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Miami Geological Society Publications
MGS-Altschuler 1964

Field Guidebook Geology and Geochemistry of the Bone Valley Formation and Its Phosphate Deposits, West Central Florida

By

Z. S. Altschuler, J .B. Cathcart, and E. J. Young

INTRODUCTION

The Bone Valley Formation is a shallow-water, marine and estuarine phosphorite of Pliocene age. It underlies about 2000 square miles of the coastal plain, inland from Tampa Bay, Florida, and is one of the world's most important sources of phosphate. The formation occurs on the southern flank of the Ocala uplift as a thin blanket of pebbly and clayey sands composed of quartz and clastic phosphate. This deposit is an excellent example of marine transgression during which the phosphate was derived, by reworking, from the underlying, weathered, Hawthorn Formation.

The southeastern coastal plain has been traditionally viewed as a very stable and undisturbed domain of shelf sedimentation. Study of the Bone Valley Formation, however, shows the Florida shelf to be a region of both tectonic and geochemical instability. Linear belts of structural uplift have been active in the region since the mid-Tertiary. This uplift has influenced the scenic present-day karst topography and has determined the Bone Valley's lithologic and faunal facies differentiation.

Weathering has transformed phosphatic clays and clayey sands of the Bone Valley Formation to lateritic aluminum phosphates, has created zones of supergene uranium enrichment, and also has caused a regional transformation of montmorillonite to kaolinite. Prolonged weathering leads to breakdown and removal of clay, and ultimately creates apparently unconformable mantles of residual quartz sand, which have been regarded as Pleistocene terraces. Geologic study in this area is characteristically plagued and enlivened by weathering which has destroyed real unconformities and created apparent ones.

Trips into the open-pit phosphate mines of the region will review recent studies of the U. S. Geological Survey, emphasizing the above aspects of the geology. It is pertinent to note that many of the interrelations and problems reviewed, however fundamental and interesting in themselves, were disclosed in the course of Geological Survey investigations of phosphorites as radioactive raw materials. These studies of phosphorite were motivated, in turn, solely by the occurrence of uranium as a trace substituent in the structure of apatite!

Many geologic studies pertinent to the area are cited in the following pages. A number of these, however, warrant special notice as synoptic or regional studies. Cooke (1945) in a classic and indispensable treatise covering the entire state; Vernon (1951) presents a cogent regional study of the Ocala uplift; and important data and observations on an area of comparable lithology and problems in the "hardrock" belt to the north, are presented in Espenshade and Spenser (1963).

(The entire report is available below)


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