Abstract

The Guadalupian Seven Rivers Formation (SR) is commonly thought of as a “clean” carbonate and gypsum/anhydrite unit sandwiched between the siliciclastic Shattuck Sandstone and Yates Formation on the northern shelf of the Delaware Basin. However, thin (<1 to 20 ft) siliciclastic units are present in the lower and middle parts of the shelf interior Seven Rivers Formation. The purpose of this presentation is to summarize new observations about the distribution, character, and deposition of these siliciclastic units in the Rocky Arroyo outcrop area and in the shallow subsurface of eastern Eddy County, NM.

The most prominent and correlative Seven Rivers siliciclastic unit is the Bowers Sandstone. The Bowers Sandstone comprises two siliciclastic members separated by a medial non-siliciclastic member. Each siliciclastic member changes from dolomitic siltstone facies near the shelf margin through sandstone facies in the shelf interior carbonate facies to anhydritic siltstone facies farther from the shelf margin. The Bowers sandstone to anhydritic siltstone facies transition is parallel to, but slightly updip from the regional SR dolomite-evaporite transition. The medial Bowers unit has the dolomite or evaporite lithology of surrounding SR strata.

The Bowers dolomitic siltstone facies dilutes with carbonate as it reaches the shelf edge. Bowers sandstone facies comprises about 10 ft of very fine-grained sandstone and interbedded argillaceous siltstone. On outcrop, bedding is very thin and distinctly non-planar. Sedimentary structures indicate a predominantly non-marine, episodic wadi deposition. The two sandstone units form two bands of slightly greener vegetation traceable on outcrop, but the sandstones are rarely well exposed. Bowers siltstone facies comprise poorly sorted argillaceous siltstones about 6 ft thick. On outcrop, the siltstone facies is brick red with faint, irregular lamina, possibly indicating pond deposition. The facies are typically heavily disrupted by weathering. The two Bowers siltstones form the prominent red bands in the grey SR gypsum outcrop exposures.

Other SR siliciclastic units are predominantly argillaceous siltstone. Siliciclastic units are most abundant near and just downdip from the regional SR carbonate-evaporite facies change. Most siliciclastic units thin updip from the SR regional carbonate-evaporite facies change and terminate down dip by dilution with carbonate. In areas downdip from the regional SR carbonate-evaporite facies boundary, a few siliciclastic units above the Bowers Sandstone are low-porosity sandstones, but these sandstones rarely exceed 2 feet thick.

Two persistent siliciclastic units below the Bowers Sandstone can be traced in the subsurface to the outcrop and along outcrop across the carbonate-evaporite facies transition. Where interbedded with the carbonate, these units are very fine-grained sandstone and silt-stone with sedimentary structures similar to those in the Bowers Sandstone.

Thin (1 – 10 cm) argillaceous siltstone beds on outcrop are intercalated within the peloidal dolopackstones comprising most of the shelf interior carbonate in the lower part of the SR. Some and perhaps most of the thin siltstones are dissolution lags resulting from evaporitic argillaceous pond or mud-flat deposits undergoing dissolution prior to complete lithification of the carbonate.

The SR siliciclastics were probably eolian sourced. Wind-blown silt and sand-sized material settled into ponds to form the poorly sorted argillaceous siltstones, whereas predominantly sand and coarse silt settled in subaerial settings. The sand and coarse silt was reworked into wadi and pond-margin.