Field Relations and Relative Age Dating

Geology is the primary scientific field associated with dinosaur studies, but people with little understanding of geology have found fossils of plants and animals, including dinosaurs (Chapter 3). However, some advanced knowledge of this subject is certainly helpful for continued success. Paleontology is the most readily recognizable subdivision of geology that applies to dinosaur studies, but other specialties, such as sedimentology, stratigraphy, and tectonics, are also essential to form a more complete, contextual view of dinosaurs (Table 4.3).

LE 4.3 Some subdisciplines of geology and their relevance to the geosciences (especially economic applications). All of these subdisciplines overlap with one another in some way, so the divisions between them are often artificial.

Subdiscipline

Definition and Application

Geochemistry

Geophysics Mineralogy

Marine geology Paleoclimatology

Petrology Sedimentology

Seismology Stratigraphy

Structural geology

Study of properties and changes in elements in the earth: interpreting the ages of rocks, the cycling of elements in earth systems (including pollutants), and the original parameters of paleoenvironments and burial conditions of organic material (for petroleum exploration).

Study of the earth through principles of physics: determining presence of subsurface oil or mineral deposits.

Study of minerals, such as their geological occurrence and crystallinity: examining formation of metallic ore deposits and precious stones and determining industrial applications of minerals, such as clays in ceramics and paper.

Study of the Earth's oceanic processes: interpreting sea-level fluctuations, earth history, and interactions of oceanic environments with other global systems.

Study of ancient climates and their changes through time:

interpreting patterns in climate change and factors affecting climate in the past, including factors that might affect modern climate.

Study of rocks, which can be subdivided into study of igneous, metamorphic, or sedimentary rocks: interpreting conditions for formation of ore deposits.

Study of sediments, including methods for their transport and deposition and post-depositional history: interpreting paleoenvironments and modern processes (formation of shorelines, rivers).

Study of the interior of the earth and how it releases energy: evaluating and predicting earthquakes. Study of sedimentary rocks through mapping of their vertical and horizontal extents: evaluating resources such as coal, aquifers, and oil.

Study of the deformation of earth materials: describing and predicting the extent and nature of folded and faulted rocks, which assists with interpreting Earth history and location of earth resources.

The basic principles of geology, which were formed through many repeated observations made by field geologists through the early nineteenth century (Chapter 3), are still used by geologists and paleontologists today and are responsible for what is seen in a geologic map. These principles are original horizontality, superposition, lateral continuity, inclusions, cross-cutting relationships, and biologic succession (Fig. 4.2). They comprise the techniques for relative age dating of rocks, that is, determining the relative order in which geologic phenomena occurred, without necessarily knowing the exact ages of the phenomena.

Original horizontality is the concept that sediment, unconsolidated material occurring at the Earth's surface, when originally deposited, settled under the influence of gravity into more or less horizontally-oriented layers. Once such layers become

Relative Age Rocks

FIGURE 4.2 Idealized diagram of basic field relations of rocks that can be used to determine relative ages, using original horizontality, superposition, lateral continuity, inclusions, cross-cutting relationships, and biologic succession. Phenomena are labeled from oldest (1) to youngest (19).

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