Bucket Welding: Where Does All That Salt Go?

Abstract

Thick bucket minibasins are a relatively common feature in the northern Gulf of Mexico. The two main end members are isolated minibasins that formed above a feeder, or were transported into a feeder, and progradational minibasins that formed as a sedimentary wedge encountered a feeder. In both cases the minibasins sink down the throat of a diapir. Salt within the feeder must be expelled from from the feeder in order to facilitate welding. One way to do this is by extension that creates space for the minibasin to sink, but it is difficult to reconcile extension with some examples of bucket minibasins in the downdip parts of northern GoM. We used physical models to investigate this process. Our findings are as follows: 1. In the isolated minibasin scenario bucket welds formed successfully only where the minibasin did not exceed the width of the neck of the feeder and the isolated minibasin stayed in situ, see-sawing it's way down the feeder as it was loaded from above. This scenario is probably rare in nature. 2. Progradation across a circular feeder results in significant deepening of the minibasin but failed to evacuate all the diapiric salt before the sedimentary wedge formed a canopy weld on the seaward side of the feeder, inhibiting further expulsion. Even with the addition of deep ridge networks to interconnected diapirs, to allow deep outflow of diapiric salt, progradation far outpaced salt evacuation resulting in significant trapped salt. 3. Doubling the length of the feeder so it became an elliptical wall in order to enhance the area of the prograding wedge that lay above thick salt, coupled with deep ridge networks, successfully resulted in subsidence of bucket minibasins. Deep outflow of diapiric salt was minor compared to the breakout and surface back flow of salt that allowed the progradational minibasin to rotate and continue to subside into the feeder. Major extrusion onto the minibasin (air) took much less work than evacuation of diapiric salt by intrusion into the seaward diapirs along the deep ridge network or by inflating the canopy and pushing up its roof. The surface back flow also provided an additional load on the feeder, accelerating expulsion of salt. An additional model, identical in all aspects except for the absence of the deep ridge network, also successfully formed a bucket minibasin through the same process. It took an additional 5 days, indicating that deep salt flow, whilst not a necessity, does accelerate welding.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017