Current controversies in the Caledonides

No abstract available

Timing of deposition, orogenesis and glaciation within the Dalradian rocks of Scotland: constraints from U-Pb zircon ages

The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U-Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ñ 4 Ma), and on the late Grampian 'Newer Gabbros' (470 ñ 9 Ma) of Insch and Morven-Cabrach in Aberdeenshire. These age data, together with the existing 590 ñ 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic (<600 Ma) age and the Ordovician rocks of the Highland Border Complex. (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected

Detrital muscovite Ar-40/Ar-39 ages from Carboniferous sandstones of the British Isles: Provenance and implications for the uplift history of orogenic belts

Major progradations of elastic sediments are recorded in the sedimentary record of the Famennian and the Visean-Namurian of the United Kingdom acid surrounding waters. We have determined Ar-40/(39)A, ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 Ma, spanning both progradations. Detrital mica ages are dominated by a peak at 415 Ma, with minor peaks at 440 Ma and 390 Ma. The 415 Ma muscovites are derived from the unroofing of the Scandian nappes during the compressional phase of the Caledonian orogeny in Scandinavia. The 440 Ma muscovites record pre-Scandian orogenic activity, which is rarely preserved in the orogenic record. Thermochronological evidence suggests that episodic postorogenic uplift, and exhumation events kept the Scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the elastic sediment progradations. The near total absence of detrital muscovites with ages lt 415 Ma suggests that the Scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension. The river(s) which supplied the sediments probably ran parallel to the strike of the major Scandian thrusts, along the length of the Caledonian orogen, in a manner analogous to the major river systems of contemporary orogenic highlands

The Orange River, southern Africa: an extreme example of a wave-dominated sediment dispersal system in the South Atlantic Ocean

Sediments delivered to the South Atlantic Ocean by the Orange River are fractionated and dispersed northwards and westwards by a vigorous longshore drift system and a number of ocean currents. Gravels are accreted to the coastline for a distance >300 km north from the Orange River mouth. Sands are transported alongshore for >700 km but are, in places along this transport path, returned onshore by coastal winds to form the main Namib Sand Sea and other smaller dune fields. Mud is more widely dispersed westwards, northwards and southwards, probably by slow-moving, ocean-scale currents into basins on the shelf and onto the continental shelf edge. This dispersal system, operating since at least Eocene times, is believed to have originated during a time when there was a Late Cretaceous–Early Cenozoic uplift of southern Africa, which resulted in: (1) intensification of the existing southerly wind system; (2) incision of the Orange River, which, coupled with a shift in climate, resulted in a coarsening of its sediment load delivered to the coast; (3) a broad, weakly subsiding or mildly uplifting inner continental shelf with little accommodation space for the sediment load of the incising Orange River

Some observations on diamondiferous bedrock gully trapsites on Late Cainozoic, marine-cut platforms of the Sperrgebiet, Namibia

Namibia's southwestern coast, the Sperrgebiet, hosts one of the world's largest gem diamond placer deposits consisting of fluvial, marine and desert deflation/aeolian placer types. To date, the richest onshore placer discovered in the Sperrgebiet comprises several, Plio-Pleistocene to Holocene, littoral marine packages distributed northwards from the Orange River mouth for some 120 km to Chameis Bay. In this zone, known as Mining Area No. 1 (MA1), these Quaternary marine deposits are floored predominantly by siliciclastic rocks of the Late Proterozoic Gariep Belt that have been bevelled into a number of marine-cut platforms during the different Quaternary high sea level stands (notably, at + 30 m, + 8 m, + 4 m and + 2 m). In many places, these bedrock platforms are extensively gullied and potholed, forming abundant fixed trapsites that promote diamond concentration in the Late Cainozoic littoral sediments. Some observations on the distribution, orientation and incision of bedrock gullies in MA1 are presented here. Three principal bedrock gully types, all of which constitute good diamond trapsites, have been identified in the bedrock footwall between the Orange River mouth northwards to Chameis Bay, a distance of some 120 km: (i) swash-parallel gullies in the southern, proximal sector where the bedrock s2 dips 45°W and the abrasive gravels are most abundant, thus overriding lithological and structural controls in the footwall; (ii) strike-parallel gullies in the central reach where the bedrock s2 dips between 80° and 85°W and the abrasive bedload is reduced, resistant lithological units promote the formation of such features along the orientation of the metsediments; and (iii) joint gullies in the northern, distal portion where the bedrock s2 dips 45–60° to the E and coarse abrasive bedload is virtually absent, the structural pattern in the bedrock is exploited by marine erosion. In these Late Cainozoic placers, diamond concentration is further linked to the depth of the gullies incised into the marine-cut platforms. Maximum incision is noted on the seaward margins of the marine-cut platforms where high-energy littoral processes, acting in a micro-tidal range of ca. 1.8 m under strong prevailing southerly wind and South Atlantic derived southwesterly swell regimes, promote marine erosion and deep gully formation. Therefore, the most favourable diamondiferous trapsites in the Late Cainozoic marine packages of the southwestern Sperrgebiet are those developed in deep bedrock gullies of either swash-parallel, strike-parallel or joint pattern origin on the seaward margins of marine-cut platforms that represent discrete high sea level stands during the Quaternary