IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG) e-ISSN: 2321–0990, p-ISSN: 2321–0982.Volume 8, Issue 1 Ser. III (Jan – Feb 2020), PP 11-18 www.iosrjournals.org DOI: 10.9790/0990-0801031118 www.iosrjournals.org 11 | Page Sedimentology of the Fluvial deposits, the Old Brahmaputra River, Mymensingh, Bangladesh SUDIP SAHA 1 , S. BIPULENDU BASAK 2 1 Department of Geology and Mining, University of Rajshahi, Rajshahi-6205, Bangladesh. 2 Department of Environmental Science and Engineering, Jatiya Kabi Kazi Nazrul Islam University, Trishal, Mymensingh, Bangladesh. Abstract: The Old Brahmaputra is a tributary of the Brahmaputra River that finally falls into the mighty Meghna River at Bhairab Bazar, Bangladesh. The morphology of this river indicates that it is a meandering river. The sedimentary sequences are composed of alternate beds sands and clay. The coarser sand beds were deposited at high energy condition of the river. The present study reveals that Fm, Sh and Sp facies constitute the lithological succession of the study area. The massive clays with peds were formed in the backswamp when the depositional basin became shallow and lost its energy. Textural studies of the sand samples exposed along the left bank of the Old Brahmaputra near Mymensingh town shows that the graphic mean sizes vary from 3.41 to 3.50ɸ, very fine sand. The sediments are well sorted that indicates the marine transgression of the Bengal Basin at the time of deposition. The sands are strongly coarsely skewed. The scatter plots of the deposits decipher that they deposited either in river or beach environments. Keywords: Old Brahmaputra, sand, well sorted, river, beach. --------------------------------------------------------------------------------------------------------------------------------------- Date of Submission: 29-01-2020 Date of Acceptance: 14-02-2020 --------------------------------------------------------------------------------------------------------------------------------------- I. Introduction The Old Brahmaputra River is a tributary of the Brahmaputra River that originates from Bahadurabad Ghat and flows through Gaibandha, Jamalpur, Sherpur, Mymensingh, Kishoreganj, Norshindi districts of Bangladesh and falls into the Meghna River at Bhairab Bazar. The average width of the Old Brahmaputra River is 200 meter and it is a meandering river. The present study is conducted on the left bank of the Old Brahmaputra River, Mymensingh, Bangladesh. The study area is bounded by the latitude 24.751677ºN and 24.770391ºN, longitude 90.401125ºN and 90.423170ºE (Figure 1). The sedimentary sequences composed of mainly alternate beds of sands and clays. The sediments exposed along the left bank and bars of the Old Brahmaputra River near Mymensingh town can be classified as Older and Younger deposits. The sediments exposed along the river bank are older than the present day bar sediments. The color of the present day bar sediments, especially the finer part (clays), indicate that they are derived from the near source sediments of Pleistocene deposits, Madhupur clay. The textural analysis is important tools to decipher the depositional environments of the sediments. Grain size analysis is also important to the engineers, agriculturalists and hydrogeologists. The present research work is conducted to determine the mean grain size of the sands and their distributions in the study area. The frequency curves were drawn to show the mode of the sediments and the cumulative frequency curves were made. Different bivariate plots were constructed to reveal the depositional environments (Friedman, 1967).