Sedimentary?

Question 1

Conditions of sedimentary rock formation:

Answer 1

• Sedimentology controlled by weathering and therefore climate
• More likely to see stronger minerals in sedimentary rocks as they survive the weathering
o Haematite and quartz most stable and weather slowest
o Halite and calcite least stable and weather fastest
• Grains/Clasts = framework
o Cement and matrix fill in the gaps = matrix

Question 2

Transportation

Answer 2

• Further transport = loses clay and sands become better sorted
• Sands have maturity
o fluvial overbank, allubia and mairne turbidite sands tend to be immature to submature
o Fluvial channel sands tend to be submature to mature
o Aeolian and beach sands tend to be mature to supermature
• The longer distance travelled = the more well rounded the grains. If angular, the source is close

Question 3

Sandstone occurrences?

Answer 3

• Quartz arenites: shallow marine (above SWB), windblown dune deposits, narrowly restricted in time and widespread in extent when present
• o Arkoses: alluvial fan and fan delta deposits, desert arkose (mineralogically immature, texturally supermature), adjacent to magmatic arcs, etc.
• o Lithic arenites: alluvial deposits (adjacent uplifted mountainous sources), orogenic clastic wedges, turbidites, etc.
• o Wackes: sediment gravity flows, submarine fan complexes, dominant sandstone of the Archean

Question 4

Controls on carbonate depth?

Answer 4

• Temperature: raising the temperature of seawater promotes limestone deposition
• Pressure: reducing the pressure (or depth) of seawater promotes limestone deposition
• Degree of agitation: breaking waves in the surf zone mix water with and promote limestone formation because additional CO2 is absorbed by the atmosphere
• Organic activity: Organisms precipitate calcium carbonate or modify the geochemical environment enough for precipitation to occur (CO2 removal)
• Sediment masking and clogging: Slow terrigenous accumulation rates and little mud in suspension favour carbonate production and accumulation
• Light: photosynthetic organisms require light (shallow, clear water)
• Calcite compensation depth: Calcarerous sediment accumulates only above the CCD
• Allochems (transported and deposited as clasts)
• Grains
o Skeletal grains (bioclasts)
o Coated grains (oöids,pellets,etc.)
• Nonskeletal
o Four major types o coated grains
o Oöids (< 2 mm)/pisoids (> 2 mm) o oncoids
o Pellets/peloid
o Aggregates (grapestones) o clasts
o Noncarbonate grains

Question 5

Pelagic Sediments?

Answer 5

o Sediments in the deep ocean
o Pelagic rain: debris from decaying organisms which falls to the bottom of the ocean
o Colour of matter indicates amount of total inorganic carbon, need less than 1 % TOC for petroleum
o Places in Dorset where 40% TOC - Kimmeridge Clay
o Deep sea sediment containing at least 30% biogenic material is called an ooze
o Accumulate slowly but they accumulate faster as deep sea terrigenous clay
o Surface seawater is depeleted of CO2 due to photosynthesis
o Deep water contains more CO2 because of the decay of sinking dead organisms and colder water can hold more CO2
o Increase in hydrostatic pressure also increases the solubility of calcium carbonate; CCD is the depth at which calcite dissolves
o Red clay is distinct to the deep sea
o SCD - silica dissolves in shallow water and preserves in deep water

Question 6

Mudstone structure?

Answer 6

o Two kinds of “layers” within the “sheets” o “T” layers – tetrahedral layers
o Tetrahedral coordination of Si & Al o “O” sheets – octahedral layers
o Octahedral coordination of mostly Al & Mg, occasionally Fe
o T and O layers bonded to form sheets that are repeated in vertical direction, and the spaces between the sheets may be:
o Vacant
o Filled with interlayer cations, water, or other sheets.