Bulletin 89—Geology of the Chama Quadrangle, New Mexico

By W. R. Muehlberger, 1967, 114 pp., 1 table, 17 figs., 2 plates (including color geologic map), 2 appendices, 1 index.

The Chama quadrangle, in the central part of the Chama Basin along the northeast margin of the San Juan Basin, contains outcrops of Precambrian rocks; Pennsylvanian, Triassic Chinle, Jurassic Entrada, Todilto, Morrison, Cretaceous Dakota, Mancos, Mesaverde, and Lewis strata; the Blanco Basin, El Rito, Conejos Quartz Latite, and Cenozoic gravels, moraines, and alluvium. Northwest-trending folds and faults dominate, cut by the west-trending Brazos fault zone. Willow Creek and Horse Lake anticlines, about 10 mi long, and 2–4 mi wide, are asymmetrical with crests 1,000 ft above adjoining troughs. Structural relief between the Chama syncline and the Laramide monoclinal upwarp of the Tusas Mountains is about 5,000 ft.

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 44 ft of beds containing Desmoinesian fossils crop out just east of the quadrangle in Chaves Canyon, with only the lower 250 ft 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 consists of about 250 ft 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 grayish-green 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 ft 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 ft 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 ft conformably overlies the middle unit. The topmost unit consists mainly of siltstone and shale with a capping sandstone that commonly contains pelecypod casts.

The Mancos Formation lies conformable 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 1,700 ft. The lower part can be subdivided into mapable members which approximate those of Dane. The Graneros Shale member, generally poorly exposed, consists of about 120 ft of fissile, finely bedded, slightly sandy, slatey shale, with a few concretion zones in the lower part. Conformably overlying this, the Greenhorn Limestone Member consists of about 20 ft of limestone beds separated by shale units. The Carlile Shale Member consists of from 400–500 ft of olive-gray to black shale with a zone of highly fossiliferous, calcareous sandy beds bear the middle. 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. The base is designated at the lowest appearance of Inoceramus grandis prisms covered with Otrea congesta; this also coincides with a color break from dark gray below to light-gray above. A thin, yellow-buff, medium- to coarse-grained, calcareous sandstone or arenaceous calcithite crops out 10–20 ft 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.