Iza, an Unusual Diapir in Northern Spain1
Interspersed among the folded structures in the Cantabrian Mesozoic basin of northern Spain are at least 12 diapirs with cores of plastic Triassic shale, evaporite, and ophitic igneous rock exposed at the surface. They are in a generally east-west alignment over a distance of about 130 km (81 mi). The Iza diapir is in the east part of the basin at the eastern termination of the diapiric trend.
Surface evidence, seismic work, and the data from four wells drilled on the structure have enabled workers to outline this unusual diapir. Most diapirs in northern Spain are expressed on the surface as irregularly circular depressions representing the intrusive mass. The surrounding beds dip away from the central depression, commonly very steeply, to form sharp ridges. In contrast, the Iza diapir is a buried wall or ridge of plastic rock—at least 5 km (3 mi) long by less than 1.5 km (1 mi) wide— intruded into a sedimentary section more than 4,410 m (14,468 ft) thick. Only the uppermost tip is exposed, and it is in a belt of indistinct outcrops up to 30 m (102 ft) wide. One of the wells drilled on the structure penetrated an inverted block of Upper Cretaceous sandstone above Paleocene carbonate rock; the block apparently was incorporated into the diapir.
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“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.