9.6 EVAPORITES
439
shelf, phosphate placers occur that derived from Eocene Moroccan and Pliocene Saharan phosphorites (Tooms et al., 1971).
Detrital phosphate gravels not only occur on marine shelves, but also as alluvial gravels. The "river pebble" deposits of South Carolina and Florida are a case in point.
Guano is a deposit rich in phosphates and nitrates formed from the excreta of sea
birds and bats (Plate 5C). Guano has been a significant source of these minerals notably
from the Chilean coasts and on Pacific islands such as Nauru, where solutions rich in
phosphates have percolated down from guano to replace reefal limestones.
These examples illustrate three characteristic features of phosphate minerals: their
genesis by replacement of carbonates, their ability to be eroded and reworked, and
their occurrence on continental shelves.
9.5.2 Mode of Formation of Phosphates
Seawater is generally nearly saturated with phosphate ions, ranging from 0 to 3 ppm
PO4 in deep cold water to about 0.01 ppm in warm surface water. The solubility of phosphate decreases with increasing temperature and increasing pH. These changes occur,
and phosphates thus tend to be precipitated, where deep cold oceanic water wells up into
shallower warmer waters. These conditions are fulfilled in several situations (Fleming,
1957). The most significant locus at the present time appears to be along the western
coasts of South America and Africa, where the cold currents of the Humbolt and Benguela move northward. Rich in nutrients, including phosphates, these waters generate
blooms of phytoplankton, which in turn support shoals of fish and flocks of sea birds.
Phosphate removed from the seawater by organisms returns again when they die, and
settles on the sea bed within miscellaneous organic matter. Phosphates become concentrated during early compaction of the mud. Constant agitation winnows out the lighter
material to leave denser incipient phosphate mud pellets. These continue to become
enriched with phosphate, as do bones, teeth, and shell debris. In this manner, bedded
phosphate rock--phosphorite- is formed (Fig. 9.28).
Eight factors favor phosphate formation: a broad shallow shelf, an adjacent major
ocean, a low latitude (<40~ high organic productivity, shallow marine sedimentation,
minimal terrigenous input, a marine transgression, and a suitable environmental trap,
such as a bay, estuary or carbonate bank (Brown, 1994).
Cognizant of these factors it is now possible to model episodes of phosphate formation throughout the earth's history, by integrating paleoclimate, paleooceanography,
and plate movement. This is done in conjunction with studies of total organic carbon
productivity and preservation, in an attempt to locate petroleum source beds (Parrish,
1995).
9.6 EVAPORITES
9.6.1 Introduction
The evaporites include the mineral salts such as anhydrite and halite. As their name implies it was once widely assumed that these rocks form by the evaporation of salt-rich
439
shelf, phosphate placers occur that derived from Eocene Moroccan and Pliocene Saharan phosphorites (Tooms et al., 1971).
Detrital phosphate gravels not only occur on marine shelves, but also as alluvial gravels. The "river pebble" deposits of South Carolina and Florida are a case in point.
Guano is a deposit rich in phosphates and nitrates formed from the excreta of sea
birds and bats (Plate 5C). Guano has been a significant source of these minerals notably
from the Chilean coasts and on Pacific islands such as Nauru, where solutions rich in
phosphates have percolated down from guano to replace reefal limestones.
These examples illustrate three characteristic features of phosphate minerals: their
genesis by replacement of carbonates, their ability to be eroded and reworked, and
their occurrence on continental shelves.
9.5.2 Mode of Formation of Phosphates
Seawater is generally nearly saturated with phosphate ions, ranging from 0 to 3 ppm
PO4 in deep cold water to about 0.01 ppm in warm surface water. The solubility of phosphate decreases with increasing temperature and increasing pH. These changes occur,
and phosphates thus tend to be precipitated, where deep cold oceanic water wells up into
shallower warmer waters. These conditions are fulfilled in several situations (Fleming,
1957). The most significant locus at the present time appears to be along the western
coasts of South America and Africa, where the cold currents of the Humbolt and Benguela move northward. Rich in nutrients, including phosphates, these waters generate
blooms of phytoplankton, which in turn support shoals of fish and flocks of sea birds.
Phosphate removed from the seawater by organisms returns again when they die, and
settles on the sea bed within miscellaneous organic matter. Phosphates become concentrated during early compaction of the mud. Constant agitation winnows out the lighter
material to leave denser incipient phosphate mud pellets. These continue to become
enriched with phosphate, as do bones, teeth, and shell debris. In this manner, bedded
phosphate rock--phosphorite- is formed (Fig. 9.28).
Eight factors favor phosphate formation: a broad shallow shelf, an adjacent major
ocean, a low latitude (<40~ high organic productivity, shallow marine sedimentation,
minimal terrigenous input, a marine transgression, and a suitable environmental trap,
such as a bay, estuary or carbonate bank (Brown, 1994).
Cognizant of these factors it is now possible to model episodes of phosphate formation throughout the earth's history, by integrating paleoclimate, paleooceanography,
and plate movement. This is done in conjunction with studies of total organic carbon
productivity and preservation, in an attempt to locate petroleum source beds (Parrish,
1995).
9.6 EVAPORITES
9.6.1 Introduction
The evaporites include the mineral salts such as anhydrite and halite. As their name implies it was once widely assumed that these rocks form by the evaporation of salt-rich