essential of geology

Topic 5: Sediments and Sedimentary Rocks

(Interlude B and Chapter 6)

October 9th, 2015

Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. Norton Chapter 6: Pages of Earth’s Past: Sedimentary Rocks

Four classes of sedimentary rock:

Clastic—loose rock fragments (clasts) cemented together.

Biochemical—cemented shells of organisms.

Organic—carbon-rich remains of once living organisms.

Chemical—minerals that precipitate directly from water

solutions.

Review

Chemical Clastic Organic Biochemical


Top Hat Review

Table 6.1


How do grain size and shape, sorting, sphericity and

angularity change as sediments move downstream?

Review


By the end of this lecture you

should know:

Select examples of

sedimentary structures and

the clues they provide about

the depositional environment.

Different types of depositional

environments and the

character of sedimentary

rocks that accumulate them.

What a sedimentary basin in

and how they form in the

context of plate tectonics.

Objectives


Sedimentary Structures

Sedimentary structures refers to:

The layering of sedimentary rocks

Surface features on layers formed during deposition

Arrangement of grains within layers

Sedimentary structures provide evidence about

conditions at deposition.

Bedding and Stratification

A single layer of sediment or sedimentary rock with a

recognizable top and bottom is called a bed.

The boundary between two beds is called a bedding plane.

Several beds together constitute strata.

The layering in a sequence of beds is called bedding or

stratification.


Why does bedding form?

Bedding reflects changing conditions during deposition.

Changes in water depth, current velocity, climate, sediment

source and sediment supply causes bedding to form

These alter:

Sediment composition

Grain size

Sorting, etc.

Fig. 6.10b


If a sequence of strata (rock package) is distinctive

enough to be traced as a unit across a region it is called

a stratigraphic formation.

Formations are able to be mapped.

Formations are named for places they are best exposed.

Geologic maps display the distribution of formations.


Fig. 6.11a, b


Deposition in a Current

Many clastic sediments accumulate in moving fluids

Sedimentary structures can develop at the interface

between sediment and fluid.

Water or wind flowing over sediment creates bedforms.


Ripple marks—cm-scale

ridges and troughs.

Develop perpendicular to

flow.

Commonly found on modern

beaches

Ripple marks can be

preserved

Found on bedding planes of

ancient sedimentary rocks

Dunes—similar to ripples

except much larger.

Range in size from tens of

cm (streams) to hundreds of

m (deserts). Fig. 6.13c


Cross beds—created by ripple and dune migration.

Sediment moves up the gently sloping, upstream face of a

ripple or dune.

Sediment deposited on the steep downstream face (slip

face).

Ripples and dunes continually migrates downcurrent.

Fig. 6.13a, b


Bed Surface Markings

Occur during or after deposition while sediment is still

soft.

Mudcracks—polygonal desiccation (drying) features in

mud indicating alternate wet and dry conditions

Scour marks—troughs eroded in soft mud by current flow

Fig. 6.15


Top Hat


The setting in which sediment accumulates

Depositional Environments

Fig. 6.16

Two main categories of

depositional

environments include:

Terrestrial (non-

marine)

• Deposited above

sea-level

Marine

• Deposited at or

below sea level

• Includes coastal

environments

Terrestrial Environments

Sand-dune environments—

wind-blown sand.

Result in well-sorted, well

rounded sandstones with

gigantic cross beds.

Glacial environments—due

to movement of ice.

Ice carries and dumps

every grain size.

Creates glacial till; poorly

sorted gravel, sand, silt,

and clay.



Mountain stream environments

Fast-flowing water carries

large clasts during floods.

During low flow, these cobbles

and boulders are immobile.

Coarse conglomerate is

characteristic of this setting.

Alluvial fan—sediments that

pile up at a mountain front.

Rapid drop in stream velocity

creates a cone-shaped wedge.

Sediments become

conglomerate and sandstone.



River (fluvial) environments

Sand and gravel deposited in channels.

Fine sand, silt, and clay are deposited on banks nearby flood

plains.

Fig. 6.16e



Lake—large ponded

bodies of water.

Gravels and sands

deposited near shore.

Well-sorted muds

deposited in deeper

water.

Delta—sediment piles up

where a river enters a

lake.

Marine Environments

Marine delta - sediment accumulates 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.

Much more complicated than simple lake deltas.

Fig. 6.18a


Marine Environments

Coastal beach sands—constant wave action

Well-sorted, well-rounded sandstone.

Beach ripples often preserved on bedding planes.

Fig. 6.18b


Marine Environments

Shallow-marine clastic deposits—

finer sands, silts, muds.

Fine sediments deposited

offshore where energy is low.

Finer silts and muds turn into

siltstones and mudstones.

Shallow water carbonate

environments.

Warm, clear, marine water,

relatively free of clastic sediments.

Sediments are mostly shells of

organisms.

Source of limestones.


Marine Environments

Deep marine deposits—fines settle out far from land.

Skeletons of planktonic organisms make chalk or chert.

Fine silt and clay lithifies into shale.

Fig. 6.20

Sedimentary Basins

Sediments vary in thickness across Earth’s surface.

Thin to absent in some locations.

Thicken to 10–20+ km in sedimentary basins.

Sediments accumulates in special regions where the

Earth’s surface sinks (subsidence) providing a space in

which sediment collects.

Basins are places that accumulate sediment.

Fig. 6.21


Sedimentary Basins

Foreland basins—continent side of collisional mountain

belt.

Flexure of the crust from loading creates a downwarp.

Fills with debris eroded off of the mountains.

Fluvial, deltaic, and lake sediments fill foreland basins.

Fig. 6.21


Sedimentary Basins

Rift basins—divergent plate boundaries.

Crust thins by stretching.

Thinned crust subsides.

Sediment fills the down-dropped troughs.

Fig. 6.21


Top Hat

Schedule and Midterm Info

Week 6 Oct-12 Thanksgiving – no lecture

Oct-14 In Class Review Activity

(to be handed in at the end of class- 1% of final grade)

Oct-16 Midterm 1: In Class

(40 multiple choice questions, 20% of final grade)

You will not be required to know the chemical formulas of the minerals

we learned about in Topic 3.

Check out the “Midterm I Information” document posted on D2L.

Office Hours: 09:00-09:50 Mo, We, Fr (excluding holidays) or by

appointment.

Office: ES 530 (open door policy)

essential of geology

Documents

sedimentary rocks deposition

character of sedimentary

sedimentary basin

layers sedimentary structures

sedimentary rocks interlude

classes of sedimentary

norton chapter

pages of earths past