Deep-Sea Trace Fossils In The West Crocker Formation, Sabah (Malaysia), And Their Palaeoenvironmental Significance

Mazlan Madon

doi:10.7186/bgsm71202103

Abstract

In the “flysch” series of the West Crocker Formation (Eocene–Oligocene), Kota Kinabalu, Sabah, trace fossils are fairly common although not ubiquitous. The trace fossils commonly occur as hypichnial semi- or full-reliefs on the sole of thin turbiditic sandstone beds (mainly Bouma Tc division) in the thinly bedded heterolithic sandstone-mudstone facies interpreted as submarine fan lobe deposits. Their presence in mainly the thinly bedded facies of the fan system suggests preferential production and preservation in the fine-grained “distal” parts of the Crocker submarine fan system. Trace fossil assemblages characteristic of the Nereites ichnofacies indicate sedimentary environments mainly in bathyal to abyssal water depths (>2000 m). This ichnofacies is dominated by horizontal grazing, farming and feeding traces, ranging from solitary to branching tubular burrows (Ophiomorpha, Palaeophycus and Planolites) to meandering trails and tunnels (Nereites, Cosmorhaphe, Helminthopsis), as well as the spiriform burrows Spirophycus. Graphoglyptids are the most diagnostic of the Nereites ichnofacies, produced by sediment grazers and farmers (agrichnia) and often displaying intricate networks of mainly horizontal tunnels preserved as hypichnial semi-reliefs. They include the delicate spiral traces of Spirorhaphe, as well as the enigmatic hexagonal network burrow Paleodictyon. Other ichnogenera include Planolites, Thalassinoides and Ophiomorpha which are facies-crossing and not environment specific. Detailed observations of the trace fossil assemblages and the degree of bioturbation enabled different sub-ichnofacies of the Nereites ichnofacies to be distinguished. Ophiomorpha is more common in sandy “proximal” facies and tend to penetrate deeply into pre-existing turbidite beds, its presence suggests a well-oxygenated newly deposited turbidite substrate, probably in the axial region of the fan lobes. Hence, channel axis and proximal fan deposits tend to be dominated by the Ophiomorpha rudis sub-ichnofacies. The Paleodictyon sub-ichnofacies is more typical of the lower energy lobe/fan fringe subenvironments. Proximal but off-axis areas are characterized by a mixture of the Ophiomorpha rudis and Paleodictyon sub-ichnofacies.

Highlights

The deep-sea environment is relatively stable, dominated by pelagic/hemipelagic deposition of clay-rich sediments
This paper describes some of the trace fossils found in the outcrops of the West Crocker Formation around the city of Kota Kinabalu, Sabah, and briefly discusses their palaeoenvironmental significance in relation to the deep-sea depositional processes in the Eocene–Oligocene NW Borneo margin
In a later study of sedimentary structures of the West Crocker Formation sediments, Tajul Anuar (1989) described some trace fossils which he identified as Skolithos, ?Scoyenia, Helminthoida labyrinthica, Cosmorhaphe fuschi, Taphrelminthopsis auricularis, and ?Thalassinoides sp

Summary

INTRODUCTION

The deep-sea environment is relatively stable, dominated by pelagic/hemipelagic deposition of clay-rich sediments. Description: This trace fossil (Figure 9G) is identified as a variety of Halopoa, which has features distinct from H. imbricata described above These are horizontal to subhorizontal traces, occurring as cylindrical tubes preserved as hypochnial semi- or full-relief on the sole of thin turbidite beds (Bouma Tc). They are long, straight, and bilobate – with a characteristic median furrow which is not always centred along the axis of the trail but appears to be curved or winding (Figure 9G). Glockerichnus Pickerill, 1982 Glockerichnus isp Description: These intricate star-like, radial trace fossil (Figure 15A) appears to be a branching network of small tubes or tunnels, radiating outwards from a central area, in a fan-like fashion, making a large (20 cm in diameter) stellate (star-like) network on the sole of thin turbidite beds. Helminthoida is an obsolete term not recommended for further use, as all are included in Nereites (Uchman, 1998)

DISCUSSION

CONCLUSIONS