Petrography, Mineralogy, and Geochemistry of Greenish- and Reddish-Gray Siltstones Straddling the Vertebrate-Defined Permian–Triassic Boundary in South Africa

Author (Your Name)

Jiawen Li, Colby CollegeFollow

Date of Award

2016

Document Type

Honors Thesis (Open Access)

Department

Colby College. Geology Dept.

Advisor(s)

Robert A. Gastaldo

Second Advisor

Johann Neveling

Third Advisor

Walter A. Sullivan

Abstract

The Permian–Triassic Mass Extinction (PTME) is considered the largest in Earth’s history, yet the terrestrial response is not well understood. The Karoo Basin, South Africa, hosts abundant exposures of terrestrial deposits that span the Permian– Triassic Boundary (PTB). The terrestrial PTB is defined by a turnover in vertebrate assemblage zones and has been associated with a transition from greenish-gray to reddish-gray siltstone. The current PTME model states that the siltstone color change is a reflection of aridification and eolian deposition, which is believed to represent rapid climate change directly linked with the event. Greenish- and reddish-gray siltstones were collected from below, at, and above the vertebrate-defined PTB at Old Lootsberg Pass, Eastern Cape Province, and Bethulie, Free State Province, to test this hypothesis.

Samples are examined petrographically, mineralogically, and geochemically. The presence of primary structures attributed to fluvial processes indicates in-channel, fluvial origin of most samples. Greenish- and reddish-gray siltstones are chemically indistinguishable for most major elements, except for Na, Ti, and Mn. Mössbauer spectroscopy reveals that the reddish-gray color is likely due to the presence of finely- dispersed hematite, not a lower Fe2+/Fe3+ ratio. Hematite primarily occurs as coatings on illite and chlorite. Titanite, rutile, and ilmenite are also found in the reddish-gray siltstones. CIA-K values indicate that these rocks represent deposits in environments with a constantly high water table. Combined, the results indicate that the reddish-gray coloration is secondary to greenish-gray color, and is a product of diagenetic hematite formation under saturated-sediment conditions, likely due to interaction with water in the environments.

Comments

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Keywords

Karoo Basin, Permian-Triassic Mass Extinction, climate, siltstone coloration, hematite

Recommended Citation

Li, Jiawen, "Petrography, Mineralogy, and Geochemistry of Greenish- and Reddish-Gray Siltstones Straddling the Vertebrate-Defined Permian–Triassic Boundary in South Africa" (2016). Honors Theses. Paper 814.
https://digitalcommons.colby.edu/honorstheses/814