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Permian Reef Complex Virtual Field Trip
Stop III-9: Queen/Seven Rivers Fm. Collapse Breccia

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

Excellent exposures of Shattuck Sandstone (Queen Fm.) on the right (see Dunham, 1972, Stop I-5; Pray and Esteban, 1977, southwest end of Stop VIII). The sandstone has broad, channel-like structures with northwest-southeast axes. Note bimodal eolian lag(?) deposits at some horizons and small adhesion ripples, suggesting that much of the sand may have been deposited on sabkhas or interdune flats. Other sedimentary structures, in lower horizons, indicate sheet-flood or ephemeral stream sedimentation; marine trace fossils are also present in basal exposures of the sandstone. Thus, there may be an upward progression of depositional environments from marine to fluvial to eolian.

Walk uphill along the continuation of the outcrop. These medium-to thin-bedded dolomites, about 19 km (12 mi) shelfward of the Capitan scarp, are generally placed in the Seven Rivers Fm.; Sarg (1976), however, included them in the Queen Fm. Some interesting collapse breccias occur in these strata (see photo), which were included by Dunham (1972) in his "calcisphere dolomite wackestone" facies. The largest breccia occurs as an isolated pocket in a thick, light-tan dolomite bed. The breccia has large, angular clasts of dolomite with corroded and altered borders (see photo). The clasts are held in a partial matrix of microcrystalline calcite, internal sediment (green illite-kaolinite clay and quartz silt), and coarsely crystalline, blocky, late, sparry calcite cement. Considerable remnant porosity also is present in the breccia zones. The presence of the greenish clays and abundant evaporite crystal molds (see photo) indicate that these breccias are probably the result of interstratal evaporite dissolution. The presence of some zones with calcitic nodules with geopetal collapse features associated with evaporite removal (see photo) and other zones with gypsum pseudomorphs (see photo) reinforces that interpretation.

Solution collapse breccia in lower part of the Seven Rivers Formation.
Solution collapse breccia in lower part of the Seven Rivers Formation. View is from outcrops along an ephemeral stream bank just north of Dark Canyon road, eastern edge of Seven Rivers Embayment, Eddy Co., New Mexico.
© Peter A. Scholle, 1999
evaporite solution-collapse breccia
Polished rock slab showing an evaporite solution-collapse breccia in the basal part of the Seven Rivers Fm. The clasts are dolomite and residual clays, calcite cements, and epoxy fills the interstices. Dissolution and collapse probably took place during Tertiary uplift but some dissolution may also have taken place during latest Permian time. Roadside outcrop along New Mexico Highway 137 in western Rocky Arroyo, Eddy Co., New Mexico.
© Peter A. Scholle, 1999
Evaporite crystal molds in dolomicrites from the basal Seven Rivers Formation.
Evaporite crystal molds in dolomicrites from the basal Seven Rivers Formation. Loss of evaporites (dissolution and/or calcitization) is probably related to modern-cycle weathering. View is from outcrops along New Mexico Highway 137 in western Rocky Arroyo, Eddy Co., NM.
© Peter A. Scholle, 1999
irregular vugs inferred to be former evaporite nodules in the basal Seven Rivers Fm
Thin section photomicrograph (plane-polarized light) of calcite (stained pink with Alizarin Red S) filling irregular vugs inferred to be former evaporite nodules in the basal Seven Rivers Fm. Note the cloudy, geopetal fabric within the calcite — the "cloudy" material consists of kaolinite booklets that have fallen to the bottom of the vugs when the evaporites were leached. Sample from outcrop along New Mexico Highway 137 in western Rocky Arroyo, Eddy Co., New Mexico. Long axis of photograph = 3.5 cm.
© Peter A. Scholle, 1999
Calcitized evaporite rosettes in sandy dolomite of the Seven Rivers Formation.
Calcitized evaporite rosettes in sandy dolomite of the Seven Rivers Formation. Replacement by calcite is interpreted to have occurred relatively recently, in association with Late Tertiary uplift and ground water circulation. View is from outcrops along New Mexico Highway 137 in western Rocky Arroyo, Eddy Co., NM. Coin is 1.8 cm in diameter.
© Peter A. Scholle, 1999

Pray and Esteban (1977) argued for a modern karstic origin for these features; Dunham (1972) postulated a Permian origin, presumably related to weathering and dissolution of evaporite minerals during pluvial intervals. Note the abundant evidence of associated evaporite (mainly gypsum) crystal casts in these dolomites and decide for yourself when dissolution took place.

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