Stratigraphy of the Morrison and related formations, Colorado Plateau region, a preliminary report

Three subdivisions of the Jurassic rocks of the Colorado Plateau region are:
the Glen Canyon group, mainly eolian and fluvial sedimentary rocks; the San
Rafael group, marine and marginal marine sedimentary rocks; and the Morrison
formation, fluvial and lacustrine sedimentary rocks.

In central and eastern Colorado the Morrison formation has not been differ-
entiated into members. In eastern Utah, northeastern Arizona, northwestern
New Mexico, and in part of western Colorado, the Morrison may be divided
into a lower part and an upper part; each part has two members which are
di1Ierentiated on a lithologic basis.

Where differentiated, the lower part of the Morrison consists either of the
Salt Wash member or the Recapture member or both; these are equivalent
in age and inter tongue and intergrade over a broad area in the vicinity of the
Four Corners area of New Mexico, Colorado, Arizona, and Utah. The Salt
Wash member is present in eastern Utah and parts of western Colorado, north-
eastern Arizona, and northwestern New Mexico. It was formed as a large
alluvial plain or 'fan' by an aggrading system of braided streams diverging
to the north and east from an apex in south-central Utah. The major source
area of the Salt Wash was to the southwest of south-central Utah, probably in
west-central Arizona and southeastern California. The member was derived
mainly from sedimentary rocks. The Salt Wash deposits grade from predomi-
nantly coarse texture at the apex of the 'fan' to predominantly flne texture at
the margin of the 'fan'.

The Salt Wash member has been arbitrarily divided into four facies: a con-
glomera tic sandstone facies, a sandstone and mudstone facies, a claystone and
lenticular sandstone facies, and a claystone and limestone facies.

The Recapture member of the Morrison formation is present in northeastern
Arizona, northwestern New Mexico, and small areas of southeastern Utah
and southwestern Colorado near the Four Corners. It was formed as a large
alluvial plain or 'fan' by an aggrading system of braided streams. The Recap-
ture deposits grade from predominantly coarse texture sedimentary rocks to
predominantly fine texture and have been arbitrarily divided into three facies:
a conglomeratic sandstone facies, a sandstone facies, and a claystone and
sandstone facies. The distribution of the facies indicates that the major source area of the Recapture was south of Gallup, N. Mex., probably in west-central New Mexico. The Recapture was derived from an area of intrusive and
extrusive igneous rocks, metamorphic rocks, and sedimentary rocks.

The upper part of the Morrison formation consists of the Westwater Canyon
member and the Brushy Basin member. The Westwater Canyon member forms
the lower portion of the upper part of the Morrison in northeastern Arizona,
northwestern New Mexico, and places in southeastern Utah and southwestern
Colorade near the Four Corners, and it intertongues and intergrades northward
into the Brushy Basin member. The Westwater Canyon member was formed
as a large alluvial plain or 'fan' by an aggrading system of braided streams.
The Westwater deposits grade from predominantly coarse-textured sedimentary
rocks to somewhat finer textured sedimentary rocks, and have been arbitrarily
divided into two facies: a conglomeratic sandstone facies and a sandstone
facies. The distribution of the facies indicates that the major source area
of the Westwater was south of Gallup, N. Mex., probably in west-central New
Mexico. The Westwater was derived from an area of intrusive and extrusive
igneous rocks, metamorphic rocks, and sedimentary rocks. The similarity of
the distribution and composition of the Westwater to the Recapture indicates
that the Westwater represents essentially a continuation of deposition on the
Recapture 'fan'; the Westwater contains, however, considerably coarser
materials.

Whereas the S