Research on Cyclostratigraphic and Depositional Model of Sedimentary Manganese Deposits (Triassic) in Southwestern Guangxi Area

Key words: 
• southwestern Guangxi area /
• Triassic /
• manganese-bearing sequence /
• cyclostratigraphy /
• sea level change

Abstract: The manganese deposits are mainly distributed in Southwestern Guangxi area, which was regarded as one of the most predominant ore types in this area. However, the Triassic stratigraphic frameworks of manganese deposit in the southwestern Guangxi area is rarely discussed. The spectral analysis and filter methods can be adopted to divide cyclic sequence hierarchy and discuss depositional mechanism, and the cumulative residual curves from average amplitude variations of good logs are introduced and demonstrated as a valid graphical method to define depositional regime transformation and check boundary surfaces of cyclic sequences. The cumulative residual curves results show the positive half waves and negative half waves of cumulative residual curves are, respectively, corresponds to upward-deepening and upward-shallowing sedimentary successions in low frequency cyclic sequences in a well interval. Transform points of positive half waves and negative half waves are calibrated to boundary surface depth of cyclic sequences. The GR log data from Well 0302 of Southwestern Guangxi Area as a case example, cumulative residual curves are calculated and constructed on well log data. The low and high frequency cyclic sequences are distinguished and its procedure and methods are illustrated in detail. The significance and interpretation of amplitudes and wavelength changes of sedimentary cycles are discussed in this paper. The Beisi Formation of the Lower Triassic can be divided into four forth-order sequences, and the manganese ore deposit developed in the regression cycle. The origin model of manganese ore can be divided in three stages:1) Sea-level fall-the oxidizing manganese is formation; 2) Sea-level rise-the oxidizing manganese is transformed in manganese carbonate; 3) The maximum flooding surface-a little manganese carbonate is formed. The investigation provides a new effective approach to build the correlating framework of high-resolution sequence in Southwestern Guangxi Area.