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

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.

Rock Magnetic Signature of Heterogeneities Across an Intraplate Basal Contact: An Example From the Northern Apennines

“…We suggest that the axes interspersion in a plane perpendicular to the cleavage reveals the preservation of the lozenge‐shape petrofabric formed by crosscutting normal shear faults/fractures during the initial brittle stage of deformation, when the marly sediments were not completely lithified (Remitti et al., 2007; Vannucchi et al., 2010); P5 represents a local variation observed close to the contact with the Fiumalbo shale (T1‐B4) and without any relationship with the structural data. Possible explanations are: (a) higher contribution of ferromagnetic minerals and the presence of a mixed/intermediate magnetic fabric (Ferré, 2002; Pueyo Anchuela et al., 2010), considering that this pattern shows the highest χ m values; (b) percolation of ferruginous fluids (Ejembi et al., 2020); and (c) preservation of a strong original fabric.…”

“…P5 represents a local variation observed close to the contact with the Fiumalbo shale (T1‐B4) and without any relationship with the structural data. Possible explanations are: (a) higher contribution of ferromagnetic minerals and the presence of a mixed/intermediate magnetic fabric (Ferré, 2002; Pueyo Anchuela et al., 2010), considering that this pattern shows the highest χ m values; (b) percolation of ferruginous fluids (Ejembi et al., 2020); and (c) preservation of a strong original fabric.…”

Paleomagnetic studies of Jurassic and Cretaceous rocks in the Eastern Qiangtang Terrane, Tibetan Plateau, China

“…In the past few decades, many studies have demonstrated that anisotropy of magnetic susceptibility (AMS, also termed magnetic fabric), a comparatively rapid tool, has delivered crucial information on basin evolution (e.g., Mattei et al, 1997;Cifelli et al, 2005;Oliva-Urcia et al, 2010, 2011, 2013Yu et al, 2014;Tang et al, 2015;Li et al, 2020aLi et al, , 2020cLi et al, , 2021Jiang et al, 2022;Zhang et al, 2022a), emplacement of igneous bodies (e.g., Hrouda, 1982;Bascou et al, 2005;Antolín-Tomás et al, 2009;Cañón-Tapia and Mendoza-Borunda, 2014;Yan et al, 2019), fold/fault deformation (e.g., Aubourg et al, 1999;Saint-Bezar et al, 2002;Evans et al, 2003;Luo et al, 2009;García-Lasanta et al, 2015), and paleocurrent directions (e.g., Tarling and Hrouda, 1993;Pueyo Anchuela et al, 2013;Ejembi et al, 2020;Bilardello, 2021). A fundamental step prior to interpreting AMS data is to ascertain what minerals give rise to the magnetic fabric.…”