107-5 Facies shifts and sequence architecture of the Adriatic Carbonate Platform, Croatia, during and after the Toarcian Anoxic Event (TOAE)

Session: Sedimentary Geology Division/SEPM Student Research Poster Competition: Dynamics of Stratigraphy and Sedimentation

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Abstract:

The Early Jurassic Toarcian Oceanic Anoxic Event (TOAE; ~183 Ma) was a major episode of global warming and carbon cycle disturbance. We document the response of the Adriatic Carbonate Platform to this event and its aftermath based on a ~170 m-thick Toarcian–Lower Aalenian succession in Croatia. The section was studied bed-by-bed for facies, Dunham textures, facies stacking patterns, δ¹³C fluctuations, and sequence stratigraphy. Nine depositional sequences were identified, each comprising 1–6 parasequence sets. The δ¹³C values range from  +0.8 to +4‰ VPDB (mean +2.1‰), and δ¹⁸O from -3 to 0 ‰VPDB (mean -1.2‰). A pronounced negative δ¹³C excursion marks the lower Toarcian foreshoal–outer ramp deposits, the platform equivalent of coeval organic-rich basinal facies. These are succeeded by skeletal-oncoid and later ooid-peloid grainstones, reflecting an overall shallowing-upward trend during global post-TOAE cooling. Oncoids occur across a variety of settings and display morphological diversity, indicating microbial activity and variable bottom-water oxygenation during and after the TOAE. The shift from oncoid- to ooid-dominated facies reflects both improved oxygenation and changes in platform morphology, from rimmed to flat-topped, across the Toarcian–Aalenian boundary. Parasequences are generally poorly cyclic and subtidal, often lacking tidal-flat caps, likely due to low long-term accommodation and bypass of Milankovitch-scale signals. The scarcity of transgressive parasequence parts suggests submarine erosion in a high-energy setting where sedimentation lagged behind sea-level rise. This study provides high-resolution insight into how shallow carbonate systems responded to global anoxia and subsequent climate cooling, with implications for interpreting microbial carbonate proliferation, oxygenation trends, and platform evolution during major oceanic perturbations.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8220

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