Geology Reference
In-Depth Information
Plate 9 Skeletal Grains (Bioclasts): 'ThinSection Fossils' and Taphonomic Pitfalls
Skeletal grains are the prime source of carbonate sands and muds deposited in marine environments. The value
of bioclasts in facies analysis depends largely on the level of systematic assignment. Determination of 'thin-
section fossils' at a genus or species level depends on the paleontological background of the student and her or
his ability to reconstruct a three-dimensional picture from random cuts. Reconstruction and taxonomic attribu-
tion is facilitated by comparison of 'thin-section fossils' with 'acid-residue' fossils (-> 1-6). The joint use of
thin-section and acid-residue techniques is strongly recommended - last but not least because thin-sections
reflect only parts of the total fossil record of the limestone sample. The poor preservation of taxonomically
important criteria caused by taphonomic and diagenetic processes (-> 7) is responsible for generalized taxo-
nomic assignments (e.g. 'limestone with gastropods and foraminifera').
Two- and three-dimensional aspects of fossils.
Selective silicification of a Late Pennsylvanian shelf limestone (Carnic Alps,
Austria) allows of thin-section and acid-residue data to be compared (Fohrer 1991).
1
Thin section: Median section of the microgranular calcareous foraminifera
Deckerella
Cushman and Waters (Palaeo-
textulariacea, Fusulinina). Genus criteria: Elongate test, short and wide chambers, biserially and later uniserially ar-
ranged. Microgranular wall dark in transmitted light; exterior part of the wall agglutinated. Aperture of later stages with
parallel slitlike openings. The thin section provides all the information necessary for generic determination. The genus is
restricted to the Carboniferous.
2
Acid residue of a specimen of
Deckerella
. The morphology of the test and the arrangement of chambers are shown more
distinctly than in the thin section.
Thin section: Oblique axial section of the dasyclad green alga
Anthracoporella
Pia (compare Pl. 59/6).
'
Stem cell
'
filled
with sediment; the thick wall exhibits tangential, longitudinal and oblique sections of pores (branches). The arrow points
to encrusting foraminifera.
3
4
Acid residue of
Anthracoporella
. Shape and arrangement of pores are more obvious than in the thin section. Quantitative
species-diagnostic data (e.g. pore diameter) are different from thin-section data.
5
Thin section: Fenestrate bryozoan exhibiting the obverse side of the net-like colony, fenestrules (F) and cross sections of
zooecia apertures (ZA); compare Pl. 85/10. These criteria allow them to be designated only as 'fenestrate bryozoa'.
6
Acid residue.The taxonomically important arrangement, size and shape of the zooecia are more obvious than in thin
sections and allow them to be assigned to the genus
Fenestella
. Measurements can be done more exactly than in thin
sections.
Taphonomic and diagenetic controls on the preservation of skeletal grains
.
Essential criteria for describing the
'microtaphofacies' of this sample are abrasion, fragmentation, bioerosion and encrustation. Microtaphofacies refers to the
taphonomic biasing caused by displacement, redeposition and mixing of grains as well as selective dissolution which in
turn is controlled by the primary skeletal mineralogy.
7 The limestone conglomerate (Early Tertiary, Eocene: Croatia) consists of fossils (gastropods - G, bivalves - P, foramin-
ifera, dasyclad algae), micritic limestone pebbles (MLP), peloids and small reworked and rounded micrite clasts (MC).
Gastropods and alveolinid foraminifera exhibit different preservation due to different primary mineralogy: The aragonitic
gastropod shells are dissolved, but the shape of the test is still preserved. Molds are infilled with sediment and microfos-
sils. In contrast, the porcellaneous calcitic tests of the miliolid foraminifers
Alveolina
(A) and
Orbitolites
(O) exhibit no
diagenetic changes. Low-Mg calcite is less susceptible to dissolution than aragonite or High-Mg calcite. The example
exhibits a common relative order of bioclast preservation within an environment affected by oscillating marine, phreatic
and vadose diagenetic processes: Low resistance of primary aragonitic skeletal grains (e.g. gastropods, scleractinian
corals, dasyclad algae) to dissolution, and higher resistance of primary calcitic grains represented by miliolid foramin-
ifera, coralline algae, and fibrous lamellar bivalve shells (Gindy 1987; Bathurst 1994).
Taphonomic history of gastropods: Reworking and redeposition of the 'steinkerns' (lithoskels) took place subsequent
to dissolution of aragonitic shells and mold formation together with the deposition of well-rounded micritic limestone
pebbles (MLP). Interstices between larger skeletal grains and pebbles were filled with alveolinid and other foraminifera,
dasyclads and small bioclast debris. Gastropod molds were encrusted by foraminifera (F) and spirorbid worms (W). Other
organisms settling on hard substrates are barely recognizable barnacles (B), compare Pl. 93/6, 7. Borings of shells are
evident. Meteoric dissolution is recorded by irregular solution voids (SV) filled with silt.
Temporal sequence of events: Reworking of back-reef biota (indicated by
Alveolina
and
Orbitolites
) and deposition of
molds of aragonitic shells, pebbles and micrite clasts in a high-energy environment -> encrustation and boring of larger
fossils -> cementation and meteoric dissolution -> deposition of foraminiferal sand. The mixed deposition of worn
fossils, allochthonous lithoclasts and blackened peloids as well as strong encrustation and boring characterize a 'lag
deposit', formed in an environment with slow accumulation of coarse material within a winnowing zone (Standard
Microfacies Type: SMF 14). This is the case in coastal zones which are prone to rapid changes in sedimentation rates,
destruction, dissolution and renewed marine deposition.
-> 1-6: Fohrer 1991
