Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 685

TABLE 1 Relevant Differences Between Carbonates and Siliciclastics
with Respect to Formation and Preservation of Trace Fossils. Modified
from Curran (1994) —Cont'd

Carbonates Siliciclastics

biofilms, and early-diagenetic
cementation.

Early
diagenesis

8. Rapid lithification; cementation
process accentuates trace-fossil
definition, that is, differential
cementation between burrow
matrix and fill materials.

Lithification may never occur or
be delayed for a significant
amount of geological time.

9. Normally little to no sediment-
color contrast between burrow
materials and matrix (except in
cases of diagenetic staining),
limiting the outlining of trace-fossil
forms.

Sediment-color contrast is the
norm, commonly outlining trace-
fossil forms.

10. High potential for diagenetic
alteration of rocks bearing trace
fossils, including the development
of ichnogenic macroporosity.

Lower potential for diagenetic
alteration and development of
ichnogenic macroporosity,
although it can be locally
important.

General 11. Some depositional

environments minor or essentially
unrepresented, such as glacial,
fluvial, fluvial-deltaic, and
deepest sea.

A wider range of depositional
environments represented in the
rock record.

Marine siliciclastic deposition commonly takes place on a well-defined gra-
dient from the shoreline to the shoreface, across the shelf, and down the slope to
a basin floor. In contrast, warm-water carbonate systems are chiefly controlled
by the growth of organisms with prime dependence on available light. Conse-
quently, their platforms have low-gradient geometries resulting in a shelf and
ramp sea-floor with very low relief and facies development as broad belts.
Given the overall geometry of carbonate platforms with broad facies zona-
tion, favorable conditions for the preservation of traces occur when different
factors come together. Many shallow-marine carbonate rocks consist of fine-
grained carbonate (micrite) generated from a degradation of skeletal material
combined with in-situ precipitation which covers extensive areas in tropical
and subtropical zones with abundant benthic life. The fine-grained and homo-
geneous sediment within peritidal facies is typically subject to periodic subaer-
ial exposure, which leads to gradually changing substrate consistencies from
soft to stiff and firm over short time intervals (e.g., diurnally). This prograding