The Burma Circle of the Geological Survey of India and their Contributions to the Geology of Myanmar
By THAN HTUN (GEOSCIENCE MYANMAR)
EPISODE: 45
The Tertiary Succession and its Classification
This is a continuation of the previous Episode 44, entitled “The Tertiary Succession and its Classification”, contributed by Prof LD Stamp in 1922.
Post-Eocene (Pegu-Irrawadian)
As the main petroliferous horizon, the Pegu has been studied in great detail, but it is only very recently that anything like a clear conception of the deposits as a whole has been obtained, and much remains to be done. In this respect, Mr E Vredenburg’s recent work is extremely important in removing misconceptions caused by previous writers.
The Post-Eocene beds may be treated as a whole; they are entirely continental in the extreme north, except for the very highest beds, which are completely marine in the south. For the present, the term “Pegu System may be taken to correspond essentially with the marine facies of the post-Eocene Tertiary. It ranges in age from the lowest Oligocene to Pontian or lower part of the Pliocene”. The corresponding continental facies may be termed the Irrawadian. However, until recently, the Pegu and Irrawadian were believed to be superimposed systems of different ages, and when it was found that the Pegu beds passed northwards into freshwater deposits, the latter were termed “Freshwater Pegus”. For the present, this term may conveniently be retained for those freshwater beds that are shown by their vertebrate remains to be earlier than the well-known Irrawadian of the Oilfield region of Yenangyaung, Minbu, etc.
1.The Marine Post-Eocene or Pegu System (sensu lato)
Several horizons can now be distinguished on a palaeontological basis (“Standard Faunas”), and at least some of the stages based on a study of these faunas can be traced through the changes of facies from Lower to Upper Burma. The relationship of the lithological divisions to the stages is far more easily expressed in diagrammatic form, and a mass of detail has been incorporated, which is not repeated in the text below: -
The total thickness of the Pegu System in Lower Burma may be roughly 10,000 feet.
Basal or Kyet-u-bok Bed: This bed, which the writer regards as the base of the Pegu System, is a well-marked calcareous band, conglomeratic in places with small quartz pebbles. It is characterized by four species of foraminifera, always present but in very variable proportions. They are Nummulites yawensis Cotter (formerly described as N cf beaumonti and N beaumonti), Orthophragmina omphalus Fritsch, Operculina cf canalifera D’Arch., and Gypsina globulus Reuss. The associated mollusca include species common to the underlying Yaw Stage, notably Velates orientalis. Whilst the species of foraminifera mentioned occur in enormous numbers in the bed itself, they do not appear to range higher. It will be noticed that at least two of the foraminifera are Eocene forms. Taking a narrow palaeontological view, one would be tempted to regard the Kyet-u-bok Bed as Eocene. It is, however, distinctly a basal bed stratigraphically, and its characteristics are very similar to those of other typical basal beds. As a general rule, one may say that the fauna of a basal bed of a formation comprises: -
(a) Survivors from the preceding stage occur in enormous numbers before their final extinction. Their number is frequently increased by the presence of numerous rolled specimens.
(b) Forerunners of the succeeding faunas. It is the presence of these new elements in the fauna that decides its age
Shwezetaw Stage: In Lower Burma, the lowest Pegu Beds are shales, which form the lower part of the Sitsayan Shales of the Henzada and Prome districts. Passing northwards to the neighboured of Yenanma and Ngape’, one finds a series of shales that succeed the Basal bed and then by a group of sandstones – the Shewzetaw Sandstones. The shales are usually unfossiliferous, but the writer found an interesting fauna a few hundred feet above the base in Magyisan Chaung (latitude 19° 57’). The fossils have not yet been examined in detail, but they include forms closely allied to, but specifically distinct from, species described from higher horizons of the Pegu. Further north, the sandstone facies invade the whole of the Shwezetaw Stage and become of shallow-water type. About latitude 21° the sandstone resting on the fossiliferous Basal Bed contains coal seams, and the principal fossil is the brackish-water Batissa (Cyrena). The more marine type of Shwezetaw Sandstone (about latitude 20° 5’), seems to be characterized by Ampullina birmanica Vred, and this fossil is also found in the lowest Pegu (Shinmadaung Sandstones) where they rest on pre-Cambrian in the Shinmadaung area. Interbedded igneous rocks also occur in this district.
Padaung or Sitsayan Stage: This stage seems to mark the period of greatest extension of marine conditions during Pegu times, being still definitely marine as far north as latitude 21° 6’(Dudaw Taung) or even to 21° 40’ (West of Myaing). The stage is characterized by Tritonidea martiniana, and, towards the higher part, calcareous bands with Lepidocyclina theobaldi. In the south, this division is represented by the upper part of the Sitsayan Shales, which has thick bands of Lepidocyclina Limestone. The latter were sometimes mistaken by Theobald (1874) for Nummulitic Limestones – as in Henzada and at “Lime Hill” near Thayetmyo. Vredenburg believes that the lower part of the exposed Pegu in the oilfields of Minbu and Yenangyat belongs to this stage.
Singu Stage: Vredenburg has adduced evidence to show that the exposed Pegu rocks in the Oilfields of Yenangyaung and Singu and the higher beds in the fields of Minbu and Yenangyat are the equivalent of the Prome Beds (Division A of Theobald) of Lower Burma. The latter are a sandy group about 1,500 feet thick. The faunas of all the Pegu stages discussed up to the present are of distinctly Oligocene character.
Kama Stage: A predominantly shaley group found in Lower Burma which has recently yielded a magnificent series of fossils. The most fossiliferous beds occur about 700 to 1,000 feet above the top of the Prome Beds “A”. The fauna is distinctly Miocene and corresponds to the Gaj of Western India. This stage is probably represented by the highest Pegu Beds with brackish-water fossils (Batissa) in the oilfields of Upper Burma. The presence of the Miocene mammal Docatherium in the higher part of the Pegu of Yenangyaung supports this correlation. The “Freshwater Pegu” above the beds with Tritonidea martiniana in the Myaing region have yielded the lower Miocene mammal Cadurcotherium.
Pyalo Stage: A sandy group restricted to Lower Burma, which has yielded numerous examples of Ostrea latimarginata, a fossil characteristic of the Upper Gaj of India.
Akauktaung Stage: Formerly called “Marine Irrawadian” by Stuart, and corresponding to Theobald’s Mogaung Sands. Yields Ostrea virleti, O. digitata, and O. gingensis. It is doubtful whether this stage is separable from the Pyalo Stage.
Summarizing, it may be said that, independently of their exact age, the higher beds of the Pegu (as at Minbu, Yenangyaung, and Yenangyat) are characterized by the presence of large Cyrenae – C (Batissa) crawfurdi, C (B) noetlingi, C (B) petrolei, etc. These fossils also occur in the basal Irrawadian, which is probably derived. Throughout the Pegu, there is a marked increase in shallow-water characters northwards. The more homogeneous clays and shales of the south give place to the alternating shales and sandstones of the great oilfields. At Singu, conglomeratic bands and “Bone Beds” are frequent, whilst further north, remains of crocodiles and land vertebrates are found, and lateritic “Red Beds” appear.
2.The Continental Post-Eocene or Irrawadian System and “Freshwater Pegu”.
The “Freshwater Pegu” is naturally restricted to the more northern parts of Upper Burma. North of latitude 21° 30’, the Eocene Yaw Stage passes up gradually into a mass of somewhat coarse sandstone. At the base, logs of wood bored by molluscs and afterwards silicified are frequent, whilst in the higher part, and especially further north, silicified wood is abundant. North of latitude 22° 45’, the Freshwater Pegu rests directly on the Pondaung Sandstone, and bands of quartz-pebble conglomerate and lateritic “Red Beds” become frequent. Vertebrate remains – especially crocodilians – are occasionally found, and the occurrence of Cadurcotherium in the higher beds near Myaing has already been mentioned.
The Irrawadian of Upper Burma comprises a thick series – certainly more than 5,000 feet in the neighbourhood of Yenangyaung – consisting mainly of coarse, current-bedded sands. At the base, there is usually a well-marked “Red Bed” or old lateritic land surface. Associated with this band, either above it or below, there is frequently a bed of white sand rich in kaolin. Interbedded bands or even beds of some thickness of clay, which approach pipeclay in general characters, are frequent in the lower beds and again in the higher part of the Irrawadian. The Irrawadian is famous for the enormous quantity of silicified fossil wood that it contains- hence the old name, “Fossil Wood Group” (Theobald). The series has also yielded several interesting vertebrate remains, notably near Yenangyaung. Specimens from this locality come from two distinct horizons: -
(a) Lowest beds containing Hipparion punjabiense Lyd. (Hippotherium antilopinum of Noetling and earlier writers), Aceratherium lydekkeri Pilg. (A. perimense of Noetling), crocodilian and chelonian remains. At this lower horizon, Mastodon and Hippopotamus seem to be rare or absent; no undoubted occurrence is known to the writer.
(b) A conglomeratic band some 4,500 feet higher in the series and exposed along the banks of the Irrawaddy between Yenengyaung and Nyaunghla yielding numerous Mastodon latidens, Stegodon clifti, and Hippopotamus irravaticus.
A rough correlation of the Burmese and North-Western European Tertiaries is as follows: -
This is the end of “An Outline of the Tertiary Geology of Burma.”
References:
Stamp, L Dudley. 1922: An Outline of the Tertiary Geology of Burma, the Geological Magazine, Vol LIX.
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