Talking about shallow oceans, here is an excerpt (couple of paragraphs) from that article by the geophysicist from Caltech:

The first significant step in bringing these clues together was the close examination of another up-and-down example from Bond's global survey. In the late 1980's this work inspired Christopher Beaumont, a geologist at Dalhousie University in Nova Scotia, to tackle a baffling observation about Denver, Colo. Although the city's elevation is more than a mile above sea level, it sits atop flat, undeformed marine rocks created from sediments deposited on the floor of a shallow sea during the Cretaceous period. Vast seas covered much of the continents during that time, but sea level was no more then about 400 feet higher than it is today. This means that the ocean could never have reached as far inland as Denver's current postion--unless this land was first pulled down several thousand feet to allow waters to flood inland.

Based on the position of North America's coastlines during the Cretaceous, Beaumont estimated that this bowing downward and subsequent uplift to today's elevation must have affected an area more then 600 miles across. This geographic scale was problematic for the prevailing view that plate tectonics alone molded the surface. The mechanism of plate tectonics permits vertical motions within only 100 miles or so of plate edges, which are thin enough to bend like a stiff fishing pole, when forces act on them. But the motion of North America's interior happened several hundred miles inland--far from the influence of plate collisions. An entirely different mechanism had to be at fault.

Beaumont knew that subducted slabs of ancient seafloor might sit in the mantle below North America and that such slabs could theoretically drag down the center of a continent. To determine whether downward flow of the mantle could have caused the dip near Denver, Beaumont teamed up with Jerry Mitrovica, then a gaduate student at the University of Toronto, and Gary T. Jarvis of York University in Toronto. They found that the sinking of North America during the Cretaceous could have been caused by a plate called the Farallon as it plunged into the mantle beneath the western coast of North America. Basing their conclusion on a computer model, the research team argued that the ancient plate thrust into the mantle nearly horizontally. As it began sinking, it created a downward flow in its wake that tugged North America low enough to allow the ocean to rush in. As the Farallon plate sank deeper, the power of its trailing wake decreased. The continent's tendency to float eventually won out, and North America resurfaced.

When the Canadian researchers advanced their theory in 1989, the Farallon plate had long since vanished into the mantle, so its eistence had only been inferred from geologic indications on the bottom of the Pacific Ocean. At that time, no seismic images were of high enough resolution to delineate a structure as small as a sinking fragment of the seafloor. Then, in 1996, new images of the mantle changed everything. Stephen P. Grand of the University of Texas at Austin and Robert D. van der Hilst of M.I.T., seismologists from separate research groups, presented two images based on entirely different sets of seismic measurements. Both pictures showed virtually identical structures, especially the cold-mantle down-wellings associated with sinking slabs of seafloor. The long-lost Farallon plate was prominent in the images as an arching slab 1,000 miles below the eastern coast of the U.S.

end-of-excerpt

You might fine the below link interesting on the Afican uplift.

Dennis

• Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume