Chapter 5

Chapter 5

Chapter 5 Sedimentary Environments 1 Guiding Questions How does vertical stacking of distinctive types of strata provide clues to environments of deposition? What are the distinctive features of marginal marine and shelf deposits? What are the characteristics of deep-sea

sediments? 2 3 Paleogeography Reconstruction of ancient environments from the stratigraphic record Distribution of land and sea Identification of localized environmental features

Framework for interpretation of past life Employ actualism 4 Nonmarine Environments Soil Loose sediment containing organic matter and accumulated in contact with atmosphere Topsoil Upper zone of many soils

Sand and clay mixed with humus Humus Organic matter that gives topsoil its dark color Derived from decay of plant debris by bacteria 5 Nonmarine Environments Soils Type of soil depends on climatic conditions

Caliche Calcium carbonate produced by evaporation of groundwater Laterite Iron oxide rich soil produced in moist tropical regions 6 Nonmarine Environments Burrows

Plant Animal Aid in identifying ancient soils Devils corkscrews 7 Nonmarine Environments Lakes Lower elevation, more

likely preservation Indicates abundant precipitation Sediments Coarsest around lake margin Finest at center Often layered Freshwater fossils

8 Nonmarine Environments Glaciers Indicate cold climates Scratches produced by glacial motion Record direction of motion 9

Nonmarine Environments Till Unsorted, heterogeneous material Tillite Lithified till Moraine Ridges of till plowed up at the farthest edge

of the glacier 10 Nonmarine Environments Meltwater Transports sediments from glacier Forms streams and lakes Outwash Well-stratified layers of sediment

Varve Annual layers of alternating coarse and fine sediments 11 Nonmarine Environments Varves Annual record Count!

Dropstones Scattered coarse sediments found in sediment matrix Ice-rafted debris 12 Nonmarine Environments Desert soils Little organic matter Evaporite minerals

Interior drainage Precipitation does not leave the basin Playa lake Temporary lakes Associated with evaporites 13 Nonmarine Environments Dunes

Piles of sand < 1% of deserts Moves with prevailing wind direction Associated with deserts and beaches 14 Nonmarine Environments Dune migration

Trough cross- stratification Moves downwind Direction changes with Sands move up and over prevailing winds Beds accumulate on curved top; accumulate on downwind side surface cut through older beds 15 Nonmarine Environments

Alluvial fans Low, cone-shaped structures Develop where mountain slope meets valley floor Change in slope reduces stream velocity Sediments settle out Braided streams form 16

Nonmarine Environments Mudcracks Polygonal shape Form from alternate wetting and drying Associated with evaporites Halite Gypsum Anhydrite

17 Nonmarine Environments Waters move from mountains to sea through a variety of depositional environments Braided streams Meandering rivers Marginal marine systems

18 Nonmarine Environments Braided streams More sediment available than the water can transport Forms numerous channels and bars 19

Nonmarine Environments Meandering River Abundant water relative to sediment Backswamps Flood plain Mud settles out when stream overflows 20

Nonmarine Environments Point bar Slowest flow on inner bend Accumulate sands Fastest flow on the outer bank Cuts away bank Natural levees Form during floods Coarsest sands deposited

first, then fines 21 Nonmarine Environments Vertical sequence of accumulation Coarse channel sediments at base Fine backswamp muds at top

Illustrates Walthers Law 22 Marginal Marine Delta Depositional body of sand, silt, and clay formed when river empties into the sea Sediments settle out

in sequence 23 Marginal Marine Delta plain Layers of sand and silt deposited as river nears sea Distributary channels Separated by levees

Delta front Silt and clay slope deposits Prodelta Clays often deposited by a freshwater plume Progrades into basin Sediments coarsen upward

24 Marginal Marine Mississippi River Delta River dominated delta Progrades into Gulf of Mexico Lobes Growing portion of the delta

25 Marginal Marine Mississippi River Delta Active lobe Growing portion of delta Switched lobes in the past

Abandoned lobe Sediments compact Lobe sinks New lobe forms on top 26 Marginal Marine Deltaic Cycles Sequence of deposition Coarsens upward

Erosion can remove tops 27 Marginal Marine Barrier-Island Lagoon Complex Barrier Islands Waves and currents pile up sands Longshore Current

Lagoons Protected from strong waves behind barriers Muds and muddy sands 28 Marginal Marine Progradation Shoreline builds out into sea High supply of

sediment Builds over deeper water environments Illustrates Walthers Law 29 Marginal Marine Fossils Useful in

reconstructing environments of past 30 Marine Tempestites Storm deposits on shelf Sands deposited within normal muds

and muddy sands 31 Carbonate Systems Organic reefs Modern reefs formed from coral Ancient reefs formed from different organisms

32 Carbonate Systems Reef front Seaward side Often rubble called talus Reef flat Lagoon On leeward side Patch reef

33 Carbonate Systems 34 Carbonate Systems Barrier reefs Elongate reefs with lagoon behind

Fringing Reefs Grow along coast Lack lagoon 35 Carbonate Systems Atolls Reefs on volcanic islands Darwin

Formed by sinking island Up to 65 km across Often open at one end 36 Carbonate Systems Buried atolls Often important

petroleum reservoirs 37 Carbonate Systems Carbonate Platform Broad carbonate structure above seafloor Windward side Nutrient rich Abundant reefs

Buffered system CO2 + H2O = H2CO3 HCO3 + CaCO3 = 2HCO-3 38 Carbonate Systems Stromatolites Cyanobacteria mats trap sediments Grows up through

sediments to produce new one Layered organic-rich and organic-poor muds 39 Carbonate Systems Living Stromatolites Found in hypersaline,

supratidal and intertidal settings Little competition and predation Shark Bay, Australia 40 Deep Sea Environments Turbidity current Dense sediment-laden

flow driven by gravity Turbidite Produces graded deposit Poorly sorted coarse grains at base Fine grains at top 41 Deep Sea Environments

Turbidites are common in canyons Drop sediment load at base Form deposit similar to alluvial fan 42 Deep Sea Environments Pelagic sediment Fine-grained

sediments that accumulate by settling through the water column Calcium carbonate Silica Clay 43 Deep Sea Environments Calcareous ooze Accumulations of singlecelled planktonic

organisms Foraminifera Calcareous nannoplankton Common < 4 km w.d. Dissolution increases below 4 km w.d. 44 Deep Sea Environments Siliceous ooze Diatoms Radiolarians Common in upwelling regions Accumulations can alter to opal then chert 45 46

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