Abstract

A series of ten zones based on brachiopods are defined in the late Givetian-Frasnian strata of north-central and eastern Iowa. This zonal succession is proposed as a standard biostratigraphic sequence for the U.S. Midcontinent region, and facilitates correlations between faunal sequences in western and eastern North America. The late Givetian-Frasnian strata of Iowa were deposited during four transgressive-regressive cycles represented by the Coralville, Lithograph City, Shell Rock, and Lime Creek formations.

The Strophodonta (Strophodonta) plicata Zone is proposed to encompass the fauna of the late Givetian Coralville Formation, and it correlates with the upper part of the conodont disparilis Zone. The late Givetian Strophodonta (S.) iowensis and late Givetian-early Frasnian Strophodonta (S.) n. sp. A zones are proposed within the brachiopod succession of the Lithograph City Formation. Brachiopod assemblages assigned to the first zone occur with the conodont Pandorinellina insita fauna (Lowermost asymmetricus Zone), and conodonts correlated with the Lower asymmetricus Zone occur in the upper part of the latter brachiopod zone. These three brachiopod zones are correlated with Faunal Intervals 27L (lower Tecnocyrtina Fauna), 28 (upper Tecnocytrina Fauna), and 29 (lower Allanella Fauna), respectively, proposed within the Devonian of central Nevada (Johnson & others, 1980; Johnson & Trojan, 1982; Johnson, Klapper, Murphy, & Trojan, 1986).

As currently defined (Subcommission on Devonian Stratigraphy) the Middle-Upper Devonian boundary in Iowa falls at some point within the Strophodonta n. sp. A Zone, probably at the position where the Eleutherokomma Fauna (containing E. jasperensis) enters the upper part of the Idlewild Member in north-central Iowa, and high in the upper Strophodonta Fauna of the Andalusia Member of extreme eastern Iowa.

The Shell Rock Formation is restricted to (northern and central) Iowa. Two zones are proposed within the brachiopod succession established in the Shell Rock Formation; these are the Tenticospirifer shellrockensis and Strophodonta cicatriosa zones. The first zone encompasses the fauna of most of the Mason City Member, and the second encompasses the fauna of the uppermost Mason City, Rock Grove, and Nora members. Both zones appear to correlate with Faunal Interval 30 (upper Allanella Fauna) of Nevada (Johnson & others, 1980; Johnson, Klapper, Murphy, and Trojan, 1985).

Five zones are proposed within the brachiopod succession of the type Lime Creek Formation; these are the Nervostrophia thomasi, Douvillina arcuata, Cyrtospirifer whitneyi, Elita inconsueta, and Iowatrypa owenensis zones. The conodont Palmatolepis semichatovae first appears near the base of the Nervostrophia thomasi Zone defined in the lower part of the Juniper Hill Member, and last occurs in the basal 45 cm of the Cyrtospirifer whitneyi Zone of the Cerro Gordo Member. The conodont Palmatolepis foliacea first appears in the Cerro Gordo Member at, or just above the base of the Elita inconsueta Zone.