Arapahoe Formation

The Arapahoe Formation is a geological formation of latest Cretaceous (Maastrichtian) age that is present within the Denver Basin that underlies the Denver, Colorado, area.

[3][4] The formation includes fossil leaves[5] and dinosaur remains, although none of the latter have yet been referred to a specific genus.

[7] The Arapahoe Formation consists of alluvial fan and fluvial deposits that prograded eastward from the growing Rocky Mountain Front Ranges.

Medium grey to brown claystone makes up the majority of the formation, with lesser amounts of light grey to light brown quartzose sandstone.

[5] In 2002 the Arapahoe Formation was included as part of a larger unconformity-bounded unit named the D1 Sequence, in order to facilitate basin-wide studies and avoid confusion arising from the lateral and vertical facies changes that occur within the Denver Basin.