Charles F. Stehman

A study of the paleontology and sedimentology of eight cores from the Eastern Crestal Mountains of the Mid-Atlantic Ridge near 45^o North and one core from the vicinity of the Dalhousie Seamount to the West of the Ridge demonstrates that sediments in these areas result from bottom reworking and concentration of pelagically deposited material.

Paleontological analysis using planktonic Foraminifera indicates a lack of water mass change over the study area during the deposition of the cored sediment. Thus conventional methods of marine Pleistocene stratigraphy are inapplicable to the sediment sections studied. Further quantitative examination of the faunal content of the cores reveals a changing planktonic foraminiferal succession through time, which is common to all the cores; this allows stratigraphic interpretation and correlation among the cores. The stratigraphy and a test based on Planktonic foraminiferal numbers make possible detection of undisturbed and disturbed sediment sequences. Identification of disturbed sediment sections and the relative stratigraphic positions of the undisturbed sections in the cores is combined with a climatic index, based on coiling direction of the species of planktonic Foraminifer Globigerina pachyderma, to construct a chronology of the stratigraphy. This indicates: 1) The cores represent sediment accumulations from a time no greater than 132,000 years B.P.; 2) The original stratigraphy is not related to climatic change; 3) Net sedimentation rates in the study area range between 4.7 and 57.0 centimeters per 1000 years in different localities, and 4) Sediment accumulations are not truly pelagic.

Textural and calcium carbonate analysis of the sediments suggest that sediment accumulation processes involve bottom reworking mechanisms other than turbidity current deposition. Little correlation is found between disturbed and undisturbed sediment and texture changes. Good correlation is found between climatic fluctuation and textural changes, involving a change in silt content. Correlation is found in some localities between climatic fluctuation and relative calcium carbonate accumulation rates.

The character of sediment accumulations in different provinces of the Mid Atlantic Ridge near 45^o North is dissimilar and tectonic activity associated with ridge spreading in the 45o North area is confined to the immediate area of the median valley.

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Supervisor:  Franco Medioli