The rate of growth of the modern highstand prograding shoreline has been compared to that of the ancient Stage 3 and Stage 5d highstand prograding shorelines. The modern shoreline averages 2.5 m-thick and has prograded 7 km offshore. This equates to an average rate of growth of 0.4 km3/1000 years. The Stage 3 highstand shoreline prograded 8.1 km offshore, averages 7.8 m-thick, and prograded for 35,000 years. This equates to a volume of preserved sediment of 0.1 km3/1000 years. The Stage 5d shoreline prograded 10,000 years, averages 9.4 m-thick, and has prograded 16.5 km offshore. This equates to an average rate of growth of 1.2 km3/1000 years.
The Stage 3 and Stage 5d highstand shorelines prograded in response to high sediment yields from the Colorado and Rio Grande rivers. This sediment was reworked and transported by longshore currents into the study area. The greater thickness and extent of the Stage 5d shoreline compared to that of the modern and Stage 3 may be due to a high sediment supply from the wave-dominated Rio Grande HST 1 delta. The Stage 3 volume is an order of magnitude lower than Stage 5d due to a rapid base-level fall and stranding of the Stage 3 shoreline on the shelf.
It follows that there is a high preservation potential of a highstand shoreline if the
sediment supply is great enough to overcome erosion and ravinement associated with sea-
level rise and fall. It appears that the critical factor that determines the thickness and overall
extent of the shoreline is the amount of sediment input into the system. Given that the
modern highstand shoreline has a low sediment input (no major deltas currently empty into
the Gulf) compared to Stage 5d time, and continues to prograde despite this low sediment
input, the likelihood of preservation is uncertain. It may be inferred that the modern
shoreline will be preserved if sediment supply to the system increases.
Near-shore
core dataset