Abstract
The frequency distribution of turbidite thickness records information on flow hydrodynamics, initial sediment volumes and source migration and is an important component of petroleum reservoir models. However, the nature of this thickness distribution is currently uncertain, with logânormal or negativeâexponential frequency distributions and powerâlaw cumulative frequency distributions having been proposed by different authors. A detailed analysis of the Miocene Marnoso Arenacea Formation of the Italian Apennines shows that turbidite bed thickness and sandâinterval thickness within each bed have a frequency distribution comprising the sum of a series of logânormal frequency distributions. These strata were deposited predominantly in a basinâplain setting, and bed amalgamation is relatively rare. Beds or sand intervals truncated by erosion were excluded from this analysis. Each logânormal frequency distribution characterizes bed or sandâinterval thickness for a given basal grainâsize or basal Bouma division. Measurements from the Silurian Aberystwyth Grits in Wales, the Cretaceous Great Valley Sequence in California and the Permian Karoo Basin in South Africa show that this conclusion holds for sequences of disparate age and variable location. The median thickness of these logânormal distributions is positively correlated with basal grainâsize. The powerâlaw exponent relating the basal grainâsize and median thickness is different for turbidites with a basal A or B division and those with only C, D and E divisions. These two types of turbidite have been termed âthin beddedâ and âthick beddedâ by previous workers. A change in the powerâlaw exponent is proposed to be related to: (i) a transition from viscous to inertial settling of sediment grains; and (ii) hindered settling at high sediment concentrations. The bimodal thickness distribution of âthinâbeddedâ and âthickâbeddedâ turbidites noted by previous workers is explained as the result of a change in the powerâlaw exponent. This analysis supports the view that A and B divisions were deposited from highâconcentration flow components and that distinct grainâsize modes undergo different depositional processes. Summation of logânormal frequency distributions for thinâ and thickâbedded turbidites produces a cumulative frequency distribution of thickness with a segmented powerâlaw trend. Thus, the occurrence of both logânormal and segmented powerâlaw frequency distributions can be explained in a holistic fashion. Powerâlaw frequency distributions of turbidite thickness have previously been linked to powerâlaw distributions of earthquake magnitude or volumes of submarine slope failure. The logânormal distribution for a given grainâsize class observed in this study suggests an alternative view, that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters, in addition to the settling velocity of the grainâsizes present.
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