T174 Precambrian Basin Analysis

Thursday, 29 October 2020
10:00 AM - 12:00 PM
Meeting Rooms

CORIOLIS DEFLECTION OF HYPERPYCNAL FLOW IN DELTA FRONT SEDIMENTS IN THE MID-EDIACARAN (~580 Ma) WONOKA CANYON AT UMBERATANA SYNCLINE, SOUTH AUSTRALIA

Paper number: T174-6
Program: Technical Sessions
Time: 10:25 AM – 10:30 AM

Authors

Abstract

A re-analysis of more than 1,000 paleocurrent measurements from tabular sandstone-siltstone event layers in the mid-Ediacaran (~580 Ma) Wonoka Formation of the northern Flinders Ranges, South Australia has yielded an unexpected result. Swaly 3–D climbing ripples (tops of beds, lower flow regime) indicate flow directions systematically to the right of flute marks (bases of beds) and parting lineation (upper flow regime). The differences are reproducible at two different sets of outcrops (+3.2° and +6.6° for ripples vs flutes, and +10.0° and +14.5° for ripples vs parting, with respective 95% confidence intervals for circular mean directions of ±1.82° and ±1.24° for ripples, ±0.92° and ±1.32° for flutes, and ±2.29° and ±3.32° for parting). They are also observable in individual beds. The depositional setting is the basal 250-300 m fill of a sinuous, kilometer-deep paleocanyon system that is generally regarded as comparable to the submarine canyons of contemporary continental margins. Details of the facies and organization into nine upward-fining conglomerate-based cycles 27.5 ± 3.7 m and 28.6 ± 2.0 m thick, respectively reinforce an alternative interpretation: that the canyons were cut by rivers during a short-lived Messinian-style drawdown of sea level in a temporarily isolated marine embayment. The event layers are interpreted as delta front hyperpycnites fed by one or more high-order incised valleys (fluvial channels) with ~5-10 m and ~3-6 m of local erosional relief, respectively, and widths in the order of 100 m. Simple back-of-envelope calculations are consistent with Coriolis deflection of individual flows over length scales of hundreds of meters to in excess of 10 km from valley termini, and flow-parallel gradients in the order of 0.1° to 0.5°. The magnitude of the Coriolis deflection is determined by the relative strength of the Coriolis acceleration to the frictional drag of the current. The deflection θ can be approximated as tan θ = f h/CD U, where f = 1.45 × 10-4 sin (latitude), CD ~3 × 10-3 is the drag coefficient, h is the thickness of the sediment-laden flow, and U is its speed. Published paleomagnetic data suggest a paleolatitude of ~15°-35° N. Assuming a speed of 0.5 m/s and a thickness of 5 m yields a 7° deflection to the right at 15° N, and a 15° deflection at 35° N. Stronger and thinner currents would be deflected less.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-354107

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