Geology of the Chama quadrangle, New Mexico
Authors: Muehlberger William R. 
Published: March 2023

The Chama quadrangle lies astride the central part of the Chama Basin, an elongate structure along the northeastern margin of the San Juan Basin.

Massive cross-bedded Precambrian quartzite of the Kiawa Mountain Formation crops out on the eastern margin of the quadrangle, the westernmost of the extensive exposures in the Tusas Mountains.

Nearly 400 feet of beds containing Desmoinesian fossils crop out just east of the quadrangle in Chaves Canyon, with only the lower 250 feet well exposed. Each recognizable group of beds thins onto the Precambrian surface. The lowest beds, mainly sandstone and siltstone, appear to be of talus origin. A disconformity separates these beds from the overlying arkosic sandstone and fossiliferous nodular limestone.

The Triassic Chinle Formation is about 500 feet thick and consists of red arkosic siltstone and sandstone with conglomeratic zones near the base and center. This unit lies on a smooth surface of Precambrian rocks, except for the inlier of Pennsylvanian rocks in Chaves Canyon.

The Jurassic Entrada Formation consists of about 250 feet of very well-sorted, friable, very fine-grained sandstone. Color zoning roughly divides the unit into three parts: a lower light reddish-brown, a middle pale yellow-orange, and an upper grayish-orange zone. Cross-bedding is most obvious in the middle part, although it is present throughout.

The Todilto Formation is either absent or covered throughout the quadrangle; it crops out east of the quadrangle south of Chaves Canyon. Where present, it is included on the map as part of the Morrison Formation.

Where exposed along the eastern margin of the quadrangle, the Morrison Formation is composed of interstratified units of mudstone and sandstone, with sandstone more abundant near the base. The mudstones are grayish green to pale red and are interbedded with grayishgreen to light-gray quartz sandstone.

The Cretaceous Dakota Formation crops out on all major anticlines and domes, as well as holding up long dip slopes along the eastern margin of the quadrangle. It can be subdivided roughly into three units: a lower sandstone, a middle shale, and an upper sandstone. The lower cliff-forming sandstone member, about 185 feet thick, is pale orange to moderate yellowish brown, massive, cross-bedded and parallel-bedded, very fine- to coarse-grained, and locally conglomeratic. From 70 to 125 feet of dark-gray, platy, carbonaceous silty shale and very fine-grained sandstone conformably overlie this unit. A very light-gray, medium- to fine-grained, thick, parallel-bedded, cliff-forming sandstone ranging in thickness from 27 to 45 feet conformably overlies the middle unit. The topmost unit (which has been stripped from the domes and anticlines within the quadrangle) consists mainly of siltstone and shale with a capping sandstone that commonly contains pelecypod casts.

The Mancos Formation lies conformably above the Dakota and consists almost entirely of shale with some silty limestone beds, concretions, and numerous fossils. Within the Chama quadrangle, the total thickness could not be measured but is estimated to be at least 1700 feet. The lower part can be subdivided into mappable members which approximate those of Dane (1948). The Graneros Shale Member, generally poorly exposed, consists of about 120 feet of fissile, finely bedded, slightly sandy, clayey shale, with a few concretion zones in the lower part. Conformably overlying this, the Greenhorn Limestone Member consists of about 20 feet of limestone beds separated by shale units. The Carlile Shale Member consists of from 400 to 500 feet of olive-gray to black shale with a zone of highly fossiliferous, calcareous sandy beds near the middle (Juana Lopez Member of Dane, Cobban, and Kauffman, 1966). Thin bentonite beds occur throughout the section;, septarian concretions are common in the lower part. The Upper Mancos Undifferentiated includes all beds between the Carlile Shale Member and the Mesaverde Group, the Niobrara Calcareous Shale and Upper Shale members of Dane (1948). The base is designated at the lowest appearance of Inoceramus grandis prisms covered with Ostrea congesta; this also coincides with a color break from dark gray below to light gray above (caused by an increase in quartz sand content). A thin, yellow-buff, medium- to coarse grained, calcareous sandstone or arenaceous calclithite crops out 10 to 20 feet above the base in the central part of the quadrangle. The upper contact is gradational and is chosen at the base of the lowest continuous massive sandstone.

The Mesaverde Group is not divisible in the Chama quadrangle, except for the southwest corner where a sharp transition upward into the coal-bearing sequence was found. Elsewhere it consists of 200 to 300 feet of massive-bedded, very fine-grained to fine-grained sandstone, commonly glauconitic, with a 10- to 15-foot shaly interval near the middle and thinner shaly intervals elsewhere in the section.

The top of the Mesaverde Group grades into the overlying Lewis Shale. A maximum thickness of 600 feet of Lewis is preserved in the northwestern part of the quadrangle. Erosion has removed the upper part of the Lewis Shale as well as any younger latest Cretaceous or earliest Tertiary units that may have been deposited prior to the deposition of the post-Laramide Eocene (?) Blanco Basin Formation.

The Blanco Basin Formation consists of 250 to 350 feet of arkosic conglomerate, sandstone, and shale. It comprises the basal prevolcanic Tertiary formation along the west front of the San Juan Mountains and northern Tusas Mountains. The El Rito Formation, a conglomerate composed of rounded Precambrian quartzite clasts in a red matrix, is the basal Tertiary formation along the west base of the southern Tusas Mountains and interfingers with the Blanco Basin Formation just north of Canones Box on the eastern edge of the quadrangle. The Blanco Basin Formation is an alluvial fan deposit that is the upslope, coarse-grained facies of the Wasatch Formation of the eastern San Juan Basin.

Only the Conejos Quartz Latite of the extensive Tertiary volcanic and sedimentary sequence of the Tusas Mountains extends into the quadrangle. It consists of two recognizable subdivisions: a lower slopeforming unit of tuff and conglomerate and an upper, blocky, cliffforming unit of flow breccia and agglomerate. Conglomerate clasts are principally of granitic and metamorphic rocks set in a matrix of tuff. Clasts of volcanic rocks are found near the eastern margin of the quadrangle. The flow breccia and agglomerate are composed of a dark purplish-brown, porphyritic-aphanitic quartz latite.

Gravels capping ridges 400 to 600 feet above the modern drainage levels and well above the terrace deposits assigned to the glacial events are here classified as Tertiary (?) terrace deposits. Two distinct levels can be recognized. Both sets slope south to southwest and suggest that an ancestral Chama River drained this area during the late Tertiary(?).

Quaternary deposits form an extensive veneer, in places completely obscuring the underlying bedrock relationships. Three major sequences of terrace deposits, each correlated with a glacial period; moraines where still recognizable as such; landslides commonly composed of glacial detritus, as well as the underlying formations; and recent alluvium have been delineated on the map.

Northwest-trending folds and faults superimposed on a larger structural terrace, the Chama Basin, dominate the Chama quadrangle. The Basin plunges northward into Colorado where it is partly buried by the Tertiary rocks of the San Juan Mountains. The Brazos fault zone, a west-trending transverse structure, lies just south of the quadrangle and can be traced for 50 miles from south of the Brazos Box westward into the San Juan Basin. It appears to be a feature along which there has been recurrent activity and which appears to be controlled by Precambrian stratigraphy and structure. South of the fault zone, the Chama Basin opens into a very shallow, broad, northtrending syncline.

The largest anticlinal structures, Willow Creek and Horse Lake anticlines, are in the southwest corner of the quadrangle. They bound the Brazos fault zone and constitute part of the Archuleta anticlinorium, the structural unit that separates the Chama Basin from the San Juan Basin on the west. Both anticlines are about 10 miles long, from 2 to 4 miles wide, and asymmetrical, with their crests immediately adjacent to narrow synclinal downwarps, whose troughs structurally are as much as 1000 feet lower.

The central part of the quadrangle has very low structural relief, except for elongate, narrow bands of faulting and tight folding. Dips as high as 32 degrees accentuate the contrast between these structures and the surrounding, nearly horizontal beds. Up to 5000 feet of structural relief between the Chama syncline and the Laramide monoclinal upwarp of the Tusas Mountains can be demonstrated in the Chama and adjacent Brazos Peak quadrangles. This slope is broken by a series of northwest-trending basement faults which have developed zones of faulting and elongate folding in the overlying sedimentary cover. These fault zones vary in throw, but most are less than a few hundred feet.

The Tertiary units of the Tusas Mountains are deposited across the truncated edges of these Laramide structures. The Tusas Mountains have a regional, gentle, east dip and are broken by long zones of northwesttrending faults. Some of these faults have had several periods of movement during Tertiary as well as Laramide time and possibly also even older movements.