Shelf-slope sediment transport in medium-sized basin margin clinoforms, with a focus on slope channel facies and architecture
Creators: name: Gan, Yuqian
name: 0000-0001-8934-1300
Publisher: name: The University of Texas at Austin
Year: 2022
Subjects: Slope channel
Clinoform
Sediment gravity flow
Deep water
Shelf-slope transition

description: Once sediment is brought beyond the shelf-edge, its transport into deep water is dominated by sediment gravity flows passing through slope channels and canyons, as well as from finer grained overbanking systems diverted out of these conduits onto the generally muddy slope. Channels and channel complexes are well studied at the scale of continental-slope clinoforms, albeit often at relatively low resolution. This thesis focus on the facies and architecture of slope-channel fills within medium-sized clinoforms (height &lt; 1km) that were supply-dominated (low-angle progradting shelf edge trajectory). The main study database is a set of well exposed, ‘walkable’ clinoforms in the Early Jurassic, back-arc Neuquén Basin of Argentina. Key results from this work are that (1) upper slope channels have a tendency to be deeper than the lower slope channels within the same clinoform set, (2) a downslope fining of slope channel fill from conglomerate to sandstone, with conglomeratic debris flows accumulated preferentially in upper slope channels, whereas lower slope channels mostly host sand-rich turbidites, except for the cases of coarse sediment (pebble) bypass to lower slope and basin floor (3) individual channel-element infills (especially within upper slope channel complexes) are strikingly dominated by sharp-based, decimeter thick, sediment gravity flow units that internally show an upward change from conglomeratic to sand-rich beds. Clast sizes decrease upwards and sandy matrix percent increases upwards through each infill unit, as debrite beds change upward to sandy turbidites. We found that this prominent multi-flow, fining-upward sequence is especially common on upper slope sites and was possibly related to a eustacy-regulated decreasing sediment supply from the nearby shelf into the slope channels. This tentative explanation uses the landward-stepping action of a transgressive shoreline to explain the decimeter scale upward-fining architecture. The coarse sediment is initially brought to the shelf edge and slope by shoreline regression but sediment flux to the channels then reduced during drowning of the shelf. Autogenic explanations for the channel architecture are also possible, but the very prominent repetition of 10-20m thick regressive-to-transgressive cycles (kms lateral extent) in the adjacent shoreline-shelf stratigraphy make the allogenic drive hypothesis feasible. The slope portion of the Maastrichtian Lewis-Fox Hill clinoforms in Washakie Basin is used as a second case study for analysis of similar, medium-sized, supply-dominated clinoforms, with overall finer grains (sandstone dominant). Sandstone thickness from the slope section is extracted from closely spaced well log data to approximate slope channel thickness. In this case, the impact on channel architecture made by differential lateral subsidence across the basin and of lowering of the shelf-edge trajectory during clinoform growth was a focus of the analysis. Results suggest that (1) channels commonly reduced in thickness from upper to lower-slope sites, (2) more rapid subsidence across the same clinoform set increased the numbers of channels but the channels became relatively small and (3) flattening of the shelf-edge trajectory (from progradation-dominant to aggradation-dominant) caused more deeply downcut channels, (4) greater degree of progradation of the shelf edge is linked with deeper slope channels, explained by greater amount of sediment being ‘forced’ into deep water with a more limiting accommodation on the shelf.