6.16: Sedimentary Structures Preserved in Bedding

Sedimentary structures preserved in bedding

Sedimentary deposits (including sediments and sedimentary rocks) commonly preserve evidence of how they were deposited. Anyone who has been to the beach or a sand dune area have seen ripple marks created by the movement of sand under the influence of wind or water. Listed below are examples of sedimentary structures preserved in bedding of ancient sedimentary rocks. The processes that created them are the same that can be observed occurring today.

• ripple marks—a series of small ridges produced in sand by water currents or by wind (Figure 6.59).
• cross bedding—inclined sedimentary structures in a horizontal unit of rock. These tilted structures are deposits from bedforms such as ripples and dunes, and they indicate that the depositional environment contained a flowing fluid (typically, water or wind) (Figure 6.60 and 6-61).
• desiccation cracks—mudcracks; irregular fracture formed by shrinkage of clay, silt, or mud under the drying effects of atmospheric conditions at the surface (Figure 6.58).
• graded bedding—bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Large fragments tend to settle out fastest from a slowing turbulent flow.
• biological structures—many kinds of organisms burrow or bore into sediments creating holes for feeding or for shelter (or both). Most marine sedimentary beds preserve bioturbation features - bioturbation means "churning of the sediments" as organisms, typically worms, shrimp, and other invertebrates work through the sediments to eat decaying organic mater (or other organisms feeding there). They also use the burrows as shelter or nesting site. Very often the traces are preserved as structures in the sediment. Trackways, burrows, or resting sites are also common structures preserved in marine sediments.

Figure 6.59. Ripple marks on sand dune sand in water deposits form from current flow (air or water)	Figure 6.60. Cross bedding in ancient sand dune deposits Zion National Park, Utah	Figure 6.61. Formation of cross bedding caused by the migration of ripples or dunes	Figure 6.62. Desiccation mud cracks in Precambrian rocks, Grand Canyon, Arizona

Turbidity Currents and Development of Submarine Canyons and Fans

A turbidity flows is a turbid, dense current of sediments in suspension moving along downslope and along the bottom of a ocean or lake. In the ocean, turbidity currents can be massive episodic events. They typically form and flow down through a submarine canyon (carved by previous turbidity flows) and accumulate near the base of the continental slope on deep-sea fans. Turbidity flows produces deposits showing graded bedding (Figure 6.63 and 6-64). Slowing turbid currents drop their coarser fractions first (gravel and sand) and the finer silt and clay fractions settle out last.

Figure 6.63. Appearance and example of graded bedding in sedimentary deposits. Graded beds will "fine-upward" as currents slow down. They may "coarsen upward" if the energy of the depositing flow (current) increases.	Figure 6.64. Turbidity currents flow down slope under water under the influence of gravity. At peak flow, turbidity currents will scour the seabed, but as flow slows and stops, coarse sediments are deposited first, and finer material last.