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. The formation includes fossil leaves and dinosaur remains, although none of the latter have yet been referred to a specific genus. It also includes aquifers that are important sources of water for the area.

The Arapahoe Formation consists of alluvial fan and fluvial deposits that prograded eastward from the growing Rocky Mountain Front Ranges. It rests unconformably on the Laramie Formation, and is conformably overlain by the Denver Formation. It reaches a maximum thickness of about .

The base of the Arapahoe Formation is marked by a discontinuous conglomerate, or where the conglomerate is absent, by sandstone beds that commonly contain large ironstone concretions. The conglomerate is composed principally of chert pebbles, but pebbles of granite, gneiss and schist are also present. Medium grey to brown claystone makes up the majority of the formation, with lesser amounts of light grey to light brown quartzose sandstone. The formation top is marked by a change from sandstone and claystone to the tuffaceous sediments of the overlying Denver Formation.

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. The base of the D1 coincides with the base of the Arapahoe Formation and is marked by the abrupt change to conglomerate at the top of the Laramie Formation. The top D1 is placed at the base of a regional paleosol series. The Denver Formation and the Dawson Arkose are also included in the D1 Sequence.
Q736917
formation
Concept
Q338199
latest Cretaceous
TIME
Q278141
Maastrichtian
TIME
Q5259679
Denver Basin
LOC
Q16554
Denver
LOC
Q1261
Colorado
LOC
Q430
dinosaur
Concept
Q34740
genus
Concept
Q465088
alluvial fan
Concept
Q1434465
fluvial
LOC
Q876798
unconformably
Concept
Q3077404
Laramie Formation
LOC
Q5259726
Denver Formation
LOC
Q191704
conglomerate
Concept
Q13085
sandstone
Concept
Q2509719
ironstone
Concept
Q1093379
concretion
Concept
Q1070438
chert
Concept
Q41177
granite
Concept
Q166409
gneiss
Concept
Q15315
schist
Concept
Q6931872
claystone
Concept
Q43010
quartz
Concept
Q484924
tuff
Concept
Q742139
facies
Concept
Q2048517
paleosol
Concept
Q17379999
Dawson Arkose
LOC
Q4784197
Arapahoe Formation
LOC
Q4784197
Arapahoe Formation
LOC
Q4784197
Arapahoe Formation
LOC
Q4784197
Arapahoe Formation
LOC
Q4784197
Arapahoe Formation
LOC
2002-01-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
2002
DATE
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1
NUMBER
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1
NUMBER
+1^^http://www.w3.org/2001/XMLSchema#decimal
1
NUMBER
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NUMBER
instance of
located in the administrative territorial entity
underlies
underlies