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.

The Cura Mallín basin is part of a chain of sedimentary basins that formed within the Andean volcanic arc between 33° and 43°S during the late Oligocene and early Miocene. Most previous studies of these basins have suggested that they are pull-apart–type basins, produced by strike-slip deformation of the Liquiñe-Ofqui fault zone and other structures, all of which are currently active. However, no direct evidence has been cited for a correlation between formation of the Oligocene-Miocene basins and concurrent strike-slip faulting. The Cura Mallín basin lies more than 100 km north of the modern Liquiñe-Ofqui fault zone and is one of the largest and best exposed of the Southern Andean Oligocene-Miocene basins, making it a promising study area for distinguishing between Oligocene-Miocene tectonic activity that produced the basin and subsequent tectonic activity. Stratigraphic and structural data presented here from the Cura Mallín basin and its surroundings include facies variations, stratal thickness patterns, internal and external structural features, 40 Ar/ 39 Ar radiometric ages, and apatite and zircon fission-track ages. Based on the distribution of sedimentary facies and their relation to geologic structures, we conclude that the Cura Mallín basin formed as a result of normal faulting, with little or no significant strike-slip deformation in the area. Due to the lack of supporting evidence for interpretations of the other Oligocene-Miocene basins as pull-apart basins, we suggest that the entire chain of Oligocene-Miocene sedimentary basins formed in response to extensional tectonics on the Southern Andean margin.

Abstract The central Colorado landscape bears a strong imprint of post-Laramide (late Eocene to Quaternary) tectonics, volcanism, climate change, and drainage rearrangement. This field trip will examine the post-Laramide evolution of central Colorado, traversing the Front Range, from the Colorado Piedmont on the east to the upper Arkansas valley segment of the Rio Grande Rift on the west (Fig. 1 ). The first day of the trip will involve a transect from the Denver-Colorado Springs section of the Piedmont across the southern Front Range, South Park, and Mosquito Range to the upper Arkansas valley. On this day we will focus on questions concerning the roles of tectonics and climate in driving post-Laramide landscape changes, examining structural, sedimentological, paleontological, geomorphic, and fission track evidence that has been used to reconstruct post-Laramide history. We will end the day with an initial overview of rift-related structures, sediments, and geomorphology as we enter the upper Arkansas valley. We will spend the second day in the southern portion of the upper Arkansas valley and the adjacent Poncha Pass transfer zone, examining structural and sedimentological evidence for the nature and timing of Neogene and Quaternary faulting and graben formation, and the character of the transfer zone. On our final day we will traverse back to the Piedmont, this time traveling down the canyons of the Arkansas River. We will examine rift-related structures and sediments in the Pleasant Valley graben and at the northern end of the Wet Mountain Valley, and will discuss the history of Cenozoic and earlier faulting in the area, the evolution of the Arkansas River drainage, and its recent downcutting history. We will end the trip with a discussion of the Neogene and Quaternary erosional history of the High Plains and Piedmont, and possible implications of this history for the driving mechanisms of landscape change.

A systematic relationship exists between seismically defined, master normal faults of the Albuquerque Basin and the structural style of rift shoulders on the basin margins. Rift shoulders occupying the footwalls of these master faults have at least three characteristics distinguishing them from the other shoulders of the basin: 1. They are the source areas for the basin’s major fanglomerates in the syn-rift Santa Fe Group. 2. They display distinctly Neogene apatite fission-track ages. 3. They have undergone the greatest amount of rift-related uplift in the Neogene. These relationships imply that uplift of these rift shoulders may be in part, the result of isostatic rebound of the lithosphere, accompanying tectonic unloading by the seismically defined master faults of the basin.