Laramide fluvial evolution of the San Juan Basin, New Mexico and Colorado: Paleocurrent and detrital-sanidine age constraints from the Paleocene Nacimiento and Animas formations

Abstract

Understanding the tectonic and landscape evolution of the Colorado Plateau−southern Rocky Mountains area requires knowledge of the Laramide stratigraphic development of the San Juan Basin. Laramide sediment-transport vectors within the San Juan Basin are relatively well understood, except for those of the Nacimiento and Animas formations. Throughout most of the San Juan Basin of northwestern New Mexico and adjacent Colorado, these Paleocene units are mudstone-dominated fluvial successions intercalated between the lowermost Paleocene Kimbeto Member of the Ojo Alamo Sandstone and the basal strata of the lower Eocene San Jose Formation, both sandstone-dominated fluvial deposits. For the Nacimiento and Animas formations, we present a new lithostratigraphy that provides a basis for basin-scale interpretation of the Paleocene fluvial architecture using facies analysis, paleocurrent measurements, and 40Ar/ 39Ar sanidine age data. In contrast to the dominantly southerly or southeasterly paleoflow exhibited by the underlying Kimbeto Member and the overlying San Jose Formation, the Nacimiento and Animas formations exhibit evidence of diverse paleoflow. In the southern and western part of the basin during the Puercan, the lower part of the Nacimiento Formation was deposited by south- or southeast-flowing streams, similar to those of the underlying Kimbeto Member. This pattern of southeasterly paleoflow continued during the Torrejonian in the western part of the basin, within a southeast-prograding distributive fluvial system. By Torrejonian time, a major east-northeast–flowing fluvial system, herein termed the Tsosie paleoriver, had entered the southwestern part of the basin, and a switch to northerly paleoflow had occurred in the southern San Juan Basin. The reversal of paleoslope in the southern part of the San Juan Basin probably resulted from rapid subsidence in the northeast part of the basin during the early Paleocene. Continued Tiffanian-age southeastward progradation of the distributive fluvial system that headed in the western part of the basin pushed the Tsosie paleoriver beyond the present outcrop extent of the basin. In the eastern and northern parts of the San Juan Basin, paleoflow was generally toward the south throughout deposition of the Nacimiento and the Animas formations. An important exception is a newly discovered paleodrainage that exited the northeastern part of the basin, ∼15 km south of Dulce, New Mexico. There, an ∼130-m-thick Paleocene sandstone (herein informally termed the Wirt member of the Animas Formation) records a major east-flowing paleoriver system that aggraded within a broad paleovalley carved deeply into the Upper Cretaceous Lewis Shale. 40Ar/ 39Ar dating of detrital sanidine documents a maximum depositional age of 65.58 ± 0.10 Ma for the Wirt member. The detrital sanidine grains are indistinguishable in age and K/Ca values from sanidines of the Horseshoe ash (65.49 ± 0.06 Ma), which is exposed 10.5 m above the base of the Nacimiento Formation in the southwestern part of the basin. The Wirt member may represent the deposits of the Tsosie paleoriver where it exited eastward from the basin. Our study shows that the evolution of Paleocene fluvial systems in the San Juan Basin was complex and primarily responded to variations in subsidence-related sedimentary accommodation within the basin.

Early Paleocene Magnetostratigraphy and Revised Biostratigraphy of the Ojo Alamo Sandstone and Lower Nacimiento Formation, San Juan Basin, New Mexico, USA

“…Differences in stratigraphic terminology related to the Ojo Alamo Sandstone and underlying Naashoibito Member have created confusion about the relationship between these two units (for example see discussions in Powell, 1973;Williamson andWeil, 2008a, 2008b). Previous authors recognizing the Naashoibito Member and Ojo Alamo Sandstone as unique stratigraphic units representing different formations (e.g., Baltz et al, 1966;Williamson, 1996;Williamson and Weil, 2008a;Williamson et al, 2008) or as different members of the same formation (Powell, 1973;Sullivan et al, 2005;Fassett, 2009;Cather et al, 2019). Herein we recognize the Ojo Alamo Sandstone as a separate stratigraphic formation (sensu Baltz et al (1966).…”

“…These data, coupled with previous sedimentological analyses, demonstrate that the Naashoibito Member is late Maastrichtian in age and the Ojo Alamo Sandstone is early Paleocene in age with an erosive unconformity, that cuts out the Cretaceous-Paleogene boundary, separating the two units The Nacimiento Formation conformably overlies the Ojo Alamo Sandstone and unconformably underlies the Eocene San Jose Formation. The Nacimiento Formation is subdivided into six members: (1) The Kutz , (2) Tsosie, (3) Angel Peak, (4) Arroyo Chijuillita, (5) Ojo Encino, and (6) Escavada Members (Williamson and Lucas, 1992;Williamson, 1996;Cather et al, 2019). This study focuses on the Kutz, Tsosie, Arroyo Chijuillita and Ojo Encino Members.…”

“…It is a thick succession of crossbedded channel sandstones, splay sandstones, and floodplain mudstones. The lower Kutz Member tends to be drab in color and becomes more variegated with reddish mudstones in the upper portion (Cather et al, 2019). The Tsosie Member is exposed in the southwestern part of the basin and is characterized by thick, cross-bedded channel sandstone complexes separated by mostly drab floodplain mudstones.…”

“…The Tsosie Member is exposed in the southwestern part of the basin and is characterized by thick, cross-bedded channel sandstone complexes separated by mostly drab floodplain mudstones. The channel sandstones were interpreted as deposits from a river with a maximum depth of at least 5 m (Cather et al, 2019). Regional dip for the Ojo Alamo Sandstone and Nacimiento Formation varies between 1-5° (Baltz, 1967).…”

“…The Ojo Alamo Sandstone and the Nacimiento Formation from the San Juan Basin (SJB) in northwestern New Mexico and southwestern Colorado (Fig. 1) preserve a nearly continuous succession of lower Paleocene terrestrial deposits (e.g., Baltz et al, 1966;O'Sullivan et al, 1972;Williamson, 1996;Williamson et al, 2008;Cather et al, 2019). The SJB also preserves one of the most complete records of early Paleocene mammalian evolution following the Cretaceous-Paleogene (K-Pg) mass extinction and has been extensively studied for over a century (e.g., Granger, 1917;Matthew, 1937;Simpson, 1959;Williamson and Lucas, 1992;Williamson, 1996;Williamson et al, 2015).…”