摘要
An investigation into lake terraces and their sexiimentary features in the Fen River Graben Basin shows that several paleolake regression-transgression cycles took place during the mid to late Quaternary. The horizontal distribution of the lowest loess/paleosol unit overlying each lake terrace indicates the occurrence of four rapid lake regressions when paleosols $8, $5, $2, and S1 began to develop. The horizontal distribution of the topmost loess/ paleosol unit underlying the lacustrine sediment in each transition zone between two adjacent terraces indicates that following a lake regression, a very slow lake transgression occurred. The durations of three lake transgressions correspond to those of the deposition or development of loess/paleosols L8 to L6, L5 to L3, and L2. It is thereby inferred that regional tectonic movement is likely the primary factor resulting in the cyclical process ofpaleolake regressions and transgressions. Taking these findings along with published geophysical research results regarding the upper mantle movements underneath the graben basin into account, this paper deduces that a cause and effect relationship may exist between the paleolake regression- transgression cycles and the tectonic activity in the upper mantle. The occurrence of a rapid lake regression implies that the upwelling of the upper mantle underneath the graben basin may be dominant and resulting in a rapid uplifting of the basin floor. The subsequent slow lake transgression implies that the thinning of the crust and cooling of the warm mantle material underneath the graben basin may become dominant causing the basin floor to subside slowly. Four rapid paleolake regressions indicate that four episodic tectonic movements took place in the graben basin during the mid to late Quatemary.
An investigation into lake terraces and their sexiimentary features in the Fen River Graben Basin shows that several paleolake regression-transgression cycles took place during the mid to late Quaternary. The horizontal distribution of the lowest loess/paleosol unit overlying each lake terrace indicates the occurrence of four rapid lake regressions when paleosols $8, $5, $2, and S1 began to develop. The horizontal distribution of the topmost loess/ paleosol unit underlying the lacustrine sediment in each transition zone between two adjacent terraces indicates that following a lake regression, a very slow lake transgression occurred. The durations of three lake transgressions correspond to those of the deposition or development of loess/paleosols L8 to L6, L5 to L3, and L2. It is thereby inferred that regional tectonic movement is likely the primary factor resulting in the cyclical process ofpaleolake regressions and transgressions. Taking these findings along with published geophysical research results regarding the upper mantle movements underneath the graben basin into account, this paper deduces that a cause and effect relationship may exist between the paleolake regression- transgression cycles and the tectonic activity in the upper mantle. The occurrence of a rapid lake regression implies that the upwelling of the upper mantle underneath the graben basin may be dominant and resulting in a rapid uplifting of the basin floor. The subsequent slow lake transgression implies that the thinning of the crust and cooling of the warm mantle material underneath the graben basin may become dominant causing the basin floor to subside slowly. Four rapid paleolake regressions indicate that four episodic tectonic movements took place in the graben basin during the mid to late Quatemary.
