SEQUENCE STRATIGRAPHY OF THE OLIGOCENE LIMESTONES EXPOSED AT HTONDAUNG AREA, THAYET TOWNSHIP, MAGWAY REGION, MYANMAR
Abstract
- The study area is situated in Thayet Township. The present work mainly emphasizes on the limestone bodies in the Okhmintaung Formation of the area. The carbonate rocks exposed at Thayet area contains stacked depositional cycles consisting of characteristic succession of sedimentary facies and facies associations. The stratigraphic succession starts on delta abandonment phases together with an erosional surface at the base of facies, Grobigerina packstone-grainstone facies in the lower part. Delta abandonment phase is indicated by the occurrence of swamp development and the subsequent formation of coal at the base of Limestone. Through the succession, repetitive of inner, middle and outer ramp facies associations are present, and then they are followed by an alternation of shale and sandstone facies at the top. The term "cycle boundary" was used to determine the fourth-order cycles. The individual cycles were numbered 1-5 and the same numbers were used for their respective boundaries. The fourth–order cycles (parasequence sets) can be recognized by environmental changes within section; by the graduality of the change in fauna and the co-occurrence or normal marine perforates and platforminterior imperforates, and a relatively large amount of planktonic foraminifera. The first level or base level 'C-1' of the Limestone shows the shallowing upward, start with basinal blue-grey shale, Globigerinid wackestone, Globigerinid packstone-grainstone, or Lepidocyclina packstone. The outer ramp sediment shifted just above outer ramp (basinal shale) sediment within the section. The cycle 'C-2' observes an outer to mid-ramp community of planktonic fauna in the lower part, and a gradual shift from a planktonic dominated fauna toward perforate larger benthic one of the midramp in the upper. The cycle 'C-3' of the lower part consists of the retrogradation of the facies belts (from mid-ramp sediment to algal shoal of the inner ramp) marks the following TST aggradation and slow progradation of the corresponding HST. The cycle 'C-4' consists of bluishgray shale with high amount of planktonic foraminifera (Globigerina sp., Globotruncana sp., Nodosaria sp.).The upper shows inner to mid-ramp sediments containing a rich perforate foraminifera (Lepidocyclina sp., Operculina sp.) packstone-grainstone containing coal and gypsum bearing shale. The base of the 'C-5' consists of yellowish brown calcareous shale with lime bands. The upper part shows middle to inner ramp sediments having rich in benthonic foraminiferal assemblages.
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Year
- 2020
Author
-
Mi Mi Ko
Subject
- Geology
Publisher
- Myanmar Academy of Arts and Science (MAAS)