Mj. Kraus, Development of potential acid sulfate paleosols in Paleocene floodplains, Bighorn Basin, Wyoming, USA, PALAEOGEO P, 144(1-2), 1998, pp. 203-224
Floodplain paleosols in the Paleocene Fort Union Formation in the Bighorn B
asin are distinguished by a dark claystone interval with yellow jarosite mo
ttles, relatively high total organic carbon values, and woody debris and le
aves along bedding planes. The paleosols also have various redoximorphic fe
atures, including low matrix chromas, grey root mottles, and local concentr
ations 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 avai
lable. The former presence of pyrite and the redoximorphic features suggest
that the Fort Union paleosols first developed as potential acid sulfate so
ils (Brinkman and Pens, 1973). The dark claystones with jarosite are interp
reted as permanently reduced or Gr horizons. The Gr horizon is overlain by
mudstone that is lighter colored and lacks jarosite. This represents an anc
ient Bg horizon or A/Bg sequence. The jarosite indicates that the paleosols
underwent a subsequent episode of oxidation and acidification. Those proce
sses 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 waterlogge
d soils were pushed even farther below the groundwater table. Only with lat
e 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 sug
gest that the floodplain swamps were not close to the active channel. The b
ipartite nature to the profiles (Bg horizon or A/Bg sequence above a Gr hor
izon) probably resulted from two different kinds of floodplain deposition.
Grain size trends indicate that the Gr horizons formed on sediment deposite
d 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 avul
sion. The greater permeability and higher topographic position of this sedi
ment caused it to be better drained than the Gr horizon. The paleosols are
unusual because they formed in a freshwater setting, whereas modern potenti
al acid sulfate or acid sulfate soils form in coastal areas where sulfate i
s readily available. The source of the sulfate may have been Cretaceous mar
ine shales, exposed along the flanks of mountains rising around the Bighorn
Basin during Paleocene time. Other Upper Cretaceous and Paleocene formatio
ns in the Rocky Mountain area contain similar rocks, suggesting that sulfid
ic soils were relatively common in the mid-continent at this time. Most of
these other examples probably reflect deposition associated with the retrea
ting Western Interior Seaway and humid climates. (C) 1998 Elsevier Science
B.V. All rights reserved.