L6: Sedimentary Structures 2

Question 1

5 types of Dunes

Answer 1

1) Barchan Dune - is a horseshoe-shaped dune with the front curve facing into the wind. Barchan dunes are often found in groups, but occasionally alone. They often move over a flat surface of pebbles or bedrock. The slip face of a barchan is away from the wind.
2) Transverse Dune - form long lines of ridged dunes that are perpendicular to the wind direction. They have steep slip faces at the back sides of the ridges. These wavy dunes form in areas with plenty of sand and not much plant life. The sand dunes found behind beaches are often transverse dunes formed by strong ocean winds. The farther inland you travel, the more plant life you encounter.
3) Linear Dune - are mostly parallel to the wind and form long, straight ridges. These dunes can be over 100 meters tall and go on for many kilometers. Geologists think linear dunes are caused by winds that blow from one direction in one part of the year (northwest), and then shift and blow from a different direction (southwest) during another part of the year. The overall dune movement is easterly with a long, thin shape. The sand bedding is criss-crossed and the slopes are unequal. the different types of sand dunes most commonly seen by geologists.
4) Star Dunes - are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around the margins of the sand seas, particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 meters tall and may be the tallest dunes on Earth.
5) Parabolic Dunes - U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. Sometimes these dunes are called U-shaped, blowout, or hairpin dunes, and they are well known in coastal deserts. Unlike crescentic dunes, their crests point upwind. The elongated arms of parabolic dunes follow rather than lead because they have been fixed by vegetation, while the bulk of the sand in the dune migrates forward. The longest known parabolic dune has a trailing arm 12 kilometers long.

Question 2

3 categories for dunes (Alone, together etc.)

Answer 2

Simple dunes - Individual dunes spatially separated from others
Compound Dunes - 2 or more dunes of the SAME type coalesced or superimposed
Complex Dunes - 2 or more TYPES of dunes coalesced or superimposed

Question 3

Explain Barchan Dunes (Form, Formation etc.)

Answer 3

Crescentic in plan view and wedge-shaped in cross-section
Steeply-dipping lee side facing downwind and gently dipping stoss side facing upwind
Dune gradually moves downwind
Formed with insufficient sand to build larger dune forms and 1 prevailing wind direction

Question 4

3 forms of aeolian sedimentary structures

Answer 4

1) Grainfall laminae - Fine Grains dropped out of suspension and form thin laminae
2) Wind ripple lamination - Common on dunes and sand sheets, deposits are the most abundant type of aeolian strata. 1-10mm parallel laminae with slight changes in grain size inversely graded.
3) Avalanche (Grainflow) - Coarse, Inversely graded, discontinuous, 2cm thick cross strata formed in avalanching noncohesive sand on dune surfaces once the sand builds up past the angle of repose

Question 5

Imbrication

Answer 5

Flattened clasts dip in the upstream direction to resist the flow of current

Question 6

Desiccation cracks

Answer 6

Muddy or water-laden sediments dry out causing them to contract, causing curls and desiccation cracks. Indicates an arid climate and way up.

Question 7

Secondary sedimentary structures

Answer 7

Structures at are formed after deposition but before lithification

Question 8

3 common soft-sediment deformations

Answer 8

1) Dewatering structures
2) Degassing structures
3) Load structures - dense sediment over soft sediment causing the soft sediment to shift

Question 9

Bioturbation

Answer 9

Record in a sediment left by the various activities of an organism

Question 10

ichnofacies

Answer 10

An ichnofacies is an assemblage of trace fossils that provides an indication of the conditions that their formative organisms inhabited.

Question 11

Stromatolites

Answer 11

Rock formed by layers of microbial mats in a laminated fabric