ABSTRACT: Quaternary Sediments and Facies Relationships in the Sound of Iona, Western Scotland

Maurice A. Cucci

This study presents the sedimentological relationships between interbedded carbonate marine sediments and siliciclastic glacial deposits in the Sound of Iona. In this area, there is an abrupt vertical and lateral facies transition between siliciclastic sediments deposited in turbid, cold water and biogenic limestones deposited in clear, warm water. The reason for this abrupt transition is that highly productive, carbonate-secreting organisms were rapidly deposited during the temperate-climate marine transgression. The carbonates abruptly cover preexisting glacial lowstand deposits. Terrigenous influx is low during the transgression because glacial erosion has removed siliciclastic sediments from coastal source areas.

The Sound of Iona is carpeted by a Holocene deposit of carbonate sand, estimated from seismic data to be 3-5 m thick. Point-count data show that the sand averages 75% carbonate, with comminuted barnacle shells comprising over 50% of the sand.

Four facies are recognized: (1) rippled carbonate sands lying on the margins of the sound, which grade laterally into (2) centrally located carbonate sand waves and sand ribbons, which lie downstream to (3) in-situ coralline algae deposits (maerl) which in turn overlie (4) relict glacial drift deposits.

The sand-wave facies forms the thickest deposit (^sim5 m). The sand is localized over a bathymetric high formed from an older deposit of glacial drift. Carbonate-sand deposits are thin or absent distal to the sand-wave facies; relict glacial deltaic deposits lie exposed in these outlying localities.

The barnacle-rich, rippled carbonate-sand facies is deposited over preexisting glacial deposits, producing an abrupt vertical facies transition. These carbonate sands are transported by wind-generated wave and low-velocity tidal currents. As these sands are further transported by high-velocity tidal current toward the central bathymetric high, they are sculpted into a hierarchy of bed forms dominated by sand waves. A sand ribbon associated with the sand waves is created by tidal currents accelerating around a midchannel barrier island. Maerl grows in a wave-sheltered location where there is a very low ambient sedimentation rate. Glacial deposits are exhumed in proximal areas by scouring or lie exposed in distal areas of very low sedimentation.

Recent carbonate sedimentation is the local sediment response to both the Holocene, temperate-water marine transgression and terrigenous-trapping catch basins created by Pleistocene glaciation. Glacial scouring has created local bathymetric deeps and convolute coastlines which inhibit siliciclastic influx to the Holocene Scottish shelf. Shallow-water, shelfal island areas free of siliciclastic influx are exploited by high-productivity, carbonate-producing organisms. At death, the organisms shed their shells, creating abundant bioclastic sands that cover preexisting glacial deposits. This creates an abrupt vertical change in lithology.

These Holocene carbonate and Pleistocene glacial deposits are modern analogs to the interbedded Precambrian carbonate and siliciclastic glacial rocks seen in outcrop on west Scottish islands.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990