Discerning halokinetic from autocyclic sequences in deep-water sedimentary systems

Abstract

Halokinetic stratigraphy refers to sedimentary deposits affected by the growth or retreat of salt-cored topography. Deep-water sediment gravity flows can be influenced by seafloor topography associated with salt structures, and the interaction between the two ultimately controls the depositional architecture of their successive deposits. Where these deposits onlap salt diapirs they can form reservoirs with combined structural-stratigraphic traps. Typical-ly, these halokinetic sequences are poorly-imaged in seismic data due to steep dips, salt over-hangs and near-diapir deformation. In addition, they are not well represented in outcrop, largely due to dissolution of the associated halites. Therefore, the facies and architecture of these halokinetically-influenced deep-water successions are challenging to investigate, but are antici-pated to differ from those in unconfined basins, or those where topography is generated by non-halokinetic processes. This study aims to understand how and why halokinetically-influenced deep-water systems dif-fer from those in unconfined settings, using a multi-scalar and multi-method approach. Out-crop data from the Cretaceous Basque-Cantabrian Basin are compared with subsurface data from the Paleocene North Sea Central Graben. The results of these studies are compared to two-dimensional Discrete Element Models of different sedimentation patterns influenced by salt growth. Key insights derived from this approach include: 1) salt-related, active topography, and the de-gree of confinement are shown to be important modifiers of depositional systems, affecting the degree of flow confinement which results in predictable facies variability and remobilisation of deposits; 2) facies analysis reveals that channels and lobes are influenced (re-routed and con-fined) by salt growth at a range of scales, from centimetre-scale sedimentological characteristics identified in core and outcrop, to kilometre-scale geomorphological attributes visible in seis-mic; 3) axially-derived deep-water depositional systems are heavily modified by laterally im-pinging mass transport deposits formed in response to salt-controlled topographic growth of the sea bed; 4) recognition criteria for deep-water halokinetically-influenced settings include: mul-tiple directions of ripple lamination, injectites, fluidisation structures, presence of hybrid beds, range of MTD types, stratigraphic thickness variations, onlap of deposits against halokinetical-ly-deformed substrate, and abrupt juxtaposition of deep-water depositional facies and MTDs; 5) modelled thinning rates are up to six times greater within 350 metres of a salt diapir, com-pared to further afield, and typically decrease upwards (with time) and laterally (with distance) from the diapir; and 6) in both subsurface and modelled scenarios, stratigraphy deposited be-tween closely-spaced diapirs (3 km), a flat, plateau-like zone is developed between the deformation zones, where halokinetic-modulation is reduced. Salt basins are complex, due to: the presence and variability of the top and base salt; early diapiric stratigraphy; non-piercing diapirs; and salt-related faults, therefore, even these ‘minimal modulation’ zones are unlikely to contain stratigraphy which is completely undeformed by halokinesis. The integration of these diverse techniques allows for the spatial and temporal distribution of deep-water facies and architectures in salt-influenced basins to be recognised. These character-istics are compared to their halokinetically-influenced counterparts from different depositional environments, and unconfined, or non-halokinetic topographically-confined deep-water basins. In most deep-water successions the dominant controls are allocyclic (external), but these suc-cessions are heavily influenced by halokinesis (salt growth) which drives autocyclic (internal) modulation of the primary depositional signa

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mads Huuse (Supervisor)