Wednesday, March 31, 2010

Accommodation Space

Key Concept:  Accommodation Space

Accommodation space is the area available for sediment to be deposited, usually the space between the sedimentary surface and sea level.  This concept is very important because it provides the intellectual context for connecting depositional environments to relative sea level change.  Tracking lateral and temporal changes in depositional environments and using them to recreate accommodation allows one to interpret relative sea level changes and thus use sequence stratigraphy.  To a first order approximation, accommodation space depends on global sea level, subsidence, and sediment supply. 

Sediment Deposition
Sediments are deposited in areas with low flow speed relative to neighboring environments. The relative flow speeds in an environment are most important for deposition.  Sediment can accumulate in a high energy environment if it is next to an even higher energy environment.  For example, along a wave-washed coast line, promontories are very high energy and bays between them are medium high energy.  Sediment erodes from the promontories and is deposited in the bays (and washed off shore).
Rivers are the major source of sediment transport on land and sediment supply to the oceans (except carbonate platforms.)

Base Level (the elevation at the end of a river)
Base level is the point below which the river can’t erode.

Rivers have equilibrium profiles where the volumes of sediment coming into each segment of the river and leaving it are about the same; the same amount of sediment gets deposited as gets eroded on average.  Areas above the equilibrium profile tend to be eroded.  Areas below the equilibrium profile tend to have deposition.  Over long periods of time, areas at the equilibrium grade also experience net erosion, which tends to flatten the overall grade through time.  Rivers can only sense the local profile or flow speed gradients, so there are lots of changes required for an entire river to reach an equilibrium gradient.

As base level changes, the equilibrium gradient shifts up (increase in base level) and down (decrease in base level), changing where sediments are eroded and deposited.

Sediment Supply
Sediment supply depends on: 1) paleogeography, e.g. watersheds, transport processes, topography, etc. that are capable of feeding sediment to an area; and 2) climate, which influences the production of sediments through weathering, and the transport of sediment through water run-off volume.  For carbonates, it depends on ocean chemistry and ecology.

Sea Level
There are 3 different ways that we think about sea level: 1) global sea level (eustacy) measured relative to something like Earth’s geoid; 2) relative sea level which is measured relative to something in a basin; and 3) water depth, which is the distance from the sediment surface to the water surface at a point in a basin.

Eustacy (global sea level change)
Eustacy depends on several factors.  The volume of ocean basins affects global sea level and depends on continental distributions, average age of oceanic crust (which depends on rates of sea floor spreading), and the amount of sediment deposition in ocean basins.  Changes in the distribution of continents affected eustacy by at most 0.1 mm/kyr during Phanerozoic time. In contrast, the average age of oceanic crust caused a maximum of ~ 7 mm/kyr fall in sea level since Late Cretaceous time. Changes due to variations in sediment deposition are on the order of a few mm/kyr in sea level.

Changing the amount of water stored on continents is a much faster way of changing global sea level.  Water can be stored as ice, in lakes, or as groundwater.  Ice is one of the most important storage mechanisms, and changes in ice volume can cause sea level changes of up to 180 m on orbital periods of about 100,000, 40,000, and 20,000 years.  Rapid ice volume changes result in meters of global sea level change per thousand years.  Melting of the Antarctic ice sheet would raise sea level by 60-75 m, whereas melting of the Greenland ice sheet would raise sea level by 5 m.  Sea level changes associated with Pleistocene ice sheets are on the order of 150 m.  Because these changes are commonly driven by Earth’s orbital parameters, ice-related sea level changes are commonly cyclical, which is very useful for interpretations of basins. 

Lakes can also store significant amounts of water.  When the Mediterranean Sea dried up, it caused a ~12 m global sea level rise.  The amount of ground water is poorly constrained, but I expect that long term changes could measureably affect sea level.

Relative Sea Level

Relative sea level as observed within a basin is a combination of eustatic sea level plus the influences of subsidence or uplift measured from some reference point like the basement-sediment contact.  Relative sea level is the main control on accommodation space.  As it goes up, more accommondation space is created, and accommodation space is removed as relative sea level goes down.  Sediment supply influences how much of the accommodation space is filled.  Compaction of sediment and loading of the basement (which increases subsidence) also increase accommodation space.

Water Depth
Water depth is the amount of water above the sediment-water interface.  It varies by location in the basin, and depends on relative sea level and sediment supply.  It tracks how much accommodation space is filled and where in the basin.

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