Shallowing-upward sequences in Purbeckian peritidal carbonates (lowermost Cretaceous, Swiss and French Jura Mountains)



Purbeckian carbonates in the Swiss and French Jura (Goldberg Formation, lower Berriasian) comprise shallow-subtidal, intertidal, supratidal, low-energy, high-energy, marine, brackish, freshwater, and hypersaline facies. These facies are arranged in small (0–2–1.5 m thick) sequences which display a dominant shallowing-upward component, and which form the fundamental units of the highly structured Purbeckian sedimentary record.

Six types of small-scale sequences can be recognized. A: intertidal to supratidal overprinting of shallow lagoonal facies; B: algal-marsh sequence with frequent dolomitization; C: sabkha sequence, often associated with collapse breccia; D: tidal-flat sequence with desiccation features; E: lacustrine sequence; F: terrestrial overprinting of subtidal or intertidal facies. Episodic event deposits such as tempestites are superimposed] Thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences. Green marls and black pebbles are common at the top and indicate long subaerial exposure. The sequences are often incomplete, as subtidal facies may be absent, or their upper part can be eroded. Lateral facies changes are common, which is due to the very shallow and partly emergent Purbeckian platform where various depositional environments were juxtaposed. However, many sequence boundaries are well developed and can be correlated over large parts of the study area.

The Purbeckian shallowing-upward sequences were generated by climatically controlled sea-level changes. Autocyclic processes occurred locally, but were overprinted by drops of sea-level affecting the entire platform. The small-scale sequences are most probably related to the 20 000-year cycle of the precession of the equinoxes. Larger sequences with usually well-developed emersion surfaces are attributed to the 100 000 and 400 000-year eccentricity cycles of the Earth's orbit. Identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces (which often cut across facies boundaries), and thus to interpret in detail the palaeogeographic, sedimentological and diagenetic evolution of the Purbeckian peritidal carbonate environments.