Is stage of Lower Cretaceous Series. Geochronologic work indicates that time-span of stage may have been 18 m.y. or more. K-Ar dating of two biotite-bearing bentonites collected along northwest flank of Black Hills uplift [eastern Powder River basin], WY. Upper bentonite (99.6 Ma) from base of Newcastle Sandstone lies in barren interval between HAPLOPHRAGMOIDES GIGAS (below) and MILIAMMINA MANITOBENSIS (above) foraminiferal zones. Lower bentonite (104.4 Ma) from bottom of Skull Creek Shale lies near base of H. GIGAS Zone. H. GIGAS Zone is younger than GASTROPLITES Zone (ammonite), and lower part of M. MANITOBLENSIS Zone is older than NEOGASTROPLITES Zone (ammonite). GASTROPLITES fauna is of earliest late Albian age. Therefore, middle-late Albian boundary is older than 104 m.y. With Albian-Cenomanian boundary at 96 Ma, the late Albian spans at least 8 m.y. A preliminary K-Ar age of 108 Ma on plagioclase from a third bentonite in middle Albian of British Columbia, CN indicates latter half of Albian may have spanned 12 m.y.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Revised NEOGASTROPLITES ammonite zonation by Cobban and Kennedy (1989) interprets Albian-Cenomanian boundary to fall possibly as low as base of the N. cornutus zone. * and ** denote zones with reversed order from previous zonation; [Note: the original stratigraphic order of Reeside and Cobban (1960) has been found to be correct according to discussion with Cobban this date (12/16/93)].
Neogastroplites zonations [We apologise. The figure for this synopsis is being reconstructed.]

Source: GNU records (USGS DDS-6; Denver GNULEX).