Lying on an extremely narrow continental shelf in the southwest Gulf of Mexico are two groups of living patch reefs in the vicinity of Veracruz and Anton Lizardo, Mexico. Extending from the shoreline to water depths of 50 m, these are the only major reef buildups in the clastic province of the Gulf of Mexico west of the Yucatan and Florida carbonate provinces. Terrigenous clastic sediments are the major sediments in the interreef areas, and are primarily mud and silt brought in by high-gradient rivers which drain a narrow coastal basin and high mountains within 150 km of the coast. Some sand may be left from the last Pleistocene low sea level. The clastic sediments also contain 1-2% carbonate material from planktonic and benthonic fauna. Reef-derived carbonate sediments a e present only in a narrow zone around each reef patch. The carbonate percentage ranges from 100 in the living coral areas to less than 5 within 100 m, although traces of reef-carbonate material extend considerably farther. Several areas of dead reefs are also present. If these results are applied to ancient deposits, the location of reefs by sampling from either outcrops or well cuttings would be difficult, although the presence of reefs could be inferred as being in an upcurrent direction.

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