Geology Reference
In-Depth Information
Fig. 10.15
Schematic geological cross-section through a
transgressive barrier island depositional system as exemplified
by the Wadden Sea and showing typically stacked sedimentary
facies relating to particular coastal environments and facies
(Modified after Streif
1990
)
these organisms are shell-producing and hence
contribute to the total and bioclastic material budgets
of the back-barrier depositional system. In addition,
most of the organisms are responsible for a partial or
complete destruction of primary sedimentary structures
up to the depth of their burrowing and feeding activity.
This 'bioturbation' process also includes the activity of
some higher order animals such as birds and fish.
In a geological context, bioturbation and the pro-
duction of bioclastic material are tidal flat attributes
that have only evolved in the course of the Phanerozoic
(600 Ma BP - Present), older (Precambrian) deposits
being generally devoid of such features. Finally, any
list of biological influences would be incomplete if
algal and bacterial activity were omitted. In this con-
text, true algal mats, which consist predominantly of
green algae and mostly occur on muddy tidal flats and
in salt marsh pools, must be distinguished from mats
produced around the mean high-water level by so-
called 'blue-green algae' (cyanobacteria). These latter
mats should correctly be referred to as 'microbial'
mats (e.g., Gerdes and Krumbein
1987
; Noffke
2010
).
At smaller spatial scales, the basic depositional
building blocks of back-barrier tidal flat systems
outlined above are characterized by a large variety of
physical and biological surface structures as well as
internal sedimentary structures which, if comprehen-
sively illustrated, would fill a whole topic. Good
summaries of typical clastic tidal facies and their
sedimentary structures can be found in de Raaf and
Boersma (
1971
), Klein (
1977
), and Reineck and Singh
(
1980
), while biogenic structures and ichnofacies of
temperate tidal environments are comprehensively dealt
with in Schäfer (
1972
). For the purpose of this contribu-
tion, a selection of features is presented that, alone or in
combination, have some degree of diagnostic power in
identifying tidal flat deposits in the rock record.
10.4.1 Biological Surface Structures
The evolution of organisms in the course of the
Phanerozoic, and their frequent adaptation to specific
environmental conditions, has greatly facilitated the
identification of particular depositional environments
in the rock record. Intertidal flats are no exception in
this context.
In Fig.
10.16a
, a well preserved and still rooted tree
stump in the middle of an intertidal flat suggests
transgressive inundation in the East Frisian Wadden
