By: B. J. Kowallis, E. H. Christiansen, and A. Deino
Precise correlation of volcanic ash beds, common over wide areas in continental and marine sedimentary sections, has wide application in stratigraphic, tectonic, and paleontologic studies. To establish a protocol for direct correlation of ash beds, we examined two bentonitic volcanic ashes from the uppermost Cenomanian strata of the lower Bridge Creek Limestone Member of the Greenhorn Formation near Pueblo, Colorado. The same ashes were sampled at several other localities from central Colorado to southwestern Utah from the time-equivalent lower Mancos Shale in the Four Corners region and lower Tropic Shale farther west into Utah.
The goal of this study was to establish a protocol for correlation of altered bentonitic Mesozoic ashes. Izett established a number of criteria that could be used for correlation in young ashes--relative and absolute age, mineralogy, chemical composition, glass shard morphology, uranium distribution in zircons, etc--but these characteristics, in addition to other features of the ash, needed to be evaluated for altered Mesozoic ashes. We have evaluated chemical composition, zircon morphology, absolute age, phenocryst type and abundance, abundance of inclusions in zircon and apatite, and uranium distribution in zircon and apatite for two ashes collected from several localities in the Upper Cretaceous Tropic Shale.
Pages: 22 p.
Location: Kane County and San Juan County
Media Type: Paper Publication