Deposited Sediments and Sedimentary Rocks Flashcards
What features can deposited sediments have?
Sedimentary structures: small, 3D features that can form in a sediment during or after deposition as result of how and where the sediment is deposited, or from the activities of plants and animals
Primary sedimentary structures:
form as the sediment is being deposited, most common type is strata which hare nearly horizontal layers of sediment which form as sediment accumulates over time. Layers > 1cm thick called beds, < 1cm layers are called laminations.
A graded bed is a special type of
bed, normal grading: grain fine upwards, reverse grading: grains coarsen upwards
If the strata are inclined, we call the layers:
cross-beds or cross-lamination
Cross-bedding is the result of deposition on:
inclined surfaces such as ripples (small) and dunes (large), which are produced by flowing water or wind
Symmetrical ripples:
form in areas of oscillating waves (beach)
Asymmetrical ripples:
form in areas of unidirectional fluid flow (rivers and deserts)
Imbricated pebbles:
form when flat pebbles overlap because flowing water in rivers pushes them all over to point downstream
Sole marks are grooves and indents the form either as objects are dragged across sediment by:
a current, or back-current eddies scour out patches of sediment
Secondary sedimentary structures
form after a sediment has been deposited
Mudcracks:
form when water dries up and clay minerals contract
Bioturbation:
result of animals, insects, and plant roots churning up sediment and erasing primary sedimentary structures
Soft sediment deformation:
occurs when sediment is loaded onto waterlogged sediments. Compression may cause water to squeeze out, deforming the original horizontal bedding
How do sediments turn to stone?
Lithification
What is lithification?
describes the processes that turns sediment into stone
- Compaction: due to burial
- Cementation: precipitation of chemical sediments between clasts
Cements like silica or carbonate minerals precipitate out of solution:
bind grains together.
Compactions reduces pore space (voids or space between grains) by
forcing grains and matrix closer together. Helps to consolidate the grains (hold clasts together).
Sedimentary rocks type is based on:
- Texture (clastic or crystalline)
- Composition (the type of sediment it is made of)
Siliciclastic sedimentary rocks are distinguished by:
- Grain size and shape (texture)
- In sandstones, also specify grain type (relative proportion of quartz, feldspar, lithics, and clay)
- Gravel = conglomerate or breccia
- Sand = sandstone
- Silt = siltstone
- Clay and slit = shale (fissile) or mudstone (massive)
Chemical sedimentary rocks are distinguished by:
Crystal type (carbonate, halite, microcrystalline quartz)
Bioclastic sedimentary rocks are distinguished by:
- Grain size and shape (texture)
- Grain type (chemical sediments like shells, or organic sediments like plants)
Naming Clastic Sedimentary Rocks Based on Grain Size
Clast name + rock name = name of rock
Clast names of clastic sedimentary rocks
Conglomerate (rounded clasts) or Breccia (angular clasts) for sizes: boulder to granule
Sandstone for very coarse sand to very fine sand
Siltstone/Mudstone/Shale for microscopic grain sizes
Naming Sandstones (for very coarse sand to very fine sand sedimentary rocks!!!!)
Feldspathic arenite, quartz arenite, lithic (rock) arenite INCLUDE GRAIN SIZE MODIFIER
Classification of Carbonate Rocks
- Use Dunham classification of limestones diagram
- All have modifier i.e oolitic or fossiliferous or dolomitic if including dolomite/calcite followed by type of stone: mudstone, wackestone, packstone, grainstone, or boundstone
