Lecture 14: Miri Zone PDF

Geology Lecture 14: Miri Zone MIRI ZONE Estimated geological timings and characteristics of four prominent unconformities in the Neogene sediments of greater Miri area.  Miri Zone The boundary between the Sibu Zone and the Miri Zone is represented by...

Geology Lecture 14: Miri Zone MIRI ZONE Estimated geological timings and characteristics of four prominent unconformities in the Neogene sediments of greater Miri area.  Miri Zone The boundary between the Sibu Zone and the Miri Zone is represented by the Tatau-Mersing line. This is a structurally complex zone consisting of Palaeocene to Eocene ophiolitic rocks, including spilite, basallt, tuff, and radiolarian chert. The Tatau-Mersing Line also represents a major unconformity between the Belaga, Mulu, and Kelalan Formations (some of Rajang Group) and the overlying Upper Eocene-Recent sediments of the Miri Zone. The unconformity is Late Eocene in Sarawak and Middle-Late Miocene in Sabah, suggesting that the deformation associated with the collision of the Luconia Block with the northern margin of Borneo was diachronous northward. An important hiatus has also been documented within the Melinau Limestone. This Late Eocene unconformity represents a major phase of tectonism that deformed the deep marine rocks of the Rajang Fold-Thrust Belt. Overlying these deformed rocks are Oligocene-Miocene, mainly shallow water, deltaic-marine sediments that crop out in central Sarawak (Tatau, Nyalau, Setap, and younger formations. Further to the northeast towards Miri, similar shallow water Miri, Lambir, and Tukau Formations occur. The Oligo-Miocene sediments are predominantly silliciclastics except for isolated outcrops of carbonates such as the Melinau Limestone. Rocks in Miri Zone represent the lower part of the Sarawak Basin succession which extends offshore into the continental margin. MULU FORMATION  Mulu Formation This formation of sub-metamorphosed shales and slates, inter-bedded with hard sandstones, occupies the large inlier of the Mulu Anticlinorium and the highest mountain, Gunung Mulu (2376 m) (Haile, 1962). It is a monotonous succession of intensely folded turbidite composed of slaty and phyllitic shales and quartz sandstones. Bedding of the thin sandstones is commonly obliterated by well-developed cleavage. The best outcrops are in the Tutoh Gorge (Figure 31), where the sandstone beds may be as much as 150-m thick, inter-bedded with thin slates. Quartz veining is important locally. In the Tutoh Gorge, the strike is NE-SW and nearly vertical dips towards the SE predominate (Haile, 1962). The formation is poorly fossiliferous, but Discocyclina sp.; Globorotalia wilcoxensis Cushman & Ponton have been recovered. Liechti et al. (1960) have concluded that the Mulu Formation is Palaeocene to Lower Eocene in age. KELALAN FORMATION  Kelalan Formation The inlier occupies the Temala anticlinorium and is exposed at Batu Gading, up river from Marudi along the Baram River (Haile, 1962). It is a turbidite sequence mainly of shale and sandstone, with subordinate limestone, tuffite and tuffaceous limestone. In many places the shale is slaty. The formation is intensely folded. The age of the limestone ranges from Upper Cretaceous to Upper Eocene. Globotruncana sp. has been recorded only at one locality beneath the Melinau Limestone at Bukit Besungai (Northeast) indicating an Upper Cretaceous age. MELINAU FORMATION  Melinau Limestone Formation The Melinau Formation (Late Eocene-Early Miocene) is exposed on the northwest flank of the Mulu range. The formation reaches a maximum thickness of 2.1 kilometres. It consists of well cemented, partly recrystallised and brecciated, grey to white, shallow marine carbonates. The unit reflects depositional conditions outside the areas of substantial clastic input. It is inferred to have been deposited over bathymetric highs, at the depositional surface overlying the developing accretionary / collisional Crocker-Rajang belt. The formation ranges in age from Late Eocene to Early Miocene although most of the Oligocene is missing. Detailed stratigraphic studies (Adams, 1965, 1983) however, failed to produce any marked lithological or faunal breaks that might indicate the presence of an erosional unconformity. The Melinau Formation is in part time equivalent to the Temburong and Meligan Formations and is overlain by the Setap Formation. Its type locality is at Gunung Melinau on the NW flank of Gunung Mulu, where the outcrop length is 37 km and width 8 km. The formation details vary regionally and detailed studies have been made of the highly fossiliferous limestone. Batu Gading and Bukit Besungai Outcrop (Figure 30): The sequence, which overiies the strongly folded Kelalan Formation with angular unconformity, dips 12-15° towards the north (Figure 30). The Upper Eocene limestones are massive, mainly unbedded, of high purity, dark grey and highly fossiliferous. The principal organisms are larger Foraminifera, algae, fragmented corals, together with echinoid and bryozoan debris (Adams and Haak, 1962). The following fauna have been identified, of definite Upper Eocene age: Pellatispira spp. including Pellatispira crassicolumnata Umbgrove; Discocyclina sp.; Aktinocyclina spp.; Nummulites spp. including Nummulites cf. semigloblus (Doornink) and Nummulites javanus Verbeek and Operculina spp. Calcareous algae occur profusely throughout the limestone, and conmionly make up the bulk of the rock. They are mainly Melobesieae and the genus Archaeolithothamnium occurs in great profusion. The overlying sequence is conformable, but there is a major hiatus; the Oligocene is totally absent! The disconformity is not always readily seen, except where the Eocene limestone is overlain by calcareous silty shales (Figure 30), immediately north of the cave entrance. The shales, which contain irregular clasts of Eocene limestone, have been deposited on an eroded surface of Eocene limestone. The calcareous shales contain a rich Lower Miocene fauna: Globigerina dissimilis Cushman & Bermudez var.; Globigerina binaiensis Koch; Globigerinoides spp.; Globorotalia mayeri Cushman & EUisor and Globoquadrina venezuelana (Hedberg) var LAMBIR FORMATION  Lambir Formation In northeast Sarawak, the Baram Delta begins with a very dramatic change from Setap Shale to very sandy Lambir Formation at the southerm margin of the Lambir Hills (Figure 42). The Middle Miocene (Tf) Formation consists of sandstone, shale and some limestone. The formation occupies comparatively gentle synclines and is little changed diagenetically. Maximum thickness is around 1600 m. The basal Lambir Formation thick-bedded sandstones have a sharp erosive base overlying medium dark-grey mudstone and calcareous mudstone (Sibuti or Setap Shale Formation). The calcareous mudstone contains Foraminifera and crabs. The environment was shallow marine, as indicated by an abundance of gypsum. The basal sandstone is well sorted, made up of a number of sandstone cycles with hummocky cross-bedding (Figure 49). The top sandstone shows low-angle planar crossbedding, and may be a beach deposit. The sequence therefore shows a distinct shallowing upwards transition from marine to coastal, concomitant with the change in orientation of the coastline and uplift of interior Borneo Fossils in the Lambir area are generally of little time- stratigraphic value. A Tf^(Middle Miocene: Langhian) age for limestone in the Bakong area is indicated by Flosculinella bontangensis Rutten and a Tf2-3 (Middle to Upper Miocene) age for the upper part of the formation is indicated in the Bakong area by the absence of Flosculinella botangensis, Austrotrillina howchini, Spiroclypeus sp. and Miogypsinoides sp. (Wilford, 1961). Headland Dua, beach SW of Miri, Tusan Cliff Area. Note the razor-sharp abrasion unconformity above the Lambir sandstone. MIRI FORMATION  Miri Formation The Miri Formation is predominantly arenaceous, with clay and shale restricted mainly to the lower part. The Miri Formation ranges from Middle to Upper Miocene. It has been divided into a Lower and Upper Miri Formation. The Lower part has well-defined beds of shale interbedded with sandstones; the upper part is more sandy. The outcrops around Miri Hill provide an invaluable analogue for the offshore Baram Delta, and they have been studied in great detail (Tan et al., 1999). The base of the formation is a gradual transition from the argillaceous Setap Shale to the sandy Miri Formation. The Lower Miri Formation, known as the Miri Shale, is dated by the Foraminifera of the Loxostoma 1 zone. The basal sand of the Upper Miri Formation (105 sand) is identified as the Bolivinita zone. The overlying Upper Miri Formation sands and clays contain microfossils attributed to the Nonion 3 zone. The unconformably overlying Seria Formation is recognized by the presence of Triloculina 18. Ten different sedimentary facies have been recognized and described (Tan et al., 1999): Medium-scale trough cross-bedding The trough wavelengths vary from 0.1 to 3.0 m. The facies contains the trace fossils Ophiomorpha nodosa, Ophiomorpha irregulaire, Palaeophycus, Teichichnus and Planolites, all indicating a shallow marine environment. The facies was deposited in shallow sub-tidal areas of an estuary. Small-scale trough cross-bedding This is a subordinate facies containing mixed mud clasts and mud-draped cross-bedding. The environment could have been muddy tidal flats.  Herringbone cross-bedding It occurs within the planar to tabular cross-bedded units. The bi-polar dip directions suggest a tidal environment as current directions reverse. There is a confirmatory presence of flat topped ripples. Flaser-bedded fades These are cross-bedded sands with numerous intercalated mud flasers. Sand and mud were continuously supplied into the shallow sub-tidal to inter-tidal environment.  Wavy-bedded fades This facies is of alternating continuous wavy layers of mud draping the sandy ripples. The sand layers are 5-10 cm thick, while the mud layers are less than 2 cm. Sand-day alternation fades Regular interbedded fine- grained thin-bedded sands and mud. The sands range from 1 to 23 cm, clays 1 to 5 cm thick. Boundaries are sharp. Load casting is common. The current action was minimal during deposition and trace fossils are rare.  Lenticular-bedded fades Subordinate irregular sand bodies embedded in mud. The amount of sand does not exceed 25%. The environment was sub- to inter-tidal with low current activity. Mudcrack fades and associated mudstones This uncommon facies shows periodic exposure in between periods of deposition.  Hummocky cross-stratified (HCS) sandstone facies Fine- to very fine-grained sheet sandstones displaying low-angle cross- stratification. The sandstone beds range from 15 to 90 cm thick. Bioturbation is common. The environment was shallow marine. The long low-angled undulating hummocky cross-stratification can be produced by storm waves. The bases of these sands are abrupt indicating sudden emplacement into a muddly environment. The already existing clays were actively reworked by the ingressing sands. Massive coarse sandstones Poorly sorted medium to coarse grained sandstones with no internal structures. De-watering and collapse structures are common, suggesting deposition was in a fluidized condition. These sands are thought to result from storms of typhoon strength; the transport being by turbidity current in very shallow water. But the sands may subsequently be reworked. The Tukau Formation uncomformably overlain the Miri Formation NYALAU FORMATION  Nyalau Formation The Nyalau Formation (Middle Miocene) of Bintulu area, Sarawak occurred as: (i) offshore-subtidal estuarine sandstones, sandy shales, and shales with dispersed lignite bands and marls, (ii) silty sandstone interval, which are partly calcareous, and grading into sandy-limestone, (iii) Biban sandstone Member, of Oligocene-Miocene age, which consists of fine- to medium-grained sandstones and siltstones with calcareous nodules (iv) Kakus Member, of Lower-Middle Miocene age, consisting of massive sandstone intervals, laminated clays, and brackish-shales and lignites LIANG FORMATION  Liang Formation The Liang Formation aged Pliocene to Middle Pleistocene. Origin name of this formation is based on Sungai Liang, Brunei. Reaches thicknesses of nearly 0.6 kilometres it outcrops in the Lumut Hills, in the Berakas syncline and near Limbang. Boundaries: 1. In the coastal area, the formation rests unconformably on calcareous Setap Shale Formation with an angle of unconformity ranging from 12o to more than 40o. 2. In the Klias subsurface the formation rests partly conformably on the Seria Formation. In the Balingian-Mukah area, it trangresses successively onto the Belaga Formation (Metah Member and probably also Bawang Member), and the Tatau and the Nyalau Formations, but only the contact with the Metah Member is exposed. 3. In the Suai Coast, the basal boundary is an unconformable onlap on the Lambir Formation. Locally a small unconformity and disconformity between the formation and the Lumut Member is in evidence, e.g. in the Tudan Area. 4. The top boundary of the Liang Formation and its member is either purely erosional, or an unconformity with the overlying Quaternary terraces or Alluvial deposits Thickness: +60 to 1,500m. Estimation from seismic data 1,900 to +3,000m. Lithology: succession of sands, clays, gravel, beds, with some lignites, of very young appearance and poor consolidation. Fossils: bryozoa, echinoid, foraminifera (ammobaculites sp., ammodiscus sp., haplophragmoides sp., etc.), gastropoda, ostracod, and plant remains. Depositional environment: very shallow, near shore type of marine deposition which, however, changes inland into paralic or even continental conditions. Remarks: the ‘Sikat Formation’ and the ‘Penian Formation’ are now include in the Liang Formation.