Initial Laramide tectonism recorded by Upper Cretaceous paleoseismites in the northern Bighorn Basin, USA: Field indicators of an applied end load stress

W.T Jackson,B Shaulis,J.A Vantongeren,M.J Bartholomew,J.B Setera,M.P Mckay,D.T Allison,D.L Spurgeon

doi:10.1130/g46738.1

Abstract

Abstract Soft-sediment deformational structures associated with paleoseismicity (e.g., planar clastic dikes) exist within Upper Cretaceous Mesaverde Group strata in the Laramide Elk Basin anticline, northern Bighorn Basin (Wyoming, USA). Retrodeformation of the Elk Basin anticline to a horizontal Mesaverde Group position indicates that all basement offset is removed and that clastic dikes exhibit a dominant northeast trend. The trend of clastic dikes corresponds to the interpreted northeast-southwest direction of early Laramide layer-parallel shortening, suggesting that the development of clastic dikes recorded initiation of basement deformation and Laramide tectonism. To determine the timing of clastic dike development, we present zircon U-Pb geochronology from the stratigraphically lowest sand-source bed generating upwardly injected clastic dikes and a volcanic bentonite bed (Ardmore bentonite) above the stratigraphic interval containing clastic dikes. Weighted mean ages bracket clastic dike development between 82.4 and 78.0 Ma. Our results imply initiation of basement deformation ∼8–15 m.y. prior than other estimates in the Bighorn Basin. Therefore, we interpret the development of clastic dikes in the Elk Basin anticline to represent an initial phase of Laramide tectonism associated with an applied end load stress transmitted from the southwestern North American plate margin in response to the collision of the conjugate Shatsky Rise oceanic plateau ca. 90–85 Ma. Results demonstrate how sedimentary responses in the foreland can be used to understand tectonic processes at plate boundaries and provide spatial-temporal parameters for models of Laramide deformation.

Highlights

Late Cretaceous through Eocene Laramidestyle deformation occurred east of the Sevier thrust front in the western United States Cordillera (Fig. 1A; Dickinson and Snyder, 1978; DeCelles, 2004; English and Johnston, 2004; Yonkee and Weil, 2015)
We suggest a connection exists between the development of paleoseismites in the Elk Basin anticline and the collision of the conjugate Shatsky Rise along the southwestern North American plate margin ca. 90–85 Ma (e.g., Saleeby, 2003; Liu et al, 2010)
Because paleoseismites indicate Laramide tectonism 8–15 m.y. prior to estimations for Bighorn Basin, we propose that paleoseismites are surficial features that record initial phase of basement deformation

Summary

Introduction

Late Cretaceous through Eocene Laramidestyle (basement-involved) deformation occurred east of the Sevier thrust front in the western United States Cordillera (Fig. 1A; Dickinson and Snyder, 1978; DeCelles, 2004; English and Johnston, 2004; Yonkee and Weil, 2015). The onset and duration of Laramide deformation are temporally bracketed by the transition from marine to nonmarine sedimentation (Dickinson et al, 1988; Raynolds, 2003; Cather, 2004), crosscutting structural and stratigraphic relationships (Wiltschko and Dorr, 1983; Stone, 1993; Hoy and Ridgway, 1997; Cather, 2004; Tindall et al, 2010), basin subsidence (Mitrovica et al, 1989; Lawton, 1994; Heller et al, 2003; Leary et al, 2015), lulls in magmatic activity (Dickinson and Snyder, 1978; Humphreys, 2009), deposition of synorogenic strata (DeCelles et al, 1991), exhumation of basement arches (Omar et al, 1994; Crowley et al, 2002; Peyton et al, 2012), and paleoelevation estimates (Fan and Carrapa, 2014; Fan et al, 2014) To describe these spatial and temporal relationships, models propose basal friction (e.g., Bird; 1998; Yonkee and Weil, 2015; Behr and Smith, 2016; Copeland et al, 2017), hydrodynamic stresses and flow in the asthenosphere (e.g., Liu et al, 2008; Jones et al, 2011; Heller and Liu, 2016), and plate-margin end load stresses (e.g., Livaccari and Perry, 1993; Erslev, 1993; Tikoff and Maxson, 2001; Axen et al, 2018) as driving forces of Laramide tectonism. It remains unclear how the application of an end load stress would be recorded in the rock record and, if so, how the end load stress would be distributed throughout the Laramide belt

Objectives

Discussion

Conclusion