Geology Reference
In-Depth Information
Development of hardground formation over time
Box 5.5 indicates higher numbers of studies of Or-
dovician, Jurassic and Cretaceous hardgrounds than
other Phanerozoic hardgrounds. This pattern reflects
not only the interest in specific time intervals but also
the distributional pattern over time (Brett 1988; Wil-
son and Palmer 1990, 1992).
Hardgrounds were particularly abundant in Middle
and Late Ordovician, Jurassic and Late Cretaceous.
Spectacular Ordovician examples are the beautifully
colored
Orthoceras
limestones of Scandinavia and the
Baltics (Lindström 1979).
The general community structure of Ordovician
hardgrounds shows relative little change during the Pa-
leozoic. Mesozoic encrusting communities differ
strongly in their composition from middle Paleozoic
counterparts. The general diversity and guild structures
observed in modern hardgrounds were firmely estab-
lished by the Jurassic. Late Cretaceous chalk hard-
grounds occur as well as cemented hard beds within a
succession of porous, slightly cemented soft chalk. The
upper surfaces of these hardgrounds are strongly bored
and encrusted, and often show signs of abrasion and
corrosion. Glauconitization and phosphatization are
common features. The duration of the depositional hia-
tus associated with the formation of hardgrounds has
been estimated at only several hundreds to thousands
of years for the chalk, as compared to about 200 000
years for Ordovician hardgrounds. Surprisingly little
is known about Late Paleozoic and Tertiary hard-
grounds.
Mechanical concentration
is achieved by filtering
(sieving), screening, bypassing, or scouring sediment.
Condensation by sieving is well-known from neptu-
nian dikes (Wendt 1976; Lehner 1992) and from car-
bonate ramps in times of sea-level rise when the clas-
tic input is reduced (Dromart 1989; Schlager 1989;
Leinfelder et al. 1993). Condensation by screening is
common across strongly rifted passive continental mar-
gins (Bernoulli and Jenkyns 1974; Seyfried 1980; Bice
and Stewart 1990). Condensation by bypassing occurs
on carbonate ramps during sea-level lowstands. Con-
densation by winnowing is a common feature on clas-
tic shelves where burial-exhumation-reworking pro-
cesses are effective (Fürsich 1979; Hallam 1988). A
combination of winnowing and filtering processes dur-
ing rising sea level may lead to shell concentrations on
clastic shelves and bonebeds on carbonate platforms
(Kidwell 1985, 1986, 1987; Nummedal and Swift 1987;
Fürsich et al. 1991). Concentration of lithified bottoms
by scouring requires extense current activity (Wilson
and Palmer 1992).
Chemical condensation
can be caused by dissolu-
tion below the carbonate compensation depth, as
strongly favored by Jurassic cephalopod limestones
from the Tethys (Hollmann 1964; Garrison and Fischer
1969). But precisely these examples were questioned
with respect to the water depths and the processes in-
volved in the production of the condensed deposits
(Schlager 1974). Many authors now favor relatively
shallow depositional environments for the Tethyan con-
densed sections (e.g. Wendt and Aigner 1985; Martire
1992). Relative sea-level fluctuations and a combina-
tion of winnowing and bioerosion of differently lithi-
fied substrates could be responsible for the condensa-
tion features attributed predominantly to dissolution
effects at the sea bottom until now.
5.2.4.2 Condensation Surfaces and
Condensed Sections
Condensed sections are characterized by strata which
are much thinner than their coevally formed strati-
graphic equivalents, due to greatly reduced sedimen-
tation rates. Condensed deposits accumulate over a
large time span and should have a recognizable and
chronological biostratigraphic record. It must be kept
in mind that condensation reflects different factors -
the decrease of the sedimentation rate (stratigraphic
condensation: Heim 1924), the decrease of the accu-
mulation rate (sedimentary condensation), and a mix-
ture of different age fossils (taphonomic condensation:
Brett and Baird 1986).
Condensation through biological concentration
is
caused by bioerosion on hardgrounds as shown by the
common record of microborings at pelagic Triassic and
Jurassic discontinuity surfaces (Seyfried 1981; Schmidt
1990).
Microfacies criteria pointing to condensation and re-
duced or omitted sedimentation in pelagic environ-
ments:
• Centimeter- to meter-sized rock units consisting of
bedded, variously colored limestones and limestone/
marl sequences.
• Alternation of intensively burrowed beds and beds
exhibiting current transport and quiet-water deposition.
•
Controls on condensed sections:
Major processes involved in condensation include
mechanical concentration, chemical concentration and
biological concentration (Fels and Seyfried 1993):
Variously spaced intercalation of hardgrounds ex-
