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Dolomite Textures in Lateral Transition Zones: Record of Diagenetic Fluids from Smackover (Alabama) and Ladinian (Northern Italy) Dolomites

Mary L. Barrett, Edith Newton

Comparative observations of dolomite textures resulting from different temperature regimes of the Smackover Formation (Jurassic), Alabama, and the Ladinian Dolomite (Triassic) of northern Italy, indicate the usefulness of fabrics to understand flow pathways of diagenetic fluids and the resulting pattern of dolomite nucleation and growth. The transition from limestone to dolomite is present on many scales, from a few meters adjacent to vertical fractures in the Triassic platforms, to a few kilometers on the flanks of paleotopographic highs of the Jurassic Manila Embayment.

The lateral transition consists of massive dolomite to selective dolomite to scattered rhombs in limestone, which may result from a mass flux difference of dolomitizing fluids during a specific time frame, a temperature change, or a combination of both. The selectively dolomitized zones reflect micropermeable paths through grains or matrix, grain/matrix boundaries, or microfractures that are locked in before being obscured by continued replacement. In the Smackover, the area of massive dolomitization is characterized by both porous and tight zones altered under the same conditions. Dolomite exhibits end member fabrics from textural reorganization to preservation of original textures. Microfractures in the Italian carbonates also influence the distribution of massive dolomite.

Pore and rhomb size variations plus transition-front geometries imply differences between these two carbonate provinces, such as specific fluid pathways, rock permeabilities, or temperatures. However, characterizing transitions and responses of original textures yields much information on physicochemical processes responsible for porosity distribution on a dolomitized carbonate platform.

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