島根県西部・中新統益田層群における断層に規制された堆積盆地の発達過程  [in Japanese] Depocenter migration controlled by faults in the Masuda Group, western Japan  [in Japanese]

The Masuda Group distributed around Masuda City, western Japan, consists of blackish to shallow sea deposits more than 270 m thick. Field evidence as follows indicates that the group has deposited in the basin formed by depression caused by faulting. (1) The group is bordered mainly by the high-angle surface of unconformity which inclines at an angle 10 to 60 degrees and is represented by a straight line on the geological map. (2) Breccia deposits which consist mainly of debris and rock fragments drived from the adjacent basement rocks and reveal no effect by currents are distributed in a wedge-shaped profile at the foot of the high-angle surface of unconformity. (3) The high-angle surface of unconformity is concordant with ferred faults within the basement and consolidated sheared zones recognized occasionally in the basement near the contact. (4) The surface of unconformity is very rugged and parts of the surface are traced to ones of consolidated fractures in the basement. Breccia deposits are recogneized at some horizons along faults bounding the basin or basement highs. They indicate that some of them have been newly born or revived during sedimentation. The thickest parts of 4 units which are separated by means of 3 marker-beds are different from each other. Divergence of the thickest part of each unit suggests that the depocenter of each stage has migrate northward with time. Slumped masses more than 4 m are developed nearly at horizons of breccia deposits, suggesting that paleosubmarine slope inclined toward the depocenter of the overlying unit. Migration of the depocenter and changes of inclination of the paleosubmarine bottom were caused by newly-born or rivived faults which have originated the basin. Faulting which had occured during deposition and caused migration of the depocenter have been reported from other basins so that it is thought to play an important role in tectonic evolution of a sedimentary basin in a tectonic active area.