Post-Depositional Fluid Flow in Jurassic Sandstones of the Uncompahgre Uplift: Insights From Magnetic Fabrics

John I Ejembi,Sara Satolli,Eric C Ferré,Sarah A Friedman

doi:10.3389/feart.2020.601415

Abstract

The anisotropy of magnetic susceptibility (AMS) in sedimentary rocks results from depositional, diagenetic, syn- and post-sedimentary processes that affect magnetic grains. Some studies have also shown the potential role played by post-depositional fluid flow in detrital and carbonate formations. Here we present a new case study of Middle-Upper Jurassic sandstones where secondary iron oxides, precipitated from fluids that migrated through pores, give rise to the AMS. These sandstones are well exposed in the Uncompahgre Uplift region of the Central Colorado Trough, Colorado. The magnetic foliation of these undeformed, subhorizontal strata consistently strike NE-SW over a large distance with an average 45° dip to the SE. This steep AMS fabric is oblique with respect to the regional subhorizontal bedding and therefore does not reflect the primary sedimentary fabric. Also, outcrop-scale and microscopic observations show a lack of post-depositional plastic (undulose extinction) or pressure-solution (stylolites) deformation microstructures in these sandstones, hence precluding a tectonic origin. The combination of magnetic hysteresis, isothermal remanent magnetization, and thermal demagnetization of the natural remanent magnetization indicate that these rocks carry a chemical remanent magnetization born primarily by hematite and goethite. High-field magnetic hysteresis and electron microscopy indicate that detrital magnetite and authigenic hematite are the main contributors to the AMS. These results show that post-depositional iron remobilization through these porous sandstones took place due to the action of percolating fluids which may have started as early as Late Cretaceous along with the Uncompahgre Uplift. The AMS fabric of porous sandstones does not systematically represent depositional or deformation processes, and caution is urged in the interpretation of magnetic fabrics in these types of reservoir rock. Conversely, understanding these fabrics may advance our knowledge of fluid flow in porous sandstones and may have applications in hydrocarbon exploration.

Highlights

IntroductionFluid-flow through permeable sandstones is a key process in multiple economically important geological processes such as, hydrocarbon migration (e.g., Oliver, 1986; Hunt, 1990; Ungerer et al, 1990), diagenetic alteration through fluid-rock interaction (Chan et al, 2000; Beitler et al, 2005; Potter and Chan, 2011; Potter-McIntyre et al, 2014), groundwater contamination of aquifers (e.g., Massei et al, 2002), or fluid-driven precipitation of ore minerals (e.g., Wilkinson, 2001)
For materials deposited under a strong current, a linear fabric parallel to the current may be imparted, such fabrics are typically reported in shales and rarely in sandstones (e.g., Ellwood and Howard, 1981; Schieber and Ellwood, 1993; Tarling and Hrouda, 1993), and the amplitude of the imbrication angle of deposited sediments is very small compared to the strongly oblique magnetic fabric observed in our specimens (Figures 11B)
The magnetic mineralogy of the Jurassic rocks on the Eastern flank of the Uncompahgre Uplift rocks attests to a primary, detrital magnetite subsequently altered into hematite and goethite

Summary

Introduction

Fluid-flow through permeable sandstones is a key process in multiple economically important geological processes such as, hydrocarbon migration (e.g., Oliver, 1986; Hunt, 1990; Ungerer et al, 1990), diagenetic alteration through fluid-rock interaction (Chan et al, 2000; Beitler et al, 2005; Potter and Chan, 2011; Potter-McIntyre et al, 2014), groundwater contamination of aquifers (e.g., Massei et al, 2002), or fluid-driven precipitation of ore minerals (e.g., Wilkinson, 2001). Musgrove and Banner (1993) used elemental and stable isotopic signatures in groundwater samples from a Cambro-Ordovician and Mississippian aquifer that were collected from different wells to evaluate the origin and mixing of meteoric and saline fluids in the United States Midcontinent. This approach relies on direct sampling of fluids for geochemical analysis and modeling whereas the hydrodynamic model approach requires multiple topographic relationships to be maintained for an accurate prediction of fluidflow directions. The Cenozoic and some Late Mesozoic sedimentary formations in this region have been eroded through successive uplifts (Figures 1, 2; Pederson et al, 2002)

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Discussion

Conclusion