ABSTRACT: A Quaternary Analog for a Mixed Carbonate and Siliciclastic Sequence on the W. Scottish Shelf
Maurice A. Cucci
Quaternary sediments and temperature biogenic carbonate sediments occur as part of a short-duration (estimated at 20 Ka) sequence in and beneath the shallow (<20 m) marine waters of the Sound of Iona (the inner western continental shelf of Scotland). The deposits were studied using high-frequency seismic, sonar, scuba, dredge, core, video, radiometric, and onshore surveys.
These deposits unconformably overlie Paleozoic metasediments. They form three system tracts: (1) a lowstand siliciclastic tract composed of a sediment gravity flow and an overlying deltaic unit, (2) a trangressive siliciclastic cobble and boulder horizon and (3) a transgressive to highstand bioclastic sand. The siliciclastic deposits are poorly sorted, glacially derived deposits which formed during a Pleistocene lowstand. The terrigenous transgressive deposits, formed during the early Holocene rise, are the winnowed strata of the earlier lowstand delta. Carbonate sediments (barnacles, mollusks, and coralline algae) were deposited during the later transgression and succeeding highstand. The carbonates overlie and terminate against the siliciclastic sediments via onlap and downlap.
The study supports a hypothesis that siliciclastic deposits can predominate during lowstands, whereas biogenic sediments occur during transgressions and highstands; high-productivity carbonate organisms exploit shelfal, terrigenous-sediment-depleted locales. This stratigraphic relationship helps explain the anomalous existence of ancient interbedded limestone and glacial deposits that occur near this study area and other localities worldwide.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990