Geology Reference
In-Depth Information
Fig. 10.14
Shear strength as a function of mud content (
a
), porosity
(
b
), wet bulk density (
c
), and dry bulk density (
d
). Note the
clearly defined maximum and minimum shear strengths for any
value of the given parameters, and the increasing range (standard
deviation) in shear strength with decreasing mud content and
porosity, but increasing bulk density (Based on Xu
2000
)
10.4
Depositional Facies
and Sedimentary Structures
or shoreface/ebb-delta/inlet deposits not shown here,
depending on where the cross-section is located relative
to the barrier shoreline. The bulk of the back-barrier
depositional system consists of channel fills and tidal-
flat deposits, the latter getting progressively finer-grained
toward the mainland shore. The land-sea transition is
commonly marked by extensive salt marsh deposits. In
the immediate rear of the coastal barriers, salt marshes
and overwash deposits complete the depositional
sequence. Intercalated brackish-water deposits and
peat horizons may occur up to variable distances from
the mainland shore, indicating temporary sea-level still-
stands or short-lived regressions. Superimposed on
this depositional system is a variety of biofacies com-
prising particular invertebrate animal communities
adapted to exist in particular parts of the system (Frey
and Howard
1969
; Schäfer
1972
; Howard and Frey
1975
; Hertweck
1994
). In addition to depending on the
geographic (climatic) location, the community struc-
ture also depends on the energy gradient (current and
wave exposure), on the tidal gradient (exposure or
immersion period), and on sediment composition,
including organic matter (food resources). Many of
Barrier island depositional systems include a variety of
facies that are intimately related to the morphological
elements illustrated in Fig.
10.4
. In the course of vertical
accretion, the individual facies take on the form of three-
dimensional, interfingering sedimentary units that define
the internal architecture of the depositional system. In
Fig.
10.15
, the most important sedimentary units of
back-barrier tidal flats are illustrated in a schematic
geological cross-section of a Holocene barrier system
(Fig.
10.15
) that is aligned perpendicular to the shore and
cuts through the middle section of a typical barrier island
system of the Wadden Sea. Similar examples can be
found in van Straaten (
1964
), Reineck and Singh (
1980
),
Beets et al. (
1996
), and Vos and van Kesteren (
2000
).
The base of the depositional system is commonly
formed by an erosional unconformity (here above
Pleistocene deposits). In some places, pre-existing and
partly eroded brackish-water deposits and basal peat
can still be found. On the seaward side, the barrier
system commences with shoreface/beach/eolian dune
