Document Type

Article

Publication Date

1989

Publication Title

U.S. Geological Survey Professional Paper 1499

Abstract

The southern portion of Trail Ridge in Clay County, Fla., has been an important domestic source of altered ilmenite, zircon, and other minerals since 1949. The ridge as a physiographic feature extends in a north-south direction over 160 km and is 1 to 2 km wide. The ore is a fine- to medium-grained sand body that extends down from the crest of the ridge about 20 m. Average heavy-mineral content is about 4 percent. Over half of the ilmenite and zircon is contained in thin, dark laminae dipping 26° to 41 ° southwest; these laminae extend deep within the ore and outline the slip faces of a great eolian dune complex. The subordinate dark laminae differ in heavy mineralogy and grain size from the encasing light-colored sand. The dark laminae have a modal grain size of 0.2 mm and average about 6 percent heavy minerals, among which altered ilmenite, zircon, and rutile predominate. The light-colored laminae have a coarser modal grain size of 0.3 mm and average only 1 percent heavy minerals, among which the lighter heavy minerals staurolite, sillimanite, and tourmaline form half. Grain size variables and grain surfaces are appropriate for eolian sand. Over­printed on these original features of the dune are a surficial weathering zone over 3 m thick, where tan leucoxene takes the place of black altered ilmenite, and several underlying humate-cemented zones that probably represent water-table stillstands.

The immediately underlying unit is a lignitic peaty layer 1.5 m thick, here referred to as peat. The organic fraction is derived entirely from freshwater plants. In-place tree stumps have been noted, but the predominant component of the peat is fragments of wood and other transported plant debris. The peat and its constituents indicate depo­sition in a swamp environment, and local horizons enriched with charcoal and fungal remains indicate periodic subaerial exposure. Vegetation varied from open shrub swamp to cypress forest.

The age of the peat has been determined palynologically as post­Miocene. Its carbon-14 age is greater than 4.5 x 104 years; that is, pre-latest Pleistocene.

The upper portion of the peat layer contains admixtures of sand. This sand is present as isolated grains embedded in laminated organic matrix and is dominantly well rounded and frosted. As a grain population, the sand in the peat matches the overlying Trail Ridge ore sand in mineralogy and grain morphology but is slightly finer in grain size. This sand we regard as an important clue to the history of the area. The sand was apparently deposited from aerial suspension, and its characteristics were acquired in the adjacent high-energy eolian environment. Upward increase of sand in peat records the approach of the dune that eventually prograded the swamp. The peat and the overlying sand are essentially the same age. Fine sand found in peat represents the sand fraction remaining suspended in flow separation at the top of the slip face of the dune. Ore represents the traction-load fraction. The Trail Ridge dune itself is probably the drainage dam that impounded the swamp it later overrode.

The sand embedded in peat is also an important clue to the weath­ering history of Trail Ridge heavy minerals, as entombment in peat probably arrested oxidation. The presence of leucoxene, and other features of the mineral assemblage, shows that the minerals were already weathered at deposition. This evidence is in accord with grain-surface features and grain-size-density relations. Thus, mineral alteration at Trail Ridge occurred in two stages, one before and one after deposition.

Trail Ridge apparently represents a coast-parallel transgressive dune complex, analogous to younger dunes elsewhere that have become completely decoupled from parental shorelines. The Trail Ridge body was probably composite, made up of individual parabolic dunes, each migrating southwestward. The base of the Trail Ridge body was probably originally higher at the southern end than at the northern end.

Comments

USGS-authored or produced data and information are considered to be in the U.S. public domain.

This professional paper was obtained from U.S. Geological Survey.

Download

Share

COinS