BIOFACIES AND DEPTH OF DEPOSITION OF THE DECORAH SHALE, TWIN CITIES BASIN

The Twin Cities Basin is a section of the 1.1 Gy Keweenawan Rift, bounded on the northwest by the 1.1 Gy Penokean Mountains, to the Southwest by the Belle Plain Fault, and the southeast by the Hudson-Afton Horst. The early Chatfieldian (454-451My) Decorah Shale has its depth of deposition determined by the rate of subsidence of the rift, the rate of sediment supply from the Penokeans, and the sea level.

The depth of deposition of the Decorah can easily be estimated from the biofacies. Because the red light, on which green (Chlorophyta) algae depend for photosynthesis is extinguished at shallower depths than other wavelengths, these algae are usually restricted to depths less than 50 m. Chatterton and Ludvigsen (1976) developed a classification of Ordovician trilobite biofacies which they interpreted as a bathymetric gradient. From shallowest to deepest these are (1) Bathyurus biofacies; (2) Isotelus biofacies;(3) Calyptaulax-Ceraurinella biofacies, typified by those genera and Sphaeroexochus and Cybeloides; and (4) Dimeropyge biofacies, typified by that genus and Dolichoharpes, as well as by the acanthoparyphinids and Carrickia. The fourth facies is about 50 meters deep on the basis of coexisting small green algae

Cisne and others (1984) investigated the depth estimates inferred from variations in biofacies. The extremes of their biofacial components were bryozoans and Doleroides for shallower and Paucicrura and Sowerbyella for deeper.

The ratio of benthic mollusks to articulate brachiopods represents a depth gradient, with the benthic mollusks a shallower and articulate brachiopods a deeper facies. This is best measured by absolute counts of specimens, but can also be measured by numbers of taxa. The simple ratio of articulate brachiopods divided by the sum of clams and snails agrees with other estimates. The reason appears to be the excellent methods of clearing gills of mud in mollusks versus the lack of an anus in articulate brachiopods. Lingulids prefer shallower depths than Articulates.

Depth of deposition of the Decorah varies from 0 to 30 meters, mean 15 meters.