EFFECTS OF SEA-LEVEL CHANGES ON DELTA FORMATION: A SMALL-SCALE PHYSICAL MODEL APPROACH

As a means of investigating the geologic factors that affect the geometry, facies, and sediment grain-size distribution of deltas, a tank measuring 4-feet by 2-feet by 1-foot was assembled in which small-scale physical models of deltas can be built. Constructed of ¼-inch acrylic, the tank was designed with options to adjust the inclination of the shelf-slope, raise and lower sea level, alter discharge, and change the location of sediment input. Each sediment size fraction was color-coded for ease of identification and visual contrast. This study focuses on how sea-level changes affect internal and external attributes of deltas. Prior to experimenting with sea-level changes, control deltas were built under static sea-level conditions to provide a baseline for comparison. Development of these deltas was photodocumented to record the history of progradation. When fully developed, water was drained from the tank to access, photograph, and sample those facies formed in subaqueous environments. To analyze facies associations, facies successions, facies architecture, and grain-size distributions, the deltas were cored to basement in strategic locations. Once cored, the deltas were physically sectioned in a series of longitudinal and transverse profiles to reveal internal morphology. With the understanding gained from studies of the control deltas, experiments were performed wherein sea-level fluctuations were introduced to model changes in delta formation under conditions of eustatic sea-level rise and fall. For comparison, photographs and experimental data were collected using the methods applied to the control deltas. Results of this investigation illustrate that sea-level changes affect delta geometry, thickness of slope deposits, position of the shelf-slope break, occurrence of faults, nature of base of slope deposits, and bedding characteristics.