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Introduction to carbonate hardgrounds and their faunas:
Sedimentological definition: Hardgrounds are synsedimentarily
lithified carbonate sea-floors that became hardened in situ by the precipitation
of a carbonate cement in the primary pore spaces. To the sedimentologist
studying Recent carbonate sediments, the term describes the consequence
of the precipitation of cement within a soft sediment on the sea floor,
contemporaneously with or soon after deposition (Bathurst, 1971; Bromley,
1975). Essentially, sedimentation and cementation occur in the same
submarine environment, giving rise to a hard sea-floor which may then
be colonized by a fauna and flora that show adaptations to hard-substrate
dwelling. This happened often in the "calcite seas" of the Ordovician-Silurian and Jurassic-Cretaceous. Ideally, the term should be used for the same circumstances
when applied to the ancient record, but it is not always certain that
the cementation of the surface of a unit necessarily took place in the
same submarine environment in which the bed was originally deposited.
Cementation episodes, for example, may also represent intervals of exposure
to meteoric waters or burial, with colonization and renewed sedimentation
following resubmergence or erosion. Only detailed study of cement textures
and fabrics, and maybe trace-elements and isotopes, can distinguish
such cases from true hardgrounds formed by synsedimentary submarine
cementation.
Paleontological concerns: To the student of the
faunas of ancient hard substrates, the details of the environment of
substrate cementation or the precise timing of substrate hardening relative
to colonization may or may not be important, depending on the type of
study. Primary descriptions of community composition, species interactions,
succession, or spatial distributions do not necessarily require the
elucidation of the substrate's cementation history. Comparative studies,
however, in which hardground faunas of different ages are compared in
order to determine long term evolutionary trends in community composition
and structure, require maximum information about environments of cementation
and colonization in order to ensure that the communities being compared
across the ages come from as similar environments as possible (Palmer,
1982; Wilson and Palmer, 1992). Just as sedimentological detail may
or may not be of concern to the student of hardground faunas, so the
faunas may or may not be of concern to the hardground sedimentologist.
The principal reason why sedimentologists ignore the fauna at their
peril, however, is that it is often the fauna that unequivocally points
the finger at the lithified character of the original sediment surface.
Although many hardgrounds show a characteristic mineral staining (most
commonly by iron minerals) at or just below their surface, which can
draw attention to the surface in outcrop, this is not always present
(particularly in coarser-grained lithologies). In the absence of such
staining it is the presence of the boring and encrusting fauna alone
that testifies at the macroscopic level to the originally hard character
of the surface. |
