Statistical analysis of turbidite cycles in submarine fan successions

To statistically test and evaluate the significance of asymmetric upward thickening and thinning trends and other cyclic patterns in turbidite successions, twenty-eight bed-by-bed sections with a wide coverage in geological time, tectonic settings, facies characteristics, and depositional environments were measured and described. First, 286 sandstone packets were selected from the 28 turbidite sections through statistically based segmentation. Then, these packets were examined by three powerful correlation tests (Kendall's, Spearman's, and Pearson's correlation tests) and four tests for randomness. Only 34 (11.9%) of the sandstone packets pass tests designed to identify asymmetry at the 10% significance level. Monte Carlo simulation and the binomial probability analysis indicate that the number of asymmetric sequences identified in the original set of turbidite sandstone packets is indistinguishable from the number which can be produced by random processes. -- Eighty-six sandstone packets were tested for upward coarsening and fining trends. It was found that as many as 50% of sandstone packets from coarse-grained channel fills fine or coarsen upward. Upward fining sequences dominate, which is interpreted as the result of channel filling or the stacking of onlapping deposits at a channel mouth. -- The Hurst statistic provides a measure of long-term persistence. Sixteen (84.2%) of 19 turbidite sections show the Hurst phenomenon, i.e., irregular and non-periodic clustering of high and low values of bed-thickness, grain-size, and sandstone percentage. This clustering is related to vertical changes of sedimentary facies caused by lateral shifting of environments on the fan surface. The strength of the clustering, inferred from Hurst K values, might be useful as an index to distinguish submarine fan environments: channel-Ievee complexes tend to have strong clustering; lobe-interlobe deposits tend to display moderate clustering; and basin-floor sheet sand systems tend to have weak clustering. -- Combining facies characteristics observed in the field with the statistical results provides some criteria for the identification of submarine fan environments, and yields four preliminary fan models. (1) Type 1 sandy fans typically form in forearc basins, are fed by littoral sources or fan deltas, and are characterized by very coarse (pebbly sandy) sediments. Both channel and lobe deposits are well developed. A significant number of upward fining sequences can be found in channel deposits. Channel-interchannel complexes, particularly channel-levee complexes, display strong clustering of bed-thickness, grain-size and other parameters. (2) Type 2 sandy fans mainly form in forearc basins but also in foreland basins, and are fed by littoral sources or small rivers. The basic features of this model are similar to those of model 1, but pebbly sediments are absent or rare, and upward fining sequences lack significance. (3) Type 1 muddy fans tend to form in passive margins, foreland basins or foredeeps, and are fed by a large river delta. They are typically characterized by nested sandy bodies of channel and crevasse-splay-lobe deposits in very well-developed muddy levee deposits, which causes strong clustering of bed-thickness, grain-size and other parameters. Channels might gradually die out without a sandy lobe at the end. (4) Type 2 muddy fans form in the same tectonic settings as model 3, but sediments are mainly derived from failure at the shelf break, resulting in well-developed megaturbidite beds with thick mud caps. The large, muddy turbidity currents responsible for these beds are usually unchannelized and efficiently transport sandy sediments to form wide spread basin-floor sand sheets