Time and fossils

Geological time can be determined absolutely or relatively. The ages of rocks are estimated numerically using the radioactive elements that are present in minute amounts in particular rocks and minerals. Relative ages of different units of rocks are established using the sequence of rocks and zone fossils. Sediments are deposited in layers according to the principle of superposition, which simply states that in an undisturbed sequence, older rocks are overlain by younger rocks.

Zone fossils are fossils with a known relative age. In order for the zone to be applicable globally, the fossils must be abundant on a worldwide scale. Most organisms with this distribution are pelagic - that is they live in the open sea. The preservation potential of the organism must also be high - that is they should have some hard tissues, which are readily preserved.

Stratigraphy

The study of sequences of rocks is called stratigraphy. There are three main aspects to this study: chronostratigraphy, litho-stratigraphy, and biostratigraphy (Fig. 1.2).

Chronostratigraphy establishes the age of rock sequences and their time relations. Type sections are often established. These are the most complete and representative sequences of rocks corresponding with a particular time interval. For example, outcrops along Wenlock Edge in Shropshire, UK, form the type section for the Wenlock Series of the Silurian.

A point in a sequence is chosen for a boundary between one geological time interval and the next. It represents an instant in geological time and also corresponds with the first appearance of distinctive zone fossils. Relative timescales can then be established with reference to this precise point. These points are called "golden spikes".

The differentiation of rocks into units, usually called formations, with similar physical characteristics is termed lithostra-tigraphy. Units are described with reference to a type section in a type area that can be mapped, irrespective of thickness, across a wide geographic area.

In biostratigraphy, intervals of geological time represented by layers of rock are characterized by distinct fossil taxa and fossil communities. For example, the dominant fossils in Palaeozoic rocks are brachiopods, trilobites, and graptolites.

Chronostratigraphy

Lithostratigraphy

Era

Period

Series

Silurian

Llandovery

Birkhill Shale

u o

E

s

Ordovician

Ashgill

Upper Hartfell Shale

Detailed description of the sediments

| Black shale

^ Gray mudstone

Note that the boundary between the shales is below the Ordovician-Silurian boundary

UnU Volcanic ash

Biostratigraphy: graptolite biozones

A Parakidograptus acuminatus

B Glyptograptus persculptus

C Climacograptus extraordinarius

D Paraorthograptus pacificus

E Dicellograptus complexus

Biostratigraphy: graptolite biozones

A Parakidograptus acuminatus

B Glyptograptus persculptus

C Climacograptus extraordinarius

D Paraorthograptus pacificus

E Dicellograptus complexus

Zone species ranges

Lines show species duration. Note that most biozones are defined by a range of coexisting zone fossils

Fig. 1.2 Stratigraphic description of the sequence of rocks that crosses the Ordovician-Silurian boundary at Dobb's Linn, Southern Uplands, Scotland. Geological time is split into different zones depending on the method of analysis. Chronostratigraphy divides the section into two periods. Lithostratigraphic analysis divides the sequence into two shales. Biostratigraphy, as determined by the zone fossils, gives a more detailed division of relative age within the sequence.

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