Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 157

architecture, referred to as “autostratigraphy” ( Muto et al., 2007 ). For large-
scale and long-lived deltaic successions this is practical, but practitioners should
be aware that such autogenic parasequences will not correlate along strike into
contemporaneous shoreline successions such as shoreface settings. Allogeni-
cally induced FS, however, will bound parasequences and parasequence sets
that will extend along strike into non-deltaic portions of the shoreline.
Autogenic production of a highly firm substrate in the marine and marginal-
marine realm is generally difficult to accomplish, because the eroding event must
be capable of exhuming a compacted sediment layer without burying the exposed
surface during the same event. That is, the firmground must be colonized during a
hiatus between the eroding event and subsequent deposition in order to create an
ichnologically demarcated omission surface ( Fig. 1 ). In siliciclastic settings, auto-
genic shallow-marine firmgrounds may be associated with the margins and bases
of channels (e.g., tidal channels, tidal inlets, distributary channels, and estuaries);
with mud beds that are temporarily buried beneath large, periodically moribund
bedforms (e.g., tidal ridges, tidal shoals, and storm-mobilized bedforms;
Dashtgard et al., 2008; Yang et al., 2009 ); andwith high intertidal mud flats, which
may endure longperiods of subaerial exposure and desiccation aroundneapcycles.
Another source of firm, shallowly buried substrates may be found in the foreshore
of fine-grained sandbeaches, wherewave pounding contributes to the repacking of
sand grains and dewatering (e.g., Dalrymple, 1979; Kindle, 1936 ). This results in
surprisingly firm media that may favor palimpsest softground omission suites.
However, as the sands are still water-saturated, they are unlikely to produce firm-
grounds unless incipiently cemented during early diagenesis prior to exhumation.
In arid settings, some intertidal siliciclastics may be prone to incipient calcite
cementation, forming firmgrounds or even hardgrounds ( Gryszczynski, 1986 ).
Carbonate settings are more susceptible to cementation during autogenic expo-
sure (e.g., beachrock), or to some occurrences of subaqueous cementation due to
sediment starvation (e.g., some submarine hardgrounds; Bromley, 1975 ). In con-
trast to siliciclastic deposits, carbonates mainly contain biogenic sediments that
originate due to the growth of organisms and build extensive platforms with low
gradients ( Knaust et al., 2012 ). Consequently, frequent subaerial exposure within
the peritidal facies belt is common at the top of shallowing-upward successions.
Moreover, microbial mats and biofilms thrive in such environments and contribute
to sediment binding and preferred trace-fossil preservation. Calichification and
early diagenetic cementation significantly contribute to the lithification of carbon-
ate sediment and promote the development of discontinuity surfaces.
Deep-marine settings are also susceptible to autogenic omission suites, owing
to oceanic currents (e.g., contour currents) as well as proximal turbidity currents,
which may strip away superficial soupy muds (cf. Savrda et al., 2001a,b; Stow
and Holbrook, 1984 ). Oceanic currents tend to provide a persistent source of
energy that continuously removes sediment from the area (sediment stripping),
whereas the latter process mainly leads to sediment bypass in the vicinity of
the truncated sediment, leaving only a thin veneer of sediment in its wake.