Controls on Facies Mosaics of Carbonate Platforms: A Case Study from the Oxfordian of the Swiss Jura

Abstract

Modern shallow-water carbonate systems commonly display a complex pattern of juxtaposed 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 sufficiently high time resolution. This case study from the Oxfordian (Late Jurassic) of the Swiss Jura Mountains shows a way to improve the temporal and spatial resolution for the interpretation of carbonate rocks. We have performed a sequence-stratigraphic and cyclostratigraphic analysis that allows defining 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 correlated between sections. However, identification and correlation of sequences related to the 20-kyr cycle may be difficult 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 tenthousand years and helps interpret the controlling factors such as differential subsidence, lowamplitude eustatic sea-level fluctuations, climate and ecology of the carbonate-producing organisms. Reefs and ooid shoals developed preferentially on topographic highs and thus accentuated platform morphology. Siliciclastics were shed onto the platform during sea-level drops and rainfall in the hinterland; their distribution was controlled by platform morphology. Siliciclastics and associated nutrients hindered carbonate production and thus indirectly influenced 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 to 20 kyr. They can thus be compared more easily with Holocene rates than if averaged over millions of years. Our study shows that the Oxfordian platform in the Swiss Jura was as complex and dynamic as Holocene carbonate systems.