Geology Reference
In-Depth Information
Holocene sea-level rise (Fig.
9.33
, Kim et al.
1999
;
Chang and Choi
2001
; Lim et al.
2004
; Yang et al.
2006b
). The transgressive succession, usually uncon-
formally underlain by either bedrock or pre-Holocene
deposits which are commonly stiff mud (Kim et al.
1999
), consists of three or four depositional units
including the facies of salt-marsh, mud-, mixed-, and
sand flats in an ascending order (units B
1
-B
4
in
Fig.
9.33
). The lowermost unit (B
1
), early Holocene
salt-marsh deposit, is composed of intensively biotur-
bated dark-gray mud with minor sand-rippled lamina-
tion, rich in organic matter, and small plant roots. Unit
B
2
, mudflat deposit, is characterized by intensely
bioturbated dark-gray mud with sporadic rhythmic
lamination, grading conformably both downward
and upward into units B
1
and B
3
. Unit B
3
, mixed-flat
deposit, is composed of dark-gray, moderately biotur-
bated to laminated sandy silt or silty sand with relative
abundance of shell fragments. The uppermost unit B
4
,
sand-flat deposit and unconformably overlying unit B
3
,
is characterized by greenish to olive gray, very fine
to fine sand with slight bioturbation and relatively
well-preserved lamination, typically developing storm-
generated small fining-upward successions with hum-
mocky cross-stratification (Fig.
9.28
).
The whole Holocene coarsening-upward succes-
sion was previously interpreted as the result of the con-
tinual retrogradation of the non-barred tidal flats (Kim
et al.
1999
; Chang and Choi
2001
; Lim et al.
2004
),
whereas the interpretation was questioned by Yang
et al. (
2006b
). They noted that there was generally
relative abundance of tidal creek/channel deposits
(Fig.
9.33c
) and absence of storm- or wave-generated
structures in units B
1
-B
3
. These three units were con-
sequently interpreted to be deposited in a back-barrier
tidal-flat setting, because tidal channels are commonly
rare on the modern open-coast tidal flats of the study
area. As the transgression continued, the former barri-
ers migrated landward over the back-barrier tidal flats,
which thereafter changed into open-coast tidal flats
(Fig.
9.33c
). They become a wave-dominated setting
with a volumetric majority of storm-generated beds
(Yang et al.
2006b
).
Fig. 9.32
Schematic models showing two most common pro-
gradational tidal-flat successions on the open-coast environment
(After Li and Li
1982
; Li et al.
1992
; Dalrymple et al.
2003
)
The progradational fining-upward intertidal-flat suc-
cession commonly continues with a coarsening-upward
succession toward the subtidal flats for the muddy
open-coast tidal flats with a gently smooth intertidal-
subtidal profile (Fig.
9.32a
, Li et al.
1992
). It may also
be underlain by the thick sand deposits when the muddy
intertidal flats prograde over the subtidal sand-ridge
systems like those on the central North Jiangsu coast
(Ren
1985
) and the distributary-mouth bars in the del-
tas (Fig.
9.32b
; Dalrymple et al.
2003
), or the sandy
intertidal flats prograde over the subtidal ridge-runnel
complex (Reineck and Cheng
1978
; Semeniuk
1981
).
9.7.1.2 Retrogradational Open-Coast
Tidal-Flat Successions
The retrogradational sandy open-coast tidal flats com-
monly develop along the coast receiving slight sedi-
ment input, having a concave-up profile with general
coarsening-landward trend of intertidal sediment dis-
tribution except the inner parts behind the swash bars/
ridges (Yang et al.
2005, 2008a
). They produce a trans-
gressive coarsening-upward succession in response to
9.7.1.3 Estuarine-Deltaic Channel Filling
Successions with Tidal Rhythms
A few examples have been reported to have the neap-
spring cycles in recent and Holocene estuarine-deltaic
channels, which lie between the truly open-coast and
