Lecture 8// Sedimentary Rocks Flashcards
Chapter 8
Sediment (1)
Produced from the weathering and erosion of pre-existing rock (creating smaller particles and dissolved components):
Clastic/ Detrital sediment
Solid pieces of pre-existing rocks (mostly silicates) named based on the grain size.
When looking at gravel and sand, the grain sorting and grain rounding indicates the distance of transport (for both high and low sphericity):
- very poorly-sorted: angular
- moderately sorted: intermediate
- very well-sorted: rounded
Wentworth grain size scale (7)
Gravel:
- > 256 mm: boulder
- 64-256 mm: cobble
- 4-64 mm: pebble
- 2-4 mm: granule
Sand: 1/16-2 mm
Mud:
- 1/256-1/16 mm: silt
- <1/256 mm: clay
Lithification (3)
The burial and compaction of sediment, and cements precipitation from groundwater in pore spaces.
Sedimentary rocks form from the lithifiaction of sediment.
Larger grains (clasts) and smaller gains (matrix) are separated by pore spaces that may be partly or fully occupied with cement.
Clastic Sedimentary Rock Classification (8)
Gravel:
- (Rounded) Conglomerate: Clastic, lithic fragments
- (Angular) Breccia: Clastic, lithic fragments
Sand:
- Quartz sandstone: Quartz
- Arkose: Quartz, feldspar
- Lithic sandstone: Quartz. feldspar, lithic fragments
Silt, massive:
-Siltstone: silicate minerials
Mud:
- (massive) Claystone: Silicate and clay minerals
- (laminated) Shale: Silicate and clay minerals
Chemical Sedimentary Rock Classification (Rocks composed solely of precipitated minerals.) (5)
Chert
- Microcrystalline
- Microcrystalline silica
Dolostone
- Crystalline
- Dolomite
Rock gypsum
- Crystalline
- Gypsum
Rock salt
- Crystalline
- Halite
Travertine
- Microcrystalline
- Calcite from saturated fluids
Biogenic Sedimentary Rock Classification (Rocks built from organic materials.) (4)
Chalk
- Clay or mud
- Skeletal coccolithophroids
Coal
- Massive, blocky
- Concentrated carbon
Coquina
- Sand or gravel
- Shell fragments
Skeletal Limestone
- Visible or microscopic skeletal fragments
- Calcite
Environments of Clastic Sediment Deposition (gravel/sand/mud) (8)
Marine environments:
- Shallow marine (above continental shelf / epicontinental seas)
- Deep marine
Transitional environments: (where ocean meets land)
-Deltas, tidal flats, embayments, beaches, etc.
Continental environments:
- Fluvial (river beds & banks, floodplains); lacustrine (lakes)
- Eolian (deserts); paludal (swamps); glacial
Deciphering Paleo Environments - Lithology (2)
Example: Limestone
-Limestone is mainly formed in marine settings that are relatively low energy (away from shore).
-Example: sandstone
Sandstone is formed in higher energy settings, including marine settings (close to shore), transitional settings (at the shore), and continental settings such as river beds.
Deciphering Paleo Environments – Grain Size (4)
The grain size of detrital rocks gives clues as to the energy of the medium that
deposited the sediment:
- Mud (silt and clay) = quiet/low energy environments
- Sand = higher energy environments
- Gravel = very high energy environments
Deciphering Paleo Environments - Fossils (2)
Body fossils:
The actual remains of organisms.
Biogenic structures (trace fossils): The remains of an organism’s behavior, such as footprints and burrows.
Biogenic sed. structures (9)
Traces of life formed during or soon after deposition.
These trace fossils can indicate:
- terrestrial or marine deposition
- if there were any physico- chemical stresses present at the time of deposition
-Locomotive and resting traces (such as tracks and trails)
-Dwelling structures (domiciles)
-Grazing structures (between
sediment grains)
-Deposit-feeding structures (ingesting sediment)
-Escape structures (organism) is buried and moves upwards to re- establish its burrow closer to the sediment-water interface)
Mud cracks (2)
- In the Utah desert, storms are frequent and severe. Ephemeral (dry) stream beds are flooded and at the end of the flood, mud is deposited from suspension.
- Following the storm the water evaporates. The mud shrinks as it dries, forming mud cracks.
Oscillation ripples (2)
- Symmetrical oscillation ripples are formed from the to-and-fro action of waves interacting with the seafloor/lake floor.
- Continued deposition results in wavy bedding.
Deciphering Paleo Environments – Physical Structures (Current ripples/current ripple lamination; Dunes/cross-bedding) (5)
Asymmetrical ripples/dunes form from unidirectional flow (a current) of water or wind:
-Down-current migration of the ripples or dunes forms internal cross-laminations or cross-beds.
-Eventually the pile fails, and
cascades down the slip face
(lee face). The ripple thus
migrates down current (wind
or water).
- Larger asymmetric ripples (>5cm in height are called dunes, and the internal stratification is called cross bedding.)
- The migration of very large wind dunes (meters high) creates large-scale cross bedding. Cross beds on the order of metre+ high are only formed by the migration of eolian dunes.
