Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 162

2.2 Carbonate Responses to RSL Changes
Carbonates differ from siliciclastics in a number of characteristics (see Knaust
et al., 2012 ). Most carbonate deposits result from the growth and decay of
organisms instead of through the redeposition of clastic debris. Accordingly,
carbonate depositional systems typically show very low depositional gradients
and comprise broad facies zones ( Schlager, 2005 ), although locally they may
produce higher gradients than can be achieved by siliciclastic settings, owing
to biolithic development (e.g., reef margins). This correspondingly implies
greater sensitivity to small changes in RSL, but with less lithological contrast
between facies than observed in siliciclastic systems ( Miall, 2010 ). During RSL
fall, the main “carbonate factory” of a tropical carbonate system is readily
exposed on the shelf and is, therefore, shut down. As a result, the shelf is subject
to widespread karstification (more pronounced under humid climatic condi-
tions) or pedogenesis (more dominant under arid climatic conditions), while
the basin center is starved of sediment and/or prone to pronounced evaporite
precipitation. During transgression, the “carbonate factory” resumes production
with the submergence of the shelf. Slow transgressions may allow the carbon-
ates to catch up with the rise in RSL, whereas rapid transgressions may result in
the drowning of the carbonate platform and the formation of drowning uncon-
formities, which are essentially flooding surfaces in the carbonate realm
( Catuneanu et al., 2011 ). Following drowning, sediment starvation may result
in the development of hardgrounds or in the accumulation of condensed
sections.
All types of sequence-stratigraphic surfaces and systems tracts may develop
within carbonate successions ( Bover-Arnal et al., 2009; Catuneanu et al., 2011;
MacNeil and Jones, 2008 ). Among the array of sequence-stratigraphic surfaces,
SUs and MRSs are arguably surfaces with most prominent physical and/or ich-
nological expression within carbonate systems. Subaerial exposure surfaces
originate where marine deposition is temporarily interrupted by terrestrial
conditions, with at least incipient soil formation. Deeply penetrating rhizoliths
are a common delineating phenomenon, typically associated with karstification,
caliche, and vadose diagenesis ( Fig. 2 A and B). Such exposure surfaces are
potential candidates for SUs.
MRSs occur at the tops of shoaling-upward trends maximized in the shallow-
marine, intertidal, or supratidal environments. These indicate a change to deeper-
water environments or more distal, open-marine conditions. MRSs are identified
by the transition from upward-shallowing intervals to upward-deepening facies.
Ichnology is well suited, at the facies level, to identify such transitions ( Giannetti
and Monaco, 2004; Knaust, 1998; Laporte, 1969; Lukasik and James, 2003;
Olivero, 1996 ). Suites in the underlying regressive interval are characterized
by classical transitions of Seilacherian ichnofacies from distal low-energy
archetypes (e.g., Zoophycos or Cruziana ichnofacies) to higher-energy suites