Geol 135 Sedimentation
J Bret Bennington
Updated 12/99

Depositional Basins

Although sediments can be seen accumulating in many places in the modern world, much of this sedimentation is probably transient and will not end up forming layers in the stratigraphic record. Generally, to preserve sediments on a geologic time scale requires deposition within a long-lived, tectonically-generated, depositional basin.

Because tectonic basins develop over millions of years, they can be continually subsiding (creating accomodation space) and can accumulate vast thicknesses of sedimentary rock, far in excess of the actual depth of the basin at any point in time. In fact, it is the subsidence and burial of sedimentary layers within tectonic basins that protects them from the forces of erosion at the surface.

Geosynclines

The existence of vast thicknesses of sedimentary rock has been recognized for a long time. To explain the basins that accumulated these sediments and the folded and faulted mountains that often seemed to have evolved from these basins, geologists originally envisioned the development of large scale folds in the Earth's crust caused by vertical subsidence and uplift. These folds were called geanticlines and geosynclines. It was envisioned that once a geosyncline began to form it would continue to subside, partly due to the weight of sediments accumulating within it. Subsidence would continue until the layers at the bottom of the geosyncline reached metamorphic conditions. Eventually, isostatic rebound would cause the geosyncline to invert into a geanticline, uplifting and folding the once-buried rock.

Geosyncline theory and terminology has been largly supplanted by plate tectonics and basins formation and evolution is now explained by the interactions of lithospheric plates.

Plate tectonic basins

  1. Extensional - related to rifting within and between plates
  2. Convergent - related to plate collisions
  3. Strike-slip - related to transform plate boundaries
Extensional Basins

Extensional tectonics can develop beneath both oceanic and continental crust. Rifting occurs along oceanic plate margins forming mid-ocean ridges. When rifting occurs beneath continental crust a variety of scenarios may unfold.

Mantle plumes concentrate rising heat flow in the mantle at the base of the crust, creating an upwarping of the crust that results in crustal extension along a three-part triple junction. Usually, two of the three rifts continue to develop while extension along the third rift (failed arm) dies out.

Rift basins

Terrestrial rifts start as a series of block faulted rift valleys called horst and graben or half-graben (basins that are deeper on one side than the other). Sediments deposited in the rift graben are initially alluvial, fluvial and aeolian, and subsequently may be lacustrine as large rift lakes form within the closed basins. As extension continues, subsidence of the fault blocks creates new accomodation space which fills with sediments. This process can create thousands of feet of accumulated strata (for example, the Newark basin in eastern North America preserves a 3 mile thick pile of terrestrial strata).

Intracratonic sag basins

If rifting fails along a rift arm prior to the separation of continental crust, then the flow of heat from the mantle is cut off and the partially rifted crust cools and subsides over time, creating an intracratonic sag basin. Such basins tend to be broad, but not very deep and they subside slowly over time. The Mississippi Valley in central North America is an example of this type of basin.

Incipient ocean basins and passive margins

If rifting continues along a rift zone, eventually basaltic crust will form within the rift. As this new oceanic crust cools and subsides the rift will be flooded by the sea and become an incipient ocean basin. Rifting now becomes seafloor spreading and the ocean basin will widen over time.

Along the margin of the new ocean basin the oceanic crust attached to the continental margin continues to cool and subside, downwarping the continental margin. This subsidence creates accomodation space on the continental shelf, resulting over time in a thick sequence of passive margin sedimentation. The southeastern margin of North America preserves a textbook example of a rift-drift passive margin sequence.
 
 

Convergent Margin Basins

In general, these basins are related to collisional plate margins where one plate is being subducted beneath another. Although it seems logical to associate uplift and erosion with plate collision, the process also creates a variety of actively subsiding basins.

Subduction and trenches - This process results from a slab of oceanic crust descending downward into the mantle along an oceanic trench. The descending slab causes partial melting within the rock of the overlying plate, resulting in the formation of a volcanic arc near the trench.

The trench itself is a depositional basin that can accumulate sediments washed oceanward from the continental margin or volcanic arc of the overriding plate. Also, as the descending plate is subducted, slabs of oceanic sediment can be scraped off and thrust faulted against the margin of the overriding plate (obduction), forming an accretionary prism.
 
 
 
 

Forearc basins

These basins form between the subduction zone and the volcanic arc and their size depends primarily on the angle of subduction of the descending slab. Subsidence is due to the weight of the accretionary prism and the sediment load of the basin.

Backarc basins

If the angle of subduction is steep, the crust behind the volcanic arc will come under tension and the hot, weak crust near the volcanic arc will rift, forming a backarc basin. As extension continues, oceanic crust will be erupted at the rift, forming a small ocean basin.

Retroarc foreland basins

In the case of western South America, compression of the overriding plate bearing South America has resulted in crustal thickening and shortening, forming the Andes Mountains. Thickening has caused masses of rock to move eastward along thrust faults, loading the lithosphere on the eastern margin of the Andes and causing downward flexure of the crust to form a basin.

Peripheral foreland basins

During continent-continent collisions, an orogenic belt develops as crust thickens at the collision zone. Thrust-faulted blocks move outward from the suture zone onto the crust of the foreland region, loading it and causing it to flex and subside.

Initially, as the thin crust of the continental margin is thickened to normal depth, topography in the orogenic belt is subdued and erosion does not generate much sediment. At this early stage in the orogeny, subsidence of the foreland outpaces sediment supply and the basin rapidly deepens. As the crust continues to thicken in the orogenic belt, mountains rise up and begin shedding sediments into the foreland basin. Sediment supply rate catches up with and finally overtakes subsidence rate, causing the basin to fill with sediment. A typical foreland basin stratigraphic sequence is a thick, coarsening upward succession, with a thin base of shallow water deposits, overlain by deep water shales and turbidites that shallow into siltstones and sandstones, capped by deltaic and fluvial continental deposits.

Across the foreland basin, the sedimentary fill coarsens and thickens overall toward the source in the highlands. The overall mass of clastic deposits accumulated in the foreland basin is often called a clastic wedge.
 
 

Transform Fault Basins

Strike-slip motion along transform faults can produce relatively small regions of extension that result in sedimentary basins. These are most common where transform faults are curved or where they branch or terminate. The Dead sea and the Sea of Galilee are both examples of transform fault basins.