Sedimentary Facies of the West Crocker Formation North Kota Kinabalu-Tuaran Area, Sabah, Malaysia

Newly outcrops exposed in the West Crocker Formation have led to the detail sedimentolgical analysis of the formation. Eight sedimentary facies have been recognised in which it was divided into three main groups: (1) sand-dominated facies (F1-F2), (2) poorly- sorted unit mixed sand and mud-dominated facies (F3), and (3) mud-dominated facies (F4-F5). These are: F1- graded sandstone (massive to planar laminated), F2-ripple-cross laminated, wavy and convolute lamination sandstone, F3-chaotic beds of mixed sandstone and mudstone blocks and clasts, F4-lenticular bedded of sandstone, and F5-shale. The studies of the formation has come out that it was deposited in a sand-rich submarine fan with specific location located at (1) inner fan channel-levee complex; (2) mid-fan channelised lobes, and (3) outer fan.

West Crocker Formation. The deformed, folded, imbricated and thrusted deep-water of the West Crocker Formation (Leong, 1999) which represents a northeast-southwest trending, elongate trough, part of a large submarine fan, built out northward from the Sunda Shelf in Eocene to Oligocene times, which was accreted against Borneo in the Late Oligocene to Early Miocene to form part of the Northwest Borneo Subduction Complex (Tongkul, 1987).
The sedimentology logging was documented with emphasis on the lithology, sandstone thickness, grain size, morphology of the beds contact, sedimentray structures, bioturbation style and internal charactericties. These features will be used to interpret facies and facies association of the succesions and the depositional will be construcuted for the West Crocker Formation, NW Sabah, Borneo.

Sedimentary facies
The West Crocker Formation can be divided into eight sedimentary facies.

Graded Sandstone (Massive to Parallel Laminated)
This facies consists of light grey, fine to coarse grained with moderate sorted of sandstone. It shows medium to thick-bedded with individual thicknesses between 3 to 5 meters (Figure 2A). Basal contacts of beds are sharp, or locally erosive with abundant of granule to pebbles which can result the normal grading ( Figure 2B). Generally, these beds are massive or display as feint horizontal stratification ( Figure 2C). Soft sediment deformation such as dewatering structures, dish structures, dewatering pipes and mudclasts are common either at the middle or at the top of beds. The sandstones show parallel lamination, current-ripple and climbing current ripple lamination developed in the finergrained at the upper part of this beds ( Figure 2D). The flute cast and groove marks are common. Massive character implies of rapid deposition from highly concentrated turbulent flow (Tongkul, 1987). The poor development of internal deposition structures indicates that these sandstones were deposited rapidly by processes of traction and fallout from short-term turbidite flow (Collenette, 1958;Tongkul, 1987).

Ripple-Cross Laminated, Wavy and Convolute Lamination of Sandstone
F3 contains of light grey, very fine-grained sandstone with interbedded with hemipelagic mudstones. This facies resembles alternation of laminated mudstone with sharp, planar bases with lack of flute casts or tool marks. This facies is typically 0.5 to 1 m in thickness. Internally, F3 consists of intervals of current-rippled to climbing current ripple very fine-grained sandstone which repetitively alternate with wavy and convolute mud drapes of flaser bedding ( Figure 3E).
This F2 can be interpreted as lower density turbidity current deposit with extensive uni-directional flows in respond to sediment supply and hemipelagic setting. Current-ripples to climbing-current ripples indicates that late stage uni-directional traction modification processes occurred during deposition of F2 suggesting that sediment supply to the bed was relatively high with some additional energy and balanced the transport of sediment across the bed which allowing ripples to aggrade (Middleton, 1993). The presence of wavy mud drapes indicate the more extensive tractional modification with more incoming sediments input and these uni-directional flow were pulsatory in nature and allowing mud-sized particles to settle out or fallout of suspension on a form of mud drapes (Lowe, 1982).

Chaotic Bed of Mixed Sandstone-Mudstone Blocks and Clasts
This facies refers to a chaotic combination of sandstone and mudstone. Mudstone commonly appears in dark grey-coloured while sandstone is yellowish or light-grey in colour. Fine-grained sandstone significantly contains more mud than high-density turbidite sandstone. The contacts may be gradational or sheared. This facies consists of clast rich units, with 1 to 2 m in thickness. Internally, this facies is always chaotic and poorly sorted, structureless, and ungraded ( Figure 3F). The sandstone and mudstone clast shows tabular to sub-spherical. The coal form as finely-disseminated chips within matrix exists in this facies. This facies is sharply overlain by sandy high-density turbidites of facies 1 sandstone unit. Facies 3 is interpreted as a debrite, which was deposited by a flow either in laminar or weakly turbulent. The ungraded nature of both of the structures and clasts in different size suggested that this facies is deposited as a single phase of the flow in which whole mass undergoes large and continuous deformation by freezing off en masse, rather than by progressive settling (Muddler and Alexander, 2001). The variety clast shapes within this facies and their internal structure information is strong evidence to show that this facies is formed of debris flow in which normally tend to be supported by matrix strength, dispersive pressure, and buoyancy (Middleton, 1993). Clasts in this facies are internally deformed suggesting that the clasts are poorly lithified at the beginning of the deposition and later it may be deformed by internal shearing or the interaction of clasts within the flow.

Lenticular Bedded Sandstone
This sandstone lenses is yellowish in colour while the shale is dark grey. These occurrences are generally fine-to medium-grained sandstone. The composite thickness of this facies ranges from 3 to 5 m. The sandstone lenses are about 0.5 to 2 cm and shale layer is about 4 to 8 cm ( Figure 3G). The unit extends laterally about 5 m. Most of sandstone beds have sharp basal and top contacts and are capped with shale layers. Internally, the sandstone beds consist of parallel lamination and climbing lamination. The deposition of the alternations between sandstone and shale requires fluctuating flow conditions, where it represents a quiet and low energy of turbidity current, which has fall-out and repetitively. The variable thickness in this facies suggested that it formed at a moderately strong, intermediate cycle of flow. The existence of parallel lamination and climbing ripples in the sandstone beds imply a waning turbidity current.

Shale
F5 is shale in dark grey-coloured. The F5 unit range from 0.5 to 1 m in thickness. Its displays as a sharp top and bottom contact of the beds. It lateral continuity is about 5 m. Stratigraphically, F8 commonly occure above F1 or F2. F5 is interpreted to present deposition of suspension fallout of finer grained particles. In turbidite system, deposition of coarse sediments may leave a residual suspension of fine grained sediments. This residual fine grained sediment will wane and tend to deposit under lower energy of turbidity currents. This fine grained material could also associate with and transported within the flow with response to a rapid settle down (Amy et al., 2005).

Depositional Environment
The West Crocker Formation, NW Sabah is characterised by high amount of sandstone content dominated by thick amalgamated, channel-levee, channelized and distal lobe sandstones. Based on the documented five sedimentary facies, a sand-rich submarine fan depositional model is proposed for the West Crocker Formation, as following (Mutti and Normark, 1987) model. This model is generally divided into Inner, Mid, and Outer fan (Walker, 1978).