Sedimentary Rocks —The Archives of Earth

History

?

Disconformity

There are actually 3 types of rocks…

• Igneous – cool from liquid (magma or lava)

• Metamorphic – pre-existing rocks that have been altered by intense temperature or pressure

• Sedimentary – form mainly from deposition of sediments

Historical geology focuses on sedimentary rocks

• Only rocks that contain fossils

• Indicate ancient depositional environments

•Why???

What is a sediment?

• Fragment of pre-existing rock (or animal shell)

• Why does water off Galveston look murky, while water off Florida looks clear?

What kind of rocks do we find around Houston?

• Not many rocks!

• Lots of unlithified sediment

• Why do many houses in Houston have foundation problems?

What is a sedimentary rock?

• Rock that forms at or near Earth’s surface

• 3 types– Clastic– Chemically-precipitated– Biogenic

How do clastic sedimentary rocks form?

• Weathering

• Transport

• Deposition

• Lithification

How do other sedimentary rocks form?

• Chemical – precipitation of dissolved materials

• Biogenic (organic) – accumulations of organic material

Environments of Deposition

• At or near surface of Earth

• Marine

• Continental

• Transitional (deltas, barrier islands, beaches)

Marine

• Coastal

• Shelf

• Deep water

www.ngdc.noaa.gov/mgg/image/sedthick.jpg

Continental

• Fluvial– Meandering– Braided

• Desert

• Lacustrine

• Glacial

www.uoregon.edu/~millerm/depenv.html

Continental

• Fluvial– Meandering– Braided

• Desert

• Lacustrine

• Glacial

www.uoregon.edu/~millerm/depenv.html

Continental

• Fluvial– Meandering– Braided

• Desert

• Lacustrine

• Glacial

www.uoregon.edu/~millerm/depenv.html

• Large-scale cross-beds in a Permian-aged wind-blown dune deposit in Arizona

Dune Cross-Beds

Continental

• Fluvial– Meandering– Braided

• Desert

• Lacustrine

• Glacial

www.mikelevin.com/DLBlissParkTahoe.jpg

Continental

• Fluvial– Meandering– Braided

• Desert

• Lacustrine

• Glacial

www.peakware.com/encyclopedia/peaks/photos/everest7.htm

• Moraines and poorly sorted till

Moraines and Till

• Origin of glacial drift

Delta

• Form in oceans or lakes (marine and non-marine)

www.uoregon.edu/~millerm/depenv.html

• Stream/river-dominated deltas – long

distributary channels extending far seaward

– Mississippi River delta

Stream/River-Dominated Deltas

Where would you find different sedimentary rocks?

Where would you find different sedimentary rocks?

Where would you find different sedimentary rocks?

• Grain size is controlled by energy

• High energy– River– Beach

• Low energy– Lake– Deep ocean

Large grains

Small grains

• If the size range is not very great, – the sediment or rock is well sorted

• If they have a wide range of sizes, – they are poorly sorted

• Wind has a limited ability to transport sediment so dune sand tends to be well sorted

• Glaciers can carry any sized particles because of their transport power, so glacier deposits are poorly sorted

• Grains more rounded with longer transport

Sorting, Rounding

• A deposit of well rounded and well sorted gravel

Rounding and Sorting

• Angular, poorly sorted gravel

• Tabular cross-bedding forms by deposition on sand waves

Cross-Bedding

• Tabular cross-bedding in the

Upper Cretaceous Two Medicine Formation in

Montana

• form in response to water or wind currents flowing in one direction

• asymmetric profiles allowing geologists to determine paleocurrent directions

Current Ripple Marks

http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch03/chapter03-

5sedstr.html

• As the waves wash back and forth, symmetrical ripples form

• Wave-formed ripple marks in shallow seawater

Wave-Formed Ripple Marks

Modern Deposition near Houston

• Fluvial– Brazos, Colorado, Trinity, San Jacinto

Rivers

• Transitional– Deltas, barrier islands

• Marine– Gulf of Mexico

Ancient Environments

• Important for historical geology

• Important for oil companies (need to know where sand was deposited)Why are we looking at

modern depositional environments?

Present is the key to the past

• Study modern depositional environments to learn about ancient ones

• Knowledge of ancient environments helps oil companies and historical geologists

Brazos River

• Longest river in Texas – 1450 km

• Highest sediment supply of any Texas river

• Originates in New Mexico

Where does deposition occur?

www.uwsp.edu/geo/courses/geog391/toriv/Diagrams.htm

Point Bars

• Sediment deposited within the inside bank of a meander loop

• Fining upward sequence (grain size decreases)

• Coarsest sediment deposited by highest energy

Brazos River – Point Bar

Brazos River – Cut Bank

Coastal – Galveston Island

Barrier Islands

• Formed during sea level rise

• Rate of SL rise and rate of sediment deposition approximately equal

• Wave-dominated environment

• On broad continental margins with abundant sand, long barrier islands lie offshore separated from the mainland by a lagoon

• Barrier islands are common along the Gulf and Atlantic Coasts of the United States

• Subenvironments of a barrier island complex: – beach sand grading offshore into finer deposits– dune sands contain shell fragments (not found in

desert dunes)– fine-grained lagoon deposits

Barrier Islands

• Subenvironments of a barrier island complex

Barrier Island Complex

Texas Coast

• Most Texas beaches relatively fine-grained

• Low gradient of rivers like Brazos

• Why is this a problem?

Brazos Delta

gulf.rice.edu

• Present-day gravel deposits by a swiftly-flowing stream(Most transport and

deposition takes place when the stream is higher)

Environmental Interpretations and Historical Geology

• Nearby gravel deposit probably less than a few thousand years old

• Conglomerate more than 1 billion years old – shows similar

features

Environmental Interpretations and Historical Geology

• We infer that it too was deposited by a braided stream– Why not deposition by glaciers or along a

seashore?– No evidence for either glacial activity or

transitional environment