Page 25 - Petroleum Geology

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ment to be taken permanently beyond the reach of fluctuations of baselevel.
The more rapid the subsidence, the larger the proportion of sediment ac-
cumulated permanently (up to the limit, of course, of total accumulation of
the sediment supplied).
Sediment accumulation may therefore be viewed as the difference between
the supply of sediment and the capacity of the environment to remove it.
Sedimentary basins are areas that, over a span of time, have had a supply of
sediment and a capacity to retain all or part of it.
It is clearly erroneous to consider sediment accumulation generally as
resulting from sediment falling directly from suspension. Sediment ac-
cumulates in this way only when the depositional surface lies below base-
level, and then the sediment usually is of very fine grade. In general, sedi-
ment is redistributed on the sea floor of the continental shelf, and this
generalization applies to muds and other fine-grained sediment because few
areas of the continental shelves are below the baselevel of muds, which lies
deeper than those for coarser grades.
The work of Barrell forms the basis for an understanding of sedimentary
basins. The concept of fluctuations of baselevel, leading to discontinuous
sediment accumulation, reconciles the long-standing observation that the
maximum net rate of sediment accumulation over major intervals of geolog-
ical time (Eras, Periods) is very much slower than that suggested by the sedi-
ments themselves. The maximum net rate of accumulation, obtained by
dividing the period of time in years by the maximum known thickness of
sediment accumulated during that time, is given for the Phanerozoic in
Table 1-1. In contrast with the rates in this table, Holocene sediment of the
US. Gulf Coast accumulated at about 1 m in 500 years, that of the Orinoco
delta at Pedernales accumulated at about 1 m in 100 years (Kidwell and
Hunt, 1958) and, as Barrell pointed out, the preservation of tree trunks in
the geological record of many parts of the world, in rocks of various ages,

TABLE 1-1
Phanerozoic rates of sediment zccumulation (based on data of Holmes, 1960)

Phanerozoic: 1 m per 4400 yrs
1 ft per 1300 yrs
Palaeozoic: 1 m per 5700 yrs
1 ft per 1700 yrs
Mesozoic: 1 m per 4100 yrs
1 ft per 1200 yrs
Cenozoic: 1 m per 2100 yrs
1 ft per 600 yrs
The apparent increase in accumulation rates with time must be interpreted with caution.
It may be real, but there has been less opportunity for younger rocks to be destroyed and
more opportunity for them to be measured.