depositional environments...2/22/15 2 depositional environments environmental conditions: kind &...

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Depositional Environments Geographic area in which sediment is deposited

Characterized by a combination of geological process & environmental conditions

Depositional Environments Geological processes:

Nature of currents Water, wind, ice

Plate tectonic setting Presence of volcanic activity

Generates abundant sediment Climate

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Depositional Environments Environmental conditions:

Kind & amounts of water Ocean, lake, river, ice

Topography Lowland, mountain, shallow ocean, deep ocean

Biological activity

Non-marine Depositional Environments

Alluvial Fans Braided and Meandering Fluvial

Systems Deserts Lacustrine Systems

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Desert

Depositional Environments Continental Environments ~ Desert

Desert - dunes and arid-region streams Transporting agent - wind Sediments - sand and dust (loess)

•  Sand dunes are large features with steep faces Ripples, cross-bedding formed from sand dunes

•  Well sorted, quartz-rich sands •  May have internally draining lakes •  Mudcracks in wet area between sand dunes •  Oases are spring that show up in rock record

Climate - arid Organic processes - little organic activity, mainly

surface tracks and trails

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Eolian stratigraphy

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web.ncf.ca

Eolian Dune internal architecture Eolian dunes commonly are built by climbing wind ripples that produce climbing translatent strata. The angle of climb determines the angle of the layers. Visually the laminae have a “pinstripe” appearance and internally they commonly are inversely graded.

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Eolian dunes change as climate changes

eas.unl.edu

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Eolian Cross-bedding

drc.oberlin.edu

Eolian Cross-bedding

Geol.umd.edu

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Figure 2. Composite stratigraphic section of exposed Nepean Formation at study

location.

MacNaughton R B et al. Geology 2002;30:391-394

©2002 by Geological Society of America

Evolutionaryfairytale.com

Eas.unl.edu

Navajo Sandstone - Southwestern

U.S.

This well-known sandstone (it occurs in Grand Canyon, Zion National Park in south-western U.S.) records arid conditions that were common during the Jurassic!

Monsoonal circulation is recorded in these rocks!

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Navajo Sandstone has large-scale cross-bedding produced by the migration of large sand dunes, similar to those in

modern Sahara desert

The cross-bedding in the Navajo Sandstone indicates it formed during the yearly migration

of the monsoon circulation; large extent of deserts, as well as widespread storm deposits caused by hurricanes suggests “Megamonsoon”

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rationalskepticism.org

Criteria to identify Eolian Facies

1. 2. 3. 4. 5.

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Lakes are standing bodies of water formed on land and as such they can contain many of the same features that form along shorelines

(e.g., deltas, turbidites, slumps)

Lacustrine Delta Deltas form anywhere that running water enters a standing body of water

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Fluvial system entering a lake makes an elongate delta

Inclined cross-bedding was produced by Gilbert-type delta

prograding into a lake

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Green River Formation above Cretaceous strata in Utah preserves

lacustrine features

Leaf fossils can be common in some lacustrine facies

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The Solenhofen Limestone in Germany is an important unit for it preserves

Archeopteryx fossils - the first evidence of birds, feathers

Stromatolites can grow in lakes and carbonate can accumulate along

shorelines

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Fish fossils can be very common in lacustrine deposits

Rhythmically bedded lacustrine sediments (varves), colors relate to

grain size and oxygenation of the lake

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Triassic - Jurassic Rifting of Eastern N.A.

Triassic-Jurassic Rifting

Asymmetric basins were filled with non-marine and marine sedimentary rocks

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Triassic-Jurassic Rifting Evaporites

formed in some of the rift basins

Evaporites were

formed from evaporation of sea water

Triassic-Jurassic Rifting

Louann Salt!

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Triassic - Jurassic Rifting of Eastern N.A.

Basalt sills and dikes are common within the rift basins developed as North America and Africa separated!

Triassic-Jurassic Rifting

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Sedimentary Cycles

Criteria to identify lacustrine facies

1. 2. 3. 4. 5.