Geology Reference
In-Depth Information
Box 9.5.
Describing microbialites, encrustations and borings.
Microbial carbonates
Micrite
: Inhomogeneous or homogeneous texture? Peloidal micrite? Differences in color? With or without undulation?
Peloids:
Within the micritic matrix? In interskeletal or intraskeletal protected cavities? Silt- or fine sand-sized grains?
Peloids with dark center surrounded by a clear rim of euhedral calcite crystals?
Clotted fabric:
Frequency of micritic and peloidal micritic areas? Relation of the clotted fabrics to non-clotted fabric
areas? Remains of tubular and filamentous cyanobacteria?
Calcimicrobes:
Tiny tubes with micritic walls, often bifurcated. Morphological type (see Pl. 53), frequency, irregular
occurrence or in layers?
Fabric:
Non-laminated or laminated fabric (-> microbialites or stromatolites; see Fig. 9.1 and Box 9.1)?
Non-laminated fabric:
Irregular, sometimes dome-shaped structures consisting of mesoclots (-> thrombolites)? Dome-
shaped, sometimes bushy structures exhibiting micritic, peloidal and coarsely zoned microstructures (-> dendrolites)?
Dome-shaped structures consisting predominantly of undifferentiated micrite (-> leiolites)?
Laminated fabric
: Macroscopic growth form (dome shaped, columns, high or low forms; fine- or coarse-grained
laminae; agglutinated laminae)? Internal structures (individual laminae composed of micrite peloids or mesoclots)?
Disruptions of the lamination by sediment, borings or biogenic encrustations? Geometrical arrangement of basic
growth elements (see Logan et al. classification, Fig. 9.3)?
Biogenic encrustations
Occurrence
of the encrusters:
On or within substrates (epibenthic or endobenthic)?
Location of the crusts:
Upward growing crusts on a surface or pendant crusts in cavities?
Thickness of the crust:
Uniform or laterally decreasing or increasing?
Interruptions of crustbuilding:
Layers separated by sediment or cement? Bored surfaces of layers?
Substrate:
Lithified surface, fossil or other sedimentary grains? Carbonate cement? Boundary encruster/substrate?
Growth forms:
Thin or thick crusts, sheets, mounds, plates, semi-erect or erect forms)?
Sequential patterns:
Composition of crusts consisting of different encrusters? Relation between parts formed by spe-
cific encrusters? Thickness of the individual crust layers?
Relation between encrusters and microbialites:
Crusts embedded within microbialites? Distinct or vague boundaries?
Encrusting organisms:
Identity (encrusting organisms should be differentiated to the lowest possible taxonomic level)?
Species diversity and percent cover of upper and undersides of host organisms (e.g. corals, stromatoporoids, shells)?
Proportion of autotrophic to heterotrophic encrusters?
Bioerosion and borings
Microborers:
SEM characterization. Which groups (bacteria, cyanobacteria, algae, fungi, foraminifera, bryozoans)?
Relative frequency? Growth sequences?
Macroborers:
Thin-section characterization. Simple tubes or branched networks? Shape of the bore holes (cylindrical,
dumb-shaped, amphora-like)? Mode of the exterior opening (single or multi-apertured openings; circular, elliptical
or slit-like openings)?
Assignment of the trace fossils to major groups:
Sponges, worms, bivalves or cirripedians? Dominating group?
Dimension of the borings:
Lengths and diameters of the bore holes?
Orientation of the borings:
Vertical, oblique, horizontal, irregular, or fluctuating?
Substrate type:
Rock surfaces? Lithified surface, e.g. hardgrounds and disconformity surfaces? Fossils? Sedimentary
grains?
Relation borings and substrate:
Depth of the penetration below the bored substrate? Borings only in specific fossils?
Cross-cutting patterns?
Association of boring organisms and the organisms of the host substrate:
Repeated co-occurrence of particular borers
and specific host fossils? Borings only in specific biofacies types (e.g. only in coral reef limestones)?
Quantitative data:
Percentage of macroborer groups within a defined area? Density of macroboring measured by point-
counting or image analysis considering macroborers versus the non-bored substrate areas?
depending on reef type, environmental setting and
dominant macroborers.
The abundance of borings in coral reefs expressed
by the ratio of the point counter percentage of borings
to the percentage of non-attacked substrate areas per
cm
2
is low and moderate in the Late Triassic (Northern
Alps: 0.01 - dominating macroborers bivalves and
sponges, and 0.09 - mainly worms; Nayband Forma-
tion, Iran: 0.14 - worms dominate over bivalves and
cirripedians) and in the late Early Jurassic (Morocco:
0.08 and 0.12 - dominated by cirripedians), moderate
in the Middle Jurassic (Iran, India, Chile: 0.12 - bi-
valves and worms equally important), and increases in
the Late Jurassic, Oxfordian, and Kimmeridgian mi-
crobial-coral reefs (central and western Europe: 0.33-
0.59 - dominated by bivalves and worms followed by
