Development of potential acid sulfate paleosols in Paleocene floodplains, Bighorn Basin, Wyoming, USA

Abstract

Floodplain paleosols in the Paleocene Fort Union Formation in the Bighorn Basin are distinguished by a dark claystone interval with yellow jarosite mottles, relatively high total organic carbon values, and woody debris and leaves along bedding planes. The paleosols also have various redoximorphic features, including low matrix chromas, grey root mottles, and local concentrations of goethite and hematite, especially associated with roots and other organic matter. Jarosite is usually an oxidation product of pyrite, which forms under waterlogged conditions when sulfate and organic matter are available. The former presence of pyrite and the redoximorphic features suggest that the Fort Union paleosols first developed as potential acid sulfate soils ( Brinkman and Pons, 1973). The dark claystones with jarosite are interpreted as permanently reduced or Gr horizons. The Gr horizon is overlain by mudstone that is lighter colored and lacks jarosite. This represents an ancient Bg horizon or A/Bg sequence. The jarosite indicates that the paleosols underwent a subsequent episode of oxidation and acidification. Those processes require that the soils be drained, which probably did not occur during Paleocene time and may be a recent event. Rather, the soils were probably buried as the Fort Union Formation continued to aggrade, and the waterlogged soils were pushed even farther below the groundwater table. Only with late Cenozoic excavation of strata in the basin and exposure of the Fort Union deposits, did jarosite precipitate. Pedogenic and paleobotanical features suggest that the soils formed in floodplain swamps dominated by coniferous trees. The high organic content and clay-rich nature of the Gr horizons suggest that the floodplain swamps were not close to the active channel. The bipartite nature to the profiles (Bg horizon or A/Bg sequence above a Gr horizon) probably resulted from two different kinds of floodplain deposition. Grain size trends indicate that the Gr horizons formed on sediment deposited by overbank flooding. The impermeable, fine-grained sediment and its low topographic position led to waterlogging. The overlying A/Bg or Bg horizons formed on coarser sediment that was probably deposited during channel avulsion. The greater permeability and higher topographic position of this sediment caused it to be better drained than the Gr horizon. The paleosols are unusual because they formed in a freshwater setting, whereas modern potential acid sulfate or acid sulfate soils form in coastal areas where sulfate is readily available. The source of the sulfate may have been Cretaceous marine shales, exposed along the flanks of mountains rising around the Bighorn Basin during Paleocene time. Other Upper Cretaceous and Paleocene formations in the Rocky Mountain area contain similar rocks, suggesting that sulfidic soils were relatively common in the mid-continent at this time. Most of these other examples probably reflect deposition associated with the retreating Western Interior Seaway and humid climates.