The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin


Chapter 5 - page 7

The Sedimentary Archives


Sedimentary Structures

Sedimentary structures are larger features which form during (or shortly after) deposition of the sediment, but before lithification.

Some sedimentary structures are created by the water or wind which moves the sediment. Other sedimentary structures form after deposition — such as footprints, worm trails, or mudcracks.

Sedimentary structures can provide information about the environmental conditions under which the sediment was deposited; some structures form in quiet water under low energy conditions, whereas others form in moving water or high energy conditions.

  1. Stratification (= layering or bedding) is the most obvious feature of sedimentary rocks. The layers (or beds or strata) are visible because of differences in the color or texture of adjacent beds.

    Strata thicker than 1 cm are commonly referred to as beds.

    Thinner layers are called laminations or laminae.

    The upper and lower surfaces of these layers are called bedding planes.

  2. Graded bedding results when a sediment-laden current (such as a turbidity current) begins to slow down. The grain size within a graded bed ranges from coarser at the bottom to finer at the top. Hence, graded beds may be used as "up indicators."
  3. Graded bedding
    Graded bedding

  4. Cross-bedding or cross-stratification is an arrangement of beds or laminations in which one set of layers is inclined relative to the others. The layering is inclined at an angle to the horizontal, dipping downward in the downcurrent direction. Hence, cross-beds may be used as indicators of ancient current directions.

    Cross-beds usually curve at the bottom edge, becoming tangent to the lower bed surface. The upper edge of individual inclined cross-beds is usually at a steep angle to the overlying bedding plane. Hence, cross-beds may also be used as "up indicators."

  5. Cross-stratification Zion
    Cross-bedding or cross-stratification. Cross bedding. Zion National Park, Utah.
    Public domain image courtesy of National Park Service.

    Cross-bedded Permian Sandstone
    Cross-bedded Permian Sandstone, northeastern Arizona.

  6. Ripple marks are undulations of the sediment surface produced as wind or water moves across sand.

    Symmetric ripple marks are produced by waves or oscillating water.

    Asymmetric ripples form in unidirectional currents (such as in streams or rivers). Asymmetric ripples have a steep slope on the downstream side, and a gentle slope on the upstream side. Because of this unique geometry, asymmetrical ripples in the rock record may be used to determine ancient current directions or paleocurrent directions.

  7. Diagram of symmetrical ripple marks and asymmetrical ripple marks
    A. Symmetric ripple marks or wave ripples.
    B. Asymmetric ripple marks or current ripples.

    Modern asymmetric ripple marks
    Modern asymmetric ripple marks.

  8. Mud cracks are a polygonal pattern of cracks produced on the surface of mud as it dries. The mud polygons between the cracks may be broken up later by water movement, and redeposited as intraclasts (particularly in lime muds).
  9. mudcracks

    Modern and ancient mudcracks
    Modern and ancient mudcracks. Photos courtesy of Pamela Gore.

  10. Scour marks are depressions or erosional features formed as a current flows across a bed of sand.

    Sediment may be deposited over the scoured layer, filling the depressions. When the overlying sediment becomes consolidated, you can see positive-relief casts on the base of the overlying bed. These casts are termed "sole marks," because they appear on the bottom (or sole) of a bed of sediment.


Determining "up direction"

You should examine carefully the sedimentary structures in any dipping sedimentary sequence, because the rocks can be overturned by tectonic forces, and what initially appears to be younger because it is on top, may in fact turn out to be at the bottom of the section!

Sedimentary structures can be used to determine "up direction."

Sedimentary structures such as graded beds, cross beds, mudcracks, flute marks, symmetrical (but not asymmetrical) ripples, stromatolites, burrows, tracks, and others can be used to establish the original orientation of the beds. Features which can be used to determine "up direction" are called geopetal structures.

Fossils can also be used to establish up direction, if they are present in the rock in life position.


Indicators of "which way is up" in sedimentary rocks

 


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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

September 17, 2005