The Metaline Formation is a Cambrian unit that crops out in Stevens and Pend Oreille Counties, northeastern Washington. The 985 to 1,250-m (3,200 to 4,100-ft) thick formation has been divided into lower, middle, and upper members, although structural complications and numerous covered intervals have made measurement of complete stratigraphic sections impossible. Based mainly on trilobite studies, early workers assigned a Middle Cambrian age to the 120 to 290-m (390 to 950-ft) thick lower member. The present study divides the lower member into the following five lithofacies: (1) gray mudstone, (2) ooid-arenite, (3) gray packstone-wackestone, (4) black packstone-wackestone, and (5) black mudstone facies.

The gray mudstone facies grades upward from underlying fine-grained siliciclastics, and it is composed of bioturbated lime mudstones with a small admixture of thin-shelled trilobite fragments, chitinophosphatic inarticulate brachiopods, and disarticulate echnioderms. The ooid-arenite facies occurs as pods within the gray mudstone facies, and is composed of spherical quartz grains and ooids developed around quartz cores in a neomorphic spar matrix. The facies is extremely well-sorted. The gray packstone-wackestone facies is composed of ooid-oncoid packstones alternating with argillaceous, fossiliferous wackestones. Rounded intraclasts and peloids are secondary components. Fossil allochems include robust trilobite fragments, echinoderm plates, and rare (?) Epiphyton algae. The black pac stone-wackestone facies contains the same components as underlying rocks but is characterized by an increase in carbonaceous material and pyrite causing the black color. Fossils are less robust than in the underlying facies. The black mudstone facies is characterized by bioturbated lime mudstones with rare fossil fragments. The top of the member is marked by a red-stained, well-cemented zone below a disconformable contact with the middle member.

The lower member of the Metaline Formation represents the first carbonates deposited on a ramp-type shelf margin during a major Middle Cambrian transgression. The observed lithologies suggest deposition under conditions of changing water depth, agitation, and oxygenation in the shallow subtidal zone. The mudstones and wackestones in all facies were deposited under low-energy conditions. Shallowing allowed increased agitation, and oxygenation suitable for the local development of ooids and oncoids. Periodic storms produced high-energy packstone deposits composed of concentrations of the components found in the typically low-energy subtidal zone.

Along with the transgression of marine environments, oxygen-starved waters migrated shoreward from an offshore, lower shelf basin. This transgression of the pycnocline caused dysaerobic and anaerobic conditions in shallow subtidal waters of the upper shelf. Aerobic and dysaerobic conditions produced gray rocks, and anaerobic conditions produced black rocks rich in organic material. Biofacies were also affected by the low oxygen levels. Thin-shelled and chitinophosphatic forms dominated the epifauna when dysaerobic conditions occurred. With decreasing oxygen content, the epifauna was progressively excluded from the shallow subtidal zone. The result of maximum anoxia was the cessation of carbonate sedimentation and the formation of a submarine hardground at the end of "lower Metaline de osition."

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