264-3 Depositional Gradients Shape Early Triassic Carbonate Geochemistry: Insights From a 500 km Transect of the Virgin Limestone (USA)

Abstract:

The Early Triassic reflects the prolonged scars of the end-Permian mass extinction, as recorded in depauperate marine communities and marked geochemical instability. Geochemical records have long been employed to explore the drivers of this crisis, and have been used to frame hypotheses about the extinction and subsequent delayed environmental recovery, typically centered around global-scale environmental processes such as expanded shallow-marine anoxia, extreme hothouse conditions, and widespread ocean acidification. As an alternative, a growing body of evidence suggests that facies-dependent and post-depositional factors may exert a dominant influence on Early Triassic geochemical signals, particularly given the widespread occurrence of ‘anachronistic’ carbonate facies during this interval. As such, distinguishing the impact of local and diagenetic vs. global signatures would be useful for understanding the timing and drivers of Early Triassic paleoenvironmental recovery.

The Lower Triassic Virgin Limestone Member of the Moenkopi Formation and the Union Wash Formation (collectively, the Moenkopi Platform) span a transect from shoreline to deep basin. Collectively, they contain both a variety of anachronistic carbonate facies – including microbial bioherms, seafloor carbonate fans, subtidal laminated micrites, and flat-pebbled conglomerates – and evidence for biotic recovery, including the oldest Triassic metazoan bioherms. The Moenkopi Platform therefore provides an ideal venue to explore how depositional gradients and facies-level environmental variability shape Early Triassic geochemical records. 

Here, we leverage a multi-proxy carbonate geochemical framework (δ¹³C, δ¹⁸O, δ^44/40Ca, δ²⁶Mg, trace-metals) to help distinguish the relative importance of global paleoceanographic change, local environment variability, and diagenetic processes on Moenkopi Platform carbonate geochemistry. Preliminary results suggest that there are strong depositional gradients in both primary geochemical signals and the extent of diagenetic resetting. Nearshore sections are generally typified by fossiliferous facies, and ‘altered’, seawater-buffered geochemical signatures. In contrast, deeper shelf, slope, and basin sections preserve a diverse assortment of facies and generally capture ‘primary’, sediment-buffered geochemical signatures. These patterns coexist with an asymmetric δ¹³C gradient, in which δ¹³C values tend to be heaviest in nearshore and slope settings, and significantly lighter on the shelf. Collectively, these results suggest that depositional gradients may have a profound influence on preserved sedimentary geochemistry. Put another way, the massive environmental variability recorded in the Moenkopi Platform may be partially facies- and depth-dependent, suggesting Early Triassic records must be interpreted through this lens.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-18158

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