a relatively high-velocity condition of the wave backwash
when the depth of the water sheet is less than 1.5 cm. They
appear on the sandy platform when the wave backwash
moves down leaving the area exposed.
Current crescent
These are crescent-shaped (U or V shaped) structures
which widen in the flow direction. They have been shown
to form around dead gastropod or pelecypod shells with
their arms open downslope toward the river direction
(Bhattacharya, 1993). The tapering end of the dead gastropod shells always points downcurrent and in a downslope
direction. Current crescents form around scattered tools of
permeable or impermeable bluff bodies like pellets, shell
fragments, mineral matters, lithic fragments, vegetation
hummock, and decomposable organic matter in the aqueous estuarine environment.
Internal sedimentary structures
These structures in the estuarine environment are only
seen in several trench sections cut through the
mid-channel bar and washover flat sediments during short
periods of their surface exposure at falling water stages.
Internal sections from dugout trenches of mid-channel
bars exhibit large-scale tabular cross-stratifications with
dip directions controlled by the dominating flow. Some
trench sections in the mid-channel bars clearly reveal
penecontemporaneous deformations of sandy laminae
toward the crestal part of the arrested megaripples. The
deformations are marked by slightly disturbed layers with
minor undulations or folding. The megaripples bedding
is followed upward by parallel laminations as internal
structures with distinct tidal bundles characterized by
alternations of sand-mud couplets and sets of horizontal
stratifications. In some occasions, convex-upward
reactivation surfaces are present within cross-bedded
units.
Sedimentary units depicting a number of internal sedimentary structures have been recognized in washover flats
following sequences from bottom to top.
Hummocky cross-stratification
These are low-angle (2–3
) undulating crossstratifications. The lamina sets (15–20 cm thick) are both
concave and convex upward with wavelength of 1–2 m
and height of 6–15 cm. Texturally, the hummocky crossstratifications are comprised of sand-sized particles.
Planar tabular cross-stratification
The planar tabular cross-beds form an isolated set in
between the underlying hummocky cross-stratifications
and the overlying sand-mud laminated units. The foreset
dips landward at an angle of 15–18
and therefore indicates their flood-tidal origin. The mud drapes and mud
laminae punctuated within foreset laminae refer to slackening structure. The planar cross-beds are not laterally
persistent. They grade to flaser beds and farther away
(15–20 m) into parallel-laminated units.
Flaser bedding
These structures are cross-stratified ripple bedding
containing thin streaks of mud in the crests and troughs
of the ripples. Most flaser laminations are the wavy flaser
type of Reineck and Wunderlich (1968) in which the mud
flasers are concave upward when they occupy the crests of
ripples and concave downward in the overlying ripple
crests. The flaser-bedded unit continues laterally for
a length of 4–5 m. The mud flasers are often of several
millimeters in thickness.
Conclusion
Sedimentary structures in estuarine environments have
been utilized for interpreting the hydrodynamic conditions
of their formation. These structures reflect the effects of
macrotidal regimes, moderate wave energy, and longshore
currents (at river mouth bars). The various scales of tidal
cycles involving neap-spring and ebb-flood, together with
wave swash and backwash, impart some modifications in
the bedforms. Internal manifestation of bedforms is the
direct consequence of surface features, and many of these
are interpreted as features of the tidal domain. The morphology of the bedforms of Hugli-Matla estuary in India,
with some exception, closely resembles those described
for the Bay of Fundy, Canada, and Loughor estuary, UK.
It is concluded that the perpendicular-to-shoreline
alignment of bedforms like backwash ripples, swash
marks, rhomboid marks, current crescents, and internal
sedimentary structures of estuarine environments may be
interpreted incorrectly as beach features in the rock record.
Such a misinterpretation may lead to a 90
error in the
mapping of a local palaeo-shoreline at the mouth of an
estuary.
Bibliography
Allen, J. R. L., 1968. Current Ripples: Their Relation to Patterns of
Water and Sediment Motion. Amsterdam: North Holland.
Bhattacharya, A., 1993. Backwash and swash oriented current crescents: indicators of beach slope, current direction and environment. Sedimentary Geology, 84, 139–148.
Collinson, J. D., and Thomson, D. B., 2006. Sedimentary Structures. London: George Allen.
Conybeare, C. E. B., and Crook, K. A. W., 1968. Manual of Sedimentary Structures. Bureau of Mineral Resources, Geology
and Geophysics, Canberra A.C.T. Australia, Bull. No 102.
Dalrymple, R. W., Knight, R. J., and Lambiase, J. J., 1978.
Bedforms and their hydraulic stability relationships in a tidal
environment, Bay of Fundy, Canada. Nature, 275, 100–104.
Das, G. K., and Bhattacharya, A., 2000. Bedform morphodynamics
in the meso-macrotidal Thakuran River of Sunderbans, NE
India. Coastal Zone Management, S.D.M.C.E.T., Dharwad &
IGCP, 367, Special publication number, 2, pp. 101–106.
Davis, R. A. D., 1983. Depositional Systems. Englewood Cliffs:
Prentice-Hall.
Elliot, T., 1983. Facies, sequences and sand-bodies of the principal
clastic depositional environments. In Parker, A., and Sellwood,
B. W. (eds.), Sediment Diagenesis. Dordrecht: D. Reidel Publishing Company, pp. 1–56.
Elliot, I., and Gardiner, A. R., 1981. Ripple, megaripples and sand
wave bedforms in the macrotidal Loughor Estuary, South Wales,
SEDIMENTARY STRUCTURES
571
when the depth of the water sheet is less than 1.5 cm. They
appear on the sandy platform when the wave backwash
moves down leaving the area exposed.
Current crescent
These are crescent-shaped (U or V shaped) structures
which widen in the flow direction. They have been shown
to form around dead gastropod or pelecypod shells with
their arms open downslope toward the river direction
(Bhattacharya, 1993). The tapering end of the dead gastropod shells always points downcurrent and in a downslope
direction. Current crescents form around scattered tools of
permeable or impermeable bluff bodies like pellets, shell
fragments, mineral matters, lithic fragments, vegetation
hummock, and decomposable organic matter in the aqueous estuarine environment.
Internal sedimentary structures
These structures in the estuarine environment are only
seen in several trench sections cut through the
mid-channel bar and washover flat sediments during short
periods of their surface exposure at falling water stages.
Internal sections from dugout trenches of mid-channel
bars exhibit large-scale tabular cross-stratifications with
dip directions controlled by the dominating flow. Some
trench sections in the mid-channel bars clearly reveal
penecontemporaneous deformations of sandy laminae
toward the crestal part of the arrested megaripples. The
deformations are marked by slightly disturbed layers with
minor undulations or folding. The megaripples bedding
is followed upward by parallel laminations as internal
structures with distinct tidal bundles characterized by
alternations of sand-mud couplets and sets of horizontal
stratifications. In some occasions, convex-upward
reactivation surfaces are present within cross-bedded
units.
Sedimentary units depicting a number of internal sedimentary structures have been recognized in washover flats
following sequences from bottom to top.
Hummocky cross-stratification
These are low-angle (2–3
) undulating crossstratifications. The lamina sets (15–20 cm thick) are both
concave and convex upward with wavelength of 1–2 m
and height of 6–15 cm. Texturally, the hummocky crossstratifications are comprised of sand-sized particles.
Planar tabular cross-stratification
The planar tabular cross-beds form an isolated set in
between the underlying hummocky cross-stratifications
and the overlying sand-mud laminated units. The foreset
dips landward at an angle of 15–18
and therefore indicates their flood-tidal origin. The mud drapes and mud
laminae punctuated within foreset laminae refer to slackening structure. The planar cross-beds are not laterally
persistent. They grade to flaser beds and farther away
(15–20 m) into parallel-laminated units.
Flaser bedding
These structures are cross-stratified ripple bedding
containing thin streaks of mud in the crests and troughs
of the ripples. Most flaser laminations are the wavy flaser
type of Reineck and Wunderlich (1968) in which the mud
flasers are concave upward when they occupy the crests of
ripples and concave downward in the overlying ripple
crests. The flaser-bedded unit continues laterally for
a length of 4–5 m. The mud flasers are often of several
millimeters in thickness.
Conclusion
Sedimentary structures in estuarine environments have
been utilized for interpreting the hydrodynamic conditions
of their formation. These structures reflect the effects of
macrotidal regimes, moderate wave energy, and longshore
currents (at river mouth bars). The various scales of tidal
cycles involving neap-spring and ebb-flood, together with
wave swash and backwash, impart some modifications in
the bedforms. Internal manifestation of bedforms is the
direct consequence of surface features, and many of these
are interpreted as features of the tidal domain. The morphology of the bedforms of Hugli-Matla estuary in India,
with some exception, closely resembles those described
for the Bay of Fundy, Canada, and Loughor estuary, UK.
It is concluded that the perpendicular-to-shoreline
alignment of bedforms like backwash ripples, swash
marks, rhomboid marks, current crescents, and internal
sedimentary structures of estuarine environments may be
interpreted incorrectly as beach features in the rock record.
Such a misinterpretation may lead to a 90
error in the
mapping of a local palaeo-shoreline at the mouth of an
estuary.
Bibliography
Allen, J. R. L., 1968. Current Ripples: Their Relation to Patterns of
Water and Sediment Motion. Amsterdam: North Holland.
Bhattacharya, A., 1993. Backwash and swash oriented current crescents: indicators of beach slope, current direction and environment. Sedimentary Geology, 84, 139–148.
Collinson, J. D., and Thomson, D. B., 2006. Sedimentary Structures. London: George Allen.
Conybeare, C. E. B., and Crook, K. A. W., 1968. Manual of Sedimentary Structures. Bureau of Mineral Resources, Geology
and Geophysics, Canberra A.C.T. Australia, Bull. No 102.
Dalrymple, R. W., Knight, R. J., and Lambiase, J. J., 1978.
Bedforms and their hydraulic stability relationships in a tidal
environment, Bay of Fundy, Canada. Nature, 275, 100–104.
Das, G. K., and Bhattacharya, A., 2000. Bedform morphodynamics
in the meso-macrotidal Thakuran River of Sunderbans, NE
India. Coastal Zone Management, S.D.M.C.E.T., Dharwad &
IGCP, 367, Special publication number, 2, pp. 101–106.
Davis, R. A. D., 1983. Depositional Systems. Englewood Cliffs:
Prentice-Hall.
Elliot, T., 1983. Facies, sequences and sand-bodies of the principal
clastic depositional environments. In Parker, A., and Sellwood,
B. W. (eds.), Sediment Diagenesis. Dordrecht: D. Reidel Publishing Company, pp. 1–56.
Elliot, I., and Gardiner, A. R., 1981. Ripple, megaripples and sand
wave bedforms in the macrotidal Loughor Estuary, South Wales,
SEDIMENTARY STRUCTURES
571