--> Modern Analog for Ancient Ooid-Grainstone Diagenesis and Pore Networks; Holocene Cancún Eolianite Ooids, Isla Cancún

AAPG ACE 2018

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Modern Analog for Ancient Ooid-Grainstone Diagenesis and Pore Networks; Holocene Cancún Eolianite Ooids, Isla Cancún

Abstract

The Holocene Cancún Eolianite along the eastern shore of Isla Cancún in Yucatán, Mexico, is an excellent natural laboratory for investigating early diagenetic processes in aragonite ooids. The eroded outcrops, composed of loose ooid sand to lithified ooid grainstone, present well-exposed dune stratification consisting of climbing translatent stratification, grainfall laminations, and sandflow cross-stratification. The sediments are composed of aragonitic ooids that are undergoing extensive early diagenesis in the vadose zone by meteoric water. Different parts of the eolianite have achieved various stages of diagenesis, which allows the observation and understanding of the progression of aragonite ooid sand to grainstone stabilization. Dissolution of the unstable aragonite and Mg-calcite nuclei creates oomoldic pores, and the generated calcium carbonate is reprecipitated as very fine to fine-crystalline equant calcite in interparticle pores. In ooid cortices, aragonite needles undergo dissolution by separating into nanoballs (irregular spherical particles 30 to 50 microns in diameter). No cement is being reprecipitated in the dissolving nuclei for lack of suitable nucleation sites. Also, the cortex bands show differential dissolution resulting from packing arrangement of aragonite needles. The tangential packed needle layers show a closer packing arrangement and contain smaller pores than the randomly packed needle layers that contains larger pores. The layers containing larger pores appear to be undergoing more rapid dissolution than the layers with smaller pores. The vadose zone of the Cancún Eolianite is a closed system relative to conservation of calcium carbonate. The amount of dissolution of aragonite and Mg-calcite approximates the amount of very fine to fine-crystalline equant calcite cement in the interparticle pore space. The early pore network has many similarities to ancient pore networks in ooid grainstones, such as cement-reduced interparticle pores, cortex-dissolution-band pores, oomoldic pores, and abundant nano- to micropores in altered ooids. Concepts derived from investigating the Cancún Eolianite can be applied to understanding how ancient ooid-reservoir pore networks formed and evolved, especially “heartbreak rock” where all the interparticle pore are cemented, while the oomoldic pores are cement free. The resulting pore network can have high porosity (up to 25 percent) and associated low permeability (less than a millidarcy).