486 L. Dudley Stamp — 



wards of deltaic conditions from the Eocene to the present and 

 a consequent retreat of the sea towards the south. ^ It follows that : 



(») the Tertiary succession is predominantly marine in the south and 

 mainly continental in the north, 



(6) each horizon can be traced laterally from marine through deltaic 

 into fluviatile or seolian deposits as one goes northwards, 



(c) there is, on the whole, a tendency for continental conditions to move 

 soythwards as one ascends in the succession. The continental type of 

 Pliocene (Irrawadian) is the most widely spread of all the continental 

 deposits. 



Certain other important factors must, however, be considered. 

 Throughout the Tertiary period there seem to have been movements 

 of folding and uplift along the line of the Arakan Yomas, culminating 

 in the great folding movements which affect all the Tertiary deposits 

 of Burma and which took place towards the end of the Pliocene. 

 The Burma Gulf commenced as a geosyncline, and, like all 

 geosynclines, has undergone " buckling " and deepening at intervals. 

 The principal effects of this intermittent deepening have been to 

 allow the accumulation of a vast thickness of sediment and to 

 cause the return of marine conditions northwards. Consequently, 

 although there has been a general retreat of the sea to the south, 

 there is evidence of periodic marine invasions northwards. These 

 marine invasions are marked by wedge-shaped masses of marine 

 strata in the midst of the beds of continental type. This is well 

 seen m the case of the marine Yaw Series of Upper Burma. In 

 other words, there is a series of cycles of sedimentation. An effect 

 of the " buckling " is seen in the overlap of certain beds when 

 traced from the margins towards the centre of the basin. That is 

 to say that normally two beds are in direct and conformable super- 

 position ; locally the lower bed may be absent and the higher bed 

 may be seen resting on some older strata owing to the effect of slight 

 folding and consequent denudation just prior to the deposition of the 

 higher bed. It is not j»ossible to point to any great unconformities 

 amongst the Tertiary strata of Burma, though much has been 

 written on their occurrence or otherwise. The observed cases fall 

 into two groups : — • 



(a) " ravinements " such as occur almost invariably at the base of 

 Cycles of Sedimentation.^ Examples : the base of the Yaw Stage in the 

 Lower Chindwin District,^ and probably also the base of the Irrawadian 

 in the Oilfields of Minbu, Yenangyaung, etc. The intra-formational 

 " unconformities " frequently observed in the Pegu System belong to this 

 class, 



(6) local unconformities due to the denudation of minor folds produced 

 as a result of the " buckling " of the floor of the geosyncline during the 

 Tertiary period. A good example is seen in the Myaing region, where the 

 Irrawadian rests directly on the Pondaung Sandstone.* 



^ Cotter, Journ. and Proc. Asiatic Soc. Bengal, N.S., vol. xiv, 1918, p. 414. 

 2 Stamp, Geol. Mag., Vol. LVIII, 1921, p. 109. 



' For details see Stamp, " The Geology of Part of the Pondaung Range, 

 Burma ", Trans. Mining and Geol. Inst. India, vol. xvii, 1922. 



* Geol. Surv. India, 1 in. map. Sheet 84K./14, geologically coloured. 



