Sequence stratigraphic framework for mixed aeolian, peritidal and marine environments: Insights from the Pennsylvanian subtropical record of Western Pangaea

Abstract

Due to difficulties in correlating aeolian deposits with coeval marine facies, sequence stratigraphic interpretations for arid coastal successions are debated and lack a unifying model. The Pennsylvanian record of northern Wyoming, USA, consisting of mixed siliciclastic–carbonate sequences deposited in arid, subtropical conditions, provides an ideal opportunity to study linkages between such environments. Detailed facies models and sequence stratigraphic frameworks were developed for the Ranchester Limestone Member (Amsden Formation) and Tensleep Formation by integrating data from 16 measured sections across the eastern side of the Bighorn Basin with new conodont biostratigraphic data. The basal Ranchester Limestone Member consists of dolomite interbedded with thin shale layers, interpreted to represent alternating deposition in shallow marine (fossiliferous dolomite) and supratidal (cherty dolomite) settings, interspersed with periods of exposure (pedogenically modified dolomites and shales). The upper Ranchester Limestone Member consists of purple shales, siltstones, dolomicrites and bimodally cross‐bedded sandstones in the northern part of the basin, interpreted as deposits of mixed siliciclastic–carbonate tidal flats. The Tensleep Formation is characterized by thick (3 to 15 m) aeolian sandstones interbedded with peritidal heteroliths and marine dolomites, indicating cycles of erg accumulation, preservation and flooding. Marine carbonates are unconformably overlain by peritidal deposits and/or aeolian sandstones interpreted as lowstand systems tract deposits. Marine transgression was often accompanied by the generation of sharp supersurfaces. Lags and peritidal heteroliths were deposited during early stages of transgression. Late transgressive systems tract fossiliferous carbonates overlie supersurfaces. Highstand systems tract deposits are lacking, either due to non‐deposition or post‐depositional erosion. The magnitude of inferred relative sea‐level fluctuations (>19 m), estimated by comparison with analogous modern settings, is similar to estimates from coeval palaeotropical records. This study demonstrates that sequence stratigraphic terminology can be extended to coastal ergs interacting with marine environments, and offers insights into the dynamics of subtropical environments.

An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex)

“…Fissile purple shales with similar interpretations are also documented in the Ranchester Limestone Mbr of the underlying Tensleep Fm. (Blanchard et al, 2016).…”

Tidal dynamics in palaeo‐seas in response to changes in physiography, tidal forcing and bed shear stress

“…Indeed, small relative sea‐level changes in low‐gradient basins lead to the migration of facies belts over large horizontal distances (Midtkandal & Nystuen, 2009; van Yperen et al ., 2019). The effects associated with the migration of the facies belts can be further amplified in arid, paralic environments, when these relative sea‐level variations are associated with arid–humid climatic oscillations (Mountney, 2006; Anthony et al ., 2010; Jordan & Mountney, 2010, 2012; Blanchard et al ., 2016; Vieira & dos Santos Scherer, 2017). Increased periods of aridity facilitate the deposition and progradation of sand flats and aeolian dunes, which can subsequently be reworked by tidal currents as the sea transgresses the previously‐exposed coastal areas (Anthony & Dobroniak, 2000; Anthony et al ., 2010).…”

“…General trends, however, are expected to remain relatively constant, even with the addition of more complex input parameters. Differences between the geology and the model could also suggest that sediment in the innermost parts of the Curtis Sea was transported and deposited by other processes (for example, wind or flash floods; Anthony et al, 2010;Blanchard et al, 2016;Rivers et al, 2020), but whose signatures were not preserved in the rock record. Indeed, small relative sea-level changes in lowgradient basins lead to the migration of facies belts over large horizontal distances (Midtkandal & Nystuen, 2009;van Yperen et al, 2019).…”

Tidal dynamics in palaeo-seas in response to changes in bathymetry, tidal forcing, and bed shear stress

“…These differences between the geology and the model could also suggest that sediment in the innermost parts of the Curtis Sea was transported and deposited by other processes (e.g. wind, or flash floods; Anthony et al, 2010;Blanchard et al, 2016;Rivers et al, 2020), but whose signature (s) were not necessarily preserved in the rock record because tidal currents could have overprinted the original sedimentary structures. Indeed, small relative sea-level changes in low-gradient basins lead to the migration of facies belts over large horizontal distances (Midtkandal & Nystuen, 2009;van Yperen et al, 2019).…”

“…Indeed, small relative sea-level changes in low-gradient basins lead to the migration of facies belts over large horizontal distances (Midtkandal & Nystuen, 2009;van Yperen et al, 2019). The effects associated with the migration of the facies belt can be further amplified in arid, paralic environments, when these relative sea-level variations are associated with arid-humid climatic oscillations (Mountney, 2006;Anthony et al, 2010;Jordan & Mountney, 2010Blanchard et al, 2016;Vieira et al, 2017). Increased periods of aridity facilitate the deposition and progradation of sand flats and aeolian dunes, which can subsequently be reworked by tidal currents as the sea transgresses the previously-exposed coastal areas (Anthony & Dobroniak, 2000;Anthony et al, 2010).…”