The Timing of Diagenesis and Thermal Maturation of the Cretaceous Marias River Shale, Disturbed Belt, Montana

Abstract

The hypothesis that chemical remanent magnetization (CRM) in argillaceous rocks may be due to release of Fe during smectite illitization has been tested by study of spatial and temporal relationships of CRM acquisition, smectite illitization, and organic-matter maturation to deformation in the Montana Disturbed Belt. New K-Ar ages and stacking order and percentages of illite layers in illite-smectite (I-S) are consistent with conclusions from previous studies that smectite illitization of bentonites in Subbelts I and II of the Disturbed Belt was produced by thrust-sheet burial resulting from the Laramide Orogeny. Internally concordant, early Paleogene, K-Ar age values (55À57 Ma) were obtained from clay subfractions of thick bentonites which were significantly different in terms of their ages (i.e. Jurassic Ellis Formation and late Cretaceous Marias River Shale), further supporting a model of smectite illitization as a result of the Laramide Orogeny. Internally concordant K-Ar ages were found also for clay sub-fractions from a thick bentonite at Pishkun Canal (54 Ma) and from an undeformed bentonite near Vaughn on the Sweetgrass Arch (48 Ma). In Subbelts I and II, a greater degree of smectite illitization corresponds to increased thermal maturation, increased natural remanent magnetization intensity, and increased deformation (dip of beds). A dissolutionÀprecipitation model over a short duration is proposed for the formation of illite layers in Subbelts I and II. A characteristic remanent magnetization was developed before or just after folding began in the early Paleogene. More smectite-rich I-S, low thermal maturity, and the absence of a CRM were noted in one outcrop of an undeformed rock on the Sweetgrass Arch. Strontium isotope data allow for the possibility that internal or externally derived fluids may have influenced illitization, but the K-Ar age values suggest that illitization was probably in response to conductive heating after the overthrusting had occurred. The differences in K-Ar dates among the bentonites studied herein may be due to differences in the timing of peak temperature related to differences in distance below the overthrust slab, in rates of burial and exhumation, and in initial temperature.

Molecular modeling of the effects of 40Ar recoil in illite particles on their K–Ar isotope dating

“…They clearly prove that a predominant fraction of 40 Ar remains in its initial interlayer position and that the recoil-induced 40 Ar l oss is negligible. This is experimentally supported by numerous K-Ar dating studies on the finest illite-smectite subfractions showing no considerable Ar loss (Clauer et al, 1997;Środoń et al, 2002;Osborn et al, 2014) and by 40 Ar/ 39 Ar illite dating model of the 39 Ar recoil random walk (Hall et al, 2000), which has been remarkably successful in explaining stereotypical age spectral shapes.…”

“…In the case of K-Ar age variability among finest subfractions that should represent purely authigenic clay (i.e. < 0.2 µm in bentonites), their crystallization history and illite-smectite crystallite growth may play a major role (Clauer et al, 1997;Środoń et al, 2002;Osborn et al, 2014). On the other hand, Lerman and Clauer 5 (2005) and Lerman et al (2007) have suggested post-crystallization diffusion in the diagenetic zone as a mechanism responsible for lower K-Ar isotope age estimates of finer particles, as compared to coarser ones.…”

Tracing hydrocarbons in gas shale using lithium and boron isotopes: Denver Basin USA, Wattenberg Gas Field

“…Furthermore, if there were detrital contaminants they have been shown to diminish in abundance in the progressively smaller size fractions of clay (Pevear, 1999). The bentonite, while fairly impermeable relative to sandstones, is infiltrated by porefluids from the surrounding shale, allowing alteration of the volcanic glass (ash) to smectite first, then illite with increasing temperature and K supply (Altaner et al, 1984;Elliott et al, 1987;Altaner, 1989;Williams and Ferrell, 1991;Clauer et al, 2014a;Osborn et al, 2014). Crystallization of illite in bentonites occurs at 60˚-150˚C; temperatures of hydrocarbon generation (Tissot and Welte, 1984;Hower et al, 1976) when B and Li are released from source rocks (Williams et al, 2001;.…”

XRD-based 40Ar/39Ar age correction for fine-grained illite, with application to folded carbonates in the Monterrey Salient (northern Mexico)

“…Ar/ 39 Ar illite dating is commonly used to analyze the timing of various geological processes in the upper crust, including diagenesis and hydrocarbon formation (Pevear, 1999;Dong et al, 2000;Clauer et al, 2011;, low grade metamorphism (Dong et al, 1997;Merriman and Peacor, 1999;Verdel et al, 2011), epithermal ore formation (Hall et al, 1997 and, faulting (e.g., van der Pluijm et al, 2001;Solum and van der Pluijm, 2005;Haines and van der Pluijm, 2008;Osborn et al, 2014), and folding (Fitz-Díaz and van der Pluijm, 2013;Fitz-Díaz et al, 2014a). Illite is one of the few K-bearing minerals that form under diagenetic and anchizonal conditions ( Fig.…”