Page 78 - Sedimentology and Stratigraphy

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Erosional Sedimentary Structures 65

Fig. 4.33 Syneresis cracks in mudrock, believed to be
Fig. 4.32 Mudcracks caused by subaerial desiccation of mud.
formed by subaqueous shrinkage.
cracks are easily removed by later currents and may that marks the base of the channel. The size of chan-
be preserved as mud-chips or mud-flakes in the nels can range from features less than a metre deep
overlying sediment. Desiccation cracks are most and only metres across to large-scale structures many
clearly preserved in sedimentary rocks when the tens of metres deep and kilometres to tens of kilo-
cracks are filled with silt or sand washed in by water metres in width. The size usually distinguishes chan-
or blown in by the wind. The presence of desiccation nels from other scour features (see below), although
cracks is a very reliable indicator of the exposure of the key criterion is that a channel confines the flow,
the sediment to subaerial conditions. whereas other scours do not.
Syneresis cracks are shrinkage cracks that form Small-scale erosional features on a bed surface are
under water in clayey sediments (Tanner 2003). As referred to as sole marks (Fig. 4.34). They are pre-
the clay layer settles and compacts it shrinks to form served in the rock record when another layer of sedi-
single cracks in the surface of the mud. In contrast to ment is deposited on top leaving the feature on the
desiccation cracks, syneresis cracks are not polygonal bedding plane. Sole marks may be divided into those
but are simple, straight or slightly curved tapering that form as a result of turbulence in the water caus-
cracks (Fig. 4.33). These subaqueous shrinkage ing erosion (scour marks) and impressions formed by
cracks have been formed experimentally and have objects carried in the water flow (tool marks) (Allen
been reported in sedimentary rocks, although some 1982). They may be found in a very wide range of
of these occurrences have been re-interpreted as desic- depositional environments, but are particularly com-
cation cracks (Astin 1991). Neither desiccation mon in successions of turbidites where the sole mark
cracks nor syneresis cracks form in silt or sand is preserved as a cast at the base of the overlying
because these coarser materials are not cohesive. turbidite.
Scour marks Turbulent eddies in a flow erode into
4.7 EROSIONAL SEDIMENTARY the underlying bed and create a distinctive erosional
STRUCTURES scour called a flute cast. Flute casts are asymmetric
in cross-section with one steep edge opposite a tapered
A turbulent flow over the surface of sediment that has edge. In plan view they are narrower at one end,
recently been deposited can result in the partial and widening out onto the tapered edge. The steep, nar-
localised removal of sediment. Scouring may form a row end of the flute marks the point where the eddy
channel which confines the flow, most commonly initially eroded into the bed and the tapered, wider
seen on land as rivers, but similar confined flows edge marks the passage of the eddy as it is swept away
can occur in many other depositional settings, right by the current. The size can vary from a few centi-
down to the deep sea floor. One of the criteria for metres to tens of centimetres across. As with many
recognising the deposits of channelised flow within sole marks it is as common to find the cast of the
strata is the presence of an erosional scour surface feature formed by the infilling of the depression as