paleozoic and mesozoic seas

grand canyon

The Paleozoic sedimentary rocks of the Grand Canyon contain evidence of marine transgressions and regressions, but even there the rock record is not complete. Look at the sequence in the Figure 1.2 and see if you can determine whether the sea was transgressing or regressing. At the bottom, the Tonto Group represents a marine transgression: sandstone (11), shale (10), and limestone (9) laid down during 30 million years of the Cambrian Period. The Ordovician and Silurian are unknown because of an unconformity. Above that is freshwater limestone (8), which is overlain by limestone (7) and then shale (6), indicating that the sea was regressing. After another unconformity, the rocks of the Supai Group (5) include limestone, siltstone, and sandstone indicative of a regressing sea. Above those rocks are shale (4), sandstone (3), a limestone and sandstone mix (2) showing that the sea regressed and transgressed and finally limestone (1) indicating that the sea had come back in.

marine transgressions and regressions

Some of the most important events of the Paleozoic and Mesozoic were the rising and falling of sea level over the continents. Sea level rises over the land during a marine transgression. During a marine regression, sea level retreats. During the Paleozoic there were four complete cycles of marine transgressions and regressions. There were two additional cycles during the Mesozoic (Figure 1.1). One of two things must happen for sea level to change in a marine transgression: either the land must sink or the water level must rise. What could cause sea level to rise? When little or no fresh water is tied up in glaciers and ice caps, sea level is high. Sea level also appears to rise if land is down dropped. Sea level rises if an increase in seafloor spreading rate buoys up the ocean crust, causing the ocean basin to become smaller. What could cause sea level to fall in a marine regression? Six marine transgressions and regres- sions have occurred during the Phanero- zoic. Geologists think that the Paleozoic marine transgressions and regressions were the result of the decrease and increase in the size of glaciers covering the lands. Click image to the left or use the URL below. URL:

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rock facies

Geologists know about marine transgressions and regressions from the sedimentary rock record. These events leave characteristic rock layers known as sedimentary facies. On a shoreline, sand and other coarse grained rock fragments are commonly found on the beach where the wave energy is high. Away from the shore in lower energy environments, fine-grained silt that later creates shale is deposited. In deeper, low-energy waters, carbonate mud that later hardens into limestone is deposited.

instructional diagrams

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questions

a marine transgression is when

a) sea level retreats

-->  b) sea level rises over land

c) sea level remains the same

d) sea level rises and falls

during the paleozoic era, there were _ complete cycles of marine transgressions and regressions.

a) one

b) two

c) three

-->  d) four

what can cause sea level to rise?

a) melting glaciers and ice caps.

b) down dropping land.

c) increasing seafloor spreading rates.

-->  d) all of the above.

the rock facies that represents a marine transgression from bottom to top is

a) limestone, shale, sandstone

b) shale, limestone, sandstone

-->  c) sandstone, shale, limestone

d) limestone, sandstone, shale

shale is the rock that forms from halite and sand deposits.

a) true

-->  b) false

geologists think that the paleozoic marine transgressions and regressions were caused by decreasing and increasing glaciers.

-->  a) true

b) false

carbonate mud later hardens into __ in deep, low energy waters.

a) shale

b) silt

c) sandstone

-->  d) limestone

a sequence of sandstone-shale-limestone-shale indicates

-->  a) a marine transgression and partial regression

b) a marine regression and partial transgression

c) a marine transgression and regression

d) a marine regression and transgression

much of what geologists know about paleozoic marine transgressions and regressions is from what is displayed at the grand canyon.

-->  a) true

b) false

diagram questions

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