The Golden Spike

In geological parlance, the word "Cambrian" can have two meanings. The first, and most commonly used, designates an interval of geologic time spanning from the second in which the first Cambrian sand grain was deposited until the instant that the first sediments of the Ordovician were laid down. No one was there with a stopwatch to mark these events, so these moments must remain theoretical instants of the geological past. Their importance is that they mark the beginning and ending of the sum total of Cambrian time. This time interval is called the Cambrian Period, and it is subdivided into the Early Cambrian, the Middle Cambrian, and the Late Cambrian Epochs. Periods and epochs are subdivisions of geologic time, and could have been measured with a stopwatch had anyone been there to time them. Radiometric dating of rocks can provide estimates of these times, but there are tens of millions of years of uncertainty with all radiometric dates of Cambrian age. It is not possible to know the beginning of the Cambrian Period with anything approaching stopwatch precision.

The second usage of "Cambrian" is as a designation of a particular body of rocks. These are all the rocks (both sedimentary and igneous) that were formed during the Cambrian Period. The term Cambrian System is used to refer to the sum total of rocks deposited during the Cambrian Period. With the help of a magic bulldozer, one could theoretically excavate all the patches of Cambrian rock worldwide and pile them into an enormous heap that would constitute the entire Cambrian System. One would need a legion of bulldozers to do this, because many surviving Cambrian rocks are buried quite deeply below the present-day surface of the earth.

The Cambrian System is divided into the Lower Cambrian, the Middle Cambrian, and the Upper Cambrian Series. The "Lower" and "Upper" used for series refer, naturally, to the physical positions of the strata from different parts of the Cambrian System. The Cambrian System is a "time-rock unit." In contrast to the Cambrian Period which is purely a time unit, a time-rock unit refers to all the rocks that formed within a certain period of time, fust as time units like "Cambrian Period" can be subdivided into epochs, the time unit "Cambrian System" can be subdivided into series. Series correspond exactly to the Early, Middle, and Late epochs introduced above. In this time-stratigraphic hierarchy, the "Early Cambrian Epoch" corresponds to the "Lower Cambrian Series." In other words, "Cambrian Period" and "Early Cambrian Epoch" are used in the same sense as "the year 1963," whereas the "Cambrian System" and "Lower Cambrian Series" are used in the same sense as "all the red wine bottled in 1963." The relationships between period, system, epoch and series are shown in figure 5.1.

Time Units Time Rock Units

Era ~ Erathem

Period ~ System

Age ~ Stage

Rock Units

WESTERN TERMS SOVIET TERMS

Group Svit (Suite)

Formation Gorizont (Horizon)

Member Bed

FIGURE 5.1. Comparison of time, rock, and time-rock units. Both Western and Soviet rock terms are shown.

There is one problem regarding the recognition of the beginning of the Cambrian Period and its time-rock equivalent, the base of the Cambrian System. Neither the instant of time nor the point in the layered sedimentary sequence has yet been formally defined. A great deal of effort has been expended in recent years trying to locate the "best" point in rock to define the base of the Cambrian. Criteria for what is "best" include the ability to locate in other stratigraphic sections a point which is the same age as the point located in the formally defined standard section or stratotype section. When a stratotype point and section is finally agreed upon, geologists will be able to "drive the golden spike," and thus formally define the Pre-cambrian-Cambrian boundary. The definition of a stratotype point is merely a formal, legalistic variant of the technique that resulted in the breakthrough in correlation made by European geologists in the 1830s (discussed in the first chapter). The golden spike is very useful, nevertheless, as the objective criterion against which a stratigraphic boundary can be correlated the world over.

The Cambrian System was a product of the pioneering geological research of the 1830s. Its initial definition involved two great early stratigraphers, Adam Sedgwick and Roderick Murchison. These British geologists began their studies of ancient fossil-bearing rocks in friendly collaboration, but their relationship deteriorated into a bitter feud over a terminological dispute. Secord (1986) has written an analysis of this tempestuous episode in Victorian science.

Using the new and powerful technique of correlating strata with fossils, Sedgwick and Murchison both began to study the stratigraphy of some of the most ancient sedimentary rocks known in Britain. At the time, these rocks were referred to as the "transition" rocks. "Transition" refers to the idea current before the 1830s that many of the most ancient sedimentary rocks known were transitional between the crystalline, igneous "primary" rocks (then thought to be the most ancient rocks on earth) and the "secondary" rocks that contained abundant fossils. Sedgwick began his work in North Wales; Murchison began his in the Welsh Borderlands. As this research progressed, Sedgwick named his rocks Cambrian; Murchison designated his, Silurian. The scientists agreed to a boundary between their two systems (with the Cambrian being the older system), and in 1835 they jointly published a short paper for the British Association for the Advancement of Science entitled "On the Silurian and Cambrian Systems, exhibiting the order in which the older Sedimentary Strata succeed each other in England and Wales" (Sedgwick and Murchison 1835). With continued study, it became clear that

Sedgwick's Upper Cambrian was equivalent to Murchison's Lower Silurian. Murchison was the more aggressive of the two, and, in an undiplomatic move, he tried to subsume Sedgwick's Cambrian into his Silurian. To Sedgwick's profound dismay, the newly created British Geological Survey adopted Murchison's classification of the disputed interval.

For many years, Sedgwick expressed his unhappiness with the situation to other prominent scientists. Much of Sedgwick's problem resulted from the fact that the Cambrian (in modern reckoning! rocks of North Wales are very poorly fossiliferous. This made it difficult for Sedgwick to demonstrate a distinctive fauna for the Cambrian. Stubblefield (1956) has shown that much of Sedgwick's original Cambrian System in Wales belongs to the Precambrian or to younger systems. As currently defined, the Cambrian in Wales consists primarily of fine-grained sedimentary rocks which have yielded only sparse upper Lower Cambrian faunas.

Some of Sedgwick's contemporaries favored retaining the term Cambrian, and others opted for abandoning it and, following Murchi-son's urgings, using Silurian to refer to all the oldest animal fossils. In the first edition of Origin of Species, Charles Darwin refers to the most ancient strata with animal fossils as Silurian (Darwin 1859). In a letter (dated May 30, 1857) to Sedgwick, the great anatomist Richard Owen (the scientist who gave dinosaurs their name) wrote:

I have stuck to the "Cambrian" more instinctively, than through cold conviction, being bothered by the diversity of flat assertions as to fossil evidence from men whom I felt to be my masters in regard to that. In an old Diagram used a score of years ago nearly, at the Coll. of Surgeons, the base "Cambrian" supported, next, "Silurian." I used to be appealed to, to do away with the fundamental name, but could not make up my mind to it, and I hoisted the old flag again at my "Lectures," this year, in Jermyn Street.

I began to think . . . that "(Cambrian] will come to be thought" all right, after all. But as "America" will never now be called "Columbia" . . . you will probably have your name attached to a very small part of your discoveries (S. Rachootin, personal communication, 1987).

With the passage of time, "Cambrian" was retained and Sedgwick was at least partly vindicated. "Primordial" fossils such as the trilo-bites described by the American geologist Ebenezer Emmons (1847;

see chapter 1, figure 1.2) were discovered at stratigraphic levels well below those at which anything had been found before, and were eventually assigned to the Cambrian. Interestingly, Emmons was quite correct in his assessment of the age of the New York trilobites, and he even went so far as to correlate the Taconic trilobites to the European system "known under the term Cambrian (p. 48)." In a passage that seems to predict the outcome of the Cambrian-Silurian dispute, Emmons challenged Murchison's claims that the Silurian contains the oldest known fossils:

peculiar fossils [occur] on both sides of the Atlantic, which, so far as discoveries have yet been made, are confined to the slates of the Cambrian . . . system; and now the great object of the writer is to show that the above question [i.e., Murchison's proposal to call the oldest fossils Silurian] has not been settled right, or according to facts; or, in other words, ... all the Cambrian rocks are not Lower Silurian (1847:48-49).

When a major unconformity (gap in the sedimentary record) was recognized between Murchison's Lower and Upper Silurian, many geologists switched to a three-part classification of this part of the geologic time scale, with the middle part often being referred to as "Lower Silurian." The Silurian-Cambrian dispute ended in 1879 when Charles Lapworth proposed the Ordovician Period and System for the contested strata. By the turn of the century, the terms Cambrian, Ordovician, and Silurian were in wide usage, and they remain today as the universally used lowermost three periods or systems of the geologic time scale. The basic, period-level geologic time scale has not changed since the term Ordovician was proposed.

More recent work has led to calls for the creation of a sub-Cambrian period and system. Abundant Ediacaran fossils occur in the western Soviet Union, and Soviet geologists have proposed the term "Vendian" as a period before, and a system below, the Cambrian (Sokolov 1952; Sokolov and Fedonkin 1984). A counterproposal was offered by Cloud and Glaessner, suggesting that this interval be called the Ediacarian Period and Ediacarian System (note the additional "i" in this spelling), with a type section in the Flinders Ranges of South Australia. Cloud and Glaessner discount the Vendian System because the stratotype section for the Vendian is known only from borehole cores, and is "inaccessible to direct observation" (1982:791). It is true that accessibility for collecting and study is one of the criteria for a suitable stratotype, but in our opinion this should not affect the priority of the Vendian System. It makes good sense to find a Vendian stratotype that is more accessible than the original borehole sequences, but we don't think the term needs to be replaced with the unwieldy "Ediacarian."

Rowland (1983) has examined some terminological implications of the Vendian/Ediacarian dispute. For many decades, introductory geology students have been obliged to memorize the periods of the geologic time scale (figure 1.1), a duty that many of them found, and continue to find, onerous. One strategy for memorizing is a mnemonic aid that uses the first letter of every period to create a clever sentence. Some professors offer an award for the most clever and original mnemonic devised by student in each year's historical geology course. This contest almost always results in the creation of hilarious and innovative mnemonics. (Some entries cannot be read in public.) Many students fall back on old standard mnemonics, such as "Carl's Old Shirt Doesn't Match Pete's Pants" for Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian. According to Rowland, with the adoption of a Precambrian period, the old mnemonics (some of which have been in use for over 100 years) must go.

With Carl's Old Shirt headed for the trash, what's the next generation of geology students supposed to do to keep the periods straight? If Ediacarian prevails, try this as an aid: Every Class of Students Detests Memorizing Pointless Periods. Or, if Vendian is the choice: Very Cold Or Snowy Days May Prompt Pneumonia. (1983:82)

Regardless of the outcome of the latest terminology debate, there is as yet no formally defined Precambrian-Cambrian boundary. Both practical and philosophical approaches have been applied to the problem of defining the beginning or the base of the Cambrian. Some earth scientists have philosophically argued that all metazoan body fossils visible to the naked eye should be included in the Cambrian (Cloud and Nelson 1966). The unlikelihood of finding the earliest metazoans visible to the naked eye as fossils poses practical difficulties for pinpointing a correlatable boundary in sedimentary rocks. They were probably small, as well as soft-bodied. On the other hand, practical solutions to the Cambrian lower boundary problem that permit the presence of common and easily recognized fossils such as trilobites above and below the boundary (Bjorlykke 1982) seem overly pragmatic. With Bjorlykke's approach, fossils which many paleontol ogists would consider as being characteristically Cambrian would have to be assigned to the Precambrian. Bjorlykke makes an important point, however, because first appearances of fossil groups are not a suitable basis for a major boundary.

The concept of a Lower Cambrian Stage and Series was developed by continued application of the powerful (and pragmatic!) principles of biostratigraphy in the works of Barrande (1852-1911), Bragger (1886), and Lapworth (1879, 1888). Walcott (1891) was first to divide the Cambrian into lower, middle, and upper series and his lead has been followed ever since. Walcott also (1889, 1890) succeeded in correlating the unique successions of Cambrian faunas with fossilif-erous strata in North America, following the precedent of transAtlantic correlation set by Emmons (1847). Walcott's correct (1890) recognition of the Lower Cambrian trilobite fauna containing the trilobite Olenellus in North America was the first instance of successful intercontinental correlation of demonstrably Lower Cambrian sediments.

Following Walcott's (1890, 1900) success with intercontinental correlation, the Olenellus Fauna became widely accepted as a formal biostratigraphic zone, synonymous with the concept of the Lower Cambrian Series. But it soon became apparent that shelly fossils extended below the range of the Olenellus Fauna. G. F. Matthew was probably the first person to realize that there are shelly fossils stratigraphically below the abundant trilobites and other typically Cambrian fossils of the Olenellus Fauna (E. Landing 1987, personal communication). Matthew referred to this stratigraphically low interval (with few or no trilobites) of shelly-fossil-rich strata as the "Etcheminian." Matthew was not pleased when Walcott (1900), in a Murchisonian move, tried to include this low interval within the Olenellus Fauna (Matthew 1900). Matthew noted (1900:256) that "tube worms and brachiopods seem the most striking fossils of this lower" stratigraphic interval, and he was justifiably upset when Walcott (1900) tried to include this lower interval in the Olenellus Fauna without having demonstrated the presence of appropriate trilobites at this stratigraphic depth.

By the 1940s, in confirmation of Matthew's (1900) views, it was recognized that Precambrian-Cambrian boundary sections in many parts of the world possessed faunas of small shelly fossils underlying the lowermost trilobite-bearing beds. For example, Howell et al. (1944) called this interval the Obolella zone {Obolella is a small, oval-shaped inarticulate brachiopod). The global biostratigraphic im portance of small shelly fossils was not appreciated until the late 1960s, when V. V. Missarzhevskii, A. Yu. Rozanov, and others began presenting the results of their studies on small shelly fossil faunas occurring before the first trilobites in the central and western USSR.

Unlike Matthew, who considered the early shelly faunas to be "distinct from the Cambrian" (1900:255), the Soviet workers in the 1960s assigned most of the early, pretrilobite shelly faunas to the Cambrian. This research culminated in the publication of the "brown bible" (Rozanov et al. 1969). In this book, the Tommotian Stage was defined, and the type section for the stage was along the banks of the Aldan River area in southeastern Siberia. The "brown bible" uses a combination of archaeocyathan and small shelly fossil range data to biostratigraphically subdivide the bottom of the Siberian Cambrian sequence into a number of biostratigraphic zones.

Faunas comparable to those of the Siberian Platform are now known from Scandinavia, many parts of the USSR, Poland, England, France, Newfoundland, Nova Scotia, India, Iran, New England, northwestern Canada, California and Nevada, northwestern Mexico, Mongolia, China, and Australia. Many of these faunas have been referred to as "Tommotian shelly faunas," although precise correlation with the Tommotian Stage of the Siberian platform is often questionable. It is in this sort of situation that the need for a "golden spike" is most apparent.

The International Union of Geological Sciences—International Geological Correlation Programme (IUGS —IGCP) Working Group on the Precambrian-Cambrian Boundary is commissioned with trying to locate a stratotype boundary for the base of the Cambrian. The three best candidate stratigraphic sections are sections: (a) along the Aldan River in Siberia; (b) in the Yunnan Province, Peoples' Republic of China; and (c) at Fortune Head in southeastern Newfoundland. None of these sections is completely satisfactory as a stratotype section. Both the Chinese and Soviet sections are rich with shelly fossils, but the sections are thin and are marred by significant gaps, manifest as unconformities. The Newfoundland section is thicker and the preserved sediments potentially represent much more time than is represented by the Chinese and Soviet sections (Signor et al. 1988). But the Precambrian-Cambrian boundary in Newfoundland is defined solely on trace fossil data (Narbonne et al. 1987), and the lowest shelly fossils occur more than 400 meters above the proposed boundary.

There are political aspects to the selection of a stratotype bound-

FIGURE 5.2. Canadian geologist Guy Narbonne points to the proposed Pre-cambrian-Cambrian stratotype in a sea cliff at Fortune Head, southeastern Newfoundland.

ary. The boundary should be politically, as well as geographically, accessible. The Canadians, Soviets, and Chinese are vying with one another for the stratotype "prize." After all, who wouldn't want to have the "golden spike" in one's own geologic backyard? (Figure 5.2 shows a Canadian geologist eagerly promoting the Newfoundland boundary.) This stratotype boundary is the most important one in the entire geologic time scale, and once defined, all questions concerning the age and correlation of the Precambrian-Cambrian boundary must be referred to the sedimentary exposure into which the "golden spike" has been "driven." No doubt some enthusiastic strat-igrapher will wish to erect a sizeable monument on the hallowed spot.

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