REGIONAL GEOLOGY AND GEOLOGY OF THE STUDY AREA

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1 CHAPTER - 2 REGIONAL GEOLOGY AND GEOLOGY OF THE STUDY AREA 2.1 REGIONAL GEOLOGY A detail account of the Tertiary succession of Assam has been given by P. Evans (1932). The rock formations of the area belongs to the Cenozoic era, which comprises mainly of clastic sediments rather than chemical and biogenic sediments. The generalised Cretaceous-Quaternary stratigraphic succession in Assam Arakan Region (after Das Gupta, 1977) is given in Table-2.2 The Tertiary sequence in Assam belongs to two facies - Shelf facies and Geosynclinal facies. The shelf facies comprises of the Dhansiri valley and the Upper Assam plain. The geosynclinal facies comprises of the Surma valley of the Fold belt. The Disang Group is thick succession of shales with thin bands of fine grained sandstone which is micaceous at places. The sedimentary structures and composition are suggestive of flysch facies. The Barail Group is arenaceous in character with the consistent development of carbonaceous facies. In Surma valley it is divided into Laisong, Jenam and Renji Formations. The Laisong conformably overlies the Disang Group and grades upward to Jenam Formation. In North Cachar distinction between Laisong and Jenam is difficult. Lower contact of Renji 8

2 Formation is gradational while upper contact is unconformable to conformable with the overlying Surma Group. Surma Group exposed in the Fold Belt is most extensively developed and divided into two Formations - Bhuban Formation and Bokabil Formation. Bhuban is arenaceous and is divisible into three units consisting of very well bedded fine sands with numerous development of thin carbonaceous shale beds. The Bokabil Formation typically consist of clay, grey laminated shales with minor sandstone intercalations in the Surma valley. The Tipam Group conformably overlies the Bokabil Formation consisting of coarse ferruginous sandstones. These are well bedded with large scale planar cross stratifications. In Cachar it is divisible into Jaipur Formation and Gobindpur Formation. The upper part, Girujan clay consists of clay, shales and fine to medium grained sandstones. Dupitila Formation unconformably overlies the Tipam Group, consisting of medium to coarse grained pebbly sandstones with soft clay bands. Namsang Formation exposed in Upper Assam is equivalent to the Dupitila Formation. The Dihing Group unconformably overlies the Dupitila in Surma valley and the Namsang Beds in Upper Assam. It comprises of pebble beds with loose feldspathic sandstones. The pebbles and boulders of Dihing Group include quartzite, gneiss and other crystallines. 9

3 2.2 GEOLOGY OF THE STUDY AREA Outcrops of Tertiary rocks are exposed in and around Mandardisa, N.C. Hills. These are well represented by Barail Group of rocks. Moreover, scanty exposures of rocks of Surma Group are also present in the area. My study is confined within Barail Group of rocks due to inaccessible, unsafety and difficult terrain. The tertiary rock exposures in the study area belong to the Barail Group as mentioned in the passing reports. The rock exposures dominantly composed of monotous sandstones with intercalated shale /shales represents the Barail Group. The Surma Group of rocks is exposed in the area, represented by an alternate sequence of grey laminated shales, siltstones and sandstones. A conglomerate bed occurs as a marker horizon in the area separating the two Groups. Based on the field observations, lithological characters and structural configuration, a tentative stratigraphical succession is proposed in the study area. The study is confined within the rocks of Barail Group (Table-2.1). 10

4 2.3 LITHOSTRATIGRAPHIC UNITS OF THE STUDY AREA Barail Group Although a thick arenaceous sequence of rocks have been mapped in the area, it becomes very difficult to classify them with the existing nomenclature proposed by Evans (1932). The Barail Group of rocks occupy 85% of the total area mapped. The rock formations are mostly represented by sandstones and shales. Good exposures are seen along Lumding river near Mandardisa pump house. Barail Group is characterised by three formations in some parts of NE India. These three formations namely Laisong, Jenam and Renji are differentiated by their distinct lithological characters. The Laisong Formation is represented generally by bedded and massive sandstones with subordinate shale. Jenam Formation consists of sandstones altering with shale and carbonaceous shale. Renji Formation is characterised mostly by sandstones (Evans, 1964). So distinct characters are not observed to any formations. Hence, Group is found as undifferentiated Barail, though there are some similarities with Laisong (Lower) Formation. Similar experiences were also observed by Srivastava and Pandey (2011) and Singha (2002) Sandstones Bedded and massive sandstones (Fig.Nos.4,5,6 and 7) intercalated with shales in the area are under study. Good exposures of sandstones are 11

5 seen along Lumding Nala (Lumding River) near about 5 kms. distance and the colour of the sandstones vary from yellowish brown to brown at places. The colour variation is due to different oxidation processes. The oxidation is so intense that at places the rocks become a highly ferruginous. Concretionary nodular structures are common (Fig. No.18). The nodules are ferruginous and reddish in colour and grow concentrically with a nucleus of sand at the core. The cross bedded varieties of sandstones (Fig. No.8) show alterations of fine and medium grained in the included set of beds. Laminations in the sandstones are very common at the top (Fig. No.11). The individual lamellae vary in thickness from 2 cm. to a few mm. and consist of fine to medium grained sand of different colour due to differential weathering. Megascopically, the rock is fine to medium grained, compact, grey to bluish grey and consist of sandy quartz grains with little biotite and feldspar. In some places streaks of carbonaceous shale have been noticed in bedded and massive sandstone deposits. Matrix appears to be argillaceous with fragments of shales Shale Shale occurs both as band partings intercalated in the sandstones. Thickness of individual beds vary from a few cms to as much as 2 m. Good exposures were seen along the Lumding nala. The shales are generally grey in colour with thin layers. Dark colour carbonaceous shale were noticed at few places. 12

6 The major rock units of the area are bedded to massive and medium to fine grained sandstones, current bedded to laminated sandstones Conglomerate bed Conglomerate or Ferruginous pebble beds are seen with the sandstone formation of the Barail Group which acts as an unconformity between the two formations that is the Barail and the Surma Groups Surma Group The Surma Group of rocks are characterised by sandstones and shales Quaternary and Recent alluvium These deposits are best developed along the Lumding river, forms an escarpment and are characterised by pebble beds and loose admixtures of sand, silt and clay. The pebbles and finer sediments are bounded by iron rich ferruginous materials. These deposits are generally known as Newer and Older alluvium whose age assigned as Quaternary to Recent. 2.4 LITHOFACIES OF THE STUDY AREA INTRODUCTION A facies is a body of rock with specific characteristics and defined on the basis of colour, composition, texture, fossil contents and sedimentary 13

7 structures (Reading, 1996). In other words, facies is defined as a distinctive body of rock that forms under certain set of environmental conditions reflecting the sedimentary processes (Cant and Walker, 1976). The purpose of the facies analysis is to prepare a simplified facies model that can be used as a tool for the interpretation and discrimination of the depositional environment. The exposures of study area are predominantly arenaceous (sandstone) with intercalations of grey and carbonaceous shales. The sandstones show both massive and bedded and well laminated character. Sandstones are fine to medium grained, grey to brownish grey coloured. Laminations are found within the fine-grained sandstone. The sandstones show planar to trough cross-stratifications. The sandstone layers show blocky as well as slabby bedded characters METHODOLOGY At each exposure, individual lithounits were identified based on gross lithology, sediment texture (grain-size) and sedimentary structures. At least six lithofacies were identified based on the scheme suggested by Miall (1978, 1990), Allen (1990), Blatt et. al, (1980). The sedimentary structures were recognized following Reineck and Singh (1980) and Collinson and Thomson (1982). A generalised lithofacies column is shown in Fig Bedded (stratified) sandstone lithofacies (Sh) This facies is represented by light grey coloured medium grained sandstone. Individual beds are about 40 cm. in thickness. It occurs at the 14

8 lowermost part of the sandstone. This facies shows the maximum thickness about 4 m Massive Sandstone Lithofacies (Sm) This facies is represented by grey coloured medium to fine grained massive sandstone. At places sandstone is reddish brown and ferruginous. It occurs in the middle unit of Barail Group exposed near Lumding river. The thickness is about 6m Planar trough Cross-bedded Sandstone Lithofacies (Sp & St) It is associated with the middle unit of the sandstone. Presence of cross-bedded occur in the brownish to grey medium grained sandstone. Planar cross strata occur in small scales. These are associated with trough cross stratification. The top and bottom contacts are sharp. The beds are laterally extended. These cross sets commonly occur as single sets but occasionally cosets also. The thickness of the whole sets are on an average of about 15 to 20 cms. whereas foresets vary from about 6 to 9 cm.. This lithological facies thickness extend about 3 m Laminated fine grained sandstone (Sl) It consists of laminated reddish brown to dark grey, fine grained sandstone associated with upper unit of Barail Group exposed near Mandardisa cave. It has gradational contact with the overlying facies (conglomerate). In the laminated facies each lamina is almost parallel to the lower set of boundary. The laminations show variation in colour (light and 15

9 dark). Thickness of individual lamination is about 2 cm. This facies shows the maximum thickness of about 3 m Shale Lithofacies (Fl & Fm) It is associated with the lower to upper unit exposed near Lumding river and Mandardisa cave and characterised by the presence of thin layers subfacies of greyish to carbonaceous dark shales. Layers and massive both characters are present within shales. Its thickness vary between about 2 cm. to 3 cm Conglomerate Lithofacies (Gms) This facies is found at the top of Barail Group marked the unconformity between Barail and Surma Groups. It comprises matrix supported pebbles Facies Association and Facies interpretations Facies association is a group of facies that occur together and considered to be genetically or environmentally related and give more information than a single facies (Reading, 1996). The analysis of facies is thus the fundamental to environmental interpretations and palaeogeographic reconstruction (Miall, 1986, 1990). Based on grain-size, sedimentary structures, water depth and genesis of individual facies, there are six types of facies associations in the study area. The present study gives a picture of the distribution of various facies as well as the water level conditions throughout the area. The facies model 16

10 commonly indicates the sedimentation and deposition under high energy conditions for the basal stratification through cross-strata and ultimately finely laminated sands in the wanning stages of a flood. Facies observations indicate that majority of the sequences towards the base may be a product of braided river condition under moderate to high energy condition during the deposition of sediments. The abundance of massive and horizontally stratified sands suggest the presence of an upper flow regime. The laminated fine sands along with shale layers may also be interpreted as sediments filling abandoned channels. These channels are active only at the flood stage and these were the sediments deposited mainly from suspension mainly as overbank facies. (Miall, 1978). A gradual decline in the size of successive trough and planar foresets in vertical order implies progressive decline in current bars, a characteristic feature of stream deposits. The thin occurrence of associated shale lithofacies is suggestive of rapid shifting of channel bars through time. The sedimentary characteristics and presence of current formed structures like current beddings may be suggestive of intertidal sandflat origin, where bedload transport under tidal action dominated over slack water (suspension) depositions with moderate to high energy condition (Das and Singha, 2012). However, considering the nature of the finding of this study the present lithofacies of the area may also be considered to have an influence of a distributary network of channels. Moreover, probably local tectonism took 17

11 place in the depositing area due to orogenic activity. Hence shallow marine sedimentation prevailed in the study area. The marine environment finally retreated from the area towards the beginning of the deposition in the continental environment. 2.5 TECTONO-SEDIMENTARY SETUP OF THE AREA From the old literature, it is found that the Shillong Plateau, Karbi- Anglong Hills / Mikir Hills are an autochthon consisting of crystalline rocks partly covered by Tertiary and later sediments. This mass has been termed as the Foreland spur, and is overthrust from the north-west by the Eastern Himalaya and from the south-east by the Naga Hills (Evans, 1964). Recent literature said that the Barail trough, Surma Valley, the Bengal basin and the Cachar-Tripura-Mizoram Fold Belt gradually evolved in the north-eastern part of India under the influence of compressional regime through time (Nandy, 1982; Dasgupta et. al, 2005). The Barail-Surma and Tipam Groups throughout the Assam-Arakan Fold Belt (AAFB) had been interpreted as deltaic to shallow marine deposits for a long period. 18

12 Table : STRATIGRAPHIC SUCCESSION OF THE STUDY AREA. Group and Formation Lithology Age Alluvium Newer and older alluvium Pleistocene to Recent Unconformity Surma Group sandstones with shale Miocene Unconformity (Conglomerate) Barail Group Grey to bluish grey, Oligocene medium to fine grained bedded, massive and laminated sandstones with thin bands of shale and carbonaceous shales and cross-bedding in the sandstones. BASE IS NOT EXPOSED 19

13 Table 2.2 -GENERALIZED CRETACEOUS-QUATERNARY STRATIGRAPHIC SUCCESSION IN ASSAM ARAKAN REGION (after DAS GUPTA, 1977) (FIGURES INDICATE MAXIMUM THICKNESS IN METERS) Age (Approximate) Group STAGE AND LOCAL FACIES GEOSYNCLINAL SEDIMENTS SURMA VALLEY NAGA-PATKAI OROGENIC BELT Pleistocene - Recent A LLUVIUM Present Day Alluvium Older Alluvium and High-level Terraces SHELF SEDIMENTS (Shillong-Mikir Hills Fig.aue and Upper Assam Valley).UNCONFORMITY Plio-Pleistocene DIHING Not subdivided 400 (locally developed on the outskirts of the Barail Range only) Not Subdivided 900 Dhekiyajuli Beds 1800 in Upper Assam Valley.UNCONFORMITY. Mio-Pliocene DUPI TILA Upper Dupi Tila 2800 Upper Dupi Tilas in Garo Hills 1000 UNCONFORMITY Lower Dupi Tila 500 Namsang Beds 800 Namsang Beds 930 in Assam Valley....UNCONFORMITY.... Miocene TIPAM Girujan Clay 1500 Tipam Sandstone 1600 Girujan Clay 1800 Tipam Sandstone 2300 Girujan Clay 600 Tipam Sandstone 900 SURMA Boka Bil 1500 Bhuban 4000 Not Subdivided 900 Not Subdivided UNCONFORMITY.... Oligocene Eocene Cretaceous BARAIL DISANG Renji 1000 Jenam 1200 Laisong 2400 Over 1500 Tikak Parbat 600 Baragolai 3300 Naogaon 2200 (Possible local unconformity in the Eastern zone of Naga-Manipur Hills) Probably over 3000 Local unconformity in the Eastern zone of Naga-Manipur Hills) Not Subdivided Kopili Alternations 800 Jaintia - Sylhet series Limestone 600 (including Theria) Danign Maestrichtion & lower Eastern Disangs, Makwari Beds (Mai-T Beds, Negrais Series in Arakan) Longpar Stage 200 Mahadek Stage

14 Fig.2.1 GEOLOGICAL MAP OF THE BARAIL SANDSTONES OF THE STUDY AREA 21

15 Fig.: 2.2 LITHOFACIES COLUMN OF THE SEDIMENTARY ROCKS OF THE STUDY AREA 22

16 2.3 TECTONIC SETTINGS OF THE NORTHEAST INDIA AND VICINITY (after KAYAL,1998) 23

17 Fig. 2 FIELD PHOTOGRAPH SHOWING WEATHERED SANDSTONE. Fig. 3 FIELD PHOTOGRAPH SHOWING WEATHERED SANDSTONE. 24

18 Fig. 4 FIELD PHOTOGRAPH SHOWING BEDDED SANDSTONE. Fig. 5 FIELD PHOTOGRAPH SHOWING BEDDED SANDSTONE. 25

19 Fig. 6 FIELD PHOTOGRAPH SHOWING MASSIVE SANDSTONE. Fig. 7 FIELD PHOTOGRAPH SHOWING MASSIVE SANDSTONE. 26

20 Fig. 8 FIELD PHOTOGRAPH SHOWING TROUGH CURRENT BEDDING SANDSTONE. Fig. 9 FIELD PHOTOGRAPH SHOWING SANDSTONE WITH PLANAR STRUCTURE 27

21 Fig. 10 FIELD PHOTOGRAPH SHOWING LAMINATED SANDSTONE WITH COLOUR BANDS Fig. 11 FIELD PHOTOGRAPH SHOWING MASSIVE & PLANAR NATURE OF SANDSTONE 28

13 FIELD PHOTOGRAPH SHOWING BEDDED (B), MASSIVE

22 Fig. 12 FIELD PHOTOGRAPH SHOWING FINE GRAINED MASSIVE SANDSTONE (F.M.S) ALONG WITH CONGLOMERATE (C) Fig. 13 FIELD PHOTOGRAPH SHOWING BEDDED (B), MASSIVE (M) AND CURRENT BEDDED (Cr) SANDSTONE 29

23 Fig. 14 FIELD PHOTOGRAPH SHOWING CONGLOMERATE BED Fig. 15 FIELD PHOTOGRAPH SHOWING CONTACT BETWEEN BARAIL AND SURMA GROUP 30

24 Fig. 16 FIELD PHOTOGRAPH SHOWING POT HOLE STRUCTURE Fig. 17 FIELD PHOTOGRAPH SHOWING CONCRETIONARY STRUCTURE 31

Facies Analysis Of The Reservoir Rocks In The. Sylhet Trough, Bangladesh. Abstract

Facies Analysis Of The Reservoir Rocks In The Sylhet Trough, Bangladesh Joyanta Dutta Petroleum Geoscience Program, Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand