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6 DEPOSITIONAL SYSTEMS
carbonate and siliceous oozes and to red manganiferous clays. Though the ancient facies can be interpreted as pelagic, their actual depositional environment is a matter for
debate.
Another important ancient type of pelagic deposit that occurs throughout the stratigraphic record from the Pre-Cambrian to Recent is black shales (Wignall, 1994). These
contain pelagic micro- and macrofossils. Benthonic body fossils and bioturbation are
generally absent. The black color is in part due to the presence of organic carbon, but
largely due to abundant disseminated pyrites (FeS2). Noted black shales include the
Lower Paleozoic Alum shales of the Baltic, the Liassic and Kimmeridgian shales of
northwest Europe, the Cretaceous shales of the Atlantic Ocean margin basins, and the
Devonian shales of North America. The fauna and lithology of these black shales clearly
indicate deposition in anoxic conditions. These may occur in various water depths and
environments that include lakes, lagoons, continental shelves, and deep oceans (Tyson
and Pearson, 1991). Research has particularly concentrated on establishing the global
and stratigraphic distribution of black shales. These data have been integrated with
the movements of tectonic plates and computer-generated paleoclimatic models (Huc,
1995).
6.3.2.10.3 Economic aspects of pelagic deposits
Pelagic deposits are of considerable economic interest as petroleum source beds, and
because they often contain phosphates, barytes, and a variety of metallic ores. Phosphate deposits will not be described here because they are dealt with elsewhere (see
Section 9.5). When discussing shelf deposits it was mentioned that modern ocean beds
are not anoxic, but that there is an anoxic zone between some 180 and 220 m that corresponds to the modern continental shelf margin. Modern deep oceans are moderately
oxygenated because of a constant downward flow of oxygenated cold polar waters. Thus
one would not normally expect ancient pelagic deposits to be organic-rich potential
petroleum source beds. There appear to be two ways in which pelagic environments
generate petroleum source beds (Katz, 1995). Some geologists have noted that source
rock formation was a worldwide event in late Jurassic and early Cretaceous times. They
have correlated this with an equable global climate. With no polar ice caps there would
be no cold water descending into the deep ocean basins to keep them oxygenated.
Anoxic conditions might thus favor source rock formation (Schlanger and Cita, 1982;
Stein, 1986). This idea has not received universal support. In particular it is noted that
most Upper Jurassic and Lower Cretaceous source beds occur around the modern Atlantic Ocean. They may therefore reflect restricted oceanic circulation as the American
plates separated from Europe and Africa (see Section 10.2.4).
There is another way in which deep-water organic-rich source beds may form. In the
Miocene the Mediterranean dried out to deposit basin-wide evaporites. This is termed
the Messinian Salinity Crisis. Today organic-rich muds are found in deep parts of the
Mediterranean, such as the Strabo trench, and the Bacino and Bannock basins. These
occur because of the presence of anoxic brines at depths greater than about 30003200 m. Messinian evaporites crop out on the adjacent sea floor, and are leached out by
the seawater. The resultant dense brines flow down into the basinal lows, where there
is insufficient current activity to rework them with waters of normal salinity. Organic
matter is thus preserved in these troughs, like pickled herring in a barrel.
6 DEPOSITIONAL SYSTEMS
carbonate and siliceous oozes and to red manganiferous clays. Though the ancient facies can be interpreted as pelagic, their actual depositional environment is a matter for
debate.
Another important ancient type of pelagic deposit that occurs throughout the stratigraphic record from the Pre-Cambrian to Recent is black shales (Wignall, 1994). These
contain pelagic micro- and macrofossils. Benthonic body fossils and bioturbation are
generally absent. The black color is in part due to the presence of organic carbon, but
largely due to abundant disseminated pyrites (FeS2). Noted black shales include the
Lower Paleozoic Alum shales of the Baltic, the Liassic and Kimmeridgian shales of
northwest Europe, the Cretaceous shales of the Atlantic Ocean margin basins, and the
Devonian shales of North America. The fauna and lithology of these black shales clearly
indicate deposition in anoxic conditions. These may occur in various water depths and
environments that include lakes, lagoons, continental shelves, and deep oceans (Tyson
and Pearson, 1991). Research has particularly concentrated on establishing the global
and stratigraphic distribution of black shales. These data have been integrated with
the movements of tectonic plates and computer-generated paleoclimatic models (Huc,
1995).
6.3.2.10.3 Economic aspects of pelagic deposits
Pelagic deposits are of considerable economic interest as petroleum source beds, and
because they often contain phosphates, barytes, and a variety of metallic ores. Phosphate deposits will not be described here because they are dealt with elsewhere (see
Section 9.5). When discussing shelf deposits it was mentioned that modern ocean beds
are not anoxic, but that there is an anoxic zone between some 180 and 220 m that corresponds to the modern continental shelf margin. Modern deep oceans are moderately
oxygenated because of a constant downward flow of oxygenated cold polar waters. Thus
one would not normally expect ancient pelagic deposits to be organic-rich potential
petroleum source beds. There appear to be two ways in which pelagic environments
generate petroleum source beds (Katz, 1995). Some geologists have noted that source
rock formation was a worldwide event in late Jurassic and early Cretaceous times. They
have correlated this with an equable global climate. With no polar ice caps there would
be no cold water descending into the deep ocean basins to keep them oxygenated.
Anoxic conditions might thus favor source rock formation (Schlanger and Cita, 1982;
Stein, 1986). This idea has not received universal support. In particular it is noted that
most Upper Jurassic and Lower Cretaceous source beds occur around the modern Atlantic Ocean. They may therefore reflect restricted oceanic circulation as the American
plates separated from Europe and Africa (see Section 10.2.4).
There is another way in which deep-water organic-rich source beds may form. In the
Miocene the Mediterranean dried out to deposit basin-wide evaporites. This is termed
the Messinian Salinity Crisis. Today organic-rich muds are found in deep parts of the
Mediterranean, such as the Strabo trench, and the Bacino and Bannock basins. These
occur because of the presence of anoxic brines at depths greater than about 30003200 m. Messinian evaporites crop out on the adjacent sea floor, and are leached out by
the seawater. The resultant dense brines flow down into the basinal lows, where there
is insufficient current activity to rework them with waters of normal salinity. Organic
matter is thus preserved in these troughs, like pickled herring in a barrel.