Sedimentology/Stratigraphy (GEOL-407)

- Glossary of Sedimentary Structures -

Note: I have indicated the student author/compiler for each entry by including their initials in brackets after the glossary terms. SL = Steve Lehner.

Oscillation Ripples [SL]

Oscillation ripples form in sandy beds near the shores of lakes, and near
shore in the ocean, in depths up to 200 meters. Here the current alternates
in direction due to waves washing back and forward. Symmetrical ripples
form when the current is equally strong in both directions. They have either
broad flat troughs or rounded troughs.  Symmetrical ripples can be up to a meter
and a half long and tend to have long slightly wavy crests. They can be as
tall as 25 cm (The Encyclopedia of Sedimentology). Asymmetrical ripples
resemble straight ripples formed in unidirectional current. They indicate
alternating waves of unequal strength. Ripple size tends to increase with
increasing coarseness of sand. However wave intensity, water depth and
other factors influence ripple dimensions.

Herring Bone Cross Bedding [SL]

Cross beds form as sand dunes or ripples migrate due to wind or water
currents. Sand grains saltate up the windward or upstream side of the dune
and collect at the crest creating a current shadow downstream or downwind. Here
fine sediment settles out of the water or air on the leeward side of the
dune creating a dark layer. When the crest grows too large and collapses, the
dark layer is covered by a thicker layer of sand creating an individual cross
bed. The process then repeats. Herringbone cross bedding forms when current
periodically flows in opposing directions such as in a tidal flat or tidal
channel. The result is alternating layers of cross beds dipping in opposite
directions. The pattern created resembles the backbone and ribs of a
herring or fish; hence the name.

Lenticular Bedding [SL]

Lenticular bedding consists of lense-shaped deposits of sand interbedded
with mud which has been lithified into sandstone and shale respectively. It
is believed that it forms in tidal and delta front environments that
fluctuate between high and low energy. The lens shaped deposits of sand are
actually discontinuous crests of ripples, presumably deposited during the
high energy events, which lack sufficient sand to form complete ripples. Ripple
cross lamination is sometimes evident in well preserved examples. In environments
where there is more sand, complete ripples are interspersed with mud in
their troughs. Known as flaser ripples, they grade into wavy ripples, and then
into the starved ripples or 'lenses' of lenticular bedding as the ratio of mud
to sand in the depositional environment shifts in favor of mud.
 

Gutter Casts [SL]

At the time of deposition beds of soft mud and sandy material are subject
to deformation by a variety of methods. Eddies in swift currents can leave
scour marks in the form of channels, grooves, and gutters. These depressions are
usually steeper and wider where they begin due to the swirling current and
then taper off in the downstream direction. Rocks, sticks, and other
objects, referred to as tools, also make indentations as they are tossed about in
the current.  These low places are subsequently filled in with sediment that is
often of different texture than the surrounding mud or sand. After burial
and lithification this sediment fill often leaves a positive impression of the
original gutter, or depression. Usually visible in cross section sometimes
these casts are exposed when attached to an overlying bed. Thus they are
called sole marks. Sole marks can be important indicators of paleo-current
direction. Gutter casts and scour fill channels are common in storm
dominated shelf environments as well as fluvial settings.