ABSTRACT

This report describes late Neogene (post-Oligocene) deposits in southwestern Florida with special emphasis on the Caloosahatchee River area. The Caloosahatchee and Ft. Thompson formations are described in detail and their relationship to the Tamiami formation, Pamlico and other Late Neogene deposits is discussed.

In southern Florida the Caloosahatchee marl rests unconformably on the Tamiami formation (Late Miocene). In most places the Caloosahatchee marl is less than 40 feet thick, but east of Ortona Locks it reaches a thickness in excess of 60 feet. The formation is composed of a succession of shallow-water marine and brackishwater marls, sandstone and limestone, and a few intercalated thin beds of fresh-water marl. Most of the deposits were formed close to shore on the inner continental shelf or in bays and inlets, but the middle member (Bee Branch member) represents a relatively deep water (15-20 fathoms deep) outer shelf environment. The Bee Branch member can be traced for many miles along the Caloosahatchee River.

Typically, the Ft. Thompson formation, which overlies the Caloosahatchee marl unconformably, is represented by a thin sequence of alternating and intertonguing fresh-water and marine beds deposited along a fluctuating shoreline. The uppermost member, the Coffee Mill Hammock marl, is the most extensively developed unit, and was deposited in the shallow water (5 fathoms deep) of a bay or inlet. The lower member, the Okaldakoochee marl, is typically represented by fresh-water beds and one shallow-water marine unit.

The Caloosahatchee marl, heretofore considered Pliocene in age, is assigned to the Pleistocene epoch. The most compelling evidence supporting a Pleistocene classification is derived from vertebrate remains. Most of the vertebrate species never have been recorded from deposits older than Pleistocene. The presence of Equus (Equus) sp. cf. E. (E.) leidyi and Holmesina septentrionalis in the formation is regarded as strong evidence for a post-Kansan (probably Sangamonian) age. Paleoecological and paleogeographical studies indicate that the Caloosahatchee marl was deposited under tropical conditions suggestive of an interglacial period, and that the deposition of the formation should be, at least in part, correlated with the making of the Wicomico shoreline (100 feet above sea level), generally regarded as Sangamonian in age.

The Ft. Thompson formation is classified as Wisconsinan in age, and was possibly deposited during a temporary recession of the Wisconsinan glaciers. Paleogeographical and paleoecological studies suggest that the formation should be correlated, at least in part, with the making of the Pamlico shoreline (25-30 feet above sea level).

The overlying Pamlico sands may represent littoral sediments deposited from the retreating Ft. Thompson sea. The Lake Flirt marl, that overlies the Pamlico formation is a very recent deposit.