Publications
Salt-flank deformation and sedimentation during passive rise of the exposed El Papalote diapir, La Popa Basin, Mexico, GeoCanada2000, Calgary, M. Rowan, K. Giles, T. Lawton, R. Ratliff
El Papalote is an exposed salt stock located in La Popa Basin, northeast Mexico. It is elliptical in plan view (approximately 1x2 km) and occurs on the limb of a major anticline formed during the Tertiary Hidalgoan Orogeny, with a minor flexure centered over the diapir. Shortening postdated much of the diapir growth, so that the diapir and adjacent growth strata are now tilted about 30i to the northeast.El Papalote diapir is surrounded by Maastrichtian to Eocene rocks that are deformed in a halo of about 1 km width. These strata include marine to nonmarine siliciclastics that thin toward the diapir, as well as locally developed carbonate "lentils" that pinch out away from the diapir. The bases of the lentils are debris flows that include diabase clasts that were sourced from large, Mesozoic diabase inclusions within the body of the diapir. The thinning and onlap of the siliciclastics, the local development of the carbonate reefs, and the diabase clasts all attest to the bathymetric elevation and periodic exposure and erosion of the diapir, proving that the diapir was growing passively during at least the Maastrichtian to Eocene.
The halo of deformation around the diapir consists of beds that gradually increase in dip from regional values to vertical or overturned as they approach the diapir. Removal of the shortening deformation shows that the diapir flared in all directions, but especially to the southeast, where the edge of salt originally had dips that ranged from 45¡ to horizontal. In this area, the beds closest to the diapir are folded through vertical to completely overturned before being truncated by a vertical unconformity and overlying beds. These beds, in turn, are also folded to completely overturned before they too are cut by younger vertical beds. In other words, the geometry consists of a series of stacked and folded "J"-unconformities (Figure 1).
Figure 1: Cross section through southeastern edge of El Papalote diapir (in grey) showing folded siliciclastic units and carbonate lentils (1-5) and "J"-unconformities. No vertical exaggeration.
The geometry of the unconformities and the existence of the overlying carbonate debris flows show that the deformation accompanied passive diapirism at the sea floor and was not the result of active intrusion of the salt upward through a preexisting column of rock. However, the sharp angular unconformities create problems for restoring the observed geometry and determining the evolution of the passive diapir. Standard restoration algorithms either require slip surfaces at high angles to bedding or result in significant bed-length changes, neither of which is justified by field observations. Therefore, we employ "complex geometry", a new kinematic algorithm, to forward model and restore the sequential evolution while maintaining all bed lengths and cross-sectional area. The results show the progressive interaction between diapirism and sedimentation, which consists of repeated cycles of diapir inflation and sediment onlap, failure of the oversteepened bathymetric high and erosional truncation, and salt extrusion.
The results of this study provide important new insights into the geometry and evolution of diapir-flank structures. Overturned beds and complex, folded unconformities adjacent to and beneath vertical and flaring diapirs cannot be imaged on seismic data, even modern 3-D data. The lessons learned from exposed diapirs such as those in La Popa Basin should be used in the exploration and production of hydrocarbons around analogous subsurface diapirs in salt basins throughout the world.
