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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cont_environs_c8_part_2 - Bounding surfaces in eolian...

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