An Illustration of a Sea Level Control Model from a Subsiding Coastal Fan System: Pleistocene Ogasayama Formation, Central Japan

Abstract

The sea level control model is a framework to explain coastal alluvial fan processes controlled by relative sea level changes and the resulting stratigraphic sequences. The model generally predicts three types of stratigraphic units, namely: (1) open fan deposits with tripartite organization for each relative sea level rise beyond the previous high stand, and subsequent sea level fall (A-type); (2) fan valley fill deposits, constructed during sea level rise up to the previous high stand (B-type); and (3) dissection-fill deposits consisting predominantly of overflow deposits (C-type). Combination of the model with a eustatic fluctuation curve suggested from an oxygen isotope record, modified for tectonic subsidence, allows a semi-quantitative estimation of numbers and dimensions of respective stratigraphic units. Estimations for the time interval recorded by the Ogasayama Formation, which shows all the depositional units predicted by the model, compare favorably with the field observations. Thicknesses predicted are directly related to the suggested amounts of relative sea level changes, which are mostly around for all types of stratigraphic units. The average recurrence period for A-type bodies is and for B-type bodies , time intervals generally assumed to be in the order of eustatic sea level fluctuations. The general agreement between model and observations in terms of both stratigraphic architecture and semi-quantitative analysis illustrates the direct control of relative sea level fluctuations on the construction of coastal fan deposits. Alternate occurrences of humid and dry climatic conditions associated with the eustatic cycle may be responsible for progradation of Gilbert-type deltas within the fan valleys, which show finer grain size than the coarsest open fan deposits.