Page 26 - Petroleum Geology

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indicates that they were buried before the wood rotted, implying a rapid
rate of sediment accumulation to be measured in tens of years per metre
rather than thousands.
Barrell’s concept also requires us to exercise extreme caution when inter-
preting the geological record in terms of present-day environments. Only a
small proportion of the sediment at present in transport along the sea floor
will normally accumulate near where it is at present, and not all of that that
accumulates will accumulate for a significant time-span geologically. More-
over, the post-Pleistocene has been a time of generally rising sealevel and
rising baselevel, so it has been a period of active, but not necessarily per-
manent, sediment accumulation at an average rate of about 1 m in 1000
years perhaps (if we accept the figure Gutenberg, 1941, p. 729, derived from
tide gauge records as being representative of the Holocene).
In general, it is erroneous to consider rock units in the stratigraphic record
as analogous to sand banks, shoals, bars, tidal flats, etc., of the seafloor
today - although there are clear exceptions to be found in thin units with
ripple marks and animal tracks and burrows. It is generally much more ac-
curate to regard rock units as the incomplete record of the passage of sedi-
ment of different compositions with bedding planes representing diastems. A
clean sand did not necessarily accumulate from a clean sandy sea floor be-
cause the mud with it might have been winnowed out, to accumulate else-
where.
The migration of a large sand bank over the sea floor, for example, may be
recorded in the stratigraphic sequence by a thin, laterally discontinuous sand
unit that consists only of that portion that came to be permanently below
baselevel. The dune shape will not be apparent, and there may even be no
trace of current bedding. Likewise, on a smaller scale, the preservation of
ripple marks requires that a surface that was close above the baselevel of that
material on one tide be buried under a protective layer of sediment by the
next tide, and for baselevel to be elevated permanently above the surface of
the ripple marks. Only in areas of extremely rapid subsidence are ripple-
marked surfaces likely to be common, and it is hard to escape the feeling that
they are a common feature preserved by uncommon events - that the or-
dinary ripple-marked surface in shallow water and between the tide lines is
unlikely to be preserved. Worm burrows are sometimes interpreted as evi-
dence of slow sediment accumulation, but consideration of baselevel suggests
that it is more likely to have been very rapid.
While the concept of a physiographic basin is quite distinct from that of a
sedimentary basin, a sedimentary basin is necessarily situated within a phys-
iographic basin because it is the latter that is the dominant influence on
sediment supply. The combined concept is dynamic: changes in the physiog-
raphic basin with time affect the type and character of the sediment that
accumulates in the sedimentary basin.
Sediment supply is often apparently in very close balance with subsidence,