Late Oligocene-Early Miocene Palynological Succession From Marginal Marine Deposits, Nyalau Formation, Bintulu Sarawak: Palynostratigraphy, Paleovegetation And Paleoclimate Significance

Abstract

Palynological analysis was conducted on one hundred and twenty-four samples collected from ten sedimentary outcrops in Sangan, Tatau, Bintulu and Similajau areas of central Sarawak. These outcrops form part of the Oligocene-Miocene marginal marine successions of the Nyalau Formation. Rich palynomorph assemblages were recovered from the samples and they provide well-preserved palynological record for the Oligocene-Miocene boundary in the onshore of northwest Borneo. Three distinct palynological zones were identified based on changes in pollen assemblages. The three zones are informally named FT-I, FT-II and FT-III. The FT-I zone is characterized by high frequency of Florschuetzia trilobata with common Florschuetzia semilobata including sporadic Meyeripollis naharkotensis. High percentage of F. trilobata and sporadic occurrence of F. semilobata are typical of the FT-II zone. Another important characteristic for this zone is characterized by common to abundant Casuarina type pollen while M. naharkotensis is virtually absent. The FT-III zone is marked by the presence of Florschuetzia levipoli. The zone is also characterized by abundant F. trilobata including regular occurrence of F. semilobata with common montane elements represented by Pinus, Picea, Alnus, Ephedra and Tsuga. All the identified zones record regular occurrence of Gramineae throughout with strong variation in diversity and frequency of peat swamp taxa represented by Blumeodendron type, Calophyllum type, Stemonurus type Calamus type and Dactylocladus type. The mangrove element such as Rhizophora type exhibits gradual increased in number throughout the FT-I and FT-II zone and showing reduced abundance in FT-III zone. These three zones are correlatable to the Late Oligocene to Early Miocene palynostratigraphy of the Cycle I and Cycle II in the Sarawak Basin. The palynological successions from these areas suggest a typical wet climate with intermittent seasonally dry conditions throughout the Late Oligocene - Early Miocene. This is evident from the regular occurrence of grass pollen i.e., Gramineae/Poaceae with common rain forest and peatswamp elements. This is supported by the common to high frequency of mangrove pollen notably Rhizophora type (Zonocostites ramonae). The Early Miocene may have experienced occasional super wet climatic conditions where rain forest pollen assemblages were prevalent including intermittent acme of Casuarina type pollen.