Lower Pennsylvanian (Morrowan) Ammonoids from the North American Midcontinent
Authors: Manger Walter L. , Saunders W. Bruce 
Published: May 2019

The Lower Pennsylvanian Hale Formation, which comprises the lower portion of the type Morrowan Series, in northwestern Arkansas, is subdivided into the Cane Hill and overlying Prairie Grove Members. The Cane Hill Member includes interbedded shale and sandstone with a basal conglomerate containing clasts reworked from underlying, truncated Mississippian formations. The Prairie Grove Member is highly variable, but includes sandy biosparite and calcareous sandstone. Highly fossiliferous pebble conglomerate and calcirudite lenses occur sporadically throughout the Hale Formation. Ammonoids and conodonts show that the Cane Hill-Prairie Grove boundary is unconformable.

Several thousand ammonoids collected from more than 100 localities in the Hale Formation show that four ammonoid zones and two subzones are recognizable in the Hale succession, and consequently in the redefined Halian Stage of the Morrowan Series. These are, in ascending order, theRetites semiretia, Quinnites henbesti, Arkanites relictus(including theArkanites relictus relictusand overlyingCancelloceras huntsvillenseSubzones) andVerneuilites pygmaeusZones. Halian Stage ammonoids are known primarily from northern Arkansas, but an upperArkanites relictusZone (Cancelloceras huntsvillenseSubzone) ammonoid assemblage occurs in the Primrose Member of the Golf Course Formation in south central Oklahoma.

Conodont-ammonoid associations in the Hale sequence provide a basis for integration of independently based zonal information.Rhachistognathus primusZone conodonts occur in theRetites semiretiaZone; theIdiognathoides sinuatusZone ranges through theQuinnites henbestiandArkanites relictus relictusSubzone. The overlyingCancelloceras huntsvillenseSubzone andVerneuilites pygmaeusZone both contain conodonts of theNeognathodus symmetricusZone.

The Hale ammonoid succession has few, if any, species in common with the type Namurian of Europe, but numerous genera are common to both sequences and the generic successions coincide and are equivalent in degree of development. TheRetites semiretiaZone is equivalent to theReticuloceras circumplicatileZone (R1a); theQuinnites henbestiand lowerArkanites relictusZones correspond to some portion of the R2b–R2c interval; and the upperArkanites relictusZone and theVerneuilites pygmaeusZone correlate to Zone G1. TheRetites semiretiaZone correlates to the lowerReticuloceras-BaschkortocerasGenozone (Nm2b1) of the upper Namurian in the south Urals; theQuinnites henbestiZone is equivalent to some portion of the Nm2b2–3intervals of this zone; the lowerArkanites relictusZone is equivalent to the lowerBilinguites-CancellocerasGenozone (Nm2c1) and the upperArkanites relictusandVerneuilites pygmaeusZones correspond to the uppermost interval (Nm2c2) in the south Urals sequence.

Systematic descriptions of biostratigraphically significant Halian taxa, includingReticuloceras tiroGordon,R. wainwrightiQuinn,Retites semiretiaMcCaleb,Arkanites relictus relictus(Quinn, McCaleb, and Webb),A. relictus redivivusn.subsp.,Quinnitesn.gen. (type speciesQ. henbesti(Gordon)),Q. textum(Gordon),Bilinguites eliasin.sp.,Cancelloceras huntsvillensen.sp., andVerneuilites pygmaeus(Mather) are also presented.