Page 79 - Sedimentology and Stratigraphy

Processes of Transport and Sedimentary Structures
66 Nichols/Sedimentology and Stratigraphy 9781405193795_4_004 Final Proof page 66 26.2.2009 8:16pm Compositor Name: ARaju

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Fig. 4.34 Sole marks found on the bottoms of beds: flute marks and obstacle scours are formed by flow turbulence;
groove and bounce marks are formed by objects transported at the base of the flow.
it is to find the depression itself (Fig. 4.34). The by simply stating the measurements in millimetres,
asymmetry of flute marks means that they can be centimetres or metres. This, however, can be cumber-
used as palaeocurrent indicators where they are pre- some sometimes, and it may be easier to describe the
served as casts on the base of the bed (5.3.1). An beds as ‘thick’ or ‘thin’. In an attempt to standardize this
obstacle on the bed surface such as a pebble or shell terminology, there is a generally agreed set of ‘defini-
can produce eddies that scour into the bed (obstacle tions’ for bed thickness (Fig. 4.35). A bed is a unit of
scours). Linear features on the bed surface caused by sediment which is generally uniform in character and
turbulence are elongate ridges and furrows if on the contains no distinctive breaks: it may be graded
scale of millimetres or gutter casts if the troughs are (4.2.5), or contain different sedimentary structures.
a matter of centimetres wide and deep, extending for The base may be erosional if there is scouring, for
several metres along the bed surface. example at the base of a channel, sharp, or sometimes
gradational. Alternations of thin layers of different
Tool marks An object being carried in a flow over a lithologies are described as interbedded and are
bed can create marks on the bed surface. Grooves are usually considered as a single unit, rather than as
sharply defined elongate marks created by an object separate beds.
(tool) being dragged along the bed. Grooves are shar-
ply defined features in contrast to chevrons, which
form when the sediment is still very soft. An object
saltating (4.2.2) in the flow may produce marks F !! @ *
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known variously as prod, skip or bounce marks at
the points where it lands. These marks are often seen
in lines along the bedding plane. The shape and size of
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all tool marks is determined by the form of the object
which created them, and irregular shaped fragments,
such as fossils, may produce distinctive marks.
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4.8 TERMINOLOGY FOR SEDIMENTARY 7 ! @
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STRUCTURES AND BEDS
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When describing layers of sedimentary rock it is useful
to indicate how thick the beds are, and this can be done Fig. 4.35 Bed thickness terminology.