Upper Devonian and Lower–Middle Mississippian strata of the North American midcontinent are ubiquitously fine-grained and silt-rich, comprising both so-called shale as well as argillaceous limestone (or calcareous siltstone) that accumulated in the Laurentian epeiric sea. Although long recognized as recording marine deposition, the origin and transport of the fine-grained siliciclastic material in these units remains enigmatic because they do not connect to any proximal deltaic feeder systems. Here, we present new data on grain size, whole-rock geochemistry, mineralogy, and U-Pb detrital-zircon geochronology from units across Oklahoma; we then integrate these data with models of surface wind circulation, refined paleogeographic reconstructions, and correlations from the greater midcontinent to test the hypothesis that wind transported the siliciclastic fraction to the marine system. The exclusively very fine silt to very fine sand grain size, clear detrital origin, widespread distribution over large regions of the epeiric sea, Appalachian sources, and paleogeographic setting in the subtropical arid belt far-removed from contemporaneous deltaic feeder systems are most consistent with eolian transport of dust lofted from subaerial delta plains of the greater Appalachian orogen and incorporated into subaqueous depositional systems. Delivery of dust that was minimally chemically weathered to Devono-Mississippian epeiric seas likely provided essential nutrients that stimulated organic productivity in these commonly organic-rich units.
The origin and production of silt are key factors in the formation of
loess deposits. Although many processes can potentially lead to silt
generation, few are known to produce silt in the volumes and
particle-size modes required to form geologically significant loess
deposits. Here we investigate the hypothesis that pedogenic weathering
in tropical and Mediterranean climates can generate abundant in situ
silt, and therefore contribute significantly to loess formation
throughout geologic time. We utilize granulometric and geochemical data
from soils formed in Puerto Rico (hot-humid) and Southern California
(hot-arid) to discern whether the mud fraction (<62.5 μm) is
generated from bedrock (autochthonous) or sourced from eolian
contributions (allochthonous). Our study demonstrates that the Puerto
Rico soil contains abundant (up to 72%) silt- and clay-sized grains
compared to the Anza Borrego soil (<6%). However, the silt
fraction of the Puerto Rico soil is at least partially derived from
eolian inputs, and the silt fraction of the Anza Borrego soil is
geochemically indistinguishable from allochthonous dust sources.
Furthermore, while intense chemical weathering in a tropical climate can
produce abundant fines, the majority are significantly finer (average
mode ~15 µm) than the modes of most “typical” loess
deposits (modes more than 20 – 30 µm). In contrast, weathering in
Mediterranean climates produces volumetrically sparse silt. Hence,
pedogenic weathering in hot climates appears to be ineffectual for
producing the volume and size distributions of silt-sized material
needed to generate significant loess deposits.
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