Geology Reference
In-Depth Information
Box 16.4.
Guide for the description of reef limestones.
Describe biota, matrix (sediment) and cement with regard to their
specific types and abundance. Take care that even a large thin section may not provide full information on the frequency
of the three main constituents. Growth forms of reefbuilders observed on a thin-section scale should be checked by field
data.
Biota
•
Reef-building organisms.
Describe type (on higher or
lower taxonomic level). Try to estimate ranking and the
volumetric importance of potential reefbuilders.
•
Growth forms and guild structure.
Try to define the
growth forms of sessile organisms using Fig. 12.2. Growth
forms provide clues to hydrodynamic conditions and rela-
tive sedimentation rates. Assign reefbuilders to guild groups
(Fig. 16.6).
•
Encrusting organisms.
Micro-encruster associations are
environment-sensitive (Sect. 9.2.3; Fig. 9.6 and 9.10). Are
encrusters completely absent? Are encrusters common or
abundant (pointing to low sedimentation rates)? Are en-
crusters restricted to reef-building organisms? Or do they
occur on particles of reef debris?
•
Micro- and macroborings in reefbuilders
(indicating
biological destruction and potential sediment production).
Boring patterns can provide clues to paleo-water depth
(Sect. 9.3.4).
•
Microfossils in the matrix between reefbuilders.
Spe-
cific environments and subenvironments of reefs are often
characterized by facies-diagnostic benthic foraminifera,
algae and microproblematica (Sect. 14.2.2; Fig. 14.13).
•
Associated fossils.
Which fossils occur in association
with reefbuilders? Can you see recurrent patterns in the
frequency and taxonomic composition of these fossils? Are
these organisms constant reef dwellers using the niches
within the reef or were they just visiting the reef environ-
ment?
•
Relationship between matrix and reef organisms.
Can
you suggest a causal relationship between the type or abun-
dance of matrix and the existence, growth form, size or
distribution of reef organisms?
•
Potential microbial contribution
. Can you find criteria
supporting a microbially triggered origin of the reef rock
matrix (see Box 9.1)?
Carbonate cement
Cements are essential constituents of ancient reefs. Reef
cementation is controlled by water pumping and biologi-
cal activity (e.g. microbes, organic matter), and depends
on the size and connection of pores. Syngenetic Mg-cal-
cite and aragonite cements contribute to the rigidity of reefs
and assist in maintaining steep reef slopes. Cementation is
particularly abundant at seaward margins of high-energy
reefs, e.g. in forereef areas (Fig. 7.7), characterized by a
high rate of water movement and low sedimentation rates.
•
Distribution of cement and pores.
Do reef cements oc-
cur on free surfaces or in cavities? Are they found only in
the interspace between autochthonous or parautochthonous
reefbuilders (e.g. growth-framework pores), in intraskeletal
pores (Pl. 29/2), or do they occur within reef cavities (e.g.
stromatactis)? Are these cavities primary pores or solution
cavities (e.g. biomolds)?
•
Cement types and fabrics.
Describe cement types, ce-
ment sequences and cement fabrics (Fig. 7.12). Common
types are fibrous, bladed, radiaxial and botryoidal cements.
Radiaxial and botryoidal cements may dominate volumetri-
cally over reefbuilders and sediment (Sect. 7.4.4.1, 7.4.4.2;
Fig. 7.14; Box 7.7). Be aware that 'micrite' may not repre-
sent sedimentary matrix, but early diagenetic microcrys-
talline or peloidal microcrystalline cement!
•
Cement and fossils.
Look for the interrelationship be-
tween cement and fossils, e.g. layered cement sequences
(Fig. 7.14; Fig. 16.10) that indicate (often microbially
mediated) changes in water flow and chemical composi-
tion of fluids within the pore system of reef structures.
Intergrowth of micritic (potentially microbial) crusts or
organisms depending on hard substrates and cement crusts
indicates rapid synsedimentary cementation (Pl. 145/1).
•
Diagenetic environments
. Evaluate cement types and
cement distribution in terms of marine, meteoric and burial
diagenesis (Sect. 7.2.1; Box 7.6). Ecologic shallow-water
reefs tend to be affected by meteoric-vadose and marine-
vadose diagenesis, reflected by dripstone and meniscus
cements, and deposition of crystal silt within solution cavi-
ties.
Matrix and sediment
A close look at the matrix is necessary in assigning your
sample to guild types and classifying the sample as baffle-
stone, bindstone or framestone. The abundance of matrix
assists in differentiating reef types, e.g. mud mounds and
reef mounds.
•
Abundance of matrix.
Estimate or measure the abun-
dance of matrix in comparison to reef biota and synge-
netic carbonate cement.
•
Texture of the matrix.
Describe matrix and reef sedi-
ment in terms of the Dunham classification (e.g. lime mud-
stone, wackestone, packstone, floatstone; Sect. 8.3.2).
•
Matrix composition.
Characterize type, distribution and
preservation of grains within the matrix (see Fig. 16.9).
•
Can you
allocate the matrix to a specific genetic cat-
egory?
The matrix may be allomicrite, automicrite formed
on free surfaces or as internal micrite within cryptic cavi-
ties, or calcisiltite (Sect. 4.1.4). See Fig. 4.1 and Box 4.4.
