A multiscale characterisation of the Early Jurassic upper member of the McCarthy Formation, Southcentral Alaska

A multiscale characterisation of the Early Jurassic upper member of the McCarthy Formation, Southcentral Alaska

Heinhuis, Y.

Utrecht University

Molengraaff Fonds

2023-05-26

Ocean circulation, reflected in bottom current strength, is an essential factor in determining the global climate. Therefore, the deposits of ancient bottom currents are of great paleoclimatic importance. The sparse evidence of bottom currents in the Mesozoic has been linked to sluggish ocean circulation. Arguably, the sparse evidence is caused by the fact that recognition of ancient bottom current deposits is notoriously difficult, especially in mixed depositional systems. In this study, I present a multiscale characterisation of the upper member of the McCarthy Formation, an Early Jurassic record of bottom current deposition. The upper member of the McCarthy Formation has been interpreted as a glass ramp, deposited in the tropical Panthalassa Ocean, now exposed in the Wrangell Mountains, Southcentral Alaska.I studied the upper member along two sections near the Grotto Creek and subdivided itinto a cliff succession and a slope succession.I interpreted thecliff successionas the record ofa turbidity current–dominated mixed systemand the slope succession as the record of abottom current–dominated mixed system. The turbiditycurrent–dominated succession is inferred from the lobe architecture of the cliff succession, as well as the dominant presence of spiculitic sandy mudstone, indicative of down-ramp transport of grains by turbidity currents. The shift towards a bottom current–dominated mixed system is inferred from the dominant presence of laminated and bioturbated siliceous mudstones as well as the presence of bigradational bedding, interpreted as contourites, reworked turbidites, and hemipelagites, indicative of a laterally migrating sediment drift.The shift towards a bottom current–dominated mixed system is dominantly caused by a decrease in turbiditic activity rather than an increase in bottom current speeds. This is evident from an absence of sponge spicules, and medium and large scale sedimentarystructures in the cliff succession, as well as paleocurrent speed reconstructions. Bottom current speeds ranging from 9–51cm/s have been reconstructed from grain size distributions obtained from the siliceous mudstone of the upper member of the McCarthy Formation. Evidence of bottom currents suggests that circulation in the Panthalassa Oceanduring the hothouse conditions of the Early Jurassic was likely more vigorous than previously thought.

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