The complete geologic history of the Fossil Basin involves many
stratigraphic units. Most of these are not exposed within the monument
itself but are well known from outcrops outside the boundaries of the
monument and from deep wells (Fig. 4).
At the base of the stratigraphic section are rocks of Precambrian age.
These have not been penetrated by wells within the Fossil Basin but
should occur as much as 25,000 ft beneath the surface. They probably
consist of metamorphic and intrusive rocks. Beginning about 600 million
years ago, at the beginning of Cambrian time, sedimentary deposits
accumulated. These are both marine and continental in origin and range
in age from early Cambrian (600 million years old) to early Tertiary (50
million years old). Some 34 geologic formations are recognized in this
thick section, a list of which follows:
Eocene Fowkes Formation
Wasatch-Green River Formations
Paleocene Evanston Formation (upper part)
Upper Cretaceous Evanston Formation (lower part)
Adaville Formation
Lazeart Sandstone
Hilliard Shale
Frontier Formation
Lower Cretaceous Aspen Shale
Bear River Formation
Gannett Group
Jurassic Stump Sandstone
Preuss Sandstone
Twin Creek Limestone
Nugget Sandstone (upper part)
Triassic Nugget Sandstone (lower part)
Thaynes Limestone
Woodside Formation
Dinwoody Formation
Permian Phosphoria Formation
Pennsylvanian Wells Formation
Mississippian Brazer Limestone
Madison Limestone (upper part)
Devonian Madison Limestone (lower part)
Darby Formation
Silurian Not Present
Ordovician Leigh Dolomite
Bighorn Dolomite
Cambrian Gallatin Limestone
Gros Ventre Formation
Precambrian Complex of metamorphic and intrusive rocks
Within the monument itself the oldest rocks exposed are those of the
Thaynes Limestone near Prow Point. As will be seen in the discussion of
the structure of the Fossil Basin, all of the rocks beneath the Evanston
Formation are folded and faulted and within the monument are buried by
deposits of Tertiary age. For this reason detailed discussion of
stratigraphic units will be confined to those rocks that can be seen
within the boundaries of the monument or closely adjacent to it (Fig. 5
). The distribution of the various formations within the monument is
illustrated in Figs. 7 and 8.
[Illustration: Fig. 4. Geologic time scale.]
Era/Period/Epoch Age Major Biological & Geologic Events
(Millions
of years)
CENOZOIC
Quaternary Ice Age begins and evolution of
modern mammals; man reaches N. A.;
general uplift of continent.
Holocene (Recent) .01
Pleistocene 3
Tertiary Evolution and Development of major
mammal groups; Cascadian Orogeny in
northwestern United States; in latest
Cenozoic, Eocene lake system.
Pliocene 5.5
Miocene 25.5
Oligocene 36
Eocene 54
Paleocene 65
MESOZOIC
Cretaceous 130 Extinction of dinosaurs; flowering
plants flourish; height of
Cordilleran Orogeny.
Jurassic 185 First birds, dinosaurs and mammals
continue to evolve; beginning of
Cordilleran Orogeny which formed the
Rocky Mountains.
Triassic 230 Conifers evolved; first dinosaurs;
first mammals; Appalachian Orogeny;
some orogeny in Cordillera.
PALEOZOIC
Permian 265 Archaic reptiles and mammal-like
reptiles evolve. Appalachian Orogeny.
Pennsylvanian 310 First reptiles, coal swamps formed;
beginning of the Appalachian Orogeny.
Mississippian 355 Expansion of amphibians; much
carbonate deposition; Antler Orogeny
in Cordilleran Mobile Belt (Rocky
Mountain area).
Devonian 413 First land plants; first jawed
fishes, sharks and bony fish;
amphibians appear; Acadian Orogeny in
the Appalachian belt.
Silurian 425 Jawless fishes evolved; much reef
development; evaporite mineral
development.
Ordovician 475 First vertebrate remains known from
this time; Taconic Orogeny in
Appalachian belt.
Cambrian 600 First shelled invertebrates; major
flooding of North America.
Notes:
Reformatted table to combine ‘Era’, ‘Period’, and ‘Epoch’ columns
into one column in outline form.
Beneath heading ‘Epoch’: “(Cenozoic only)”; beside other periods,
“Epochs for these periods are not definitely established”
As the last line, “Precambrian time began 5 billion years ago.”