MIDDLE-UPPER DEVONIAN (LATE GIVETIAN-EARLY FRASNIAN) BRACHIOPOD SEQUENCE IN THE CEDAR VALLEY GROUP OF CENTRAL AND EASTERN IOWA by Jed Day Department of Geography-Geology Illinois State University Normal, Illinois 61761 INTRODUCTION characterized by the influx of cosmopolitan species. Regressive phases ofeach relative sea-level cycle (Fig. This study focuses on the composition and 3) are characterized by: restriction or elimination of biostratigraphy of the brachiopod faunas of the Little interbasinal cratonic seaway migration routes; reduction Cedar, Coralville, and Lithograph City formations of in overallgeographicextentofshallow subtidal platform the Cedar Valley Group in central and eastern Iowa habitats; evolution ofendemic taxa; and elimination of (Figs. 1, 2). Three new brachiopod zones are defined specialized taxa. Faunas of succeeding sequences are on the basis of the brachiopod sequence in the Little composed ofcarryover species, new endemic elements Cedar Formation (Fig. 3). Eighteen brachiopod faunas of lineages founded by older migrants, and new or associations are described from the Coralville and immigrants. Lithograph City formations. Revisions to the As many as 135 species, representing 49 genera stratigraphy and brachiopod zonation of Coralville and (Tables1-3)areknownfromlateGivetian-earlyFrasnian Lithograph City formations are included that are age strata ofthe Little Cedar, Coralville, and Lithograph necessitated by new paleontologic and stratigraphic City formations. Initial taxonomic revisions outlined data collected since the studies by Day (1989a, 1989b). in Table 1 indicate that the fauna of the Little Cedar Knowledge of concurrent brachiopod and conodont Formation consists at least 91 species representing 42 ranges allows for the calibration of Cedar Valley Group genera. The Little Cedar brachiopod fauna occurs in brachiopod and conodont sequences (Fig. 3), which association with conodonts spanning the interval of the provides the basis for refined correlations of the late Middle varcus Subzone-Lower subterminus Fauna (Fig. Givetian-Early Frasnian strata of the Iowa Basin with 3; Klapper and Bunker, 1984, 1985; Witzke et al., relatedsuccessionsthroughoutNorthAmerica(Figs.1, 1989; Bunker, 1989; Johnson and Klapper, 1992). 3). The brachiopod fauna ofthe Little Cedar Formation Four relative-sea-level events are recorded by strata (Table 1) is characterized by occurrences of important of the Little Cedar, Coralville, Lithograph City, and species of Rhyssochonetes, Striatochonetes, Shell Rock formations of the Cedar Valley Group. Productella, Dichacaenia, Meristella, Echinocoelia, Relationships between thelowaBasinDevonianrelative Charionella, Orthospirifer, Tylothyris, Eosyringothyris, sea-level curve of Witzke et al. (1989) and the Elita, /ndependatrypa, Neatrypa, Pseudoatrypa, Euramerican Devonian sea-level curve of Johnson et Seratrypa, Spinatrypa (Spinatrypa), Hypothyridina, al. (1985) and Johnson and Klapper (1992) are shown andAtribonium (Fig. 2). Diverse, but largely endemic, inFigure3. Cedar Valley Groupdepositionwasinitiated species groups of Strophodonta (Strophodonta), by the TaghanicOnlap(Johnson, 1970), corresponding Pentamerella, and Cranaena also serve to characterize toT-R Cycle Ila (Devonian Depophase II) of Johnson the Little Cedar fauna (Table 1). Seventy-eight etal. (1985). Duringperiodsofmaximum transgression, brachiopod species are restricted to the Little Cedar widespread migration routes (open-marine cratonic Formation. In the Iowa Basin (Figs. 1-3), fourteen seaway connections) were established between the (15%) Little Cedar species carry over into younger Iowa Basin and other North American cratonic basins deposits of the Coralville Formation, Lithograph City (Fig. 1). Formation, or younger Frasnian units (Table 1). The The lower, transgressive, portions of Cedar Valley extinctions of large numbers ofbrachiopod species in Group depositional sequences (Figs. 2, 3) are the interval corresponding to the Upper hermanni 53 MIDDLE DEVONIAN (Late Givetian) Cratonic Seaway Emergent 0 1000 Km Figure 1. Middle Devonian Qate Givetian) paleogeographic reconstruction of Euramerica showing extent of cratonic seaways established during the late Givetian sea-level rise of T-R Cycle Ila of Johnson et al. (1985), and locations of important Middle-Upper Devonian successions. Key: IB=Iowa Basin (Iowa, central and northeast Kansas, eastern Nebraska, central and northern Missouri); MB=Michigan Basin (lower Peninsula of Michigan, eastern Wisconsin), APB=Appalachian Basin, EPB=Eik Point/Williston Basins (southern Manitoba, north Dakota), OS=Ofiate Seaway (southern New Mexico), GB=Great Basin (Nevada), AB=Alberta Basin. Modified from fig. 2bofWitzke& Heckel, 1989, fig. 5 of Day, 1990a. Zone-Lower disparilis Zone? as seen in Iowa, mark a species groups/lineages founded by Little Cedar ances­ significant period of extinction of tropical, cratonic­ tors (including Athyris, Cranaena, Elita, platform, shallow-water benthic organisms in the Iowa Independatrypa, Pseudoatrypa, Pentamerella, Basin in the late Givetian, prior to the Seratrypa), or migrated into the Iowa Basin during the Frasnian-Famennian Mass Extinction. Similar Coralville transgression (e.g. Tecnocyrtina). extinction patterns at this time are noted in the records Brachiopod assemblages in strata of the lower part of oftabulateandrugosecorals(Pedder, 1977),andNorth the Coralville Formation (fable 2) occur in association American Givetian trilobite faunas (Hickerson, this with conodonts of the Upper subterminus Fauna (Fig. guidebook). 3;Bunker, 1988;Witzkeetal., 1989,Braunetal., 1989; The brachiopod fauna of the Coralville Formation Day et al., 1991). The Upper subterminus Fauna is consists of28 species representing 21 genera (fable 2). taken to correlate with a position in the upper part ofthe Taxonomic revisions of generic or species nom ens of disparilis Zone of the standard Givetian conodont Coralville brachiopod species identified in Day (1989a, sequence (Johnson et al., 1980, 1985; Witzke et al., 1989b) are summarized in Table 2 and discussed in 1989). later sections ofthis study. Most Coralville brachiopod Strata of the Lithograph City Formation (defined taxa are Little Cedar carryovers, or elements ofpersistent by Bunker et al., 1986; Witzke et al. 1989) contain a 54 Minnesota North­ A I 1 I Central lowal ~----! : : I I1 _.r----r t ----:.,_ ____ •,__ _ E :__ ~r.tt{----~ : LL I I I I -" I 1 1 I Cl) ~ : .- --~----:----T ...Cl) r----, I I I (.) N : : e 1 ~ l r Illinois Lf--~--:•- ~--~-T_ Cl) .. .. _r_. E I I I r------ _J : I 1 I ,_ ___ J __ :-----1I ·-t·---< 1 oo : a..~. r---- ... 30 I ---- -L.,. L--· : I 'r·I L- 0 • r-L., r. • I ·---- ::I c. e ... I I 0 ~ u.. I : a... 01 L---' (!) 0 > ::i 0 Q) = til > Fm. a... til "0 Q) ... ·o "0 "'CD 0 u..e ~ Basset ::i Datum-base of Cedar Valley Group Figure 2. Generalized cross-section of the late Givetian and Frasnian strata of north-central and eastern Iowa. Only the Frasnian portion of the Sweetland Creek Shale is included in the sequence shown in extreme eastern Iowa (modified from fig. 1, Witzke et al., 1989; fig. 2 of Plocher & Bunker, 1989; after fig. 1 of Day, 1989a, fig. 3 of Day, 1989b). brachiopod fauna of at least 29 species representing 19 The position of the Givetian-Frasnian genera (fable 3). Most notable of these are species of (Middle-Upper Devonian) boundary within the Allanella, Cyrtina,Devonoproductus,Eleutherokomma, Lithograph City Formation can not be established Eosyringothyris, Hadrorhynchia, Independatrypa, precisely because conodont faunas in the suspected Pseudoatrypa, Spinatrypina, Schizophoria, interval lack diagnostic species ofAncyrodella, and are Strophodonta (S.), and Tecnocyrtina. Species of these dominated by Pandorinellina insita. It is suggested genera occur in most parts of the the Lithograph City here that this boundary be provisionally placed at a Formation in Iowa and its equivalents in Missouri position corresponding to the base oftheStrophodonta ("upper Callaway Fm." of Witzke et al., 1989, and (S.) callawayensis Zone in the Andalusia Member in Snyder Creek Shale). The Lithograph City brachiopod extreme eastern Iowa (Fig. 3). fauna occurs in association with conodont faunas spanning the interval ofthe norrisi Zone (latest Givetian, see Klapper, in Johnson, 1990) through Zone 3 (early UTILE CEDAR FORMATION Frasnian) of the Frasnian conodont sequence in the Montagne Noire of southwestern France recently In its type area in north-central Iowa, the Little outlined by Klapper (1989). Cedar Formation is divided into (in ascending order) 55 Conodont Brachiopod DEVONIAN u ~ Iowa Devonian Devonian Zone/Fauna Zone STRATIGRAPHY ~ E Qualitative Eustatic CJ) W Witzke et al. Fauna oc01 0> a..~ Sea-Level T-R Cycle w (.!) (19 89) IOWA BASIN E::: c0 Curve Johnson e1 al. a:: <( Day (1990) Day ( 1989) oa. (1985) Wi1zke e1 al. (1989) ..!!! E ~ Witzke et al. Johnson B Klappe W ~ Klapper B This ~~ ft. (1989), CJ) Johnson (1990) Study (1992) ~-'-'-...._.__L...I.-L...L.(/ WJ.io~! I I I I l I I I LU t LJ\ -5 owenensis ~ Owen ( ~ ~Ll < ~ ~ (~ £/ito inconsueto Z. W~ ( ~ 1-------4 Cerro Gordo ) u ( N Cyrtospirifer W ( 7 li-d z ) :c: whitneyi Z. 1----------7\ <( Z.5 5 -g - < ;::. Douvillino orcuoto Z. E z z l-.,.,-::-:,...-:-:=-:-::-=:--1 w Q; 0 <( ( Nervostrop_hio ~ Juniper Hill 0.1 1-------4 thomosi Z. :::::i 3: ~ Z~ ___z._ 4___ ~ir~ll1l1l1l1]1~1~12~1rrTTTIII-rrrTTTIII,.r~rl c ~ ILII lJJlllliJIJ J a:: 0:: s. cicotricoso z. __, ~ 1----=-~·c;.;_ :cN_o_r_a ____ -1 W l1.. undioQnostic