Geology 3120 - Sedimentary Structuresgeode.colorado.edu/~structure/teaching_GEOL3120/coursenotes/3120... · Outline • Review the geologic history exercise from last time • Contacts,

Geology 3120 Geology 3120 -- Sedimentary Structures Sedimentary Structures

OutlineOutline

• Review the geologic history exercise from last time

• Contacts, primary structures, and secondary structures

• How to determine which “way is up”…

• Cross- bedding, graded- bedding, reverse graded- bedding

• Determining way up using top surface features - ripples, mudcracks, raindrops, footprints

• Determining way up using bottom surface features - load casts

• Determining way up using features within rocks -geopetal, bioturbation, stromatolites, flame structures, pillow lavas

Block model for exerciseBlock model for exercise

Geologic HistoryGeologic History

•• 12 Ma dike12 Ma dike•• 15 Ma dike15 Ma dike•• Normal faultNormal fault•• 20 Ma 20 Ma sedsed•• 22 Ma 22 Ma sedsed•• ErosionErosion•• Thrust faultThrust fault•• FoldingFolding•• Layer BLayer B•• 60 Ma 60 Ma sedsed•• Layer GLayer G•• 70 Ma 70 Ma sedsed•• 80 Ma 80 Ma sedsed

Contacts, Primary Structures & Secondary StructuresContacts, Primary Structures & Secondary Structures

• Contact - a boundary between rock units• Primary structure - structures that form during lithification• Secondary structure - structures that form after lithificaiton

Contacts, Primary Structures & Secondary StructuresContacts, Primary Structures & Secondary Structures

• Contact - a boundary between rock units• Primary structure - structures that form during lithification• Secondary structure - structures that form after lithificaiton

WhicWhich h

way way

is is up?up?

Today Option 1Overturnedsyncline

Option 2“up side down”overturnedanticline

CrossCross--beddingbedding

Checkerboard Mesa, Zion National Park

Y

Older

YoungerFlow direction

Concave up

Graded beddingGraded bedding

Y

Older

Younger

• Decrease in depositional energy with sedimentation

• Example: flood deposits, turbidity currents

Graded Graded

beddingbedding

• Decrease in depositional energy with sedimentation

• Example: flood deposits, turbidity currents

Younger

Y

Older

Reverse (Inverse) Graded BeddingReverse (Inverse) Graded Bedding

Y

Older

Younger

• Increase in depositional energy with sedimentation

• Example: debris flows (a lot less common than normal graded beds)

Ripple marksRipple marks

• Symmetric ripples indicate

bi-modal current

•Concave = up

• Asymmetric ripples indicate

unidirectional current

Y

Mud cracksMud cracks

5 cm5 cm

• Mud cracks

• Desiccation of muddy sediments

RaindropsRaindrops

Y

• Limited to terrestrial sediments

FootprintsFootprints

Y

• Limited to terrestrial sediments

Load castsLoad casts

• Protrusion of material into a layer below

• Load casts indicate the base of a layer, notthe top of a layer

• Determining the current direction may be possible

GeopetalGeopetal StructuresStructures

• A “natural” carpenter’s level

Shell or cavity in the rock Matrix

Infill material (I.e., calcite)

Y

BioturbationBioturbationY

• Habitation burrows

• Feeding burrows

• Movement

StromatolitesStromatolites

Y

• Sharks Bay, Australia

• Cyanobacteria grow upward toward the surface

Flame structuresFlame structures

Y

• Less dense material intrudes into material above • Caused by rapid loading of turbidite sands

Pillow lavaPillow lavaY

Upper curved surface

“V “ notch

ReferencesReferences

Slide 8http://www.utahpictures.com/Checkerboard.html

Slides 9-15, 17Busch, R. M. and D. Tasa, Laboratory Manual in Physical Geology, 3rd. Ed., American Geological Institute and National Association of Geology Teachers, 260 p., 1990.

Slide 18http://www.discoverwest.com.au/hablin.html

Slide 20http://volcanoes.usgs.gov/Products/Pglossary/PillowLava.html

http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html