Geology 3120 - Sedimentary Structures

Outline Contacts, primary structures, and secondary structures 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 exercise

Geologic History 12 Ma dike 15 Ma dike Normal fault 20 Ma sed Erosion Thrust fault Folding Layer B 60 Ma sed Layer G 70 Ma sed 80 Ma sed

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

Which way is up? Today Option 1 Option 2 Overturned “up side down” syncline Option 2 “up side down” overturned anticline

Y Cross-bedding Checkerboard Mesa, Zion National Park Younger Flow direction Y Concave up Older

Y Graded bedding Younger Older Decrease in depositional energy with sedimentation Example: flood deposits, turbidity currents

Y Graded bedding Younger Older Decrease in depositional energy with sedimentation Example: flood deposits, turbidity currents

Y Reverse (Inverse) Graded Bedding Younger Older Increase in depositional energy with sedimentation Example: debris flows (a lot less common than normal graded beds)

Y Ripple marks Symmetric ripples indicate bi-modal current Concave = up Y Asymmetric ripples indicate unidirectional current

Mud cracks Desiccation of muddy sediments Mud cracks 5 cm

Raindrops Y Limited to terrestrial sediments

Footprints Y Limited to terrestrial sediments

Load casts Protrusion of material into a layer below Load casts indicate the base of a layer, not the top of a layer Determining the current direction may be possible

Y Geopetal Structures A “natural” carpenter’s level Shell or cavity in the rock Matrix Y Infill material (I.e., calcite)

Bioturbation Habitation burrows Feeding burrows Movement Y

Y Stromatolites Sharks Bay, Australia Cyanobacteria grow upward toward the surface

Y Flame structures Less dense material intrudes into material above Caused by rapid loading of turbidite sands

Pillow lava Y Upper curved surface “V “ notch

References Slide 8 http://www.utahpictures.com/Checkerboard.html Slides 9-15, 17 Busch, 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 18 http://www.discoverwest.com.au/hablin.html Slide 20 http://volcanoes.usgs.gov/Products/Pglossary/PillowLava.html http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html