The Late
Palaeogene to Neogene tectonic evolution of Central East Sabah has
been studied in recent fieldwork. The aim was to determine the
mechanisms of formation of the Central Sabah basin and the later so
called 'circular basins' in order to link them within the regional
tectonic framework of North Borneo.
The deep
water/oceanic sediments of the Labang Formation (Eocene to Lower
Miocene) are extensively affected by tight folds trending
approximately E-W. The Gomantong limestones (Upper Oligocene to
Lower Miocene), which overlie the Labang Formation, show thrusts to
the NE which are compatible with the previous folds. On top of these
formations are chaotically disrupted rock units (melanges), of
disputed origin. Tectonic, diapiric and mass flow origins have all
been considered. However, the tight folds, thrusts and melange
emplacement could be related to a single compressive event at the
end of the Lower Miocene.
Unconformably
resting on top of the Labang Formation, is the Tanjong Formation
(late Lower Miocene to Middle Miocene) of the Bukit Garam Basin
which is slightly folded. This formation mainly exhibits a set of
N-S to NNE-SSW fractures. To the northeast the Sandakan Formation
(Middle to Upper Miocene) unconformably rests on the Labang
Formation and is characterised by beds which dip gently towards the
west. At the scale of the outcrop, synsedimentary NE-SW to E-W
normal faults have been recognised, as well as a set of NW-SE
fractures with no obvious sense of movement. In the Sandakan Basin
the Sandakan Formation is cross-cut by a series of steep NNE-SSW
scarps interpreted as normal faults. These are associated with
intense fracturing with the same trend. Minor folding and faulting
and subsequent erosion of a previously more extensive basin explain
the development of the now isolated Bukit Garam and Sandakan basins.