Sedimentology and sedimentary processes

Sedimentology and
Sedimentary Processes
Virginia T. McLemore


Sedimentology
• "The scientific study of sedimentary rocks and of
the processes by which they were formed; the
description, classification, origin and
interpretation of sediments" (Glossary of
Geology, AGI, 1974)
• Study of modern sediments such as sand, mud
(silt),and clay
• Understanding the processes that deposit them
• Studies of ancient sedimentary rocks


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Sedimentary rock types
• Clastic rocks
– particles derived from the weathering and erosion of
precursor rocks and consist primarily of fragmental
material
– classified by grain size and composition

• Carbonates

– precipitated by a variety of organic and inorganic
processes

• Evaporites
– evaporation of water at the Earth's surface

• Chemical sedimentary rocks (chert, jasperiod)


Principles
• Uniformitarianism, which states that the
sediments within ancient sedimentary rocks
were deposited in the same way as sediments
which are being deposited at the Earth's surface
today
• Principle of superposition Sedimentary layers
are deposited in a time sequence, with the
oldest on the bottom and the youngest on the
top.
• Principle of original horizontality sediments are
deposited at their angle of repose which, for
most types of sediment, is essentially horizontal


Principles
•

Principle of lateral continuity states that layers of sediment initially
extend laterally in all directions unless obstructed by a physical
object or topography

•

Principle of cross-cutting relationships states that whatever cuts
across or intrudes into the layers of strata is younger than the layers
of strata



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Methodology
•

Measuring and describing the outcrop and distribution of the rock
unit

•

Descriptions of rock core

•

Sequence stratigraphy (Describes the progression of rock units
within a basin )

•

Describing the lithology of the rock

•

Analysing the geochemistry of the rock


Sediment characteristics
• Porosity is the volume of voids within a rock
which can contain liquids.
• Permeability is the ability of water or other
liquids (e.g. oil) to pass freely through a rock.
• Roundness refers to the roughness of the
surface of the sedimentary grain.
• Sorting refers to the range of particle sizes in a
sediment or sedimentary rock.
• Matrix is the fine-grained material (usually clays
or silt) that is deposited originally with the
coarser-grained material


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Classification by GRAIN SIZE
•

Gravel > 2mm

•

Sand >1/16 mm < 2 mm

•

Mud <1/16 mm

•

Decide what the relative proportions of each size


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Grain Size Distribution


Folk (1980)


COMPOSITION
•

Gravel

•

Sand

•

Mud



TEXTURE
• Grain shape (attributes which refer to the external
morphology of particles)
– surface texture,
– roundness
– form. is determined by:

• Grain shape (Bustin, 1995)
– internal structure
– mineral cleavage

• characteristics of source rock such as jointing and
bedding
• lithology
• hardness
• fracture
• transport




COLOR
•

Munsell color chart

Any other features?
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Is there anything outstanding about this rock?
Does it have any important minerals or clasts?
Anything worth making note of?
Any important clasts?
Are there any fossils?
Any visible sedimentary structures (ie cross bedding)?