ORGANIC PRESERVATION OF VASE-SHAPED MICROFOSSILS FROM THE LATE TONIAN CHUAR GROUP, GRAND CANYON, ARIZONA

Abstract

Author(s): Tingle, Kelly Elizabeth | Advisor(s): Porter, Susannah M | Abstract: Vase-shaped microfossils (VSMs) are a group of early eukaryotic microfossils found globally in middle Neoproterozoic (800–730 Ma) marine strata and are interpreted as the earliest evidence for testate (shell-forming) amoebozoans. VSM tests are hypothesized to have been originally organic in life, and VSMs are occasionally reported preserved with carbonaceous tests. However, many of these reports are not confirmed. Bloeser (1985) reported carbonaceous VSMs from shales of the Walcott Member, Chuar Group, Grand Canyon, Arizona, USA, but Porter and Knoll (2000) suggested these specimens were organic-coated siliceous casts after analyzing stratigraphically equivalent VSM material. The goal of this study was to re-examine the microfossils reported by Bloeser (1985) and determine if these VSMs were actually preserved with organic tests. We identified VSMs from two samples of black shales from the Walcott Member in transmitted light microscopy and used scanning electron microscopy (SEM) to image VSMs. Energy dispersive x-ray spectroscopy was used to gain a qualitative understanding of VSM composition and wavelength dispersive x-ray spectroscopy was used to assess elemental spatial variation at a finer resolution. Raman spectroscopy was used to identify mineral components and to compare the thermal maturity of carbonaceous material within the samples. Of the VSM tests sampled, 29 possess carbonaceous tests and 37 have tests that are either partially or completely replaced by pyrite and an iron-phosphate mineral, possibly vivianite. Within some tests, the pyrite has been irregularly oxidized to jarosite. Carbonaceous material is found within the tests of all VSMs from both samples, which we suspect may be bitumen generated during the Phanerozoic (Cretaceous or later) within the Walcott Shales. Based on our observations and chemical analysis, we conclude that the VSMs are preserved with original organic test material, and speculate that sulfurization may have promoted organic preservation, consistent with observations of organic VSMs associated with sulfur, high total organic carbon content, and low concentrations of iron.