Download view the Lecture Presentation

Pages of Earth’s Past:
Sedimentary Rocks
Sedimentary Cover

Earth is covered by a thin ‘veneer’ of sediment.

The veneer caps igneous and metamorphic “basement.”

Sediment cover varies in thickness from 0 to 20 km.

Thinner (or missing) where ig and meta rocks outcrop.

Thicker in sedimentary basins.
Sedimentary Rocks

Sediments are the building blocks of sedimentary rocks.

Sediments are diverse, as are the rocks made from them.

Four classes:

Clastic – Made from weathered rock fragments (clasts).

Biochemical – Cemented shells of organisms.
 Organic

Clastic
– The carbon-rich remains of plants.
Chemical – Minerals that crystallize directly from water.
Biochemical
Organic
Chemical
Clastic Sedimentary Rocks

Clastic sedimentary rocks reflect several processes.
 Weathering
– Generation of detritus via rock
disintegration.
 Erosion
– Removal of sediment grains from rock.
 Transportation
– Dispersal by wind, water, and ice.
 Deposition
– Settling out of the transporting fluid.
 Lithification
– Transformation into solid rock.
Clastic Sedimentary Rocks

Lithification – Transforms loose sediment into solid rock.
 Burial
– More sediment is added onto previous layers.
 Compaction
– Overburden weight reduces pore space.
Sand – 10 to 20%
Clay – 50 to 80%
 Cementation
sediments.
– Minerals grow in pores, “gluing”
Clastic Sedimentary Rocks

Classified on the basis of texture and composition.
 Clast
(grain) size.
 Clast
composition.
 Angularity
and sphericity.
 Sorting.
 Character

of cement.
These variables produce a diversity of clastic rocks.
Clastic Sedimentary Rocks

Clast (grain) size – The average diameter of clasts.
 Range
from very coarse to very fine.
 Boulder,
 With
cobble, pebble, sand, silt, and clay.
increasing transport, average grain size decreases.
Clastic Sedimentary Rocks

Clast composition – The mineral makeup of sediments.
 May
be individual minerals or rock fragments.
 Mineral
identities provide clues about…
The source of the sediment.
The environment of deposition.
Clastic Sedimentary Rocks

Angularity and sphericity – Indicate degree of transport.

Fresh detritus is usually angular and non-spherical.

Grain roundness and sphericity increases with transport.
 Well-rounded
 Angular
– Long transport distances.
– Negligible transport.
Clastic Sedimentary Rocks

Sorting – The uniformity of grain size.
 Well-sorted
 Poorly

– Uniform grain sizes.
sorted – Wide variety of grain sizes.
Sorting becomes better with distance from source.
Clastic Sedimentary Rocks



Cement – Minerals that fill sediment pores.

Fluids with dissolved solids flush through pore system.

Dissolved ions slowly crystallize and fill pores.
Cementation varies from weak to strong.
Common cements:

Quartz

Calcite

Hematite

Clay minerals
Clastic Sedimentary Rocks

Coarse clastics – Composed of gravel-sized clasts.
 Breccia
– Comprised of angular fragments.
Angularity indicates a lack of transport processing.
Deposited relatively close to source.
Clastic Sedimentary Rocks

Coarse clastics – Composed of
gravel-sized clasts.
 Conglomerate
–
Indicates water transport.
Collisons round angular
corners and edges of
clasts.
Conglomerates are
deposited at a distance from
the source.
Clastic Sedimentary Rocks


Sandstone – Clastic rock made of sand-sized particles.

Forms in many depositional settings.

Quartz is, by far, the dominant mineral in sandstones.
Sandstone varieties.

Arkose – Contains abundant feldspar.

Quartz sandstone – Almost pure quartz.
Clastic Sedimentary Rocks


Fine clastics - Composed of silt
and clay.

Silt-sized sediments are lithified
to form siltstone.

Clay-sized particles form shale.
Fine clastics are deposited in
quieter waters.

Floodplains, lagoons, mudflats,
deltas, deep-water basins.

Organic-rich shales are the
source of petroleum.
Biochemical and Organic Rocks

These are sediments derived from living organisms.
 Biochemical
 Organic
– Hard mineral skeletons.
– Cells of plants, algae, bacteria and plankton.
Biochemical Rocks

Biochemical limestone – CaCO3 skeletal (shell) remains.

Warm, tropical, shallow, clear, O2-rich, marine water.

Diverse organisms (plankton, corals, clams, snails,
etc.).

Many textural varieties.
Reefs.
Shell debris.
Lime mud (micrite).
Biochemical Rocks

Chert – Rock made of cryptocrystalline quartz.
 Formed
from opalline silica (SiO2) skeletons.
Diatoms.
Radiolarians.
 Opalline
 Silica
silica added to bottom sediments dissolves.
pore fluids solidify to form chert nodules or beds.
Organic Rocks

Made from organic carbon.
 Coal
– Altered remains of
fossil vegetation.
Accumulates in lush
tropical wetland settings.
Requires deposition in
the absence of oxygen.
 Oil
shale – Shale with heat
altered organic matter.
Chemical Sedimentary Rocks

Comprised of minerals precipitated from water
solution.

Evaporites – Created from evaporated seawater.
 Evaporation
triggers deposition of chemical
precipitates.
 Examples
include halite (rock salt) and gypsum.
Chemical Sedimentary Rocks

Travertine – Calcium carbonate
(CaCO3) precipitated from
groundwater where it reaches the
surface.
 Dissolved
calcium (Ca2+) reacts
with bicarbonate (HCO3-).
 CO2
expelled into the air causes
CaCO3 to precipitate.
Thermal (hot) springs.
Caves.
Chemical Sedimentary Rocks

Dolostone – Limestone
altered by Mg-rich fluids.

CaCO3 altered to
dolomite CaMg(CO3)2 by
Mg2+-rich water.

Dolostone looks like
limestone, except…
It has a sugary texture
and a pervasive
porosity.
It weathers to a buff,
tan color.
Chemical Sedimentary Rocks

Replacement chert – Nonbiogenic in origin.

Many varieties.
 Flint
– Black or gray from organic matter.
 Jasper
– Red or yellow from Fe-oxides.
 Petrified
 Agate
wood – Wood grain preserved by silica.
– Concentrically layered rings.
Sedimentary Structures

Features imparted to sediments at or near deposition.
 Layering.
 Surface
features on layers.
 Arrangement

of grains.
Help decipher conditions at or near time of deposition.
Sedimentary Structures

Sedimentary rocks are usually layered or “stratified.”
 Arranged
 Bedding
 Beds
in planar, close-to-horizontal “beds.”
is often laterally continuous for long distances.
are often similar in composition, color and texture.
Sedimentary Structures

Bedding reflects changing conditions during deposition.

These can be changes in…
 Energy
conditions, and hence, grain size.
 Disturbance

by organisms.
Bedding may also reflect non-deposition or erosion.
Sedimentary Structures

A series of beds are referred to as strata.

Formation: Strata recognized on a regional scale.

Geologic maps display the distribution of formations.
 i.e.
Coconino Formation
Sedimentary Structures

Water flowing over loose sediment creates bedforms.

Bedforms are linked to flow velocity and sediment size.
 Ripples,
cm-scale ridges, and troughs, indicate flow.
Asymmetric ripples – Unidirectional flow.
Symmetric ripples – Wave oscillation.
Ripples are commonly preserved in sedimentary rocks.
Bedforms

Cross beds – Created by ripple and dune migration.
 Sediment
moves up the gentle side of a ripple or dune.
 Sediment
piles up, then
slips down the steep face.
The slip face
continually moves
downstream.
Added sediment
forms sloping
“cross-bedded” layers.
Bedforms

Dunes – Similar to
ripples except much
larger.
 Form
from windblown sand in
desert or beach
regions.
 Often
preserve
large internal crosslaminations.
Bedforms

Turbidity currents.
 Sediment
 As
pulse wanes, water loses velocity and grains settle.
 Coarsest

moves on a slope as a pulse of turbid water.
material settles first, medium next, then fines.
This process forms graded beds in turbidite deposits.
Bed-Surface Markings

Occur after deposition while sediment is still soft.

Mudcracks – Polygonal desiccation features in wet
mud.
Indicate alternating wet and dry conditions.
Necessitate deposition in a terrestrial setting.

Scour marks – Troughs eroded in soft mud by current
flow.

Fossils – Evidence of past life.
Footprints.
Shell impressions.
Depositional Environments

Locations where sediment accumulates. They differ in…
 Energy
regime.
 Sediment
delivery, transport, and depositional
conditions.
 Chemical,

physical and biological characteristics.
Environments range from terrestrial to marine.
Depositional Environments

Terrestrial environments – Deposited above sea level.
 Glacial
– Due to movement of ice.
Ice carries and dumps every grain size.
Creates glacial till; poorly sorted gravel, sand, silt, and
clay.
Depositional Environments

Terrestrial environments –
Deposited above sea level.
 Mountain
streams.
Water carries large
clasts during floods.
During low flow, these
cobbles and boulders
are immobile.
Course conglomerate
is characteristic of this
setting.
Depositional Environments

Terrestrial environments –
Deposited above sea level.
 Alluvial
fan - Sediments
that pile up at a mountain
front.
Rapid drop in stream
velocity creates a coneshaped wedge.
Sediments are immature
conglomerates and
arkoses.
Depositional Environments

Terrestrial Environments– Deposited above sea level.
 Sand
dunes – Wind-blown piles of well-sorted sand.
Dunes move according to the prevailing winds.
Result in uniform sandstones with gigantic cross beds.
Depositional Environments

Terrestrial environments– Deposited above sea level.
 Rivers
– Channelized flow transports sediment.
Sand and gravel fill concave-upward channels.
Fine sand, silt, and clay are deposited on nearby
floodplains.
Depositional Environments

Terrestrial environments– Deposited above sea level.

Lake – Large ponded bodies of water.
Gravels and sands trapped near shore.
Well-sorted muds deposited in deeper water.
Often capped with wetland muds.
Depositional Environments

Marine environments – Deposited at or below sea level.
 Deltas
– Sediments dropped where a river enters the sea.
Sediment carried by the river is dumped when velocity
drops.
Deltas grow over time, building out into the basin.
Often develop a topset – foreset – bottomset geometry.
Depositional Environments

Marine environments – Deposited at or below sea level.

Coastal beaches – Surf zone.
Sediments are constantly being processed by wave attack.
A common result? Well-sorted, well-rounded medium sand.
Beach sandstones may preserve oscillation ripples.
Depositional Environments

Marine environments – Deposited at or below sea level.
 Shallow
marine – Finer version of beach sediment.
Fine silts and muds turn into siltstones and
mudstones.
Usually support an active biotic community.
Depositional Environments

Marine environments – Deposited at or below sea level.
 Shallow
water carbonates – Tropical.
Skeletons of marine invertebrates.
Born in the carbonate factory.
Warm, clear, shallow, normal salinity, marine water.
Depositional Environments

Marine environments – Deposited at or below sea level.
 Deep
marine – Fines predominate far from land
sources.
Skeletons of planktonic organisms make chalk or
chert.
Fine silts and clays turn to shale.
Sedimentary Basins



Sediments vary in thickness across Earth’s surface.

Thin to zero edge where non-sedimentary rocks outcrop.

Thicken to 10 to 20+ km in sedimentary basins.
Subsidence – Sinking of the land during sedimentation.

Due to crustal flexure and faulting.

Compounded by the weight of added sediments.
Basins are important locations for natural resources.

Coal.

Petroleum.

Natural gas.

Uranium.
Sedimentary Basins

Basins form where tectonic activity creates space.
 Rift
basins – Divergent (pull-apart) plate boundaries.
Crust thins by stretching and rotational normal faulting.
Thinned crust subsides.
Sediment fills the down-dropped basin.
Sedimentary Basins

Basins form where tectonic activity creates space.

Passive margin basins – Non-plate-boundary continental edge.
Underlain by crust thinned by previous rifting.
Thinned crust
subsides as it cools.
Sedimentary Basins

Basins form where tectonic activity creates space.
 Intracontinental
basins – Interiors far from margins.
Result from differential thermal subsidence.
May be linked to failed crustal rifts.
Sedimentary Basins

Basins form where tectonic activity creates space.
 Foreland
basins – Craton side of collisional mountain belt.
Flexure of the crust from loading creates a downwarp.
Fills with debris eroded off of the mountains.