Laurasian Seaway History During the Early Jurassic From Quantitative Mineralogy and Detrital Zircon Dating

Abstract The Laurasian Seaway comprises a N‐S oriented trans‐Pangaean corridor linking Tethys to the Boreal Sea during the Jurassic, and direction of water flow has been the subject of considerable debate. The Llanbedr (Mochras Farm) borehole, Cardigan Bay Basin, Wales, is a uniquely informative record that comprises an expanded deep‐marine silty argillaceous succession of Early Jurassic age. Deposition there is inferred to have been from contour currents under conditions that were deeper and more open than the restricted basins of the eastern seaway. Quantitative mineralogy and detrital zircon ages define five mineral assemblage units. Units 1 and 2 (Hettangian–Sinemurian) have mixed age spectra including abundant pre‐Cadomian zircons (>950 Ma) sourced from the north; associated mineralogy includes high albite and biotite content indicating lack of chemical weathering. Unit 3 (uppermost Sinemurian) is similar to strata below but contains mostly Caledonian zircons (∼460 Ma) of more local origin. Unit 4 (Pliensbachian) has predominance of Cadomian zircons (750–540 Ma), and quartz‐rich sediment. The Unit 4 base corresponds to a sea‐level rise and opening of the E‐W Hispanic Corridor connecting equatorial Panthalassa with Tethys and change in configuration of sediment transport with sourcing postulated from the south and west. Unit 5 (uppermost Pliensbachian–Toarcian), shows greater dominance of Cadomian zircons coincident with severe global change at the Pliensbachian–Toarcian boundary and Toarcian Oceanic Anoxic Event. Timing and direction of inferred provenance change suggests that seaway circulation was strongly influenced by the opening of the Hispanic Corridor at the Sinemurian–Pliensbachian boundary and relatively stable thereafter.