The Fence Lake Formation is a geological formation in western

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

whose strata were deposited in the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

Description

The formation consists of two informal

members Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

. The lower member is of coarse conglomerate consisting mostly of fragments of

volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...

. The upper member is of calcareous grayish-pink

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

, poorly sorted and very fine- to coarse-grained. The upper member also includes scattered lenses of conglomerate. The formation

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

overlies the Moreno Hill Formation. It is relatively resistant to

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

and so forms cliffs and shelves and caps mesas in the area. Drainages eroded through the formation are filled by younger beds of the Quemado Formation. The formation is interpreted as

fluvial A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

deposits. The volcanic rock fragments making up the formation are thought to come from the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

Mogollon-Datil volcanic field The Mogollon-Datil volcanic field is a large () silicic volcanic field in western New Mexico (Mogollon Mountains-Datil, New Mexico). It is a part of an extensive Eocene to Oligocene volcanic event which includes the San Juan volcanic field in sou ...

and the formation itself is thought to be

in age. from before 14.5 to 7 million years old.

History of investigation

The beds making up the unit were originally included in the

Bidahochi Formation The Pliocene to Late Neogene Bidahochi Formation lies at an elevation of about to at the southeast of the Colorado Plateau; the deposits are from Hopi Lake (also called Bidahochi Lake), and the deposits extend southwards to the region at the ...

by Charles Repenning and James Irwin in 1954, along with all other Tertiary beds of the Zuni Plateau. In !956, R.J. Marr informally named the gravel beds of the lower member as the Fence Lake Gravel, and in 1958, Repenning, Lance, and Irwin excluded the high-level gravels near

Fence Lake, New Mexico Fence Lake is a census-designated place located in southwestern Cibola County, New Mexico, United States. Designated in 1980 by the United States Census Bureau, Fence Lake had a population of 42 as of the 2010 census. The area is largely ranchl ...

from the Bidahochi. In 1982, Marguerite McLellan and coinvestigators recognized that these beds were older than the beds inset in nearby drainages (identified at the time as Bidahochi Formation and later redefined as Quemado Formation) and defined them as the Fence Lake Formation.

References

{{Reflist Neogene formations of New Mexico Sandstone formations of the United States Conglomerate formations of the United States