Interbedded on a scale o f 3cm . M udstones are com p ressed & shaley
12m.
Colour Remarks
KEY
I I m udstone
porous radiolarian claystone — siltstone
radiolarian sand interbedded unit — parallel lamination
e rosive base ■=>--- sam ple number <1--- 10789
pink & w hite
= pink & w hite
brown/pink
pink/brown
Sands, silts and muds regularly and finely interbedded on a scale of 1-lOcm and often laminated may be seen commonly occurring with massively-bedded (beds up to 1.5m), of well lithified mudstone or siltstone. The mudstones are generally coloured grey-beige, buff/brown or olive/grey and the siltstones buffipeach. Figure 5.8, Locality 20, shows regularly interbedded mudstones and siltstones overlain by a massive bed of buff coloured siltstone. A radiolarian siltstone from this locality was dated as Aptian. A sample from a similar sequence at Locality 5 yielded an age of late Valanginian to late Hauterivian.
Also common are sequences of lightweight, porous, fine sand grade radiolarian claystones interbedded with porous radiolarian siltstones and radiolarian mudstones. These sequences are often coloured light pink/brown, dark pink/brown, pink/white, grey and white. Beds are colour banded and there is typically a lack of clear distinction between beds as can be seen in Figure 5.9. In dark coloured mudstones Radiolaria are often clearly visible to the naked eye as small white grains. Radiolaria frequently appear abundant in the mudstones of these sequences. However, when samples collected in the field were washed in the laboratory the Radiolaria simply disintegrated upon contact with fluids. Thin section of these sofi; mudstones show Radiolaria to be common. However, the walls of the Radiolaria are very thin to almost non existent and many Radiolaria are not infilled. The matrix is fine grained, largely composed of clay minerals with small amounts of quartz and calcite and is coloured brown probably due to the presence of iron oxide. Radiolaria obtained from a porous pink/brown claystone (TUK 10843, Locality 20) were dated as Hauterivian. In thin section the claystone contains abundant and diverse Radiolaria largely infilled by microcrystalline quartz. In some cases specimens are infilled by chalcedonic quartz. Rarely, radiolarian tests are partially or almost completely replaced by calcite. The matrix is very fine grained consisting of a mixture of clay minerals and fine grained quartz.
Less common are outcrops consisting of thin sandstone horizons interbedded with thick horizons of shale. Such sequences were well exposed at Localities 3 and 17. Locality 3 comprises a sequence of distal turbidites. Lithologies may be summarised as follows:
F ig u re 5.8: Regularly and finely interbedded radiolarian mudstones and siltstones interbedded with massive, well-lithified mudstone. Locality 20 (length o f ham m er - 30cm).
F ig u re 5,9: Pink/brown and white, porous, fine sand grade radiolanan claystones interbedded with porous radiolanan siltstones and mudstones. There is a lack o f clear distinction between beds which are colour banded (length o f ham m er - 30cm).
• Deep pink/orange, non calcareous shales.
• Dark brown and light brown, non calcareous shales.
• Deep pink/orange and brown laminated shales (also shale horizons where colours are intermixed due to slipping).
• Pale pink/peach, medium-fine grained, slightly calcareous sandstone horizons often with parallel laminations bearing rare planktonic foraminifera.
• Pink/peach and brown mudstone horizons, slightly cleaved but more competent than the shales.
The shales lack microfossils. The upper contacts of the sandstone beds are sharper than the lower contacts implying that the sequence is upside down. This is further supported by the presence of tool marks visible on the upward facing surface of one of the sandstone beds. The sands are clearly turbiditic as can be seen by the presence of toolmarks, parallel and ripple laminations and graded bedding. Towards the top of the outcrop a structural dislocation may be observed above which a sandstone unit grades upwards into a mudstone indicating the sediments above the disconformity are the correct way up. Horizons of small black manganese nodules are present throughout the sequence supporting the interpretation of a deep sea, distal environment. The sequence was logged in detail (Figure 5.10) and is illustrated in Figure 5.11. Planktonic foraminifera recovered from the calcareous sands are rare and
of very low diversity (dominated by the genus Hedbergelid).
A similar sequence may be observed at Locality 17 where a series of thick dark brown and red/brown shaley mudstones are interbedded with less frequent thin calcareous sandstone and siltstone horizons. The mudstones vary from non calcareous to slightly calcareous.
These sequences of distal turbidites are considered to be more proximal in relation to the mud and clay rich facies which occurs less commonly in the Noil Tuke river but which crops out extensively in the Noil Bunu river to the north.
Colour 2 m pink O - 10711 d irk brown pink/orange pink/orange dark brown pink/orange dark brown pink/orange pink pink/orange I m dark brown 0710 p in k
silty, harder bed Contains broken sandier horizons dark brown C alcite v ein in g
Contains fractured
dark brown C alcite v ein in g
dark brown C alcite v ein in g