Synthesis of late Paleozoic and Mesozoic eolian deposits of the ...

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the logs so the contour lines are not extended into
the subsurface.
Recapture Member of Morrison Formation
Eolian strata are most abundant in the lower
part of the Recapture Member of the Morrison
Formation in the southern part of the Colorado
Plateau (Figs. 34 and 35B), although scarce lenses
occur higher and, at one locality in the southern
part of the San Juan basin, a small lens is at the
top of the member. Because many Recapture eolian
beds are lenticular and irregularly distributed, they
could not be readily distinguished as a separate
entity that could be isopached by itself. For this
reason, the thickness of the entire member is
depicted on Fig. 34. Because some of the eolian
beds are at a stratigraphically higher position than
the Salt Wash Member, eolianites in the Recap-
ture are late Oxfordian, Kimmeridgian and early
Tithonian in age (Imlay, 1980), ranging slightly
younger than those in the Bluff-Junction Creek
complex.
The method of depicting the percent cross-
bedded eolian sandstone is different on this map
than for the other units in this study where the
percentages are based solely on surface measure-
ments. Recapture measurements made at the
surface show the percent cross-bedded eolian
sandstone reported in the literature but, because it
was felt that the underground extent of the eolian
beds should be shown, subsurface drill hole logs
had to be used even though they are not entirely
satisfactory and the results obtained from them
are not directly comparable to surface measure-
ments. The percentages obtained from the logs
were based on electric-log characteristics of the
eolian beds, which have a blocky pattern (low SP
and high resistivity) reflecting higher porosity of
the eolian sandstone compared to that in fluvial
and overbank sandstone. A distinction between
cross-bedded and flat-bedded sandstone cannot be
made by this method so the values obtained are
considered maximum possible values of eolian
sandstone in the Recapture. Although this method
has its obvious drawbacks, it yields a reasonably
accurate indication of the extent of the eolian
strata.
Where studied, the Recapture reaches a maxi-
mum thickness of at least 207 m in northeastern
Arizona (Fig. 34). Abrupt southwestward thinning
reflects beveling beneath the early Late Creta-
ceous Dakota Formation. The entire extent of the
member was not studied as only the part shown
on the map contains eolian strata.
The percent cross-bedded eolian sandstone de-
creases northeastward, southwestward and north-
westward, reflecting both facies changes and post-
depositional beveling (the member is beveled by
Cenozoic unconformities to the southeast). The
northeast and northwest drop in percentages re-
flects loss of eolian sandstone in those directions
where the eolian strata grade into beds interpreted
as fluvial, overbank floodplain, mudflat and
lacustrine in origin. Northeastward gradation into
non-eohan deposits most likely reflects subsidence
along the axis of the ancestral San Juan basin,
whose structurally deepest part lay between the
eolian deposits of the Recapture and Bluff-Junc-
tion Creek ergs in Jurassic time (Santos and
Turner-Peterson, 1986). Presumably, wetter en-
vironments inimical to eolian sedimentation ex-
isted there as a consequence of the structural and
related topographic setting.
Interpretation of the southwestward reduction
in values is obscured by southwestward regional
beveling beneath the Dakota Sandstone. The
greater part of the Recapture dune field lay in the
area between the Zuni uplift to the southwest and
the deepest part of the San Juan basin to the
northeast and its location was determined, partly
or largely, by these structural elements. Addition-
ally, the Zuni and Black Mesa uplifts appear to
have sheltered the dune field from any
northward-flowing streams that might have flowed
toward the San Juan basin from the Mogollon
highland in south-central Arizona.
Recapture eolian beds lie east and downwind
from the Late Jurassic Black Mesa and Zuni up-
lifts in northeastern Arizona (Peterson, 1986; and
this volume) and northwestern New Mexico. The
upwind position of these structures appear to have
played an important role in the genesis of the
dune field, both by diverting streams from the site
of the dune field and by providing sufficient topo-
graphic relief to disrupt atmospheric circulation