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The Dakota Formation
(early
Cretaceous)
The Dakota Formation (~150ft) is generally thin and highly variable
throughout the central Colorado Plateau. Where it is present, it
generally consists of a lower unit of sandstone or conglomerate, a
middle layer of shale and channel sandstone, and an upper layer of
marine shale and sandstone. Several paleosoils and numerous types of
burrows have been reported from the Dakota Formation. Rare fossils
include plants (dominately ferns and horsetails) and petrified wood.
The Dakota sandstone is interpreted as a shoreline facies of the
Cretaceous Western Interior Seaway. While thin in the Colorado Plateau
region, correlative formations are found throughout western North
America. Numerous dinosaur
tracksites occur in these deposits, and
trackways attributed to birds and small, coeleasaur theropods have been
found in the Dakota Group as well. Although theropod and large sauropod
tracks are present as in the Morrison, a conspicuous feature of these
Cretaceous deposits, both in the Colorado Plateau region and elsewhere,
is the presence of numerous large
ornithopod (Iguanodontid)
trackways (~50cm).
Trackways are found at numerous levels. For instance, 18 track
bearing levels have been mapped at one site near Eldorado Springs,
Colorado, 11 at Roxborough State Park, Colorado. However, no body
fossils have been found at these sites. Often these exposures show
parallel, monospecific trackways (up to 55 at Mosquero Creek) suggesting
gregarious behavior. Lockley and Hunt write:
"Overall, the Dakota Group, from outcrops in Boulder and Eldorado
Springs in the north to Mosquero Creek in the south, proves to be a
track-rich zone that extends for hundreds of miles along the trend
of the western shoreline of the western interior seaway . . . most
of the dinosaur tracks are very similar. They are predominantly
those of Iguanodon-like ornithopods. Many are virtually identical to
the footprints of an early Cretaceous ornithopod from the Carir
basin of Brazil and have therefore been named Caririchnium. Others
are slightly or subtly different, resembling so called Igunaodon
tracks from early Cretaceous strata in England and other parts of
the world. Throughout the Dakota megatracksite we also find the
tracks of carnivorous dinosaurs. Most of the well-preserved examples
are those of medium-sized, slender toed, gracile theropods of the
bird-like type" (p. 209).
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