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How did high sea levels create shallow inland seas during the Cretaceous Period?

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Cretaceous paleogeography — Cretaceous Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cretaceous Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

During the Cretaceous Period, sea levels were significantly higher than they are today—reaching approximately 200 to 250 metres (660 to 820 feet) higher during the Late Cretaceous. This rise in sea level was primarily driven by the enlargement of [midoceanic ridges].1

As these ridges expanded, they occupied a greater volume within the [ocean basins], effectively displacing water and forcing it to spill over onto the [continents]. This process, known as transgression, resulted in the inundation of continental margins and the creation of shallow [epicontinental seas]. These inland seas covered vast areas of North America, South America, Europe, Russia, Africa, and Australia, at times connecting Arctic waters to the [Tethys seaway]. This phenomenon was further influenced by high global seafloor spreading rates, which caused oceanic ridges to occupy more space and pushed marine waters onto landmasses that are now far from modern coastlines.12