ROTLIEGEND FACIES, SEDIMENTARY PROVINCES, AND STRATIGRAPHY, SOUTHERN PERMIAN BASIN UK AND THE NETHERLANDS: A REVIEW WITH NEW OBSERVATIONS

The environments of deposition of the Upper Rotliegend in the Southern Permian Basin have been of continuing interest since the discovery of the Groningen Gas Field in 1959. This is because the quality of the porosity and permeability of be sands is in most places dependent upon the original depositional environments. In the earliest years, the emphasis of much work was on mapping broad depositional realms of aeolian, fluvial, and playa environments. As new discoveries came under development, the geometry of small-scale features such as dune (good reservoir) and interdune (poor reservoir) became important, informed in part by studies of modern deserts. As the Rotliegend play matured, the emphasis of studies shifted to approaches that recognised rock units below the level of the known members of the Rotliegend that were tied into process framework elements such as climate change. The present study uses modem analogues for the Rotliegend at Bristol Dry Lake and Cadiz Dry Lake, in the California desert, U.S.A., as a basis for the discussion of ancient Rotliegend depositional environments. At these sites active aeolian, fluvial, and playa depositional environments have produced a sedimentary record similar to that in the Rotliegend. Additionally, there exists at Bristol Dry Lake a sand-flat sedimentary domain that constitutes a transition zone between the playa and surrounding fluvial and aeolian sedimentation, where the processes associated with all three major environments are roughly in balance. A sand flat also exists in the ancient Rotliegend, although it is more commonly referred to as the "transition zone" in North Sea literature. At Cadiz Dry Lake the geographical resemblance to the Rotliegend is impressive, with winds blowing parallel to the elongation of the playa and ephemeral streams entering from the margins in a manner analogous to the Rotliegend of the Southern North Sea, where fluvial and wind transport directions are at roughly right angles in much of the basin. Our work suggests that in our study area the Silverpit Formation has the qualities of a desert playa lake that was episodically flooded. We illustrate here with core examples the major lithofacies of the Rotliegend, including aeolian, fluvial, and playa sediments. These are tied to a stratigraphic scheme that is based on the major formations of Upper and Lower Slochteren, and a new, if minor, member known as the Hyde Sandstone, representing the last "regression" of the playa in the UK. Isopach and palaeogeographic maps of the major sedimentary domains that are dominated by aeolian, fluvial, or playa processes illustrate the evolution of the Rotliegend. The maps show the irregularity of the Sand Flat and its lateral shifts through time, as well as the distribution of the other domains due to dominance of wind or fluvial sedimentation locally. They also illustrate the dependence of much of the facies distribution on the palaeotopography of the Carboniferous. The development of dune fields that became the best reservoir rock was dependent upon the location of fluvial systems that supplied sand to the wind, and upon topographic slope. Much of the best dune reservoir was deposited not in the basin centres, where it became deeply buried and compacted, but on the flanks of major structures as windward and leeward sand seas and dune fields.