Groove Casts
Introduction
Groove casts are the casts of long, narrow scratches left on a muddy seafloor when tools - shells, wood fragments, pebbles - were dragged along the bottom by a current. They are simple, robust paleocurrent indicators: wherever they occur, the long axis of the groove tells you the orientation of the flow. Most groove casts are limited to showing flow axis, not flow direction - but the chevron variant overcomes that limitation.
Description
Groove casts are elongate, nearly straight ridges that result from the infilling of erosional grooves produced by a pebble, shell, piece of wood, or other object being dragged or rolled across the surface of cohesive sediment. [1]
They typically range in width from a few millimetres to tens of centimetres and have relief of a few millimetres to a centimetre or two, though much larger groove casts also occur. Groove casts are greatly elongated relative to their width. [1]
Paleocurrent Significance
Groove casts are directional features oriented parallel to the flow direction of the ancient currents that produced them. Groove casts on the same bed commonly have the same general orientation, though they may diverge at slight angles and even cross. Most groove casts do not indicate which end is upcurrent and which is downcurrent - they show the flow axis but not the sense of flow. [1]
Chevrons: The Directional Variant
Chevrons are a variety of groove casts made up of continuous V-shaped crenulations in which the V points downstream. This geometry makes chevrons unambiguous paleocurrent indicators - unlike ordinary groove casts, they show the true direction of flow, not just its axis. Chevrons are thought to form by tools moving just above the sediment surface without touching it, causing the sides of the groove to be rucked up into the characteristic V pattern. [1]
Occurrence
Groove casts are especially common on the soles of turbidite beds, because shell fragments, pieces of wood, and other tools are carried in the base of turbidity currents and dragged across a mud bottom. They also occur on the soles of beds deposited in shallow-water environments such as tidal flats and floodplains, where floating tools may briefly touch bottom and leave grooves. [1]
The turbidite association is dominant because a turbidity current simultaneously provides the energetic flow needed to entrain and transport tools, the high suspended-sediment load needed to quickly bury the depressions left by those tools, and a cohesive mud substrate that preserves the grooves until they are cast. All three conditions must be met simultaneously, and turbidites meet them almost by definition.
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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