Geology Reference
In-Depth Information
in the Swiss Jura have been studied extensively
by Dupraz (1999) and Dupraz & Strasser (2002).
Coral diversity is low to moderate. The corals
are commonly bioeroded and covered by micro-
encrusters (cyanobacteria, foraminifera, bryozo-
ans, serpulids, sponges) and by microbialites.
Conditions varied from oligotrophic allowing
for healthy coral growth to meso- and eutrophic
when nutrient levels were high and microbialite
growth was favoured. Nutrient input was coupled
with terrigenous input, which in addition dimin-
ished water transparency. This situation occurred
preferentially when sea level was low (see above).
Consequently, the trophic and photic conditions
that controlled coral growth were directly linked
to climatic and sea-level changes.
Micro-encrusters and microbialites also occur
in oncoids, which are important components
in the sections studied (Fig. 6). Other carbonate
producers include echinoderms, foraminifera,
brachiopods, bivalves, gastropods, ostracods and
serpulids. Dasycladacean algae are rarely found,
possibly due to early diagenetic aragonite dis-
solution. Charophytes occur only in the upper-
most part of the Günsberg/Röschenz Member at
Hautes-Roches and point to local development of
freshwater ponds. Oncoids and the heterotrophic
fauna are found in pure carbonates as well as in
the facies containing siliciclastics, while the coral
reefs fl ourished in the absence of siliciclastics.
maximum-fl ooding surfaces formed 10 kyr after
the sequence boundaries.
In order to best explain the observed features
along the chosen time lines, an initial tectonic
control has to be postulated, which determined
the position of the reefs and ooid shoals at
Hautes-Roches and Gorges de Court (Fig. 10).
Periodically, there was also emersion in this area,
as testifi ed by charophytes at Hautes-Roches and
tidal fl ats at Court. This topographic high was very
close to the open-marine realm because steno-
haline brachiopods and echinoderms are found
together with charophytes. A barrier system of
unknown nature must be assumed south of Pertuis
and Savagnières, in order to produce the protected
lagoonal facies there. Occasionally, ooid shoals
developed close to these sites. At the time when
sequence boundary (a) formed, climate must have
been humid, and siliciclastics were shed onto the
platform. Savagnières was less affected by this
input, and a slight topographic high is assumed
in that area. This high could be the result of dif-
ferential subsidence but may have been enhanced
by the accumulation of thick ooid shoals 300 kyr
before (Fig. 6). Siliciclastic input then diminished
rapidly, partly due to a drier climate, partly to sea-
level rise at the beginning of a 400-kyr eccentricity
cycle. Biogenic carbonate production was high
because the ecological conditions were favourable,
and ooids formed because the water was warm
and well saturated. Sediment accumulation kept
pace with rising sea level. Sea-level falls related
to the 20-kyr precession cycle were attenuated by
the general rising trend and infl icted only minor
channel formation at Vorbourg. Hautes-Roches
was dominated by ooid shoals, with a sand wave
forming in a tidal pass. The development of a tidal
fl at with microbial mats on top of the coral reef
at Gorges de Court possibly implies a seasonally
wet climate at sequence boundary (d) (Wright &
Burchette, 1996).
These reconstructions, albeit interpretative,
clearly show that at any time the platform was
structured and hosted several, juxtaposed depo-
sitional environments. Seafl oor topography was
created by differential subsidence but also by ree-
fal constructions and accumulation of ooid shoals.
The input of siliciclastics, triggered by falling sea
level and by increased rainfall in the hinterland,
slowed down carbonate production. It is therefore
not a contradiction to postulate relatively deep
lagoons at sequence boundary (a) and shallower
environments at the time of maximum fl ooding (c)
(Fig. 10). The source areas for the siliciclastics
Facies mosaics
Reconstructing the facies distribution on an entire
platform and its evolution through time based on
only fi ve sections is of course a highly speculative
enterprise. Nevertheless, there are certain clues
that are given through the time lines established
by the cyclostratigraphical analysis, and by the
logical interpretation of the facies occurring along
these time lines. For example, a wackestone with
oncoids and ooids implies a low-energy lagoon,
which was protected from the open ocean by an
ooid shoal from where the ooids were washed
in. If the ooids are corroded and/or encrusted, a
time lag between ooid formation and deposition
in the lagoon has to be postulated. For the recon-
struction presented in Fig. 10, four time lines
have been chosen: three sequence boundaries and
a maximum-fl ooding surface (Fig. 9). According
to our cyclostratigraphic analysis, the intervals
between these lines represent 20, 30 and again
30 kyr, assuming in a simplifi ed manner that the
sea-level cycles were symmetrical and that the
