Page 27 - Petroleum Geology

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because we find considerable thicknesses of sediment of a particular environ-
ment. The balance is more apparent than real, and true balance is probably
rare. It is commonly said that the rate of subsidence was equal to the rate of
sediment accumulation, and this leads to the idea of balance. More correctly,
and more usefully, one should think of the rate of accumulation being equal
to the rate of subsidence. The accumulation of a considerable thickness of
sediment of the same facies merely reflects the constancy of the physio-
graphic environments over the area of the sedimentary basin, the surplus
sediment being removed.
The primary control on sediment accumulation is subsidence (assuming
sediment supply) because sediment accumulation without subsidence is vul-
nerable to subsequent dispersal. The primary controls on the nature of the
sediment that accumulates are the sediment sources and the environments,
and energy of the environments, of the physiographic basin on the sediment-
ary basin. The matter of lateral continuity of a particular rock type involves
considerations of the three dimensions of space and that of time. A rock unit
may be discontinuous over an area for a number of reasons:
- The sediment was not distributed over the whole area.
- The sediment was distributed but did not accumulate over the whole
area.
- The sediment was distributed and accumulated over the whole area, but
accumulation was only temporary over parts of the area due to changing
energy patterns.
One conclusion is clear: the margin of a discontinuous rock unit in the
sedimentary record is not necessarily the margin of the environment it re-
presents in the physiographic basin. The sequences of rock types and their
areal distribution in a sedimentary basin constitute a variable and very in-
complete record in space of the variations of the environments in the physiog-
raphic basin over that area with time.

TRANSGRESSIONS AND REGRESSIONS

When subsidence relative to baselevel in a physiographic basin exceeds the
supply of sediment, that is, the area has a capacity to accumulate a larger
volume of sediment than is supplied, the sea tends to deepen over the depos-
itional surface, and facies tend to migrate towards the land (Fig. 1-3). This
is a transgressive phase of sedimentary basin development, leading to the
accumulation of a transgressive sequence of sediments. Where this process
raises baselevel above the depositional surface, pelagic sediments may accum-
ulate (and may accumulate to a considerable thickness). Sedimentary basins
tend to be enlarged during transgressive phases, but the enlargement is only
permanent if the subsidence relative to baselevel is permanent.
If the sea over part of a physiographic basin becomes shallower, and the