Why is a seismic reflection a stratigraphic time line?

What causes a reflection

Most seismic reflections occur at a more-or-less flat interface between two stratigraphic units with differing acoustic impedance (velocity times density). The "more-or-less-flat" (but not necessarily level) limitation is because complicated surfaces are very difficult to image. Gently curving surfaces produce reflections; rough surfaces usually do not, because energy is scattered instead of being reflected.

The most common geological feature which is a smooth, flat (or gently curved) surface marking a change between two materials with differing acoustic impedance is the boundary between two beds of different sedimentary lithologies: between a shale and a sandstone, between shale and limestone, between coal and shale, etc.

Why this is a time line

Consider the simplest example: a marine shale overlying a marine sandstone. As the sedimentary basin is filling with sand, the sand supply is suddenly replaced by a clay supply. The only geologically plausible reasons for this are a change in water depth or a change in sediment supply. A change in water depth is most likely to be caused by a change in sea level; a change in sediment supply might be caused by a change in climate (precipitation), a change in the drainage (such as a river breaking through to a new channel, or a blockage of a river by a landslide or a fault). Most other lithological changes are caused by the same two agents: a coal swamp is replaced by clastic deposition when the water becomes deeper; a limestone is replaced by clastics when the influx of clastic material becomes significant; or aeolian sands are covered with water from which evaporites are deposited.

Each of these two agents occur simultaneously over large areas. Changes in water depth may occur on a global scale, as the sea level changes, or on a local scale due to local tectonics or (especially in lakes) climatic changes. Changes in sediment supply are usually local, but still occurring over large areas relative to the size of seismic recording geometry.

Gradual changes in sea level or in sediment supply deposit transgressive or regressive lithological units, but in almost all cases the gradual changes are superimposed on more rapid changes, so that the surfaces of such units are not smooth, and so do not produce reflections.

Were time lines originally flat and at the base level?

Time lines representing changes of water depth or changes in sediment supply can usually be assumed to be originally at sea level provided the deeper layer was deposited in shallow water or subaerially. If the older side of the interface represents deep water deposition, it cannot be assumed that the deposition occurred on a flat horizontal surface. Common examples of this are the dipping layers seen at the shelf edge, and the distinctive layer often produced by a volcanic eruption.