Discussion
Stratigraphic sections S1 and S2, separated by tens of kilometers, are shown in the schematic. The line segments in this schematic represent biostratigraphic correlation between S1 and S2. Which one of the segments (P, Q, R, S) of this correlation line represents an unconformity?
Answer: B
The segment representing an unconformity is Q.
In a graphic correlation plot, each point on the line pairs a level in S2 with the level of the same biostratigraphic horizon in S1, and the slope of a segment compares how much rock the same time interval left in each section. Along Q the line is horizontal: the S2 thickness keeps increasing while the S1 level does not change at all. So a time interval represented by tens of metres of strata in S2 is represented by no strata in S1, which is a hiatus, an unconformity in S1.
Segments P, R and S all slope, so both sections were accumulating sediment during those intervals; a steep or gentle slope only means S1 accumulated faster or slower than S2, not that time is missing.
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