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Mesozoic Carbonate Rafts in the Northern Gulf of Mexico


Seismic correlations and well data confirm that deep-water carbonate beds of Mesozoic age have been found above the shallow allochthonous salt canopy in the northern Gulf of Mexico. Publicly available wells in the Garden Banks (Norton GB 754, Vienna GB 840, Sumatra GB 941) and Keathley Canyon (Hadrian KC 911) OCS protraction areas penetrate Mesozoic carbonates situated above the salt canopy or equivalent salt welds. The seismic character of these carbonates strongly resembles that of in situ Mesozoic carbonates. Carbonate seismic facies have been mapped at shallow levels on seismic data in Garden Banks, Keathley Canyon, Green Canyon, and Walker Ridge OCS protraction areas. The presence of displaced Mesozoic carbonate rafts above the canopy raises an important question: how did Mesozoic strata get to such a shallow level in the basin statigraphy? A geologic mechanism for displacing Mesozoic carbonates from their normal position and transporting them as rafts mirrors that for creation of the allochthonous salt canopy. As salt inflates to form large, broad, diapiric structures, overlying strata (i.e. Jurassic and Cretaceous carbonate) are lifted above adjacent subsiding minibasins containing equivalent strata. At later times in the Paleocene, Eocene, Oligocene, and Miocene, salt from the inflated structures broke out to form shallow canopies. As salt travelled laterally it carried the roof material with it. Radial spreading of the salt broke the roof material into multiple smaller units. Seismic mapping suggests some rafts have travelled many tens of kilometers away from the diapiric structures that lifted them to the shallow salt canopy.