lecture 5 Sedimentary Rocks

Lecture 5: Sedimentary Rocks

Useful Web page -- UBC provides an excellent homepage for Sedimentary Rocks

Uniformitarianism

James Hutton -- "the present is the key to the past"
The same Earth processes operating today have been operating throughout Earth history. -Widely accepted.
Hutton thought that rates were the same as well, but now we know some rates are different. Especially those related to tectonics. Earth used to be faster, so convection and plate movement may have been faster.

Uniformitarianism vs Catastropohism

Interpretation of sedimentary rocks is where Uniformitarianism is most important. Processes today are similar to those in the past.
Catastrophism important for extinctions, major volcanic eruptions, climate change?
Key to interpreting sedimentary rocks is to compare rocks to present-day environments
Slides will make some of these comparisons.

Stratification refers to the layers of sedimentary (or volcanic) rocks. Owes to sedimentation of clastic material at the Earth's surface, one layer at a time.

Clastic -- rocks composed of detritus - the result of the mechamical breakdown of older rocks
Chemical -- precipitation from solution in water -- evaporating seas
Biogenic -- copmposed of animal/plant activity -- often shells or body parts -- bioclastic

Clastic Rocks

Weathering -- mechanical or chemical breakdown of rock during exposure to air, water or organics.
Mechanical weathering produces detritus. This detritus is then transported by water or wind. It is later deposited and then becomes lithified to form clastic sedimentary rocks
Lithification is the process that turns sediments into rocks

How do sediments turn into rocks?

Diagenesis -- collective name for the chemical, physical, biological changes that occur after initial deposition.
Compaction -- self-explanetory
Cementation -- SiO2 or carbonate cements infiltrate the sediments
Recrystallization -- less-stable minerals re-crystallize into more stable forms.

Tools for Environmental Reconstruction

Sed rocks are one of the most accurate picture of past environments

Composition
Texture
Fossils
Sedimentary structures

1) Composition

Qtz and feldspar is more resistent to wxing than mafic minerals, therefore ends up in "clean" or mature sandstones. Often found in continental settings where transport times are high. One indication of maturity or amount of time the grains have spent being actively worked in the sedimentary environment.

2) Texture

Grain Size is a measure of the energy of the system. Beaches deposit sands or small cobbles; high-energy rivers can deposit large cobbles or boulders. Glaciers and landslides, probably the highest energy environments on Earth, can deposit house sized blocks.

--Conglomerate/Breccia ---- sandstone ---- siltstone - shale
--cobbles------------------ sand --------- silt/mud -- clays, mud

In general, the smaller the grains the lower the energ of the depositional system.

Sorting is a measure of the range of partical sizes.
Poor sorting reflects sudden changes in the energy of the system
--according to specific gravity -- gold placer deposits
--according to size -- most common minerals are the same specific gravity
Poor sorting in tills, landslide deposites.
Good sorting in beach sands deposits.

grain shape indicates length of transport.
round vs angular
round means repeated abrasion during transport
angular meants short transportation distance

3) Fossils

Tell us about the paleo-environment if we know something about the living habits of similar present-day organisms.

4) Bedding and Sedimentary Structures

Cross-bedding is formed from dunes (wind, water)-- indicates down-stream or down-wind direction.
-draw pictures of process, steep-sided ripples, symetric ripples.
-Ripples small, low velocity
-Dunes big, higher velocity
Graded -- heavy comes out first -- turbidites, mudslides, -- indicates which way is up in old rocks
-- gradational vs sharp contacts.
Mud Cracks-- seen from the top. Like columnar jointing

Biogenic Sediments

Limestone
Siliceous ooze in the deep sea -- Chert
Fossil Fuels -- compressed plant matter
Peat accumulation of plant material in a reducing (no Oxygen) environment turns into coal

Sedimentary Facies

Facies refers to a distinctive set of characteristics that differs, as a group, from those elsewhere in the same rock unit.

Use barrier islands as an example.

In the rock record, we see vertical records of various facies. How can this happen?
-Rising or falling sea level

Transgressive Sequence or Transgression is when the sea-level rises and the shore-line moves away from the center of the basin, leaving its sedimentary record

Regressive Sequence or Regression is when the sea-level falls and the shore-line moves towards the center of the basin, also leaving its sedimentary record

Succession of Facies (see fig. 4.23)
-Superimposed facies are those that can be observed beside each other in present-day environments.

Examples of Sedimentary Environments and Rocks
Essentially what we did with slides
See book for more complete descriptions of each and every environment
Brief outline follows + some tectonic significance

Generalizations
Contintental -- very mature, qtz,feldspar-rich, well sorted, the more mature -- the more qtz.
-Limestone -- warm, shallow seas.
Convergent margins and tectonically active regions -- immature, greywackys, lots of voclanic detritus, less qtz, muddy matrix, less well sorted, turbidites at continental margin.
-Large accumulations of sedimentary rocks
Oceanic -- oozes, mostly siliceous, chert, made from Silica shells of micro-organisms.

Examples

Streams --Point Bar deposits. Rounded conglomerates
Lakes - varves
Glacial -- unsorted till, matrix supported
Alluvial fans -- breccias/conglomerates, with channels, clast supported.
Eolian -- wind-blown sand dunes
Lagoon -- where fresh and salt water mix -- sand and mud
Deltaic -- where sediment-rich rivers meet the sea -- stream channel and over-bank deposits
Beaches -- mature qtz,feldspar-rich sands -- some cross bedding
Carbonate shelves
Evaporite basins -- Mediterranean
Continental slop -- tubidity currents, turbidites
Deep-sea oozes -- carbonates and/or silicates, depending on depths.
Turbidites -- undersea landslides. Thinnly layered beds tracable for large distances (kms).
-Formed by turbidity currents.
Deep-Sea Oozes -- the only thing that sediments the deep oceans.

Carbonate Compensation Depth - about 4 kilometers
only SiO2 below, can have carbonates above.