Orbital Forcing Cyclostratigraphy of Miocene Formation in the South of Albert Rift,Uganda

Abstract

Albert Rift is located at the northern end of the west branch of the East African Rift in Uganda. The coherent stratigraphic and chronological framework for Albert Rift has not been established yet right now. This paper provides a particular approach of Milankovitch cyclostratigraphy research for determining the starting time of deposition. Firstly, Laskar solution proposed in 2010 is used to calculate the orbital elements value (eccentricity, obliquity and precession) since Miocene. The theoretical value of orbital forcing periods are derived from spectrum analysis and used as the basis of comparing standards. Then, spectrum analysis is applied on the natural gamma data from two wells which penetrated into granite basement to identify Milankovitch cycle in the Miocene formation. Thirdly, filtering is employed to extract eccentricity and obliquity cycle curve to calculate the initial time of deposition on the basement. Finally, the sedimentary rate variation of in the Miocene formation is analyzed by the cycle thickness and time interval. According to theoretical cycle analysis, the Miocene theoretical orbital periods include eccentricity periods: 405ka, 125ka, 95ka, and obliquity periods: 53ka, 40ka, precession periods: 24ka, 22ka and 19ka. These theoretical orbital periods serve as a benchmark for comparison with the actual cycle thickness on wells. Spectrum analysis on natural gamma data show that the orbital forcing cycles preserve in the Miocene of Albert Basin. The eccentricity and obliquity cycle curve corresponding to the cycle thickness are filtered from natural gamma data and the cycle numbers are recorded to calculate geological time. The initial sedimentary process of the rift may begin at approximately 15Ma and the sedimentary time interval of the oil-bearing layer was to be about 810ka. After correction of strata according to the stratigraphic dip and angle, the cycle stratum thickness is calibrated to true formation thickness to calculate deposition rate. It is about 0.125 m/ka in Miocene formation. Coupled with the characteristics of structure evolution, lake level and paleontology research results, the sedimentary rates are gradually increasing vertically. This study illustrates an accurate determination of the initial time of the basin to accept sediments for the first time, thus it sets a significant foundation for establishing a uniform stratigraphic framework of Albert Rift. The vertical variation of deposition rate is helpful to further understand the coupling relationship between sedimentation and tectonic evolution of Miocene formation in this basin.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019