Page 226 - Sedimentology and Stratigraphy

Fossils in Coastal and Estuarine Environments 213

Lagoonal faunal and floral assemblages are therefore Beach/barrier systems
often limited in numbers of taxa, being dominated by . lithology – sand and conglomerate
those that are adapted to either brackish or hypersa- . mineralogy – mature quartz sands and shelly sands
line conditions. Although the diversity of fauna may . texture – well sorted, well rounded clasts
be severely limited by brackish or hypersaline waters . bed geometry – elongate lenses
in the lagoon, those species that are tolerant flourish . sedimentary structures – low-angle stratification
in the absence of competition in waters rich in nutri- and wave reworking
ent from the surrounding vegetation. These special- . palaeocurrents – mainly wave-formed structures
ised organisms may occur in very large numbers and . fossils – robust shelly debris
fossil assemblages in lagoons are typically of very low . colour – not diagnostic
diversity or even monospecific. . facies associations – may be associated with coastal
The traces of organisms can commonly be found plain, lagoonal or shallow-marine facies
and the ichnofacies (11.7) present will depend upon
the energy of the environment and the nature of the Lagoons
substrate. In lagoons the fine, organic-rich sediment . lithology – mainly mud with some sand
provides a favourable feeding area for organisms that . mineralogy – variable
are able to tolerate the reduced/enhanced salinity, . texture – fine-grained, moderately to poorly sorted
and bioturbation may be common. In sandy intertidal . bed geometry – thinly bedded mud with thin sheets
areas the predominant style of trace is typically a and lenses of sand
vertical structure created by animals moving up to . sedimentary structures – may be laminated and
wave rippled
the surface when the area is covered by water and
. palaeocurrents – rare, not diagnostic
down within the sediment body when the water
. fossils – often monospecific assemblages of hypersa-
recedes. This form of trace fossil is known as the
line or brackish tolerant organisms
Skolithos assemblage, after the simple vertical tubes
. colour – may be dark due to anaerobic conditions
that are found in these settings. Other ichnofacies
. facies associations – may be associated with coastal
assemblages occur if the substrate is relatively firm
plain or beach barrier deposits
(Glossifungites assemblage) or hard (Trypanites assem-
blage). Trypanites-type traces are borings made in
Tidal channel systems
solid rock (bedrock or loose boulders) by molluscs, . lithology – mud, sand and less commonly conglom-
and these are characteristic of rocky coastlines. erate
The association of marine and continental condi- . mineralogy – variable
tions is one of the characteristics of estuaries, and this . texture – may be well sorted in high energy settings
is reflected in the fossil assemblages found in deposits . bed geometry – lenses with erosional bases
in these environments. Some shelly debris may be . sedimentary structures – cross-bedding and cross-
brought in from the marine environment, but shelly lamination and inclined heterolithic stratification
fauna is also often abundant in estuarine settings. . palaeocurrents – bimodal in tidal estuaries
As well as body fossils, evidence for biogenic activity . fossils – shallow marine
is also present in the form of trace fossils, which range . colour – not diagnostic
from very abundant and diverse in tidal mudflats to . facies associations – may be overlain by fluvial,
sparse in the high energy, sandy environments of the shallow marine, continental or delta facies
outer parts of estuaries. Vegetation growth may be
prolific in tidal mudflats, especially on the upper parts, Tidal mudflats
and plant remains may be present as organic material . lithology – mud and sand
or as root traces. . mineralogy – clay and shelly sand
. texture – fine-grained, not diagnostic
. bed geometry – tabular muds with thin sheets and
Characteristics of coastal and estuarine systems
lenses of sand
These complex, heterogeneous depositional environ- . sedimentary structures – ripple cross-lamination
ments are divided into four elements for the purposes and flaser/lenticular bedding
of summarising their characteristics. . palaeocurrents – bimodal in tidal estuaries