Abstract

Regional study of the Guelph Formation (middle Silurian) in the subsurface of southwestern Ontario highlights the role of recrystallization during burial diagenesis in producing massive dolomites with relatively uniform petrographic features but complicated geochemistry. The dolomite is composed mainly of replacive, unimodal to polymodal mosaics characterized by uniform, dull to weak red cathodoluminescence. Whereas patch reefs on the platform and most pinnacle reefs on the upper basin slope are pervasively dolomitized, reefs on more basinward parts of the slope may be partially dolomitized, usually in their lower portions, or they remain limestone. The lithology of overlying carbonates in the lower Salina Group mimics closely that of the underlying Guelph Formation. Initial shallow burial dolomitization occurred in the Late Silurian in response to isolation and hydrological drawdown of the Michigan Basin. The basinward-decreasing dolomite trend resulted from continuous seawater flux from the middle (back reef) platform downward through Guelph limestones to the discharge areas on the slope and beyond. Reefs at the distal end of the regional paleoflow system, or those that were poorly connected to the system remained limestone. Both dolomite δ13C values, which range from +1.1 to +5.0 ‰ (PDB), and 87Sr/86Sr ratios, which range from 0.70845 to 0.70910, reflect a Silurian seawater source.

Values for δ18O range from −5.2 to −9.7‰ (PDB) but cannot be explained by the above mechanism alone. Recrystallization likely occurred during the progressive burial of these rocks to depths of up to 2 km, and may at least in part explain the lack of correlation among dolomite crystal size, cathodoluminescence characteristics and the various geochemical parameters measured in this study. The reason for the wide spread in δ18O values both geographically and stratigraphically reflects varying degrees of recrystallization as a function of macro- to micro-scale permeability and porosity differences in the precursor lithology and their control on fluid-rock interactions during recrystallization. The cause of the apparent decrease in dolomite δ18O values from patch reefs on the platform (average δ18O = ~ −6‰) to the lower slope (average δ18O = ~ −8.5‰) is unknown, but may have resulted from a combination of a basinward increase in burial depth and a basinward increase in heat-flow, which are now only subtly preserved through the “noise” of multiple recrystallizations.

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