266-9 Petrographic Analysis via Point Counting and Scanning Electron Microscopy of a Johnnie Formation Transect in Point of Rocks, Spring Mountains, Nevada

Session: The Neoproterozoic Earth and Life Co-evolution (Posters)

Poster Booth No.: 115

Presenting Author:

Brandon Rodriguez

Authors:

Wang, Clare¹, Lau, Andy², Jory, Billyjack³, Rodriguez, Brandon⁴

(1) Pasadena City College, Pasadena, CA, USA, (2) Pasadena City College, Pasadena, CA, USA, (3) University of California - Los Angeles, Los Angeles, CA, USA, (4) Caltech, Pasadena, CA, USA,

Abstract:

Petrographic analysis of a 60m transect below the Johnnie oolite was performed in the Spring Mountains – Point of Rocks locale, Nevada. This region has long been of interest due to its potential correlative deposition with that of the Acraman impact event in Australia and itspotential temporal tie to the largest carbon isotopic excursion (CIE) in geologic history, the Shuram-Wonoka CIE. Recent work by our group has detected locally enriched iridium at the oolite boundary at several outcrops of the Johnnie Formation in California and Nevada, potentially the result of meteoritic ejecta from the Acraman event and in line with original ejecta site geochemical analysis. However, poor age-bounding in the literature due to lack of detrital zircons in the Johnnie limestone leave much to be desired towards a reconstruction of the events towards the end of the Ediacaran, including the Acraman impact, the Gaskiers glaciation, and the Shuram-Wonoka CIE. In the absence of clear temporal fingerprints, we aim to better understand the sequence of events and the nature of the relationship between SW Laurentia and Australia via petrographic identifiers unique to the Acraman impact site, such as the Yardea dacite found within the original Bunyeroo target rock. To this end, a series of samples were collected moving down-column from the oolitic capstone, yielding nine unique sandstones and quartzites. The samples were stained and optically quantified using point counting to determine their feldspar distribution, and then analyzed via scanning electron microscopy (SEM).  Down column samples were found to exhibit roughly a 3-5:1 ratio of albite to K-spar, with one unique exception at roughly 40m below the oolite, wherein there is a sudden spike to a ratio of 14:1 albite to potassium feldspar. However, even after conducting numerous elemental maps using SEM back scatter techniques no calcium-bearing feldspars are observed, precluding the presence of residual dacite being transported via impact or tsunamigenic event for deposition within the Johnnie Formation.

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

doi: 10.1130/abs/2025AM-4320

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