El Rito Research Articles

Drainage reorganization and Laramide tectonics in north‐central New Mexico and downstream effects in the Gulf of Mexico

AbstractThe El Rito and Galisteo depocenters in north‐central New Mexico archive tectonically‐driven Paleogene drainage reorganization, the effects of which influenced sedimentation along the northwestern margin of the Gulf of Mexico. Although separated by ~100 km and lacking depositional chronology for the El Rito Formation, the two aforementioned New Mexican depocenters are commonly considered remnants of a single basin with coeval deposition and shared accommodation mechanism. Detrital zircon U‐Pb maximum depositional ages indicate that the El Rito and Galisteo formations are not coeval. Moreover, stratigraphic thickness trends and mapping relationships indicate different accommodation mechanisms for the Galisteo and El Rito depocenters; tectonically‐induced subsidence versus infilling of incised topography, respectively. The regional unconformity that bounds the base of both the El Rito and Galisteo formations is a correlative surface induced by local tectonic activity and associated drainage reorganization in the early Eocene, and was diachronously buried by northward onlap of fluvial sediments. Detrital zircon distributions in both depocenters indicate increased recycling of Mesozoic strata above the unconformity, but diverge upsection as topographic prominence of local basement‐involved uplifts waned. Sediment capture in these depocenters is coeval with deposition in other externally‐drained Laramide basins. Further, it corresponds to a period of low Laramide province‐derived sediment input and replacement by Appalachian‐sourced sediment along the northwestern margin of the Gulf of Mexico during a basin‐wide transgression. This illustrates the potential effect that pockets of sediment storage within the catchment of a transcontinental drainage system can have over the sedimentary record in the receiving marine basin.

Identifying the regional extent and geochemical evolution of interbasin groundwater flow using geochemical inverse modeling and 87Sr/86Sr ratios in a complex conglomeratic aquifer

Interbasin groundwater flow (IGF) occurs when water that is recharged in one watershed or basin discharges into an adjacent watershed or basin. IGF can directly impact the water and solute mass discharged from a watershed and consequently, skew estimates of watershed-scale weathering rates. It is extremely difficult to determine the impact of IGF on weathering processes in adjacent watersheds because it is difficult to differentiate between solute mass originating within the watershed from solute mass transported into the watershed from sources outside the watershed. We quantify the regional extent of IGF in three watersheds (Canjilon, El Rito, and Vallecitos) draining the Tusas Mountains of northern New Mexico, USA (sites where IGF has been shown to occur) using spatial trends in 87Sr/86Sr in springs and streams and geochemical modeling using PHREEQC. 87Sr/86Sr ratios show a strong connection from the Canjilon watershed to the El Rito watershed and suggest that the IGF connection may extend to the Vallecitos watershed. Geochemical modeling constrained by XRD analysis of aquifer mineralogy indicates that IGF from the Canjilon watershed is the dominant control on perennial flow in springs and streams in the adjacent El Rito watershed. However, the geochemical signature of an IGF connection from El Rito to the Vallecitos watershed is weak to non-existent. The maximum possible IGF contribution from El Rito to Vallecitos is during snowmelt, contributing as much as 20% of the water and solute mass to Vallecitos. However, the maximum IGF contribution is <5% during summer and fall months. In the El Rito watershed, the solute load is controlled by weathering kinetics occurring from within and outside the drainage area of the watershed. This diverges from the traditional watershed weathering model which assumes that the solute mass comes from only within the drainage area of the watershed. In comparison, the Vallecitos watershed conforms more to the traditional watershed weathering model. Our methodology is unique because it accounts for the background geochemical evolution of IGF in addition to mixing with locally-recharged groundwater. The mineral-weathering models, when combined with 87Sr/86Sr ratios, appear to be a robust methodology in sorting out the geochemical kinetics of watersheds connected through IGF and may also offer unique capabilities to investigate the regional extent of IGF in other settings.

Surface Water and Groundwater Interactions in Traditionally Irrigated Fields in Northern New Mexico, U.S.A.

Better understanding of surface water (SW) and groundwater (GW) interactions and water balances has become indispensable for water management decisions. This study sought to characterize SW-GW interactions in three crop fields located in three different irrigated valleys in northern New Mexico by (1) estimating deep percolation (DP) below the root zone in flood-irrigated crop fields; and (2) characterizing shallow aquifer response to inputs from DP associated with irrigation. Detailed measurements of irrigation water application, soil water content fluctuations, crop field runoff, and weather data were used in the water budget calculations for each field. Shallow wells were used to monitor groundwater level response to DP inputs. The amount of DP was positively and significantly related to the total amount of irrigation water applied for the Rio Hondo and Alcalde sites, but not for the El Rito site. The average irrigation event DP using data for the complete irrigation season at each of the three sites was 77.0 mm at El Rito, 54.5 mm at Alcalde and 53.1 mm at Rio Hondo. Groundwater level rise compared to pre-irrigation event water levels ranged from 3 to 1870 mm, and was influenced by differences in irrigation practices between sites. Crop evapotranspiration estimates averaged across irrigation events were highest in Rio Hondo (22.9 mm), followed by El Rito (14.4 mm) and Alcalde (10.4 mm). Results from this study indicate there are strong surface water-groundwater connections in traditionally irrigated systems of northern New Mexico, connections that may be employed to better manage groundwater recharge and river flow.

The End of Erendira's Prostitution

Ends are ends only when they are not negative but frankly transfigure the events in which they are immanent. (Frank Kermode 175) HE criticism of Gabriel Garcia Marquez' works usually accomplishes the difficult task of considering both his formal innovations and his criticisms of Latin American society and politics, but in the secondary literature on La increible y triste de candida y de su abuela desalmada (1972), the magical, fairy-tale elements have been analyzed much more thoroughly than its main theme of oppressive child prostitution. For example, Joel Hancock discusses Erendira and the Brothers Grimm (1978); Barbara B. Aponte examines el rito de initiation (1983); Marta Morello-- Frosch analyzes the funci6n de lo fantAstico (1984-85); Efren Ortiz gives the story una lectura mitica (1980); Antonio Benitez Rojo calls la Bella Durn-dente de Garcia (1987); Roberto Reis writes that the estructura de Poder is questioned through the use of the fantastic (1980); Jasbir Jain calls it Reversal of a Fairy Tale (1987) and Mario Vargas Llosa places his comments on within a chapter titled the Hegemonia de lo imaginario in his 1971 biography of Garcia Marquez, Historic de un deicidio. The social problem that La increible historic addresses deserves a more frontal treatment. (I will use this short form of the story title to distinguish it from the film, for which I will use the character's name within quotation marks: Erendira). Overlooked in comparison to genre and myth, the story's social themes and realist strategies reveal Garcia Marquez' early interest in criticizing aspects of women's oppression. In this article I first remind readers of the Colombian author's insistence on the autobiographical origins of the Erendira-Abuela pair, and then discuss prostitution in several other texts by him. Finally I seek to explain the controversial ending of La increible historia by highlighting its faithfulness to its genesis and analyzing where the fiction diverges both from those origins and from literary norms. My contention is that Erendira!s prostitution is comparable to the massacre of the banana workers in Cien anos de soledad in that they are narrathemes based on real events which are treated by the fictions as unreal. In Cien anos, the historical massacre is represented, but then the sole survivor cannot find anyone to believe his testimony. All deny his tale and declare him mad. Hyperbole and improbability weight the evidence in the fiction in favor of the massive coverup, the Big Lie in which a massacre on such a scale never could have happened. Yet history has witnessed such an event. Similarly, the nature of Erendira's slavery is couched in the language of fable and fairy tale, giving the appearance of fictionality, of its impossibility or untruth in any literal sense. Hyperbole and improbability have led to reader incredulity. Yet Erendira!s barbaric treatment by her grandmother and the huge number of men who have paid-sex with her in fact could happen and does happen in reality. According to published interviews with the author, La increible historic is based on an experience when, as a sixteen-year-old, Garcia Marquez saw an eleven-year-old girl working as a prostitute. She was accompanied by a woman whom he presumed to be a relative. At a decisive moment in the fictional story, the text similarly anchors itself in the reporters look, in personal testimony, rather than the omniscient authority of a storyteller. The episode with the bordello prostitutes is witnessed by a first-person, participant narrator who enters the text on the page before (126), interrupting the third-person omniscient narration. Like Garcia Marquez' newspaper account of seeing the young prostitute abused by a relative, this unnamed participant narrator is a young man. In this segment, a group of sex workers from the local brothel are angry because has attracted all their customers to wait in lines at her tent. …

A Model for Evolution of Laramide Axial Basins in the Southern Rocky Mountains, U.S.A.

The structural, stratigraphic, and sedimentologic development of Laramide axial basins of the southern Rocky Mountains is inconsistent with previous models relating them to transpressional tectonics. Axial basins are better explained as broad synclinal troughs in the hanging walls of large pop-up structures, in contrast to perimeter and ponded basins formed in the footwalls of these same structures. Laramide faults in the southern Rocky Mountains can be divided into the E-dipping Park Range thrust system and the W-dipping Sangre de Cristo thrust system. The W-dipping Front Range fault is the backthrust of the Park Range system, and the E-dipping Nacimiento fault is the back-thrust of the Sangre de Cristo system. The broad synclinal troughs formed between these two pairs of oppositely verging thrust systems were the locations for sediment deposition in axial basins, as exemplified by the Galisteo-El Rito basin of northern New Mexico. The Eocene Galisteo and El Rito formations were deposited on a broad syncline of Phanerozoic strata with crystalline basement along the basin margins. Most sediment was derived from the northeast, north, and northwest, except near the NE-trending Tijeras-Canoncito transfer zone along the southeast margin of the basin. Uplift of the Sangre de Cristo Range occurred first, followed by backthrusting to uplift the Sierra Nacimiento. This model explains Laramide uplifts and basins as the direct response to rapid NE-SW convergence between the North American and Farallon plates, without multiple reorientation of Laramide stress fields, and without major strike-slip along the eastern side of the Colorado Plateau.

Persistence of the Meadow Jumping Mouse, Zapus hudsonius luteus, in New Mexico

Zapus hudsonius luteus is a subspecies of the meadow jumping mouse (Hafner et al., 1981) that is restricted to New Mexico and Arizona, where it occupies mesic habitats in lowland valleys and along montane streams (Bailey, 1913; Findley et al., 1975; Hafner et al., 1981; Hoffmeister, 1986). The subspecies has been reported from 14 localities in New Mexico (Hafner et al., 1981; R. D. Ohmart and V. C. Hink, in litt.) and 11 localities in Arizona (Hafner et al., 1981; Hall, 1981; Hoffmeister, 1986). In New Mexico, these records are from the San Juan Mountains (El Rito area), Sangre de Cristo Mountains (North Williams Lake), Jemez Mountains (upper Guadalupe River drainage), Sacramento Mountains (Rio Penasco and Silver Springs Creek), and the Rio Grande Valley (Espanola to the Bosque del Apache National Wildlife Refuge; Fig. 1). Southwestern populations of the genus Zapus were initially described by Miller (1911) as a distinct species, Zapus luteus, then as a subspecies of the western jumping mouse, Zapus princeps (Z. p. luteus), by Krutszch (1954). Based on genetic relationships between southwestern Zapus and other representatives of the genus, Hafner et al. (1981) concluded that these peripheral isolates were instead conspecific with the meadow jumping mouse, Z. hudsonius, and, therefore, considered them to be Z. h. luteus.

A geophysical model of the Española Basin, Rio Grande rift, New Mexico

A model of the subsurface structure of the eastern part of the Española Basin in the northern Rio Grande rift of New Mexico was constructed from geophysical data obtained since 1983 by the Summer of Applied Geophysical Experience (SAGE) field course. Approximately 742 new gravity observations, 1276 ground magnetic stations, 30 km of seismic refraction lines, 19 km of seismic reflection lines, 22 magnetotelluric stations, and several Schlumberger and dipole‐dipole resistivity lines were established. Our studies provide new information on one boundary of a major continental rift and on the depositional and structural style of an extensional basin within the rift. Integration of these data sets into a single transect indicates that the Española Basin is asymmetrical with approximately 2 to 3 km of sediments and sedimentary rocks near the center, thinning eastward to the Precambrian outcrop of the flanking Sangre de Cristo uplift. Several minor faults with throws of less than 200 m were found, but no major eastern bounding fault was observed. Thus, the Española Basin could be an asymmetrical, west‐dipping half‐graben. However, major fault offset, down toward the basin axis, may occur within Precambrian rocks of the Sangre de Cristo uplift. In either case, the geometry of the basin does not agree well with current models for the structural evolution of continental rifts, which emphasize low‐angle detachment faults which create asymmetrical, hinged half‐grabens. These models predict that major shoulder uplift should occur adjacent to the side of the graben bounded by a listric master fault rather than adjacent to the hinged side. In contrast, for the Española Basin major uplift occurred adjacent to the eastern side, which could be the hinged side of the basin. A thick wedge of older sedimentary rocks with high P‐wave velocity (4.4 km/s) and low electrical resistivity (5 Ω⋅m) was discovered under the younger Tertiary sediments and sedimentary rocks near the center of the basin. This wedge has maximum thickness of 1.2 km at the western end of the profile and thins eastward. The physical properties suggest this layer could be older Tertiary, or possibly a Mesozoic‐Paleozoic, section of rocks. If the latter, it has potential economic importance because of the possible presence of a Cretaceous section which is known to produce oil and gas in the Albuquerque Basin to the south and the San Luis Basin to the north. However, based on data from the Yates La Mesa no. 2 well, 10 km south of the transect, this wedge is likely middle Tertiary lacustrine deposits (NMOCD, 1986). The great thickness of lake deposits may represent a major lacustrine facies of the Eocene Galisteo and El Rito formations, exposed around the southern, southwestern, and northwestern margins of the basin. Magnetotelluric data suggest the crystalline basement underlying the central Española Basin may be more conductive than near the eastern margin. The entire Española Basin is also underlain by a highly conductive layer of about 1 Ω⋅m at a depth of 15 km. Both the shallow and deep low‐resistivity zones may result from hot, saline fluids. Such fluids deep within the crust may reduce the shear strength of the crust significantly and concentrate crustal extension on the west side of the rift.

Dialogue with the Past: Thoughts on Some University of New Mexico Field Schools

THE EARLY HISTORY OF field schools in Southwestern archaeology has recently been addressed by Gifford and Morris (1985), who emphasize the period between 1920 and 1940 when these institutions provided nearly the only preprofessional, practical experience for archaeology students. The kinds of field classes offered by Edgar L. Hewett, founder of the University of New Mexico field school, have been described by Chauvenet (1983). Field schools have a recognized long and venerable history and have provided American archaeology with many of its most acclaimed practitioners. By the time I assumed directorship of the UNM summer field school in archaeology in 1974, between ten and sixteen institutions offered summer field schools in the Southwest, and a large and growing number of schools offered fieldwork for credit throughout North America and elsewhere. Some Southwestern field schools, such as the University of Arizona's at Grasshopper, primarily addressed the needs of advanced students. Others, such as Southern Methodist University's program at Fort Burgwin, offered specialized laboratory training in addition to general field experience. The UNM field schools of the 1970s and early 1980s remained committed to undergraduate students and to providing these students with as broad a range of field experiences as possible. The following is a personal reflection on the UNM field school as I knew it. A field school often provides a student's first field experience, as it did for me in 1964 when, as one of five undergraduates from George Washington University, I journeyed to New Mexico to join Dr. Florence Hawley Ellis in her excavations at Sapawe, near El Rito. It is a tribute to the seriousness of this venture that three of the five of us earned Ph. D.'s and pursued professional careers in anthropology, although I was the only one to to do so in archaeology.' The field school may also furnish the first supervisory experience for graduate students, as it did for me in 1965 when I worked for Dr. J.J. Brody excavating pit house sites near Taos. My fellow teaching assistants included John Beardsley, Nancy Hammack, and Michael Marshall, all of whom subsequently found careers in archaeology. A host of graduate students, far too numerous to mention here, also supervised field school students before going on to become professional archaeologists. Finally, of course, field schools supply the human resources essential to the archaeological research of the faculty member responsible for the project. Research in ethnology, linguistics, and biological anthropology is generally undertaken by one person working alone. Although some archaeological projects can be completed with the labor of one or a few individuals, most excavation

Annotated Bibliography of the Eocene Paleontology and Stratigraphy of New Mexico

This bibliography is not comprehensive, but it includes all major, and most minor, references on the stratigraphy and paleontology of the Eocene formations in New Mexico. Papers discussing the following formations are included: Baca, Blanco. Basin, Cub Mountain, El Rito, Espinaso, Galisteo, Love Ranch, McRae, Orejon, Palm Park, and San Jose. Although the Blanca Basin, Cub Mountain, El Rito, Espinaso, Love Ranch, McRae, and Orejon Formations are not demonstrably Eocene, their assignment, to that epoch by various workers, past and present, justifies their inclusion here.

Supplementary Data for: A Preliminary Basin Analysis of the El Rito Formation (Eocene), North-Central New Mexico

Supplementary Data for: A Preliminary Basin Analysis of the El Rito Formation (Eocene), North-Central New Mexico

A preliminary basin analysis of the El Rito Formation (Eocene), north-central New Mexico

The Eocene(?) El Rito Formation in north-central New Mexico is the basal Tertiary unit of the Chama basin and the northwestern Espanola basin. Deposition of the El Rito Formation occurred in the final stages of the Laramide orogeny. The formation is predominantly an alluvial-fan deposit, showing two principal facies: quartzite conglomerate and quartzose sandstone. Postdepositional pore filling by calcite, zeolite, and hematite, alteration of feldspars, and removal of heavy minerals are observed in the sediments of the El Rito Formation. The extensive diagenetic effects are interpreted as resulting from diagenesis under semi-arid to arid conditions. Paleocurrent directions determined from imbricated clasts, cross-bedding, and parting lineations indicate that the primary sources of the El Rito Formation were in the north and northeast. The clasts of the early, conglomeratic facies were derived from the Precambrian crystalline highlands of the Brazos-Sangre de Cristo uplift. The conglomerates represent debris-flow, sieve, and high-energy braided-stream deposits. Point counts of thin sections of four sandstone samples show high QFL% quartz and QpLvmLsm% metamorphic lithic fragments. These results also indicate that the provenance was the Precambrian metamorphic rocks of the Brazos uplift. The sandstone units represent sheet-flow sediments deposited from lower-energy, braided streams with more easterly sources than the sources for the conglomerates. Only sandstones in the western portion of the El Rito depositional basin may have had significant sources in the Nacimiento uplift, the Gallina-Archuleta arch, or the southern San Juan uplift at the northwest. The El Rito Formation was deposited in a south-southeast-trending, asymmetrical basin with through-flowing drainage in the southern Rocky Mountains east of the Colorado Plateau. The basin formed in response to compression associated with the north-northeast jamming of the Colorado Plateau into the stable craton during the Laramide orogeny. The El Rito depositional basin probably was continuous with the Galisto basin before northeast-trending normal faulting of the Rio Grande rift separated the two basins in the Miocene to Holocene.

Preliminary Basin Analysis of El Rito Formation (Eocene), North-Central New Mexico: ABSTRACT

The Eocene El Rito Formation of north-central New Mexico is the basal Tertiary unit of the Chama basin and the north-western Espanola basin. Deposition of the El Rito occurred in the final stages of the Laramide orogeny. The formation is an alluvial-fan deposit, showing two principal facies, quartzite conglomerate and quartzose sandstone. Paleocurrent directions determined from imbricated clasts, cross-bedding, and parting lineations indicate that the primary sources for the sediment were in the north and northeast. The clasts of the early, conglomeratic facies were derived from the reworking of sediments eroded from the Precambrian crystalline highlands of northern New Mexico early in the Laramide orogeny. The conglomerates represent debris-flow, sieve and high-energy, braided-stream gravel deposits. The immature, sand-sized particles were derived from proximal sources in the Brazos uplift during the final stages of the Laramide orogeny. The sandstone units represent sheet-flow sediments deposited from lower energy, braided streams with more easterly sources than the sources for the conglomerates. Point counts of thin sections of four sandstone samples show high QFL% quartz and QpLvmLsm% metamorphic lithics. These results are consistent with a provenance in the Precambrian metamorphic core of the Brazos uplift. Extensive diagenetic effects typical of Cenozoic desert alluvium include calcite, zeolite, and hematite pore-filling and replacement of grains, alteration of feldspars, and removal of heavy minerals. End_of_Article - Last_Page 563------------

Roscoe R. Hill, 1880-1960

THE DEATH of Roscoe R. Hill on October 26, 1960, at his home in Washington, D. C, ended a long and productive career, both in and out of the archival field, that covered almost half a century in academic and public service. For the Society of American Archivists Dr. Hill's passing is a special loss—the loss of one of our founding members (among the oldest of those surviving) and of one who was active in the Society up to within hours of his death, when he was working on the latest installment of his American Archivology series. Almost every year our quarterly had at least some word from him, usually a note of praise for a new Latin American archival guide or documentary historical publication or a gentle but persistent gibe at some of the frailties of our professional argot—a favorite pastime in his dialogs with fellow archivists. He also served on several of the Society's committees: Classification and Cataloging, International Relations, and Membership. In 1951 he was elected an honorary member in the Society, with lifetime tenure, and in 1958 he was enrolled in the Society's first selection of distinguished Fellows. Like most leading archivists in America and Europe in this century, Dr. Hill's academic training and intellectual interests were basically historical. He worked variously and tirelessly as a teacher, writer, administrator, and consultant, and he traveled extensively and had a wide acquaintance abroad, especially in Latin America. He had grown up in post-Civil War Illinois, had attended Eureka College (A.B., 1900) and the University of Chicago (1903-4, 1907), and had initially taught in midwest high schools and in a mission school in Cuba (1901-8 passim). His first major archival project, and the one that established his reputation, was in Seville, in 1911-13, for J. Franklin Jameson's Department of Historical Research at the Carnegie Institution of Washington: the compilation of the Descriptive Catalogue of documents relating to United States history in a major collection of Spain's colonial records, the Papeles Procedentes de Cuba. He then taught history at universities—Columbia, California, New York, and New Mexico—1913-20 passim; headed the Spanish-American Normal School at El Rito, N. Mex., 1917-19; and served in the U. S. State Department, 1920-28, first

A Reconstruction of Northern Rio Grande Prehistory

of the Rio Grande where, according to the earliest Spanish accounts (Winship 1896), there were over forty villages in seven districts: Tiguex, with twelve pueblos located in the vicinity of modern Albuquerque and probably represented by the surviving Tiwa towns of Isleta and Sandia; seven pueblos in the Galisteo and along the eastern foothills of the Sandia Mountains; the Keres district with seven pueblos; Jemez, also with seven pueblos; three villages at Aguas Calientes, exact location unknown; Yunque-yunque, probably corresponding to San Juan, one of the present Tewa villages; Picuris; and Taos. Closely related to these groups, but situated in the adjacent Pecos drainage, was the pueblo of Cicuye, or Pecos, the easternmost occupied pueblo (Figure 1). As an archeological region, the northern Rio Grande may be defined as a roughly rectangular area extending from a line through Isleta in the south, and the Colorado line on the north; and from the Canadian River on the east, to the Rio Puerco and Rio Chama on the west (Figure 1). Physiographically this area is complex, containing features of the Basin and Range, Southern Rocky Mountain, and Colorado Plateau provinces. The aboriginal occupation was confined largely to the broad alluvial valley found within the Rio Grande Depression, a tectonic unit belonging to the Basin and Range province, and the adjacent tributary streams. The Rio Grande Depression extends northward well up into southern Colorado, separating the southern Rockies into two prongs: an eastern prong-the Sangre de Cristo Range, a steep anticlinal uplift, and a western prong-the Nacimiento-Brazos-Conejos-San Juan system. In part, the structure of the western prong is obscured by extensive volcanics (Northrop in Colbert and Northrop 1950: 41). The higher elevations (above 7,500 feet) were not permanently occupied, so far as is presently known, but were utilized for hunting. The Upper Sonoran life zone embraces almost all of the occupied area. It is characterized by extensive open country of grama grass and sagebrush, and forested areas of pinyon and juniper. Precipitation is meager-averaging less than twenty inches in much of the area-and specialized farming techniques, either floodwater farming or irrigation, are required for the successful practice of agriculture.

An Exhumed Erosion Surface in the Jemez Mountains, New Mexico

The plateau-like surface of San Pedro Mountain in the northwestern part of the Jemez Mountains of New Mexico is a remnant of an exhumed erosion surface. It is overlain in places by a thin veneer of chert, here named the Pedernal chert, with underlying gravel. The chert and gravel are a part of a thin wedge of Tertiary beds which thickens eastward and extends across the northern spurs of the Jemez Mountains. The rocks of the wedge are correlative with the El Rito formation, Abiquiu tuff, and Santa Fe formation, found in the Rio Grande depression east of the border faults of that trough. The stratigraphy and structure of the Tertiary sedimentary rocks of the wedge reveal in some detail the history of the deformation of the Rio Grande depression. The mountains formed during the Laramide revolution were reduced in early Tertiary time to a low-lying erosion surface or peneplain. A slowly subsiding basin then formed and was maintained until late Tertiary time, when it was broken by the normal faults which now bound the Rio Grande depression, leaving some of the Tertiary basin deposits on the western mountain block. Erosion of the larger part of these Tertiary rocks has left exposed on the summit of San Pedro Mountain a large area of the exhumed early Tertiary peneplain.

Tertiary Geology of the Abiquiu Quadrangle, New Mexico

The Abiquiu quadrangle is located in north-central New Mexico, largely in Rio Ar-riba County. Tertiary volcanic and sedimentary rocks overlie pre-Tertiary rocks with angular unconformity. Oldest of the volcanics is the Chicoma formation, consisting mainly of andesite and latite flows, with a maximum thickness of several thousand feet. Tertiary sedimentaries, heretofore undifferentiated, are divided into three formations: (I) the El Rito formation, grading from talus breccia to conglomerate and sandstone, maximum thickness about 200 feet; (2) the Abiquiu tuff, consisting of more than 1,000 feet of stream-laid tuff and volcanic conglomerate, with a few small interbedded lava flows; and (3) the Santa Fe formation, consisting of a great thickness of alluvial deposits with some interbedded basalt flows. Toward the close of Tertiary time, extensive deformation by normal faulting took place. Tertiary sedimentaries were downfaulted and downwarped against pre-Tertiary rocks on the west, and against Tertiary volcan...