Interspersed among the gentle fold structures of the Cantabrian Mesozoic basin of northern Spain are at least 12 diapirs with cores of plastic Triassic shales, evaporites, and ophitic igneous rocks exposed at the surface. These occur in an S-shaped belt in a generally east-west direction over a distance of some 130 kilometers (80 miles). The Iza diapir is located in the extreme east portion of the basin at the eastern termination of the diapiric trend.

Surface evidence and seismic work plus the data from four wells drilled on the Iza structure have outlined this unusual diapir. Most diapirs in northern Spain are expressed on the surface as nearly circular depressions

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representing the intrusive mass. The surrounding beds dip away from the central depression, often very steeply, to form a series of cuestas. In contrast, the Iza diapir is a buried wall or ridge of plastic rock at least five kilometers (three miles) long by less than 1.5 kilometers (one mile) wide intruded into a sedimentary section over 4,410 meters (14,470 feet) thick. Only the uppermost tip is exposed at the surface in a belt of indistinct outcrops up to 30 meters (98 feet) wide. One of the wells drilled on the structure encountered an inverted block of Upper Cretaceous sandstone above Paleocene carbonates, apparently incorporated into the diapir.

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