Paleocurrents of the Middle–Upper Jurassic strata in the Paradox Basin,
Colorado, inferred from anisotropy of magnetic susceptibility (AMS)
Abstract
The Middle–Upper Jurassic sedimentary strata in the southwestern
Colorado Plateau recorded pervasive eolian to fluvio-lacustrine
deposition in the Paradox Basin. While paleocurrents preserved in the
Entrada Sandstone, an eolian deposition in the Middle Jurassic, has been
well constrained and show a northwesterly to northeasterly migration of
ergs from the south onto the Colorado Plateau, there is yet no clear
resolution of the paleocurrents preserved in the Wanakah Formation and
Tidwell Member of the Morrison Formation, both of which are important
sedimentary sequences in the paleogeographic framework of the Colorado
Plateau. New U–Pb detrital zircon geochronology of sandstones from
these sequences suggests that an abrupt change in provenance occurred in
the early Late Jurassic, with sediments largely sourced from eroding
highlands in central Colorado. We measured the anisotropy of magnetic
susceptibility (AMS) of sediments in oriented sandstone samples from
these three successive sequences; first, to determine the paleocurrents
from the orientations of the AMS fabrics in order to delineate the
source area and sediments dispersal pattern and second, to determine the
depositional mechanisms of the sediments. Preliminary AMS data from two
study sites show consistency and clustering of the AMS axes in all the
sedimentary sequences. The orientations of the Kmin – Kint planes in
the Entrada Sandstone sample point to a NNE–NNW paleocurrent
directions, which is in agreement with earlier studies. The orientations
of the Kmin – Kint planes in the Wanakah Formation and Tidwell Member
samples show W–SW trending paleocurrent directions, corroborating our
hypothesis of a shift in provenance to the eroding Ancestral Front Range
Mountain, located northeast of the Paradox Basin, during the Late
Jurassic. Isothermal remanence magnetization (IRM) of the samples
indicate that the primary AMS carriers are detrital, syndepositional
ferromagnetic minerals. Thus, we contend that AMS can be successfully
deployed in constraining paleocurrents in lacustrine sedimentary strata,
which lacks traditional sedimentary structures for paleocurrent
analyses.