Sedimentary Environments

• Organisms buried in sediment decay, leaving a hollow space, which when filled, forms a cast.

Sedimentary Environments

• Any area where sediment accumulates is a depositional environment.
• Each depositional environment has physical, chemical, and biological characteristics.
• Continental, transitional, and marine settings influence deposition.

Continental Environments

• Continental environments refer to deposition on land.
• This includes environments such as alluvial fans, fluvial systems, lakes, deserts, and areas adjacent to glaciers.
• Alluvial fans are fan-shaped deposits formed at the base of mountains, common in arid and semi-arid regions.
• Fluvial environments are braided and meandering river and stream systems.
• River channels, bars, levees, and floodplains are parts of the fluvial environment.
• Channel deposits are coarse, rounded gravel and sand.
• Bars are made of sand or gravel.
• Levees are made of fine sand or silt.
• Floodplains are covered by silt and clay.
• Lakes are diverse, large or small, shallow or deep, fresh or salt water, filled with terrigenous, carbonate, or evaporite sediments.
• Mudcracks, wave ripples, laminations, and varves may be present in lakes.
• Fine sediment and organic matter in some lakes produces laminated oil shales.
• Deserts have little or no rainfall and contain vast areas where sand is deposited in dunes.
• Dune sands are well sorted, well rounded, and frosted/polished without associated gravel or clay.
• Cross-bedding is common in deserts.
• Swamps are areas of standing water with trees.
• Decaying plant matter accumulates to form peat, which may become coal.

Transitional Environments

• Transitional environments are at or near where the land meets the sea.
• These include deltas, beaches, barrier islands, lagoons, salt marshes, and tidal flats.
• Tidal flats are low-lying areas alternately covered by water and exposed to air.
• Deltas are fan-shaped sediment deposits formed where a river flows into a body of water.
• Coarser sediment (sand) is deposited near the river mouth; finer sediment is carried seaward.
• Well-known deltas include the Mississippi River delta and the Nile River delta.
• Beaches and barrier islands are shoreline deposits exposed to wave energy and dominated by sand, with marine fauna.
• Barrier islands are separated from the mainland by a lagoon.
• They are commonly associated with tidal flat deposits.
• Lagoons are bodies of water on the landward side of barrier islands.
• These are protected from ocean waves by barrier islands
• They contain finer sediment than beaches.
• Lagoons can also be found behind reefs, or in the center of atolls.
• Tidal flats are periodically flooded and drained, low-relief areas cut by meandering tidal channels.
• Laminated/rippled clay, silt, and fine sand may be deposited in tidal flats.
• Intense burrowing is common in tidal flats.
• Stromatolites may be present on carbonate tidal flats with high salinity.
• Salt marshes are associated with tidal flats behind barrier islands.

Marine Environments

• Marine environments are in the seas or oceans.
• Marine environments include reefs, continental shelf, slope, rise, and abyssal plain.
• Reefs are wave-resistant, mound-like structures from calcareous skeletons of organisms like corals and algae.
• Modern reefs are in warm, clear, shallow, tropical seas between 30°N and 30°S.
• Atolls are ring-like reefs surrounding a central lagoon.
• The continental shelf is the flooded edge of the continent, relatively flat and shallow, up to hundreds of miles wide.
• Continental shelves are exposed to waves, tides, and currents, covered by sand, silt, mud, and gravel.
• The continental shelves were flooded when glaciers melted 10,000 years ago.
• The continental slope and rise are seaward of the continental shelf.
• The continental slope is steep (5-25°), passing seaward into the gradual continental rise.
• The abyssal plain is the deep ocean floor, flat and covered by very fine-grained sediment from foraminifera, radiolarians, and diatoms.
• Abyssal plain sediments include chalk, diatomite, and shale.

Facies

• A facies is a body of rock with specific characteristics, defined based on color, bedding, composition, texture, fossils, and sedimentary structures.
• The term "facies" is used in strict observational and genetic senses.

Facies Sequence

• A facies sequence is a series of facies that pass gradually from one to another.
• A sequence can have an abrupt boundary, or be bounded by a hiatus.
• Clastic environments have two types of sequences : coarsening upward and fining upward sequences.
• Grain size reflects hydraulic power during deposition; a coarsening upward sequence indicates increased flow power.
• Fining upward sequences form by migrating point bars or filling abandoned channels.

Walther's Law

• Walther's Law states that the deposits of the same facies area, and the rocks of different facies areas, were formed beside each other in space and lie on top of each other in a crustal profile.
• Facies in a conformable vertical sequence are products of neighboring environments.

Lithologies and Facies

• Shallow water carbonate shelf facies are variable.
• Listed are types of these facies.
• Basin (fine clastics, carbonates, evaporites): dark shale/silt, thin limestones; even lamination.
• Open Marine Neritic (carbonates, shale): fossiliferous limestone with marl interbeds.
• Toe or Slope Carbonates: fine grained limestone; minor lamination; lenses of graded sediment; lithoclasts.
• Foreslope: bedded fine grained sediments with slumps, lime sands; variable lithology with sedimentary breccias; slumps.
• Organic Build-Up : boundstone, encrusting masses; massive limestone/dolomite; organic structure; injection dikes.
• Sand on Edge of Platform : oolitic lime sand or dolomite; medium to large scale cross bedding.
• Open Platform: variable carbonates and terrigenous clastics; intense bioturbation.
• Restricted Platform: lithoclastic sand; fine-grained interbeds; dolomite; fine laminations; cross bedded sand.
• Platform Evaporites: nodular anhydrite and dolomite on salt flats; laminated evaporites; irregular lamination; caliche.
• Carbonate sediments generally occur in shallow, tropical environments, while siliciclastic sediments can occur worldwide.
• Carbonate sediments reflect organism skeleton size, siliciclastic sediments reflect hydraulic energy.
• Lime mud means prolific growth of organisms, siliciclastic mud means settling out.
• Carbonate sediments get cemented, while siliciclastic sediments remain unconsolidated.
• Carbonate sediments get intensive diagenesis versus unaffected siliciclastic sediments.
• Carbonate sedimentary facies is destroyed during metamorphism but siliciclastic are stable.

Geologic time

• Geologic time and Earth's geologic history relate to the petroleum industry.
• Organic and sedimentary materials require millions of years to convert to recoverable hydrocarbons.
• The late eighteenth century is generally regarded as the beginning of modern geology.
• James Hutton introduced uniformitarianism, where geologic processes act the same through time.
• This is reflected in the key concept "the present is the key to the past".

Age dating

• Geologists used the principles of stratigraphy to determine relative ages before the discovery of radioactive materials.
• Once Radioactivity was discovered, geologists were able to pinpoint a rock's absolute age.
• Absolute dating supplements relative dating.
• Common dating techniques:
  • Carbon-14: organic materials.
  • Potassium-Argon and Rubidium-Strontium organic materials.
  • Thorium-230: deep ocean sediments and aragonite.
  • Protactinium-231: ocean sediments and aragonite.
  • Uranium-238: apatite, volcanic glass, zircon.

Basic Age Dating Principles

• Stratigraphy is the study of sedimentary rock layers.
• Stratification is the layering or bedding of sedimentary rocks
• Two principles are used to interpret geologic events: the principle of original horizontality and the principle of superposition.
• The principle of original horizontality states that sediment layers are deposited horizontally.
• The principle of superposition states that the deeper the layer the older it is.
• These two principles provides a means of constructing a vertical timeline.