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Permian Reef Complex Virtual Field Trip
Stop III-6: Seven Rivers Fm. Gypsum Deposits

— Peter Scholle
Geographic location and stratigraphic position of this stop.
Geographic location and stratigraphic position of this stop.

Borrow pit in gypsum of Seven Rivers Formation. Surface weathering makes viewing of gypsum outcrops a frustrating exercise; most are altered to a very great degree with a solution residue covering most fabric elements. This borrow pit exposes the freshest samples easily accessible to a field trip group. Gray-white gypsum with a nodular, enterolithic texture (see photo) can be seen in isolated blocks scattered around the pit. This "chicken-wire" fabric may be related to a sabkha origin of the evaporite, but may also be a consequence of dehydration-rehydration reactions during burial and uplift (although the unit is gypsum on outcrop, it is generally anhydrite in the subsurface). The "chicken-wire" texture, with thin clay films between gypsum nodules, also been interpreted as the product of displacive growth of subaqueous gypsum in silty-clayey sediments in a shallow-water lagoon (Sarg, in Pray and Esteban, 1977). Other blocks show fine examples of gypsum rosettes (see photo). These clusters of crystals characterize bottom-nucleated gypsum growth from standing brine bodies in shallow salina settings.

Massive nodular gypsum in the Seven Rivers Formation.
Massive nodular gypsum in the Seven Rivers Formation. Although nodular fabrics usually are interpreted as indicative of formation in coastal sabkha settings, such nodular structures can also be produced diagenetically in evaporites of other origins. These evaporites probably have been converted from anhydrite to gypsum during Tertiary/Quaternary uplift. Outcrop in borrow pit along Dark Canyon road near intersection with New Mexico Highway 137, Eddy Co, NM.
© Peter A. Scholle, 1999
Gypsum rosette fabrics from evaporites facies of the Seven Rivers Formation.
Gypsum rosette fabrics from evaporites facies of the Seven Rivers Formation. These are bottom- nucleated crystals that grew into shallow, standing water bodies (salinas). Outcrop in borrow pit along Dark Canyon road near intersection with New Mexico Highway 137, Eddy Co, NM. Coin is 1.9 cm in diameter.
© Peter A. Scholle, 1999
Nodular of red siltstone and nodular gypsum from the Seven Rivers Formation
Nodular of red siltstone and nodular gypsum from the Seven Rivers Formation. Siltstone shows vague horizontal lamination, largely disrupted by growth of evaporite minerals. Outcrop in Seven Rivers embayment on New Mexico Highway 137 about 10 km north of the intersection with Dark Canyon road, Eddy Co., NM.
© Peter A. Scholle, 1999

On the left, one can see a reentrant of the Seven Rivers Embayment, an extensive planar topographic feature developed by dissolution of the evaporites of the Seven Rivers Fm. The reddish hillslopes consist of silty redbeds with bedded or nodular gypsum (see photo) although these are very difficult to examine because the slopes typically are covered with a thick residue of evaporite dissolution.

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