Changes in Sedimentary Architecture from the Paleocene-Eocene Thermal Maximum (PETM), Bighorn Basin, Wy
The PETM was a dramatic, transient episode of global warming. The initial rise in temperatures probably took less than several thousand years. This episode of warming has been attributed to the addition of massive amounts of greenhouse gases to the atmosphere. Excellent exposures in the Bighorn Basin, WY provide an opportunity to examine the impact of this event on the architecture of fluvial strata, particularly floodplain deposits.
In the Cabin Fork area of the basin, the PETM interval overlies the Paleocene Fort Union Fm, which is characterized by drab paleosols that indicate poorly drained conditions. The PETM interval is found at the base of the Willwood Fm and can be divided into three lithologic units. The lower ~10 m of the interval has thin, red paleosols with small carbonate nodules. Vertically, the paleosols alternate with relatively thick deposits attributed to channel avulsion. The overlying ~20 m of section has weakly developed paleosols with abundant, large CaCO3 nodules. This part of the section also has thick avulsion intervals that are dominated by cut-and-fill deposits generated by crevasse-splay channels. The uppermost ~10 m of the PETM consists of thick, purple and red paleosols with no CaCO3 nodules. Intervening avulsion deposits are thin.
Our preliminary interpretation is that, with onset of the PETM, floodplains became better drained as a result of less rainfall or more seasonal precipitation as well as higher temperatures. The thinness and morphologic features of the paleosols indicate that pedogenesis was modest, thus, sediment accumulation rates were moderate. Weakly developed paleosols in the middle part of the section indicate rapid rates of sediment accumulation, and the cut-and-fills indicate highly unstable channels and floodplains. The high sediment flux may reflect intensified seasonal precipitation, as also suggested by the large CaCO3 nodules. The upper part of the PETM section, with the absence of CaCO3 and presence of thick, purple paleosols, was probably deposited in a wetter climate with more stable floodplains. The thick paleosols and their close spacing suggest relatively slow sediment accumulation.
Paleosols show that the lower ~30m of the PETM interval represent far less time than the upper 10m. Thus, sediment accumulation rates appear to have been more rapid when the climate was drier and more seasonal; rates slowed with increased wetness towards the end of the PETM.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009