80 Sources and Composition of Marine Sediments
3.5.2 Sand is typical of beach and shelf deposits (Fig. 3.6) Like gravel, it may
consists of rock fragments, a striking example being provided by black beach sands
on Hawaii, which are made of volcanic rock. Commonly, however, sand consists of
fragments of the minerals quartz, feldspar, mica, and others. Except for quartz, these
are compounds of the type [Na, K, Mg, Ca]a [Si, AI]b Dc, that is, alumino-silicates.
Quartz (Si02) is the mineral most resistant to abrasion and to leaching, and sands
which have been extensively reworked are therefore greatly enriched in quartz (mature sands). The first minerals to be destroyed during weathering, transport, and
reworking are the iron-rich minerals. Feldpar also is not very resistant to chemical
destruction.
In many tropical beaches, sand may consist entirely of fragments of calcareous
skeletons, of mollusks, corals, and algae. One might argue whether a deposit made up
of such "bioclastic" material should be counted as "lithogenous", as derived from
pre-existing rocks or as "biogenous". Strictly speaking, there is a cycle of mechanical
breakup and abrasion involved (hence lithogenous), but for geochemical balance
calculations we might include this material with chemical deposits. Carbonate particles are easily abraded and chemically eroded. Thus, in a mixture of carbonate and
quart sand, the quartz will soon dominate when the sand is reworked.
The source areas and dispersal history of sands can also be explored by noting the
compositional types of "heavy minerals" (densities greater than 2.8 glcm 3 ; examples:
the silicates hornblende, pyroxene, olivine, and also magnetite, ilmenite, rutile) (Appendix A4). The heavy mineral association allows the mapping of depositional provinces, which, in tum, provides clues to the action of shelf currents and other factors
(Fig. 3.7).
The shape of sand grains, especially quartz grains, can give clues to their origin.
For example, grains in glacial deposits have sharp edges, whereas reworking by
waves leads to rounding. Dunes especially collect well-rounded grains, and such
grains may be "frosted". One problem with a straightforward application of these
concept is that quartz grains are so resistant that they can be recycled many times by
erosion and redeposition of old sedimentary deposits (polycyclic sands). Another
problem is that etching of grains can take place after deposition, within the sediment,
obliterating and confusing the surface markings.
3.5.3 Silt-Sized sediment is very characteristic of continental slope and rise, but may
occur at any place on the shelf where conditions are quiet, so that it is not washed out
by wave or current action. The composition of the silt is much like that of sand in the
coarser end of the range, and like that of clay in the finder end. Mica is especially
abundant in terrigenous silt.
Sand is commonly studied with a binocular microscope, and the composition of
clay is investigated by X-ray diffraction. The study of silt traditionally fell into the
crack between these two methods and has had a rather low popularity rating. More
recently, the scanning electron microscope has made it possible to investigate this
size fraction in more detail (Fig. 3.8). The composition of the silts is usually closely
related to that of the associated fine sand fractions.
3.5.2 Sand is typical of beach and shelf deposits (Fig. 3.6) Like gravel, it may
consists of rock fragments, a striking example being provided by black beach sands
on Hawaii, which are made of volcanic rock. Commonly, however, sand consists of
fragments of the minerals quartz, feldspar, mica, and others. Except for quartz, these
are compounds of the type [Na, K, Mg, Ca]a [Si, AI]b Dc, that is, alumino-silicates.
Quartz (Si02) is the mineral most resistant to abrasion and to leaching, and sands
which have been extensively reworked are therefore greatly enriched in quartz (mature sands). The first minerals to be destroyed during weathering, transport, and
reworking are the iron-rich minerals. Feldpar also is not very resistant to chemical
destruction.
In many tropical beaches, sand may consist entirely of fragments of calcareous
skeletons, of mollusks, corals, and algae. One might argue whether a deposit made up
of such "bioclastic" material should be counted as "lithogenous", as derived from
pre-existing rocks or as "biogenous". Strictly speaking, there is a cycle of mechanical
breakup and abrasion involved (hence lithogenous), but for geochemical balance
calculations we might include this material with chemical deposits. Carbonate particles are easily abraded and chemically eroded. Thus, in a mixture of carbonate and
quart sand, the quartz will soon dominate when the sand is reworked.
The source areas and dispersal history of sands can also be explored by noting the
compositional types of "heavy minerals" (densities greater than 2.8 glcm 3 ; examples:
the silicates hornblende, pyroxene, olivine, and also magnetite, ilmenite, rutile) (Appendix A4). The heavy mineral association allows the mapping of depositional provinces, which, in tum, provides clues to the action of shelf currents and other factors
(Fig. 3.7).
The shape of sand grains, especially quartz grains, can give clues to their origin.
For example, grains in glacial deposits have sharp edges, whereas reworking by
waves leads to rounding. Dunes especially collect well-rounded grains, and such
grains may be "frosted". One problem with a straightforward application of these
concept is that quartz grains are so resistant that they can be recycled many times by
erosion and redeposition of old sedimentary deposits (polycyclic sands). Another
problem is that etching of grains can take place after deposition, within the sediment,
obliterating and confusing the surface markings.
3.5.3 Silt-Sized sediment is very characteristic of continental slope and rise, but may
occur at any place on the shelf where conditions are quiet, so that it is not washed out
by wave or current action. The composition of the silt is much like that of sand in the
coarser end of the range, and like that of clay in the finder end. Mica is especially
abundant in terrigenous silt.
Sand is commonly studied with a binocular microscope, and the composition of
clay is investigated by X-ray diffraction. The study of silt traditionally fell into the
crack between these two methods and has had a rather low popularity rating. More
recently, the scanning electron microscope has made it possible to investigate this
size fraction in more detail (Fig. 3.8). The composition of the silts is usually closely
related to that of the associated fine sand fractions.