(Dis)Order in Submarine Fans at Different Observational Scales: An Example From Unit a of the Laingsburg Formation, Karoo Basin

Abstract

Perceived order in submarine fans is traditionally used to support simple sequence stratigraphic interpretation of allogenic forcing but evidence is typically limited to qualitative evaluation of the geological dataset. Quantitative analysis of the well-studied ‘Unit A’ basin-floor fan from the Laingsburg Formation, Karoo Basin, South Africa suggests a more complex interpretation. We use Markov facies analysis to test for order in the facies successions, and runs tests and power spectral analyses to assess order in the succession of bed thicknesses. Some bed-by-bed order is detectable in the facies successions, but thickness successions are more complex; no order is detectable as simple consistent bed-by-bed runs of thickening or thinning-upwards. Spectral analysis does detect some significant periodicities, but the periods are different in each vertical section analysed, ranging from 35-45 beds, 50-55 beds and from 60-70 beds in respective vertical sections. This perhaps indicates that complex autogenic compensational stacking processes control the details of how patterns emerge at specific locations, even when larger-scale trends do occur in regional stacking of lobes and lobe complexes and there is no evidence for a single, dominant allogenic control the Karoo submarine fan strata. More comprehensively, this analysis suggests that sequence stratigraphic interpretations of outcrop and core deep-water strata will be more robust with integrated quantitative bed thickness and facies trend analysis as a standard approach. Sequence stratigraphic interpretations of deep-water strata should avoid a model-driven single dominant control approach, and instead use the quantitative analysis to more fully explore the typical complexity of these strata.