Geology Reference
In-Depth Information
stones. The thin, curved elements exhibit a single-lay-
ered or multilayered calcitic prismatic microstructure.
These accumulations within distinctly defined beds
were originally regarded as algal filaments and there-
fore termed 'filaments'. The fossils have been now
proved to be protoconchs or juvenile valves of thin-
shelled bivalves (Peyre 1959; Conti and Monari 1992).
Most authors ascribe Jurassic filaments to juvenile
shells of posidoniid bivalves (
Bositra, Posidonia
) and
Triassic findings to halobiid bivalves. Interpretations
of the life habits of
Bositra
and related taxa are still
controversial and include nektoplanktonic or pseudo-
planktonic as well as benthic modes of life. 'Limestone
with filaments' is common and widely distributed in
the Jurassic (ranging from Toarcian to Kimmeridgian;
abundant in the Middle Jurassic and early Late Juras-
sic) of the Tethys (Pl. 87/3; Pl. 113/2; Pl. 137/1). The
depositional settings of filament-rich limestones cor-
respond to the standard facies zones 1, 2 and 3.
dense clusters with mutual cementation), clingers
(growing alone, in close associations or in dense clus-
ters with mutual cementation. Attached to other rud-
ists, corals or hardgrounds), and recumbents (horse-
shoe-like forms with the long axis of both valves lying
parallel to the sediment-water interface). There is cur-
rently a lively discussion on whether or not rudists
formed rigid reef frameworks or were bafflers form-
ing bafflestones. The depositional settings of rudist
limestones usually correspond to the standard facies
zones 4, 7 and 8.
Rudists are common in Cretaceous platform, ramp
and reef carbonates exhibiting a biofacies response to
the windward/leeward position and shallowing-up-
wards cycles. Their habitats stretched from protected
and open-marine parts of platforms to shelf margins
where they produced huge amounts of bioclastic rud-
ist sands. Rudists formed small bouquets, larger clus-
ters or extended thickets, and contributed significantly
to the formation of extended reef belts (e.g. Oman; Fig.
10.42). Rudist density reflects differences in sedimen-
tation rates.
Elevator rudists were efficient producers of bioclas-
tic sands that accumulated near Late Cretaceous car-
bonate platform margins. Boundstones and associated
bioclastic sand piles form major oil and gas reservoirs
along shelf margins (Mexico, Dubai, Abu Dhabi,
Oman). Rudist shells have a high intraskeletal poros-
ity. Additional fracturing increases the reservoir poten-
tial. Leaching of the aragonitic parts or of the whole
shell during freshwater diagenesis can cause moldic
porosity (Cestari and Sartorio 1995).
Rudist limestones are widely used as decorative
building stones all over the world.
Cretaceous rudist limestones:
Rudists are sessile
gregarious bivalves known from Late Jurassic (e.g.
Diceras
limestones) until the end of the Cretaceous,
characterized by a lower attached and an upper oper-
cular valve of different sizes. Three morphotypes are
distinguished based on life orientation (Skelton and Gili
1991): conical or cylindrical elevators (vertical upward
growth; growing alone, or in close associations or in
Basics: Bivalves
Carter, J.C. (1980): Guide to bivalve microstructures. - In:
Rhoads, D.C., Lutz, R.A. (eds.): Skeletal growth of
aquatic organisms. - 645-673, New York (Plenum Press)
Carter, J.C. (1990): Evolutionary significance of shell mi-
crostructure in the Palaeotaxodonta, Pteromorpha and
Isofilibranchia (Bivalvia: Mollusca). - In: Carter, J.C.
(ed.): Skeletal biomineralization: patterns, processes and
evolutionary trends. - Volume I, 136-199; volume II (at-
las), pls. 1-121. New York (Van Nostrand)
Cestari, R., Sartorio, D. (1995): Rudists and facies of the
Periadriatic domain. - 207 pp., S. Donato Milanese (Agip)
Conti, M.A., Monari, S. (1992): Thin-shelled bivalves from
the Jurassic Rosso Ammonitico and Calcari a Posidonia
formations from the Umbrian-Marechean Apennine (Cen-
tral Italy). - Palaeopelagos,
2
, 193-213
Elorza, G., Garcia-Garmilla, F. (1998): Paleoenvironmen-
tal implications and diagenesis of inoceramid shells (Bi-
valvia) in the mid-Maastrichtian beds of Sopelana,
Zumaya and Bidard sections (coast of the Bay of Biscay,
Fig. 10.42.
Monospecific association of rudists
(
Vaccinites
vesicularis
Woodward) contributing to the formation of spec-
tacular Late Cretaceous reefs that may represent outstanding
reservoir rocks. Campanian: Saiwan, Central Oman. Ham-
mer for scale. From Schumann (1995).
