From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, February 18, 2008
Tracking the Migration of Salt Diapirs using
AAPG Distinguished Lecturer, New Mexico State University
The progressive migration of diapiric salt bodies can be interpreted using stratal and structural relationships present in the sedimentary packages that surround them. Distinctive near-diapir growth stratal packages associated with vertically moving passive diapirs are referred to as “halokinetic sequences”. Halokinetic sequences are angular unconformity bounded, growth-stratal packages that form due to temporal variations in relief over passively rising diapirs. These stratal packages document the dynamic interplay between salt movement and adjacent sedimentation.
Two end-member types of halokinetic sequences (Type A and Type B) have been recognized on outcrop in the shelfal strata of La Popa salt basin, Mexico. The types differ in depositional facies, maximum degree of internal folding, amount of fault reactivation on unconformities, overall sedimentation rate, and distance of halokinetic sequence termination from the salt/sediment interface. Type A sequences are associated with periods of overall very low sediment accumulation rates typical of marine transgression on the shelf. They contain basal, diapir-derived debris flows encased in outer shelf fine-grained sandstones deposited by hyperpycnal flows that are abruptly overlain by outer shelf black shales. These strata are locally tightly folded with truncation angles of up to 90o at sequence boundaries. Sequence boundaries show significant fault reactivation during later halokinesis and evidence of brittle shear. Type A sequences terminate directly against the diapir. Type B sequences are associated with periods of moderately high sediment accumulation rates typical of marine regression on the shelf. They contain basal, lower shoreface sandstone that shallow upward to tidal and lagoonal sandstone. These strata display minimal folding with truncation angles that are <15o. There is little or no reactivation of the sequence boundaries during later halokinesis. Type B diapir-proximal sequence terminations are spatially separated from the diapir by an average of 250m.
Both styles of halokinetic sequences are seen on seismic lines and can be used to “fingerprint” the fluctuating conditions present near the diapir during migration. The characteristics of the two types of sequences and their stratal arrangement into composite sequences have important implications for reservoir quality, geometry, continuity, and charge potential in diapir-related traps.