LITTLE LAGERSTÄTTEN — ACRITARCH ABUNDANCES IN A CHERT NODULE FROM THE LOWER MIDDLE DEVONIAN ONONDAGA LIMESTONE NEAR PHELPS, NEW YORK, USA

Shallow, tropical seas covered much of North America during Devonian time (419-359 Ma). The lower Middle Devonian (Eifelian, 398-392 Ma) Onondaga limestone represents the carbonate bank environments that dominated the northern end of the Appalachian basin during this time. Chert nodules within the Onondaga offer excellent three-dimensional preservation of marine organic-walled microfossils; most show little or no post-depositional crushing, decay, or other damage, suggesting very early chert diagenesis. The Thermal Alteration Index (TAI) is approximately 3+/4-, which is consistent with regional thermal history.

We used transmitted light microscopy of thin sections to investigate the acritarchs preserved in a chert nodule from the Edgecliff member of the Onondaga near Phelps, in the Finger Lakes region of New York.

A total of 2,392 acritarchs were identified from a single thin section. Acritarchs were grouped into morphotypes by overall geometry (acanthomorphs, sphaeromorphs, netromorphs, and polygonomorphs), size (< 10 microns, 10-30 microns, > 30 microns), process type, and surface ornamentation. Initial results show that sphaeromorphs dominate (63%), followed by acanthomorphs (20%), and polygonomorphs (16%). Netromorphs (1%) comprise a very minor part of the assemblage. Within these large-scale groupings, some finer details may be seen. Medium and small-sized forms dominate the assemblage: medium (37%) and small (21%) sphaeromorphs are the most common, followed by medium acanthomorphs (13%) and medium polygonomorphs (9%) and small acanthomorphs (4%). Large sphaeromorphs (4%), large polygonomorphs (4%) and large acanthomorphs (2%) round out most of the rest of the assemblage. Forms larger than 60 microns are very rare.

Leiosphere-type acritarchs dominate the assemblage, but the presence of acanthomorphs suggests a shallower, rather than deeper, assemblage. High quality organic preservation coupled with early diagenesis suggests future work may include fruitful stable isotope analysis.