Use of the Computer for Quantitative Analysis of Fossil Distribution

Abstract

Time-trend analysis is a mathematical technique used to smooth out irregularities in geologic data based on a sequence of observations taken along a traverse or on a vertical measured section. A highspeed digital computer is used to calculate smoothed values and plot the time-trend curves, which in turn are used to study the underlying trends of sedimentation. Nine smoothing equations containing from 5 to 21 terms are available to compute curves with varying degrees of smoothing. The smoothing sum of squares is a measure of the total variability accounted for by each of the smoothing equations and can be used to detect a rhythm or cycle in the data. Several closely spaced, measured sections are being investigated in the Upper Ordovician of southeastern Indiana to reconstruct the sequence of environments which existed during their deposition. Time-trend curves are being used to study the relationship between physical properties of the environment and faunal distribution. The percentage of limestone in a sequence of thin limestone and shale layers is used as an index of environmental conditions. The corresponding time-trend curves are plotted for ten genera of brachiopods to examine their vertical distribution. There appears to be a definite relationship between the amount of limestone in the section and the occurrence of certain brachiopod genera. The maximum on the limestone percentage curve corresponds closely to the disappearance of one brachiopod genus and the introduction of two new genera, indicating a significant shift in environmental conditions.