The Black Shale Unit in the Maymyo Formation, Pyin Oo Lwin Township

Transcription

1 Universities Research Journal 2011, Vol. 4, No. 5 The Black Shale Unit in the Maymyo Formation, Pyin Oo Lwin Township Khaing Khaing 1 San and Ko Ko Gyi 2 Abstract Very limited outcrops of fossil dated Middle Devonian beds occur in Wetwin-Padaukpin area, Pwepon area (east of Kyadwinye Iron Mine) and near Hkai Hsin in Yadanatheigi area in Northern Shan State. That is probably due to the regional dolomitization that has erased most of the fossil record in the carbonates of the Shan State. Padaukpin Limestone and Wetwin Shale are the fossiliferous unit of Middle Devonian age which escaped from dolomitization. These two units sandwiched between the dolomite or dolomitic limestone of the Maymyo Formation. The age and stratigraphic position of the Wetwin Shale is still a problem and it must be needed to reinvestigate in details from the paleontological point of view. The Paleozoic units of Middle Ordovician to Upper Devonian in Wetwin-Padaukpin area and Pwepon area, Pyinoolwin Township have been mapped in detail and discussed for the occurrence of the Black Shale Unit (Wetwin Shale) in the Maymyo Formation of both areas resulting from the current investigation. Key words: Middle Devonian, Wetwin Shale, Maymyo Formation Introduction In Myanmar, Middle Upper Devonian Wetwin Shale unit is exposed as isolated outcrops especially in the Wetwin-Padaukpin area, Pyinoolwin Township (Fig.1). Stratigraphically, this shale unit is sandwiched in the limestone and dolomite of the Maymyo Formation. The present work pointed out the new occurrence of black shale unit in Pwepon that is situated about 12 miles northeast of Pyin Oo Lwin. In this area, these black shales are exposed as the sandwiched unit between Maymyo Formation. Lithologically and faunastically, these black shales are generally identical with the one which exposed in the Wetwin-Padaukpin area. Actually, these black shales are conformably overlain by the Padaukpin Limestone. The stratigraphic position of black shale is rather interesting for detailed investigation, as the previous literature described the overlying unit on the Padaukpin Limestone. 1. Lecturer, Dr., Department of Geology, Kengtung University 2. Professor, Department of Geology, Pakokku University

2 60 Universities Research Journal 2011, Vol. 4, No. 5 Figure 1. Location map of the study area. Stratigraphy Wetwin Shale in Wetwin-Padaukpin Area General Information La Touche, 1913, has been placed the Wetwin Shale in the lower part of the Plateau Limestone (lower dolomitic part) of Devonian age. This unit has been placed in the lower part of the Shan Dolomite of Devonian age (Brunnschweiller, 1970), lower division of the dolomitic part of the Plateau Limestone (Anderson, 1969) and the Maymyo Dolomite Formation (Amos, 1975). Aye Ko Aung, 2000 proposed that the Wetwin Shale and the Padaukpin Limestone were considered as the sub-unit of member rank to be included in the Maymyo Formation. On the basis of faunal evidences and their stratigraphic position, the fossils bearing limestone of Padaukpin Limestone and buff to black coloured Wetwin Shale are sandwiched in the Maymyo Formation (Khaing Khaing San, 2005).The geological map of the Wetwin-Padaukpin area is shown in (Fig.2).

3 Universities Research Journal, 2011, Vol. 4, No Figure 2. Geological map of the Padaukpin area (Khaing Khaing San, 2005).

4 62 Universities Research Journal 2011, Vol. 4, No. 5 Explanation of the Maps

5 Universities Research Journal, 2011, Vol. 4, No Recently accepted condition is that the Plateau Limestone of northern Shan State is composed mainly of two units viz. Upper Plateau Limestone (calcitic part) of Anthracolitic age (Permo-Carboniferous) and Lower Plateau Limestone of Devonian age. The latter consists essentially of dolomite and partly calcitic limestone and shale which are richly fossiliferous. Previous Works The name Wetwin Shale was initially given by La Touche (1913) for a fossiliferous shaley sequence near the Wetwin village, 12 miles northeast of Pyinoolwin. The Wetwin fauna collected by La Touche (1913) were later identified and described by Reed (1908). Some other notable works for the description of the stratigraphy of the Wetwin Shale were done by La Touche, 1913; Pascoe, 1959; Anderson, 1969; Brunnschweiler, 1970; Amos, 1975; Bender, 1983 and Wolfart et. al., Since the year 1981 through 2000, the second year Honours and final year Geology students of Mandalay University and the year 2001 on wards, the geology students from many Universities of Myanmar had been carried out the field work for the course of field geology in Wetwin-Padaukpin area. As the Wetwin Shale exposed in the very limited area, the outcrop mapping for this unit is very scarce. Outcrop distribution Fairly good exposure are encountered at the following localities: - near mile post 53/7 to 54/4 on the Mandalay-Lashio car-road - Cut-section on the Wetwin-Kyuntabin cart-track - Wetwin dam site (about one mile NE of Kyuntabin) - Flat terrain near Wetwin (Budaywa) Lithology Gross lithology for this unit is thin- to medium-bedded, yellow to buff colour with mottled pink colorations in some places; occasionally shows dark grey to black coloured, hard and compact, fissile shales. Conchoidal fractures on moderately hard shaley beds are also noted. Desiccation cracks on black carbonaceous, argillaceous beds are good indication marks for sub-aerial exposure or shrinkage of mud under

6 64 Universities Research Journal 2011, Vol. 4, No. 5 atmospheric condition. This indicates the shallowness of water at the last stage of deposition of muddy sediments (Figs.4, 5, 6&7). Fauna, age and correlation Pelecypods, brachiopods, bryozoans and crinoid stems are abundant in this shaley horizon. Reed, 1908 described the following fauna assemblage from the Wetwin Shale. Brachiopods : Athyris sp. cf. A. spiriferoides, Camarotoechia sp., Chonetes subcancellata, C. sp., C. sarcinulata, Lingula sp. Bryozoans : Fenestrella polyporata, F. wetwinensis, Polypora sp. Lamellibranchiata : Janeia birmanica, Nucula sp., Paleoneilo sp., Paracyclas sp. cf. P. poavia, P. sp. cf. P. rugosa, Phthoria sp., Prothyris sp. Gastropods : Bellerophon shanensis, B. sp. Trilobita : Prionopeltis sp. cf. P. cyclurus On the basic of these fauna assemblage, Wetwin Shale is correlated with those of the Hamilton group in North America and Naples fauna of western New York and designated the age provisionally either to the lower part of the Upper Devonian (Frasnian) or to the Upper part of the Middle Devonian (Givetian) (Pascoe, 1959). As the type locality of Wetwin Shale in Wetwin Padaukpin area, it can be traced to within half a mile of the Padaukpin Limestone. Pascoe (1959) described that the contact between the Wetwin Shale and the Padaukpin Limestone is faulted in nature. The present study observed that this fault is passing through the centre of the Wetwin Shale Unit near Kyuntabin village. It underlies the Padaukpin Limestone but has dips of different direction. They exposed on each side of the Wetwin anticline. Therefore, it is not possible to observe clearly the relationship of these two units. They may be the same stratigraphic horizon of different facies. According to the present status of investigation, it is proposed that the Wetwin Shale and the Padaukpin Limestone may possibly be analogus, but the facies is different and its age should probably be designated as Eifelian.

7 Universities Research Journal, 2011, Vol. 4, No Black Shale Unit in Pwepon Area General Information The black shale unit is exposed in the Pwepon area, as the lower member of Maymyo Dolomite Formation (Win Win Kyi, 1990). She also described that the Maymyo Dolomite Formation is divisible into two sub units of member rank namely lower black shale and biostromal limestone member which laterally passed into brecciated dolomite, dolomitic limestone and sandy limestone Member and upper dolomite and dolomitic limestone unit which are highly brecciated and very porous. Distribution Although thick soil and vegetation had been covered in the Pwepon area, black shales are cropped out as very limited exposures on the low denudational hill slope along the cart-track to Pwepon cave. These outcrops are traceable laterally and their exposed thickness is rather mappable (Fig.3). Lithology This unit is thinly-bedded, yellow to buff, dark grey to black, closely spaced, highly jointed fissile shale mainly composed of calcareous silty shale and rarely carbonaceous shale. The outcrop nature is lenticular in exposure. The measured exposed outcrop is 600 feet long and 150 feet wide (Figs. 8&9). Stratigraphic Position The underlying and overlying lithologies are mainly dolomitic limestone of Maymyo Formation. As mentioned above, there are two sandwiched members within the Maymyo Formation and the present investigation points out the position of Black Shale Unit which underlies the biostromal limestone member. Fauna, age and correlation Fossils are rare and they are hardly seen on outcrops because of intensive weathering, mainly physical weathering. Some identifiable fossils collected are: Brachiopods: Lingula sp., Camarotoechia sp.

8 66 Universities Research Journal 2011, Vol. 4, No. 5 Figure 3. Geological map of the Pwepon area (Khaing Khaing San, 2005

9 Universities Research Journal, 2011, Vol. 4, No On the basis of faunal content, lithologic similarity and same stratigraphic situation, this black shale unit can be correlated with the Wetwin Shale of Wetwin-Padaukpin area. Conclusion As the type locality of Wetwin Shale in Wetwin-Padaukpin area, it can be traced to within half a mile of the Padaukpin Limestone. Pascoe (1959) described that the contact of Wetwin Shale and Padaukpin Limestone is faulted contact. The present study observed that this fault is passing through the centre of the Wetwin Shale Unit near Kyuntabin village. It underlies the Padaukpin Limestone but have dips of different direction. They exposed on each side of the Wetwin anticline. Therefore, the relationship of these two units cannot be observe clearly. They may be the same stratigraphic horizon (formed of lateral facies changes) or may be either overlie or underlie with each other in Wetwin-Padaukpin area. But in the Pwepon area, the Black Shale Unit (equivalent unit of Wetwin Shale) directly follows conformably the Quartzose Sandstone Unit of the Zebingyi Formation and underlies conformably the biostromal limestone. These three units also dip in the same direction of southeast amounting This fact strongly indicates that the Black Shale (Wetwin Shale) conformably underlies the biostromal limestone (equivalent unit of Padaukpin Limestone). As aforementioned the type locality of Wetwin Shale in Wetwin-Padaukpin area also underlies the Eifelian Padaukpin Limestone. The comparative table for these two shale units is summarized in (Table 1).

10 68 Universities Research Journal 2011, Vol. 4, No. 5 The Wetwin Shale previously designated as Givetian to Frasnian in age. The present investigation of the stratigraphic position of Wetwin Shale, its age must be needed to reinvestigate in detail. Pascoe, 1959 described that the fauna of Wetwin Shale is quite distinct in composition and specific characters from that of the Padaukpin Limestone, but it is undoubtedly of Devonian age, and there are neither Silurian nor Carboniferous elements in it. These fauna allied to the Hamilton group of North America and Naples 4 fauna of western New York and this shale unit probably overlies the 5 Padaukpin Limestone, and its age refered to Givetian to Fransian. For the exact age determination of the Wetwin Shale, there was no occurrence of Figures 4&5 Buff colored Wetwin Shale with thin laminations are developed as spheroidal structure due to differential weathering 6 7 Figure 6. Closed up view showing "Desiccation cracks" in black shale. (Loc: Wetwin Dam) Figure 7. Thin-bedded, buff coloured shale exposed along the cart track, near Kyuntabin village 8 9

11 Universities Research Journal, 2011, Vol. 4, No Figure 8. Thin-bedded shale layers with intercalation of buff and black colored layers show closely spaced joints of Black Shale Unit Figure 9. Thin-bedded, buff to black colored, calcareous silty shale composed the Black Shale Unit Table (1) Comparisoin of two shale units in Wetwin and Pwepon areas Wetwin Shale Location Near mile post 55/7-54/4, section on the Wetwin- Kyuntabin cart-track Lithology Stratigraphic situation Outcrop nature Faunal content Thin- to medium-bedded, yellow to buff shale or calcareous shale, and black carbonaceous shale Underlain by dolomitic limestone, overlain by dolomitic limestone and biostromal limestone (Padaukpin Limestone) Isolated exposures in the dolomitic limestone Brachiopods: Athyris sp. cf. A. spiriferoids, Chonetes sp.,lingular sp., Camerotoechia sp. Bryozoans: Fenestrella polypora, F. wetwinensis, Polypora sp. Pwepon Shale Near Pwepon cave, 5 km east of Kyadwinye Iron Mine Thinly bedded, yellow to buff, dark grey to black, closely spaced jointed silty shale Underlain by dolomitic limestone, overlain by dolomitic limestone and biostromal limestone (Padaukpin Limestone) Lenticular bodies in the dolomitic limestone Brachiopods: Lingula sp., Camerotoechia sp. Age Eifelian?Lower Eifelian index fossil for either Givetian or Frasnian and its stratigraphic position can not describe clearly in the previous works. According to the present status of investigation, the age of Wetwin Shale (Black Shale Unit) of Maymyo Formation should probably be designated as lower Eifelian or older than this age. This paper intended to describe only for the information of new locality of Wetwin Shale and its

12 70 Universities Research Journal 2011, Vol. 4, No. 5 stratigraphic position. More information must be needed to confirm the age of the Wetwin Shale from various points of view. Acknowledgements We would like to express out gratitute acknow to Rector Dr. Thar Tun Aung, Kengtung University for his kind permission and encouragement to do this research work. Special thanks are also due to Professor (Retired) Dr. Aye Ko Aung, Department of Geology, Dagon University for his valuable discussions and suggestions. References Amos, B.J. (1975). Stratigraphy of Some Upper Paleozoic and Mesozoic carbonate rocks of the eastern Highlands. Newsl. Stratigr., 4(1): Anderson, M.M., A.J. Boucot A.J. and Johnson J.D. (1969). Eifelian brachiopods fron Padaukpin, northern Shan States, Burma. Bull. Br. Mus. Nat. Hist., 18: Aye Ko Aung (2001). Handbook of the Paleozoic units in Pyin-Oo-Lwin District. Upper Myanmar (unpub.), Dept. of Geol., Yadanabon University. Bender, F. (1983). Geology of Burma: Geburiider Borntraeger, Berlin Stuttgart. Brunnschweiler, R.O. (1970). Contribution to the post-silurian geology of Burma, northern Shan State and Karen State. Journal of geological Society of Australia, 17: Geology Field Party ( ). Geology of the Wetwin-Padaukpin area, Pyinoolwin Township. (Unpublished) Field Reports, Mandalay University. Khaing Khaing San (2005). Middle Devonian Rugose Corals of the Padaukpin Limestone, Pyinoolwin Township, Mandalay Division. Ph.D. Thesis (unpublished), Geology Department, University of Mandalay. 282p. La Touche, T.H.D. (1913). Geology of the northern Shan State. Mem. Geol. Surv. India, 39 (2):397p. Pascoe, E.H. (1959). Annual of the geology of India and Burma: Third ed., v.2. Govt., India press. Reed, F.R.C. (1908). The Devonian faunas of the northern Shan State. Memory of Geological Survey of India. 2: Win Win Kyi (1990). Geology and Mineral Resources of Pathin-Kyadwinye Area, Pyinoolwin Township. M.Sc. Thesis, Geol. Dept., Mandalay University, 100p.