Expression of the Permo-Triassic Boundary in a High Accommodation, High Southern Paleolatitude Continental Margin Setting

Allen Peter Tevyaw II, University of Nebraska-Lincoln

Document Type Article

This thesis moved to the embargo series by author's request.

Abstract

The end-Permian was marked by multiple environmental perturbations that ultimately resulted in the greatest life crisis of the geological record, the end-Permian Extinction (EPE). This paper evaluates the Permo-Triassic boundary (PTB) interval in the high accommodation, high southern paleolatitude paleocontinental margin setting of the northern Sydney Basin, eastern Australia. Recent investigations into the PTB interval in the Sydney Basin place the EPE somewhere above the top of the last Permian coal, which in Sydney’s northern coal field is the Vales Point Coal (VPC). Measured vertical sections and stratigraphic relationships from drill cores, open pits and natural surface exposures, in conjunction with geochemical analyses were used to investigate the expression of the PTB and EPE in the rapidly subsiding foredeep of the Sydney Basin. Our data reveals a coastal to lowland alluvial floodbasin setting proximal to an orogenic hinterland to the east and experienced progressively increasing estuarine influences to the west. Prior to the VPC, alluvial channel bodies are predominantly conglomerate filled, compensationally stacked and lensoid shaped in a direction perpendicular to paleoflow. Vertically separating these channel bodies are thick tuffs, coals, and gray root-penetrated mudrocks that indicate gleyed conditions. Immediately before and after the VPC, channel bodies show reduced bed thickness and a change from conglomerate to pebbly sandstone fill but record no change in fluvial style, stacking pattern, and/or sediment dispersal direction. Coals and tuffs are noticeably absent above the VPC, and mudrocks become more strongly colored but remain indicative of gleyed conditions. The interval immediately above the VPC is highly complex, with some sections preserving up to 18 meters of mudrock in conformable contact with the VPC, whereas others are truncated by fluvial downcutting. Weathering proxies and δ¹³C values of organic matter through conformable sections suggest periods of intense chemical weathering and at least one pulse of ¹³C depletedorganic matter above the VPC. This study finds that the depositional environment of the northern Sydney Basin did not change significantly in response to climatic perturbations following the top of the VPC, which is substantially different than the traditional view of abrupt, severe alterations to the continental landscape.

Advisors: Christopher Fielding & Tracy Frank