Tool Marks
Introduction
Tool marks are small gouge marks on muddy bedding surfaces produced when objects - shells, wood fragments, or other debris - carried by a current make intermittent contact with the bottom. Unlike groove casts, which record continuous dragging, tool marks record brief, discrete impact events. Their shape depends on the angle at which the tool struck and the way it interacted with the surface, giving each type a characteristic and recognisable geometry.
Overview and General Asymmetry
Small gouge marks are produced by tools that make intermittent contact with the bottom, creating small individual marks. Brush and prod marks are asymmetrical in cross-sectional shape, with the deeper, broader part of the mark oriented downcurrent. Bounce marks are roughly symmetrical. Roll and skip marks are formed by a tool bouncing up and down or rolling over the surface to produce a continuous track. [1]
The Five Tool Mark Types
Each type of tool mark reflects a distinct mode of tool-bottom interaction.
Bounce marks form when the tool approaches the sediment surface at a low angle and immediately bounces back into the current. Because the tool hits and leaves at similar angles, the resulting mark is roughly symmetrical in cross-section. [1]
Brush marks form when the tool approaches at a very low angle with its axis inclined upcurrent, then is lifted away by the current. This geometry creates a ridge of mud on the downcurrent side of the mark - the tool skims the surface and pushes sediment forward. [1]
Prod marks form when the tool reaches the surface at a fairly high angle and is then lifted up and away by the current. The steep approach produces a deep, narrow gouge with the deep end pointing downcurrent - the asymmetry is pronounced because the entry angle is steep. [1]
Roll marks form when the tool rolls along the sediment surface, producing a continuous track. Unlike the other types, roll marks are not isolated impact features but continuous impressions. [1]
Skip marks form when a tool travels downcurrent with a saltating movement - bouncing and hitting the surface at nearly regular intervals. The result is a series of regularly spaced impact marks aligned with the current direction. [1]
Paleocurrent Significance
The asymmetric types - brush marks and prod marks - are particularly useful because the deeper, broader end of each mark points downcurrent. This makes them unambiguous indicators of both flow axis and sense, similar to flute casts. Bounce marks, being symmetrical, indicate the flow axis but not its direction. Roll and skip marks record a directional track aligned with the current.
Like groove casts, all tool marks are found as positive-relief casts on the bases of overlying coarser-grained beds, and are most common in turbidite sequences where the conditions of energetic currents and cohesive mud bottoms are both consistently met.
Related Topics
Flute Casts
Flute casts are among the most prized paleocurrent indicators in sedimentary geology - not merely because they show the orientation of flow, but because they show which direction the current was...
Groove Casts
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...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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