No abstract

The Late Cretaceous depositional history of the continental margin across the Goban Spur can be divided into two episodes, sediment records of which are separated by an upper Campanian unconformity traceable from the foot to the top of the margin. Below the unconformity, there was no deposition at the shallowest site (548), and deposition of relatively short sequences of foraminiferal and nannofossil-bearing white chalks, with indications of hiatuses or condensation, took place at the two sites of intermediate depth (549 and 551). The varied clay-mineral associations show a marked terrigenous influx superimposed on the mostly biogenic calcareous sedimentation. A 1-m-thick bed of carbonaceous shale and radiolarian chert interbedded with low-TOC white chalks was drilled at the boundary between the Cenomanian and the Turonian of Sites 549 and 551. Interpretations of this lithology are discussed. Above the unconformity, layers of nearly pure chalks (upper Campanian to Maestrichtian), with frequent signs of redeposition, blanket the whole margin, except for the steeper slopes like the Pendragon Escarpment. The Albian ocean crust drilled at Site 550 lay farther from the contemporary shore line than the other drill sites. In the Cenomanian the still narrow oceanic trough received nearly pure calcareous mudstones under alternating conditions of poorly and normally oxygenated bottom water. A sediment gap at Site 550 spans the upper Cenomanian and the whole of the Turonian. From the lower Senonian to the lower Paleocene, chalky turbidites are interbedded within hemipelagic mudstones. Deposition probably took place beneath the CCD in the early Senonian. The mineralogical composition of the hemipelagic part of these turbidites shows a significantly greater terrigenous influence than coeval deposits at shallower sites.

We determined concentrations of trace elements in the carbonate fraction by atomic absorption analysis following the procedure described by Renard and Blanc (1971, 1972). The age of the samples studied range from Barremian to middle Eocene (Holes 390, 390A), Tithonian to Aptian (Hole 391C), and late Campanian to Barremian (Hole 392A). Mineralogically, they all contain low magnesian calcite. The results of the analysis are given in Tables 1 to 4. HOLE 391C Relationship With Insoluble Residues In some samples, we obtained sufficient insoluble residues, following treatment with acetic acid, to relate certain element concentrations to percentage of insoluble residue (Figure 1). We noted a distinct, positive correlation between concentrations of potassium, sodium, and magnesium and the percentage of insoluble residue. (Some of these elements may have been flushed into the insoluble fraction in spite of the precautions taken.) We found a positive, but less pronounced, relationship between zinc and strontium concentrations and insoluble residue, whereas we detected no correlation between manganese concentrations and insoluble residue. Because of this relationship we consider the results obtained for potassium, sodium, and magnesium, and those obtained for strontium, and zinc, when the samples contain more than 10 per cent insoluble, to be unreliable. Strontium The concentration curve of strontium contents is very characteristic for samples containing more than 90 per cent carbonates (Figure 2). The amount of strontium decreases continuously from the Aptian to the Tithonian sediments; a rapid decrease occurs in the lower Valanginian-upper Barremian sediments. We attribute this to increased diagenesis with greater depth which reflects the degree of diagenesis as a function of age. This is consistent with earlier claims of Sheeiman and Shirmohammadi (1969), Kinsman (1969), and Renard (1972) that strontium is a reliable measure of carbonate diagenesis. The similarity of the strontium concentrations and the sediment accumulation curves illustrates that the highest rate of sedimentation (2.8 cm/10 3 years) corresponds to the greatest loss of strontium. A rough correlation between the gradient of the strontium concentration curve and the sedimentation rate is given in Figure 3.

No abstract

No abstract