A. J. BUDDING

ORIGIN AND AGE OF SUPERFICIAL STRUCTURES, JICARILLA MOUNTAINS, CENTRAL

Abstract: In the Jicarilla Mountains, sheets of influence of gravity. Outliers of consolidated Upper Cretaceous Dakota (?) Sandstone have been gravels, equivalents of the Ogallala Formation in involved in gliding on an erosional surface. Such southeastern New Mexico, partly cover the super- structures are restricted to the vicinity of partly ficial structures and indicate a pre-Pliocene age for eroded domes formed by Tertiary intrusions. The the gliding process. The term, superficial structure, doming created the structural relief necessary for or dicoiffement, is preferred over collapse structure, sheets of sandstone to move down a slope under the currently in use.

in the northern part of section 8 is composed of Introduction Dakota Sandstone. The northern end of this While studying the geology of the Jicarilla feature terminates about 100 feet south of Mountains in Lincoln County, New Mexico, limestone outcrops of the San Andres Forma- during 1959 and 1960, the writer recognized tion which is exposed extensively in section 5. several unusual gliding structures which have On the preliminary geologic map of the south- not been described in the literature. Although eastern part of New Mexico (Dane and Bach- little geologic information has been published man), the juxtaposition of these beds has been on the area, a preliminary geologic map of the explained as a fault contact. Field investiga- southeastern part of New Mexico (Dane and tion, however, shows that the San Andres Bachman, 1958) contains most of the Jicarilla Formation in section 5 forms a southward Mountains proper. On this map, however, a plunging anticline, and that successively young- different interpretation of the structures is er formations are exposed farther south (Fig. 1). given. This paper is an attempt at rectifica- The Dakota Sandstone, which has an almost tion, as well as a discussion of the gliding struc- horizontal attitude, forms a sheet which over- Hires and a comparison with similar structures lies, from north to south, rocks of the Bernal elsewhere. Formation, the Santa Rosa Sandstone, and the Chinle Formation (Fig. 1, cross-section). The Geology of Gliding Structures anomalous position of the Dakota Sandstone The Jicarilla Mountains, a northern exten- can be explained as the result of gliding on an sion of the Sierra Blanca, are underlain by erosional surface. The downslope creep may Permian, Triassic, and Cretaceous sediments have been aided by the lubricating action of which have been intruded by sills, dikes, and the underlying Chinle mudstones. Numerous other plutons of predominantly monzonitic joints, many with slickensided surfaces, attest composition. Table 1 lists the lithology and to intense internal movements in the sandstone thickness of the exposed stratigraphic units. during the gliding process. In Tertiary time, doming of the sediments by Superficial features similar to the one de- monzonite intrusions and a general uplift of scribed can be observed in other parts of the the area was followed by a period of erosion. Jicarilla Mountains. A large sheet of Dakota The presence of hard resistant beds, such as the Sandstone, present in sec. 11, T. 6 S., R. 13 E. sandstones of the Dakota, underlain by a less (Fig. 1, loc. 3), has been involved in landslid- resistant formation, the Chinle mudstones, ing, judging from its present anomalous posi- created conditions favorable for gravity gliding. tion with respect to older strata. In the north- A particularly well-exposed landslide block ern half of sec. 3 and 4, T. 5 S., R. 12 E. (Fig. 1, is exhibited in sec. 8, T. 4 S., R. 12 E., about loc. 2), the same formation occurs in an anom- a half mile north-northwest of the hamlet of alous position. Both outcrops are located near Luna, New Mexico (Fig. 1). A low hill, ap- the edge of the ficarilla Monzonite, a con- proximately 2200 feet long and 1400 feet wide, cordant intrusion which has been partially ex-

Geological Society of America Bulletin, v. 74, p. 203-208, 2 figs., February 1963 203

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posed by erosion. Gravitational gliding took (Bretz and Horberg, 1949; Sellards and others, place where the emplacement of monzonite 1932) have pointed out that the Ogallala bodies created sufficient structural relief in the Formation, now exposed along the edge of the overlying sediments. Llano Estacado in southeastern New Mexico and west Texas, once may have extended west Age of Structures of the Pecos River into the eastern foothills of A thin veneer of consolidated gravels over- the Guadalupe and Sacramento mountains. lies the Dakota and Santa Rosa sandstones in The San Andres dip slope, which extends from the northern half of section 8 (Fig. 1). Similar Lincoln County east to the Pecos River, is in gravels are exposed in the west half of section effect an exhumed and only slightly modified 16, and boulders weathered from these con- erosion surface which has existed since pre- TABLE 1. EXPOSED STRATIGRAPHIC UNITS, JICARILLA MOUNTAINS, NEW MEXICO Period Formation Average thickness Character Tertiary Ogallala at least Boulder and pebble conglomerate with (Pliocene) Formation 50 feet sandy matrix and calcareous cement; frag- ments predominantly quartzite, limestone, monzonite, chert, and sandstone; gray to white in color Mesa Verde at least Interbedded gray shales and buff sand- Formation 420 feet stones Mancos Shale 410 feet Bluish-gray calcareous shales with thin Upper limestone layers near base Cretaceous Dakota (?) 150 feet Predominantly thick-bedded, white to buff, Sandstone quartz sandstone, with siliceous cement; some cross-bedding Chinle 280 feet Maroon and red mudstones with some layers Formation of variegated siltstone and sandstone Triassic Santa Rosa 200 feet Thin-bedded, red to brown, micaceous Sandstone sandstone with lenses of chert pebble con- glomerate Triassic (?) Bernal (?) 300 feet Brick-red to buff, fine- to medium-grained, Formation calcareous sandstone San Andres 600 feet Thick-bedded to massive gray limestone, Formation vuggy in places; gypsum in upper portion; light-brown sandstone lenses near base Permian Glorieta 120 feet Well-sorted, medium-grained quartz sand- Sandstone stone; intertongues with overlying San Andres Formation and underlying Yeso Formation Yeso at least Pale-red and yellow, friable sandstone and Formation 60 feet siltstone

glomerates are abundant on the plains farther Ogallala time. Although the Ogallala Forma- east. tion (or Panhandle Formation of Sellards and The majority of the boulders in the con- others) is predominantly shaly in composition, glomerate is composed of limestone of the San it has some conglomerate members near its Andres Formation, monzonite, and sandstone base. As the western source area of the clastic and chert derived from the Dakota and Santa components of the formation is approached, a Rosa sandstones. The sandy matrix is gray to change to coarser particles can be expected. white in color and has a calcareous cement. A pebble count of the conglomerate exposed The formation was deposited, as the result of in section 8, gave the following results: chert rapid uplift and erosion, on a surface of late and jasper, 30. 2 per cent; monzonite, 13.2 per Paleozoic and Mesozoic rocks. Several authors cent; rhyolite, 0.9 per cent; sandstone, 5.7 per

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GENERALIZED GEOLOGIC MAP OF THE MEDICINE BOW MOUNTAINS, WYOMING

HOUSTON AND PARKER, PLATE 1 Geological Society of America Bulletin, volume 74

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0 1000 2000 3000 4000 5000 Feet

Figure 1. Geologic map and cross section of vicinity of Luna, Lincoln County, New Mexico. Base map taken from semi-controlled photo-map compiled by U. S. Soil Conservation Service from aerial survey flown in 1946. Localities 1, 2, and 3 are referred to in text.

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cent; limestone, 32.1 per cent; mudstone, 1.9 tion is found in the superimposed character of per cent; and quartzite, 16.0 per cent. These some of the dry stream channels. In the south figures compare favorably with the boulder half of section 5, Red Hill Canyon is deeply in- counts reported by Bretz and Horberg from cised in the limestones of the San Andres Ogallala gravels and residuals from the Sacra- Formation, which here form a southward mento Plain. The higher percentage of chert plunging anticline. The meandering stream and jasper may be attributed to the widespread channel cuts through one of the highest parts occurrence of chert pebble conglomerate in the of the limestone ridge. The meanders probably Santa Rosa Sandstone. developed on the Ogallala surface, and during It is, therefore, concluded that the consoli- subsequent erosion the canyon incised its dated gravels represent a coarser facies of the course in the Permian and Triassic rocks. Ogallala Formation. If this conclusion is cor- rect, the time of landsliding can be established Sequence of Events within certain boundaries. The detachment of The formation of the landslide and subse-

Figure 2. Block diagrams illustrating development of superficial structure: (a) arching of sedimentary beds by laccolithic intrusion and uplift of area; (b) erosion of dome and forma- tion of gliding structure; (c) deposition of sand and gravel of Pliocene Ogallala Formation; (d) renewed erosion, partially exposing pre-Tertiary formations and gliding structure. Dakota Sandstone (vertical hatching) and upper contact of San Andres Formation are indicated on front and side of block diagrams.

the Dakota Sandstone block and the downhill quent events are illustrated in a series of block transport to its present position must have diagrams (Fig. 2). taken place after the period of monzonitic During early or middle Tertiary time the igneous activity which created the domes, but region was intruded by monzonitic magma prior to the widespread deposition of erosional which formed laccolithic masses, sills, and dikes debris during Ogallala time. In accordance (Smith and Budding, 1959). This caused arch- with current opinion, the monzonite bodies are ing of the sediments overlying the laccoliths regarded as Tertiary in age, whereas the (Fig. 2a) which were then attacked by erosion. Ogallala Formation is lower to middle Pliocene Mesozoic post-Dakota sediments, such as the (Sellards and others). The formation of the Mancos Shale and Mesa Verde Formation ex- gliding structures is thus Tertiary, but the age tensively exposed 15 miles south, were stripped is pre-Pliocene. off, and along the flanks of the dome erosion Further evidence of the once extensive dis- proceeded down to the Triassic and Permian tribution of the gravels of the Ogallala Forma- beds. Near the south flank of the dome, a large

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block of Dakota Sandstone became detached (Knetsch, 1957). Gliding structures in the from the main body of the Dakota and began Swiss Alps and French Alps have been recog- sliding down the slope (Fig. 2b). After descend- nized by Lugeon (1949) and Goguel (1952). ing almost 1000 feet and traveling an unknown Lugeon introduced the name, decoiffement, distance horizontally, the landslide block came which is more applicable to these structures but to rest upon the eroded surface of Permian and lacks an easy translation. The term, gravita- Triassic rocks. Onto this degraded surface, sand tional gliding, is appropriate when large tec- and gravel beds were deposited, the fragments tonic units have been moved several miles of which were derived predominantly from the under the influence of gravity; such move- San Andres limestone and monzonite intrusives ments are now suspected for parts of the Swiss which by this time were exposed over large Alps, the Apennines, and the northern Rocky areas in the Jicarilla Mountains. These deposits Mountains. As the structures described from graded farther east into the finer-grained silts the Jicarilla Mountains are shallow in extent, and clays which formed the Ogallala Forma- the term, superficial structure, has been chosen. tion. The burial process (Fig. 2c) may have From the southwestern United States, taken place during deposition of this forma- Toreva block-sliding (Reiche, 1937) is a well- tion, which occurred in Pliocene time. Subse- known phenomenon. Toreva-blocks rotate into quent erosion removed most of the consolidated the slope on their downward movement and gravels, but small outliers are still preserved rest on an underlying formation, usually a (Fig. 2d). Some of the main drainage channels shale, at some elevation below the outcrop oi of the area, such as Red Hill Canyon, represent the resistant bed. superimposed drainage which developed on Superficial structures are caused by the ac- the Ogallala surface. tion of gravity on primary structures, which are usually fairly simple folds. In the Jura Comparison with Similar Structures Elsewhere Mountains, as well as in the examples from Structures of similar magnitude and origin Iran, the initial folding is due to a regionally as those discussed here have been described directed stress field; in the example discussed from other parts of the world. Harrison and herein, the folds are domal structures in sedi- Falcon (1934; 1936) drew attention to certain mentary rocks that overlie intrusions. Anti- structures in Iran, some of which are the result clinal folds and their subsequent erosion, to- of large scale gliding under the influence of gether with a resistant bed overlying a shaly gravity. These authors introduced the name, lubricating substratum, are the conditions collapse structure, a term which has since found necessary for large slabs or sheets to move its way into literature. However, the term, slowly down a slope under the influence of collapse, connotes a disintegration of internal gravity. Superficial structures are of a sec- structural elements, and since only superficial ondary origin and can result in complications parts of a larger structure are involved in the which, if not recognized, may give rise to process of gravity gliding, objections can be erroneous structural interpretations. raised to the continued use of this term

References Cited Bretz, J Harlan, and Horberg, Leland, 1949, The Ogallala formation west of the Llano Estacado: Jour. Geology, v. 57, p. 477-490 Dane, Carle H., and Bachman, George O., 1958, Preliminary geologic map of the southeastern part of New Mexico: U. S. Geol. Survey Misc. Geol. Inv. Map 1-256 Goguel, Jean, 1952, Traite de tectonique: Paris, Masson et Cie, 383 p. Harrison, J. V., and Falcon, N. L., 1934, Collapse structures: Geol. Mag., v. 71, p. 529-539 1936, Gravity collapse structures and mountain ranges, as exemplified in south-western Iran: Geol. Soc. London'Quart. Jour., v. 92, p. 91-102 Knetsch, Georg, 1957, Gravitative Denudation von unter geringcr Bedcckung gebildeten Struktiiren: Geol. Rundschau, v. 46, p. 557-563 Lugeon, Maurice, 1949, Question de mode en geologic et autres histoires; le decoiflement: Annales Hebcrt et Haug, v. 7, p. 261-274

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Reiche, Parry, 1937, The Toreva-block—a distinctive landslide type: Jour. Geology, v. 45, p. 538-548 Sellards, E. H., Adkins, W. S., and Plummer, F. B., 1932, The geology of Texas, I, Stratigraphy: Univ. Tex. Bull. 3232, 1007 p. Smith, Clay T., and Budding, A. J., 1959, Little Black Peak fifteen-minute quadrangle, east half: N. Mex. Bureau Mines and Min. Res. Geol. Map 11

DEPT. GEOLOGY, NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY, SOCORRO, N. MEX. MANUSCRIPT RECEIVED BY THE SOCIETY, MARCH 27, 1962

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