cont_environs_c8_part_2

cont_environs_c8_part_2 - Bounding surfaces in eolian...

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Bounding surfaces in eolian deposits Bedforms rarely preserved Usually preserve lowest part of bedform foreset (cross bedding) 3 types of bounding surfaces Ractiviation surfaces Formed by erosion of lee face Superposition surfaces Formed by dune migration Interdune surfaces Between sets of cross-strata that separate accumulations of different bedforms Super surfaces Regional unconformities marking regional interruption
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Ancient desert deposits: Navajo/Nugget Sandstone One of thickest, most widespread, best exposed ancient eolian erg in world 700 m thick 265,000 km 2 (5 states) Fine- to medium quartz sand Well rounded Frosted grains Huge tabular crossbeds 20° foreset dips 5 - 35m tall foresets (preserved lower foreset) Freshwater invertebrate fossils Dinosaur tracks
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Ancient desert deposits: Navajo/Nugget Sandstone Navajo Formation eolian deposits intertongued with fluvial deposits of the Kayenta Formation in NE Arizona Three fluvial to eolian drying- upward cycles Each represents advance of Navajo ergs across Kayenta alluvial plain Possible resulted from wet to dry climate shifts Preserved cross-bedded Navajo eolian dune/interdune deposits interbedded vertically with floodplain and other fluvial deposits of Kayenta Formation 1 2 3
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8.4 Lacustrine systems Lakes cover ~2% of Earth’s surface More prevalent today than during much of past Continents are presently in an emergent state Only minor proportion of overall stratigraphic record Lake chemistry is sensitive to climatic conditions Ancient episodes of wet and dry climates can be deciphered on basis of lake chemistry/mineraology
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Origin of lakes • Origin – Tectonic movements (e.g., Lakes Tanganika and Baikal) – Glacial processes (ice scour, ice damming, moraine damming) – Landslide – Volcanic processes (lava damming, crater explosion/collapse) – Wind deflation – Fluvial activity (formation of oxbow lakes)
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Size of Modern and ancient lakes • Modern Few 10s - 1000s km 2 surface areas Few m to 1700 m water depths Caspian Sea is largest modern lake (436,000km 2 ) • Ancient Small ponds to 100,000 km 2 surface areas Popo Agie Lake (late Triassic in Wyoming/Utah) T’oo’dichi’ Lake (Jursassic in Colorado) Green River Basin (Eocene in northern Utah)
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Lake settings Modern lakes occur in a variety of environmental settings – Glaciated and non glaciated – Plains and mountains – Very hot to very cold – Very arid to very humid – Mostly fresh but some highly saline – Siliciclastic and chemical sedimentation
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Principle kinds of lakes Open lakes = those inflow and precipitation is balanced by outflow and evaporation – stable shoreline Closed lakes = those with no major outflow – evaporation and infiltration commonly exceed inflow – fluctuating shorelines
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3 Factors controlling lake sedimentation 1. Physical processes causing transport/deposition
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