EAS 102 / BIO G 170 Lecture 5, Page 1 of 6
The “How To” of Reading the Geological Record: Tracking Rocks in Four Dimensions
GEOCHRONOLOGY: TELLING TIME IN HISTORICAL PERIODS
Where Are We? Whether the aim is to reconstruct the solar system’s formation (as it was in the last lecture), to chart continental drift in a movie-like series of world maps (as it will be in Lectures 9-12 and 16-18), or to trace the evolution of our own species (as it will be in Lectures 19-26), the technical problem generally comes down to reconstructing the four-dimensional historical trajectories of the rocks, minerals, and other debris that record the story. To accurately map the assembly and breakup Pangaea, for instance, one must be able to track the supercontinent’s constituent terranes by measuring (1) paleolatitude, (2) paleolongitude, and (3) paleoelevation as functions of (4) time. The more fully and accurately a terrane’s or a mineral grain’s four-dimensional trajectory can be reconstructed, the more precisely geological processes’ operation can be studied and the more exactly historical sequences of events can be reconstructed. Labs 3-8 illustrate with some of the original examples how geologists learned how to recognize plate-tectonic processes and reconstruct their operation. The basic concepts in this, the first of four lectures on reading the geological record, are discussed and illustrated in greater detail in Labs 3 and 4.
The Discovery of Rocks as Historical Records It seems not to be obvious that rocks are products of everyday processes. Once the connection is made, however, it becomes clear that rocks are historical records. Herodotus (“The Father of History,” 484- c. 425 B.C.) and Leonardo da Vinci (1452-1519), among others, observed fossils in rocks and drew the obvious conclusions: the rocks are ancient sediments, and the Mediterranean Sea must have been higher in the past. More than once when leading Ithaca gradeschoolers on fossil-collecting trips, your professor has watched as the face of a latter-day Herodotus or Leonardo lit up with a corresponding realization: Eureka! Fossils show lake level must have been higher in the past! Among other things, this example shows that measuring geological time in years as well as in historical periods is important to avoid associating many-times-older fossils with a water-filled basin formed only in the last few ka of postglacial time, in the case of Cayuga Lake, or , in the case of the Mediterranean, one that had been as dry as Death Valley for much of the intervening few million years.
Importance of the Uniformity Principle — As first discussed in Lecture 1, the discovery of the Earth’s historical record was intimately linked to recognition of the uniformity principle, and so was the realization that recorded events could be put in order on a geological time scale. The uniformity principle’s main implication in this regard is that rocks can be read back as historical records by applying a knowledge of time-invariant rock-forming processes — a knowledge that can be obtained from studying these processes and their products in the present. James Hutton’s phrase “The present is the key to the past” perhaps too concisely sums up the uniformity principle’s implications (or at least too concisely for literal-minded geologists who have mistakenly concluded the implication that processes always have proceeded at more or less the same rate).
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Although the uniformity principle is implicit in Herodotus’ and Leonardo’s interpretations of rocks and fossils, many early scientists seemed to expect rocks to be Genesis written in stone. Isaac Newton’s Principia (Mathematical Principles of Natural Philosophy, in Latin; 1687) put the understanding of physical phenomena on a foundation firm enough to give definite, quantitative, testable predictions about the outcome of experiments; but it took another few generations for the understanding to become well and widely enough disseminated among natural philosophers to yield much in the way of applied science, much less technology. During the Enlightenment (c. 1687-1789), natural philosophers began to examine rocks with an eye to explaining them as products of newly understood processes. Ever before them was the prevailing belief that the Earth had been created around 4000 B.C., as many had deduced from chronological information in the Bible1. Sedimentology, marine ecology, and oceanography — the sciences most concerned with those aspects of the “present” most useful as keys to sedimentary rocks— did not develop until the mid- to late 19th Century, by which time “hard-rock” geology was already well advanced. What held up the birth of modern geological science until the end of the 18th Century was precisely what was holding up the Industrial Revolution: chemical knowledge good enough to be put into practice, whether as a key to the past in geology, or as a key to the future in chemical manufacturing. Pure chemistry advanced rapidly during the late 18th Century2. Its first industrial success was in dyestuffs manufacture in later 18th Century Edinburgh. Here was the very beginning of the Industrial Revolution. This success was due largely to a close-knit circle of natural philosophers who made it their business to understand the process behind their chemical products. The men applied the same common-sense approach to process-product relations in their shared hobby, geology. Among them was James Hutton, who made his money manufacturing ammonium chloride [NH4Cl] for the dyestuffs industry. Hutton’s Theory of the Earth (1788, 1795) founded modern geology squarely on the uniformity principle. Through him, the uniformity principle has come to be known in geology as the doctrine of uniformitarianism (see Lecture 1).
What the Uniformity Principle’s Application Revealed — The first modern geological explorations of the 18th Century were in a sense an extension of geographic explorations. By Hutton’s time, Europeans’ systematic mapping of the globe by latitude and longitude had been under way for three centuries. Australia and Antarctica were still to be discovered when geologists began to take over from miners and sailors in systematically exploring the third spatial dimension, depth. Hutton’s interpretation of rocks in accord with the uniformity principle revealed time to be an unexpectedly, indeterminately vast fourth dimension. Uniformitarian reading of the geological record led Hutton to a further, socially more revolutionary conclusion: as consistent with the notion that nothing supernatural happens, nothing supernatural seems to have happened around 6 ka ago, when Earth was then generally supposed to have been created, or at any other time in a much longer history.
1 Creation is the starting point for the Hebrew calendar, which dates it to 7 September 3760 B.C. on the everyday Gregorian calendar. The Venerable Bede established the convention of numbering years relative to the birth of Christ in his De Temporum Ratione (725; by far the best early medieval computistic work, and the standard work on chronology and the calendar for centuries to come), thus setting the alarm for episode after episode of millennial hysteria. Bede estimated Creation to have taken place on or about 18 March 3952 B.C. For general review, see N. Dershowitz and E.M. Reingold, Calendrical Calculations (Cambridge, 1997). Apparently as unaware of the religious associations as they are of the associated scholarship, some geologists celebrate 23 October as “Bishop Ussher Day” in recognition of the 17th Century protestant archbishop who reckoned Creation to have taken place on or about that date in 4004 B.C. 2 See A. Clough and N.L. Clough, The Chemical Revolution (Batchworth, 1952).
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The Geological Time Scale Geological Timekeeping Processes — Any process that generates a time-varying, geologically preserved product is potentially useful in telling geological time. The most useful processes have proved to be (1) sediment accumulation, the usual basis for establishing layers’ relative age within one rock outcrop; (2) evolutionary change in organisms as documented by fossils (change within lineages, including origination and extinction of species), the usual basis for establishing secdimentary rocks’ age relationships between outcrops; (3) radioactive decay, the usual basis for estimating absolute geological age (e.g. ages in years), as will be discussed in the next lecture; (4) sea level change; (5) change in the geomagnetic field’s strength and polarity. In geologically young organic material, (6) chemical decay is sometimes useful. Change in sea water’s 87Sr/86Sr ratio (7) is proving useful in geochronology, though the underlying geochemical processes involved are several and complicated. Deceleration of the Earth’s rotational rate (8) is potentially useful, but hardly ever used. Seasonal growth in organisms (9) can be useful in dating (e.g. tree-ring dating, which in some areas has been carried back a few ka’s), and so can seasonal sedimentation (10) (e.g. annual varves in lake sediments, which have been counted back a few ka’s in some areas, and annual layers in glacial ice, which have been counted back as much as 150 ka in some ice cores).
Geological Time Scales — Today’s geological time scale is really several time scales in one. The relative geological time scale is concerned with geochronology, the measurement of time in historical periods (e.g. Enlightenment, Industrial Revolution, Victorian Period, Gilded Age, Cold War Era, Devonian Period). It includes the stratigraphic time scale (the one calibrated in eras, periods, epochs, and other units), the biochronological scale (a collection of many separate scales, each calibrated in the biochrons of variously related species), the geomagnetic time scale (the one calibrated in geomagnetic chrons, periods defined by reversals in the geomagnetic field’s polarity), the sea-level time scale (the one calibrated in cycles of sea-level rise and fall), and various isotopic time scales (e.g. the seawater 87Sr/86Sr scale or the seawater 18O/16O used in Quaternary ice-age dating, as discussed in Lecture 15). The absolute geological time scale, in contrast, is concerned with geochronometry, the measurement of time in years or other absolute units. It is based almost entirely on dating samples from the extent of radioactive decay within them, but sometimes on counting daily, monthly, or annual varves in layered sediments or daily monthly, or annual growth increments in shells. The geological time scale is, among other things a tool like a calendar which can be used to establish the age relationships among rocks the world over, and among the events that the rocks record. The two parts, geohronological and geochronometric, are used together to establish such things are the the age relationships between a region’s radiometically dated igneous rocks and paleontologically dated sedimentary rocks.
Development of the Relative Time Scale The Law of Superposition — What kids discover in mud puddles, some geologists seem to treat as a Law almost literally brought down from mountaintops: layers of sediment are laid down one on top of another in chronological order. Not until the late 17th Century did the literati scoop the kids. This generalization, known as Steno’s Law of Superposition, was the original basis for dating rocks and constructing a time scale.
The First Global Time Scale — Recognition of superposition in sedimentary rocks opened the way to measuring geological time in terms of sediment acumulation. By about 1760, geological exploration of Western Europe had revealed a fundamental generalization about the Earth’s crust: crystalline rocks (termed “Primary”) underlie sedimentary rocks (“Secondary”) which in turn underlie unconsolidated sediment (“Tertiary”). Based on superposition, the three were understood to represent distinct historical periods: Primary (≈ Precambrian), Secondary (≈ Paleozoic + Mesozoic), and Tertiary (≡ Tertiary).
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Biotic Succession — Going up through a stratigraphic section, new kinds of organisms appear and old ones disappear. The phenomenon, known as biotic succession, provided the first means of determining age relationships among rocks in separate outcrops, and thus of constructing globally applicable time scale for sedimentary rocks. In building this scale, superposition has been used to establish the sequence of fossils in each outcrop; and the time scale itself has been pieced together from all outcrops collectively. The scale thus amounts to a theory of the age relations among the stratigraphic sections that define it. Although biotic succession was noted as early as the 17th Century, it was not actually applied to regional geochronology until the end of the 18th Century. In 1815, William Smith, an English civil engineer who studied and applied biotic succession while laying out canals, published the first large-scale map showing the distribution of rocks by age, the first geological map of Great Britain.
Biological Systematics and Geochronology — Telling time with fossils comes down to determining the ages of strata in relation to events in evolution, namely, the the originations and extinctions of species. A fossil-based chronology is no better than the biological classification behind it. But what is a species of fossil? According to the Biological Species Definition, a species is a group of actually or potentially interbreeding populations— an entity difficult enough to recognize among living organisms, much less fossils, and inapplicable to asexually reproducing forms, such as those on which the Cretaceous and Cenozoic time scales are mainly based. In practice, a paleontological species is an anatomically defined entity that, so far as paleontologists can tell, could represent a “real” species. Paleontologically based dating works all the same, as confirmed by innumerable cross-checks through dating by other methods. Fossils are fallible timekeepers. No species of organism occurs everywhere all the time throughout its existence, though some plants may come close with their pollen. Most species do not leave fossils, and most are beetles. Relatively few species are stratigraphically useful, and some are more useful than others in telling time. Short-lived, geographically and environmentally wide-ranging species that are common as fossils are most valuable in time keeping. They are sometimes called index fossils or guide fossils, and their durations are typically on the order of a few Ma. The perfect index-fossil species would be abundant everywhere for a geological split second, would leave virtually indestructible remains, and would not have an ecology (at least in the sense of no strong association with any particular environment)— in short, something much more like stratospheric dust from a major volcanic eruption than an assemblage of living organisms.
Stratigraphy, Stratigraphic Correlation, and the Relative Time Scale — As further discussed and illustrated in Labs 3 and 4, stratigraphic correlation is the demonstration of correspondence in character and position between strata in different areas. Correspondence in age does not necessarily have much to do with correspondence in type of depositional environment, and thus with correspondence in lithology or fossil content. Early geologists, however, did not clearly distinguish correspondences in lithology, fossil content, and age. The result was one conundrum after another in working out locally and globally applicable relative time scales, as in the 18th Century Primary-Secondary-Tertiary scheme in which rock type was equated with age. By the mid-20th Century, stratigraphy had become divided into its three traditional branches— lithostratigraphy, biostratigraphy, and chronostratigraphy (or rock- stratigraphy, fossil-stratigraphy, and time-stratigraphy)— in recognition of their fundamentally different character. Newer branches are magnetostratigraphy and tectonostratigraphy, and we will be concerned with them here only as they touch on chronostratigraphy.
Stratigraphic Classification — In lithostratigraphy, biostratigraphy, chronostratigraphy, magnetostratigraphy, and tectonostratigraphy, rocks are classified respectively by rock type, fossil content, age, remanent magnetism, and association with a particular terrane (e.g. alone among North America’s terranes, its southeastern coastal plain used to be attached to Africa as part of the supercontinent Gondwanaland).
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A rock unit (or lithostratigraphic unit) such as a formation or bed is a mappable body of strata which is defined by a type section and, ordinarily, named for the type section’s location (e.g. the Cascadilla Member of the Ithaca Formation, named for its type section in Cascadilla Creek’s gorge) The basic biostratigraphic unit is the biostratigraphic zone (biozone) of a single species, the actual body of rock in which that species occurs. Chronostratigraphic units are comprised of all rock formed between two events. The hierarchy runs from chronostratigraphic zone (chronozone), stage, series, system, erathem, to eonothem; the corresponding hierarchy of chronological units runs from chron, age epoch, period, era, to eon. The more inclusive units are defined by internationally agreed-upon points in specific stratigraphic sections (boundary stratotypes). In practice, the most frequently used chrons and chronozones are those corresponding to biozones.
Events, Event Markers, and Chronostratigraphic Correlation — In addition to originations and extinctions of species, events commonly made use of in chronostratigraphy include geomagnetic reversals, sea level changes (particularly reversals between transgression and regression), and many kinds of depositional event. So-called key beds are event markers. They include volcanic ash falls, turbidite beds, and layers reworked during storms. Perhaps the most famous is the so-called Cretaceous-Tertiary Boundary Clay, a worldwide (but spatially intermittent) deposit that many now identify with ejecta from the impact crater at Chicxulub, Mexico. Varves or similar, more or less annual bands can be traced long distances in some deposits (e.g. varves in glacial lake deposits, bands in banded iron-formation and banded gypsum). Chronostratigraphic correlation can be biostratigraphically, lithostratigraphically, or magnetostratigraphically based. Lithostratigraphic correlation of a key bed, for instance, amounts to chronostratigraphic correlation. The important point is that litho-, bio-, magneto-, or tectonostratigraphic correlation is not necessarily chronostratigraphic correlation.
The Stratigraphic Time Scale — Application of biotic succession to geochronology opened the modern phase in development of the relative time scale, and work proceeded rapidly in the years following the Napoleonic Wars. Fossiliferous metamorphic rocks had revealed the inadequacy of the old Primary- Secondary-Tertiary classification, and geologists sought a new order based on fossils. The characteristically flat-lying, fossiliferous strata of continental platforms were the first to be thoroughly investigated. At the time, it was not realized that sediments had been laid down in these areas only at times of highest sea level, and that most of the Earth’s history was unrepresented there. Rocks do indeed seem to fall out in discrete units not unlike the seven geologists once sought for the seven days of Genesis. In setting up the geological systems, early 19th Century geologists did a remarkably precise job of picking out the major packages, as sea-level studies of the 1970s and ’80s have revealed. Although some earlier-named formations were set up as geological systems, the Silurian System (1835) was the first to documented in the modern sense, complete with descriptions of the fossils characterizing the time interval. By 1850, most of the familiar geological systems had been established, as well a a number that proved to be more or less synonymous with ones previously proposed. Unconformities on continental platforms perplexed many of geology’s pioneers, for it was not understood at first that sediments there represented only a small fraction of the elapsed time. The Catastrophists, who began in Revolutionary France, interpreted unconformities as indications of abrupt bursts of revolutionary change, even whole new creations— a clever way to compact geological history into what was then a socially more palatable expanse and still have an everyday, uniformitarian world most of the time. Rather like Romanticism, its counterpart in contemporary art and literature, Catastrophism strained credulity too much to remain popular for long.
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An accompanying diagram explains two actual instances of what can (and often has) gone wrong in geologists’ attempts to build and refine the relative time scale: the Cambrian Controversy, which split British geologists into warring factions for half a century and was eventually resolved by the creation of the Ordovician Period for the overlap between the Cambrian and Silurian Periods as originally defined; and Cambrian Controversy’s North American counterpart, the Taconic Controversy, which likewise turned on early geologists’ failure to recognize that available knowledge of present-day organisms and environments was not yet a workable key to the past. In Lab 3, you will have the opportunity to make the same mistake they did as you analyze rocks concerned in the Taconic Controversy. In Lab 4, you will have the opportunity to figure out the problem for yourselves, as Cornell Professor Henry Shaler Williams was the first to do.
Study Questions 0. Continuing Questions: What is the uniformity principle, and what are its implications for understanding our world and its historical record? (Lecture 1) 1. How can rocks be dated? How can a geological time scale be constructed— a time scale of worldwide applicability? What are the main processes useful in geological timekeeping and age- dating? 2. What is the relative geological time scale? …the absolute geological time scale? What are the differences? 3. What is the stratigraphic time scale? What is stratigraphic correlation? What are three main kinds of stratigraphic correlation? 4. What are five kinds of event used in determining the age relationships of strata?