A craton consists of crust that has not been affected by orogeny for at least about the last 1 billion years. As a result, cratons have cooled substantially, and therefore have become relatively strong and stable. Geologists divide cratons into two provinces: shields, in which Precambrian metamorphic and igneous rocks crop out at the ground surface, and cratonic platforms, where a relatively thin layer of Phanerozoic sediment covers the Precambrian rocks (figure above).
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| Domes and basins of the North American cratonic platform. |
In shield areas, we find widespread exposures of intensively deformed metamorphic rocks with abundant examples of flow folds and tectonic foliation. That’s because the crust making the cratons was deformed during a succession of orogenies in the Precambrian. These orogens are so old that
erosion has worn away the original topography, in the process exhuming deep crustal rocks.
In the cratonic platform, the pattern of contacts between stratigraphic formations defines regional domes and basins. These are broad areas that gradually sank or rose, respectively, over geologic time (figure above a, b). For example, in Missouri, strata arch across a broad dome, the Ozark Dome, whose diameter is 300 km. Individual sedimentary layers thin toward the top of the dome, because less sediment accumulated on the dome than in adjacent basins. Erosion during more recent geologic history has produced the characteristic bull’s-eye pattern of a dome, with the oldest rocks (Precambrian granite) exposed near the centre. In the Illinois Basin, strata warp downward into a huge bowl that is also about 300 km across. Strata get thicker toward the basin centre, indicating that the floor of the basin was subsiding as sediment was accumulating there was more room for sediment to accumulate where the basin subsided the most. The basin also has a bull’s-eye shape, but here the youngest strata are exposed in the centre. Geologists refer to the broad vertical movements that generate huge, but gentle, mid-continent domes and basins as epeirogeny.
Credits: Stephen Marshak (Essentials of Geology)
