The Cretaceous rocks of Minnesota are flat-lying, loosely
consolidated sediments of variable lithology. Two formations,
both showing wide facies variation are recognized,
the predominantly marine Coleraine Formation in northern
Minnesota and the predominantly nonmarine Windrow Formation
in southern Minnesota. Elsewhere in the State the strata
are poorly exposed and knowledge of their lithology, thickness,
and correlation is fragmentary.
The strata rest unconformably and with profound hiatus
on a surface with a maximum relief of 1,400 feet, which developed
during a long interval of erosion and weathering that
extended from sometime after the Devonian into the Cretaceous.
In general, marine sandstones and shales deposited
in the western and northern parts of the State grade eastward
into estuarine, paludal, deltaic, and lacustrine sediments.
The sediments lie. on rocks ranging in age from Precambrian
The distribution and character of the sediments are interpreted
to indicate that they formed in and adjacent to the
Late Cretaceous sea that invaded Minnesota from the west
and continued to advance eastward over an irregular terrain.
In general, the vertical succession at any particular
locality consists of a basal regolith developed on pre-Cretaceous
bedrock, an unconformity, basal nonmarine stream
deposits, and finally marine clastic sediments; but the full
succession is not present at all localities.
The fossil record indicate s that the strata can be correlated with the entire Colorado Group of the western interior
United State s. Sediments that were deposited at lower present-
day altitudes are Cenomanian in age, whereas those deposited
on bedrock at higher altitudes are Turonian and later