Geology

Menefee Badlands Terrain

NPS photo by Phil Varela

The Cretaceous Setting

The rocks exposed in Chaco Canyon record an interval in the Earth's history from approximately 85 to 75 million years ago during the Late Cretaceous Period. During this time, the region was situated at the edge of a shifting coastline of an ancient inland sea.

In the area of the Southern Rocky Mountains, mountain building activities produced a broad area of subsidence known as the Western Interior Basin, which flooded and created a vast epicontinental sea known as the Western Interior Seaway. By Late Cretaceous time, the Western Interior Seaway was hundreds of miles wide and extended from the Arctic Sea to the Gulf of America dividing North America into two separate land masses. The shorelines of this epicontinental seaway were oriented generally north-south and repeatedly shifted position to the east or west in response to continuing changes in global sea level. This sea level change resulted in alternating marine and non-marine deposition of sediments as the sea transgressed (grew) and regressed (receded) over time.

 

Paleogeography of North America about 75 Million Years Ago; box outline is approximate location of Chaco

Map created by Ron Blakey and adapted by Phil Varela

 

The climate of the Southern Rocky Mountain region during the Late Cretaceous was subtropical and supported lowland forests species thought to be similar to modern day conifer, eucalyptus, ebony, palm, cypress and magnolia. Upland areas supported hardwood forest species similar to oak, walnut, ash and birch. The landscape was much different than it is today, and resembled that of modern day coastal areas such as those found on the southeastern Atlantic and Gulf coasts of the United States. These mainland beach and barrier island environments consisted of river deltas, estuaries, marshes, swamps, and lagoons adjacent to beaches and relatively shallow coastal waters.

 

The majority of the exposed features in Chaco Canyon belong to a suite of rocks known as the Mesa Verde Group. The further subdivisions of the unit are from oldest to youngest: the Crevasse Canyon Formation, the Point Lookout Sandstone, the Menefee Formation, the Cliff House Sandstone, the Lewis Shale, and the Pictured Cliffs Sandstone. Of these formations, the Menefee and Cliff House Formations are visible within the main body of the canyon. Two younger units, the Lewis Shale and the Pictured Cliffs Sandstone, are generally exposed only near the northern boundary of the park. The Crevasse Canyon Formation is only visible in the Kin Ya'a unit of the park, which is currently closed to the public. The Point Lookout Sandstone is not visible within the park.

 

Created by Phil Varela

 

 

Menefee Formation

The Menefee Formation is the oldest exposed unit of the Mesa Verde Group within the main body of the canyon and is visible as a slope-forming unit that underlies the steep mesa walls of the more resistant Cliff House Sandstone. It is especially prominent on the south side of the canyon, with a number of excellent exposures at the base of Fajada Butte. Its layers show an upwards transition from fluvial sandstone and shale to carbonaceous shale, coal, and interbedded sandstones as it approaches the contact with the Cliff House Formation, telling the story of a very different climate from the one we experience at Chaco today.

The Menefee Formation is thought to have been formed from sediments deposited on a deltaic coastal plain by rivers flowing north and east across New Mexico toward the retreating sea. At the edge of the sea, the streams meandered through a wide, flat, coastal plain with deltas, shallow swamps, and lagoons accumulating plant material which would eventually form thin beds of lignite, a type of coal.

Vertebrate fossils found regionally within the Menefee Formation include turtles, fish, and crocodiles as well as fragmentary evidence of larger creatures such as hadrosaur, ankylosaur, and ceratopsian dinosaurs, and giant marine lizards known as mosasaurs. At Chaco Canyon, remains of a Cretaceous side neck turtle (Testudines pelomedusidae), an unidentified hadrosaur dinosaur, and an unidentified theropod dinoasur have been recovered from the upper portions of the Menefee Formation.

Plant fossils found in the Menefee include leaf impressions of palm and conifer as well as specimens closely resembling modern laurel, witch-hazel and camellia, suggesting a warm, moist, subtropical environment. At Chaco Canyon, plant fossil specimens are predominantly conifer with occasional broad leaf angiosperms (plants that produce flowers and encased seeds) and palm leaf impressions.

 

Cliff House Sandstone

The Cliff House Sandstone is a sequence of marine sandstones on top of the Menefee Formation. It formed at a time called a transgression, when sea level was rising. The three principle Cliff House units (Lower, Intermediate, and Upper) visible in Chaco Canyon represent the beach, near shore marine, and offshore marine deposits produced near a barrier island beachfront by repeated incursions of the Western Interior Seaway.

The Lower Sandstone unit, a massive marine sandstone, forms the 80 – 100 foot prominent cliffs visible throughout the canyon. This sandstone was deposited just off of a Late Cretaceous beach, in relatively shallow water still affected by wave and storm action. An abundance of ripple marks are visible in this unit. Fossils include shells and casts from clams, ammonites (including possible Placenticeras) snails, and shark’s teeth. Iron-cemented, knobby casts of burrows known as Ophiomorpha (“serpent shape”) Nodosa (“nodular”) are particularly abundant. These casts are thought to be the fossilized remains of burrows left by a small shrimp-like crustacean known as Callianasa.

Immediately above the Lower Sandstone unit is the Intermediate unit. Although primarily a shallow water marine sandstone, it has locally interbedded shales which were deposited in deeper water. This unit is less resistant to erosion than the Lower Sandstone and forms gentle slopes rather than cliffs. Fossils found in this unit are generally similar to that of the Lower Sandstone but Ophiomorpha burrows are often less abundant and accumulations of the large clam Inoceramus are often more abundant.

The uppermost unit exposed in the main body of Chaco Canyon is the Upper Sandstone unit. It is similar in composition to the Lower Sandstone and also forms steep cliffs, but is generally only 30-60 feet in thickness. This unit represents a beach and barrier island environment and was deposited in shallower water than the Intermediate unit, reflecting a lowering of local sea level as the Western Interior Seaway receded for a period of time. Fossils found in this unit include abundant invertebrate shells and shark teeth, and bone debris probably from marine lizards.