[ p. 179 ]
Following the known events of the Proterozoic era, and before the introduction of Paleozoic strata with their abundance of fossils, there is a vast blank, a break of very great significance. During the time of this Epi-Proterozoic interval, which Walcott has called Lipalian time (from a word meaning gone or missing), the continents appear to have stood well above the general oceanic level, and the chief geologic work done was that of erosion. Of course a record was being made somewhere in accumulating strata, but these deposits are as yet wholly unknown. For this reason, wherever Paleozoic or later formations lie upon the Proterozoic, there occurs a most significant break between them. This break, represented by an unconformity of the first order, must be fully understood, and a description of it will also make more clear all other intervals and breaks.
It is now the custom of geologists to speak of the xmconformities also as breaks and intervals: breaks for the shorter times of lost record represented by the disconformities and diastemata, and intervals for the greater ones seen in the angular unconformities.
Erosion intervals and unconformities are of very variable duration in the different areas of the lands. They may be so short that geologists have the greatest difficulties in discerning the breaks, and on the other hand they may appear as if embracing nearly all of geologic time. It is of common occurrence on the Canadian Shield to find the Archeozoic formations overlain by the almost recent Pleistocene glacial deposits, and even these may be absent. It then appears as if in such places no rocks had been deposited, either by the sea or by the forces of the land, since Archeozoic time, and yet geologists know that the shield has been variously covered by sheets of sediments formed at sundry times in the Proterozoic, Paleozoic, and, to a more limited extent, in the Mesozoic.
We have seen that some geologists refer the Keweenawan to the Paleozoic, but most workers hold that the formation of this series was the latest event in the Proterozoic, and that a long interval occurs between this series and the oldest deposits of the Cambrian. Further, in most places on the shield the Keweenawan was never [ p. 180 ] laid down, and the Animikian and even the Huronian may have been eroded away before the seas of Cambrian time again invaded the area. Therefore, the pre-Cambrian interval is locally very variable in its appai’ent duration, but from the standpoint of the ascertained geologic record in its most complete form, it does not seem that this break can be more important than the similar ones between the other eras. Accordingly, we may say that the interval appears to have lasted very long, but as there are so few late Proterozoic fossils at hand, and therefore no record of organic evolution to guide us, the length of the interval cannot as yet be determined.
Where the Paleozoic strata rest on the Proterozoic, there is in most places a marked and usually an angular unconformity. In western Montana, Idaho, and British Columbia, however, the Paleozoic rests without a marked unconformity on the earlier formations of the Proterozoic or Beltian series (see Fig., p. 167). This condition means that here the lithosphere was not folded toward the close of the Proterozoic, and in fact not until the close of the Mesozoic.
The marked and usually angular unconformity beneath the base of the Paleozoic means that more or less thick sheets of rock or even mountain ranges had been elevated and subsequently worn away. Therefore the land areas of Lipahan time were reduced to a low-lying plain, a peneplain, and all this before Paleozoic time. It was over these eroded and flat lands that the Paleozoic seas spread their conglomerates, sands, muds, and limestones with their fullness of fossils. We learn therefore during Lipalian time only of the destructive work of the aerial forces, a work of slow action through the agencies of the atmosphere, oxygen, carbonic acid, water, temperature, wind, and gravity. Grain by grain the high places of the lands were transported into the seas and oceans (study Frontispiece, and Fig., p. 168). How much time was consumed, no one knows, but it was long enough for much of the animal world to change its soft skin to one protected by an inflexible covering of carbonate of lime such as is seen m the corals, cystids, brachiopods, and gastropods of the Cambrian.
Walcott speaks of Lipalian time as the “ era of unknown marine sedimentation between the adjustment of pelagic life to littoral conditions and the appearance of the Lower Cambrian fauna.” In other words, the term Lipalian stands for the unrecovered Epi-Proterozoic interval, a time consumed by the marine animals in evolving from floating and swimming forms without exterior [ p. 181 ] skeletons to the manifold life of the Cambrian with its protective coverings.
Lipalian Oceans. — The North American continent throughout Lipalian time is thought to have stood ’rcll above the average of oceanic level during this time. What is true of North America appears to be equally so for all the continents, since nowhere are there any known Lipalian marine formations. These facts suggest that toward the close of the Proterozoic the earth’s lithosphere underwent one of its greatest readjustments, seemingly the greatest of its several “ critical periods ” known to geologists. Accordingly, the ocean basins were then overdeepened, greatly lowering the oceanic level and causing the continents to appear as having been much raised above it. For these reasons it is thought that all of the Lipalian marine deposits were laid down at lower levels than those of subsequent times and hence are forever buried in the oceans.
The low strand-line of Lipalian time has subsequently come to hold a higher level, in consequence partly of the wear and tear of the continents carried into the oceans, but probably mostly because of the vast quantities of new waters added through volcanic acthity from the interior rock magmas. It is thought that the amount of water so added since Lipalian time may equal 10 per cent of the present oceanic volume.
During Lipalian time, when all the continents stood well above the strand-line, erosion was very active, and through the wearing down of the lands much new salt was freed from the rocks and added to the oceanic waters. This increase in salinity may have been an added stimulus toward for ming the outer shells or skeletons in the invertebrate animals, and combined with another cause — a more profuse adaptation to the bottoms of the shallow seas, leading to crowding and a fiercer struggle for existence — brought on quickened evolution and the necessity for external armoring; hence a skeleton either of chitin with some lime or one wholly of carbonate of lime. Siliceous skeletons had long before been adopted by the radiolarians and sponges.
C. D. Walcott, Abrupt Appearance of the Cambrian Fauna on the North American Continent Smithsonian Miscellaneous Collections, VoL 57, 1910, pp, 1 - 16 .