Characteristics contemporary dating
Nevertheless, there is no greater testimony to the validity of fossil-based stratigraphic geology than the absolute dates made possible through radioactive measurements.Almost without exception, the relative order of strata defined by fossils has been confirmed by radiometric ages.Failing that, the repetition of a certain layered sequence (e.g., a black shale sandwiched between a red sandstone and a white limestone) lends confidence to physical correlation.Finally, the measurement of a host of rock properties may well be the ultimate key to correlation of separated outcrops.Correlation based on the physical features of the rock record also has been used with some success, but it is restricted to small areas that generally extend no more than several hundred kilometres.The first step is determining whether similar beds in separated outcrops can actually be traced laterally until they are seen to be part of the same original layer.Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.The most useful indication of time equivalence is similar fossil content, provided of course that such remains are present.
From an examination of such outcrops with special focus on the sequence of animal forms comes the empirical generalization that the faunas of the past have followed a specific order of succession, and so the relative age of a fossiliferous rock is indicated by the types of fossils it contains.fossil assemblages.
Such features as colour, ripple marks, mud cracks, raindrop imprints, and slump structures are directly observable in the field.
Properties derived from laboratory study include (1) size, shape, surface appearance, and degree of sorting of mineral grains, (2) specific mineral types present and their abundances, (3) elemental composition of the rock as a whole and of individual mineral components, (4) type and abundance of cementing agent, and (5) density, radioactivity, and electrical-magnetic-optical properties of the rock as a logging, involves lowering a small instrument down a drill hole on the end of a wire and making measurements continuously as the wire is played out in measured lengths.
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