5.3 Hemipelagic and Pelagic Sediments
Relative sea-Ievel changes favor such a process.
Condensed sections on shelves and upper slopes,
produced during sea-Ievel rise (cf. Sect. 7.2) can bec dtagenehc phosphogenesis. The subsequent sea-Ievel
fall then causes partial or total reworking of the condensed section leaving behind lag sediments rich in
phosphorite. In consequence, the net sedimentation
rate over a considerable time period is very slow,
approaches zero, or becomes even negative.
This m~ans that .th~ accumulation rate of P, forming a
phophonte deposIt, IS not necessarily greater than that of
normal continental margin sediments deposited at relatively
high rates but with low P contents (Filippelli 1997).
For the same reasons, the phosphorite of an individual
deposit may show a considerable age range, as identified
with the aid of inc1uded fossils or radiometrie age determinations (e.g., von Rad and Kudrass 1984; Thomson et al.
1984; McArthur et al. 1988).
With rising or falling sea level the zone of phosphorite formation can migrate upslope or downslope and
thus create a broader deposit along a shelf-slope or
bank-ridge setting (Figs. 5Ag, 5.7a, and 5.9a and b).
Generally, weathering processes in emerged formations can further enhance the concentration of marine phosphorites, e.g. by preferential dissolution of
the accompanying carbonate (e.g. Valeton 1988).
Co-Existing Authigenic Mineral Phases. The weakly
reducing environment enabling phosphogenesis to
occur, particularly in combination with repeated winnowing and reworking, is also favorable for the formation and relative enrichment of glauconitic minerals (Sect. 6.1). Glauconite is therefore frequently
associated with phosphorite deposits and may occasionally occur in large quantities as, for example, in
the upper Cretaceous of Egypt (Glenn and Arthur
1990). Pyrite mayaiso be present in phosphorites,
but is frequently precipitated subsequent to
phosphogenesis at somewhat greater burial depths,
when sulfate reduction has already started (Fig. 5.7b
and c). Carbonate nodules, calcite cement, and silicification phenomena present in phosphorite horizons
are assumed to be a result of later diagenesis.
Ancient phosphorites. During certain time periods in
the Earth's history, phosphorite seems to have formed
preferentially, for example in the late Proterozoic and
Cambrian, Permian, late Cretaceous/early Tertiary,
a~d Miocene (e.g. Valeton 1988). However, the princlpal processes of phosphogenesis probably did not
differ from the present situation (Trappe 1994).
Rather, a fundamentally different paleogeographic
configuration of the oceans and continents may have
allowed the generation of "phosphorite giants".
Prominent examples are the existence of Pangea in
the Permian and Triassic (Herring 1995), or the
205
circum-equatorial N eo-Tethys ocean in the U pper
Cretaceous. In addition, wide shelf seas, shifts of the
clirrtatic belts, and sea-Ievel changes may have favored deposition and enrichment of phosphorite in
specific regions.
The widespread and economically important Late
Cretaceous-Eocene phosphorite province of the Tethyan
equatorial belt, inc1uding Jordan and several other Middle
East countries, formed on a wide shelf or epicontinental sea
at the northern margin of the North African-Arabian continent (e.g. Abed and Kraishan 1991; Abed and Fakhouri
1996). The sea floor was characterized by lows and swells
and affected by relative sea-level changes. The abundant
phosphorites mainly occur in the inner part of this wide
carbonate shelf. They represent the top of shallowing carbonate/chert cyc1es and are associated with limestones,
dolomites, and chert. The granular phosphorite is reworked
and rests in places on erosional surfaces, locally even on
calcrete or si1crete. Reworked chert indicates that a considerable time span must have elapsed between the deposition
of biosiliceous ooze, its lithification, and reworking. Furthermore, thick oyster beds and their pore space can be
phosphatized, or granular phosphorite is in places silicified.
These observations point to a relatively long and complex
depositional, erosional, and diagenetic history of these
phophorites, in which sea-level changes and periods of
emergence playa significant part. It is, however, not c1ear
. in this case, whether upwelling waters of the Tethys ocean
or river-borne iron compounds delivered the large amounts
of phosphvrus necessary for this huge phosphorite province.
Summary (Phosphorites)
Phosphorite preferentially forms on continental
shelves and shelf-breaks, and atop submarine
highs (e.g. seamounts and drowned carbonate
buildups).
Sources of P are organic matter, bones, and
microbial mats. In some cases, rivers may have
supplied mud rich in P adsorbed to iron hydroxides.
Zones of coastal upwelling and condensed sections are favorable for phosphogenesis.
Reworking of early diagenetic phosphorite
leads to lags rich in granular phosphorite
which are frequently associated with chert,
dolomite, and glauconite.
Relative sea-Ievel changes favor such a process.
Condensed sections on shelves and upper slopes,
produced during sea-Ievel rise (cf. Sect. 7.2) can bec dtagenehc phosphogenesis. The subsequent sea-Ievel
fall then causes partial or total reworking of the condensed section leaving behind lag sediments rich in
phosphorite. In consequence, the net sedimentation
rate over a considerable time period is very slow,
approaches zero, or becomes even negative.
This m~ans that .th~ accumulation rate of P, forming a
phophonte deposIt, IS not necessarily greater than that of
normal continental margin sediments deposited at relatively
high rates but with low P contents (Filippelli 1997).
For the same reasons, the phosphorite of an individual
deposit may show a considerable age range, as identified
with the aid of inc1uded fossils or radiometrie age determinations (e.g., von Rad and Kudrass 1984; Thomson et al.
1984; McArthur et al. 1988).
With rising or falling sea level the zone of phosphorite formation can migrate upslope or downslope and
thus create a broader deposit along a shelf-slope or
bank-ridge setting (Figs. 5Ag, 5.7a, and 5.9a and b).
Generally, weathering processes in emerged formations can further enhance the concentration of marine phosphorites, e.g. by preferential dissolution of
the accompanying carbonate (e.g. Valeton 1988).
Co-Existing Authigenic Mineral Phases. The weakly
reducing environment enabling phosphogenesis to
occur, particularly in combination with repeated winnowing and reworking, is also favorable for the formation and relative enrichment of glauconitic minerals (Sect. 6.1). Glauconite is therefore frequently
associated with phosphorite deposits and may occasionally occur in large quantities as, for example, in
the upper Cretaceous of Egypt (Glenn and Arthur
1990). Pyrite mayaiso be present in phosphorites,
but is frequently precipitated subsequent to
phosphogenesis at somewhat greater burial depths,
when sulfate reduction has already started (Fig. 5.7b
and c). Carbonate nodules, calcite cement, and silicification phenomena present in phosphorite horizons
are assumed to be a result of later diagenesis.
Ancient phosphorites. During certain time periods in
the Earth's history, phosphorite seems to have formed
preferentially, for example in the late Proterozoic and
Cambrian, Permian, late Cretaceous/early Tertiary,
a~d Miocene (e.g. Valeton 1988). However, the princlpal processes of phosphogenesis probably did not
differ from the present situation (Trappe 1994).
Rather, a fundamentally different paleogeographic
configuration of the oceans and continents may have
allowed the generation of "phosphorite giants".
Prominent examples are the existence of Pangea in
the Permian and Triassic (Herring 1995), or the
205
circum-equatorial N eo-Tethys ocean in the U pper
Cretaceous. In addition, wide shelf seas, shifts of the
clirrtatic belts, and sea-Ievel changes may have favored deposition and enrichment of phosphorite in
specific regions.
The widespread and economically important Late
Cretaceous-Eocene phosphorite province of the Tethyan
equatorial belt, inc1uding Jordan and several other Middle
East countries, formed on a wide shelf or epicontinental sea
at the northern margin of the North African-Arabian continent (e.g. Abed and Kraishan 1991; Abed and Fakhouri
1996). The sea floor was characterized by lows and swells
and affected by relative sea-level changes. The abundant
phosphorites mainly occur in the inner part of this wide
carbonate shelf. They represent the top of shallowing carbonate/chert cyc1es and are associated with limestones,
dolomites, and chert. The granular phosphorite is reworked
and rests in places on erosional surfaces, locally even on
calcrete or si1crete. Reworked chert indicates that a considerable time span must have elapsed between the deposition
of biosiliceous ooze, its lithification, and reworking. Furthermore, thick oyster beds and their pore space can be
phosphatized, or granular phosphorite is in places silicified.
These observations point to a relatively long and complex
depositional, erosional, and diagenetic history of these
phophorites, in which sea-level changes and periods of
emergence playa significant part. It is, however, not c1ear
. in this case, whether upwelling waters of the Tethys ocean
or river-borne iron compounds delivered the large amounts
of phosphvrus necessary for this huge phosphorite province.
Summary (Phosphorites)
Phosphorite preferentially forms on continental
shelves and shelf-breaks, and atop submarine
highs (e.g. seamounts and drowned carbonate
buildups).
Sources of P are organic matter, bones, and
microbial mats. In some cases, rivers may have
supplied mud rich in P adsorbed to iron hydroxides.
Zones of coastal upwelling and condensed sections are favorable for phosphogenesis.
Reworking of early diagenetic phosphorite
leads to lags rich in granular phosphorite
which are frequently associated with chert,
dolomite, and glauconite.