ABSTRACT: Travertine Deposits on Pinnacle Reefs in Southwestern Ontario: Implications for Paleohydrology of Middle Silurian Reefs in the Michigan Basin

Rob Frizzell, and Mario Coniglio, University of Waterloo

Pinnacle and patch reefs in the Guelph Formation in the Michigan Basin commonly display a 15-30 m thick reef cap facies developed over stromatoporoid- and coral-bearing organic reef facies. This study focuses on the origin of distinctly laminated carbonates within this reef cap facies in 13 pinnacle reefs in southwestern Ontario. Given their stratigraphic position on the tops of reefs, and below a major evaporitic succession, these laminated carbonates, which range in thickness from < 1 m up to 10 m, have previously been described by as stromatolites, caliche, or a combination of the two. The variety of fabrics encountered, however, is remarkably similar to those developed in travertines. Kendall (1989) proposed a similar origin for laminated carbonates developed on reef buildups in the Middle Devonian Elk Point Basin in western Canada.

The fabrics encountered in this study include planar flowstone with chevron-like laminations, concentrically laminated carbonates that resemble travertine pisoids, and breccia composed of angular fragments of laminated carbonate. Initially some of these fabrics were highly porous but were subsequently filled with internal sediment and cement. Microscopically the facies mostly consists of conformable, thin laminae (0.25 mm-1.0 mm), composed of equigranular 100-400 mm dolomite crystals alternating with microcrystalline dolomite. Some of the coarser laminae also contain elongate crystals which may reflect the habit of their fibrous calcite(?) precursor. The correct interpretation of these laminated carbonates has important implications for understanding diagenetic fluid flow within Middle Silurian pinnacle reefs.

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