approximated by a logarithmic function. The outer turbulent layer takes up a majority (80–90 %) of the BBL. Flow
characteristics of this layer mainly depend on the velocity
difference with the external free flow (i.e., velocity defect)
and the overall scale of the boundary layer.
Wind waves affect the BBL by imposing a wave boundary layer wherever the water depth is less than half of the
wave length. The wave-induced oscillatory water motion is
affected by the sea bottom within the wave boundary layer.
However, in a natural continental shelf, any definition
of the BBL structure is not straightforward due to the
influences of many factors (e.g., density stratification,
internal waves, seabed topography). A practical indicator
for the BBL is a thermohaline pycnocline observed in
the water column (e.g., Stips et al., 1998; Perlin et al.,
2005). Just above the sea bottom, there is a homogenous
layer of temperature, salinity, and density, which indicates
the inner layer and the transitional layer.
Bibliography
Grant, W. D., and Madsen, O. S., 1986. The continental-shelf bottom boundary layer. Annual Review of Fluid Mechanics, 18,
265–305.
Nielsen, P., 1992. Coastal bottom boundary layers and sediment
transport. In Series Editor-in-Chief Liu, Philip L-F. (ed.),
Advanced Series on Ocean Engineering. World Scientific Publishing Co. Pte. Ltd., Singapore, Vol. 4.
Perlin, A., Moum, J. N., and Klymak, J. M., 2005. Response of the
bottom boundary layer over a sloping shelf to variations in
alongshore wind. Journal of Geophysical Research, 110,
C10S09, doi:10.1029/2004JC002500.
Prandtl, L., 1905. Verhandlungen des dritten internationalen
Mathematiker-Kongresses in Heidelberg 1904, Krazer, A.
(ed.), Leipzig: Teubner, p. 484. English trans. Ackroyd,
J. A. K., Axcell, B. P., Ruban, A. I. (eds.) 2001. Early Developments of Modern Aerodynamics. Oxford: ButterworthHeinemann, p. 77.
Stips, A., Prandke, H., and Neumann, T., 1998. The structure and
dynamics of the Bottom Boundary Layer in shallow sea areas
without tidal influence: an experimental approach. Progress in
Oceanography, 41, 383–453.
Cross-references
Sediment Dynamics
Sediment Transport Models
BOUMA SEQUENCE
Thierry Mulder
1 and Heiko Hüneke
2
1
University of Bordeaux, Talence, France
2
Institute of Geography and Geology, Ernst Moritz Arndt
University, Greifswald, Germany
The Bouma sequence (named after Arnold H. Bouma,
1932–2011) is a characteristic set of sedimentary
structures typically preserved within positively graded
sand or silt-mud couplets. From base to top,
Bouma (1962) differentiated the following intervals above
an erosion surface or sharp boundary: (Ta) massive to
graded sand, (Tb) plane-parallel laminated sand,
(Tc) cross-laminated sand and silt, (Td) parallellaminated sand to silt, and (Te) laminated to
homogeneous mud (Figure 1). Because of nonuniform
grain size distribution and flow transformations
(Fisher, 1983), the complete sequence is rare.
Turbidite beds represent the typical deposit of
low-concentration turbidity flows and related
non-cohesive density flows.
The Bouma sequence is the first model of sedimentladen gravity flows and represents the first predictive
model in sedimentology. It is a facies model of combined
suspension fallout and traction deposition by a bipartite
density flow (see Mulder, 2011, for details). The successive divisions with typical sedimentary structures display
a bottom-to-top decline in energy consistent with the
grading.
The basal surface of many turbidites, i.e., the lower
bounding surface of Ta, commonly displays erosional features produced by turbulent scouring (Lanteaume et al.,
1967).
Turbidity flows commonly develop from stratified
density flows with a strong vertical velocity
gradient. Within such bipartite flows (basal laminar flow
and a top turbulent flow), the high particle
concentration of its basal parts hinders suspension
fallout (Mulder, 2011). Consequently, a poorly graded
Ta division can be interpreted as being deposited
from a concentrated-flow basal part. Rapid
deposition and resulting unstable initial grain packing
are also indicated by the common occurrence of waterescape structures such as dish or pillar structures or
dewatering pipes.
The Bouma divisions Tb to Te record the passage of
the flow body with a fully turbulent regime and
reflect flow deceleration (Walker, 1965). The parallel
lamination (Tb division) results from plane-bed
transport of sand in the upper flow regime. The ripple
cross-lamination (Tc division) reflects settling of
sand and silt from suspension while lower-flowregime current ripples migrate on the seabed. Climbingripple cross-lamination and convolute lamination
would indicate rapid fallout and short-lived liquefaction,
respectively. The Tc division is the most common structure in turbidite beds because ripples are the easiest structures to form for a given grain size and velocity/flow
energy.
The uppermost divisions (Td and Te) are mainly products of settling from suspension. The pelitic top
(Te division) represents the interaction of ongoing pelagic
production with the fine terrigenous particle fallout from
the turbulent tail of the flow.
68
BOUMA SEQUENCE
characteristics of this layer mainly depend on the velocity
difference with the external free flow (i.e., velocity defect)
and the overall scale of the boundary layer.
Wind waves affect the BBL by imposing a wave boundary layer wherever the water depth is less than half of the
wave length. The wave-induced oscillatory water motion is
affected by the sea bottom within the wave boundary layer.
However, in a natural continental shelf, any definition
of the BBL structure is not straightforward due to the
influences of many factors (e.g., density stratification,
internal waves, seabed topography). A practical indicator
for the BBL is a thermohaline pycnocline observed in
the water column (e.g., Stips et al., 1998; Perlin et al.,
2005). Just above the sea bottom, there is a homogenous
layer of temperature, salinity, and density, which indicates
the inner layer and the transitional layer.
Bibliography
Grant, W. D., and Madsen, O. S., 1986. The continental-shelf bottom boundary layer. Annual Review of Fluid Mechanics, 18,
265–305.
Nielsen, P., 1992. Coastal bottom boundary layers and sediment
transport. In Series Editor-in-Chief Liu, Philip L-F. (ed.),
Advanced Series on Ocean Engineering. World Scientific Publishing Co. Pte. Ltd., Singapore, Vol. 4.
Perlin, A., Moum, J. N., and Klymak, J. M., 2005. Response of the
bottom boundary layer over a sloping shelf to variations in
alongshore wind. Journal of Geophysical Research, 110,
C10S09, doi:10.1029/2004JC002500.
Prandtl, L., 1905. Verhandlungen des dritten internationalen
Mathematiker-Kongresses in Heidelberg 1904, Krazer, A.
(ed.), Leipzig: Teubner, p. 484. English trans. Ackroyd,
J. A. K., Axcell, B. P., Ruban, A. I. (eds.) 2001. Early Developments of Modern Aerodynamics. Oxford: ButterworthHeinemann, p. 77.
Stips, A., Prandke, H., and Neumann, T., 1998. The structure and
dynamics of the Bottom Boundary Layer in shallow sea areas
without tidal influence: an experimental approach. Progress in
Oceanography, 41, 383–453.
Cross-references
Sediment Dynamics
Sediment Transport Models
BOUMA SEQUENCE
Thierry Mulder
1 and Heiko Hüneke
2
1
University of Bordeaux, Talence, France
2
Institute of Geography and Geology, Ernst Moritz Arndt
University, Greifswald, Germany
The Bouma sequence (named after Arnold H. Bouma,
1932–2011) is a characteristic set of sedimentary
structures typically preserved within positively graded
sand or silt-mud couplets. From base to top,
Bouma (1962) differentiated the following intervals above
an erosion surface or sharp boundary: (Ta) massive to
graded sand, (Tb) plane-parallel laminated sand,
(Tc) cross-laminated sand and silt, (Td) parallellaminated sand to silt, and (Te) laminated to
homogeneous mud (Figure 1). Because of nonuniform
grain size distribution and flow transformations
(Fisher, 1983), the complete sequence is rare.
Turbidite beds represent the typical deposit of
low-concentration turbidity flows and related
non-cohesive density flows.
The Bouma sequence is the first model of sedimentladen gravity flows and represents the first predictive
model in sedimentology. It is a facies model of combined
suspension fallout and traction deposition by a bipartite
density flow (see Mulder, 2011, for details). The successive divisions with typical sedimentary structures display
a bottom-to-top decline in energy consistent with the
grading.
The basal surface of many turbidites, i.e., the lower
bounding surface of Ta, commonly displays erosional features produced by turbulent scouring (Lanteaume et al.,
1967).
Turbidity flows commonly develop from stratified
density flows with a strong vertical velocity
gradient. Within such bipartite flows (basal laminar flow
and a top turbulent flow), the high particle
concentration of its basal parts hinders suspension
fallout (Mulder, 2011). Consequently, a poorly graded
Ta division can be interpreted as being deposited
from a concentrated-flow basal part. Rapid
deposition and resulting unstable initial grain packing
are also indicated by the common occurrence of waterescape structures such as dish or pillar structures or
dewatering pipes.
The Bouma divisions Tb to Te record the passage of
the flow body with a fully turbulent regime and
reflect flow deceleration (Walker, 1965). The parallel
lamination (Tb division) results from plane-bed
transport of sand in the upper flow regime. The ripple
cross-lamination (Tc division) reflects settling of
sand and silt from suspension while lower-flowregime current ripples migrate on the seabed. Climbingripple cross-lamination and convolute lamination
would indicate rapid fallout and short-lived liquefaction,
respectively. The Tc division is the most common structure in turbidite beds because ripples are the easiest structures to form for a given grain size and velocity/flow
energy.
The uppermost divisions (Td and Te) are mainly products of settling from suspension. The pelitic top
(Te division) represents the interaction of ongoing pelagic
production with the fine terrigenous particle fallout from
the turbulent tail of the flow.
68
BOUMA SEQUENCE