HOLOCENE STRATIGRAPHY AND GEOMORPHIC EVOLUTION OF MORGAN PENINSULA, ALABAMA

Morgan Peninsula, Alabama is an attached barrier system about 25 km long and 2 to 5 km wide separating Bon Secour Bay from the Gulf of Mexico. Approximately 10.7 km of 100 mHz Ground Penetrating Radar (GPR) data, 21 km of 2-15 kHz chirp data, 4 cores with an average penetration of 13 meters, 30 cm resolution digital elevation data, and aerial photography were collected to examine the internal facies architecture and evolution of the peninsula.

The peninsula is composed of four obliquely-aligned sets of beach ridges that suggest a complex history of formation. The peninsula is connected to the mainland in the east by a Pleistocene Gulfport Fm. barrier complex, while the rest of the peninsula is composed of Holocene sediment. The oldest Holocene portion of the peninsula consists of NW-SE trending beach ridges that are truncated by concave beach ridges with the limbs terminating in the Gulf of Mexico. The westernmost part of Morgan Peninsula consists of a relatively narrow set of recurved beach ridges that indicate spit accretion in modern times.

We interpret two GPR facies: eolian and beachface. The beachface facies consists of prograding clinoforms with slopes of 10 to 15 degrees. The overlying eolian facies consists of parallel laminations. Both these facies overlie a deeper reflector at about 15 meters, which correlates to Pleistocene fluvial sediments and represents the base of Morgan Peninsula. The seaward dipping clinoforms are indicative of a prograding shoreline and have been subdivided into progradational packages, indicating a more episodic nature of construction. Beach ridges sets have different average gradients that suggest a change in wave energy, sediment supply, or source over the history of construction. A core through the clinoforms reveals a thick section of well-sorted, humate-impregnated beach sand overlying an interbedded sand and shell-hash unit. The shell hash contains a large number of both fragments and intact shells from inlet-dwelling fauna, suggesting that the oldest set of beach ridges overlie a tidal delta. Future work will concentrate on constraining the maximum age of Morgan Peninsula, and the rates of its formation.