Abstract Oligo-Miocene volcaniclastic sedimentation in northern New Mexico occurred before and during initiation of the Rio Grande rift. Reconstruction of dispersal patterns and paleogeography of extensive volcaniclastic aprons, using geochemical, paleocurrent, textural and petrofacies analyses, places important constraints on models for the tectonomagmatic initiation and evolution of the rift. K-Ar ages and chemical analyses of volcanic clasts from conglomerate provide details concerning three primary volcanic centers: 1) San Juan Mountains (29–27 Ma, high-K andesite and rhyodacite); 2) Latir volcanic field (28–25 Ma, high-K andesite and rhyolite); and 3) the previously unrecognized Servilleta Plaza center (23–22 Ma, Iatite, high-K andesite and rhyodacite), which may have been a southern extension of the Latir field. These three volcanic centers provided detritus to the following units, respectively: I) Esquibel Member of Los Pinos Formation, upper Abiquiu Formation, middle Picuris Formation, and Bishops Lodge Member of Tesuque Formation; 2) Cordito Member of Los Pinos Formation and uppermost Abiquiu Formation; and 3) Chama-EI Rito Member of Tesuque Formation and upper Picuris Formation. Correlation of these units is based on both conglomerate and sandstone petrofacies. Detailed point counting of volcanic lithic-grain varieties (vitric, granular, seriate, microlitic and lathwork) is especially useful in discriminating petrofacies. The Esquibel petrofacies has abundant plagioclase, seriate lithics and dense minerals, with low quartz. The Cordito petrofacies has abundant sanidine, quartz and vitric-to-granular lithics, with few dense minerals. The Plaza petrofacies is similar to the Esquibel, but with higher quartz and microlitic lithics, and lower total feldspar and dense minerals. Use of petrofacies simplifies the chaos of stratigraphic nomenclature and allows regional correlations of poorly exposed units. Paleocurrent and textural data from these volcaniclastic petrosomes define volcaniclastic aprons derived from the above volcanic centers. The coordinated use of clast ages and petrology to define source characteristics, sandstone petrofacies to define petrosomes, and paleocurrent and textural analyses to define dispersal patterns provides a powerful method for reconstructing ancient volcaniclastic systems.

Two Oligocene conglomeratic units, one primarily nonvolcaniclastic and the other volcaniclastic, are preserved on the west side of the Jemez Mountains beneath the 14 Ma to 40 ka lavas and tuffs of the Jemez Mountains volcanic field. Thickness changes in these conglomeratic units across major normal fault zones, particularly in the southwestern Jemez Mountains, suggest that the western margin of the Rio Grande rift was active in this area during Oligocene time. Furthermore, soft-sediment deformation and stratal thickening in the overlying Abiquiu Formation adjacent to the western boundary faults are indicative of syndepositional normal-fault activity during late Oligocene–early Miocene time. The primarily nonvolcaniclastic Oligocene conglomerate, which was derived from erosion of Proterozoic basement-cored Laramide highlands, is exposed in the northwestern Jemez Mountains, southern Tusas Mountains, and northern Sierra Nacimiento. This conglomerate, formerly called, in part, the lower member of the Abiquiu Formation, is herein assigned to the Ritito Conglomerate in the Jemez Mountains and Sierra Nacimiento. The clast content of the Ritito Conglomerate varies systematically from northeast to southwest, ranging from Proterozoic basement clasts with a few Cenozoic volcanic pebbles, to purely Proterozoic clasts, to a mix of Proterozoic basement and Paleozoic limestone clasts. Paleocurrent directions indicate flow mainly to the south. A stratigraphically equivalent volcaniclastic conglomerate is present along the Jemez fault zone in the southwestern Jemez Mountains. Here, thickness variations, paleocurrent indicators, and grain-size trends suggest north-directed flow, opposite that of the Ritito Conglomerate, implying the existence of a previously unrecognized Oligocene volcanic center buried beneath the northern Albuquerque Basin. We propose the name Gilman Conglomerate for this deposit. The distinct clast composition and restricted geographic nature of each conglomerate suggests the presence of two separate fluvial systems, one flowing south and the other flowing north, separated by a west-striking topographic barrier in the vicinity of Fenton Hill and the East Fork Jemez River in the western Jemez Mountains during Oligocene time. In contrast, the Upper Oligocene–Lower Miocene Abiquiu Formation overtopped this barrier and was deposited as far south as the southern Jemez Mountains. The Abiquiu Formation, which is derived mainly from the Latir volcanic field, commonly contains clasts of dacite lava and Amalia Tuff in the northern and southeastern Jemez Mountains, but conglomerates are rare in the southwestern Jemez Mountains.

Geologic mapping, age determinations, and geochemistry of rocks exposed in the Abiquiu area of the Abiquiu embayment of the Rio Grande rift, north-central New Mexico, provide data to determine fault-slip and incision rates. Vertical-slip rates for faults in the area range from 16 m/m.y. to 42 m/m.y., and generally appear to decrease from the eastern edge of the Colorado Plateau to the Abiquiu embayment. Incision rates calculated for the period ca. 10 to ca. 3 Ma indicate rapid incision with rates that range from 139 m/m.y. on the eastern edge of the Colorado Plateau to 41 m/m.y. on the western part of the Abiquiu embayment. The Abiquiu area is located along the margin of the Colorado Plateau–Rio Grande rift and lies within the Abiquiu embayment, a shallow, early extensional basin of the Rio Grande rift. Cenozoic rocks include the Eocene El Rito Formation, Oligocene Ritito Conglomerate, Oligocene–Miocene Abiquiu Formation, and Miocene Chama–El Rito and Ojo Caliente Sandstone Members of the Tesuque Formation (Santa Fe Group). Volcanic rocks include the Lobato Basalt (Miocene; ca. 15–8 Ma), El Alto Basalt (Pliocene; ca. 3 Ma), and dacite of the Tschicoma Formation (Pliocene; ca. 2 Ma). Quaternary deposits consist of inset axial and side-stream deposits of the ancestral Rio Chama (Pleistocene in age), landslide and pediment alluvium and colluvium, and Holocene main and side-stream channel and floodplain deposits of the modern Rio Chama. The predominant faults are Tertiary normal high-angle faults that displace rocks basinward. A low-angle fault, referred to as the Abiquiu fault, locally separates an upper plate composed of the transitional zone of the Ojo Caliente Sandstone and Chama–El Rito Members from a lower plate consisting of the Abiquiu Formation or the Ritito Conglomerate. The upper plate is distended into blocks that range from about 0.1 km to 3.5 km long that may represent a larger sheet that has been broken up and partly eroded. Geochronology ( 40 Ar/ 39 Ar) from fifteen volcanic and intrusive rocks resolves discrete volcanic episodes in the Abiquiu area: (1) emplacement of Early and Late Miocene basaltic dikes at 20 Ma and ca. 10 Ma; (2) extensive Late Miocene–age lava flows at 9.5 Ma, 7.9 Ma, and 5.6 Ma; and (3) extensive basaltic eruptions during the early Pliocene at 2.9 Ma and 2.4 Ma. Clasts of biotite- and hornblende-rich trachyandesites and trachydacites from the base of the Abiquiu Formation are dated at ca. 27 Ma, possibly derived from the Latir volcanic field. The most-mafic magmas are interpreted to be generated from a similar lithospheric mantle during rifting, but variations in composition are correlated with partial melting at different depths, which is correlated with thinning of the crust due to extensional processes.

Abstract The purpose of this paper is to briefly review the upper Eocene and Oligocene lacustrine deposits in the southwestern part of the United States and to discuss how they have been used to understand the paleoclimate, tectonic history, and volcanic environment in this region. Two well-studied lacustrine deposits(upper Eocene Florissant and Oligocene Creede Formations)will be a primary focus because they represent two major types of lake basins that formed in the region during this time. The Florissant Formation was deposited in a lake basin formed by a blocked stream drainage (McElroy and Anderson, 1966). The Creede Formation was deposited in a lake basin formed by caldera collapse (Steven and Ratte, 1965; Larsen and Crossey, 1996). Other examples of these two types of lake basins and other lake deposits in the region area lso discussed (see Figure 1 and Table 1). Much of the southwestern part of the United States(Arizona, Utah, Colorado, and New Mexico) was elevate drelative to surrounding regions during the latest Eocene through Oligocene (Figure 1) (Christiansen and Yeats, 1992; Elston and Young, 1991; Gregory and Chase, 1992; Chapin and Cather, 1994; Wolfe et al.,1998). This was largely the result of Laramide-style high-angle faulting and filling of adjacent Laramide style basins (Miller et al., 1992; Dickinson et al., 1988).Prominent erosional surfaces existed adjacent to tectonichighlands in the Rocky Mountains (Evanoff, 1990;Chapinand Cather, 1994) and northern Arizona-southern Utaharea (Elston and Young, 1991). Voluminous volcani sminitially produced intermediate-composition compositevolcanoes in numerous areas. This was followed in the Sawatch Range, San Juan, Marysvale, Latir, and Mogollon-Datil volcanic fields by silicic calderas (seelocations on Figure 1) (Steven, 1975; Elston, 1984;Christiansen and Yeats, 1992). Latest Eocene and early Oligocene sedimentation occurred largely in a fewbroad, low-relief Laramide basins (Claron: southwestern Utah, Gold strand, 1994; Baca, central New Mexico,Cather and Johnson, 1986; Uinta: northeastern Utah,Andersen and Picard, 1972) and in volcaniclasticaprons and proto-rift basins adjacent to the volcanicregions (Anderson et al., 1975; Ingersoll et al., 1990).Few other late Eocene-Oligocene deposits are presentin the region, and most of those are associated within itial extension in the southern Rio Grande rift (Macket al., 1994) and southern Basin and Range (Grover,1984; Eberly and Stanley, 1978; Christiansen and Yeats,1992). Small, localized accumulations of fluvial and lacustrine deposits in volcanic depressions and blocked stream drainages (see Figure 1 and Table 1)preserve the only sedimentary records for understanding the tectonic, climatic, and paleoenvironmental history of this region during this time.