Is there a fluvial response to the Palaeocene-Eocene Thermal Maximum in the Bighorn Basin?

Records of the Paleocene-Eocene Thermal Maximum (PETM) have been well documented in continental sediments of the Bighorn Basin, NW Wyoming. Previous studies have concentrated on the detailed documentation of sedimentation... [ view full abstract ]

Records of the Paleocene-Eocene Thermal Maximum (PETM) have been well documented in continental sediments of the Bighorn Basin, NW Wyoming. Previous studies have concentrated on the detailed documentation of sedimentation patterns immediately prior to, during- and post- PETM at a few select sites within the basin. Each study has identified sedimentological changes over the PETM interval, however, results are not consistent across the different studies.
This study aims to assess the sedimentological response of the fluvial systems at several localities across the entire Bighorn Basin to analyse how consistent the response, if any, is to a rapidly changing climate. This has been undertaken in conjunction with a basin scale facies analysis to better place the PETM into context with the rest of the basin fill, both spatially and temporally.
In the most distal northern portion of the basin a large amalgamated channel deposit, termed the ‘boundary sandstone’ is observed at the PETM, whereas at Polecat Bench, 14 km away, a change to thicker more welded paleosol deposits are observed at the PETM. In the SE portion of the basin we see a gradual change in the colouration of soils from drab greys to red mottled soils, with the latter increasing in thickness up-section. However, laterally the red soils grade into drab grey soils, highlighting issues of extrapolating trends over even short distances. In the SW of the basin, although the PETM has not been located directly, the boundary between the Paleocene and Eocene is characterised by a gradual increased thickness of channel braid belt deposits over 200m stratal thickness. A comparison of the different sites suggests there are no ubiquitous trends across the basin at the PETM, either suggesting that the response is different depending on position within the basin, or that there is no obvious sedimentological response. Basin scale facies analysis suggests that there is no clear sedimentological response to the PETM as the sedimentary succession deposited during the PETM shows no significant difference to the succession developed either prior to or post the PETM section. It is speculated that changes that would be expected with short term oscillations in mean annual precipitation within the main body of the PETM such as increased grain size and barform size are not observed. Instead an increase in preserved channel deposits is observed which is more likely controlled by subsidence/aggradation rates rather than extrinsic climate forcing.