Thursday, March 28, 2019

Lower Ferron Sandstone and Juana Lopez Members of the Mancos Shale around the north side of the San Rafael Swell

In an attempt to show that the Juana Lopez Member of the Mancos Shale really does expose itself on the west side of the San Rafael Swell, I introduce some photos to help.

Lower Ferron east of Huntington (visible upper right).
Lower Ferron east of Huntington, these beds contain Prionocyclus hyatti near MoundsThey also form a small cliff under the Ferron Sandstone south of here from Ferron south to Last Chance Creek.

Juana Lopez Member east of Huntington (visible upper left)  Prionocyclus macombi concretions in middle of cliff  (right) and P. wyomingensis concretions at top of cuesta across wash.
Juana Lopez Member east of Huntington.  The same thin platy siltstone beds that appear on the east side of the swell appear here with the same platy rubble littering the slopes.  The concretion beds are also the same.

Two beds of the Lower Ferron Sandstone with large round concretions just north of Mounds.  Lower bed contains the ammonite Prionocyclus hyatti along with many other mollusks.
Around the north side the Juana Lopez is still there above the Lower Ferron.

Juana Lopez Member just west of Cat Canyon where Highway 6 and 191 cut through the Farnham Dome just east of Wellington.  Prionocyclus wyomingensis beds at top of cuesta in middle distance (Light cliff), P. macombi concretions in middle of slope.  Farnham Unit of Lower Ferron Sandstone in foreground.  Some minor faults also in foreground, one duplicating the orange bed in center.
The same thin siltstone beds, the same platy rubble.

Lower Ferron Sandstone just below dark Juana Lopez Member, 1 mile southwest of Grassy Siding.  Coon Spring Sandstone Bed at base.  Brown concretion to left center is what's left of the P. hyatti beds at Mounds.
On the east side of the Swell, the beds above the Lower Ferron have been refered to the Juana Lopez.  The only real difference I can see is a couple (2) of thin (2cm max) calcarenite beds.  South and east of Green River the Juana Lopez has more and thicker calcarenite beds.

Juana Lopez Member just west of Grassy Siding, Mt. Elliot in right distance.  Prionocyclus macombi concretions form edge of middle cuesta, P. wyomingensis concretions form orange lip of upper cuesta.

 Here on the east side are the same thin bedded siltstone beds, the same platy rubble, and the same concretion beds with the same fossils.  The siltstone beds on the west side maybe get a bit thicker, up to 3 or 4 cm, even more reason not to refer them to the Blue Gate Shale.


Dane, C. H., Cobban, W. A., and Kauffman, E. G., 1966, Stratigraphy and Regional Relationships of a Reference Section for the Juana Lopez Member, Mancos Shale, in the San Juan Basin, New Mexico, U.S.G.S. Bulletin 1224-H

Edwards, C., D. Hodgson, S. Flint, and J. Howell, 2005, Contrasting styles of shelf sediment transport and deposit in a ramp margin setting related to sea-level change and basin floor topography, Turonian (Cretaceous) Western Interior of central Utah, USA: Sedimentary Geology, v. 179

Molenaar, C. M., and Cobban, W. A., 1991, Middle Cretaceous Stratigraphy on the South and East Sides of the Uinta Basin, Northeastern Utah and Northwestern Colorado,   U.S.G.S. Bulletin 1787-P