Geology Reference
In-Depth Information
the degradation of reefs (Scott and Risk 1988), and di-
agenetic processes affecting reef organisms (Jones and
Pemberton 1988).
Bivalve borings can be recognized in thin sections
by vertical or oblique boreholes that often contain re-
mains of the rock-boring shell (Pl. 52/8). Records of
bivalve borings in hardgrounds, reefs and fossils are
known from the Ordovician to the Quaternary, but do
not become more abundant until the Triassic.
contribute to bioerosion. Millimeter-sized holes be-
lieved to have been made by gastropods have been re-
ported from the Paleozoic, but these data are contro-
versially discussed. Undisputable drill holes of carnivo-
rous naticid gastropods occur in the Late Triassic. Gas-
tropod drilling is very rare in the Jurassic to the Early
Cretaceous interval. Starting with the Late Cretaceous
gastropod drilling in mollusk shells become abundant
(Kowalewski et al. 1998).
Gastropods:
Carnivorous gastropods drilling into
various fossils are predators rather than borers, but still
'Worms':
Trace fossils referred to boring activities
of worms occur throughout the Phanerozoic and are
Plate 52 Bioerosion and Boring Organisms
Bioerosion has a tremendous impact on carbonate sedimentation and carbonate diagenesis. The destruction of
hard substrates by boring, rasping, grazing and browsing organisms is substantial in the breakdown of carbonate
skeletons and for the production of fine-grained to sand-sized sediments both in warm-water and cool-water
environments. A huge variety of microbes, plants and animals penetrate hard surfaces. Submarine bioerosion
increases the porosity and surface area of skeletons and makes them more susceptible to dissolution. Microborers
are highly effective in the biological degradation of subaerially exposed carbonate rocks (e.g. karst) and of
limestones used as building stones.
Microborers
(-> 1-4) include autotroph (-> 1,2) and heterotroph bacteria,
green algae (-> 3) and red algae, fungi (-> 4), as well as foraminifera and bryozoans. Important
macroborers
are
sponges (e.g. clionids; Fig. 9.1.1), bivalves, 'worms' and cirripedian arthropods (Fig. 9.13). Microborers are
known since the Proterozoic, the oldest macroborers have been reported from the Early Cambrian. Microborings
are studied by stereoscan electron microscopy. SEM photos (-> 1-4) exhibit microborings of recent bivalve
shells which were collected from an upper shelf slope covered by reefs and bioclastic sands (Günther 1990). The
samples were first impregnated by plastic material, followed by dissolution of the carbonate so that casts of the
boreholes can be studied. Macroborings are described and differentiated in thin sections and rock and fossil
samples. Many groups produce morphologically distinct borings which are valuable in evaluating paleoenviron-
mental conditions. Distribution patterns of microborers provide a highly promising tool for the differentiation of
paleodepth zones.
1
Modern shell-boring cyanobacteria (
Mastigocoleus testarum
Lagerheim). Long arched tunnels (Ø 6-10
m) growing in
all directions and forming a dense network. Short branches with terminal heterocysts (H). Generally, cyanobacterial
microborings are concentrated in shallower waters. Cozumel shelf, Yucatan, Mexico. Water depth 42 m.
2
Modern shell-boring cyanobacteria (
Hyella gigas
Lucas and Golubic). Thick short filaments with rounded tips, arranged
in clusters. Ø 10-40
m. Upper and lower photic zone. Cozumel. Water depth 10 m.
3
Modern shell-boring green algae (
Phaeophila engleri
Reinke). Filaments are characteristically branched exhibiting dis-
tinct sporangia (arrows). Boring in the uppermost layer of a bivalve shell. Note the prismatic shell microstructure. Gener-
ally, green algal borings are common in shallower waters. Upper part of the photic zone. Cozumel. Water depth 2.5 m.
4
Modern shell-boring fungi.
Spherical and stalked sporangia, oriented to the shell surface. Thin hyphae connecting the
sporangia. Fungal borings occur in shallow-marine and deeper marine environments but increase in abundance in deeper
waters. Lower photic zone. Cozumel. Water depth 20 m.
5
Macroborings in a brachiopod shell.
The primary mineralogy of the brachiopod shell was Low-Mg calcite. Note differ-
ences in size, orientation and shape of the bore holes which may be attributed to the
Trypanites
group regarded as being
excavated by polychaete worms. Early Devonian (Emsian): Anti-Atlas, southern Morocco.
6
Micrite envelope
forming cortoids: Reworked rounded echinoderm clasts are surrounded by thin micritic linings, caused
by multiple microboring (arrows), and subsequent infilling of the bore holes with microcrystalline calcite cement (see
Sect. 4.1.2.3). The microstructure of the echinoderm fragments characterized by reticulate pattern and cleavage planes
partly preserved and partly obliterated by recrystallization of High-Mg calcite to Low-Mg calcite. D: Dasyclad green
algae. Early Permian: Carnia, Southern Alps, Italy.
7
Borings in crinoid bioclasts.
The echinoderm microstructure is still preserved. Note the different infilling and the close
setting of the boreholes. Middle Triassic (Muschelkalk): Southwestern Germany.
8
Lithophagid bivalve borings
in colonial corals. Remains of the bivalves are still preserved within the boreholes (arrow).
Late Jurassic (Kimmeridgian): Lower Saxony, northern Germany.
