Understanding of stratigraphy underwent a major change in the 1950s when the American geologist Laurence L. Sloss (1913-96) proposed the concept of sequence stratigraphy. sloss and many colleagues and subsequent workers recognized numerous large, laterally extensive rock units that they named sequences that are bounded by unconformities of global significance. Some of these unconformities are so significant that they are found in almost all shallow water deposits of that particular age around the world. Studies revealed that these always occur where sea level has dropped from high to low, and the overlying sequence is always transgressive. Sloss and coworkers used index fossils to show that these unconformities have the same age on all continents, and are clearly related to changes in sea level. Sea level has been as much as 1,200 feet (350 m) higher than at present, and as many as 650 feet (200 m) lower than present. By correlating different unconformity-bound sequences on the different continents, Sloss and coworkers produced curves that showed the relative height of the sea for the past 600 million years of Earth history.
Sea-level curve showing the global average sea-level height through Phanerozoic geologic time, along with the six main unconformity bounded sequences deposited during transgressive and sea level highstands. These sequences include the Sauk (S), Tippicanoe (T), Kaskasia (K), Absaroka (A), Zuni (Z), and Tejas (T) sequences.
The simple and slow rise and fall of sea level through geologic time produces unconformity-bound sequences. When sea level is high, sediments accumulate on the continental shelf, and when sea level falls, the shelf is exposed and eroded, and the sediments move off the shelf, producing an unconcon-formity. When sea level rises again the new sequence is transgressive, deposited unconformably over the eroded shelf.
The history of the Earth and its life-forms has been based largely on the study of the stratigraphic record as described above, and by correlating the ages of different events such as changes in sea level from continent to continent using index fossils, fossil zones, geochronology, and a few other timescales such as the magnetic polarity timescale, geochemical timescales, and data from other fields such as plate reconstructions using paleomagnetism, paleoclimate, and paleoenvironmental reconstructions. From this, geologists have been able to piece together a robust history of the planet Earth, although there is still much to be learned and debated by generations of future geologists.
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