- The type section of the Mascall Formation, which is located in
the John Day Valley, is interpreted to represent a sequence of
paleosols. These fossil soils were formed on a floodplain during the
middle Miocene. The measured thickness of this section is 1340 feet,
and although the top of the section is truncated by an erosion
surface, the original thickness was probably not much more than
2000 feet. Sediment accumulation rates were high in the vicinity of
the type section with deposits being predominantly of the overbank
type. Minimum sediment accumulation time at the type section is
thought to have been several hundred thousand years.
A concretionary horizon, which occurs within the type section is
determined to represent a significant temporal hiatus. Because of the
absence of caliche in this layer and elsewhere in the type section,
and because of the occurrence of moisture-loving plants, a wet,
temperate climate is envisioned for the type section during the middle
The floodplain sediments of the type section are predominantly
composed of ash which was produced by nearby silicic volcanism. This
ash was mostly washed in from the surrounding highlands, but on
occasion the floodplain was blanketed by air fall debris. Scanning
electron microscopy demonstrates that this ash is of the type erupted
by plinian and pelean type volcanoes. The ash has been mostly altered
to clay minerals, and SEM, TEM, and XRD analyses, show these clays to
consist principally of smectite (Ca, Mg), with lesser amounts of
kaolin and tubular halloysite.
Deposits west of Picture Gorge are predominantly of floodplain
origin; however, a limited lacustrine sequence also occurs. East of
the type section the floodplain deposits tend to become coarser and
reflect main channel deposition.
Farther to the east, near the mouth of Fields Creek a 300 foot
thick lacustrine sequence occurs, representing a shallow eutrophic
lake which was at least 3.5 miles in east-west dimension.
Mascall deposits of the Paulina Basin also were formed in a
floodplain environment. The area was characterized by slow sediment
accumulation rates, and river meandering resulted in deposition of
large tabular sandstone bodies. Meander loop cut-off probably
occurred often, and as a result the floodplain was probably dotted
with oxbow lakes. Volcanoes were active nearby, and on occasion
covered the floodplain with pyroclastic debris.
The Mascall Formation is believed to have been deposited only in
the structural and topographic lows of the time. Present occurrences
of Mascall rocks in the John Day Valley, Paulina Basin, Fox Basin, and
Miocene rocks which may or may not be Mascall in the Bear Valley and
Unity Basin, are not the remnants of a huge alluvial fan. Rather each
of these structurally lows was filled in by sediment and pyroclastic
material from their respective adjacent highlands.
Pumices ranging from white to black in color, representing a
zoned eruption, were collected from Mascall deposits in the John Day
Valley. Chemical analysis of these pumices precludes magma mixing as
a means for producing the zoned eruption. It is not known whether
crystal fractionation or assimilation of wall rock material
represented the mechanism involved. High K₂0 values in the Mascall
pumice show that the magma had a continental source.