i m B.S(final year) student of geology of peshawar university
i have two Qs...
how to apply walther's law on stratigraphy of pakistan?
is milankovitch cycle remain constant or changes in the past?
Dear Mr. Salman Akbar,
Thank you very much for sending me your question.
Johannes Walther (1860–1937)had propsoed that facies that exist in conformable vertical succession of strata also occur in laterally adjacent environments. This statement is known as the Walther's law of correlation of facies.
How do you aapply the Law; prepare several cross sections across any basin with an appropriately distance covering the area with the well control that may give you information through wireline logs and biostrtigraphy and try to correlate various geological time lines (Groups, Formation, members and litho units) within a sequence (Sequence here refer as the surfaces between two unconformities)and determine various liho facies and determine sedimentary environments.
In these sections you should be able to define the paleo shore line, shelf-slope hinge line, slope of the shelf, toe of the slope and the deepr part of the basin on the basis of lithology. Age determination is possible through biostratigrphic data. In Pakistan you can select any area, for example lower Indus Basin and prepare crosss ection with wells covering Cretaceous sextion.
From East to west and north to south several cross section after completion will give you a complete paleo geography of the basin and you may be able to interpret and define key depositional strand lines. For example, Lower Cretaceous deposition have taken place at a site which was havinf land towards east and the shelf-slope-basin to the west of the present day Indus Basin. Different wells data will show the major lateral litho facies (sand rich towards east and shale dominating westward).
The Milankovitch Cycle are considered as long term climatic cycles which are smaller cycles of orbital variation of planet earth.
In the last hundred thousand years, the earth's orbit around the sun had chnaged from a thin ellipse (oval) to a circle and back again. When the orbit around the sun is most elliptical, there is larger difference in the distance between the earth and sun at perihelion and aphelion. In the Perhellion the distance of Earth from sun is around 147 million kms where in the Aphellion the distance increases to 152 million Km.
The Earth's long term climatic cycles or Milankovitch cycle depends on the variability of the axis tilt which we know at present is 23.5 degrees whereas in the past 40000 years this tilt varied between 22 to 24.5 degrees and therefore Earth had experienced extreme climatic change such as the global Ice age about 10000 years ago.
Hopefuully, both answer will help you understand your problems.
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