Early Cambrian Humid, Tropical, Coastal Paleosols from Montana, USA
Gregory J. Retallack, 2013. "Early Cambrian Humid, Tropical, Coastal Paleosols from Montana, USA", New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems, Steven G. Driese, Lee C. Nordt
Download citation file:
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.
Figures & Tables
New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems
After initial breakthroughs in the discovery of fossil soils, or paleosols in the 1970s and early 1980s, the last several decades of intensified research have revealed the much greater role that these deposits can play in reconstructing ancient Earth surface systems. Research currently focuses on terrestrial paleoclimatology, in which climates of the past are reconstructed at temporal scales ranging from hundreds to millions of years, using paleosols as archives of that information. Such research requires interdisciplinary study of soils conducted in both modern and ancient environments. These issues and many others were discussed at the joint SEPM-NSF Workshop “Paleosols and Soil Surface Analog Systems”, held at Petrified Forest National Park.