Geology Reference
In-Depth Information
Box 4.12.
How to describe rhodoids?
A combination of field and laboratory criteria is recommended. Arrows indicate
palecological indications and clues for interpreting paleo-environmental and depositional settings.
Field criteria
Depositional regime:
Predominantly carbonate, mixed siliciclastic-carbonate or siliciclastic?
Position of rhodoid beds in the section:
Occurrence within defined horizons (-> event beds, part of shallowing or
deepening upward sequences). Repetitive occurrence (-> cyclic patterns)?
Occurrence in mud-dominated or grain-dominated limestones
. (-> different settings, water depths and environmental
controls)?
Abundance and distribution of rhodoids:
Either more than or less than 50% of rock volume (-> rhodolite or rhodoid-
bearing limestone). Densely packed (grain-to-grain contacts) or scattered?
Color of rhodoids and matrix
(-> differences in composition and/or weathering)?
Sedimentary structures:
Bedding, cross-bedding, bioturbation, grading, parallel orientation, imbrication, geopetal struc-
tures (-> movement of rhodoids)?
Field and laboratory criteria
Limestone texture:
Wackestone/floatstone, packstone, grainstone/rudstone, bindstone?
Microfabrics:
Single or multiple rhodoids Composite rhodoids (e.g. algal-foraminiferal macroids)?
Orientation of rhodoids and other grains
(-> current transport?)
Size ranges, average, frequency distribution, sorting
(-> micro-, meso- or macroids)?
Variations in shape, sphericity, surface and growth forms (symmetrical, asymmetrical) of rhodoids
(-> hydrodynamic
conditions)?
Association with ooids
and/or lithoclasts
(-> reworking of rhodoids)?
Associated fossils?
Laboratory criteria
Geopetal fabrics in rhodoids
(-> overturning during growth)?
Shape, external growth forms and sphericity:
Spheroidal, ellipsoidal, discoidal (Folk and Sneath triangle diagram (->
hydrodynamic energy; environmental and biological controls). Symmetric or asymmetric growth? Contorted rhodoids?
Surface:
Smooth, bumpy, grooved, lobate; conspicuously round or rounded; bored; encrusted; surface preserved or eroded
(->
reworking and redeposition)?
Internal growth patterns:
Laminar, branched, columnar, globular; densely or openly spaced branches (-> hydrodynamic
energy). Mixed or multi-stage microfabrics?
Layers and lamination:
Configuration (concentric or non-concentric, continuous or discontinuous); thin or thick lamina
crusts?
Size:
Dominating sizes, ranges and distribution; arithmetic mean diameter: (long + intermediate + short axis lengths):3.
Biotic composition of coatings:
Taxonomic inventory (-> paleo-water depths), mono- or multispecific morphotypes
including shape and size, abundance of different groups, rhodoid diversity in relation to rhodoid size, type and
frequency of encrusting organisms; comparison with organisms occurring outside the rhodoids (-> biological con-
trols, continuous/discontinuous growth, selective biota)
Ecologic successions and growth patterns of rhodoids:
Changes in basic microfabrics and biotic contribution
(-> changes
in environmental factors)
Borings:
Totally or partially bored (-> hard and soft stages)
Cracks within the oncoids:
Type, frequency, infilling (-> desiccation, syn- or post-depositional origin)
Nucleus:
Present or absent; single or several nuclei; size; composition (bioclast, lithoclast, other grains; fragments of
coralline algae); preservation of nuclei; comparison of nuclei and non-oncoid grains with regard to grain types and
biotic diversity (identical, different -> autochthonous/allochthonous rhodoids)
Cortex and nucleus:
Size and ratio; different thickness of cortex on the top and bottom of the rhodoids (-> in-situ
growth)?
Associated fauna?
such as shown by different sizes of Late Paleozoic
Archaeolithoporella
and
Archaeolithophyllum
rhodoids.
But whether the succession of algal species represents
changing environments or a change in rhodolith size is
still an open question.
Monospecific branching rhodoids occur both in tem-
perate and arctic waters as well as in tropical environ-
ments.
Biotic composition:
Taxonomic successions may be
the result of a change in rhodolith size: the larger the
size, the higher the stability of communities over time.
