ABSTRACT: Origin of Smackover Dolomites: Southwest Alabama

D. Joe Benson

Dolomite is a major component of the Upper Jurassic Smackover Formation in southwest Alabama. Dolomite comprises most of the unit in updip areas and across paleohighs. The amount of dolomite decreases basinward, where dolomite is restricted for the most part to the upper and lower parts of the Smackover.

Paragenetic relationships indicate that the majority of the replacement dolomite formed early in the diagenetic history of the Smackover, prior to or contemporaneous with deposition of overlying Buckner evaporites. On the basis of isotopic composition, two major types of replacement dolomite can be identified. Type 1 dolomites are characterized by positive (0.0 to +3.1^pmil PDB) ^dgr¹⁸O values. They are restricted to the uppermost and lowermost parts of the Smackover in the depositional basins, but comprise virtually the entire Smackover section over paleohighs. Type 2 dolomites are characterized by negative (-0.1 to-5.9^pmil) ^dgr¹⁸O signatures and comprise most Smackover dolomite in downdip areas. ^dgr¹³C values for both types range from +6.4 to +2.6 and show a regular downward decrease within the Smackover.

The distribution, timing of formation, and isotopic composition of the dolomites indicate two distinct mechanisms were responsible for dolomite formation. The isotopically heavy type 1 dolomites were brine derived and formed during refluxion of brines from Smackover and Buckner sabkhas or brine pools. The type 2 dolomites formed from mixed marine and meteoric waters. Dolomitization occurred in isolated mixing zones that developed around shoal complexes during short-term falls of eustatic sea level. The distribution of dolomite and specific dolomitization mechanisms in any area are a function of eustatic sea level fluctuations and paleotopography.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990