The Lytle Formation or Lytle Sandstone is a
geologic formation found in
Wyoming,
,
and
New Mexico.
Description
The Lytle Formation consists of white to light gray gravels and conglomerates. It is variable in thickness but is about at the type location. It is separated from the underlying
Morrison Formation
The Morrison Formation is a distinctive sequence of Late Jurassic, Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandsto ...
by a significant regional unconformity. It is overlain by the
South Platte Formation or the
Glencairn Formation
The Glencairn Formation is a geologic formation found in Colorado and New Mexico. It preserves fossils characteristic of the Albian Age of the Cretaceous Period.
Description
The Glencairn Formation consists of dark gray shale and buff sandstone ...
.
The formation is likely an
early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
geologic unit, with its northern exposure running north and south within the
Front Range foothills and the
Dakota Hogback in northern
Colorado and southern
Wyoming where it is assigned
formation rank within the
Dakota Group.
In south-central Colorado, the Lytle is an unassigned formation. The formation is also mapped in the valley of the
Dry Cimarron in northeastern
New Mexico, where it forms a prominent band in the lower parts of the cliffs.
The Lytle was the last (youngest) non-marine unit to form in the
Denver Basin before the region was fully inundated by the
Western Interior Seaway. It was formed above sea level from sediments carried by heavily laden rivers flowing from the eroding uplifts of the
Sevier orogeny several tens of millions of years before the
Rocky Mountains rose. It is particularly noted for abundant brown chert pebbles washed in from the uplifted
Permian rock far to the west.
of the Lytle Formation in the
Raton Basin suggests a
late Jurassic age for this unit. However, it is possible that the lack of younger zircons reflects a hiatus in deposition of airfall material.
Fossils
Known fossils are fragments of
petrified wood eroded from the west as well as nondescript animal burrows, possibly ''
Skolithos'' and ''
Arenicolites''. Body fossils are extremely rare, but a few
ostracod and
bivalve
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
fossils are consistent with an
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...
to
Albian age for the formation.
History of investigation
The formation was first named as the Lytle sandstone member of the Purgatoire Formation by G.I. Finlay in 1916, for exposures near
Lytle, Colorado
Lytle is a ghost town located 12 miles south-west of Fountain in El Paso County, Colorado. The only building that remains is the Lytle School. The earlier name of the town was Turkey Creek. There was a Post Office from 1877 to 1881 and a Rural ...
. Finlay found no fossils in the unit, but regarded it as likely
early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
in age.
Waage subsequently traced the unit into northeastern New Mexico
and northern Colorado.
References
{{reflist, 30em
Cretaceous Colorado
Cretaceous formations of New Mexico