253-1 The base of the Ordovician System is defined by a Global Stratigraphic Section and Point and two Auxiliary Stratigraphic Sections and Points, but the three boundaries are at three different stratigraphic horizons

Session: Conodonts from North America and Beyond - Honoring the Career of Dr. James E. Barrick

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Abstract:

An international committee formed to choose a Global Stratigraphic Section and Point for the base of the Ordovician. In 1985 they decided conodonts would be the main group to define the boundary.  A GSSP was defined in Newfoundland in 2001 and Auxiliary Stratigraphic Sections and Point in Utah in 2017 and in China in 2021. The Utah ASSP has the best conodont fauna, with ~31,000 conodont specimens and 14 biozonal units from 59 samples in 206m of limestone.  The boundary at 160.6m is at the First Occurrence Datum of Iapetognathus fluctivagus and also of its ancestor, Iapetonudus ibexensis. The section has good Laurentian trilobite faunas and included the globally distributed basal Tremadocian olenid Jujuyaspis at 0.9m above the FAD of I. fluctivagus. Other fossils include brachiopods. An excellent carbon-isotope profile has its highest positive peak a few cm below the FAD of I. fluctivagus. The section was rejected as the GSSP due to no graptolites but was designated as ASSP in 2017. 

The Newfoundland GSSP has several flaws. It is a base-of-slope facies with debris flows and grainstone flows. The few conodonts below the GSSP boundary do not include taxa confined to the Cambrian except for several redeposited above the GSSP. Only two beds have trilobites, and both are debris flow breccias. The debris flow several meters below the GSSP has trilobites in clasts representing three different trilobite zones. The GSSP is in Bed 23 and is above a twice-published occurrences of I. fluctivagus in Bed 22.  A bedding-plane fault separates the occurrences in Beds 22 and 23.  The FAD of Cordylodus lindstromi is at the designated GSSP, but its FAD is at the next lower zonal boundary globally.  This co-occurrence results from a poor conodont succession below the GSSP.

The Chinese established a second ASSP below the FAD of C. lindstromi, which globally is a full subzone below the FAD of I. fluctivagus.  Their ASSP boundary thus does not corelate with the FAD of I. fluctivagus. The choice of an ASSP in the Chinese section demonstrates the poor quality of the GSSP, which was chosen for its excellence planktonic graptolites, which begin 4.7m above the GSSP horizon.

None of these three boundaries is at the same stratigraphic level. The International Commission on Stratigraphy should reconsider these three boundaries.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6868

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