Stratigraphy

Stratigraphy is the scientific study of rock layers and when these layers were created. It is basic to geology and is used in many fields, including archaeology and petroleum geology, or the search for hydrocarbons. Stratigraphy involves the study of how and where rock layers move under various conditions. It primarily focuses on sedimentary rock—which accounts for about 75 percent of the surface of continents—and is closely linked to sedimentology but may involve other types of rock, such as those resulting from lava flows.

Stratigraphy can determine relative ages of strata that, while inexact, are more practical and less expensive than radiocarbon dating. The field also examines how sequences of rock strata in one area relate to strata in other areas.

Background

Ancient scholars' beliefs about geology were not supported by scientific study. For example, they followed religious authorities in limiting Earth science to about a six-thousand-year period. Some thought fossilized remains had supernatural origins. Many believed that mountains grew like living organisms, such as trees.

Seventeenth-century Danish geologist Nicolaus Steno (or Nicolas) proposed several ideas about fossils and also noted strata. He suggested in a 1669 work that many rocks were formed through sedimentation. Steno's work led him to understand that strata hold a chronological history of Earth. Although Steno converted to Catholicism and did not publish many of his ideas because they did not conform to religious teaching of his time, other scholars were pursuing similar ideas.

Eighteenth-century Scottish naturalist James Hutton realized that natural forces, including volcanic activity and erosion, were constantly but slowly transforming and rebuilding Earth. His efforts to understand these actions led to the establishment of geology as a recognized science. By the late eighteenth century, scholars understood that sedimentary rock formed from layers of sediment in water. Hutton discovered that even the most ancient bedrock likely formed from sediment under extreme pressure.

Hutton realized the cycle of rock formation and erosion was a never-ending process. He dubbed it the great geological cycle. Many of his ideas were formed at Siccar Point near Edinburgh. The cliff there contains multiple vertical layers of gray shale and overlying layers of red sandstone. Hutton realized that the gray shale had originated from horizontal layers of sediment. Some force had uplifted and tilted the gray shale layers, which were then covered by an ocean that deposited the sediment that formed the red sandstone. The point where these two types of rocks meet is known as the Hutton Unconformity.

Hutton deduced that heat within Earth was a tremendous force that not only altered the composition of rock but also caused the crust to move. This movement created uplift that tilted and otherwise moved the strata as it had at Siccar Point.

Overview

The field of stratigraphy adheres to four principles based on the work of Steno:

1. superposition, which holds that younger layers are over older layers;
2. original horizontality, which holds that sedimentary layers were deposited flat;
3. cross-cutting relationships, which holds that intrusions must be younger than the layers they disturb; and
4. lateral continuity, which holds that layers of rock are continuous in all directions until they encounter other solid bodies that interrupt the continuity or are acted upon by agents that appeared later.

The principle of superposition allows geologists to define the geologic time scale by establishing the age order of layers of sedimentary rocks. The principle of original horizontality recognizes that unnatural degrees of tilt indicate that some force disturbed the rocks after they formed. Examples of these forces include intrusions and tectonics. According to the principle of cross-cutting relationships, anything that interrupts strata formed after the strata. For example, a vein of igneous rock that cuts through layers of sedimentary rock formed after the sedimentary rock. The principle of lateral continuity allows researchers to locate identical rocks that are not connected and deduce the forces that created this discontinuity—for example, erosion in a river valley or uplift that forms mountains interrupts strata.

Geologists can use clues such as the presence of igneous rocks to date rock and identify the presence of ancient oceans. Magma pushes up through cracks and fissures in Earth's crust. If magma cools and hardens as rock in these cracks, it is younger than the surrounding rock. Magma may also emerge as lava underwater. When lava extrudes at the bottom of the ocean, it forms a pillow-shaped convex top with a tip that projects down at the bottom. When pillow shapes are found in basalts, they prove that lava erupted underwater and indicate the direction the pillow originally formed.

Much research in stratigraphy since the 1980s has focused on petroleum geology and pinpointing hydrocarbon deposits. Oil comes from diatoms, single-celled aquatic creatures that produced an oil. As diatoms died, they settled into thick layers of sludge underwater. Over millennia, their remains were buried under sediment. Under high heat and pressure, the soft parts of the organisms transformed into oil and natural gas. This same heat and pressure turned the silica bodies of the organisms into rock called source rock. Oil slowly migrates into layers of reservoir rock, which is porous enough to absorb oil like a sponge. Sandstone is a common reservoir rock.

Stratigraphy has revealed that the rock within a layer may be different in various locations. One layer may contain sandstone, shale, and siltstone. These changes could develop very close together—for example, sands may have accumulated along the bends of ancient rivers and were eventually covered by mud, resulting in multiple types of stone in one layer. Companies searching for oil deposits search for layers where oil could accumulate. For example, shale is composed of clay particles and is not very porous, while sandstone is a reservoir rock. Oil that travels into a layer of sandstone will flow through the pores until it hits shale, where it stops. This is known as a stratigraphic trap. Geologists recognize these traps as very promising areas to drill for oil. They also look for trap rocks, such as shale, which prevent oil from seeping upward under pressure. Shale strata commonly contain hydrocarbons.

Stratigraphy has contributed to other areas of research as well. Alfred Wegener, who developed the theory of continental drift and the supercontinent Pangaea, used information about strata to prove his theories. He showed that mountain ranges, bands of strata containing fossil evidence, and coal deposits align on multiple continents when they are placed in close proximity to one another. These southern hemisphere continents once formed the southern half of Pangaea known as Gondwana.

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