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Flashcards in Lab Midterm 1 Deck (48):

What are sedimentary structures?

•Layering of rocks.
•Features or marks on the surface separating layers.
•Internal arrangement of grains within layer.
They help interpret depositional environment of sediments.


Why are fossils important in the study of geology?

- Relicts of organism that lived while the sediment was deposited. They give evidence of the type of environment (e.g., sea, river, etc.).
- Trace fossils : Records of biological activities


What are graded beds?

Bedding layers with gradual vertical variation of grain size.
•From repeated pulses of high-energy sediment transport.
–Sediment added as a pulse of turbid water.
–As pulse weakens, water loses velocity and sediments settle.
–Coarsest material settles first, medium next, then fine.


What are ripple marks in bedding structures?

Ripple marks are cm-scale ridges that are elongated perpendicular to the current flow direction.
–Larger versions of ripples are called dunes.
•Asymmetric ripples are associated with unidirectional flow.
e.g., river current.
•Symmetric ripples are associated with bidirectional flow.
Back and forth movement of waves along shores.
•Interference ripples can form when the currents change directions or there is interaction between waves and currents.


What is cross-stratification in bedding structures?

They are a consequence of migration and evolution of ripples.
–Sediment moves up the gentle side of a ripple or dune.
–Sediment piles up, then slips down the steep face.
–The slip face continually moves downstream.
–Added sediment forms sloping “cross-bedded” layers.
•Cross-bedding (> 1 cm); cross-lamination (< 1 cm).


What are mud cracks in bedding structures?

–Polygonal-shaped cracks formed when a layer of mud dries up.
–Indicate alternating wet and dry conditions.
–Indicate that when the sediment was deposited, it was exposed to air.


What are scour marks - flute casts in bedding structures?

the current flow can make indentations parallel to the flow direction.
-Typical of high-energy currents.
-Distinctive shape.
-Tapered end towards the downstream direction and the steeper, more bulbous end, towards the upstream direction.


What is the importance of trace fossils in biogenic sedimentary structures?

-Activity by living organisms can get in the way of preserving the layering.
-It can disrupt the layering by altering the sediments before it has been lithified.


What are track/trackways in biogenic sedimentary structures?

Result of the combination of the type of animal, its behaviour and the characteristics of the substrate


What are the different types of trace fossils in biogenic sedimentary structures?

- tracks/trackways
- trails
- coprolites
- gastroliths
- burrows
- borings
- root marks


What are stromatolites in biostratification structures?

–Formed by photosynthesizing cyanobacteria, trapping and binding sediments onto sticky biofilms.
–Among the oldest fossils known (~3.5 bya)
–Major role in oxygenation of early atmosphere and oceans.
–Require sunlight -->shallow water


What are physical characteristics used to reconstruct past depositional environments?

For detrital clastic rocks:
• Grain Size
- energy regime; travel distance; transportation medium.
• Grain shape
- angular vs. rounded: as grain is transported, its surface becomes more smooth.
• Grain sorting
- variable vs. constant energy
- as sediments are transported farther away, they tend to separate according to size /weight.
In general, can you say anything about the physical maturity of the sediments?
Physical maturity - grains have been greatly reworked / processed /weathered - well-rounded and well-sorted.
Other characteristics: e.g., glacial striations in rock


What are chemical characteristics used to reconstruct past depositional environments?

General composition of rocks:
• Terrigenous (i.e., land-derived)
- detrital clastic rocks derived from erosion of continental rocks.
- In general, can you say anything about the chemical maturity of the terrigenous sediments?
- Stable (quartz, clay minerals, iron oxides) vs. unstable minerals (feldspars, mafic minerals)
• Carbonate
- From fragments of organisms’ shells or precipitated directly in warm shallow seas.
• Organic
- Accumulation of (“soft”) organic matter (e.g., plants)
• Evaporites
- precipitation from solution; may indicate dry environment.
• Microcrystalline quartz or calcite
- from microscopic marine organism
- deep ocean.
Other characteristics: cement composition; colour


What are biological characteristics used to reconstruct past depositional environments?

Different life forms mean different environments
Shells: Exclusively marine vs. marine/freshwater
•Land plants vs. marine plants (e.g., algae, seaweed)
–Tetrapods ; dinosaurs - continental or transitional environments
- precipitation from solution; may indicate dry environment.
•Microcrystalline quartz or calcite
- from microscopic marine organisms to deep ocean.
- water environment with enough sunlight and physical protection


Describe alluvial fans

•At the mountain front, sediments carried by the fast-moving stream are emptied onto a plain region.
•Rapid drop in stream velocity creates a cone-shaped wedge, where sediments settle out and get deposited.
•Common rocks include poorly sorted conglomerate, breccia, arkose, mudstone.


Describe lake environments

•Water energy is low leading to quiet waters which means only small grain sediment is transported towards the centre of the lake.
•Lake sediments can be finely laminated and are made up of mud/clay and silt.
•Common rocks include shale and mudstone.
–Evaporites can form in dry conditions.


Describe paludal/swamps

•Lots of plant material / organic matter
•Low energy
•Common rocks include shale , coal


Describe fluvial environments

•Sand and gravel fills channels.
•Fine sand, silt, and clay is deposited on floodplains.
•Sedimentary structures include: ripples, cross-beds, mud-cracks.
•Common rocks include , conglomerate, sandstone;


Describe glacial environments

•Moving ice does not sort grains or rework them.
•Tillite is common
- Tillite: rock made of till (poorly- sorted angular glacial sediments)
•Polished/striated rocks


Describe a desert environment

•Little vegetation leads to strong wind exposure.
•Wind-blown piles of well-sorted sand.
•Dry conditions; evaporites
•Results in well-sorted sandstones with big cross-beds.


What can be found in transitional areas?

- beaches and barrier islands
- lagoons
- tidal flats


Describe delta environments

•Sediments dropped where a river enters the sea/lake.
–River flow stops at the ocean, depositing the sediments carried.
–Lots of sediment input.
•Associated with different energy regimes (low to relatively high).
–Coarser sand deposited near mouth of river
–Finer sediment carried seaward.
•Coarsening upwards sequences


Describe beaches and barrier islands

•Surf zone.
•Many sediments are being processed by wave attack.
-Usually well-sorted, well-rounded sand.
•Beach sandstones may preserve symmetrical (oscillation) ripples.
•Lots of marine shells


Describe lagoons

•Finer grain size
•Could also be affected by tidal currents
•Marine shells


Describe tidal flats

•Medium to fine grains
•Sedimentary structures include: mud cracks, ripples, cross-bedding
•Stromatolites may be present
- shallow water


What are the three areas of marine environments

•Continental shelf (shallow marine)
•Continental slope and rise
•Abyssal plain (deep marine)


Describe the continental shelf

•Continental shelf – Shallow (~0-200 m) and wide, gently sloping (~0.3o).
•Sand to clay size grains
•Also carbonates
•May show cross-bedding, laminations, marine shells


Describe continental slope and rise area of the ocean

•Continental slope – Descends from ~200 m to ~3 km at ~2o.
•Continental rise – Transition zone to abyssal plain.
•Thick accumulations of sediments – submarine fans


Describe the Abyssal plain

•Fine sediments predominate far from land sources.
•Skeletons of planktonic organisms make chalk or chert.
•Fine silts and clays turn to mudstone, shale.


What is stratigraphy?

A sequence of beds throughout geologic time
• Analysis of these sequences allows for interpretation of past depositional environments
• Vertical sequences at different locations can be correlated and lateral changes in depositional environments can be inferred.


What causes stratification?

Stratification is caused by changing conditions during deposition.
•The differences between bedding layers may be contrasting (e.g., clear difference in their composition and colour) or they could be subtle (separated by a surface representing non-deposition).
•Deposition of sediments occurs as finite events meaning deposition is not necessarily a continuous process.
–Sediments get deposited when the conditions are favorable, this can vary from time to time in a region.


What is the difference between short term or long term deposits in terms of stratification?

Changes in a deposition environment can be short-term, causing the deposition of different beds, or they can be long-term causing different sequences of beds.


What is stratigraphic formation?

Sequence of beds with distinctive properties that can be traced across a relatively large region


What is contrast?

The boundary between formations


Why is lithologic correlation important?

Equivalency of rock units from different outcrops (cores).
•Missing rock unit(s) in the sequence suggest unconformity and surface representing gap in time.


What are the different types of unconformities?

- angular unconformity
- nonconformity
- disconformity


What are sedimentary facies?

Rock unit that represents the depositional environment
– Distinctive physical / chemical / biological characteristics


Why are sea-level changes common?

Sedimentary environments shift landward or seaward in response.
•Sedimentary deposition is strongly linked to sea level.


What is transgression?

Flooding due to sea level rise.
– Depositional environments shift landward
–Strata represent progressively deeper water environments leading to“deepening” upward sequence


What is regression?

Exposure due to sea level fall.
–Depositional environments shift seaward
–Strata represent progressively shallower water environments leading to “shallowing” upward sequence.
•Sea level rise and fall creates a predictable pattern causing depositional sequence.


How do fossils form in sedimentary rocks?

–Burial of remnant of organisms in sediments.
–Marks or traces on the sediments made by ancient organisms.


What are the requirements for fossilization?

•Hard parts
– Greater preservation potential than soft parts. (Rare exceptions include the Burgess Shale of Canada (BC))
•Buried by sediment
•Preserved after burial so escape physical, chemical and biological destruction, This implies, among other things:
– No metamorphism
– No bioturbation
– No dissolution
Organisms are most likely to be preserved in environments with high sedimentation rate.


What are the type of fossil preservaton?

1. Original hard parts (e.g., shells, bones, teeth)
2. Original soft parts
a) Dried or frozen body fossils:
b) Body fossils preserved in amber or tar:
3. Permineralization
4. Replacement
5. Carbonization: thin carbon film


Explain original hard parts fossil preservation

Original hard parts (e.g., shells, bones, teeth)
– Unaltered with the following common chemical compositions:
• Carbonate: calcite and aragonite
–Calcite (calcium carbonate)
–Aragonite (calcium carbonate)
Aragonite is not stable at ambient P, T therefore transforms to calcite over time.
•Phosphate: bones, teeth, inarticulate brachiopods.
•Silica: microscopic skeletons (diatoms, radiolaria), some sponges.


Explain original soft parts fossil preservation

2. Original soft parts
a) Dried or frozen body fossils:
•Whole bodies (soft and hard parts) of organisms preserved.
•It occurs in a few environments and it applies to relatively young fossils, ~1000’s (not millions) of years old.
•In deserts, the dry climate can desiccate dead organisms; this enables their preservation.
•In cold environments, animal bodies may get buried under snowice and become “mummified”. Example: wooly mammoth remains in Russia
b) Body fossils preserved in amber or tar:
•Tree sap / resin can trap insects, feathers, etc. due to its sticky nature. Eventually the resin hardens becoming amber.
•Tar is an oily hydrocarbon fluid that seeps to the surface and has a sticky nature. It can also trap organisms.


Explain permineralization fossil preservation

Groundwater can seep through pores in bones and wood, where minerals can precipitate.
•Example: Fossilized bone


Explain replacement fossil preservation

–It occurs when the original material is slowly and gradually replaced by mineral matter.
–Fine details are preserved.
–Usually replaced by silica or pyrite; also by Fe-oxides and calcite.
Example: echinoderms, mollusks, articulate brachiopods, corals should be calcareous
Example: Petrified wood showing both permineralization and replacement


Explan carbonization fossil preservation

Carbonization: thin carbon film
–Flattened molds of soft / semi-soft body parts (e.g., leaves, shell-less invertebrates, insects, sponges, jellyfish) that occur in between sediment layers.
»Organic material is removed by chemical reactions and a thin layer of carbon remains on the surface of the impression