Rate of Deposition Shown by Relative Abundance of Foraminifera

Abstract

Estimates of the percentage by volume of Foraminifera in cores from seven deep borings in San Antonio Bay, Texas, were made and curves of these percentages versus depth were drawn to 55 feet. It was assumed that production of Foraminifera was at a constant rate and therefore that the rate of total sediment deposition was inversely proportional to the percentage of Foraminifera at any depth. Based on this assumption, depositional rates were computed at depth intervals of 2½ feet. A Carbon-14 dating at 55 feet and the average percentage of Foraminifera throughout the 55-foot column were used as a basis for the calculations. Although the basic assumption of constant production is not universally justified, it may be reasonable within some environments of fine clastic se iment and is apparently valid in San Antonio Bay. Calculations based on this assumption show good agreement with Carbon-14 datings at various depths. The curve of rate of deposition versus time drawn from these calculations shows a large variation in rate, with a maximum of 4.7 feet per century at -6,900 years, decreasing to a minimum rate of 0.2 foot per century from -3,100 to -4,100 years. This followed by a gradual increase to about 0.7 foot at present.