Geology Reference
In-Depth Information
Fig. 9.13
.
Macroborings
produced by cirripedians of the group Acrothoracica. The borings are characterized by a amphora-
like shape with a narrow neck at the anterior end. The borings occur within a reef limestone with abundant bryozoans (cell-
like structures). Note the reversed geopetal fabric in the borings indicating overturning of parts of the reef mound. These are
the oldest Mesozoic cirripedian borings subsequent to the Permian-Triassic extinction event. The association of ascothoracid
borings and trepostomate bryozoans in the Late Permian reefs and in the Anisian underlines the Paleozoic aspect of the early
Middle Triassic. Middle Triassic (Anisian): Olang Dolomites, Southern Alps, Italy. Modified from Senowbari-Daryan et al.
(1993). Scale is 2 mm.
groups in modern environments are sponges, bivalves
and 'worms', followed by specialized cirripedians (Fig.
9.13). These groups were also the principal macroborers
in the Mesozoic and Cenozoic. The identification of
the producer of macroborings in thin sections is diffi-
cult, but should by tried using the morphological crite-
ria summarized in Box 9.4.
Macroborers are responsible for extensive substrate
destruction, sediment production and the generation of
secondary porosity in reef frameworks.
Bivalves
: Boring into reef-building organisms by
shells is extremely well developed within three fami-
lies of bivalves
(Warme 1975; Kleemann 1980, 1983,
1990; Bromley 1981). The Pholadidae are exclusively
borers primarily using mechanical means of penetra-
tion. The Gastrochaeniidae and some Mytilidae pen-
etrate predominantly calcareous substrates using me-
chanical and chemical means. Bivalve borings have a
major impact on the destruction of littoral sediments,
Box 9.4.
Some morphological trace fossil criteria of
macroborer groups
(after Perry and Bertling 2000). Re-
views assisting in the identification of borings were pub-
lished by Warme (1975), Bromley (1978), Bromley and
D'Alessandro (1984, 1987), Pleydell and Jones (1988),
and Fürsich et al. (1994).
Macroboring groups
Sponges:
Boring sponges have received the most at-
tention of all groups of boring groups because of their
role in bioerosion processes, sediment production and
calcium carbonate dissolution (Neumann 1966; Futterer
1974; Rützler 1975; Torunski 1979; Reitner and Keupp
1991; Acker and Risk 1985).
Boring sponges (Fig. 9.11) form large chambers,
with smaller galleries branching off the main cham-
bers. Bromley (1978) noted that the cavities can be iden-
tified even after fossilization, allowing sponge borings
to be distinguished from other borings. Demospongid
borings are differentiated according to the size of the
external openings and the chamber system (Bromley
and D'Alessandro 1984). Traces of sponge borings re-
sembling present day Demospongiae have been found
in Early Cambrian archaeocyathid reefs and are com-
mon in Permian to Cenozoic carbonates.
Sponges:
Complex branched multi-apertured net-
works of large chambers; multi-apertured irregular net-
work without chambers; stellate structures with single
aperture.
Polychaete worms
: Cylindrical borings. Round or
dumbbell-shaped cross section. Mostly single-apertured.
Sipunculids
: Cylindrical, straight to gently curved,
round cross sections, single aperture.
Phoronids
: Cylindrical and U-shaped, multi-aper-
tures, or small, branched with cylindrical branches, multi-
apertured.
Bivalves
: Flask-shaped. Single apertured.
Cirripedians:
Small amphora-like borings with a slit-
like apertura in the narrow exterior part.
