Abstract

The history of the delta of the Apalachicola River, Florida, corresponds to the history of sea level since about mid-Tertiary time. The principal parts of the delta are: (1) early to middle Miocene deposits having no depositional surface preserved; (2) four terraces at about the following elevations: (a) a middle to late Miocene terrace, 80 m, (b) a late Miocene or early Pliocene terrace, 50 m, (c) an early to middle Pliocene terrace, 35 m, and (d) a questionable terrace, of probable middle Pliocene age, 25 m; (3) Pleistocene shorelines, well preserved, at about 9 and 6 m; (4) the beach ridge flat, near present sea level, and hence no older than Pleistocene; (5) the modern delta; (6) the deltaic rim of barrier islands, lagoons, and tidal marshes; and (7) offshore shoals.

The 80, 50, and 35 m surfaces are parts of a Mio-Pliocene cuspate delta. Each terrace, concave seaward, has preserved on it an offshore flat, barrier island, and lagoonal pattern which is almost identical with the modern pattern. Granulometry and electron microscopy have been used to confirm the origin of these distinctive topographic features. The Miocene and Pliocene dates have been obtained from a sea level curve synthesized from many field and literature data. All terraces in the area higher than about 10 m are pre-Pleistocene in age.

Structural features, although fairly common, are small and readily resolved. Many faults, including those of Miocene and Pleistocene age, have a bearing of N 50° E. Despite these structures, the area is one of considerable stability.

The modern delta is filling an estuary, and the delta rim is being shaped by breakers, tidal currents, wind, and storm activity. There is a general relationship between starved beaches, length of barrier islands, and inlet widths, such that barrier islands are shorter and inlets wider where beaches are starved.