Lab Midterm 1 Flashcards
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.
- Bioturbation
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
Examples
Shells: Exclusively marine vs. marine/freshwater
•Land plants vs. marine plants (e.g., algae, seaweed)
•Bones:
–Tetrapods ; dinosaurs - continental or transitional environments
•Evaporites
- precipitation from solution; may indicate dry environment.
•Microcrystalline quartz or calcite
- from microscopic marine organisms to deep ocean.
•Stromatolites
- 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
