Distribution of larger benthic foraminifera from Miocene carbonates in the Center and Southeast of the Nam Con Son basin

Abstract

In biostratigraphical studies, the planktonic foraminifera, nannofossils, and palynomorphs are recorded rarely in shallow water limestones. Meanwhile, larger benthic foraminifera (LBF) have an important role in defining the stratigraphic range and the depositional environment of carbonate formations is recorded abundantly. The precise identification of LBF’s taxonomy depends on the recognition of its internal structures using polarised microscopes. Results from the study in the Central and South-eastern Nam Con Son basin show that LBF occurred frequently in the Miocene and, especially, became extremely dominant in the Middle Miocene. In the Early Miocene (Te5 - lower Tf1), they appeared sparsely in the Te5 but became abundant in the lower Tf1. In the Middle Miocene (middle Tf1 - Tf3), they were predominant and diverse in many genera and species, marked by the disappearance of Miogypsina, Miogypsinoides, Katacycloclypeus, and most of the species of Lepidocyclina and Cycloclypeus at the top of the sub-epoch. In the Late Miocene (Tg), LBF was not varied, mainly Amphistegina and Operculina. However, some significant species of the genus Lepidocyclina were extant and dwelled on the isolated carbonate platforms in the Southeast area of the basin. The association of larger benthic foraminifera with small millioids and rotalids, planktonic foraminifera, coral, and/or algae, crinoids, bivalves, etc., may implicate the palaeoenvironmental zone of the carbonate deposits in the wells of the studied region, ranging from the lagoon, shelves surrounding reef to relatively deep, open marine conditions. The occurrence of LBF in carbonate deposits is the key to defining the age, identifying the stratigraphy and depositional environment, and correlating the carbonates.