4.3 Modern Examples, Sedimentary History
able situation only existed during relatively short periods, as for example during the last 7000 years (see
also Sect. 10.3.3).
4.3.2 Gulf of California
General Setting
The Gulf of Califomia is a narrow, deep oceanic basin which originated from strike-slip movement and
some lateral extension. It resembles in its tectonic
style to some extent the Gulf of Aden (Sect. 4.3.3).
The spreading axes of the two basins exhibit several
offsets and thus generate a number of deep
subbasins. Both gulfs have wide, deep openings to
the world oceans, which allow the exchange of intermediate and bottom waters without the restraints of a
barrier. For this reason, the salinity in these basins is
normal marine.
The tectonic setting of the Gulf of Califomia has been described by Moore and Buffington (1968), Curray, Moore et
al. (l982a), Dauphin and Simoneit (1991), Ferrari (1995).
Oceanographic and sedimentological aspects are treated by
Rosen (1964), van Andel (1964), Calvert (1966), Einseie
and Niemitz (1982), Baumgartner et al. (1991), Thunnel et
al. (1994).
The Gulf of Califomia shows some special features,
which are summarized in the simplified model of Fig.
4.5. After a Miocene proto-gulf stage on continental
crnst, characterized by a shallow-marine basin, the
modem gulf began to grow 4 Ma B.P. Presently, the
gulf is in an early drifting stage in which new oceanic
crnst is generated in several specific spreading
troughs (pull-apart basins) separated by comparatively deep sills (Fig. 4.5, cf. Sect. 12.8). The long
gulf is surrounded by mountainous regions with climatic conditions ranging from arid in the north to
moderately humid in the south. The sea floor of the
basin exhibits a rngged, complicated topography,
which favors the entrapment of sediments within the
gulf.
Hydrography and Sediments
Because of its wide, deep opening, the tidal range is
high at its shallowing, narrowing northern end. In
spite of the high relief and several rivers entering the
basin, the hemipelagic deep-water sediments and
redeposited gravity mass flows (mud turbidites) are
predominantly fine-grained.
The most striking hydraulic feature of this basin is
the subsurface inflow of very large volumes of cold,
nutrient-rich, intermediate ocean water and the outflow of warm surface water. Seasonally changing
offshore winds cause upwelling of nutrient-rich wa175
ter masses along the eastem or western margin of the
gulf. These coastal upwelling zones generate plankton blooms, with diatoms being the most abundant
group with minor proportions of radiolaria, calcareous nannofossils and foraminifera. In this way, one
of the most productive ecosystems of the world is
established (Thunell et al. 1994). Most of the silica
necessary for diatom skeletons is derived from
inflowing ocean water and recycling within the basin; only about 15-25% of the biogenic silica produced in surface water is preserved in the sediments.
The remainder is dissolved within the uppermost 500
m of water. Partial mineralization of the plankton
leads to an oxygen minimum zone at intermediate
depths along the slopes. This is recorded by annual
varves, consisting mainly of diatorns, indicating seasonal high plankton production. In addition, finegrained detrital material, swept into the gulf by seasonal rains, contributes to the formation of varves.
A large proportion of the slope sediments is redeposited and accumulates as mud turbidites in the
deepest troughs ofthe subbasins. The mean sedimentation rate of the 4 Ma old modem gulf is about 50
mlMa (dry, solid material).
The organic matter preserved in the gulf sediments is derived mainly from diatomaceous protoplasm. Its concentration is highest (2-4% organic carbon) in the varved sediments accumulated below the oxygen minimum zones
along the basin slopes. Sediments in the deep basins mostly
contain 1-2% organic carbon, because here the conditions
fluctuate between anaerobic and slightly aerobic. Some of
the organic matter transported by mud turbidites into the
deep subbasins is land-derived. On the average, the sediments of the gulf contain about 2% of organic carbon and
are therefore potentially good sources of oil and gas (Sect.
14.1).
The exceptionally high primary productivity in the regions of upwelling is also indicated by the enrichment of
trace elements, such as P, S, Mo, Se, and Cd, in comparison with average shales (Brumsack 1986a). Calcareous
skeletons are partially dissolved and normally constitute
only a few percent of the sediment. During diagenesis, thin,
interbedded dolomites can form.
The sedimentation rate on the slopes below the zones of
upwelling is very high (about 400 to 600 mlMa of porous
sediment, corresponding to approximately 100 to ISO
mlMa of dry, solid rock). The long-term mean value for the
sedimentation rate (50 mJMa) does not represent the
present-day sedimentation rate, but is calculated from the
total sediment volume of the young gulf, taking into account the growth ofthe basin (cf. Sect. 11.3.6: extensional
basin filling). Biogenic silica has contributed about 15% of
the total sediment volume. Locally, the sedimentation rates
have varied from practically zero on bathymetric highs to
-1000 mlMa in some of the spreading troughs.
Despite high heat flow, the organic matter is thermally
immature (cf. Sect. 14.2), except for sediments affected by
intrusion of basaltic sills and hydrothermal processes in the
actively spreading oceanic subbasin centers. Here,
thermogenie hydrocarbons occur, and diatomaceous oozes
and terrigenous turbidites may be thermally altered to an
epidote-zoisite facies.
able situation only existed during relatively short periods, as for example during the last 7000 years (see
also Sect. 10.3.3).
4.3.2 Gulf of California
General Setting
The Gulf of Califomia is a narrow, deep oceanic basin which originated from strike-slip movement and
some lateral extension. It resembles in its tectonic
style to some extent the Gulf of Aden (Sect. 4.3.3).
The spreading axes of the two basins exhibit several
offsets and thus generate a number of deep
subbasins. Both gulfs have wide, deep openings to
the world oceans, which allow the exchange of intermediate and bottom waters without the restraints of a
barrier. For this reason, the salinity in these basins is
normal marine.
The tectonic setting of the Gulf of Califomia has been described by Moore and Buffington (1968), Curray, Moore et
al. (l982a), Dauphin and Simoneit (1991), Ferrari (1995).
Oceanographic and sedimentological aspects are treated by
Rosen (1964), van Andel (1964), Calvert (1966), Einseie
and Niemitz (1982), Baumgartner et al. (1991), Thunnel et
al. (1994).
The Gulf of Califomia shows some special features,
which are summarized in the simplified model of Fig.
4.5. After a Miocene proto-gulf stage on continental
crnst, characterized by a shallow-marine basin, the
modem gulf began to grow 4 Ma B.P. Presently, the
gulf is in an early drifting stage in which new oceanic
crnst is generated in several specific spreading
troughs (pull-apart basins) separated by comparatively deep sills (Fig. 4.5, cf. Sect. 12.8). The long
gulf is surrounded by mountainous regions with climatic conditions ranging from arid in the north to
moderately humid in the south. The sea floor of the
basin exhibits a rngged, complicated topography,
which favors the entrapment of sediments within the
gulf.
Hydrography and Sediments
Because of its wide, deep opening, the tidal range is
high at its shallowing, narrowing northern end. In
spite of the high relief and several rivers entering the
basin, the hemipelagic deep-water sediments and
redeposited gravity mass flows (mud turbidites) are
predominantly fine-grained.
The most striking hydraulic feature of this basin is
the subsurface inflow of very large volumes of cold,
nutrient-rich, intermediate ocean water and the outflow of warm surface water. Seasonally changing
offshore winds cause upwelling of nutrient-rich wa175
ter masses along the eastem or western margin of the
gulf. These coastal upwelling zones generate plankton blooms, with diatoms being the most abundant
group with minor proportions of radiolaria, calcareous nannofossils and foraminifera. In this way, one
of the most productive ecosystems of the world is
established (Thunell et al. 1994). Most of the silica
necessary for diatom skeletons is derived from
inflowing ocean water and recycling within the basin; only about 15-25% of the biogenic silica produced in surface water is preserved in the sediments.
The remainder is dissolved within the uppermost 500
m of water. Partial mineralization of the plankton
leads to an oxygen minimum zone at intermediate
depths along the slopes. This is recorded by annual
varves, consisting mainly of diatorns, indicating seasonal high plankton production. In addition, finegrained detrital material, swept into the gulf by seasonal rains, contributes to the formation of varves.
A large proportion of the slope sediments is redeposited and accumulates as mud turbidites in the
deepest troughs ofthe subbasins. The mean sedimentation rate of the 4 Ma old modem gulf is about 50
mlMa (dry, solid material).
The organic matter preserved in the gulf sediments is derived mainly from diatomaceous protoplasm. Its concentration is highest (2-4% organic carbon) in the varved sediments accumulated below the oxygen minimum zones
along the basin slopes. Sediments in the deep basins mostly
contain 1-2% organic carbon, because here the conditions
fluctuate between anaerobic and slightly aerobic. Some of
the organic matter transported by mud turbidites into the
deep subbasins is land-derived. On the average, the sediments of the gulf contain about 2% of organic carbon and
are therefore potentially good sources of oil and gas (Sect.
14.1).
The exceptionally high primary productivity in the regions of upwelling is also indicated by the enrichment of
trace elements, such as P, S, Mo, Se, and Cd, in comparison with average shales (Brumsack 1986a). Calcareous
skeletons are partially dissolved and normally constitute
only a few percent of the sediment. During diagenesis, thin,
interbedded dolomites can form.
The sedimentation rate on the slopes below the zones of
upwelling is very high (about 400 to 600 mlMa of porous
sediment, corresponding to approximately 100 to ISO
mlMa of dry, solid rock). The long-term mean value for the
sedimentation rate (50 mJMa) does not represent the
present-day sedimentation rate, but is calculated from the
total sediment volume of the young gulf, taking into account the growth ofthe basin (cf. Sect. 11.3.6: extensional
basin filling). Biogenic silica has contributed about 15% of
the total sediment volume. Locally, the sedimentation rates
have varied from practically zero on bathymetric highs to
-1000 mlMa in some of the spreading troughs.
Despite high heat flow, the organic matter is thermally
immature (cf. Sect. 14.2), except for sediments affected by
intrusion of basaltic sills and hydrothermal processes in the
actively spreading oceanic subbasin centers. Here,
thermogenie hydrocarbons occur, and diatomaceous oozes
and terrigenous turbidites may be thermally altered to an
epidote-zoisite facies.