Geochemical data tables, X-Ray diffractograms from paleosol B horizon materials, and results of statistical analysis of pedotype and geochemical analysis data.
Paleosols can represent intervals of nondeposition in sedimentary packages and are used with increasing frequency as proxies for paleoenvironmental conditions during basin filling. However, the complexities of factors both internal and external to pedogenesis require consideration of paleosols in any basin or sedimentary package in a broader context than is often assumed. With this in mind, we measured and analyzed pedogenic features, stratigraphic position, geochemical composition, and petrography of paleosols in the Nacimiento Formation to gain insight into the paleoenvironmental conditions of the early Paleocene in the San Juan Basin. During this interval, the San Juan Basin was located in middle northern latitudes (∼40°N) and saw rapid terrestrial siliciclastic sedimentation related to Laramide tectonism. Evidence from earlier researchers suggests that prevailing climate conditions during Nacimiento Formation deposition were warm and humid. We used morphological properties of paleosols to categorize paleosols into pedotypes indicative of distinct pedogenic conditions. The general stratigraphic distribution of these pedotypes shows an up-section increase in soil drainage conditions through the Nacimiento Formation that cannot be correlated with known climate changes. We suggest that fluvial system evolution was the major control on pedogenic conditions. We investigated Nacimiento Formation paleosols with widely used paleosol geochemical climate analyses, which provided paleoclimate estimates that are in disaccord with independently derived estimates. We show that in alluvial depositional systems with source areas in weathered sedimentary rocks, these analyses can be difficult to interpret and likely lead to estimates that do not reflect true pedogenic conditions during the postdepositional near-surface alteration of sediments. Petrographic analysis of Nacimiento Formation paleosols showed that some likely formed under semiarid to subhumid conditions that allowed pedogenic accumulation and illuviation of smectite clays yet did not substantially chemically alter primary detrital plagioclase feldspar grains in paleosol B horizons. The paleosols of the Nacimiento Formation, when analyzed at the basin scale, show that sedimentary aggradational processes can overpower climate processes in creation of a sequence of paleosols in a stratigraphic section. In addition, the incorporation of clays from sedimentary rocks in a basin’s source area can drastically skew the geochemical signatures and therefore interpretations of paleosols in that basin.
Fluvial siliciclastic rocks bracketing the Cretaceous-Paleogene (K-Pg) boundary in the San Juan Basin, New Mexico (USA), provide records of regional fluvial and tectonic evolution during the Laramide orogeny. Petrographic analyses of sandstones from the Upper Cretaceous Fruitland Formation and Kirtland Formation and the Paleocene Ojo Alamo Sandstone and Nacimiento Formation show that the rivers depositing these sediments were sourced in areas where unroofing of crystalline basement rocks took place, introducing an increasing proportion of immature detrital grains into the fluvial system through time. After the Cretaceous-Paleogene boundary, rivers deposited an increasing amount of microcline and orthoclase feldspar relative to plagioclase feldspar, suggesting a growing source in unique crystalline basement rocks. Geochemical analyses show significant differences between Al- and K-poor Upper Cretaceous sandstones and Al- and K-rich lower Paleocene sandstones in the San Juan Basin.
The high proportion of sand-sized material in the Ojo Alamo Sandstone suggests that it was deposited in a basin with a low ratio of sediment supply to accommodation. However, magnetostratigraphic age constraints suggest it had a relatively high sedimentation and/or subsidence rate of as much as 0.38 m/k.y. The sediment supply must have been high in order to deposit a basin-wide coarse sand-dominated package, suggesting rapid creation of topographic relief in the San Juan uplift, the proposed source area of the Ojo Alamo fluvial system.
The observed sedimentary architecture and age constraints of the Ojo Alamo Sandstone, including kilometers-wide sand bodies and limited overbank mudstones throughout most of the outcrop area, are difficult to reconcile with accepted models of aggradation and avulsion in large fluvial systems, but available age and lithologic data make difficult a complete understanding of Paleocene San Juan Basin fluvial systems and basin evolution. Here, we present new lithologic, petrographic, and thickness data from San Juan Basin K-Pg fluvial siliciclastic units and interpretations of their origins.
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