Abstract: Vadose Cement Patterns in Strand-Plain Limestones of Eastern Yucatan Peninsula, Mexico

Michael J. Brady, William C. Ward, Paul Attwood

Carbonate cements in upper Pleistocene nearshore and beach calcarenites, and lagoonal carbonate rocks of eastern Quintana Roo show no evidence of early cementation in marine waters. Instead, cementation occurred predominantly in the vadose zone during a post-depositional low stand of sea level. Even so, there is some relation between the low-magnesium calcite cement and environment of deposition, because composition, grain size, sorting, and packing influence patterns and morphologies of vadose cements. Further complications of cementation are related to root penetration and calichification.

Cement patterns are highly variable even in the same sample. Many skeletal-rich layers and finer grained and/or better packed layers are preferentially cemented. Irregular patches of sparry calcite cement are more abundant in calcarenites of the shoreface than in those of the foreshore. Foreshore calcarenites and intraclasts (contained in the shoreface limestones) are cemented with vadose and/or freshwater phreatic cement types.

Meniscus cement occurs in calcarenites of all the depositional environments and is the most common sparry calcite cement. In addition, "spikey" calcite crystals irregularly encrusting grains are present throughout the sequence. Very large calcite crystals grew in coarse mollusk-rich layers of the shoreface limestones, probably because packing was such that there were large pores for crystal growth and because dissolution of aragonitic mollusks supplied material for cementation. These crystals commonly transect mollusk fragments preserving only "ghosts" of the original shells. Microcrystalline calcite and needle-fiber calcite apparently are associated with zones or areas that were invaded by rootlets. Micrite cement is most abundant in lagoonal wackestones and caliche crusts associated with the calcarenites. There has been some conversion of lagoonal muds to micrite by precipitation of calcite overgrowths on algal(?) needles.

Intragranular cements are also common in these strandline carbonate rocks. In most samples intragranular pores are preferentially cemented. These cements generally occur as blocky or elongate crystals filling forams, mollusks, or Halimeda utricles.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma