534
- Pull-apart basins in areas of high relief (Sect.
12.8.2).
- Some adjacent seas with high terrigenous sediment
input (e.g. the northem Gulf of Califomia (Sects. 4.3
and 11.3).
Sediment-Starved Basins
Sediment-starved basins evolve far away from effieient, mostly high-relief terrigenous sediment
sourees. Their upward-fining and thinning vertical
facies successions indicate deepening basins, and
their slowly deposited sediments tend to become increasingly rich in bio genie material, and sometimes
also in organic matter. Typical examples include
- Central parts of large oceanic basins and smaller
basins in the vicinity of mid-oceanic ridges (Sects.
5.3.2 and 5.3.3).
- Isolated submarine highs and platforms (Sect.
5.3.7).
- Epicontinental seas surrounded by lowlands.
- Arid adjacent seas with drainage areas of low sediment yield (e.g. the present-day Red Sea (Sect. 4.3).
- Closed lakes receiving little terrigenous sediment
(e.g. the Dead Sea, Sect. 11.3).
Basins with Balanced Sediment Supply
These basins tend to maintain their depositional environment for considerable time periods. Long-term
shallowing- or fining-upward trends are absent.
Characteristic examples are
- Siliciclastic shelf seas of moderate subsidence, a
surplus in terrigenoUs sediment influx is often removed by sediment bypassing into deeper water.
- Carbonate shelves and carbonate platforms tend to
adjust their vertical buildup to relative sea level, with
and without various types of reefs (Sect. 3.4). Excess
bio genie carbonate production is transferred by currents to deeper water.
- Some shelf areas affected by upwelling water
masses and high organic productivity (e.g. the modem shelves off Peru and South Africa, Sect. 5.3.4)
maintain these conditions for long time periods.
- Epicontinental seas with moderate influx of
terrigenous sediment and slow subsidence.
- Some foreare and backare basins with limited sediment supply.
- Subsiding lake basins with limited sediment supply.
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
Modifying Factors
The three principal trends mentioned above may become modified in many ways by changes in oceanographie and climatic conditions, and by variations in
organic productivity due to the faunal and floral evolution through time. In addition, sea-level fluctuations and synsedimentary tectonism (such as extension, convergence, strike-slip etc.) play an important
part. The great variety of sedimentary facies through
time is mainly a result of these modifying factors
which obscure the simple basic principles.
Why Are Shallow-Marine Deposits So Common?
It is surprising that shallow-marine deposits constitute a great proportion of the total sediment volume
in present-day environments as weIl as in the fossil
record. One would expect that the principle condition
(3), i.e., a balance between sediment supply and subsidence, is rarely realized for a time period of some
geological significance. Hence, long-persisting
shallow-water conditions should be an exception.
However, the present-day, high-energy shelf environments demonstrate that the hydrodynamic conditions
of the sea prevent aggradation of sediment up to sea
level (Fig. 11.23b and c). Instead, the incoming
terrigenous sediment is widely distributed over the
entire shelf area and some is bypassed into deeper
water (cf. Sect. 3.3). Significant outbuilding of the
coastline is normally restricted to zones of very high
sediment influx.
Thus, long persistence of shallow-marine conditions in the sedimentary record testifies to the fact
that the former basin was probably oversupplied with
sediment most of the time, but has lost part of its sediment influx to deeper, more poorly supplied regions.
Similarly, fluvial basins are commonly filled up to
the level of their stream gradient. Any excess in river
supply is carried farther downstream; thus a balance
between sediment aggradation and basin subsidence
is established· for a significant time period. Such a
process provides thick fluvial records. The examples
in Chapter 12 further illustrate the principles briefly
discussed here.
In contrast to shallow-marine sediments, deep-sea
sediments are under-represented in the fossil record,
because they were largely subducted at convergent
plate boundaries and transformed intometamorphic
rocks.
- Pull-apart basins in areas of high relief (Sect.
12.8.2).
- Some adjacent seas with high terrigenous sediment
input (e.g. the northem Gulf of Califomia (Sects. 4.3
and 11.3).
Sediment-Starved Basins
Sediment-starved basins evolve far away from effieient, mostly high-relief terrigenous sediment
sourees. Their upward-fining and thinning vertical
facies successions indicate deepening basins, and
their slowly deposited sediments tend to become increasingly rich in bio genie material, and sometimes
also in organic matter. Typical examples include
- Central parts of large oceanic basins and smaller
basins in the vicinity of mid-oceanic ridges (Sects.
5.3.2 and 5.3.3).
- Isolated submarine highs and platforms (Sect.
5.3.7).
- Epicontinental seas surrounded by lowlands.
- Arid adjacent seas with drainage areas of low sediment yield (e.g. the present-day Red Sea (Sect. 4.3).
- Closed lakes receiving little terrigenous sediment
(e.g. the Dead Sea, Sect. 11.3).
Basins with Balanced Sediment Supply
These basins tend to maintain their depositional environment for considerable time periods. Long-term
shallowing- or fining-upward trends are absent.
Characteristic examples are
- Siliciclastic shelf seas of moderate subsidence, a
surplus in terrigenoUs sediment influx is often removed by sediment bypassing into deeper water.
- Carbonate shelves and carbonate platforms tend to
adjust their vertical buildup to relative sea level, with
and without various types of reefs (Sect. 3.4). Excess
bio genie carbonate production is transferred by currents to deeper water.
- Some shelf areas affected by upwelling water
masses and high organic productivity (e.g. the modem shelves off Peru and South Africa, Sect. 5.3.4)
maintain these conditions for long time periods.
- Epicontinental seas with moderate influx of
terrigenous sediment and slow subsidence.
- Some foreare and backare basins with limited sediment supply.
- Subsiding lake basins with limited sediment supply.
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
Modifying Factors
The three principal trends mentioned above may become modified in many ways by changes in oceanographie and climatic conditions, and by variations in
organic productivity due to the faunal and floral evolution through time. In addition, sea-level fluctuations and synsedimentary tectonism (such as extension, convergence, strike-slip etc.) play an important
part. The great variety of sedimentary facies through
time is mainly a result of these modifying factors
which obscure the simple basic principles.
Why Are Shallow-Marine Deposits So Common?
It is surprising that shallow-marine deposits constitute a great proportion of the total sediment volume
in present-day environments as weIl as in the fossil
record. One would expect that the principle condition
(3), i.e., a balance between sediment supply and subsidence, is rarely realized for a time period of some
geological significance. Hence, long-persisting
shallow-water conditions should be an exception.
However, the present-day, high-energy shelf environments demonstrate that the hydrodynamic conditions
of the sea prevent aggradation of sediment up to sea
level (Fig. 11.23b and c). Instead, the incoming
terrigenous sediment is widely distributed over the
entire shelf area and some is bypassed into deeper
water (cf. Sect. 3.3). Significant outbuilding of the
coastline is normally restricted to zones of very high
sediment influx.
Thus, long persistence of shallow-marine conditions in the sedimentary record testifies to the fact
that the former basin was probably oversupplied with
sediment most of the time, but has lost part of its sediment influx to deeper, more poorly supplied regions.
Similarly, fluvial basins are commonly filled up to
the level of their stream gradient. Any excess in river
supply is carried farther downstream; thus a balance
between sediment aggradation and basin subsidence
is established· for a significant time period. Such a
process provides thick fluvial records. The examples
in Chapter 12 further illustrate the principles briefly
discussed here.
In contrast to shallow-marine sediments, deep-sea
sediments are under-represented in the fossil record,
because they were largely subducted at convergent
plate boundaries and transformed intometamorphic
rocks.