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Stratigraphic Modeling for Concealed Phosphate Deposits in Virginia's Stratigraphic Modeling for Concealed Phosphate Deposits in Virginia’s Coastal Plain Virginia Department of Mines, Minerals and Energy Division of Geology and Mineral Resources in cooperation with United States Geological Survey Mineral Resources External Research Program COOPERATIVE AGREEMENT: G10AP00054 FINAL TECHNICAL REPORT Aaron Cross and William L. Lassetter 2011 "Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number G10AP00054. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government." “Opportunities for new discoveries are good, and the increasing knowledge about the origin and occurrence of phosphate deposits should aid prospecting.” V. E. McKelvey, 1967 Abstract A study to outline an exploration program for phosphates beneath Virginia’s Coastal Plain was undertaken by the Virginia Division of Geology and Mineral Resources with funding support from the U.S. Geological Survey. An in-depth literature review highlighted the importance of two primary stages in the development of phosphate deposits in the geologic setting of the mid-Atlantic Coastal Plain. During stage one, primary phosphatic material was precipitated in the early Paleogene marine environment from nutrient-rich, oxygen-poor seawater. During stage two, primary phosphate deposits were exposed to weathering, reworked, and concentrated. The model presented here, at its simplest, advocates the concept of a genesis unit, the Paleocene-Eocene Pamunkey Group, bounded on top by an unconformity, which is overlain by the host unit, the Miocene Calvert Formation, whose base contains a lag deposit of phosphatic sand and gravel. Using geologic data derived from geologic maps and water wells completed in Virginia’s Coastal Plain, the depth to the base of the Calvert Formation was mapped and a target zone was defined based upon a maximum overburden thickness of one hundred feet. Reconnaissance- scale field investigations included sampling of selected outcrops along the Rappahannock, Mattaponi, and Pamunkey rivers, together with borehole drilling in the interfluvial regions. Laboratory analytical results of sediment samples indicate P2O5 concentrations up to 3.96%, and good correlation between phosphate, uranium, thallium, and yttrium. Field observations, geochemical results from laboratory analyses, and analysis of the structural morphology of the basal Calvert Formation support the concept of phosphate enrichment in the basal lag deposits and within other defined structural lows. Several areas were identified for the next phase of investigations that would include evaluations of gamma activity from existing well logs and additional borehole drill testing and sample collection. i Table of Contents 1.0 Introduction ............................................................................................ 1 1.1 Statement of Problem ................................................................................................1 1.2 Statement of Purpose ................................................................................................1 1.3 Value and Economics of Phosphate .........................................................................2 1.4 Types of Phosphate Resources..................................................................................2 1.4.1 Guano ..................................................................................................................2 1.4.2 Phosphate Rock and Mineral Apatite .................................................................3 1.4.3 Apatite in Igneous Rocks ....................................................................................3 1.4.4 Marine Sedimentary Apatite Deposits (Phosphorites) ........................................4 2.0 Atlantic Coastal Plain Phosphate Deposits ............................................ 7 2.1 Phosphate Deposits of Florida ..................................................................................7 2.1.1 Economic Interest ...............................................................................................7 2.1.2 Stratigraphy .........................................................................................................8 2.1.3 Depositional Setting ............................................................................................10 2.2 Phosphate Deposits of Georgia .................................................................................12 2.2.1 Economic Interest ...............................................................................................12 2.2.2 Stratigraphy .........................................................................................................12 2.2.3 Depositional Setting ............................................................................................12 2.3 Phosphate Deposits of South Carolina ......................................................................13 2.3.1 Economic Interest ...............................................................................................13 2.3.2 Stratigraphy .........................................................................................................13 2.3.3 Depositional Setting ............................................................................................14 2.4 Phosphate Deposits of North Carolina ......................................................................14 2.4.1 Economic Interest ...............................................................................................14 2.4.2 Stratigraphy .........................................................................................................14 2.4.3 The Lee Creek Ore Zone.....................................................................................16 2.4.4 Pungo River Formation Depositional Environments ..........................................17 2.4.5 Structure of the Pungo River Formation .............................................................18 2.4.6 Depositional Setting ............................................................................................19 3.0 Stratigraphic Setting for Phosphate in Virginia’s Coastal Plain ............ 21 3.1 Pamunkey Group ......................................................................................................21 3.1.1 Brightseat Formation ..........................................................................................22 3.1.2 Aquia Formation .................................................................................................22 3.1.3 Marlboro Clay .....................................................................................................24 3.1.4 Nanjemoy Formation ..........................................................................................24 3.1.5 Piney Point Formation ........................................................................................25 3.1.6 Chickahominy Formation ...................................................................................26 3.2 Chesapeake Group ....................................................................................................26 3.2.1 Old Church Formation ........................................................................................27 3.2.2 Calvert Formation ...............................................................................................27 3.2.2.1 Fairhaven Member ........................................................................................28 3.2.2.2 Plum Point Marl Member .............................................................................29 3.2.2.3 Calvert Beach Member .................................................................................29 3.2.3 Choptank Formation ...........................................................................................30 3.2.4 St. Marys Formation ...........................................................................................30 ii 3.2.5 Eastover Formation .............................................................................................31 3.2.6 Yorktown Formation ...........................................................................................31 3.2.6.1 Sunken Meadow Member .............................................................................32 3.2.6.2 Rushmore Member........................................................................................32 3.2.6.3 Morgarts Beach Member ..............................................................................33 3.2.6.4 Moore House Member ..................................................................................33 3.3 Pliocene/Pleistocene Gravels ....................................................................................33 3.4 Summary of Stratigraphy ..........................................................................................34 4.0 Depositional Model for Phosphate in Virginia’s Coastal Plain ............. 35 4.1 Stage One: Phosphogenesis ......................................................................................35 4.2 Stage Two: Reworking and Concentration ...............................................................36 4.3 Key Stratigraphic Controls .......................................................................................36 5.0 Data Collection and Results ..................................................................
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