4.4: Erosional and Post-Depositional Structures
Erosional Structures
Scours and Channels
Channels form where flowing water concentrates and erodes the underlying sediment. Eventually the sediment being carried by the river fills the channel and the river avulses to another location on the floodplain. Although we commonly think of channel deposits as being simple u-shaped features, their morphology is a function of the type of river, the orientation of the outcrop relative to paleoflow direction, and whether or not the older channel deposit was modified by younger ones. We will get into some of these differences when we start talking about depositional environments later in the course.
We are not aware of a strict size range applied to channels. But generally speaking, many people use the term scour to refer to small, nondescript erosional features within floodplain or beach sediments indicating a simple cut-and-fill history.
Groove Casts and Tool Marks
Objects drug by a moving current can make a variety of indentations into cohesive substrate. Groove casts refer to elongate linear features caused by dragging and tool marks refer to less linear features formed by rolling or bouncing along the sediment surface. If sand later blankets the sediment surface these marks may be infilled and preserved on the base of the overlying sandstone.
Flute Casts
Flutes form where a turbulent vortex erodes into muddy sediment. It forms a depression that is blunt upstream and tapers downstream. If later infilled with sand, they can be preserved as flute casts that stand in relief on the base of a bed of sandstone. They are useful for determining paleoflow direction because the blunt end faces upstream.
Obstacle Scours
Where you have a fixed obstruction (rock, tree, etc) a moving current will commonly scour a deep hole around the side and upstream edge of the feature forming a u-shaped depression that opens downstream. Although not terribly common, it is possible to preserve these obstacle scours in the geologic record.
Soft-Sediment Deformation Structures
The sedimentary structures described above were all “primary” sedimentary structures in that they record conditions at or near the time that the sediment was deposited. The structures described below are all "secondary" structures in the sense that they show modification of the sediment after it was deposited but before it underwent diagenesis (the process that transforms sediment to rock).
Flame Structures
When water-saturated mud is overlain by more dense sand, compaction and/or a slight disturbance can cause flame structures which are tongues of mud to inject upward into the overlying sand
Load Casts
Load casts (aka ball-and-pillow structures ) are sandy lobes of mud that can form when dense mud settles down into water-saturated mud as a result of compaction or disturbance. They may be be connected to source bed or may be isolated from it. Superficially they can appear similar to flute casts but they lack the pronounced blunted edge.
A wide variety of other types of nameless synsedimentary deformation features can form when shaking or loading of water-laden muds can cause overall chaotic bedding in rocks. The key to distinguishing these features from those formed as a result of tectonic forces is that over and underlying beds are flat-lying and relatively undeformed.
Desiccation cracks
As mentioned above, mud can be 70% water. When it dries out it contracts forming polygonal desiccation cracks (aka mudcracks ). These features can be preserved as polygonal features on bedding planes or as downward tapering wedges if viewed in cross section.
Raindrop Impressions
When raindrops fall onto fine-grained sediment they make small depressions or craters on the top of that sediment. These features can be preserved either as depressions on the top of the fine-grained sediment that they fell on or as bumpy casts on the base of the overlying bed.
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Figure \(\PageIndex{6}\): A) Oblique view of modern desiccation cracks atop a tidally-influenced point bar in the Salmon River near Truro, Nova Scotia. B) Slightly oblique bedding plane view of green reduced zones associated with desiccation cracks in the Pennsylvanian Clifton Formation, New Brunswick. C) Cross-sectional view of mudcracks in the Mississippian Mabou Group, Nova Scotia. D) Dessication crack preserved as a cast on the base of a sandstone, Inverness Formation (Pennsylvanian), Cape Breton Island, Nova Scotia. E) Modern raindrop impressions on a tidally-influenced point bar in the Salmon River near Truro, Nova Scotia. F) Fossilized raindrop impressions on the top of a wave-rippled sandstone from the Horton Bluff Formation (Mississippian), near Avonport, Nova Scotia. All images by
Michael C. Rygel
via
Wikimedia Commons
;
CC BY-SA 3.0
or
CC BY-SA 4.0
).