Geology Reference
In-Depth Information
Int. Assoc. Sedimentol. Spec. Publ.
(2009)
41
, 199-213
Controls on facies mosaics of carbonate platforms: a case study
from the Oxfordian of the Swiss Jura
ANDRÉ STRASSER and STÉPHANIE VÉDRINE
Department of Geosciences, University of Fribourg, CH-1700 Fribourg, Switzerland
(E-mail: andreas.strasser@unifr.ch)
ABSTRACT
Modern shallow-water carbonate systems commonly display a complex pattern of jux-
taposed depositional environments with a patchy facies distribution (facies mosaics).
On ancient carbonate platforms, the reconstruction of lateral facies distribution is
often hampered not only by discontinuous outcrop but also by lack of suffi ciently high
time resolution. This case study from the Oxfordian (Late Jurassic) of the Swiss Jura
Mountains demonstrates a way to improve the temporal and spatial resolution for the
interpretation of carbonate rocks. Sequence-stratigraphic and cyclostratigraphic analy-
ses have been performed that provide a basis for defi ning depositional sequences, which
formed through sea-level changes that were induced by the 400-, 100- and 20-kyr orbital
cycles. On the 100-kyr scale, sequence boundaries are well developed and can be cor-
related between sections. However, identifi cation and correlation of sequences related
to the 20-kyr cycle may be diffi cult if local processes overprinted the record of orbitally
controlled sea-level changes. The reconstruction of facies distribution along selected
time lines gives a dynamic picture of platform evolution with time steps of a few ten
thousand years and helps to interpret the controlling factors such as differential subsid-
ence, low-amplitude eustatic sea-level fl uctuations, climate and ecology of the carbon-
ate-producing organisms. Reefs and ooid shoals developed preferentially on topographic
highs and thus accentuated platform morphology. Siliciclastics were shed onto the plat-
form during sea-level falls and increased rainfall in the hinterland; their distribution
was controlled by platform morphology. Siliciclastics and associated nutrients hindered
carbonate production and thus indirectly infl uenced platform morphology. In addition
to these controls, random processes acted on the smaller-scale facies relationships.
Sedimentation rates can be estimated for each facies type over time spans of
10-20 kyr. They can thus be compared more easily with Holocene rates than if aver-
aged over millions of years. This study shows that the Oxfordian platform in the Swiss
Jura was as complex and dynamic as Holocene carbonate systems.
Keywords
Carbonate system, facies, sea-level changes, cyclostratigraphy, Oxfordian,
Switzerland.
INTRODUCTION
in Fig. 1a that is about to isolate a lagoon from
the open ocean, thus creating a drastic contrast
in energy and salinity levels. Storm waves and
storm-induced currents lead to abrupt changes by
redistributing sediment in large quantities, and
by breaching or creating barriers between deposi-
tional environments (Fig. 1b). Storms also induce
ecological changes by breaking corals, which are
later replaced by other species. On longer time-
scales (tens, hundreds, or thousands of years),
fl uctuations in climate and relative sea level
become important factors that lead to changes in
When fl ying over modern carbonate platforms,
when walking on tidal fl ats, emergent ooid shoals
or coral cays, and when snorkelling in shallow
lagoons and over the reef edge it is evident that
there is a large array of juxtaposed sedimentary
and ecological environments. None of these envir-
onments is stable: carbonate mudbanks and sand
shoals shift laterally due to tidal and long-shore
currents and by doing so modify also the environ-
ments adjacent to them. A good example is the spit
