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T ep
Normal pelagic fallout
Clays settling from suspension
up to several days afterwards
T et
T d
Upper parallel laminations
impact mark
drill holes and
comma marks
sinuous
grooves
Silty or sandy ripples, wavy
and convoluted lamination
T c
T b
Parallel laminations
muschelbr ü che
sichelwanne
V-shaped
groove
T a
Massive graded sands or
gravels
Basal eroded sole marks
pothole
hummocky
topography
Fig. 3.16 A Bouma turbidite sequence deposited on the seabed
following the passage of a turbidity current. Based on Bouma and
Brouwer ( 1964 )
trough
cavettos
3.3
Erosional Signatures of Tsunami
Fig. 3.17 Various cavitation features and S-forms produced by high-
velocity tsunami flow over headlands. Note that the features are not
scaled relative to each other
3.3.1
Small-Scale Features
Tsunami can also sculpture bedrock in a fashion analogous
to the S-forms produced by high-velocity catastrophic
floods or surges from beneath icecaps in sub-glacial envi-
ronments (Dahl 1965 ; Ljungner 1930 ). S-forms include
features such as muschelbrüche, sichelwannen, V-shaped
grooves, cavettos, and flutes (Figs. 3.2 and 3.17 ). They have
been linked to paleo-floods in Canada (Kor et al. 1991 ), the
northwestern United States (Baker 1978 , 1981 ), Scandina-
via (Dahl 1965 ), Scotland (Hall 1812 ), the Alps (Alexander
1932 ; Dahl 1965 ), and the Northern Territory, Australia
(Baker and Pickup 1987 ). Tsunami flow over rocky head-
lands, at the velocities outlined in Chap. 2 , has the hydro-
dynamic potential to generate cavitation or small vortices
capable of producing sculptured forms (Bryant and Young
1996 ). The spatial organization of S-forms on headlands,
often above the limits of storm waves, is a clear signature of
tsunami in the absence of any other definable process.
Individual S-forms and their hydrodynamics will be
described in this chapter, while their spatial organization
into unique tsunami-generated landscapes will be discussed
in Chap. 4 .
Cavitation is a product of high-velocity flow as great as
10 m s -1 in water depths as shallow as 2 m deep (Dahl
1965 ; Baker 1981 ). At these velocities, small, low pressure,
air bubbles appear in the flow. These bubbles are unstable
and immediately collapse, generating impact forces up to
thirty thousand times greater than normal atmospheric
pressure. Cavitation bubble collapse is highly corrasive, and
is the reason why propeller-driven ships cannot obtain
higher speeds and dam spillways are designed to minimize
flow depth and velocity. In tsunami environments, cavita-
tion produces small indents that develop instantaneously,
parallel or at right angles to the flow, on vertical and hor-
izontal bedrock surfaces. Cavitation features are widespread
and
consist
of
impact
marks,
drill
holes,
and
sinuous
grooves (Figs. 3.2 and 3.17 ).
Impact marks appear as pits or radiating star-shaped
grooves on vertical faces facing the flow. It would be simple
to suggest that such features represent the impact mark of a
rock hurled at high velocity against a vertical rock face;
however, such marks have also been found in sheltered
positions or tucked into undercuts where such a process is
unlikely (Fig. 3.18 ). Drill holes are found over a range of
locations on tsunami-swept headlands. Their distinguishing
characteristic is a pit several centimeters in diameter bored
into resistant bedrock such as tonalite or gabbroic diorite.
Drill holes appear on vertical faces, facing either the flow or
 
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