Smaller' benthonic foraminiferal assemblages and depositional environment:

• Lower Coastal Plain
• Coastal Belt
• Holomarine
• Holomarine Inner Neritic.
• Holomarine Middle Neritic.
• Holomarine Outer Neritic to Bathyal.

Lower Coastal Plain. The foraminifera display a low species diversity and generally occur in only small quantities. Assemblages consist mainly of arenaceous forms and include Miliammina fusca (Brady), Trochammina macrescens (Brady), Trochammina spp. and species of the genera Ammobaculites and Arenoparrella. The distinctive fluviomarine species Ammobaculites exiguus (Cushman and Bronnimann) and Eggerella cf. scabra (Williamson) may be present and some species of the genus Rotalia may occur.

Coastal Belt. A coastal environment is often characterised by a mixture or alternation of species typical of the Lower Coastal Plain and species of the shallow marine genera Rotalia, Elphidium and Nonion. In coastal areas where the influence of rivers is strongly felt, assemblages may include the fluviomarine species Ammobaculites exiguus and Eggerella cf. scabra.

Three holomarine environments, classified in terms of water depth can be recognised on the basis of foraminiferal assemblages. In each case, the influence of major river discharge may be indicated by the presence of the fluviomarine species mentioned above, a lower species diversity, and a relative sparseness or absence of planktonic foraminifera.

Holomarine Inner Neritic. The holomarine inner neritic environment (water depth less than 40 metres) supports a relatively diverse benthonic foraminiferal fauna (15-50 species), with a numerically strong and diverse component of calcareous forms (dominantly Pseudorotaliids and Asterorotaliids). Some planktonic species, usually small globigerinids, are also common.

Holomarine Middle Neritic. The holomarine middle neritic environment (water depth 40-100 metres) is characterised by well diversified (30 to 80 species) benthonic foraminifera. Pseudorotaliids and Ammonias dominate in the middle shelf together with Cibicides, Eponides, Operculina, Textulariids and Bigenerina. In addition, from 40m the first Lenticulinas, Heterolepas and Neoeponides appear. Beyond 70m the Marginulina / Saracenaria group, decorated Lenticulinas, Nodosariids and Hoeglundina dominate.

Planktonic taxa show marked increases in abundance and diversity at both the 40m and 70m isobaths. Taxa typically occurring at deeper levels within the overall water column, e.g. keeled taxa, also increase at these isobaths, particularly from 70m.

Holomarine Outer Neritic to Bathyal. The holomarine outer neritic to bathyal assemblages (water depths in excess of 100 metres) are dominated by planktonic species. They may also include the following deep water arenaceous or siliceous species: Ammodiscus grzybowskii (Emiliani), Cribrostomoides deformis (Andreae), Cyclammina amplectens (Grzybowski), C. cancellata (Brady), Eggerella bradyi (Cushman), Glomospira glomerata (Hoeglund), Psammosiphonella carapitana (Hedberg), and Trochammina cf. pacifica (Cushman).

Figure 4.4 shows some of the characteristic planktonic and benthonic foraminifera encountered in the stratigraphic succession of Brunei Darussalam.

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