PETROGRAPHY, MINERALOGY, AND GEOCHEMISTRY OF GREENISH- AND REDDISH-GRAY SILTSTONES STRADDLING THE VERTEBRATE-DEFINED PERMIAN–TRIASSIC BOUNDARY IN SOUTH AFRICA (Invited Presentation)

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 vertebrate-defined 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-gray and reddish-gray siltstones were collected from the transition interval at Old Lootsberg Pass, Eastern Cape Province, to test this hypothesis.

Samples were examined with optical petrographic, XRF, XRD, and SEM with EDS analyses. Primary structures attributed to fluvial processes indicate in-channel, fluvial origin of most samples. XRF data show that the two colored siltstones are indistinguishable in most major elements, except for Na, Ti, and Mn. Mössbauer spectroscopy reveals that the reddish-gray color is 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, and not a function of aridification.