Abstract

The Bonneterre Formation is the host for the lead deposits of southeast Missouri. The sedimentology of the formation strongly influenced the character of individual orebodies.Upper Cambrian sediments record a nearly continuous transgression onto an east-ward-sloping cratonic shelf. The St. Francois Mountain Precambrian highland formed a positive feature on this shelf. Lying to the west and north of the Precambrian high-land, a shallow intracratonic basin formed a slightly negative feature.Differential subsidence resulted in an eastward-thickening wedge of pre-Potosi sediments. The overall thickening to the east was little affected by the presence of the St. Francois Mountains, but this positive feature had a marked effect on the resulting facies patterns. The Precambrian highland, during the deposition of the Bonneterre Formation, became the nucleus for the development of low-energy, platform sedimentation. The margin of the platform was controlled by a well-developed stromatolite reef. The location of the reef was apparently influenced by a prevailing northwesterly wind direction.Stromatolites, sensitive indicators of depositional environments, recorded a prograding of sediments basinward during lower Bonneterre deposition along the Viburnum Trend. The inter-relationship of organic growth and differential subsidence resulted in dominantly outward biostomal growth of the reef along the Viburnum Trend, and a dominantly upward biohermal growth in the Old Lead Belt.During middle and upper Bonneterre deposition, accelerated differential subsidence resulted in (1) an environment less favorable for stromatolite growth, (2) an eastward shift of facies patterns, (3) an overall shoaling of the basin as recorded by a greatly expanded oolitic facies.This pattern of shifting facies resulted in a unique stratigraphic succession at each mine and became one of the controlling factors in the localization of the epigenetic ore-bodies of the Viburnum Trend.

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