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.