Arnott, R. William C.1, Suzanne F. Leclair1
(1) University of Ottawa, Ottawa, ON
ABSTRACT: Structureless, Coarse-Tail Graded Strata of the Neoproterozoic Isaac Formation (Windermere Supergroup), Western Canada: Indicators of Internal Hydraulic Jumps
The Isaac Formation was deposited near the base of the Neoproterozoic passive of western North America. Strata consist mostly of mudrocks interstratified with decameter-thick sandstone and conglomerate channel fills. In places the mudrock intervals, which consist generally of “classical” (low-density) turbidites interstratified with silty mudstone, are intercalated with units composed of sharp-based, coarse-tailed graded sandstone (up to lower coarse sandstone) that lack tractional sedimentary structures (informally termed waftites). The origin of these strata, which initially was thought to be related to rapid deposition from high concentrated dispersions, was investigated experimentally. Results showed that deposits from sustained, high concentrated suspensions (up to 35% volume-sediment concentration) were planar laminated, and therefore unlike the ancient deposits. Runs with the same sediment conditions, but with an internal hydraulic jump, however, deposited similar structureless, coarse-tailed graded beds. Video records showed that intense turbulence generated in the hydraulic jump entrained sediment from the bed and maintained coarse and fine sediment grains temporarily in suspension. Deposition of the bed proceeded rapidly as a fast upward-moving interface in the quickly consolidating, but still shearing, bed material. Analysis of the deposits, as photographed through the experimental-tank glass walls or seen on sediment epoxy peels, revealed faintly-banded, graded deposits with sand grains suspended in a matrix of silt, similar to those observed in the Neoproterozoic examples. In the geological record, therefore, strata like these, which should be easily recognized in core, may indicate similar kinds of depositional conditions, and hence may act as proximity-to-channel indicators.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004