Journey of Time in Stratigraphy
Monday, 15 December 2014
In stratigraphy, stratal surfaces (e.g. bedding planes) are generally considered as discrete time lines. However, expression of time in stratigraphic record can be quite complex. Unlike flat stratal surfaces, time surfaces are mostly undulating. Moreover, time was not only elapsed vertically across stratal surfaces, it was also elapsed horizontally along such surfaces. These complexities can generate contentions when it comes to dividing stratigraphic packages into chronostratigraphic or genetic units. Since many stratal surfaces are composite in nature, they represent a diachronous interval of time. This is particularly true for active depositional systems where undulating geomorphic units migrate in time and space to fill a basin. At each level of hierarchy from ripple- to basin-scale clinoformal packages, composite stratal surfaces can develop, where deposits above such a stratal surface are NOT everywhere younger than deposits below the same stratal surface. This may violate the Law of Superposition, which states that, in an undisturbed stratigraphic record, each layer is always younger than the layer beneath it. In fact, this Law holds true in one location moving strictly vertically through an undisturbed succession. While moving vertically, if an observer also moves laterally along a stratal surface for some distance, this Law may break down. These observations have critical implications in interpreting stratigraphic record, particularly in testing sequence stratigraphic concepts.