Sandstone body character and river planform styles of the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

ABSTRACT The lower Eocene Willwood Formation of the Bighorn Basin, Wyoming, USA, is an alluvial succession with a sand content varying around 25 palaeoenvironments and palaeoclimates, as well as sedimentological and stratigraphic analysis. Channel dynamics were studied at a relatively low resolution throughout the basin over the geological time from late Palaeocene to early Eocene. Here, a high-resolution study is reported to complement previous research at the basin scale. Efforts are made to document the characteristics and river planform styles of most sandstone bodies encountered through ca 300 m of alluvial stratigraphy in a 10 km2 area of the Deer Creek part of the McCullough Peaks area situated in the basin axis of northern Bighorn Basin. Four channel facies associations are recognized and ascribed to four river planform styles: crevasse channel, trunk channel, braided-like channel and sinuous-like channel, with the latter two types dominant. Braided-like and sinuous-like channel sandstone bodies differ significantly in thicknesses, being on average 6.1 m versus 9.0 m, but they have similar palaeoflow–perpendicular widths of on average 231 m and palaeoflow directions of on average N 003°. Braided-like and sinuous-like river planform styles show no spatial dependency in the 10 km2 study area. Results of this study are in line with existing basin-scale depositional models that are composed of a single axial system fed by several transverse systems dominantly from the west. The feeding of these systems could be influenced by palaeoclimate changes possibly controlling their contribution over time, thereby impacting river planform styles. At the same time, changing water discharge hydrograph, sediment load, and overbank cohesiveness may have equally driven the observed river planform style changes within the basin without a major role of catchments.Applied Geolog

Lateral and vertical characteristics of floodplain aggradation cycles in the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

Sedimentation on river floodplains is a complex process that involves overbank flooding, crevasse splaying, and river avulsion. The resulting floodplain stratigraphy often exhibits floodplain aggradation cycles with alternating fine-grained overbank flooding deposits that underwent significant petrogenesis, and coarser-grained, avulsion-belt deposits largely devoid of pedogenic impact. These cycles are linked to lateral migration and avulsion of channels driven by internal dynamics, external factors, or a combination of both. To better understand the spatial and vertical variability of such floodplain aggradation cycles, we map these in three dimensions using a photogrammetric model of the lower Eocene Willwood Formation in the northern Bighorn Basin, Wyoming, USA. This allows identifying 44 floodplain aggradation cycles in ∼300 m of strata with an average thickness of 6.8 m and a standard deviation of 2.0 m. All the cycles are traceable over the entire model, pointing to their spatial consistency over the 10 km2 study area. At the same time, rapid lateral thickness changes of the floodplain aggradation cycles occur with changes up to 4 m over a lateral distance of 400 m. Variogram analyses of both field and numerical-model results reveal stronger consistency of floodplain aggradation cycle thicknesses along the paleoflow direction compared to perpendicular to paleoflow. Strong compensational stacking occurs at the vertical scale of 2–3 floodplain aggradation cycles (14–20 m), while full compensational stacking occurs at larger scales of more than six floodplain aggradation cycles (>41 m). The lateral and vertical thickness variability of the floodplain aggradation cycles, as well as their compensational stacking behavior, are interpreted to be dominantly driven by autogenic processes such as crevasse splaying and avulsing that preferentially fill topographic lows. External climate forcing may have interacted with these autogenic processes, producing the laterally persistent and vertically repetitive floodplain aggradation cycles. The spatial variability of floodplain aggradation cycles demonstrated in this study highlights again the need for three-dimensional data collection in alluvial floodplain settings rather than depending on one-dimensional records.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Applied Geolog

Elevated physical weathering exceeds chemical weathering of clays during the Paleocene-Eocene Thermal Maximum in the continental Bighorn Basin (Wyoming, USA)

The Paleocene-Eocene Thermal Maximum (PETM) global warming event at ∼56 million years before present changed catchment weathering and erosion. Increased chemical weathering of silicate minerals is thought to be an important process removing CO2 from the atmosphere. However, changes in clay mineralogy can often be explained by enhanced erosion of catchment laterites during the event. Here, we investigate chemical and physical weathering and erosive flux changes through the PETM interval in the Bighorn Basin, Wyoming, a Laramide foreland basin, in a proximal continental-interior alluvial setting. These show an increase of detrital smectite with a lag time of 20-kyr after the main onset the PETM. The smectite increase continued for at least 50-kyr after the event. In-situ, post-depositional pedogenic clay mineral formation is similar between pre-PETM and PETM soil profiles, despite large macroscopic differences between soils that formed before and during the event. Drier, hotter summers during the PETM probably caused decreased vegetation cover that, in concert with more frequent and heavier rainstorms, intensified the erosion of smectite-rich Cretaceous bentonites on the margins of the catchment, which exceeded changes in chemical weathering within the catchment. The lagged response in reaching full PETM clay mineral values can be explained by the time required for upstream sediment to reach the catchment basin floodplain. The prolonged nature of smectite enhancement after the PETM event may again relate to signal propagation times that are now even longer due to lower fluvial recycling rates. Our results indicate that chemical weathering changes were probably superceded by enhanced physical weathering and clay-mineral transport from basin margins at this continental-interior study site.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Applied Geolog

Environmental impact and magnitude of paleosol carbonate carbon isotope excursions marking five early Eocene hyperthermals in the Bighorn Basin, Wyoming

Transient greenhouse warming events in the Paleocene and Eocene were associated with the addition of isotopically light carbon to the exogenic atmosphere-ocean carbon system, leading to substantial environmental and biotic change. The magnitude of an accompanying carbon isotope excursion (CIE) can be used to constrain both the sources and amounts of carbon released during an event and also to correlate marine and terrestrial records with high precision. The Paleocene-Eocene Thermal Maximum (PETM) is well documented, but CIE records for the subsequent warming events are still rare, especially from the terrestrial realm. Here, we provide new paleosol carbonate CIE records for two of the smaller hyperthermal events, I1 and I2, as well as two additional records of Eocene Thermal Maximum 2 (ETM2) and H2 in the Bighorn Basin, Wyoming, USA. Stratigraphic comparison of this expanded, high-resolution terrestrial carbon isotope history to the deep-sea benthic foraminiferal isotope records from Ocean Drilling Program (ODP) sites 1262 and 1263, Walvis Ridge, in the southern Atlantic Ocean corroborates the idea that the Bighorn Basin fluvial sediments record global atmospheric change. The ∼34m thicknesses of the eccentricity-driven hyperthermals in these archives corroborate precession forcing of the ∼7m thick fluvial overbank-avulsion sedimentary cycles. Using bulk-oxide mean-annual-precipitation reconstructions, we find soil moisture contents during the four younger hyperthermals that are similar to or only slightly wetter than the background, in contrast with soil drying observed during the PETM using the same proxy, sediments, and plant fossils. The magnitude of the CIEs in soil carbonate for the four smaller, post-PETM events scale nearly linearly with the equivalent event magnitudes documented in marine records. In contrast, the magnitude of the PETM terrestrial CIE is at least 5 % smaller than expected based on extrapolation of the scaling relationship established from the smaller events. We evaluate the potential for recently documented, nonlinear effects of pCO2 on plant photosynthetic C-isotope fractionation to explain this scaling discrepancy.We find that the PETM anomaly can be explained only if background pCO2 was at least 50%lower during most of the post-PETM events than prior to the PETM. Although not inconsistent with other pCO2 proxy data for the time interval, this would require declining pCO2 across an interval of global warming. A more likely explanation of the PETM CIE anomaly in pedogenic carbonate is that other environmental or biogeochemical factors influencing the terrestrial CIE magnitudes were not similar in nature or proportional to event size across all of the hyperthermals. We suggest that contrasting regional hydroclimatic change between the PETM and subsequent events, in line with our soil proxy records, may have modulated the expression of the global CIEs in the Bighorn Basin soil carbonate records.Applied Geolog