Analysis of Growth Strata Adjacent to an Exposed Deepwater Salt Diapir, Northern Spain

The Bakio diapir, composed of Triassic Keuper evaporites, is exposed in the Basque Pyrenees along the coast of northern Spain. It is flanked by upper-slope upper Aptian to Albian marls and siliciclastic turbidites, with intercalated sedimentary breccias of Aptian carbonates, that form a tapered composite halokinetic sequence (CHS). Strata are thinned and folded over a distance of 600 m from the diapir and capped by a prominent unconformity with gently dipping strata above.

Models and exposed examples of tapered CHS from Mexico and South Australia suggest that they develop when third-order sediment-accumulation rates equal or exceed diapir-rise rates, so that topographic relief is generally low and slumped debris is rare. A slow-down in sedimentation leads to increased relief and the development of the capping unconformity. Stratigraphic relationships show that this is not the case for the Bakio diapir. Debrites are common, first increasing in thickness and clast size and then decreasing toward the top of the CHS. Although detailed age data are lacking, the upward transition from marls to turbidites with pebble conglomerates suggests that sedimentation rate increased throughout the development of the CHS.

We propose that formation of the tapered CHS was not controlled so much by thickening of the roof during Albian deposition as by the presence of a preexisting thick roof of Aptian platform carbonates. Folding of the thick roof generated a wide halo of deformation flanking the diapir. Topographic relief first increased as marl deposition was unable to keep up with diapir rise, leading to common slump failure of the scarp and gradual unroofing of the diapir. Relief subsequently decreased due to a combination of erosional thinning of the roof and the presumably faster sedimentation of the turbidite package. The capping unconformity was likely caused by falling relative sea level and greater erosional capabilities of the associated turbidite flows on the upper slope.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California