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Early Cambrian Humid, Tropical, Coastal Paleosols from Montana, USA

By
Gregory J. Retallack
Gregory J. Retallack
Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, Texas 75275-0395, USA ntabor@smu.edu
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Published:
January 01, 2013

Abstract

A putative Precambrian paleosol mapped at the unconformity between the Cambrian Flathead Sandstone and Belt Supergroup at Fishtrap Lake, Montana, was found instead to be a succession of paleosols forming the basal portion of the Flathead Sandstone. Early Cambrian age of these paleosols comes from stratigraphic ranges of associated marine trace fossils: Bergaueria hemispherica, Didymaulichnus lyelli, Torrowangea sp. indet., and Manykodes pedum. Instead of a single strongly developed paleosol on top of the Belt Supergroup with a smooth geochemical depth function, five successive geochemical and petrographic spikes were interpreted as so many individual paleosols within a short sedimentary sequence of red beds, overlying brecciated and little-weathered Belt Supergroup. The most weathered intervals (paleosol A horizons) are purple-red in color (Munsell weak red, 7.5R 4/2) and massive to hackly, whereas intervening marine siltstones are planar bedded and purple-gray (Munsell dark reddish gray, 7.5R 4/1). The massive to hackly appearance comes from blocky to platy peds defined by argillans and is also the result of pervasive bioturbation of two distinct kinds: drab-haloed filament traces and ferruginized-organic filaments. In thin section, the filaments are circular as well as elliptical and elongate and of presumed microbial origin. The filament-rich (A) horizons are also defined by magnetic susceptibility and show petrographic evidence of significant weathering (depleted abundance of rock fragments, feldspar, and mica compared with lower horizons). Additional evidence of weathering comes from chemical analyses showing net loss of mass and weatherable elements within a profile. These lines of evidence indicate that Montana estuarine landscapes during the earliest Cambrian were colonized by filamentous organisms in a tropical humid paleoclimate, rather than the frigid conditions documented elsewhere during the Late Ediacaran and Early Cambrian.

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SEPM Special Publication

New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems

Steven G. Driese
Steven G. Driese
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Lee C. Nordt
Lee C. Nordt
Department of Geology, Baylor University, One Bear Place #97354, Waco, Texas 76798-7354, USA
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SEPM Society for Sedimentary Geology
Volume
104
ISBN electronic:
9781565763036
Publication date:
January 01, 2013

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