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Other Biogenic Sedimentary
Rocks
Chapter 13
Chapter 13- Other Biogenic Sedimentary Rocks
• In this chapter we will discuss other
biogenic deposits that are not carbonate:
– Siliceous sediments and chert
– Phosphates sediment and phosphatic rock
– Organic-rich materials such as coal and oil,
which are concentrated in sediments and
sedimentary rocks.
Chert and Siliceous sediment
• Chert is a fine-grained, hard sedimentary
rock composed of cryptocrystalline fibrous
chalcedony, lesser amounts of
microcrystalline and cryptocrystalline
quartz, and amorphous silica.
• It is the product of organic and inorganic
precipitation.
Two major types of cherts are
found in the geologic record:
• Bedded (primary) cherts and;
• nodular (replacement) cherts.
Each exhibits a distinctive suite of physical
characteristics, has its own mode of origin,
and occurs within a distinctive lithological
and tectonic setting.
Bedded cherts
• Most bedded chert is produced when silica-rich
organic oozes deposited on the seafloor are
recrystallized.
• Bedded cherts occur as individual bands, layers, or
laminae that range in thickness from a few
millimeters up to several meters.
• The thicknesses of individual chert layers are
often laterally and vertically uniform (ribbon
cherts), but they can vary.
Ribbon chert
Red ribbon cherts from Marin
County, CA.
Red radiolarites from the Gondran cirque, Chenaillet ophiolite near Briançon,
France.
• Internal sedimentary structures are rare.
• Many bedded cherts occurs as part of
assemblages of oceanic crustal rocks and
associated deep-sea sediments (ophiolite
sequences).
– Ex. Bermeja Complex, southwestern Puerto
Rico
Two categories of bedded chert are recognized
based on the presence or absence of fossils.
• Bedded fossiliferous cherts contain more or less
obvious remains of such siliceous organism as
diatoms, radiolarians, and sponge spicules. They
form when siliceous oozes recrystallize.
• Nonfossiliferous bedded cherts contain no visible
skeleton remains. Most are siliceous oozes so
extensively altered that all fossils have been
destroyed. When these nonfossiliferous bedded
cherts are etched some fossils are seen so few if
any bedded cherts are truly inorganic in origin.
The image shows the
siliceous skeletons of
diatoms (tiny planktonic
organisms) preserved in
lake sediment, Lake
Toskaljarvi, Finland.
Radiolarians
Radiolarians are single-celled protistan marine organisms that distinguish themselves with their
unique and intricately detailed glass-like exoskeletons. During their life cycle, radiolarians
absorb silicon compounds from their aquatic environment and secrete well-defined geometric
networks that comprise a skeleton commonly known as a test. The radiolarian tests are
produced in a wide variety of patterns, but most consist of an organized array of spines and
holes (pores) that regulate a network of pseudopods useful in gathering food. When observed
with an optical microscope, radiolarian tests are found to be low contrast light-scattering objects
that are best viewed using Rheinberg illumination, darkfield illumination, phase contrast, or
differential interference contrast (DIC) microscopy techniques. The diversity and beauty of
radiolarian tests was first captured and revealed in 1862 by Ernst Haeckel's monograph, Die
Radiolarien (Rhizopoda Radiaria), based on specimens gathered from the ocean by the
Challenger research cruises of Alexander von Humboldt. The work features 35 exquisite copper
plates illustrating hand-drawn radiolarians that still have not been surpassed in quality by
modern optical and electron microscopy techniques.
Sponges structure is based on microscopic interlocking spicules which are
built up from cell secretions. Depending on the species, these may take the
form of pointed hard calcareous hooks, or flexible nets. The spicules of
some tropical species can cause severe skin reactions if the sponge is
touched or handled.
Silica Geochemistry
• Most of the silica dissolved in seawater is derived
from decomposing rock, but very little of it is
produced by the solution of quartz because quartz
is almost insoluble in natural waters with pH
values of less than 11.
• Most of the dissolved silica is amorphous silica.
This silica is the product of chemical weathering,
especially hydrolysis, that occurs as feldspars is
converted to clay minerals.
• Other sources are submarine weathering of basalts
and submarine geysers that pump additional silicarich fluid.
• Diatoms, silicoflagellates, and radiolarians use up
nearly all the silica as soon as it is available.
• Striking variations in the amount of silica in
seawater clearly reflect this organic activity.
• Very low values of less than 0.01 ppm in surface
waters coincide with the photic zone and increases
progressively to a maximum of 11 ppm where
water depth exceeds 2 km.
• Microplankton extract dissolved silica to construct
minuscule endoskeletons of opaline silica.
What destroys the skeletons
• The skeletons of living silica-secreting organism
are covered with a carapace of organic matter that
keeps them from dissolving.
• Solutions begins immediately after death, as the
skeletal remains fall slowly to the abyssal ocean
floor and continues after burial. Most of the silica
that is dissolve is immediately reprecipitated
leaving no trace of the fossil.
Nodular Cherts
• Nodular cherts, also called secondary or
replacement cherts, occurs as first-shaped,
spherical, sub spherical, and ovoidal masses of
opal, chalcedony, and quartz disseminated mainly
in shallow-water limestone and dolostone.
• Nodules vary in size from a few millimeters to a
few centimeters.
• Individuals nodules are often linked together,
forming roughly planar bands that create
anastomosing networks and lenses of chert.
• Fossil remains of such silica-secreting organism as
sponges occur along with the silicified remains of
organism that secrete calcareous shells.
Nodular cherts are clearly of
diagentic origin
• They form when silica dissolves, migrates,
and reprecipitates elsewhere, replacing
older material.
Phosphorites
• Almost all sedimentary rocks contain minor
amounts of phosphate.
• For example, mudrocks and limestone
typically contain a fraction of a percent of
P2O5.
• Sedimentary phosphate deposits
(phosphorites), on the other hands are very
rare.
Phosphorites
• They are characterized by an abnormally high
concentration of P2O5 (20% or more).
• Phosphates in sedimentary rocks occurs as a
variety of minerals, but fluorapatite, Ca5(PO4) 3F,
is the principal species.
• Hydroxyapatite, Ca5(PO4) 3OH, is the principal
mineral in vertebrate bones and teeth.
• In most sedimentary rocks the minor phosphates
typically occurs as detrital clasts of the mineral
apatite, as organic fecal matter, or as transported
bone fragments.
Sources of phosphates in
phosphorites
• Exceptional circumstances are required to form
the few phosphorites known.
• Phosphate is derived either directly from
hydrothermal veins or by chemical decomposition
of such phosphate minerals as fluorapatite in
igneous and metamorphic rocks.
• Phosphates is essential for organisms because it is
an integral component or RNA and DNA.
Principal types of phosphorite
deposits
• 1- Concentration of nodular phosphorite lie
scattered on the floor of some modern outer
continental shelves. Ancient analog occur in
Morocco and North America.
• 2- Placer concentrations of transported and
reworked organic clasts (bone beds) occur rarely.
• 3- diagentically produced by the process of
phosphatization. Phosphate-rich fluids leached
from guano are concentrated and reprecipitated in
limestone.
Origin of modern deposits
• Zones of upwelling results in high productivity.
• Cold, nutrient-rich ocean water wells up onto
warmer shallow water in shelf areas.
• The abrupt influx of nutrients leads to extremely
high organic productivity at the base of the food
chain. Burst of phytoplankton activity propagate
up the food chain to produce large volumes of
organic material (waste and remains).
Origin of modern deposits
• Bone beds are placer concentrations of organic
skeleton from which other sediments has been
removed.
• Bones, coprolites, and other organic materials are
reworked, and the organic remains are
concentrated as current lag deposits. Dissolution
of some phosphate and its recrystallization as
cement produce solid phosphorite that replaces
mudrock and limestone.
• Flourishing communities of birds or bads confined
to islands and peninsulas or caves generate
meters-thick piles of layered guano.