From Ediacarian through Cambrian times, sea level was constantly rising until it covered almost all of the cratons. on North America all but the central part of the Canadian shield was covered by shallow seas. The Cambrian isopach map shows a prominent feature called the transcontinental arch, thought to have formed by the bending of the North American plate under the great weight of the sedimentary sequences deposited on the margins of the craton. The distribution of Cambrian sedimentary facies around this arch shows that the facies are generally parallel to the arch, and that sand facies are generally next to the arch whereas deeper water shale and limestone facies are further offshore, showing that this is an original feature and not formed by erosion. The name given to the Eocambrian through late Cambrian rise of sea level is the sauk transgression, which formed a shallow or epeiric sea over more than 75 percent of North America. During this period sea levels rose about one foot (0.3 m) every 20 years. As the seas rose they deposited a layer of quartz-rich sandstone over an unconformity that migrated toward the center of the craton with time, forming one of the major transgres-sive sand sequences of the past 500 million years of Earth history. Most of these sands were derived from the previous 500 million years of weathering products that accumulated on the cratonic interior of North America, and as the sea level rose, the high energy beach environment reworked these soils, sands, and other products in the regolith to a stable quartz-rich assemblage now preserved as the basal transgressive sand. Many of the sand grains in this basal quartzite are very rounded, and well sorted (meaning they have similar size to each other), suggesting that some of them were derived from windblown sand deposits before they were transported by rivers and reworked in the high-energy beach environment.
As the transgression continued most of the craton soon became covered with water, and less sand was available to be eroded and contribute to the sediments being deposited in the sauk transgressive sequence. At this time the climate was favorable to the production of carbonate sediments, and the major type of deposition in the sauk sequence switched to carbonates by the beginning of the ordovician. Before this time carbonates were already being deposited along the deeper, outer parts of the continental shelves and seas, and worked their way toward the center of the craton. These carbonates consist of limestone and dolostone, largely made of shell fragments, limey muds, algae, and carbonate-secreting organisms. one special type of carbonate is known as oolitic limestone, which consists of many sand-sized carbonate grains with a texture resembling onion skin. These carbonates form by rolling about on the seafloor in shallow agitated waters, continuously being precipitated around a hard nucleus. oolites form in waters saturated with respect to calcium carbonate, typically
Upper Cambrian map of sedimentary facies of North America showing areas of beach facies (red), shallow-water mud (yellow), limestone (purple), and deepwater facies in areas of high evaporation and agitation, which releases carbon dioxide. The sauk sea was shallow, as indicated by sedimentary structures formed by waves, shallow water fossils called stromatolites, and rare mud-cracks indicating that the sea bottom was occasionally exposed to the air.
Thick limestone sequences of the sauk sequence indicate that North America was located within about 20 degrees of the equator in the early Paleozoic, and the paleoequator must have run approximately up the center of the continent (with the continent drift ing into that position by plate tectonics). The climate was subhumid, warm, and rainy.
The sauk sequence was terminated abruptly about 490 million years ago when sea level suddenly dropped (on geological timescales, taking a few million years), leading to widespread erosion and the formation of a worldwide unconformity surface on top of the sauk sequence.
Life in the Cambrian included worms (preserved largely as worm tubes), the first mollusks, echino-derms, sponges, archaeocyathids, and coelenterates, and saw the widespread development of hard skeletons, generally phosphatic. In all probability there were also a large number of soft-bodied organisms in the Cambrian seas, but conditions did not favor their preservation. The evolutionary step of forming skeletons was very important to organisms in the Cambrian. The outer skeleton (exoskeleton) offers protection from ultraviolet rays and predators. It is also possible to make larger organisms with the presence of a skeleton since the skeleton can support the larger structures and act as a base for the development of muscles, which are needed for mobility.
Other organisms common in the Cambrian seas included the trilobites, a bottom-dwelling scavenger creature that resembled the modern horseshoe crab. These were the most abundant organism in the Cambrian, including more than 600 genera, some of which make very good index fossils. The Archaeocy-athids were also bottom-dwelling organisms, shaped like a vase, with a double skeletal wall. These became extinct at the end of the Cambrian. Brachiopods, resembling clams, and mollusks, forming caplike shells, were common in shallow-water settings of the Cambrian seas.
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