Abstract: Diagenetic Modification of Carbonates as a Result of Submarine Slumping, a Case History from the Jurassic Helmsdale Fault, Northern Scotland

BUCHEIT, ANDREA, K., and R. NOWELL DONOVAN

The western margin of the Moray Firth, northern Scotland, follows the line of the major north eastward trending Helmsdale fault. During the upper Jurassic, Kimmeridgian, the fault was active and formed a mobile submarine scarp. Slumps, debris slides, rock falls and density flows all contributed to a major stratigraphic unit, the Helmsdale Boulder Bed, that was deposited on the downthrown side of the fault. The fault formed a clear demarcation line between an oxygenated shelf to the west (in the direction of the modern Scottish Highlands) and an anoxic deep water basin to the east (beneath the waters of the modern Moray Firth). Rocks deposited in the latter setting constitute the principal source rock in the North Sea.

Nowhere are the rocks that represent the shelf facies preserved - their presence is inferred from clasts in the Boulder Bed. The clasts include a shelly fauna of molluscs, algae and echinoids together with the colonial coral, Isastraea. These carbonate fragments, together with the cements that hold them together, illustrate the profound diagenetic consequences of movement down the fault scarp into a reducing environment. Initial cementation, which presumably took place on the shelf involved cementation by non-ferroan calcite and some non-ferroan dolomite, as well as limited replacement by chalcedony. Later cementation illustrates a progressive development of iron-rich cements and polymorph infillings. The history of individual fragment can be inferred: some moved rapidly down the scarp, others more slowly.

AAPG Search and Discovery Article #90944©1997 AAPG Mid-Continent Section Meeting, Oklahoma City, Oklahoma