Topset Bed

Definition - What does Topset Bed mean?

A Topset Bed is a horizontal layer of coarse sand and gravel deposited on top of a river delta. The topset beds of an advancing delta are deposited in turn over the previously laid foresets, truncating or covering them. Topsets are nearly horizontal layers of smaller-sized sediment deposited on the top of the delta and form an extension of the landward alluvial plain. As the river channels meander laterally across the top of the delta, the river is lengthened and its gradient is reduced, causing the suspended load to settle out in nearly horizontal beds over the delta's top.

Petropedia explains Topset Bed

When a river meets the standing water of an ocean at a coast, it quickly loses velocity and the heaviest particles drop out. The fine suspended load may be carried farther out into the water before it settles out and sinks to the bottom. Sediments deposited in a delta are laid down in layers known as beds. Bottomset beds are those nearly horizontal or flat layers of fine clay and silt that form underwater far from the mouth of the river. Closer to the mouth, yet still underwater, are foreset beds of sand and gravel that slope steeply down toward the bottomset beds at an angle up to 25 degrees. Thin, horizontal layers of coarser sand and gravel that are deposited on the surface of the delta are topset beds. As a delta increases in size and advances farther out into the water, the topset beds cover the foreset beds, which in turn cover the bottomset beds. Topset beds can be further subdivided into:

  • The upper delta plain region
  • The lower delta plain region
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