A Morarian age for the ‘younger Moines’ of central and western Scotland
MUCH of our recent knowledge of the Moine metasediments1,2, which extend from the Moine Thrust across the Great Glen Fault to the boundary of the Dalradian Supergroup3,4 (Fig. 1), stems from the western Moine region. Here, late-Precambrian, Morarian ages (740–780 Myr) have been established from pegmatites in the Morar Division5, and a Grenvillian age (1,030 ±50 Myr) from the Granitic Gneiss of Ardgour in the Glenfinnan Division6. The ‘Central Highland Granulites’, generally regarded as ‘younger Moine’2,6, are little known geologically. Here we briefly outline the results of field (by M.A.J.P.) and radiometric investigations (by O.v.B.) in the Central Highland Granulites and the western Moines, which have fundamental implications for the evolution of these extensive metamorphic rocks, and for the nature of the Grenvillian–Caledonian interval in this sector of the North Atlantic Crust.
Grenville events in Moine rocks of the Northern Highlands, Scotland
A 14 point Rb/Sr whole rock isochron obtained from the Morar Pelite in the Morar area of the Northern Highlands suggests an age of 1024± 96 Ma for the main metamorphism of the Morar Moines. Within error this age overlaps that recently given for the Ardgour Gneiss and thus confirms the existence of Grenvillian elements within the Northern Highland Block. It suggests that the tectono-metamorphic history of the southwestern Moines is more complex than hitherto thought.
Paleocurrent Analysis in Relation to Modern Marine Sediment Dispersal Patterns
The conventional interpretation of paleocurrent studies is that directional current structures aid in identifying the location of source areas, transport direction, basin architecture, and paleoslope of ancient sedimentary sequences. Numerous examples from ancient fluvial deposits have reinforced this view. Because directional current structures are used in basin studies to explore for and delineate petroleum reservoirs, an understanding of modern current systems in relation to sediment dispersal patterns is economically important. The marine environment is characterized by different factors that control the dispersal of sediments. In coastal environments, directional current structures are fashioned by a combination of tidal, wave- and wind-driven currents which sometimes flow independently from slope and sediment sources. Dispersal patterns resulting from these currents are oriented normal, parallel, opposite, and diagonal to both slope and the location of sediment sources. Bimodal dispersal patterns appear to characterize estuaries, tidal inlets, beaches, and higher and lower tidal flats. Quadrimodal dispersal patterns characterize the tidal flat environment. Unimodal, slope-independent patterns dominate coastal dunes. The deltaic environment appears to be the only coastal environment where sediment dispersal is slope-dependent. The direction of flow of marine currents on continental shelves and in deep ocean basins is controlled by differences in density reflecting temperature and salinity differences, by wind action, by tidal action, and by the rotation of the earth. Some of these deep currents scour the ocean floor. Continental shelf dispersal patterns appear to be unimodal and bimodal and parallel with sea-floor topography. Deep-sea current dispersal patterns appear to be unimodal and parallel with sea-floor topography. Only turbidity currents, slump, and sand flow are characterized by downslope dispersal in deep-sea environments. Complex combinations of ocean current systems are known and include those which converge, diverge, and are stratified. Where two currents converge, one system will flow over the other because of differences in density. Converging current sole-mark orientations have been observed in flysch sediments, indicating that similar converging current systems may have existed in the past. These and other data also suggest that, possibly, many flysch paleocurrent indicators may have been formed by bottom-scouring ocean currents.