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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 67 (1983)

Issue: 3. (March)

First Page: 507

Last Page: 507

Title: Dolomitization of Upper Cambrian Bonneterre Formation, Southeast Missouri: ABSTRACT

Author(s): David J. Lown, Thomas A. Moss, Philip R. Schroeder

Article Type: Meeting abstract

Abstract:

The Bonneterre Formation of southeast Missouri contains a wide variety of dolomites and dolomite textures which are the results of multiple episodes of dolomite formation. The Bonneterre Formation crops out near the St. Francois Mountains at the core of the Ozark dome and is host for several large Mississippi Valley-type Pb-Zn deposits. The Bonneterre is predominantly a dolomite where it crops out, but contains limestone in the subsurface.

The Bonneterre was deposited by a series of shoaling-upward depositional cycles. A complete cycle was terminated by a tidal-flat complex which prograded basinward. The top of a completed cycle is a disconformity. Recognition of major transgressions which begin some of the larger cycles provides a basis for correlating the Bonneterre throughout southeast Missouri.

Several cores from diamond-drill holes drilled for Pb-Zn ore were logged and sampled for thin sections and trace-element geochemistry. Based on petrographic and stratigraphic relationships observed in the cores, the dolomites of the Bonneterre Formation can be separated into six types. (1) Tidal-flat dolomites are light gray and fine crystalline. These rocks are finely laminated and contain features indicative of evaporitic environments. (2) White-rock dolomites are coarse crystalline and very light gray. Individual crystals of this rock usually have sweeping or undulose extinction when viewed under crossed nicols. (3) Brown-rock dolomites are generally brownish gray to olive gray and fine to medium crystalline. Sedimentary features are well preserved in some of the fine-crystalline e amples of this lithology. (4) Clear rims or zones of dolomite that are epitactic overgrowths on other dolomite crystals are another type of dolomite found in the Bonneterre. This dolomite contains ferroan zones (as determined by staining with potassium ferrocyanide), and is associated with stylolites. (5) Saddle dolomite was deposited in all types of diagenetic pores. (6) Hydrothermal dolomite is associated with Pb-Zn sulfide minerals.

Stratigraphic and paragenetic relationships combined with trace-element geochemistry provide a basis for interpreting the origins of the Bonneterre dolomites. Tidal-flat and white-rock dolomites formed in the near-surface diagenetic environment. Tidal-flat dolomites formed in evaporitic environments similar to modern sabkha environments. White-rock dolomites formed in freshwater-saline water mixing zones. Clear dolomite rims with ferroan zones are interpreted to have formed in the subsurface diagenetic environment and are the results of pressure solution. The formation of brown-rock dolomites involves both near-surface and subsurface diagenetic environments. Saddle dolomite formed late in the diagenetic history of the Bonneterre formation and may be related to the introduction of basi al brines to the southeast Missouri region.

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