Megabeds: emplacement mechanics of large-volume event beds


Megabeds are the deposits of large-volume, sediment gravity flows found in both ancient and modern deep-water basins (e.g. >50 m to bathyal water depths) world-wide. Other than implication of size, megabeds are relatively poorly defined with limited knowledge of their flow processes and deposition. This thesis aims to establish the emplacement mechanics, flow process evolution, depositional settings and triggering mechanisms of megabeds by focusing on how sediment flow processes change in space and time. Event beds from two locations were studied: 1) the Waitemata Basin, northern New Zealand, and 2) the Central Graben, North Sea, UK. In addition megabed deposits from the literature were also reviewed. Extensive fieldwork (at 22 sites) was completed on deep-water sedimentary rocks of the Early Miocene Waitemata Basin, with particular attention on the Parnell Volcaniclastic Conglomerates (PVCs). Provenance and petrographic analysis suggest the turbidites and PVCs are multi-cycle sediments sourced from the north or east of the basin via a NW-SE trending slope. There is no record of any arc material in the Waitemata Group sediments although there is evidence of intraplate volcanism. Given the lack of arc derived material it is proposed that arc volcanism in the vicinity of the Waitemata Basin commenced at least post ~18 Ma. Facies analysis classified a total of 7 PVC bed types that result from single and multiple large volume gravity flows. Distinct differences in the spatio-temporal evolution of these flows result in a variety of megabed types. Emplacing flows include both cohesive and non-cohesive debris flows, turbidity currents and transitional flows. Facies associations are used to define several depositional elements including turbidite channels, levees, lobes and slumps. Analysis of core data from 13 hydrocarbon wells across the Central Graben, North Sea identified a single correlatable Palaeocene megabed that attained thicknesses around 25 m within a distal fan region. This megabed was deposited by a single large-volume turbidity current that interacted with the muddy substrate to enhance run-out distance and the ability for that flow to by-pass. Mud entrainment is interpreted to have increased flow efficiency by dampening turbulence resulting in lower flow Reynolds numbers and a faster thinner flow. However, substrate roughness created by underlying debrites and basin floor topography counteracted turbulence suppression. Megabeds are here defined as being emplaced by a single, large-volume (> 1.5 km3) sediment gravity flow (with a turbidity current component), an order of magnitude thicker than the mean thickness of the enclosing strata. This study classified 3 types of megabed deposits: A) coarse-grained, graded; deposited by a non-cohesive debris flow transforming into a turbidity current; B) coarse-grained couplet; deposited by a cohesive debris flow undergoing surface transformation to produce a residual turbid cloud; and C) fine-grained couplet; deposited by a waning, large-volume turbidity current that may be modified by substrate interactions or topography. When a megabed is the deposit of a turbidity current alone, it can been called a megaturbidite. In summary the key message of this thesis is that the nature of failed material, and the morphology and substrate of the receiving slope and basin are the primary control on megabed character. These parameters are often interlinked e.g. tectonically active margins with volcanic input and carbonate platforms are often mud-poor and receiving basins are spatially restricted and steep-sided. This produces Type A megabeds. A more mud-rich shelf edge or carbonate platform collapse has the potential to produce a Type B megabed. A mixed sand-mud passive margin with an extensive receiving basin and gentle slopes will most likely produce Type C megabeds.


TitleMegabeds: emplacement mechanics of large-volume event beds
AuthorFildes, Christine T.
Degree DisciplineGeology
Degree LevelDoctor of Philosophy
Degree GrantorUniversity of Auckland
FocusMarine geology
LocalityWaitemata Basin
Map References
NZMS 261 References
Auckland [R11]