"Gulf of Mexico" Sea Level Curve

William F. Tanner, S. Demirpolat, Frank W. Stapor, Luis Alvarez

Beach ridge plains on four sides of the Gulf of Mexico produced a new, better sea level history for late Holocene time: St. Vincent Island, Florida panhandle; Sanibel Island, lower peninsula of Florida; Mesa del Gavilan, east of Brownsville, Texas; and Isla del Carmen, in the state of Campeche, Mexico.

On St. Vincent Island there are more than 300 data points (elevation and relative age), each taken from a beach ridge or a swale. These ridges and swales can be grouped in sets; some sets stand high, and some are low. Their boundaries mark rises and drops of sea level. The St. Vincent Island data fall in a continuous time sequence. The types of data available on and adjacent to this island are topographic (plane-table work), sedimentologic (high-precision grain size parameters), photographic (air photos), and pedologic. All of these data agree in meaning, as far as sea level history is concerned. They provide a record of changes, both up and down, in the range of about 1 m to perhaps as much as 3-4 m. Three rises and two drops in the last 6,000 years are obvious. Changes as small as 1 -50 cm probably cannot be detected by the methods at hand, but it is highly unlikely, with hundreds of data points in one small study area, that major changes (1 m or more) have been overlooked. The latest change was a rise, about 800 years ago.

The St. Vincent Island sea level curve is the longest in the four study areas, going back to a highstand (+ 1.5-2.5 m) much more than 3,500 years ago, perhaps 5,000-6,000 years ago, and with details beginning to appear about 3,500 years ago, at the time of a lowstand. On this historical curve, there are three rises and two drops.

The Sanibel curve is the second longest, starting around 3,500 years ago with a lowstand. Except for the high at roughly 5,000 years ago, it shows all of the positions seen in the St. Vincent area. The Isla del Carmen curve is based on archeological data. It starts with a high roughly 2,400 years ago, followed by a low at about 1,700 years ago, and then one rise (800 years ago) to the present position. The Mesa del Gavilan curve (the shortest) has a single rise to the modern position, at roughly 800-1,000 years ago.

Because the four areas are scattered over distances exceeding 1,000 km, the cause of the changes in sea level position is taken to be actual sea level history, rather than local warping. We also have positive evidence (submerged beaches) for a Holocene sea level position at - 2 m. Because of the fluctuating history of sea level in the late Holocene, it is thought that these submerged beaches would not have survived a series of transgressions and regressions, and therefore represent the low position 1,000-2,000 years ago.

We would like to designate this history as the "Gulf of Mexico" sea level curve, to emphasize the fact that it is not local and therefore cannot be the result of warping, compaction, or other local effects.

For the Gulf of Mexico region, we can now make certain general statements.

1. The late Holocene sea level curve was not asymptotic from below.

2. There were several important fluctuations--in the 1-5 m range.

3. Changes less than 0.5-1.0 m cannot now be identified.

4. The many available data points (simple beach ridge plains) provide relatively high precision.

5. Quartz sand beach ridges yield hydrodynamic information, in a time sequence; shell ridges permit detailed radiometric dating.

6. Grain-size data show sea level stability for intervals as short as 10-40 years.

7. Use of a large number of data points from a large area produces regional average values that are probably meaningless.

8. Study of a very small area--such as St. Vincent Island--minimizes problems of warping, tilting, and compaction.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.