DINOSAUR RIDGE
Dinosaur Ridge (looking ~NNW) - a particularly dinosaur fossil-rich
section of the Dakota Hogback in north-central Colorado is called
"Dinosaur Ridge". It is a north-south trending ridge of
eastward-dipping Mesozoic sedimentary rocks.
Locality:
eastern side of Dinosaur Ridge, north of town of Morrison & south of town
of Golden, west of Denver, Jefferson County, Colorado, USA.
Dakota Hogback (above & below) - this spectacular outcrop is a
cross-section through the entire Dakota Hogback, of which Dinosaur Ridge is a
part. Triassic to Cretaceous sedimentary rocks are present, tilted by
Laramide Orogenic uplift during the Cenozoic (the Front Range of the Colorado
Rocky Mountains are immediately west of here - see the geologic
cross-section below).
The left-to-center portions of the roadcut are
principally the Morrison Formation (Kimmeridgian Stage, middle Upper
Jurassic, ~150-156 m.y.), consisting of fluvial (river/floodplain) and
lacustrine (lake) deposits, plus reddish-colored paleosol horizons.
Dinosaur bones and dinosaur tracks have been found in the Morrison Fm. in this
area.
The center-to-right portions of the roadcut are
principally the Dakota Sandstone (Albian Stage, upper Lower Cretaceous),
dominated by nearshore terrestrial to intertidal to shallow marine quartz
sandstone deposits. Dinosaur tracks are common on some Dakota Ss. bedding
planes in the area.
Locality:
"POGI" (= Point of Geologic Interest), roadcut along northern side of
Interstate-70 immediately east of the Rt. 40 exit (= exit 259), west of Denver,
Colorado, USA.
Morrison Formation (Upper Jurassic) at Dinosaur Ridge. The grayish
slopes consist of mudshales interbedded with some limestones, reprenting a
playa lake environment. The blocky-weathering unit above consists of
fluvial and lacustrine quartzose sandstones.
Locality:
western side of Dinosaur Ridge, vicinity of "Quarry 5" (= one of
Arthur Lakes' dinosaur excavation sites during the Cope-Marsh Bone Wars of the
late 1800s), between Interstate-70 and the town of Morrison, west of Denver,
Colorado, USA.
Sauropod dinosaur footprint bulges (above & below) in lacustrine quartzose
sandstones, upper Morrison Formation (Kimmeridgian Stage, middle Upper
Jurassic). Bones from several different sauropod dinosaurs have been
recovered from Dinosaur Ridge's Morrison Formation, making positive
identification of the footprint maker here difficult. Known Morrison Fm.
sauropods in this area include (not counting junior synonyms) Atlantosaurus
immanis, Apatosaurus ajax, and Camarasaurus sp. (see
Mossbrucker & Bakker, 2010, pp. 10, 19, 22).
Locality:
western side of Dinosaur Ridge, vicinity of "Quarry 5" (= one of
Arthur Lakes' dinosaur excavation sites during the Cope-Marsh Bone Wars of the
late 1800s), between Interstate-70 and the town of Morrison, west of Denver,
Colorado, USA.
Sauropod dinosaur footprint bulges in lacustrine quartzose sandstones, upper Morrison
Formation (Kimmeridgian Stage, middle Upper Jurassic). This was a shallow
lake environment that was crossed by a large sauropod dinosaur (probably Atlantosaurus
or Apatosaurus or Camarasaurus). The large footprint bulge
at above right is from a hindpaw (pes) of the sauropod.
Locality:
western side of Dinosaur Ridge, vicinity of “Quarry 5” (= one of Arthur Lakes'
dinosaur excavation sites during the Cope-Marsh Bone Wars of the late 1800s),
between Interstate-70 and the town of Morrison, west of Denver, Colorado, USA.
Reconstruction of sauropod dinosaurs crossing a sandy
substrate, leaving behind series of deep footprint impressions.
Dinosaur bones in fluvial sandstone matrix (above & below) in
the Morrison Formation (Kimmeridgian Stage, middle Upper Jurassic). These
bones occur in a river channel sandstone deposit. Paleocurrent indicators
show that the river system was flowing to the southeast.
the late 1800s), western side of Dinosaur Ridge
between Interstate-70 and the town of Morrison, west of Denver, Above
right: cross-section of a hip
element from a sauropod dinosaur, probably Atlantosaurus.
Below:
large pubis bone from a sauropod dinosaur, probably Atlantosaurus.
Dinosaurologist Bob Bakker interprets the long pubis bones in sauropods as
having a pubic mounting function. Pubic mounting is a type of dominance
behavior over other animals.
Locality:
"Quarry 5" (= one of Arthur Lakes' dinosaur excavation sites during
the Cope-Marsh Bone Wars of Colorado, USA.
Radioactive dinosaur bone (dark structure in the rock) in fluvial sandstone
from the Morrison Formation (Kimmeridgian Stage, middle Upper Jurassic).
This sauropod pubic bone is radioactive. All of the dinosaur bones at
this locality are radioactive. They have been partially
permineralized/replaced by the radioactive, uranium-bearing mineral carnotite.
Scintillometer readings on a bone at this locality: 422 counts per minute (cf.
a background reading of 140 cpm).
Locality:
"Quarry 5" (= one of Arthur Lakes' dinosaur excavation sites during
the Cope-Marsh Bone Wars of the late 1800s), western side of Dinosaur Ridge
between Interstate-70 and the town of Morrison, west of Denver, Colorado, USA.
Dakota Sandstone (Albian Stage, upper Lower Cretaceous) exposed in a
roadcut at Dinosaur Ridge.
Dakota Sandstone (Albian Stage, upper Lower Cretaceous) (above &
below) exposed in roadcuts near the crest of Dinosaur Ridge. The Dakota
Ss. consists principally of nearshore terrestrial to intertidal to shallow
marine quartzose sandstones.
Above:
shallow channelforms (probably tidal channels) in the structurally tilted
Dakota Sandstone.
Below:
cross bedding and horizontally bedding in the structurally tilted Dakota
Sandstone.
Bentonite - this is a lithified volcanic ash bed exposed along a roadcut near
the crest of Dinosaur Ridge. Radiometric dating indicates it is 105.6
million years old.
Dakota Sandstone (Albian Stage, upper Lower Cretaceous) - structurally
tilted quartzose sandstones on the eastern side of Dinosaur Ridge (Dakota
Hogback). These represent nearshore terrestrial to intertidal to shallow
marine facies.
Interference ripples in quartzose sandstones (Dakota Sandstone, Albian
Stage, upper Lower Cretaceous). Interference ripples have two sets of
symmetrical ripple marks intersecting at high angles. They form by the
influence of two sets of 2-directional (oscillatory) currents.
Fossils
in the Dakota Sandstone (Albian Stage, upper Lower Cretaceous) on the eastern
side of Dinosaur Ridge (Dakota Hogback), west of Denver, Colorado, USA.
Above left: bivalve.
Above right: invertebrate trace fossils (geology hammer for scale). Some
Dakota Sandstone beds are heavily bioturbated. Identified traces include Zoophycos,
Arenicolites, Diplocraterion, vertical burrows, etc. These
ichnogenera are shallow shoreface traces.
Below:
external mold of fossil tree trunk, possibly having Teredolites wood
borings.
Seed pod
with stem attached in the Dakota Sandstone, eastern Dinosaur Ridge, Colorado.
Crocodilian claw scratch marks in the Dakota Sandstone, eastern Dinosaur Ridge,
Colorado.
Dinosaur tracksite - iguanodontid dinosaur tracks on tilted sandstone
bedding plane, Dakota Sandstone, Cretaceous; Dinosaur Ridge, near Denver,
Colorado, USA.
Dinosaur tracksite - this well-known bedding plane in the Dakota
Sandstone on the eastern side of Dinosaur Ridge has 325 dinosaur footprints,
made by about 37 individuals. The sediments probably represent a tidal
flat that bordered the ancient Western Interior Seaway. The track-makers
are principally iguandontid dinosaurs and a small theropod dinosaur.
The tracks have been stained gray with charcoal to
make them easier to see - the charcoal doesn't harm the tracks.
Freeze-thaw weathering is destroying the tracks, and a building over this
outcrop is planned.
This tracksite is next to a modern freeway -
Interstate 470. As such, the dinosaur tracksite is often called the
"Cretaceous 470" or "Dinosaur Freeway".
This is part of a series of dinosaur tracksites that
occur along the western edge of the Western Interior Seaway & eastern edge
of the Rocky Mountains. Together, they are called the Dinosaur Freeway
Megatracksite.
Caririchnium leonardii Lockley, 1987 - the large lower footprint is the pes
(hindpaw) of an iguanodontid (ornithopod) dinosaur. The small upper
footprint is the manus (front paw) of the same dinosaur. The manus was
imprinted several moments before the pes. Dinosaur Ridge is the type
locality for this particular dinosaur trace fossil.
Magnoavipes - this is the footprint of a theropod dinosaur, which was bipedal,
unlike the iguandontid dinosaurs that made the tracks shown above.
Info. on this page was partly synthesized from
Mossbrucker & Bakker (2010) and M. Mossbrucker (pers. comm., 2010).
Reference cited:
Mossbrucker, M.T. & R.T. Bakker. 2010.
A guide to the paleontology of the Upper Jurassic Morrison Formation of
Morrison, Colorado: new interpretations and discoveries. Bulletin
of the Morrison Natural History Museum 1. 35 pp.