ASYMMETRIC CORKSCREW FLUTE CASTS IN THE UPPER DEVONIAN BRALLIER FORMATION: A LOCAL RECORD OF A CORIOLIS-INFLUENCED SEDIMENT SYSTEM?

Corkscrew flutes were initially described in the Triassic Muschelkalk by Rücklin (1938). They are infrequently reported in the literature and apparently were not produced in extensive flume experiments in the 1970's. We measured orientation data from 500 flute casts spectacularly preserved on the bases of three overturned siltstone beds within a 15-m-thick shale succession in part of the Brallier Formation at Bluefield, Virginia. Paleocurrent measurements yielded a vector mean of S79ºW consistent with previously published paleogeographic reconstructions that suggest deposition in a turbidite system near slope bottom 56 km west (down current) of a N–S-trending shelf edge. Most flutes are parabolic and linear in form, and 20% are of the corkscrew variety. Curiously, 78% of the corkscrew flutes exhibit a clockwise sense of rotation when viewed above the depositional surface from the nose of the flute tailward. Statistically, this is a non-random, preferred clockwise swirl (Χ² >99%). This suggests some physical cause that persisted throughout episodic deposition of 15 m of sediment.

Numerous studies have concluded that the Coriolis force influenced km-scale patterns of sedimentation in turbidites and contourites in modern and ancient settings. Examples include sediment asymmetry (Dorn & Werner, 1993, Sed. Geol.) and discordance between flutes and cross beds (Crowell, 1955; Vincent et al, 2005, both GSA Bull.). Experiments with density currents in rotating fluids show clockwise deflection of dense bottom flows consistent with the Coriolis effect (N. hemisphere) and the development of counterclockwise cyclonic vortices in the upper fluid layer, which periodically spin downward (Etling et al, 2000, Dyn. Atm. Oceans). Perhaps surprising, Earth rotation even causes cm-scale effects on the range of shot put and hammer throw contests (Mizera & Horváth, 2002, J. Biomech.).

Bluefield was 23ºS latitude according to Fammenian paleomagnetic reconstructions, far enough south for a significant Coriolis effect. Based on the very coarse silt grain size, turbidity currents traveled at 0.6 m/s, a 26 h flow from shelf edge allowing ample time in a low-friction setting for Coriolis influence. We will thus explore the possibility that flute asymmetry records a preference for the development of clockwise, low-pressure vortices.