Time-level plots and accumulation patterns of sediment sequences

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Abstract
Time-level plots represent changes of level of sediment surfaces over one, or more, episodes. The durations of the episodes must be specified, because they can vary over many orders of magnitude in different studies. This study is concerned with the relationships between sedimentological episodes, resulting in deposition or erosion of beds, centimetres to metres thick, and stratigraphic episodes resulting in deposition of tens to hundreds of metres. The object of this work is to develop a method of analysing patterns of sedimentological episodes which will allow the examination of their relationship with the stratigraphic rates of accumulation that reflect tectonic and/or climatic controls.The method plots the sedimentological episodes as a sequence of small time-level vectors, plotted head to tail. The resultant of this vector sequence is the large vector that represents the net change of level over the whole stratigraphic time episode. The gradient of this large vector is the rate of stratigraphic accumulation.The Chinji Formation (Miocene), Siwalik Group, Pakistan, is a fluvial unit, whose major channel sandstones occur between intervals of floodplain overbank mudstones with minor sandstones. Using time-level plots, interpretation of a 36 m interval from the formation is progressively improved in the light of our growing understanding of the surface processes. Having first outlined a simple first timelevel plot, the following modifications are introduced, and their time and rate significance can be assessed from new plots: (1) thick mudstone episodes are replaced by many thinner episodes; (2) long episodes of hiatus are removed unless marked by distinct soils; (3) episodes are inserted below the sandstones.