Juana Lopez refers to both the uppermost

member 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 ...

of the

Carlile Shale The Carlile Shale is a Turonian age Upper Cretaceous, Upper/Late Cretaceous series shale geologic Formation (geology), formation in the central-western United States, including in the Great Plains region of Colorado, Kansas, Nebraska, New Mexico ...

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

and to the environment that caused it to form. The Juana Lopez Member is calcareous sandstone dated to the

Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...

age of the

Upper Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cret ...

and is exposed in the southern and western

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

, northern and central

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 ...

, and northeastern Utah. The unit has been described as "the most enigmatic" member of the Carlile Shale.

Description

The Juana Lopez Member consists of beds of

calcarenite Calcarenite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) sand-size (0.0625 to 2 mm in diameter), Carbonate rock, carbonate grains. The grains consist of sand-size grains of either cor ...

separated by intervals of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

. Calcarenite is a form of limestone composed mostly of sand-sized carbonate grains. In the Juana Lopez Member, these are mostly fragmented coral, shells (especially inoceramid prisms), shark teeth, bone, and other fragmented fossils. The Juana Lopez Member smells of

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

when freshly broken. It formed on wide, shallow marine shelf, with strong wave action, but with little sediment coming in from land. It was exposed on the land surface between the retreat of the

Greenhorn cycle The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States, dating to the Cenomanian and Turonian ages of the Late Cretaceous period. The formation gives its name to the Greenhorn cycle ...

and the advance of the

Niobrara cycle The Niobrara Formation , also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is comp ...

of the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...

. As originally defined, the unit included only the uppermost calcarenite bed, with a thickness of less than . As currently defined, the unit includes the entire sequence of calcarenite beds separated by mudstone intervals, and is up to about thick. The unit is one of the most extensive units of the

, being present in the

Mancos Shale The Mancos Shale or Mancos Group is a Late Cretaceous (Upper Cretaceous) geologic formation of the Western United States. The Mancos Shale was first described by Cross and Purington in 1899 and was named for exposures near the town of Mancos, C ...

of the

San Juan Basin The San Juan Basin is a geologic structural basin located near the Four Corners region of the Southwestern United States. The basin covers 7,500 square miles and resides in northwestern New Mexico, southwestern Colorado, and parts of Utah a ...

, the

of the

Denver Basin The Denver Basin, variously referred to as the Julesburg Basin, Denver-Julesburg Basin (after Julesburg, Colorado), or the D-J Basin, is a geology, geologic structural basin centered in eastern Colorado in the United States, but extending into sou ...

and

Raton Basin The Raton Basin is a geologic structural basin in southern Colorado and northern New Mexico. It takes its name from Raton Pass and the town of Raton, New Mexico. In extent, the basin is approximately east-west, and north-south, in Huerfano an ...

, and the Benton Shale of the North Park Basin. Outcrops have been found as far east as

Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...

and as far south as

west Texas West Texas is a loosely defined region in the U.S. state of Texas, generally encompassing the desert climate, arid and semiarid climate, semiarid lands west of a line drawn between the cities of Wichita Falls, Texas, Wichita Falls, Abilene, Texa ...

Fossils

The unit is highly fossiliferous. Fossils identified in the unit are primarily marine molluscs, including '' Prinocyclus'', ''

Inoceramus ''Inoceramus'' (Greek: translation "fibrous shell" for the fibrous structure of the mineral crystals in the shell) is an extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of th ...

'', ''

Ostrea ''Ostrea'' is a genus of edible oysters, marine bivalve mollusks in the family Ostreidae, the oysters. Fossil records Although molecular studies suggest that ''Ostrea'' first appeared around the Eocene and originated no earlier than the Cre ...

'', '' Lopha lugubris'', '' Coilopoceras'', '' Hourcquia'', and ''

Scaphites ''Scaphites'' is a genus of heteromorph ammonites belonging to the Scaphitidae family. They were a widespread genus that thrived during the Late Cretaceous period. Morphology ''Scaphites'' generally have a chambered, boat-shaped shell. The ini ...

''. The fossils indicate that the member is upper

in age. Teeth of the shark ''

Ptychodus ''Ptychodus'' (from 'fold' and 'tooth') is a genus of extinct large Durophagy, durophagous (shell-crushing) lamniformes, lamniform sharks from the Cretaceous period, spanning from the Albian to the Campanian. Fossils of ''Ptychodus'' teeth ar ...

'' are also found.

History of investigation

The distinctive beds and fossils of the member were first noted by

John Strong Newberry John Strong Newberry (December 22, 1822 – December 7, 1892) was an American physician, geologist and paleontologist. He participated as a naturalist and surgeon on three expeditions to explore and survey the western United States. During the C ...

in 1861. The member was first designated in 1944 by C.H. Rankin, Jr., who recognized the member throughout northern New Mexico and western Colorado. Rankin established the correlation between the

and the

Colorado Group Colorado is a geologic name applied to certain rocks of Cretaceous age in the North America, particularly in the western Great Plains. This name was originally applied to classify a group of specific marine formations of shale and chalk known ...

and was able to identify most of the units of the Colorado Group within the Mancos Shale, and identified the top of the Juana Lopez Member as the top of the Carlile Shale. In 1955, Dan Bozanic revised the definition of the member to include a much larger set of beds, and this revision has been widely accepted.

References

Turonian Stage Cretaceous Colorado Cretaceous formations of New Mexico Cretaceous System of North America Upper Cretaceous Series of North America Stratigraphy of Colorado Sandstone formations of the United States Geologic members of the United States {{US-geologic-formation-stub