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

Vase-shaped microfossils (VSMs) are found globally in shales, carbonates, phosphorites, and cherts, typically occurring as casts and molds. It is hypothesized that VSM tests were originally organic in life, and VSMs are occasionally found with what appears to be the original organic wall preserved. Organically preserved VSMs were reported by B. Bloeser from shales of the Walcott Member, Chuar Group, Grand Canyon, Arizona, but Porter and Knoll (2000) suggested these may instead be siliceous casts coated in exogenous organic residue. However, they were unable to study Bloeser’s material firsthand.

We obtained the original Walcott shale samples from Bloeser’s study and performed SEM-EDS on 30 VSMs. We found that the VSMs appeared to be organically preserved, and that the degree of organic preservation varied among specimens within the same samples: two specimens had test walls composed of iron-sulfur minerals, four had walls that were entirely organic, and 28 were partly organic and partly iron-sulfur minerals. We confirmed that the iron-sulfur mineral was pyrite through preliminary Raman spectroscopic analyses.

We also found that all 30 VSMs were filled with carbonaceous masses reminiscent of bitumen, which often had mud-crack-like features and were sometimes partially attached to test walls. This suggests that this material initially filled the entire tests, then subsequently shrank and cracked. It is also of note that although Al-rich clay is limited in the shale matrix, it is abundant inside the microfossils, in the voids left between the shrunken carbonaceous masses and the test walls. Microbial activity on decaying organic matter has been shown to precipitate aluminosilicate coatings, and we hypothesize that the Al-rich clays inside the VSM tests are a result of a similar process.

Although the VSMs appear to be preserved with original organic walls, it is possible that they could be oil-filled casts, as has been recently observed in other Precambrian microfossils. Preliminary Raman spectroscopic analyses, however, suggest that the carbonaceous masses are less thermally mature than the carbonaceous walls or material in the matrix, providing evidence for a later emplacement of the masses. We suggest that VSMs of the Walcott shales are not organic-coated casts but are in fact preserved with original organic walls.