ABSTRACT: Closed-System Vadose Diagenesis in the Holocene Cancun Eolianite, Isla Cancun, Yucatan Peninsula, Mexico
Robert G. Loucks, Kim A. Patty
Ooid grainstones in the Holocene (< 2700 yr B.P.) Cancun Eolianite are actively undergoing vadose diagenesis that is producing distinctive dissolution and cementation features. A striking dissolution fabric consisting of numerous concentric dissolution bands developed in ooid cortexes. The preserved cortex layers consist of tangentially arranged aragonite needles. Both aragonite and Mg-calcite ooid nuclei have undergone partial to complete dissolution.
Results of an etching experiment of modern Cancun ooids suggest that cortex layers of randomly oriented needles are more susceptible to dissolution than layers made of tangentially oriented crystals. This solubility difference between individual cortex layers accounts for the striking, delicate dissolution bands in the Holocene Cancun Eolianite. Scanning electron microscope and petrographic analyses also support the conclusion that the layers of tangentially oriented needles are selectively preserved in the Holocene Cancun Eolianite.
Interparticle cements are predominantly very fine- to fine-crystalline, blocky to steep rhombohedra of calcite. Much of the calcite forms meniscus cement between grains, but patchy occurrences of pore-filling cement are also common. Cementation within the metastable interior of the dissolving aragonite ooids is extremely rare. This is probably related to the lack of calcite nucleation sites within the chemically metastable intraparticle microenvironment.
At the thin-section scale, nearly all dissolved aragonite and Mg-calcite is precipitated as calcite in the vadose zone. This is based on point-count data that indicate the amount of CaCO3 in calcite cement in the vadose zone is approximately equal to the amount of CaCO3 derived from aragonite and Mg-calcite dissolution. Therefore, the vadose zone is almost a closed diagenetic system and little CaCO3 is exported to the underlying phreatic zone.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990