Variation of Ooid Mineralogy in Jurassic Smackover Limestones as Control of Ultimate Diagenetic Potential

Clyde H. Moore, Ahad Chowdhury, Ezat Heydari

It has recently been suggested that the mineralogy of ooids formed in Jurassic seas should be predominantly calcite. Upper Jurassic Smackover ooids, across the gulf rim, however, exhibit evidence of both aragonite and calcite original mineralogy.

Both tangential and radial ooid cortex structures are present in the Smackover, with the tangential structured ooids associated with extensive moldic porosity development and elevated Sr²⁺ levels relative to well-preserved radial ooids. This suggests that the tangential structured ooids were originally aragonite, whereas the well-preserved radial ooids are thought to have been magnesium calcite. Cathodoluminescence characteristics and magnesium compositions of radial-structured ooids tend to support a magnesium calcite composition.

Detailed regional core-based studies of the upper Smackover reveal that aragonite ooids occupy the updip areas of the Smackover platform, whereas magnesium calcite ooids dominate in the downdip platform margins separated by a transition area of bimineralic ooids.

The distributional pattern and relative size of each ooid type (aragonite ooids larger than magnesium calcite ooids) tend to support some type of kinetic control over original ooid mineralogy.

Upper Jurassic mineralogic facies formed a distinct diagenetic potential gradient shortly after deposition that exerted significant controls over subsequent diagenetic events of economic importance to the Smackover, including distribution of secondary porosity (aragonite facies), early cement distribution (cements prevalent in aragonite facies), dolomitization (aragonite facies has higher dolomitization potential?), and distribution of primary porosity preservation (calcite facies).

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.