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Stratigraphy


Stratigraphy the study of the stratified rocks (sediment and volcanic) especially their sequence in time, the character of the rocks and the correlation of beds in different localities. The divisions of stratigraphy are lithostratigraphy, biostratigraphy, seismostratigraphy, chronostratigraphy etc.

Lithostratigraphy the element of stratigraphy that deals with the lithology of strata, their organisation into units based on lithologic character, and their correlation. Alternatively, it deals with the bodies of rocks divided and recognised on the basis of lithology and other features, such as sedimentary structure, geomorphological characteristics and geophysical considerations, but the other features may also be used to define these units, especially where the lithologic boundaries are gradational or otherwise obscure. Both vertical and lateral boundaries of the units are designated on the basis of significant breaks in these characteristics as are identifiable and traceable in the field. geological survey of bangladesh (GSB) earlier published ' A brief description of the litho-stratigraphic units in the geological literature of Bangladesh'. Recently GSB introduced 'Stratigraphic lexicon of Bangladesh' which is a record based on the compilation of the available published and unpublished data of the previous workers and the data of the lexicon authors. It also includes the analysis of the lexicon authors on the previous works along with their views.

Biostratigraphic unit a body of rock divided and identified on the basis of its fossil content or palaeontological character and thus differentiated from adjacent rock body. A few micropalaeontological and a number of palynostratigraphical works have already been done on the subcrop and outcrop Tertiary sediments of Bangladesh. Recently similar works on Quaternary sediments are undertaken. However a few researchers are also working on Permian coal bearing sediments.

Palynostratigraphy the stratigraphic utilisation of palynologic methods. It mainly involves the application of the study of pollen and spores in stratigraphy and palaeoecology. The first steps towards a biostratigraphy of the Tertiary formations in the bengal basin entailed palynostratigraphic studies in Bangladesh. A number of researchers reported the palynostratigraphy of the Neogene sediments as exposed in the western flank of the Sitapahar anticline along the Lichubagan-Silchhari road cut in chittagong hill tracts. In Bogra-X1 Oligocene strata were not identified by means of micropalaeontology due to lack of faunal index fossils and lithologic marker horizons. However, a palynostratigraphic examination of samples originating from the nearby Kuchma-X1 well revealed the presence of Oligocene strata. Although the lithology and facies of the Oligocene in the gaibandha area differ significantly from those encountered in Singra-1X, the assemblage of palynomorphs of the Oligocene in the Geological Drill Hole-31 corresponds fairly well with that found in Singra-1X. According to the results of palynostratigraphic investigations on samples from the entirely cored well, the Oligocene has a thickness of 134m. SK Baksi developed palynological biostratigraphic zonations (Bengal Palynological Zone or BPZ) of the Bengal Basin and later SK Baksi and U Deb introduced a palynostratigraphic scheme, which is valid for the Upper Cretaceous-Palaeocene sequence in the entire Bengal Basin. [Sifatul Quader Chowdhury]

Seismic stratigraphy the study and interpretation of seismic data obtained during reflection seismic profiling survey in order to construct subsurface stratigraphic cross section. In a sedimentary cross section only two types of reflection generating interfaces are present at the time of deposition-stratal surfaces and unconformities. Stratal surfaces are the depositional bedding planes, which separate the episodes of sedimentation. They represent ancient depositional surfaces and are therefore geologically synchronous over the area of their occurrence. Although the unconformities are not time synchronous and variable in the hiatus they represent, they posses chronostratigraphic significance in that they separate older rocks from younger ones.

Seismic sequences define natural depositional units bounded at the top and base by unconformities or their correlative conformities within which the primary reflectors parallel bedding planes and therefore parallel paleo-depositional surfaces. Thus the physiographic characteristics of depositional environment under which sediments were deposited are displayed on the seismic section and are used to reconstruct paleoenvirnments, predict facies and infer reservoir rock content. The process of describing and interpreting the reflection characteristics within a sequence is seismic facies analysis. The basic parameters used to define seismic facies units are those dealing with reflection geometry within a sequence and the external form of the sequence itself. Reflection continuity, amplitude, frequency and seismically derived interval velocity can be incorporated in facies definitions.

List of stratigraphic units are given below:

Category Basis Units
Lithostratigraphic units (Rock units) Lithology Supergroup, Group, Formation, Member, Bed(s)
Biostratigraphic units Fossil content Assemblage-zones (Cenozone) Range-zones (Acrozone) Acme-zones- (Epibole) Internal-zones
Ecostratigraphic unit Environment of origin Facies
Chronostratigraphic units (Time-rock units) Time of origin Eonothem
Erathem
System
Series
Stage
Substage/Chronozone
Geochronologic units (Time units) Geologic time Eon
Era
Period
Epoch
Age
Chron

A recent seismic section across the hinge-zone of south western Bangladesh demonstrates many features of sedimentary regime. The prominent reflector at C is the regional R-11 unconformity of probable Middle Miocene age which separates the flat lying shelfal sediments above from Cretaceous to lower Miocene shelf edge and slope sediments below. The Miocene section shows extensive channelling both at the R-11 and higher, at the Upper Miocene -Pliocene boundary. The high amplitude reflector at E represents the Middle Eocene Sylhet Limestone and displayed its development at the shelf, shelf and slope. Below 5 seconds of TWT at the east of the profile the reflector becomes flat and loses its high amplitude character as the Limestone facies passes into a shaley basin facies. Comparison of Cretaceous reflectors between G and F with the Paleogene section at D shows the prograding nature of the shelf edge through time and demonstrates the significant possibilities for stratigraphic trapping of hydrocarbons in up-dip wedge-outs of Cretaceous to Miocene strata. A buried topographic high of flat lying sand prone Miocene at R-11 unconformity level is trapped and sealed by shale-prone channel fill is found at B is another type of prospect. This example demonstrates the techniques of stratigraphic analysis are to be used successfully in Bengal Basin. [ASM Woobaidullah]

Chronostratigraphic unit based on geologic age or time of origin. Also known as time-stratigraphic unit. A type chronostratigraphic unit, ie in its type area, is generally a lithostratigraphic unit the boundaries of which may be considered on the basis of fossils and other features, to be isochronous over the area. [M Abu Bakr]