Sedimentary breccias are important ore-controlling structures in the lower half of the Bonneterre Formation in the southeast Missouri lead district. Four major breccia zones, each embracing over 60,000,000 tons of rock, have been delineated. Individual ore bodies in the breccia zones range from over 6000 feet long and containing several million tons to a few hundred feet with several thousand tons.
The breccias formed along the flanks of calcarenite ridges by submarine slides into near-by depositional basins. Extensive mine workings and closely spaced diamond drilling in the breccia bodies permit reconstruction of entire slides and determination of facies relationships leading to their development.
Lithofacies studies indicate that the four basic types of material composing the lower part of the Bonneterre Formation—argillaceous mud, sand-size sediment, recognizable algal deposits, and carbonate mud—may be referred to environments of deposition which are dominated by two elements, depositional ridges and basins. The ridges are composed for the most part of calcarenite and reef deposits, the basin sediments of fine-grained carbonate and argillaceous muds. Differential compaction led to oversteepening of slopes in the gradational zone between the ridge and basin deposits and initiated movement of unconsolidated and partially lithified sediments. Repeated slides at fairly close stratigraphic intervals account for great thicknesses of breccia in the lower part of the formation.