Sedimentary rocks are composed of:
Clasts vs. Matrix
Gilbert’s Mineralogical Classification
Figure 5.5
The sedimentary blanket
Spatial distribution of rock types:
• The lithosphere (upper 16 km of crust) is ~90-95%
igneous and metamorphic rocks by volume.
• However, ~90% of the Earth’s surface is mantled in
sediments or sedimentary rocks with an average
thickness of 800-4000 m.
Sources of Sediment
•Rifted Margin
•Collision Orogen
•Imbricate Subduction Complex
Figure 5.13
Provenance of siliclastic rocks
Composition of the siliclastic clasts can provide some clue to the
provenance of a rock and the processes which have acted on it.
Fig 5.14
Relative proportions of sedimentary rocks based
on grain size
Erosion, weathering, and geochemical differentiation
of the craton
• The geochemical composition of the continental crust is very similar to that of
sediment and sedimentary rocks
• Riddle: How can such a small sedimentary volume shift continental geochemistry?
• Answer: Recycling of sediments over deep time!
Evidence for global sedimentary recycling (After
Prothero and Schwab):
•
Total amount of salt ions in the sea are 40-50x geochemical estimates of weathering rates.
•
•
•
Most of the sediment in the coterminous US is derived from sedimentary rocks, not igneous or
metamorphic rocks.
A large volume of the basement rocks in the coterminous US are metamorphosed sedimentary rocks.
Young average age of the sedimentary blanket relative to the age of the Earth.
Reading the rock record
• Identify sedimentary textures and structures generated by
physical and biological processes
• Integrate observations via the scientific method to understand
rock genesis and post-depositional history
Many siliclastic classification systems have been
proposed
Texture or Grain size
Formation process
Transport
Some grain size based classification schemes
A, B: Folk
Figure 5.4
C: Robinson
Classification of sedimentary rocks based on
formation process
Class
Lithogenic
Biogenic
Authigenic
Examples
sandstone, siltstone
limestone, chert
gypsum, anhydrite
MonocrystalineQuartz
Top:
Plane-polarized light
Bottom:
Cross Polarized Light
From: Atlas of sedimentary rocks under the microscope, Adams, MacKenzie, and Guilford, Wiley, 1984.
Feldspar
Top:
Plane-polarized light
Bottom:
Cross Polarized Light
From: Atlas of sedimentary rocks under the microscope, Adams, MacKenzie, and Guilford, Wiley, 1984.
Bioclastic Limestone
Plane-polarized light
From: Atlas of sedimentary rocks under the microscope, Adams, MacKenzie, and Guilford, Wiley, 1984.
Gypsum in Dolomite matrix
Top:
Plane-polarized light
Bottom:
Cross Polarized Light
From: Atlas of sedimentary rocks under the microscope, Adams, MacKenzie, and Guilford, Wiley, 1984.
Anhydrite
Top:
Plane-polarized light
Bottom:
Cross Polarized Light
From: Atlas of sedimentary rocks under the microscope, Adams, MacKenzie, and Guilford, Wiley, 1984.
Classification of sedimentary rocks based
transport (Folk, 1980)
Class
Terrigenous-
Definition
formed from rocks and mineral
transported into the depositional basin (eg. Heavy minerals, clays, transported limestones pebbles).
Allochemical-
formed from minerals chemically precipitated within the basin, but later retransported within the basin (eg. Oolites, rip up clasts).
Orthochemical-
formed from minerals chemically precipitated within the depositional basin in
their original location (eg. Micritic calcite, evaporites).
Processes that modify sedimentary rocks
•Eodiagenesis
–Transport, Bioturbation, Cementation, and replacement
•Mesodiagenesis
–Physical Compaction, Pressure solution, Cementation
–Dissolution, replacement, clay mineral authigenesis
•Teleodiagenesis
–Dissolution, replacement, oxidation
Textural Maturity
Fig. 5.6
Compaction
Increasing overburden leads to enhanced pressure, compaction and
loss of porosity
Fig. 5.17
Pressure Solution of Quartz crystals
Top:
Plane-polarized light
Bottom:
Cross Polarized Light
Clay minerals through time
What factors might explain temporal differences in clays
abundance through time?
Fig 5.12
Geothermal gradients
Burial leads to
heating
and alteration
Fig 5.15
Typical Sequence of
Alteration and Diagenesis
Fig 5.19