Sedimentary basins

• Sedimentary basins are the subsiding areas where sediments accumulate to form stratigraphic successions

• The tectonic setting is the premier criterion to distinguish different types of sedimentary basins• Extensional basins occur within or between plates and are

associated with increased heat flow due to hot mantle plumes• Collisional basins occur where plates collide, either

characterized by subduction of an oceanic plate or continental collision

• Transtensional basins occur where plates move in a strike-slip fashion relative to each other

Sedimentary basins

Collision

• Subduction is a common process at active margins where plates collide and at least one oceanic plate is involved; several types of sedimentary basins can be formed due to subduction, including trench basins, forearc basins, backarc basins, and retroarc foreland basins

• Trench basins can be very deep, and the sedimentary fill depends primarily on whether they are intra-oceanic or proximal to a continent

• Accretionary prisms are ocean sediments that are scraped off the subducting plate; they sometimes form island chains

Sedimentary basins

Collision

• Continental collision leads to the creation of orogenic (mountain) belts; lithospheric loading causes the development of peripheral foreland basins, which typically exhibit a fill from deep marine through shallow marine to continental deposits

• Foreland basins can accumulate exceptionally thick (~10 km) stratigraphic successions

Continental margins

Fig. 3.6

Trailing-Edge Margin

Anatomy of a passive margin

Continental margin

Fig. 3.7

Continental slope and submarine canyons

Fig. 3.8a

Important Characteristics of Sedimentary Rocks

• Porosity

• Permeability

• Roundness of grains

• Sorting

• Matrix

Indices of maturity

( long system vs. short system plus the affects of time and the energy of the system )

Diagenesis

• Compaction

• Cementation

--- silica cement (quartz)

--- carbonate cement (calcite common)

--- iron oxide cement (hematite,goethite)

calcite

Of course, calcite is not cubic. The carbonate groups break up the cubic symmetry in several ways. First, their three-fold symmetry axes line up with only one of the symmetry axes of the cube (in red). Second, they alternate in orientation (shown by the two shades of gray). Most important, the wide spacing of the carbonate groups stretches the atomic planes and distorts the cube into a rhombohedron.

At left is the more familiar rhomobohedral unit cell. The calcium ions have a distorted face-centered cubic arrangement. Recall that a rhombohedron is a cube distorted along one of its three-fold axes. Ions on the nearest face are yellow, with light and dark green denoting ions on planes further to the rear.

Only one CO3 unit is shown, in the center of the rhombohedron. The others would be centered in the middle of each edge.

The radial purple lines are in the plane of the carbonate radical and connect to the midpoints of three edges related by threefold symmetry.

This view shows the complete unit cell with all carbonate radicals. Carbonate radicals centered on the front edges are lightest, those on the rear edges darkest.

This is the cleavage unit cell but it is not a true Bravais lattice cell since not all the radicals point in the same direction.

• with reef-building organisms

Present-Day Reef Community

SEA

Tidal Flat

SEA

Salt Crust

Sabkha

evaporation

• Silled Basin Model for evaporite sedimentation by direct precipitation from seawater– Vertical scale

is greatly exaggerated

Silled Basin Model

19 % remaining - calcite19-9.5 % rem. - gypsum9.5- 4.0 % rem. - halite4.0 and less - K and Mg salts

Order of Crystallization FromEvaporating Seawater

Dolomite

• primary ?

•Most is formed by replacemant of calcite

Evidence for oxygen production and accumulation in the atmospherebegins 3.5 billion years ago

what is this evidence?

1. Banded Iron Formations

a. BIFs are found in (ocean) sediments, and consist of banded layers; red bands are very high in iron oxides, Fe2O3 and Fe3O4 – form when reduced iron combines with molecular oxygen

b. only known sources of O2 in the atmosphere are oxygen-producing photosynthesis and UV reaction with H2O

c. when photosynthesis produced oxygen in the ocean, combined with Fe, producing “rusty rain” down to the ocean floor

d. so these formations are evidence for oxygen-producing photosynthesis in the oceans

e. BIFs occur in the rock record between 3.2 and 2 billion years ago, but then they suddenly disappear

Banded Iron-Formations Red Beds

Billions of years ago

0

1

2

3

4Oldest known microfossils

Schematic showing how BIFs formed, 3.2 – 2 billion years ago