PETROGRAPHIC COMPARISON OF LATE OLIGOCENE - EARLY MIOCENE SHELF-EDGE PATCH REEFS, NORTHERN GULF OF MEXICO (GOM) VERSUS MODERN PATCH REEFS OF VERACRUZ/ANTON LIZARDO, MEXICO

Thin sections from cores of Heterostegina Zone (~23.6-23.0 ma) carbonates of Late Oligocene - Early Miocene age that are interpreted as shelf-edge patch reefs in the northern GOM are compared with epoxy-impregnated recent sediments of modern patch reefs near the port of Veracruz and Anton Lizardo, Mexico. Petrographic analysis of 45 thin sections from the Heterostegina cores indicates hermatypic framework and binding organisms that constructed reefal or biohermal accumulations. One hundred and sixty miles of seismic line, perpendicular to this shelf edge, confirms the presence of these buildups. Oil shows in the thick porous and permeable carbonates both above and below the Heterostegina Zone interval indicate future hydrocarbon discoveries. In contrast, Veracruz-Anton Lizardo (VCAL) reefs include 13 reefs and 3 islands near Veracruz Harbor, and 15 reefs and 4 islands off the headland of Anton Lizardo that are contaminated by terrigenous sediments. Most of the sediments in reef top and upper flank environments comprise several types of coral-encrusted red-algal boundstones and coral-molluscan grainstones. Charophytes are also present. Packstones on reef flanks are arenaceous. Reef-derived carbonate sand and mud encircle each reef and supply the carbonate fraction of the quartzose packstone facies suite. The halo of packstones that signals the proximity of a reef is < 1 km wide. Petrographic study of 33 epoxy-impregnated samples from the sediment/water interface indicates that almost all reef tops off VCAL contain at least a few percent quartz and/or feldspar. Volcanic rock fragments (VRF) occur also. The provenance of fine, non-carbonate sand and VRF off the port of Veracruz appears to be the Jalapa (Teziutlán Province). Terrigenous sediments (and charophytes) occurring in reef tops off the Anton Lizardo salient are likely sourced from either the drainage basins of the Río Jamapa (vicinity of Mt. Orizaba/Citlaltépetl) or the Río Papaloapan (Sierra de Juaraz) source terrane. Multiple recycling of terrigenous glass shards and volcaniclastic sands from nearby coastal dunes resulted in additional siliciclastic contamination of these reef tops.