25 D18. Recent Advances in Paleontology II (Posters)

Sunday, 10 October 2021
9:00 AM - 1:00 PM
Exhibit Hall A

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

Paper number: 25-10
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-368484

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