Abstract Due to their particularly good mechanical and self-healing properties combined with exceptionally efficient cation adsorbents and exchanger capacities, clay minerals and clay rock formations are considered as suitable geological barriers for radioactive waste disposal. The Middle Jurassic Opalinus Clay Formation has been identified as a potential host rock. Logging data were measured at the Benken borehole drilled through this formation in northern Switzerland. This paper presents a statistical methodology to improve the description of the physical properties of the clay rock based on the well-log data. The methodology involves the classification of a set of local statistics, calculated from a reduced number of principal components computed from well-log properties. The use of a kernel-based method to calculate local statistics allows an analysis of spatial variability to be carried out at different scales, and with different scale effects. The first-order layering was found to be robust and independent of kernel size (i.e. observation scale), while preserving small-scale heterogeneities that are useful for further interpretation. The log units can be more clearly interpreted in terms of stationary or transitional log units, depending on the behaviour of local statistics. Finally, the derived spatial variability of the log-units properties are compared with earlier lithological descriptions and stratigraphic data. Supplementary material: A spreadsheet summary with the determination of clustering parameters for a kernel size of 3 m is available at https://doi.org/10.6084/m9.figshare.c.4315991

Abstract Oligocene and Lower Miocene deposits in the Paratethys are important source rocks, but reveal major stratigraphic and regional differences. As a consequence of the first Paratethys isolation, source rocks with very good oil potential accumulated during Early Oligocene time in the Central Paratethys. Coeval source rocks in the Eastern Paratethys are characterized by a lower source potential. With the exception of the Carpathian Basin and the eastern Kura Basin, the source potential of Upper Oligocene and Lower Miocene units is low. In general, this is also valid for rocks formed during the second (Kozakhurian) isolation of the Eastern Paratethys. However, upwelling along a shelf-break canyon caused deposition of prolific diatomaceous source rocks in the western Black Sea. Overall, Oligocene–Lower Miocene sediments in the Carpathian Basin (Menilite Formation) can generate up to 10 t HC m −2 . Its high petroleum potential is a consequence of the interplay of very high productivity of siliceous organisms and excellent preservation in a deep silled basin. In contrast, the petroleum potential of Oligocene–Lower Miocene (Maikopian) sediments in the Eastern Paratethys is surprisingly low (often <2 t HC m −2 ). It is, therefore, questionable whether these sediments are the only source rocks in the Eastern Paratethys.

Abstract Deep-water conglomeratic megabeds are recognized in the Upper Jurassic Brae member (equivalent to part of the more regionally defined Brae Formation) of the Kimmeridge Clay Formation in UK Block 16/17, within the South Viking Graben in several submarine fans, but have not been described in detail previously. The megabeds are distinguished from enveloping turbidite beds by their fabrics, their scale, and their clast composition. They are distinguished from mass-transport deposits (MTDs), slumps, and slides by being predominantly conglomeratic. The conglomeratic megabeds are compared with a series of conglomeratic megabeds from the Cerro Toro Formation of the Magallanes Basin, southern Chile, with megabeds from eastern Turkey, and with other well-known megabeds from around the world. The megabeds are interpreted as event beds that (1) occur randomly in the stratigraphy and are inferred to have been triggered by seismic events, (2) occur at the initiation of channel complex development, or (3) occur due to sea-level fall and are found at the base of large-scale fining-upward conglomeratic deep-water fans. Though many of the Brae member megabeds are clast-supported and disorganized, and interpreted as the product of avalanche scree from the basin-bounding fault escarpment into deeper water, some have more complex fabric indicating flow transformation. The Cerro Toro megabeds are predominantly more organized, ideally with tripartite or bipartite fabric, though divisions vary widely in occurrence, thickness, and composition. These lithofabrics are interpreted as the product of different flow rheologies that changed in time and space, reflecting flow transformation. The tripartite megabeds are here called transitional event deposits, or TEDs. Division 1 is preceded by erosion and the development of large flute marks, implying that the initial phase of a TED was a turbidity current. The lower part of Division 1 has a thin, clast-supported granular lag with a wavy top followed by a thicker, clast-supported interval with up to boulder-grade extrabasinal clasts and a coarse-granular, less than 10% sandstone matrix. This is interpreted as the deposit of a high-density turbidity current that largely bypassed at this point, leaving disorganized to weakly stratified and locally imbricated conglomerate. Division 2 is transitional with Division 1 over tens of centimeters (several inches) into a progressively more matrix-supported pebbly sandstone. The lower interval of Division 2 has an approximately 50% clast content, 50% coarse-grained sandstone matrix, and an increasing prevalence of mud clasts upward. The mud clasts increase in diameter and angularity upward, with a normal grading of lithic clasts and a gradual upward fining of the matrix. The upper interval of Division 2 is generally poor in extrabasinal clasts, but with an increase in rafted sandstone blocks as well as heterolithic clasts, and also a gradual increase in clay in the matrix. Some of the TEDs have a structureless mud cap with rare floating pebbles. Rafted intrabasinal sandstone clasts are particularly common in the Cerro Toro Formation TEDs and are also recognized in the Brae member. Division 2 is interpreted as the product of a debris flow, within which there has been significant grain size segregation. Division 2 marks rapid flow transformation and the development of a rigid plug as the event rapidly decelerated. Division 3, where present, is a structureless dirty sandstone, with mud chips, that thickens and thins and often pinches out, over the topographically irregular, sandstone block-rich, top to Division 2. It may be transitional with Division 2 but is much more commonly sharp, or even erosive, into it. Division 3 may have a sharp or graded top, sometimes fining to claystone, though the preservation potential for this is low. Division 3 is interpreted as a co-genetic turbidite. The TEDs form one end-member family of a range of megabeds, representing complex large-scale events that were sustained for periods that allow the flows to evolve in time and space, reflecting a progressive collapse of the feeder system in a repeatable manner. A scheme documenting the range of these thick deep-water conglomeratic event beds is proposed. Most of these events are best understood in the context of a fan delta clinoform prograding during river flood onto a muddy deep-water slope and headward-eroding scars on the retrogressive collapse front going through a series of steps that generate a range of events, from simple submarine scree avalanches such as those seen in the Brae systems through to events that changed and transformed during transport to produce complicated tripartite event beds such as a TED.