are filled by coarse sands at the base, which are covered by finer sands, then by silt, and finally by clay. This sequence, or package, indicates that the entire bed formed when a turbid flow charged with particles of many sizes—a turbidity current—swept down onto the deep seafloor at the margin of a continent. Upon deceleration it dropped its coarse debris first and its slower-settling fine debris later.

Key diagnostic features involve the associations between individual sediment units in the vertical sequence. For example, meandering rivers, as they migrate back and forth across a valley floor, produce repetitions of deposits that suggest cycles of sediment accumulation. Coarse sandy or even gravelly deposits with inclined bedding indicate an active stream channel. These pass upward into finer deposits that culminate in muds recording the lateral migration of the channel with deposition limited to flood events. The cycle is complete when evidence shows an abrupt cut through the sequence that is filled with another coarse deposit and subsequent fining upward.

By analyzing sedimentary sequences, geologists can recognize a wide range of ancient environments, ranging from alluvial fans that form along the bottom slopes of mountains to mountain belts that have incorporated sediments squeezed up when deep floors of ancient oceans converged with continents along subduction zones. Depositional environments bear witness to most ongoing earth processes—they only need accurate interpretation. And there are immediate applications. Reconstruction of sedimentary environments plays a major role in the search for petroleum and gas, indicating locations of natural traps for these fluids.

Accurate dating of rocks is critical to paleoenvironmental reconstruction on all spatial scales. When isotopic dates are not known, tight temporal correlations between areas must be dependable. Improvement of existing techniques continues, as does the invention of new ones. Only recently, for example, geologists have applied and improved quantitative techniques for correlating strata on the basis of the earliest and latest appearances of fossil species. Quantitative analysis of populations of fossil conodont—minute tooth-like structures of an extinct group of marine vertebrates—can correlate rocks more than 400-million-years old. Calculations indicate that these correlations are accurate to within just a few hundred thousand years.

Breakthroughs have come in other areas, too. Stratigraphers have lately discovered that many limestones exhibit sufficient paleomagnetism to display reversals in the magnetic field. This characteristic permits geologists to assign strata to positions in the global paleomagnetic time scale. Surprisingly, the magnetism results from forms of bacteria that colonize tropical seafloors and produce minerals containing iron. Using this approach, stratigraphers have dated limestone cores taken during drilling operations in the Bahama Banks. From these dates they can calculate rates of subsidence for this huge limestone platform and determine the times when global lowering of sea level left it standing far above marine waters.

New means of isotopic dating are also continually being developed and refined. Single grains of zircon from Precambrian rocks more than a billion years old can now be dated with a precision of just a few million years, as discussed previously. Other exciting techniques, still in the early stages of development, should soon permit dating of terrestrial sediments that are just a few tens or hundreds of thousands of years old—too old for radiocarbon dating, or lacking any carbon that might be dated, and too young for other dating methods.

Paleogeography

Researchers have proposed that not only the positions of the continents but also the uplift of mountains exert control over global climate patterns. Mechanisms that physically alter environments on a regional scale have traditionally been accepted, but theories that suggest that tectonic forces may cause climatic changes on a global scale still inspire controversy. For example, the uplift of the Sierra Nevada in eastern California exemplifies regional, and direct, effects. Today, the Sierra Nevada is an imposing structure, a block of granitic crust heaved upward to form a towering eastward-facing scarp that was a formidable barrier to pioneers attempting to reach California. Fossil plants dating from 10 million to 15-million-years ago that could not have lived as much as 1 km above sea level are found today on the crest of the range, nearly 3 km high. Only in the past 5-million-years has the Sierra Nevada approached its present height; the consequences of this elevation are enormous for the Basin and Range Province to the east, in Nevada and southern California. This area, which had been covered by broadleaf evergreen forests, came to lie in the rain shadow of the Sierra Nevada in the past 5-million-years and developed a savannah vegetation. During the past 1.8-million-years climates became drier on a global scale. The Basin and Range Province became the desert that we know today, although during glacial maxima it received substantial rainfall.



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