ABSTRACT: The Hull Limestone (Upper Ordovician) of Eastern Ontario: A Lower Paleozoic Carbonate Barrier-and-Lagoon System

Jeffrey P. Kiernan, (Crestar Energy, Calgary, AB, Canada) and George R. Dix, (Ottawa-Carleton Geoscience Centre & Dept. Earth Sciences, Carleton University, Ottawa, ON, Canada)

The Hull Limestone of eastern Ontario is a geographically restricted member of the Bobcaygeon Formation (Upper Ottawa Group, Trenton Group equivalent) in eastern Ontario. Detailed examination of the Hull strata has revealed three major depositional settings: barrier-front, barrier-top and lagoon.

Barrier-front deposits consist of predominantly crinoid-bryozoan grainstone with a suite of sedimentary structures indicative of a lower to middle shoreface setting (planar laminae, cross-laminae, rare HCS and a mixed Skolithos-Cruziana ichnofauna). The barrier-top facies consist of closely juxtaposed very high- and low-energy deposits, representing barred upper shoreface to beach environments and slackpond or intertidal mudflat. The high-energy facies consist of bryozoan-pelecypod-Solenopora-oncoid grainstone and rudstone. Sedimentary structures include low-angle, truncating planar laminae and locally dense clusters of small Skolithos burrows. Low-energy deposits are sparsely fossiliferous (ostracodes, green algae, gastropods and Tetradium) calcisiltite with probable fenestrae and very rare desiccation cracks. Small (<50 cm deep), storm-breach channels filled by bioclastic-intraclastic rudstone are incised into the calcisiltite. Most of the lagoon deposits are Tetradium and green algae-rich mudstones. Pelloidal grainstone are interpreted as representing small sand shoals/beaches around the fringes of the lagoon. Lenses of bioclastic-intraclastic rudstone, with grain-types characteristic of both the barrier-top and lagoon, are interpreted as storm washover deposits.

The general facies arrangement is similar to interpreted coeval warm- and cool-water carbonates in the Mississippian of Western Canada. Caution is suggested in extending this interpretation to the Hull Limestone unless further evidence beyond simple grain-type associations can be found for a cool-water environment for the barrier-front facies of the Hull and coeval offshore deposits of the Bobcaygeon Formation.

Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada