Environmental Engineering Reference
In-Depth Information
FIGURE 17
Cross-section view of
Glossifungites
Ichnofacies with
Skolithos
(
Sk
) below tidal-flat
flooding-surfaces. Photo is from the Early to Middle Cambrian Campanario Formation, Mes
´
n
Group, Purmamarca town, Quebrada de Huamahuaca, northwest Argentina.
1990
). The two types of parasequences also differ in their ichnological sig-
natures. Wave-dominated parasequences display an overall vertical transition
from the
Cruziana
to the
Skolithos
Ichnofacies, whereas tide-dominated para-
sequences exhibit a vertical replacement of the
Skolithos
by the
Cruziana
Ich-
nofacies (
Buatois and M´ngano, 2011; M´ngano and Buatois, 1999, 2004b
).
On the other hand, parasequences developed in the subtidal areas of a tide-
dominated shelf are mainly controlled by the migration and accretion of spa-
tially extensive sand bodies. For example, in sand sheets, parasequences are
characterized by the vertical transition from intercalated very thin- to thin-
bedded, very fine- to fine-grained sandstone and mudstone, hummocky cross-
stratified, fine-grained sandstone, and highly bioturbated
Skolithos
pipe rock
to non-bioturbated planar cross-stratified, fine- to medium-grained sandstone
of the sand-sheet core (
Fig. 16
A;
Desjardins et al., 2010b
).
The distinctive lithological and ichnological characteristics of parasequences
developed in intertidal and subtidal areas of tide-dominated environments may
be used for the recognition of sequence-stratigraphically significant surfaces.
The lowermost interval of a sandy tidal-flat parasequence is characterized by
sharp-based sandstone, which overlies the truncated mud-flat deposit of the pre-
vious parasequence. This high-energy flooding surface is commonly delineated
by firmground
Skolithos
of the
Glossifungites
Ichnofacies (
Fig. 17
).
Desjardins
et al. (2012b)
recognized that the parasequences at the top of the Lake O'Hara
Member of the Gog Group were truncated at a pipe-rock interval by an erosion
surface and that the expected capping cross-stratified sandstone was missing. On
top of this surface, fining-upward tidal-flat parasequences occur (
Fig. 6
A).
Because the missing facies would have recorded progradation due to gradual
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