9.7 CHERTS
457
CaSO4 + hydrocarbons --+ H2S + Ca 2+ + C032+ -k- H20 etc.
CaCO 3
This reaction is known to be promoted by sulfate-reducing bacteria, but there is reason to believe that it can also take place in a sterile environment, in the absence of
bacteria (Dunsmore, 1973; Eugster, 1985, 1986; Sawlowicz, 1986). In the latter case this
would be an exothermic reaction which could generate the high-temperature crystalline fabric seen in the ores and their gangue; hence, the superficial resemblance of these
bodies to hydrothermal veins. The implications of this possibility are discussed in Section 6.3.2.8.3.
In conclusion, the evaporites are a very important group of rocks. Their origin is still
a matter for debate, centering on the degree to which they form by crystal growth on
the sediment/water interface of basins, and the extent to which they form by replacement of carbonate during the diagenesis of brine-soaked supratidal sabkhas.
Regardless of their precise genesis, evaporites are economically important minerals,
both in their own right and because of their close association with petroleum and sulfide ores.
9.7 CHERTS
Chert is a nonclastic sediment composed of microcrystalline quartz. It occurs in both
beds and nodules in limestones and sandstones (McBride, 1979). Cherts pose three main
problems: (1) What is the source of the silica? (2) What is the environment of deposition of bedded cherts? (3) How do nodular cherts replace other sediments?
Four minerals are associated with chert. Opal or opaline silica is amorphous and isotropic. Its formula is variously presented as SiO2"nH20 or SiO2.nSi(OH)4. Christobalite
is isotropic or felted and has the formula SiO 2 + SiO 4 tetrahedra. These minerals only
occur in post-Cretaceous cherts. Chalcedony is microcrystalline quartz that often shows
fibrous or radial habits. Finally, microquartz, with anhedral crystals 10-50 tzm in diameter, characterizes the pre-Eocene cherts. The results of the Deep Sea Drilling Program
have provided much information on the formation of chert (Davies and Supko, 1973;
Adachi et al., 1986). Cherts are found below depths of some 60 m beneath modern ocean
floors. They occur in calcareous and terrigenous clays, in volcanic sediments, and in
siliceous oozes. They are most common in the latter. The source for the silica is partially
biogenic, largely from the solution of radiolaria. There appear to be four phases of chert
formation. It begins with the infilling of foraminiferal molds by chalcedony. This is followed by the replacement of carbonate matrix by christobalite, and then by the replacement of foraminifera by chalcedony. Finally, all remaining porosity is infilled by quartz,
and the christobalite reverts to quartz also.
One of the best known ancient chert deposits is the Paleozoic Caballos Novaculite of
the Marathon basin, Texas. While there is general agreement that this formed as a primary chert there is debate as to its environment of deposition. McBride and Thomson
457
CaSO4 + hydrocarbons --+ H2S + Ca 2+ + C032+ -k- H20 etc.
CaCO 3
This reaction is known to be promoted by sulfate-reducing bacteria, but there is reason to believe that it can also take place in a sterile environment, in the absence of
bacteria (Dunsmore, 1973; Eugster, 1985, 1986; Sawlowicz, 1986). In the latter case this
would be an exothermic reaction which could generate the high-temperature crystalline fabric seen in the ores and their gangue; hence, the superficial resemblance of these
bodies to hydrothermal veins. The implications of this possibility are discussed in Section 6.3.2.8.3.
In conclusion, the evaporites are a very important group of rocks. Their origin is still
a matter for debate, centering on the degree to which they form by crystal growth on
the sediment/water interface of basins, and the extent to which they form by replacement of carbonate during the diagenesis of brine-soaked supratidal sabkhas.
Regardless of their precise genesis, evaporites are economically important minerals,
both in their own right and because of their close association with petroleum and sulfide ores.
9.7 CHERTS
Chert is a nonclastic sediment composed of microcrystalline quartz. It occurs in both
beds and nodules in limestones and sandstones (McBride, 1979). Cherts pose three main
problems: (1) What is the source of the silica? (2) What is the environment of deposition of bedded cherts? (3) How do nodular cherts replace other sediments?
Four minerals are associated with chert. Opal or opaline silica is amorphous and isotropic. Its formula is variously presented as SiO2"nH20 or SiO2.nSi(OH)4. Christobalite
is isotropic or felted and has the formula SiO 2 + SiO 4 tetrahedra. These minerals only
occur in post-Cretaceous cherts. Chalcedony is microcrystalline quartz that often shows
fibrous or radial habits. Finally, microquartz, with anhedral crystals 10-50 tzm in diameter, characterizes the pre-Eocene cherts. The results of the Deep Sea Drilling Program
have provided much information on the formation of chert (Davies and Supko, 1973;
Adachi et al., 1986). Cherts are found below depths of some 60 m beneath modern ocean
floors. They occur in calcareous and terrigenous clays, in volcanic sediments, and in
siliceous oozes. They are most common in the latter. The source for the silica is partially
biogenic, largely from the solution of radiolaria. There appear to be four phases of chert
formation. It begins with the infilling of foraminiferal molds by chalcedony. This is followed by the replacement of carbonate matrix by christobalite, and then by the replacement of foraminifera by chalcedony. Finally, all remaining porosity is infilled by quartz,
and the christobalite reverts to quartz also.
One of the best known ancient chert deposits is the Paleozoic Caballos Novaculite of
the Marathon basin, Texas. While there is general agreement that this formed as a primary chert there is debate as to its environment of deposition. McBride and Thomson