From 1973 through 1978 the U.S. Geological Survey acquired multichannel common-depth-point (CDP) reflection seismic data along the Atlantic continental margin from southern Florida to the northern end of the Georges Bank basin for an investigation of the regional structural configuration of the basins and an assessment of petroleum potential. On one seismic line, located in blocks 974 and 975 of the Georges Bank basin at lat. 67°37^primeN and long. 41°00^primeW, a high-amplitude event approximately 9 km long is present. The top of the anomaly is at a depth of approximately 4,175 m, which roughly corresponds to the middle of the Jurassic section. This anomaly is interesting for more than its size. It apparently is a unique feature, as no similar high-amplitude ev nt has been found in the same stratigraphic interval on any other seismic line in the Georges Bank basin.

The area of study lies near the middle of the Georges Bank basin, which is trough-shaped and opens to the southwest. Major structural features in the basin appear to be related to basement structure. The lithologic sequence roughly consists of (a) as much as 1 km of poorly consolidated Tertiary sands, silts, and clay; (b) as much as 3.5 km of Cretaceous clastics; (c) as much as 4 km of Jurassic dolomite, limestone, and anhydrite; and (d) a basement of lower Paleozoic crystalline rocks, similar to those underlying the coastal plain. Well logs show a layer of salt overlying the basement. Potential source rocks are expected to include organic-rich Cretaceous shale and carbonaceous Jurassic limestone.

The anomaly was modeled as three possible lithologies that produce reflection amplitudes of the magnitude observed on the seismic line. Modeled lithologies included a porous carbonate containing gas, a salt lens, and an igneous intrusive rock. Analysis of the model study indicated that the porous carbonate and salt model produced a reflection amplitude that closely matched the reflection characteristics of the anomaly from the seismic data.