and ichnofossils have the merit to be great indicators of a variety of sub-environments within the complex shallow water system.</p><p>The methods involve standard outcrop logging of fluvial, tidal and shallow marine outcrops, identifying lithology and key sedimentary features including trace fossils. Clay-rich samples were checked for microfossil content. Laboratory work involved extracting organic (pollen, spores, dinoflagellates) and calcareous (foraminifera, nannofossils) microfossils and documented them with light microscope (LM), stereo microscope, and Scanning Electron Microscope (SEM).</p><p>The results revealed that the most common trace fossil assemblages are the <em>Ophiomorpha, Cruziana</em> and <em>Skolithos</em> ichnofacies, and they refer to proximal marine settings. Among the calcareous microfossils recovered were very few coccolithophorids (<em>Sphenolithus abies</em> and<em> Sphenolithus moriformis</em>), which indicate very rare holomarine conditions, while the following benthic foraminifera genera were identified: <em>Ammonia, Nonion, Elphdium, Elphidiella, Quinqueloculina, Ammobaculites,</em> and <em>Trochammina</em>. Each of these genera have specific environmental requirements concerning hydrodynamics, trophic resources, oxygen content, substrate-type and deltaic influence. Results on pollen and spores, mangrove vegetation is marked by <em>Sonneratia</em> and <em>Rhizophora</em>-types, mixed-dipterocarp by <em>Shorea</em> spp., while peat swamp by <em>Verrucatosporites usmensis</em> and <em>Osmunda</em> sp.. Besides few dinoflagellate cysts (<em>Achomosphaera </em>sp., cf. <em>Exosphaeridium</em> sp., cf. <em>Operculodinium</em> sp., gen indet., <em>Lingulodinium? pycnospinosum</em> and <em>Tuberculodinium vancampoae</em>) and two acrtitarch taxa (<em>Cymatiosphaera</em> sp. and <em>Cymatiosphaera </em>cf. <em>nuda</em>) were found. These findings indicate incomplete sets of parasequences with palaeoenvironments of mixed shallow marine conditions. Mangrove pollen retrieved within tidal sediments indicates mangrove-dominated tide-influenced shoreline, while shoreline with diverse ichnofossils show coastal area connected to wave-dominated upper shoreface/ delta front. The calcareous foraminifera and nannofossil differentiate sediments belonging to lower shoreface to offshore/ prodelta deposits.</p>