Lithogenous Sediments
79
Box 3.1 Classification of Marine Sediment Types
Lithogenous Sediments. Detrital products of disintegration of pre-existing rocks (igneous, meta·
morphic, or sedimentary, see Appendix A6) and of volcanic ejecta (ash, pumice; ash is < 2 mm).
Transport by rivers, glaciers, winds. Redistribution through waves and currents. Nomenclature
based on grain size (gravel, sand, silt, clay). Additional qualifiers are derived from the lithologic
components (terrigenous, bioclastic, calcareous, volcanogenic, etc.), and from the structure and
color of the deposits.
Typical examples are as follows (environment in parentheses).
- Organic-rich clayey silt. with root fragments (marsh).
- Finely laminated sandy silt, with small shells (delta-top).
- Laminated quartzose sand, well-sorted (beach).
- Olive-green homogeneous mud rich in diatom debris (upper cont. slope) (mud is the same as
terrigenous clayey silt or silty clay).
Fine-grained lithogenous sediments (which become shale upon aging and hardening) are the most
abundant by volume of all marine sediments (about 70 %). This is largely due to the great
thickness of continental margin sediments.
Biogenous Sediments. Remains of organisms, mainly carbonate (calcite, aragonite), opal (hydrated silica), and calcium phosphate (teeth, bones, crustacean carapaces) (see Table 3.3). Organic sediments, while strictly speaking biogenous, are commonly treated separately. Arrival at
the site of deposition by in situ precipitation (benthic organisms living there), or through settling
via water column (pelagic organisms; coarse shells fall singly, small ones commonly arrive as
aggregates). Redeposition by waves or currents. Redissolution common, either on sea floor or
within sediment. Nomenclature is based on type of organism and also on chemical composition.
Additional qualifiers from structure, c%r, si~e. accessory matter. Typical examples:
Oyster bank (lagoon or embayment)
- Shell sand (tropical beach)
- Coral reef breccia (slope below coral reef) (breccia is coarse broken-up material, in this case
reefal debris)
- Oolite sand, well sorted (strand zone, Bahamas)
- Light gray calcareous ooze, bioturbated (deep-sea floor)
- Greenish gray siliceous ooze (deep-sea floor)
Biogenous sediments are widespread on the sea floor, covering about one half of the shelves and
more than one half of the deep ocean bottom, for a total of 55 %. About 30 % of the volume of
marine sediments being deposited at the present time may be labeled biogenous, although they
may have considerable lithogenous admixture.
Hydrogenous Sediments. Precipitates from seawater or from interstitial water. Also products of
alteration during early chemical reactions within freshly deposited sediment. Redissolution common. Nomenclature based on origin ("evaporites") and on chemical composition. Additional
qualifiers from structure, COlO1; accessories. Typical examples:
- Laminated translucent halite (salt flat).
- Finely bedded anhydrite (Mediterranean basin, subsurface).
- Nodular grayish white anhydrite (ditto).
- Manganese nodule, black, mammilated, 5 cm diam. (deep Pacific).
- Phosphatic concretion, irregular slab, 15 cm diam., 5 cm thick, light brown to greenish, granular (upwelling area).
Hydrogenous sediments, while widespread (Ferromanganese deposits and metal sulfides around
deep-sea vents). are not important by volume at present. At times in the past. when thick salt
deposits were laid down in the newly opening Atlantic, in the Mesozoic, and much later in a
dried-up Mediterranean (end of the Miocene) the volume of hydrogenous sediments produced
was considerable. The salinity of the ocean may have been appreciably lowered during those
times of evaporite deposition.
79
Box 3.1 Classification of Marine Sediment Types
Lithogenous Sediments. Detrital products of disintegration of pre-existing rocks (igneous, meta·
morphic, or sedimentary, see Appendix A6) and of volcanic ejecta (ash, pumice; ash is < 2 mm).
Transport by rivers, glaciers, winds. Redistribution through waves and currents. Nomenclature
based on grain size (gravel, sand, silt, clay). Additional qualifiers are derived from the lithologic
components (terrigenous, bioclastic, calcareous, volcanogenic, etc.), and from the structure and
color of the deposits.
Typical examples are as follows (environment in parentheses).
- Organic-rich clayey silt. with root fragments (marsh).
- Finely laminated sandy silt, with small shells (delta-top).
- Laminated quartzose sand, well-sorted (beach).
- Olive-green homogeneous mud rich in diatom debris (upper cont. slope) (mud is the same as
terrigenous clayey silt or silty clay).
Fine-grained lithogenous sediments (which become shale upon aging and hardening) are the most
abundant by volume of all marine sediments (about 70 %). This is largely due to the great
thickness of continental margin sediments.
Biogenous Sediments. Remains of organisms, mainly carbonate (calcite, aragonite), opal (hydrated silica), and calcium phosphate (teeth, bones, crustacean carapaces) (see Table 3.3). Organic sediments, while strictly speaking biogenous, are commonly treated separately. Arrival at
the site of deposition by in situ precipitation (benthic organisms living there), or through settling
via water column (pelagic organisms; coarse shells fall singly, small ones commonly arrive as
aggregates). Redeposition by waves or currents. Redissolution common, either on sea floor or
within sediment. Nomenclature is based on type of organism and also on chemical composition.
Additional qualifiers from structure, c%r, si~e. accessory matter. Typical examples:
Oyster bank (lagoon or embayment)
- Shell sand (tropical beach)
- Coral reef breccia (slope below coral reef) (breccia is coarse broken-up material, in this case
reefal debris)
- Oolite sand, well sorted (strand zone, Bahamas)
- Light gray calcareous ooze, bioturbated (deep-sea floor)
- Greenish gray siliceous ooze (deep-sea floor)
Biogenous sediments are widespread on the sea floor, covering about one half of the shelves and
more than one half of the deep ocean bottom, for a total of 55 %. About 30 % of the volume of
marine sediments being deposited at the present time may be labeled biogenous, although they
may have considerable lithogenous admixture.
Hydrogenous Sediments. Precipitates from seawater or from interstitial water. Also products of
alteration during early chemical reactions within freshly deposited sediment. Redissolution common. Nomenclature based on origin ("evaporites") and on chemical composition. Additional
qualifiers from structure, COlO1; accessories. Typical examples:
- Laminated translucent halite (salt flat).
- Finely bedded anhydrite (Mediterranean basin, subsurface).
- Nodular grayish white anhydrite (ditto).
- Manganese nodule, black, mammilated, 5 cm diam. (deep Pacific).
- Phosphatic concretion, irregular slab, 15 cm diam., 5 cm thick, light brown to greenish, granular (upwelling area).
Hydrogenous sediments, while widespread (Ferromanganese deposits and metal sulfides around
deep-sea vents). are not important by volume at present. At times in the past. when thick salt
deposits were laid down in the newly opening Atlantic, in the Mesozoic, and much later in a
dried-up Mediterranean (end of the Miocene) the volume of hydrogenous sediments produced
was considerable. The salinity of the ocean may have been appreciably lowered during those
times of evaporite deposition.