Stratigraphy

Introduction

Stratigraphy is the study of the sequence of rock layers in any one area. These are generally described from the oldest to the youngest. Thus the geological legend on the map shows the ages of the rocks in this order. Against each age is a summary of the major rock types, subdivided into sedimentary, volcanic and metamorphic categories. The following sections (in stratigraphic order) describe where rocks belonging to a particular age are found, what sort of rocks they are, what environment they were formed in, and how they have been deformed.

The information on age, environment of deposition or mode of formation comes from the rocks themselves or their relationships to one another. It is worthwhile briefly discussing in very general terms what forms this information takes.

Fossils can tell us much about the environment in which sedimentary rocks were deposited, as well as providing age information; certain animal groups preferred fresh, brackish, or salt water, some groups lived in deep water, some in shallow, some liked warm conditions and some cold. The environment can also be indicated by the type of deposit. As rivers carry material into the sea and lose momentum they tend to drop the coarse fractions of their load first; therefore, near-shore deposits tend to be gravel and sand, while mud is deposited further offshore. However, sea-bottom currents can move material around, and the coarseness of the material carried depends on their strength. Current and wave action may form distinctive structures which are preserved in rock. For example ripple marks in sand, as seen on any present-day beach below high water mark, are commonly seen preserved in ancient rocks and where they may indicate a shallow water environment.

The type of sediment deposited in the sea can indicate what is happening on the adjacent land area. If the land area is undergoing uplift and fast erosion, there will be a large amount of gravel carried out to sea. Landscapes wearing away gradually contribute more sand and mud particles. The distance that particles making up the rock have travelled is indicated by their roundness. When particles are originally eroded they are angular in shape, but as they are carried along in rivers or sea currents the particles knock against one another and gradually the corners are worn off leaving a rounded surface.

Deposits on land include angular talus debris (scree slopes), lake sediments (usually fine-grained sand and mud), river deposits (layers of sand and gravel) and swamp deposits. Terrestrial deposits can be economically important, for swamps and deltas are generally full of dead and rotting vegetation, and depending on events after deposition this carbonaceous material can become coal, or the source of oil or natural gas.

Igneous rock does not give as many clues to the type of environment it was formed in. Plutonic rocks can be intruded into hard rocks on land or softer deposits beneath the sea. In general, plutonic rock intrusion accompanies mountain building activities. A relative age can be estimated from the nearby rocks. If the rocks in contact with the intrusion show alteration in the form of an aureole, they have been "baked" by the heat of the magma and are thus older than the intrusion. On the other hand, rocks which are in sedimentary contact with an intrusion and which show no alteration are younger than the intrusion.

Stratigraphy image

Volcanoes, by their very nature, erupt intermittently. Therefore, lava flows may be interbedded with sediments, so that the fossils within the sediments can be used to date the volcanic rocks. Volcanoes may erupt beneath the sea or on land, and the form of the lava can indicate this. Molten lava is chilled rapidly when it meets the sea, so that a skin is formed quickly; this skin takes the form of a sack or pillow which is filled with lava. Thus the outside edge of the "pillow" is fine grained while the centre, which cooled slowly, is coarser grained and has radial cooling cracks. Lava flows erupted on land are exposed to erosion and alteration by weathering; thus a weathered layer may be present between two flows. Volcanic ash falling to the ground, into the sea, or a lake, will form a bedded deposit, and is called tuff. The age of the tuff beds can be determined by their relationship with fossiliferous sediments, or by isotopic dating.

Stratigraphy image

Metamorphism takes place after the rocks have been formed, and isotopic methods are used to determine the date of formation of the metamorphic minerals. Some rocks, however, may have undergone several periods of metamorphism which make them very difficult to date, as the assessed date will nearly always be the last event. Slightly metamorphosed rocks may appear similar enough to the parent rock that their age can be assessed.