地球内部过程 Geologic Tectonics Unconformity, overstep and onlap(overlap) The geological history of an area is preserved in the strata. If there is a break in this record because there has been no deposition of sediment and probably erosion for much of the time, the gap is called an unconformity between the lower and the upper series of strata. The surface corresponding to this time gap is the plane of unconformity. If there have been no Earth movements other than a vertical rise and fall relative to sea level then the dip is the same above and below the unconformity. If tilting, folding or faulting of the lower formation(E-A in Fig. 4.28) before the upper series( K-M)is deposited on them has produced structural discordance, then the geometrical relationship at the plane of unconformity is an angular unconformity. The lowest bed(K) of the upper formation rests directly on progressively older beds of the lower formation and is said to overstep from E to B. The pattern of subcrop of E-A under bed K is identical to the outcrops once produced by the lower structure before K was deposited. A line of boreholes would show a systematic increase of the gap in the geological record, in the up-dip directions of the lower formation 环境迪顶学 minoucntalocelogu ,率地球内部过程 Geologic Tectonics • Unconformity, overstep and onlap (overlap) The geological history of an area is preserved in the strata. If there is a break in this record, because there has been no deposition of sediment and probably erosion for much of the time, the gap is called an unconformity between the lower and the upper series of strata. The surface corresponding to this time gap is the plane of unconformity. If there have been no Earth movements other than a vertical rise and fall relative to sea level, then the dip is the same above and below the unconformity. If tilting, folding or faulting of the lower formation (E-A in Fig.4.28) before the upper series (K-M) is deposited on them has produced structural discordance, then the geometrical relationship at the plane of unconformity is an angular unconformity. The lowest bed (K) of the upper formation rests directly on progressively older beds of the lower formation and is said to overstep from E to B. The pattern of subcrops of E-A under bed K is identical to the outcrops once produced by the lower structure before K was deposited. A line of boreholes would show a systematic increase of the gap in the geological record, in the up-dip directions of the lower formation