Syntectonic sedimentation in the East
Kaibab Monocline: Evidence for the timing of the onset of Laramide
deformation in South-Central Utah
Storm, Lauren P.,
Tindall, Sarah E., Simpson,
Edward L., and Wizevich, Michael C.,, 2008, Syntectonic
sedimentation
in
the
East Kaibab Monocline: Evidence for the timing of
the onset of Laramide deformation in South-Central Utah [abs]:
Cordilleran Section (104th Annual) and Rocky Mountain Section (60th
Annual) Joint Meeting (19–21 March 2008), Geological Society of
America Abstracts with Programs, Vol. 40, No. 1, p. 65.
The
East Kaibab monocline on the Colorado Plateau formed by faulting and
folding during the Late Cretaceous and early Tertiary Laramide orogeny.
Two northwest-dipping faults, each with ~0.5 km of right-lateral
separation, offset Cretaceous strata in the steep limb of the
monocline, where it trends northward along the western margin of the
Kaiparowits basin in southern Utah. Aerial photographs and maps of the
East Kaibab monocline show apparent differences in thickness of the
upper member of the Cretaceous Wahweap Formation across each fault
trace. Changes in true thickness across the faults would indicate that
the area experienced syntectonic sedimentation during Cretaceous
faulting, and that the faults were originally southeast-dipping normal
faults associated with the onset of monoclinal uplift. If this
hypothesis is correct, then sediments should increase in thickness from
north to south across each fault trace. Alternatively, it is possible
that faulting occurred after the deposition of sediment, and
differences in outcrop width represent dip variation and structural
position within the monocline. To evaluate the importance of true
thickness variation versus structural position, we collected GPS
locations and bedding orientations across the upper member along three
transects: 1) north of the faults; 2) between the faults; and 3) south
of the two faults. Data were used to construct cross sections showing
dip variation and structural position within the steep monoclinal limb.
Precise measurement of the upper member thickness revealed that the
thinnest sediment package (92m) lies north of the faults. Between the
faults, the upper member is 132m thick, and south of the southern fault
it is 262m thick. Our data confirm that faulting was active during the
deposition of the upper member of the Wahweap Formation, causing
thickness variations across normal faults immediately before monoclinal
uplift. This newly-recognized syntectonic sedimentation associated with
the East Kaibab monocline pinpoints the timing of onset of Laramide
deformation on the western margin of the Colorado Plateau.
