Geology Reference
In-Depth Information
10
Siliciclastic Back-Barrier Tidal Flats
Burghard W. Flemming
Abstract
Back-barrier tidal flats occur along micro- to mesotidal coasts landward of barrier
islands and in the shelter of coastal sand spits and bars. Tidal flats are generally
flood dominated, the grain size progressively decreasing shoreward. The sediment
can be divided into sand, slightly muddy sand, muddy sand, sandy mud, slightly
sandy mud, and mud. The mud fraction consists of non-cohesive sortable silt and
cohesive flocs and aggregates. Important physical and biological surface struc-
tures include wave- and current-generated ripples, ladderback ripples, washed out
ripples and other late-stage emergence runoff features, shell pavements, fluid mud
sheets, tool marks, crawling, feeding and resting traces of intertidal organisms, as
well as the feeding traces and tracks of birds. Internal sedimentary structures range
from rare dune cross-bedding to ubiquitous ripple cross-bedding in sand, through
flaser, wavy and lenticular bedding in mixed sediment, and homogenous or lami-
nated mud toward the high-water line. Bioturbation may be intense, but the pres-
ervation potential depends on the frequency and depth of reworking. The transition
from land to sea is typically marked by laminated versicolored microbial mats.
The interaction between sea-level rise and sediment supply defines the sediment
budget and hence the stratigraphy. Prograding, aggrading or transgressive systems
are easily distinguished by their stratigraphic architecture.
10.1
Introduction
derived from shell-bearing organisms, especially
molluscs, forms an overall subordinate sedimentary
component, although it may locally be enriched in the
form of shell beds and channel lag deposits. Back-
barrier tidal flats commonly occur along micro- to
mesotidal coasts (tidal ranges of ~0.3-3.5 m) in the
rear of barrier islands, and in the shelter of coastal sand
spits. Specifically excluded are carbonate tidal depos-
its (Pratt et al.
1992
), intertidal sand bodies occurring
along lower courses of many barred estuaries (Dalrymple
et al.
1992
), episodically flooded back-barrier wind
flats (Miller
1975
; Schneider
1975
), tidal lagoons
without substantial intertidal flats (Ashley
1988
;
This chapter deals with non-vegetated or bare (inter)
tidal flat depositional systems that occur in the shelter
of coastal barriers and which are predominantly com-
posed of siliciclastic sediments (sand and mud) of ter-
rigenous origin. In these systems, bioclastic material
