Stylolite Formation in a Tertiary Limestone (Clifton Formation), Australia
Stelios Nicolaides and Malcolm W. Wallace
The late Oligocene-early Miocene Clifton Formation in the Otway Basin, southeastern Australia, consists predominantly of skeletal carbonate grains of originally calcite mineralogy (e.g., bryozoa, echinoids, foraminifers), deposited in a shallow carbonate shelf environment. The Clifton Formation has had an uncomplicated burial history and was subject to very little uplift. Shallow-buried limestones are weakly cemented by calcite (<5% calcite cement down to 430 m depth) while more deeply buried limestones (>550 m) are almost completely cemented by calcite.
Evidence of mechanical compaction (e.g., grain breakage) occurs throughout the unit. The products of pressure solution in the Clifton Formation, which can only be observed microscopically, are interpenetration of grains (with smooth and sutured contacts), fitted fabrics, solution seams, and microstylolites (small scale stylolites only visible under the microscope). Significantly, no macroscopic stylolites are present (to a maximum depth of 670 m), suggesting that in these limestones, stylolite formation requires depths greater than about 700 m. In the grainstone intervals, interpenetration of grains is apparent in the shallowest samples (less than 160 m), whereas fitted fabrics and microstylolites first appear at approximately 550 m. In the clay-rich lithologies, solution seams occur t depths as shallow as 190 m, although they are better developed at and below 340 m.
Microstylolites appear to develop from an amalgamation of sutured interparticle contacts in grainstones with fitted fabrics. Further dissolution and amplification of these microstylolites could lead to the formation of stylolites.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California