Page 23 - Petroleum Geology

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the marine environment that *was developed by Barrel1 (1917) in one of the
more important papers on sedimentary geology.
Baselevel is the level at which sediment neither accumulates nor is eroded:
it is also the conceptual level at which this equilibrium would be achieved.
The energy of the environment is one side of the equation, as it were, while
the physical properties of the sediment are the other. Baselevel is used in the
singular, and must be thought of with reference to a particular, perfectly
sorted material. This is a device to simplify the more complex reality, which
is that there is a baselevel for each grade of sediment, each grain of sediment.
A poorly sorted sediment in a given position has a range of baselevels, and
sediment accumulates or passes on according to whether the baselevel of
each grain lies above or below the depositional surface.
Baselevel may be viewed another way. In each position on the sea floor
there is a grade that just (but only just) cannot be moved by the available
energy, and the baselevel for that grade coincides with the sea floor in that
position. Finer grades cannot accumulate there.
It is this general process that leads to the accumulation of sand in one area
and mud in another. It is a process that has been operating since the material
became incorporated in the stream load (but baselevel is a more difficult and
less useful concept outside the marine environment). The sediment that ac-
cumulates in a place is that fraction of the total sediment in that place that
cannot be transported further. Therefore, one must not think of a rock unit
as a representative sample of the sediment brought to the place.
Baselevel fluctuates with changing tides, seasons and weather. On the con-
tinental shelves, fluctuations of baselevel will accompany fluctuations of
current intensity, and wave and swell lengths (because the energy of waves
decreases exponentially with depth as a function of wave length rather than
of wave amplitude). It seems reasonable to suppose that few areas on the
continental shelves are permanently below the lowest baselevel induced by
exceptional circumstances once in a hundred or thousand years. If the shape
of the continental shelf and the level of the sea relative to it were to remain
constant, and the mean storm intensity, current strength, and the variations
about the mean, were also to remain constant, there would be a tendency for
sediment to accumulate evenly and thinly over the shelf with the bulk of the
total sediment eventually gassing off the shelf into deeper water. The in-
creasing depth of water away from the land, and hence the decreasing mean
or maximum energy at the depositional surface, would result in a general
sorting of clastic material from coarse to fine seawards, dense to less dense,
spherical to angular, from the coast towards the margin of the continental
shelf. The geological record does not show more than a general tendency
for this to happen, and it does show significant thicknesses of accumulated
sediment in some areas, not in others. There is another influence: subsidence
of the depositional surfaces relative to baselevel.