Astrid Makowitz¹, Kitty L. Milliken¹

(1) University of Texas at Austin, Austin, TX

ABSTRACT: New Data on the Role of Mechanical Burial Compaction in Diagenesis: Frio Formation, Gulf of Mexico Basin

Quartz filled microfractures in detrital grains, otherwise unobservable in conventional light microscopy are elucidated by scanned cathodoluminescence. The Frio Formation was investigated because it represents a unit deposited on a passive margin presently at maximum burial, absent of tectonic overprinting and affected by growth faulting, allowing initial observation of uniform material across a wide range of burial depths.

Quartz grains were observed (from 17 samples) ranging from 3223 to 17154 feet (1 to 5 km). Intragranular fractures observed are open Mode I associated with grain boundary contacts where grain boundaries are sites of highly concentrated stress resulting in brittle deformation during burial compaction. Percentage of grains that manifest quartz filled intragranular fractures varies from 9 to 84%. A statistically significant correlation exists between fracture frequency of quartz grains and depth.

These results demonstrate the important role of mechanical compaction in diagenesis and have implications for a variety to diagenetic problems. (1) Quartz filled microfractures sequester significant amounts of quartz cement within grains, representing a volume of authigenic quartz previously unaccounted for in silica mass balance calculations. (2) Microfractures provide avenues for fluid flow and preferential sites for quartz nucleation. Understanding the association between brittle deformation and cementation may enhance porosity and permeability prediction. (3) Mechanical compaction may directly cause silica mobilization due to increasing surface area in conjunction with decreasing grain size, contrary to all present notions of chemical compaction.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado