ABSTRACT

Two methods of time-trend analysis have been used in an attempt to elucidate cyclic relations in the Middle-Upper Triassic Molteno sediments from the upper part of the Karroo (Gondwana) System in South Africa. The first method is based on the thickness characteristics of the sandstones whilst the second method employs the mathematical technique of principal component analysis. This latter technique combines grain size and thickness as a function Z which is effectively a measure of the strength and duration of the energy levels within the sediment system. The smoothed data plotted as time-trend curves shows that the Z function is more discriminating for depicting cyclic trends. The structure of the series is essentially the same and fits in quite well with the idea of pulsating diastro hism as the cause of cyclicity. Variations in the rate of sedimentation and seasonal climatic effects such as temperature changes are capable of modifying or even obscuring the fundamental sedimentary rhythms of the time-trend curves. In this case, however, these disturbances have been subdued to some extent by the grain size and thickness attributes of the sandstones which are the only lithology suitable for depicting sedimentary trends. The depositional rates for sandstone indicate that the major cyclic component probably involves a time factor of at least 77,000 years and may therefore be of more than just regional significance; it may in fact be part of a widespread diastrophic acme punctuating southern hemisphere basinal sequences at about this time. The time-trend curves are genera ly simple in structure and easy to interpret. Differences appear to be related to geographic position in that those in the south are structurally very much alike whereas that in the east is slightly more complex probably because of its closer proximity to the major source area. Statistical correlation of the time-series is not feasible because of the unequal length of the series and the possibility of autocorrelation. Correlation therefore must be made in the usual geological manner and then compared with the real sections. Differences between the theoretical time-trend curves and the real sections, are of minor significancee and thus the stratigraphic interpretation seems to have been a reasonable one.