Four-dimensional Variability of Composite Halokinetic Sequences

Abstract

The architecture of salt diapir-flank strata (i.e. halokinetic sequences) is controlled by the interplay between volumetric diapiric flux and sediment accumulation rate. Halokinetic sequences consist of unconformity-bounded packages of thinned and folded strata formed by drape-folding around passive diapirs. These sequences are described by two end-members: (i) hooks, which are characterized by narrow zones of folding (<200 m) and high-angle truncations (>70°); and (ii) wedges, which are typified by broad zones of folding (300-1000 m) and low-angle truncations (<30°). Hooks and wedges stack to form tabular and tapered composite halokinetic sequences (CHS), respectively. CHSs were first and most thoroughly described from outcrop-based studies that, although able to capture their high-resolution facies variations, are limited in describing their 4D variability. This study integrates 3D seismic data from the SE Precaspian Basin, onshore Kazakhstan and structural restorations to examine variations in CHS architecture through time and space along diapirs with variable plan-form and cross-sectional geometries. The diapirs consist of curvilinear walls that vary from upright to inclined in cross-section, may flare-upward and locally display well-developed salt shoulders and/or laterally transition into salt rollers. CHS architecture is highly variable in both time and space, even along a single diapir or minibasin. A single CHS can transition along a salt wall and/or minibasin from tabular to tapered geometries. They can also be downturned and exhibit rollover geometries with thickening towards the diapir above salt shoulders. These variations can be linked to changes in the diapir morphology. Inclined walls present a greater proportion of tapered CHSs implying an overall greater ratio between sediment accumulation and salt-rise relatively to vertical walls. In terms of vertical stacking, CHS can present a typical zonation with lower tapered, intermediate tabular and upper tapered CHSs, but also unique patterns where the lower sequences are tabular and transition upward to tapered CHS. The study demonstrates that CHSs are more variable than previously thought, indicating a complex interplay between volumetric salt rise, diapir-flank geometry, sediment accumulation and roof dimensions. Ultimately, this improves our understanding of diapir-flank deformation and potential minibasin reservoir distribution.