Page 24 - Petroleum Geology

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ACCUMULATION OF SEDIMENT

The essence of Barrell’s concept of sediment accumulation is embodied in
his diagram, redrawn here as Fig. 1-2 (Barrell, 1917, p. 796, fig. 5). It depicts
the consequences of a generally rising baselevel that includes two minor or-
ders of fluctuation. The diagram is valid for a rising baselevel or a subsiding
sedimentary column: it is the relative change that is important. Sediment ac-
cumulates during periods of rising baselevel, and is removed during periods
of falling baselevel*. The process is both additive and subtractive, and only
the sediment that is left permanently below the fluctuations of baselevel
accumulates permanently. Diasterns result from the smaller oscillations: dis-
conformities from the larger. The actual sediment in a sequence may repre-
sent only a small fraction of the total time that elapsed for the accumulation
of the sequence because the sequence on the left of the diagram accumulated
during the black intervals along the time scale at the top.
Subsidence of a depositional surface, which we must now understand as
a surface over which sediment is or was being transported, provides a poten-
tial to accumulate sediment because there ;is a tendency for part of the sedi-

I II

Columnar
section

Fig. 1-2. Sedimentary record resulting from oscillations of baselevel. AA is the primary
curve of rising baselevel; B are second-order oscillations giving rise to disconformities;
C--C contains third-order oscillations giving rise to diastems. (Redrawn from Barrell, 1917,
p. 796, fig. 5.)

* A single, perfectly sorted material is implied in the diagram as a simplification.