CONTRASTING TIDAL DEPOSITS IN THE NEOGENE OF EASTERN BORNEO

Temporary exposures, mainly related to oil palm plantation development in eastern Sabah, have revealed some world class outcrops of tidal sediments. These range from clastic dominated mangrove deposits to stacked tidal channel deposits. The character of the tidal deposits appears to be governed by the range in relative sea level over time. They also demonstrate how an interpreted micro tidal range can nonetheless lead to relatively thick packages of tidal sediments.

The Pliocene Ganduman Formation crops out across the Dent Peninsula in SE Sabah. It was deposited in interpreted settings ranging from fluvial to shallow marine. Most striking are the thick tidal channel deposits, which fill a series of incised valleys. Most of the channels are filled with inclined heterolithic stratification (IHS), although sand filled channels occur at the base of some of the valley fills. Rare mud filled channels also occur. Giant burrows and lines of large, extremely well rounded, ironstone concretions have also been recorded in these channel deposits, which reach at least 50 metres in thickness. The incised valleys are interpreted as having amplified the micro tidal currents, having been created by a relatively dramatic fall in relative sea level in the Pliocene.

In contrast, the Miocene Sandakan Formation comprises storm dominated deposits with hummocky cross stratified sandstone beds and frequent giant gutter casts. Shoreward, the thick, scarp forming, shallow marine sandbodies give way to extensive fossil mangrove deposits. These can be identified by the presence of fossil mangrove lobsters and their associated trackways. The mangal sand bodies can be subdivided into vertically accreting channels, with width:depth ratios less than 1, and sand sheets, here termed reaches, interpreted as representing open areas within the mangrove belt. The latter are winnowed by wind driven currents. The mangroves are interpreted as having been deposited on a low gradient coastline with small changes in relative sea level.