Lateral Relationships

As was pointed out in the previous section, bedding planes represent changes in deposition. These changes are not typically localized to small areas but are indicative of environmental changes which effect large areas of the Earth and as a result affect many depositional environments. 

In 1669 Nicolas Steno proposed the principle of lateral continuity. This principle stated that all sediment is deposited outward in all directions until it pinches out or grades into different sediment types. The lateral gradation of sediment from one sediment type to another is the result of simultaneous deposition in adjacent environments. Take a shoreline for example; at the same time sand is being deposited on the beach silts and clays are being deposited in the lower energy off-shore environment. This simultaneous deposition yields sedimentary facies, a body of sediment with distinctive physical, chemical, and biological attributes. As depositional environments change over time the location of sedimentary facies on Earth's surface must also change. These changes will then be evident vertically within the rock record.

In order to examine this let's look at coastal marine environments such as that depicted in the image below. In general the coastal marine environment consists of three sedimentary facies: the sandstone facies, the shale facies, and the limestone facies (note that the further from shore the smaller the particle size is being deposited).

Depiction of a general coastal marine environment

Remember the sediment deposited is determined by the physical characterisitics of the environment in which it is deposited. So in the image above as long as the environment doesn't change you would expect the facies to remain in their current position. But what if sea level rises? How would this affect the locations of the sedimentary facies.

A marine transgression occurs whenever the sea level rises with respect to the land. During a marine transgression the shoreline would migrate landward and as a result all parallel sedimentary facies would also migrate landward. This would result in lower energy environments now occupying areas that were once higher energy environments and, as a result, previously deposited sediments would become buried under those deposited by the new low-energy environment as seen in the image below.

As sea level rises offshore facies will begin to be deposited on top of previously deposited facies as seen in the animation below

Overtime this would result in a vertical column with sandstone on the bottom followed by shale and then limestone on top. Because the particle size of the individual layers decrease as you move up the column this is known as a fining upward sequence (FUS). This same pattern can be found anytime the energy of the particular location is decreasing over time, thereby allowing finer particles to deposit in the same location.

Since a marine transgression causes lower energy environments to be superimposed on materials that were deposited in high energy environments no previously deposited sediment is eroded. However, during a marine regression-- when sea level drops with respect to land--the near shore, high energy environment, becomes superimposed on the low-energy off-shore environment this may cause some degree of erosion because sediments orignally only deposited in low-energy environments are now exposed to high-energy processes. During this process all sedimentary facies move parallel with the shore and eventually produce a vertical column with limestone on the bottom followed by shale with sandstone on top. Since the particle size is increasing from the bottom to the top this sequence is known as a coarsening upward sequence and can be produced anywhere the energy of a particular location is increasing over time.