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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 9. (September)

First Page: 1561

Last Page: 1562

Title: Early Cementation and Mineral Stability of Chipola Formation, Calhoun County, Florida: ABSTRACT

Author(s): John M. Kocurko

Article Type: Meeting abstract

Abstract:

The Chipola Formation crops out along the Chipola River, Ten Mile Creek, and Farley Creek in Calhoun County, Florida. The formation, dated at 16.1 m.y.B.P., is predominantly an unconsolidated, bioclastic wackestone. These carbonate rocks have undergone very little diagenetic alteration and as a result, fossil preservation is excellent. Original mineralogy is unchanged. X-ray analysis of unlithified sediment indicates an average composition of approximately 70% carbonate (45% aragonite and 25% low-magnesium calcite) and 30% non-carbonate (15% quartz, 6% clay, and 9% other). Most shell material is preserved as aragonite and a few sediment samples are still composed of high-magnesium calcite. Disregarding the age of the formation, the unit is considered to be in a very early diagenetic stage.

Cementation occurs in the unit by the formation of low-magnesium calcite in the form of microspar. The

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lithified patches of the formation take on a nodular appearance as bioclastic debris is cemented. Cementation by microspar and bladed pseudospar occurs in burrows, shells, and as random nodules. Microspar is typically equant and uniform with an average crystal diameter of 14 ┬Ám. Microspar formation results in an increased crystal volume characterized by exploded shells and extruded matrix. The crystal formation is an early diagenetic phenomena and probably driven by the presence of organic matter.

In addition to microspar, aragonitic shells are commonly replaced by neomorphic sparry calcite. Replacement appears to occur along an advancing front and is probably a result of thin-film reactions. There is no crystal volume increase.

Neomorphic spar (often referred to as "pseudospar") is formed by a different process than microspar (pseudospar) and the final diagenetic effects are different. Distinction should be made between types of pseudospar, and the terminology clarified.

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Copyright 1997 American Association of Petroleum Geologists