Abstract: Turbidite Sedimentation in Cambrian of North Wales
Jutta H. Binstock
Until about 25 years ago the Cambrian geosyncline of North Wales was regarded as a sequence of shallow-water sandstones and shales. Understanding of turbidity currents led to reinterpretation of the massive graded sandstones in the Lower and Middle Cambrian part of this sequence as deep water in origin. Left unresolved, however, was the nature of the depositional environment of an interbedded manganiferous shale sequence. This still was attributed to shallow-water, even lagoonal, deposition. Recent reexamination of this Lower and Middle Cambrian section, coupled with today's improved knowledge about manganese precipitation, makes clear that the manganese shales are an integral part of the deep-water turbidite sequence. In fact, the manganese enrichment is related directly to the pelagic intervals in that sequence. The Lower and Middle Cambrian succession as a whole most logically can be seen as a submarine-fan distributary system, where interdistributary areas containing pelagic shales alternate with channel sands and overbank deposits. One such area in the Harlech dome produced an ore bed with manganese oxide concentrations up to 50% in a pelagic shale interval. Other periods of pelagic deposition, although evident elsewhere in the section, did not produce the same degree of manganese enrichment, partly because they were not so long, and partly because coarser sediment was not diverted so completely.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC