Nonclimatic and extrabasinal processes controlled pedogenesis in paleosols of the Paleocene Nacimiento Formation, New Mexico, USA

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.

A physical and chemical sedimentary record of Laramide tectonic shifts in the Cretaceous-Paleogene San Juan Basin, New Mexico, USA

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.

Mechanical stratigraphy in Mesozoic rocks of the San Juan Basin: Integration of stratigraphic and structural terms and concepts

We characterize relationships between stratigraphy and natural fractures in outcrops of Mesozoic strata that rim the San Juan Basin in New Mexico and Colorado. These outcrops expose fluvial and shallow-marine siliciclastic deposits and calcareous mudstones deposited in a distal marine setting. We focus primarily on a regionally extensive fracture set formed during the Eocene to minimize localized tectonic effects on fracture development. Where possible, we supplement our observations with wireline log- or laboratory-derived measurements of rock properties. Our goals are twofold: 1) to illustrate how direct integration of data and concepts from stratigraphy and structural geology can lead to better fracture characterization, and 2) to develop thought processes that will stimulate new exploration and development strategies. Genetic beds form one scale of stratification in the outcrops we describe. For example, sandstone beds can be arranged into coarsening and thickening upward successions that are the depositional record of shoreline progradation. In fluvial settings, cm- to dm-scale sandstone beds can also be part of m-scale single-storey channel complexes that, themselves, can be arranged into amalgamated channel complexes 10s of m thick. In these and other settings, it is important to distinguish between beds and features that can be defined via wireline logs because it is the former (cm- to dm-scale) that are usually the primary control the distribution of natural fractures. The extension fractures we describe are typically bed-bound, with bedding being defined by lithology contrasts and the associated changes in elastic properties. Fracture spacing distributions are typically lognormal with average spacing being less than bed thickness. Although mechanical bedding and depositional bedding are commonly the same, diagenesis can cut across bed boundaries and complicate this relationship, especially where lithologic contrasts are small. Deposits from similar depositional environments which undergo different diagenetic histories can have substantially different mechanical properties and therefore deform differently in response to similar imposed stresses.

Stratigraphy and hydrocarbon resources of the San Juan Basin: Lessons for other basins, lessons from other basins

This paper examines the relationships between stratigraphy and hydrocarbon production from the San Juan Basin of New Mexico and Colorado. Abundant data and the long production history allow lessons to be learned, both from an exploration and development perspective, that can be applied in other basins. Conversely, as new play types and technologies are defined and developed elsewhere, the applicability of those tools in the San Juan Basin needs to be understood for well-informed exploration and development activities to continue. The San Juan Basin is a Latest Cretaceous – Tertiary (Paleogene) structure that contains rocks deposited from the Lower Paleozoic to the Tertiary, but only the Upper Cretaceous section has significant hydrocarbon, mostly gas, production. Herein I make the case for studying depositional systems, and the controls thereon (e.g., basin development, eustasy, sediment supply), because they are the first-order controls on whether a sedimentary basin can become a hydrocarbon province, or super basin as the San Juan Basin has recently been defined. Only in the Upper Cretaceous did a suitable combination of forcing mechanisms combine to form source and reservoir rocks, and repeated transgressive-regressive cycles of the Upper Cretaceous stacked multiple successions of source and reservoir rocks in a way that leads to stacked pay potential. Because of the types of depositional systems that could develop, the source rocks were primarily gas prone, like those of other Rocky Mountain basins. Oil-prone source rocks are present but primarily restricted to episodes of peak transgression. A lack of suitable trapping mechanisms helps to explain the relative dearth of conventional oil pools. Although gas production has dropped precipitously in the past decade, driven primarily by overabundance of gas supply associated with the shale-gas boom, the combination of horizontal drilling and multi-stage hydraulic fracturing is being applied to revive oil production from some unconventional stratigraphic targets with success.