Sedimentary structures are those large features recorded in the field along the bedding surfaces or within the sediment-body, formed during deposition before consolidation.
Sedimentary structures
Sedimentary structures: Macroscopic three-dimensional features of sedimentary rocks recording processes occurring during deposition or between deposition and lithification. They are probably the most critical means of interpreting sedimentary and post-depositional processes. Their recognition and application are key to defining depositional environments, geological history, and surface processes.
Sedimentary structures function as:
Geopetal structures: indicators of original verticality
Directional structures: indicators of current direction
Identifiers of the agent of transport.
Types of Sedimentary Structures: We recognize two principle types:
Primary sedimentary structures: occur in clastic sediments and produced by the same processes (currents, etc.) that caused deposition. Includes plane bedding and cross-bedding.
Secondary sedimentary structures: are caused by post-depositional processes, including biogenic, chemical, and mechanical disruption of sediment.
As sedimentologists, we care about sedimentary structures because of their wealth of information about the environment of deposition. We will focus on primary sedimentary structures in this lecture; later, we'll go into depth about some chemical and biological structures.
Primary Sedimentary Structures
Plane beddingBedding forms as a direct consequence of Steno's law of lateral continuity, that holds that a unit of sediment will extend laterally to the physical margins of the basin it is filing:
"Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way."
We perceive plane beds because of changes in the composition or grain size of sediment during deposition. This, in turn, reflects changing rates of deposition. Three basic mechanisms can form plane bedding:
Sedimentation from suspension
Horizontal accretion from a moving bedload
Encroachment into the lee of an obstacle.
Our perception of bedding is a function of scale. At the largest scale, successions of undisturbed formations may appear as superposed beds, however at finer scales, these resolve into other sedimentary structures that may not be strictly planar.
When bedding persists at fine scale (< 1cm) is called lamination.
In what depositional environment would one most likely expect to find plane bed laminations?
One possibility
Another
Factors might disrupt fine scale laminations in mudrocks include:
Flocculation of clays - clumping before particles settle
Bioturbation - disturbance by organisms (right)
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