Mesozoic Sedimentation on the Eastern Falkland Plateau

Abstract

Mesozoic sediments were encountered in two borings on the Falkland Plateau, Hole 327A and Site 330, both of which are situated in water depths on the order of 2500 meters on the southern flank of the slightly elevated eastern end of the plateau (Figure 1). At Hole 327A, calcareous ooze of Late Cretaceous (Maestrichtian) age was first encountered at a subbottom depth of 90 meters and the hole was terminated in carbonaceous claystones of Early Cretaceous (Neocomian-Aptian) age at a subbottom depth of 470 meters. At Site 330, calcareous (nanno) clay of Albian age was first cored at a subbottom depth of 129 meters and thereafter Mesozoic sediments ranging back to at least Upper Jurassic (Oxfordian) were drilled to a total depth of 550 meters. Below these a probable old soil profile and underlying basement rocks of granulitic gneiss and granite pegmatite were encountered. The Mesozoic sequences penetrated at these two sites suggest a depositional history as follows: 1) subaerial (paralic ?) sedimentation resulting in infill and aggradation of local bedrock basins; 2) Middle (?) to Upper Jurassic marine transgression followed by accumulation of predominantly terrigenous silts and clays in an open-shelf environment; 3) euxinic conditions and deposition of dark carbonaceous claystones which began in Upper Jurassic and persisted until near the end of the Lower Cretaceous (late Aptian); 4) open marine deposition of pelagic carbonate oozes and zeolitic clays through the remainder of the Mesozoic and most of the Tertiary. The following discussion summarizes the characteristics of the Jurassic and Lower Cretaceous sediments encountered at these two sites and their interpretation in terms of depositional environments and geologic events. Detailed lithologic descriptions of the sedimentary column at these sites are presented in other chapters, this volume.