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 1
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 2
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 3
Eolian stratigraphy 4
5
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. 6
Eolian dunes change as climate changes eas.unl.edu 7
Eolian Cross-bedding drc.oberlin.edu Eolian Cross-bedding Geol.umd.edu 8
Figure 2. Composite stratigraphic section of exposed Nepean Formation at study location. Evolutionaryfairytale.com MacNaughton R B et al. Geology 2002;30:391-394 2002 by Geological Society of America Eas.unl.edu Navajo Sandstone - Southwestern U.S. This well-known sandstone (it occurs in Grand Canyon, Zion National Park in southwestern U.S.) records arid conditions that were common during the Jurassic! Monsoonal circulation is recorded in these rocks! 9
Navajo Sandstone has large-scale crossbedding 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 10
rationalskepticism.org Criteria to identify Eolian Facies 1. 2. 3. 4. 5. 11
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) Deltas form anywhere that running water enters a standing body of water Lacustrine Delta 12
Fluvial system entering a lake makes an elongate delta Inclined cross-bedding was produced by Gilbert-type delta prograding into a lake 13
Green River Formation above Cretaceous strata in Utah preserves lacustrine features Leaf fossils can be common in some lacustrine facies 14
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 15
Fish fossils can be very common in lacustrine deposits Rhythmically bedded lacustrine sediments (varves), colors relate to grain size and oxygenation of the lake 16
Triassic - Jurassic Rifting of Eastern N.A. Triassic-Jurassic Rifting Asymmetric basins were filled with nonmarine and marine sedimentary rocks 17
Evaporites formed in some of the rift basins Triassic-Jurassic Rifting Evaporites were formed from evaporation of sea water Triassic-Jurassic Rifting Louann Salt! 18
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 19
Sedimentary Cycles Criteria to identify lacustrine facies 1. 2. 3. 4. 5. 20