Abstract

Professor T. N. George congratulated the authors on the high refinement of their analyses of sediments that, in their vertical and lateral variations, provided the sedimentological evidence for close palaeogeographical reconstruction.

A particular problem arose in the frame of the physical context in which the sediments accumulated. The basin of deposition appeared to be in more or less degree walled-in on three sides, with access of detritus limited to comparatively narrow routes. The analogy with tongues of the Mississippi delta could therefore only be a general one, with specific elements in the sediments offering similarities with incidental and small-scale spreads of Mississippi muds and silts. While the cycles of sedimentation were clearly defined by carbonaceous layers, it was also a problem to relate marine incursions to such a basinal environment without invoking very considerable variations in the source area of the incoming sediments, but it would be interesting to know if Dr. Read was prepared to link the rhythmic changes as pulses of subsidence, with corresponding pulses of uplift in the hinterland.

Moreover, the sustained influence of structures within the basin of sedimentation, as revealed by the parallel changes in thickness and facies seen at successive horizons, suggested that tectonic controls were built into the detailed frame of the basin and were not buried to the point of elimination as the cover of sediments thickened. Analogy with elements in the Mississippi was again at best only approximate, for clearly the growth of the Scottish Namurian deltas wasnot by simple

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