Open-file Report 232 - New Mexico Bureau of Geology & Mineral ...

Sandstone Tongue of the Dakota. All of these tongues repyesent
open marine deposits, with the sandstones representing shfllow
marine shelf deposits (Molenaar, 1977a).
The lower part of the Mancos Shale is composed predominan
of light to dark gray shale, with subordinate amounts of tan,
fine-grained sandstone and siltstone (O’Sullivan and Beiknan,
1963). This unit was deposited in open marine conditions. It
ranges in thickness from 750 to 850 ft in San Juan County. In
San Juan County the lower part of the Mancos Shale can be
subdivided into three members: Graneros Shale(?), Greenho-n
Limestone, and Juana Lopez Members (Molenaar, 1983a). Th2
Graneros Shale Member is approximately 60 ft thick in the San
Juan Basin. It is time-equivalent with the Hartland and the
lower part of the Bridge Creek Members of the Greenhorn Limes
(Formation?) on the east side of the San Juan Basin (Moleyaar,
1977a). The Greenhorn Limestone Member 40 is to 70 ft thick in
the San Juan Basin, and according to Molenaar (1977b) it
represents the time of maximum transgression, with shxeline the
located 280 miles to the west-northwest. The Juana Lopez Member
of the Mancos Shale is composed of thinly interbedded calcareni
with abundant fossil hash, sandstone, and shale (Molenaar,
1977b). In San Juan County, the Juana Lopez Member is
unconformably overlain by Gallup Sandstone.
The first major regression and second major transgression
are not well represented in San Juan County. However, th? second
major regression of the Cretaceous seaway is characterize3 by t
Gallup Sandstone. This unit is composed of tan, brown, ayd
pinkish-gray, very fine to very coarse grained sandstone, with
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