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Epeiric sea
Deep ocean
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Figure 20.6
Cambrian Paleogeography of North America Note the position of the Cambrian
paleoequator. During this time, North America straddled the equator as indicated in Figure 20.2a.
cratonwide unconformity marks the boundary between the
Sauk and Tippecanoe sequences.
Like the Sauk Sequence, deposition of the
Tippecanoe
Sequence
(Middle Ordovician-Early Devonian) began with
a major transgression onto the craton. This transgressing sea
deposited clean, well-sorted quartz sands over most of the
craton. The best known of the Tippecanoe basal sandstones
is the St. Peter Sandstone, an almost pure quartz sandstone
used in manufacturing glass. It occurs throughout much
of the midcontinent and resulted from numerous cycles of
weathering and erosion of Proterozoic and Cambrian sand-
stones deposited during the Sauk transgression.
The Tippecanoe basal sandstones were followed by
widespread carbonate deposition (Figure 20.8). The lime-
stones were generally the result of deposition by calcium-
carbonate-secreting organisms such as corals, brachiopods,
stromatoporoids, and bryozoans. Besides the limestones,
there were also many dolostones. Most of the dolostones
formed as a result of magnesium replacing calcium in calcite,
thus converting the limestones into dolostones.
In the eastern portion of the craton, the carbonates
grade laterally into shales. These shales mark the farthest
extent of detrital sediments derived from weathering and
erosion of the Taconic Highlands, which resulted from a
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