The Rifting of Rodinia

natomically modern humans first appeared on earth some time during the "Ice Age" or Pleistocene Epoch of the Ceno-zoic Period. There is no good reason to believe that this episode of glaciation is over, and it is entirely plausible that the great North American ice sheet could return in another ninety thousand years or so and grind New York City into the Atlantic seafloor. There is an interesting parallel between the origin of modern humans and the emergence of animals —both events are associated with glaciation. The first animals appeared during a long, drawn-out episode of glaciation at the end of the Precambrian.

Evidence for glaciation during the late Precambrian consists mainly of unusual sedimentary deposits called tillites. Most sedimentary rocks are composed of sediment particles that are more or less the same size. For instance, sandstone consists primarily of sand grains, or grains between one sixteenth of a millimeter and two millimeters in diameter. If the sandstone consists almost solely of sand grains, it is called a "clean" or "pure" sandstone. If the sand grains are all close to being the same size (a half millimeter in diameter, for instance), the sandstone is said to be very well sorted. Sorting of sediments occurs as a result of the winnowing action of wind and water. For example, light, tiny sediment particles can be carried off by winds and deposited together, leaving behind a lag of coarser grains too heavy to be moved by the wind. This natural segregation of sediment sizes by the action of wind and water is called sorting.

Tillites are the antithesis of a clean, well-sorted sandstone, because tillites include sediment sizes ranging from microscopic clay particles to meter-sized boulders mixed together in a helter-skelter fashion. The lack of sorting in tillites is due to deposition by glaciers. Glaciers carry a lot more than just ice as they move downhill; sediment of all sizes gets entrained in the ice mass. And since ice is a solid instead of a fluid or gas, it is unable to winnow or sort sediment the way wind and water can. When the edge of the glacier begins to melt (as when, for instance, it reaches the sea), the sediment of all sizes that was caught up in the glacier is unceremoniously dumped. This glacial debris, or till, is deposited in a pile or sheet of poorly sorted material. Assuming that the glacier is not enlarging or shrinking, large mounds of till can be deposited at the margin of the glacier. This can occur while the glacier remains at about the same size, since ice that melts at the glacier's foot is replenished by fresh snowfall at higher altitudes or latitudes. If subsequently lithified to sedimentary rock, this till becomes tillite, and can be preserved in the rock record as evidence for an ancient glaciation.

There is widespread evidence for glaciation in late Precambrian times. The rock record from one billion years ago to the Precam-brian-Cambrian boundary is replete with rocks of known or suspected glacial origin, and it is certain that during this interval there were times when many parts of the world experienced exceptionally low temperatures. Harland (1983) cites evidence for at least four major episodes of late Precambrian glaciation. This 400 million year "reign of glaciers" is unique in earth history, and our Pleistocene glacial interlude is insignificant compared to the duration of the glacial periods in the late Precambrian. Never before or since have glaciers been so prevalent or frequent. The Varangian ice age (about 700 to 600 million years ago; the third late Precambrian glacial episode by Harland's 1983 rekoning) was the most severe climatic event of the Vendian, and Varangian tillites are widely distributed in

Svalbard, Greenland, Norway, Sweden, Scotland, Ireland, the western Soviet Union, and the Appalachians (Hambrey 1983). The Varangian (sometimes called Varangerian) glaciation is named for the Varanger peninsula in northeastern Norway, which has a more or less complete stratigraphic record from the Proterozoic to the Early Cambrian (Vidal 1984).

Ever since cartographers began making accurate world maps, children and other people with open minds have noticed that the east coast of South America and the west coast of Africa could fit together quite nicely. In fact, the fit seems to be so good that the two contients might be neighboring pieces of a jigsaw puzzle. With the acceptance of the theory of plate tectonics, it became clear that the jigsaw metaphor is indeed appropriate for describing the face of the earth (Monastersky 1987). Alfred Wegener, the now famous German meterologist and geophysicist who was scorned in his day for his support of the continental drift theory (Wegener 1967, translation of 1929 edition), surmised that today's continents were once connected to form a single supercontinent in the remote geological past. Wege-ner named this supercontinent Pangaea (from the Greek for "all land"). Had he not met an untimely death in 1930, the geological community might not have had to wait until the 1960s for widespread acceptance of the fact of Pangaea's existence. Virtually all geologists now accept that Pangaea, and its corresponding super-ocean Panthallassa, was formed about 200 million years ago by the coming together and collision of all or nearly all of the continents and continent fragments covering the face of the earth. The sea floor crust that existed between the pre-Pangaea continents was destroyed by subduction (underthrusting and melting) beneath the converging continents. After its formation, Pangaea was subsequently sundered by tectonic forces that are still not fully understood. Pangaean fragments were scattered over the earth's surface as new ocean basins (such as the Atlantic) formed in the fractures that split the supercontinent.

In the 1960s, the geological community was galvanized by geophysical evidence supporting continental drift, and as part of this "plate tectonic revolution," evidence became available indicating that the modern ocean basins may not have opened up only once (during the fragmentation of Pangaea), but two or more times during the course of geological time. In a famous article entitled "Did the Atlantic close and then reopen?", the Canadian geologist J. Tuzo Wilson suggested that the split between North/South America and

Europe/Africa was merely a rebirth of an ocean that was destroyed by the formation of Pangaea (Wilson 1966). Some scientists began to suspect that many ocean basins may have shared the same fate. If this were the case, it might imply the existence of a precursor supercontinent that existed in remote Precambrian time, well before Pangaea. Evidence for a major, one-billion-year-old episode of continental collision and supercontinent formation began to accumulate in the early 1970s (Valentine and Moores 1972; Dewey and Burke 1973; Irving et al. 1974), and subsequently a large body of geological, pa-leontological, and paleomagnetic evidence has been marshalled in support of the existence of this Precambrian supercontinent (Lindsay et al. 1987). The supercontinent has been variously called "proto-Pangaea" (Sawkins 1976), "The Late Proterozoic Supercontinent" (Piper 1987), or simply the Precambrian supercontinent. The former existence of this supercontinent is now well established—in the late Proterozoic, there are unmistakable signs of its breakup (Conway Morris 1987a).

This supercontinent deserves its own name, so we here propose the name Rodinia (figure 6.1) for the Precambrian supercontinent and Mirovia for the corresponding superocean. These names are taken from Russian. A derivation from Russian seems fitting, because of the important research done by Soviet earth scientists on the Late Precambrian and Early Cambrian (particularly their creation of the Vendian System). Mirovia is derived from the Russian word mirovoi meaning "world" or "global," and, indeed, this ocean was global in nature. Rodinia comes from the infinitive rodit' which means "to beget" or "to grow." Rodinia begat all subsequent continents, and the edges (continental shelves) of Rodinia were the cradle of the earliest animals.

A supercontinent need not include all the continents. Supercontinents have existed that comprised only a few or several continents. When Pangaea began to split apart, it initially broke into two main chunks, a northern supercontinent called Laurasia and a southern supercontinent called Gondwana. Laurasia consists of most of North America, Greenland, Baltica (primarily Europe west of the Ural Mountains), Siberia, Kazakhstania (southwestern USSR), and China, plus a few other minor continental blocks. Gondwana includes South America, Africa, Arabia, Iran, India, Madagascar, Antarctica, Australia, and New Zealand (figure 6.2). The rift between Laurasia and Gondwana formed a great east-west sea known as the Tethyan seaway. Most modern oceans are elongate in a north-south direction, so

Terre Ere Glaciation Varanger

FIGURE 6.1. Rodinia, the Precambrian supercontinent, surrounded by the superocean Mirovia. The exact outline of Rodinia is unknown, but it approximately followed the continental edges exposed to Mirovia as shown in this figure. The South China platform may have fit in the gap between Baltica and India. (Reconstruction based on M. McMenamin 1982, Piper 1987, Donovan 1987, and Sears and Price 1978, created with TERRA MOBILIS)

FIGURE 6.1. Rodinia, the Precambrian supercontinent, surrounded by the superocean Mirovia. The exact outline of Rodinia is unknown, but it approximately followed the continental edges exposed to Mirovia as shown in this figure. The South China platform may have fit in the gap between Baltica and India. (Reconstruction based on M. McMenamin 1982, Piper 1987, Donovan 1987, and Sears and Price 1978, created with TERRA MOBILIS)

the Tethyan seaway of 150 million years ago was unlike anything that exists today.

The makeup, as well as the breakup, of Rodinia is less well documented than that of Pangaea. There is good evidence suggesting that the southern continents were close together, in an arrangement that was very similar to that of the Paleozoic Gondwana. Evidence for this reconstruction is primarily from three sources: comparisons of Precambrian bedrock geology, paleomagnetics, and paleobiogeogra-phy.

Use of bedrock geology to reconstruct ancient continental positions relies on the idea that if two separated continents were once joined to form a single, larger continent, then there ought to be distinctive geological terranes (such as mineral belts, mountain chains, bodies of igneous rock of similar age, and other roughly linear to irregularly-shaped large-scale geologic features) that were once contiguous but are now separated. Matching of these features can provide clues to the positions of continents that were once together. Such evidence was used by Sears and Price (1978) to argue that Siberia was connected to the western (present-day coordinates) coast of North America during the Precambrian. The main problem with using bedrock geology features to match continental puzzle pieces together is that many of the potentially most useful linear geologic features on the continents (such as volcanic arcs or chains of volcanoes, and continental margin fold belts or parallel mountain chains formed by compression of strata) are parallel to the edge of the continent. Therefore, these features generally run parallel to rift fractures, and are less likely to continue and be recognizable on any continent that was once connected to the continent in question.

Paleomagnetic evidence is an important tool for the determina-

Gondwana Jigsaw

FIGURE 6.2. Gondwana, the supercontinent as it existed before approximately 250 million years ago. (Created with TERRA MOBILIS) Note: TERRA MOBILIS is a registered trademark of C. R. Dcnham and C. R. Scotese, Earth in Motion Technologies, 1987, 1988.

tion of ancient continent positions and for the reconstruction of supercontinents. Nearly all rock types, be they sedimentary or igneous, contain minerals that contain the elements iron or titanium. Many of these iron- and titanium-bearing minerals are magnetic. A familiar example is magnetite, an iron oxide that was used (under the name lodestone) to form the earliest compasses. In a compass, of course, the magnetized compass needle has a tendency to align itself with the earth's magnetic field. The magnetization of a crystal of a magnetic mineral (such as magnetite) is established immediately after the mineral crystallizes from a volcanic melt (lava) but before it cools below the Curie point temperature. Each magnetic mineral has its own specific Curie point. The Curie point of magnetite is 578 degrees centigrade, which —by way of comparison —is hotter than the melting temperature of pure lead (327.4 degrees C) but less than the melting temperature of pure aluminum (660 degrees C). As the mineral grain passes through the Curie point, the ambient magnetic field is "frozen" into the crystal and will remain unchanged until the crystal is destroyed by weathering or once again heated above the Curie point. This "locking in" of the magnetic signal in igneous rock crystals is the crucial event for paleomagnetism, for it indicates the direction of magnetic north at the time the crystal cooled (sometime in the distant geologic past for most igneous rocks). The ancient latitudinal position of the rock (and the continent of which it is a part) can be determined by measuring the direction of the crystal's magnetization. For ancient rocks, this direction can be quite different from the direction of present day magnetic north. A major assumption of paleomagnetic studies is that the position of the north magnetic pole has stayed more or less in the same place (with the exception of magnetic polarity reversals, in which the north and south magnetic poles switch places. Polarity reversals need not concern us here,- see McElhinny 1979).

Sedimentary rocks can also be used for paleomagnetic determinations. As sediment particles settle to the bottom of the sea or lake or wherever they end up being deposited, any that are magnetic will have a tendency, like tiny compasses, to align themselves with the earth's magnetic field. If enough magnetic sediment grains are incorporated into the sediment, the sedimentary rock can retain a magnetic signal (called a primary remanent magnetization) that is as useful for paleomagnetic studies as are the magnetic remanences in igneous rocks. Relying mostly on paleomagnetic data, Piper (1987) has reconstructed Rodinia as a supercontinent consisting of all the major continents (figure 6.1). As noted above, Gondwana continents remain in essentially the same position as they were when Pangaea formed, which is not surprising since Gondwana remained intact between the breakup of Rodinia and the formation of Pangaea. Baltica is placed at one end of Rodinia, and North China is placed, with question, on the other end. There are some uncertainties regarding the exact placement of the continental pieces of Rodinia, most notably the position of China and the identity of the continent to the west (present-day coordinates) of North America (Siberia is shown in this position in figure 6.1, following the results of Sears and Price 1978). Despite uncertainty about the placement of some of the pieces of the jigsaw puzzle, the available paleomagnetic evidence favors the existence of a Precambrian supercontinent.

Paleomagnetic reconstruction is a form of geological analysis that is, unfortunately, fraught with uncertainties. The original magnetization is easily altered by weathering and metamorphism, and can confuse or obliterate the original magnetic signal. An inherent limitation of paleomagnetic reconstruction of ancient continental positions is that the magnetic remanence only gives information concerning the rocks' latitudinal position, and gives no clue as to the original longitudinal position of the rocks in question. For example, southern Mexico and central India, although nearly half a world apart, are both at about 20 degrees North latitude, and, therefore, lavas cooling in either country would have essentially the same primary magnetic remanence. One of the few ways to get information about the ancient longitudinal positions of continents is to use comparison of life forms on different continents. The study of ancient distributions of organisms is called paleobiogeography.

Debrenne and Kruse (1986) have determined that sixteen identical species of archaeocyathans are found in Lower Cambrian sediments of both Australia and Antarctica. Most archaeocyathan species have fairly limited geographic distributions, and Debrenne and Kruse (1986) state that the large number of archaeocyathans that are common to both Australia and Antarctica confirm the existence of a Gondwana supercontinent in the Early Paleozoic. These two continents are also likely to have been together in the Proterozoic. Because of their shared Cambrian biota, the Australian and Antarctican pieces of the of the Rodinia jigsaw puzzle are confidently in place with respect to one another.

Unfortunately, other Cambrian shelly fossils have so far proven less useful for determining the paleogeographic positions of conti nents, because they have very wide geographic distributions. Phos-phatic tubular small shelly fossils such as Anabarites and Hyolithel-lus (figures 4.3 and 4.5, respectively) and trilobitoid trace fossils such as Monomorphichnus (figure 6.3) are extremely widespread on a global scale. Part of the problem with using these simple shelly fossils for biogeographic study is that there is very little information available for recognizing differences between species. Different biological species of Hyolithellus may have looked very different in life, but their fossil remains (consisting of a simple annulated tubular shell) may be indistinguishable. Some species of Early Cambrian shelly fossils do seem to be endemic, or restricted to certain geographic locales. Lapworthella ftligrana (figure 4.8) is known from at least three widely separated localities in western North America, but, so far as we know, nowhere else (figure 6.4). There may have been geographic, climatic, or biological barriers that prevented this species of Lapworthella from spreading more widely.

Using Cambrian fossil distributions to infer the makeup of Rodi-nia gives a useful first approximation of Vendian continental posi-

FIGURE 6.3. Monomorphichnus, a trace fossil formed by the scratching action of trilobite legs. Lower Cambrian of Newfoundland; same specimen is figured in Narbonne et al. (1987). Scale bar is in centimeters.

tions of Australia and Antarctica, but it is better practice to use fossils of organisms that lived during the Vendian to make inferences about the positions of Vendian continents. I (M.A.S.M.) attempted to do this in 1981 by plotting the distributions of certain members of the Ediacaran fauna on a Cambrian paleogeographic base map (M. McMenamin 1982). My tabulation showed that although the frond-shaped members of the Vendian soft bodied fauna (such as Pteridinium; figure 2.2) had a global distribution, other distinctive members of the fauna [Dickinsonia [figure 2.5], Tribrachidium [figure 2.4] and several others, plus the African and Brazilian occurrences of Cloudina I figure 4.2]), seemed to be restricted to Baltica and Gondwana continents. I (M.A.S.M.) argued that the Ediacaran fauna first evolved and was most diverse in Gondwana, and that (contrary to conventional geologic wisdom) Baltica was as close, or closer, to Gondwana than to North America during the Vendian (M. McMenamin 1982).

This poses a problem for the reconstruction of Rodinia, because Baltica is generally thought to be very closely allied (in terms of Precambrian bedrock geology) to North America, a continent that has never produced the Gondwana/Baltica-type Ediacaran fossils even though Ediacaran fossils occur abundantly in several North American localities. One possible resolution to this problem was offered by the Piper (1987) reconstruction of Rodinia (figure 6.1), which brings Baltica and the eastern end of Gondwana relatively close together in southerly latitudes (Donovan 1987). The detailed positions of individual continental blocks within the Vendian supercon-

FIGURE 6.4. Distribution of the sclerite Lapworthella filigrana (illustrated in figure 4.8) on the Lower Cambrian North American continent. Triangles show localities where this species has been found. The straight line indicates the probable position of the Cambrian equator relative to ancient North America.

FIGURE 6.4. Distribution of the sclerite Lapworthella filigrana (illustrated in figure 4.8) on the Lower Cambrian North American continent. Triangles show localities where this species has been found. The straight line indicates the probable position of the Cambrian equator relative to ancient North America.

Equator tinent may shift with continued research, but the Piper (1987) reconstruction of Rodinia seems to be a reasonable approximation.

Recent results suggest that rifting and continental breakup occurred throughout the world near the Vendian-Cambrian boundary, although episodes of rift-related volcanism occur well back into the Proterozoic. When a continent splits apart, it does so along a roughly linear fracture in the earth's crust called a rift basin. Rift valleys can be persistent features on the margins of a continent; the East African Rift is a currently active rift valley (one that has not yet opened up into an ocean). In initial stages of rifting, the rift basin fills with flows of a dark colored volcanic rock called basalt. Also filling the basin are rapidly deposited sediments washed in from the steep sides of the rift valley, as well as ash and stream-carried fragments of volcanic rock. If the rifting and tectonic tension that initiates the formation of a rift valley continues to fruition, a new sea will open up as the rifted halves of what was once one continent move away from each other, marine waters enter the deepening rift, and new sea floor basaltic bedrock becomes covered by normal marine sediments such as sandstones, shales, and limestones.

Lava flows interbedded with normal marine sediments are common in Vendian-Cambrian sedimentary sequences, and are known from Arabia, Mexico, the Ural Mountains, as well as other places (Bond et al. 1985; Zonenshain et al. 1985). In Sonora, Mexico, debris from ancient volcanic eruptions (found as a volcaniclastic conglomerate composed of basaltic cobbles and boulders) occurs in the stra-tigraphic section (figure 6.5) between the oldest trilobites known in Mexico (figure 4.14) and Sinotubulites (figure 4.30), a late Vendian to earliest Cambrian tubular shelly fossil. These volcanic beds may be related to an episode of continental rifting along the west coast of North America (Bond et al. 1985). Eruption of basalts sometimes indicates continental rifting, and in many Paleozoic sedimentary sequences in eastern and western North America, the basaltic volcanic intervals are in the Vendian or Cambrian parts of the section. This pattern has been interpreted to indicate that rifting around North America occurred around 600 million years ago (Bond et al. 1985). Continental rifting is always associated with eruption of basalts, and in many Paleozoic sedimentary sequences in eastern and western North America, the basaltic volcanic intervals are in the Vendian or Cambrian parts of the section. This pattern suggests that rifting around North America occurred approximately 600 million years ago (Bond et al. 1985). This interpretation is consistent with the observation that marine sediments deposited much before the Vendian in North America are generally confined to localized, re-striced basins (Stewart 1976).

In the Vendian Tindir Group of Alaska and Yukon Territory (the extreme northwestern corner of the original North American continent), there is evidence for high-angle block faulting (Young 1982). Block faulting such as this is a common feature of continental breakup

Rifting Rodinia
FIGURE 6.5. Precambrian-Cambrian stratigraphic section of the Caborca region, Sonora, Mexico. (After M. McMenamin 1984)

and rift valley formation. For example, the East African Rift Valley (including Olduvai Gorge, the site of numerous finds of early hom-onid fossils) is characterized by numerous high-angle block faults at the valley margins. Thus, there is ample evidence suggesting that at least the North American part of Rodinia was rifting away from the supercontinent cluster near the Vendian-Cambrian boundary.

The history of the supercontinent Rodinia can be summarized, in very general terms, as follows. Sometime during the latter part of the remote Precambrian past (about a billion years ago), this supercontinent was intact and was composed of most, or all, of the present day continental blocks or cratons. The exact shape of this supercontinent is unknown, but educated guesses as to its shape have been made using the best available paleomagnetic data. By Vendian time, approaching the end of the Precambrian, Rodinia seems to have been still largely intact, but there is evidence (rift basins, volcanic deposits on continental margins) that it was beginning to feel tensional forces that would eventually break it asunder. Limited paleobiogeo-graphic evidence indicates that Baltica was near Gondwana, and in one reconstruction Rodinia has a more compact shape than the later supercontinent Pangaea. The Vendian saw four major phases of glaciation. We don't know if Rodinia had a large ice cap covering large areas of the supercontinent during the Vendian, but continental interior climate must have been extremely cold at times; supercontinents have very severe climates owing to the isolation of interior land from shoreline. Marine shorelines exert a moderating, maritime influence on climate, which is why Portland, Oregon has less severe winters than Minneapolis, even though Minneapolis is at a lower latitide.

Separate all the continents composing the supercontinent Rodinia (or Pangaea) and the total length of coastline is more than doubled, which would undoubtedly improve global climate. Therefore, if parts of Rodinia were at high latitudes or if global climate were severe, we would expect that many areas would experience glaciation. Another factor that could contribute to the Precambrian glacial record should be noted. During the early stages of rifting, the geothermal heat flow underneath the section of crust about to be rifted increases dramatically. Since hotter rocks are less dense, the crustal rocks, feeling the heat, rise up relative to surrounding, cooler areas of continental crust. As these areas are uplifted, they are carried into higher altitudes where the air is cooler and the chance that snow and ice will not all melt away during the summer months (a necessary condition for the formation of glaciers) is increased. Ironically, an increased heat flow from the earth's interior can cool the surface climate directly above it. This is in fact happening today —in the lofty Ru-wenzori Mountains of Ethiopia (5119 m high), uplifted by the bouy-ant effects of East African rifting, glaciers are forming at a latitude that is practically on the equator! Glaciation attributable to this same rifting-related process of uplift may explain some of the Vendian glacial deposits (K. Bjorlykke, personal communication, 1981).

Near the Vendian-Cambrian boundary, parts of Rodinia (including North America, Siberia, Kazakstania, and probably others) began to rift away from the main supercontinental body, although Gondwana seems to have remained largely intact. Sea level, at an extreme low point by the end of the Vendian, began to rise at the Vendian-Cambrian boundary, partly in response to changes in the depth and volume of ocean basins worldwide. These ocean volume changes were, no doubt, at least partly a result of the creation of new oceans. It may seem strange that sea level could go up as a result of the creation of new ocean basins, but there is no contradiction here. For every new square kilometer of basaltic sea floor crust created between the edges of a rift basin, old (and cooler and less bouyant) basaltic crust has to be destroyed (assuming that the surface area of the earth has remained constant). The destruction of old seafloor occurs by subduction (underthrusting and melting) at deep ocean trenches; once melted, the old crust is returned to the surface in the form of volcanic eruptions. Ocean basins floored by old, cold crust can hold more water than those floored by newer, hotter basaltic crust because the cold crust rides much lower—owing to its greater density—and is able to "sink" deeper into the pliable part of the earth's interior that is found below the earth's crust. As Rodinia was cleft into separate continents, Mirovia gradually became divided into a network of smaller, shallower ocean basins as a result of the rifting.

Although the details of the breakup are still being worked out, it appears that the rifting of Rodinia was responsible for the volcanic beds in marine deposits, the opening of new ocean basins, a major increase in sea level, and possibly, high altitude glaciation at the uplifted margins of rift basins. A fifth effect was potentially even more important for the early evolution of animals. This was the introduction into marine waters of large quantities of biologically important chemicals and elements, partly as a result of rift-related volcanism. This phenomenon will be discussed in chapter 8.

0 0


Post a comment