A high-resolution interpretation of Kimmeridgian depositional
environments, omission surfaces, and sequences (Courtedoux – Canton
Jura – Switzerland)
Christina Keller
Although much research has already been accomplished concerning palaeoclimate and sea-level
changes on the shallow Jura carbonate platform by means of sequence- and cyclostratigraphy (e.g.,
Hillgärtner & Strasser 2003; Hug 2003; Samankassou et al. 2003; Colombié 2002; Strasser et al.
2000), analyses with a time-resolution of a few thousand to a few ten thousand years and focussing
on omission surfaces are rare (Hillgärtner 1998). Omission surfaces are discontinuity surfaces that
mark temporary halts in deposition but involve little or no erosion (Bromley 1975). As they are the
only record of time of non-deposition, they are indispensable for comprehensive palaeoenvironmental
reconstructions. The purpose of this study is to examine the relationships between carbonate platform
omission surfaces and small- to large-scale sea-level changes by means of a high-resolution analysis
of microfacies and sedimentary structures. The investigated sections in Courtedoux in the Swiss Jura
Mountains consist mainly of packstones and marls, and comprise three omission surfaces. A sheltered
lagoon with favourable environmental conditions, with moderate energy and generally normal
salinity is considered as the main depositional environment, which features only minor changes in the
vertical and thus temporal succession. Laterally, however, short-distance variability is common. The
sedimentary deposits indicate the succession of the regressive part of a first depositional sequence
followed by two complete sequences, which both represent the stratigraphical record of one cycle of
relative sea-level change (Strasser et al. 1999). Based on facies evolution and stacking pattern, and in
correlation with other sections investigated and dated in other studies (Colombié 2002; Rameil 2005),
the three omission surfaces are interpreted as the sequence boundaries (SB) of the two small-scale
sequences, which are corresponding to 100-kyr orbital eccentricity cycles. Even though the three
omission surfaces are all interpreted as SBs, they have different appearances. Surface 1000 exhibits
borings possibly indicating shallow ponds, whereas laterally dinosaur tracks are common. This
implies an irregular topography and a partial and/or temporary emersion of the surface. Besides
borings and probable sauropod tracks, surface 1500 bears wave ripples, which all clearly entail a
surface of emersion with a slight relief, suggesting that the lower areas were covered with shallow
water. Surface 2000 has been bioturbated by Thalassinoides-producing animals while the sediment
was still soft. Later, a firm-/hardground developed, which displays Gastrochaenolites and Trypanites
and was subsequently colonised by encrusting organisms such as oysters and serpulids.
Consequently, surface 2000 implies maximum-flooding conditions (MF). On the other hand, the beds
between SB 1500 and surface 2000 feature the same succession as the sequence between SB 1000
and 1500, which suggests that surface 2000 also is a SB. Showing evidences for both SB and MF,
surface 2000 is interpreted as a SB of a 100-kyr sequence superimposed on the MF of a longer-term
(400-kyr) sequence. It is concluded that a carbonate platform does not correspond to a uniform
carbonate factory but represents a facies mosaic, in which sediment accumulation rates vary laterally
as well as through time. In order to accurately interpret omission surfaces, it is therefore important to
examine three-dimensional outcrops where lateral changes can be observed and, thus, a sufficient
amount of information is available.
André Strasser