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.
#TheGlobalNewLightOfMyanmar
No comments:
Post a Comment