Dear William,

In my opinion, the Miocene Jura-Nagelfluh might be regarded as an analogue to what you describe in your question. Sedimentation of the North Alpine Foreland Basin deposits was characterized by two marine-terrestial cycles (Lower Marine-Lower Freshwater and Upper Marine-Upper Freshwater Molasse). Deposits of the Lower Freshwater Molasse contain lowstand boulder-filled channels (Untere Jura-Nagelfluh, Lower Jura-Nagelfluh). The same hold true for deposits of the Upper Freshwater Molasse that also contains lowstand boulder-filled channels (Obere Jura-Nagelfluh, Upper Jura-Nagelfluh). Channel fills mainly consist of Upper Jurassic limestone boulders up to 0.5 m or so and are embedded in mudstone matrix. The evolution of these sedimetological features is interpreted as a result of the uplift of the Neogene evolution of the (Swiss and S-German) Jura fold-thrust belt and Upper Rhine Graben, caused by the Uplift of the Alps. However, an important factor for the evolution of these channel fills are also the lowstands of the Paratethys in the North Alpine Foreland Basin during sedimentation of Lower and Upper Freshwater Molasse. Unfortunately, most literature is in German language due to the fact that it is a more local phenomenon in Swiss and southern Germany.

The deposits of Lower and Upper Jura-Nagelfluh are mentioned in some English papers (and literature therein), e.g.:

Peter A. Ziegler and Marielle Fraefel. 2009. Response of drainage systems to Neogene evolution of the Jura fold-thrust belt and Upper Rhine Graben. Swiss Journal of Geosciences, Volume 102, Issue 1, pp 57–75.

Jourdan, S., Bernet, M., Tricart, P., Hardwick, E., Paquette, J.-L., Guillot, S., Dumont, T., Schwartz, S. 2013. Short-lived, fast erosional exhumation of the internal western Alps during the late early Oligocene: Constraints from geothermochronology of proand retro-side foreland basin sediments. Lithosphere. vol. 5. no. 2. p. 211-225.

Kuhlemann, J., Kempf, O. 2001. Post-Eocene evolution of the North Alpine Foreland Basin and its response to Alpine tectonics. Sedimentary Geology 152. 45-78.

Pfiffner, O.A., Kempf, O. 2004. Early tertiary evolution of the North Alpine Foreland Basin of the Swiss Alps and adjoining areas. Basin Research. vol. 16. 549-567.

Schlunegger, F., Jodan, T.E., Klaper, E.M. 1997. Controls of erosional denudation in the orogen on foreland basin evolution: The Oligocene central Swiss Molasse Basin as an example. Tectonics. vol. 16. no. 5, 823-840.

Hope this helps!

All the best,

Elmar

Dear Elmar,

I am very grateful to you for such a comprehensive reply. Thanks for taking so much trouble. I never imagined I would get such a fast response. Will now go hunting for the cited references, in the hope they describe something like my strange Purbeck facies.

Thanks again

Best wishes

Bill

Dear Bill,

I rechecked the literature I suggested to you. Unfortunatly, I realized that the deposits of "Jura-Nagelfluh" are only mentioned in the first paper referenced. Sorry for that, it's a long time ago that I worked on this topic. Indeed, it seems that most of the papers dealing with the Lower and Upper Jura-Nagelfluh are written in German language, indeed. Nevertheless, I list a few papers in German language, maybe you are able to read them.

Schreiner, A. 1960. Über die Juranagelfluh im Hegau. Zeitschrift der Deutschen Geologischen Gesellschaft Band 112 (1960), p. 568 - 592.

Hantke, René. 1986. Die Schweizer Jura-Nagelfluh : mehrere Schüttungen in kühl- bis kaltzeitlichen Klima- Einbrüchen im Mittelmiozän? Mitteilungen der aargauischen Naturforschenden Gesellschaft, 30 (1986).

For the last reference, see pdf attached. In the Figs. 3-5, Hantke shows pictures of the boulder-filled channels in Upper Jurassic limestones. Maybe this helps...

And one paper in English language:

M. Strasser, A. Strasser, K. Pelz, H. Seyfried. 2009. A mid Miocene to early Pleistocene multi-level cave as a gauge for tectonic uplift of the Swabian Alb (Southwest Germany). Geomorphology 106 (2009) 130–141. (attached).

The interpretation of the reasons for the sedimentation of the Jura-Nagelfluh are somewhat antiquated in the older papers. It's pretty obvious that the Jura-Nagelfluh deposition corresponds with lowstands of the Paratethys in the Miocene.

In their Fig. 3, they show the stratigraphic position of the JNF.

All the best,

Elmar

Dear Elmar,

This is very kind. Thank you again. I have not yet had a chance to chase the references, so this will help me greatly to refine my focus.

The challenge is finding similar facies and similar palaeogeographic setting: non-marine micrites deposited close to the marine margin, with interfingerings of marginal marine sediments, all in a tectonically quiet region.

Best wishes

Bill