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, the Retites semiretia, Quinnites henbesti, Arkanites relictus (including the Arkanites relictus relictus and overlying Cancelloceras huntsvillense Subzones) and Verneuilites pygmaeus Zones. Halian Stage ammonoids are known primarily from northern Arkansas, but an upper Arkanites relictus Zone (Cancelloceras huntsvillense Subzone) 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 primus Zone conodonts occur in the Retites semiretia Zone; the Idiognathoides sinuatus Zone ranges through the Quinnites henbesti and Arkanites relictus relictus Subzone. The overlying Cancelloceras huntsvillense Subzone and Verneuilites pygmaeus Zone both contain conodonts of the Neognathodus symmetricus Zone.
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. The Retites semiretia Zone is equivalent to the Reticuloceras circumplicatile Zone (R1a); the Quinnites henbesti and lower Arkanites relictus Zones correspond to some portion of the R2b–R2c interval; and the upper Arkanites relictus Zone and the Verneuilites pygmaeus Zone correlate to Zone G1. The Retites semiretia Zone correlates to the lower Reticuloceras-Baschkortoceras Genozone (Nm2b1) of the upper Namurian in the south Urals; the Quinnites henbesti Zone is equivalent to some portion of the Nm2b2–3 intervals of this zone; the lower Arkanites relictus Zone is equivalent to the lower Bilinguites-Cancelloceras Genozone (Nm2c1) and the upper Arkanites relictus and Verneuilites pygmaeus Zones correspond to the uppermost interval (Nm2c2) in the south Urals sequence.
Systematic descriptions of biostratigraphically significant Halian taxa, including Reticuloceras tiro Gordon, R. wainwrighti Quinn, Retites semiretia McCaleb, Arkanites relictus relictus (Quinn, McCaleb, and Webb), A. relictus redivivus n.subsp., Quinnites n.gen. (type species Q. henbesti (Gordon)), Q. textum (Gordon), Bilinguites eliasi n.sp., Cancelloceras huntsvillense n.sp., and Verneuilites pygmaeus (Mather) are also presented.