Sedimentary rocks
Geology 101
Making sediment
• Weathering = rock breakdown into smaller
rock, or minerals, or chemicals
• Sediment = result of weathering of rocks
• Erosion = movement of sediment
Physical weathering
• Physical:
Breaking apart of
rocks by a
physical force
• Also called
“mechanical”
weathering
Chemical weathering
• A chemical reaction
alters the composition
of the minerals in the
rocks (e.g., dissolving
halite or altering
feldspars into clay
minerals)
• Dissolving limestone
leads to karst
topography and caves
Caves are the product of varying water tables and
limestone dissolution; cave formations (speleothems)
are the result of calcite precipitation
Or a combination
• Physical weathering
may break up
bedrock, then
chemical weathering
may break down the
pieces into a soil
A quick word on soils
• Soils are the
weathered material
at the surface that
include both organic
and mineral
components
• Soils differ due to
the parent material,
time of weathering
and water content
Charles Darwin and soils
• Among other work, Darwin
in 1854 determined that
earthworms, through their
castings, produced about
an inch of soil per year.
• From this rate, he realized
that a great deal of
geologic time was needed
even to produce a fertile
soil.
Lithification
• Sedimentary
rocks
become
lithified
when they
are
compacted
then
cemented
Classification of sed rx
• Clastic = “broken”; sed rx made of broken-up
parts of other rocks
• Chemical sed rx are made from the
precipitation of chemicals from water
(“evaporites”) or the oxidation of chemicals
• Biological sed rx are “born”; that is, they
derive from the remains of creatures
Clastic sed rx
• Classified by dominant grain size
• Scale: “boulder”, “cobble”, “pebble”, “granule”,
“sand”, “silt”, “clay”
• Texture and composition are secondary
considerations (e.g., “shale”)
Texture: Grain roundedness
• The roundedness
(that is, how sharply
defined the
“corners” of
individual grains
are) is used to
determine the
transport distance
• Proximal = near;
distal = far
Texture: Grain sorting
• The variation between coarse and fine
particles in the sediment is called sorting
• Sorting is used to infer transport distance;
well-sorted sediments have come far
Layering in clastic sed rx
• Layers are called
beds, unless they
are thin, in which
case they are called
laminae
• Beds and laminae
represent distinct
depositional events,
like floods
Chemical sed rx
• Evaporites: limestone, rock salt
• Oxidation product: taconite (iron ore)
Biological sed rx
• Plant remains: coal
• Animal remains: limestone, chert
Water energy
• The speed of the
depositing medium
(usually water) can
be inferred from
clastic and some
other sed rx
• This is because
coarser grains settle
first in a suspension
Depositional environment
• Is the term given to
the area where the
sediment was
originally deposited
• Must be inferred
from the rock
identity
Sedimentary structures
• Macroscopic features in the rock that allow
you to infer climate, current direction or other
attributes of the depositional environment
How to make cross-beds
• And notice that you can infer the wind (or
water direction as well!
Cross-beds allow you to infer the depositional
environment; meter-scale cross-beds are eolian
Bioturbation – trace fossils
burrows get filled in with sediment
filled burrows can tell you which
way is up in a layer!