Geology Reference
In-Depth Information
Criteria
Constructor
Baffler
Binder
Importance
of criteria
1
habit, growth habit,
upwards, erect
upwards, erect
lateral, reptant
very high
growth direction
2
form, lifeform,
massive, domes,
cylinders, cones,
sheets, lenses,
high
growth form
branches cups,
blades
runners, webs,
columns
plates, umbrellas
3
skeletonization,
wellskeletonized
poorly skeletonized,
well skeletonized
very high
skeletal strength
strong, rigid
mostly as skeletal
fragments
4
skeletal volume,
large, clonal or
smaller, solitary
medium size, clonal
moderate
cloniality
gregarious
or clonal
or gregarious
5
biostratonomy,
in growth position
in situ (toppled,
in growth position,
high
taphonomy,
or in situ (toppled,
broken), commonly
encrust, roofover or
transportability
broken)
transported
trap sediment
6
skeletal packing
as in life or
highly variable
as in life
moderate
density, areal cover
less dense
from dispersed to
concentrated
Fig. 16.6.
Reef guilds.
Criteria used in recognizing and assigning reef-building guilds. The habit, form and packing/density
of reef-building organisms should be evaluated from outcrop observations combined with thin-section data. Skeletonization
and skeletal volume can be derived from digital analysis of thin sections and grid studies of acetate sheets taken in the field
(Bernecker et al. 1999; Fagerstrom 2002). After Fagerstrom and Weidlich (1999).
Large thin sections often allow guild assignment,
but field observations on a larger scale are pivotal.
tion to the skeletal volume of the reef. Bafflers have
small, more fragile skeletons. Bafflers are commonly
preserved as toppled fragments. Constructors can be
in life position, toppled on edge, toppled or overturned.
Bafflers often reduce current velocities. Slower and
more turbulent currents enhance the deposition of sedi-
ment within framework pores, larval recruitment, the
feeding-filtration efficiency of suspension feeders.
Interpreting the guild structure of fossil reef com-
munities and the roles of individuals and taxa in frame-
building is a two-step process:
• Assign the reef-building fossils to their membership
in one or more constructor, baffler or binder guilds con-
sidering the nature and degree of intraspecific morpho-
logical plasticity (a part of a colony may belong to one
guild, another part to a different guild) and the possi-
bility of variable guild assignment (a species may be a
member of several guilds). Important criteria for a cor-
rect determination of reef-building guilds are summa-
rized in Fig. 16.6.
• Determine the relative importance of each guild in
the overall frame-building process, based on skeletal
size, strength, the abundance of its assigned members
and the areal cover by analyzing large acetate sheet
tracings, close-up photos and inspecting in detail de-
fined quadrat-surfaces associated with the investiga-
tion of thin-section samples.
Strength and limitations of the reef guild concept:
Guild structure analysis has been successfully used in
interpreting the formation of various Phanerozoic reefs
and recognizing changes in the overall contribution of
constructors, bafflers and binders to reef formation dur-
ing the Earth's history.
Secular variations are obvious for the Constructor
Guild and the Binder Guild, whereas the importance
of the Baffler Guild seems not to have varied strongly
in the Phanerozoic (Kiessling 2002).
Limitations of the concept relate to the
•
role of the Baffler Guild in reducing current veloc-
ity and enhancing sediment deposition within the
reef framework. Baffler must not necessarily baffle;
•
guild assignment of microbes that may act as bind-
ers and constructors as well;
•
guild assignment of cryptic reef communities, e.g.
pendant biota within reef cavities;
•
possibility that some reef organisms may be mem-
bers both of the reef-building guild and of the
Dweller Guild;
•
good community preservation during rising sea lev-
els, limited preservation during falling sea levels.
