State of Delaware DELAWARE GEOLOGICAL SURVEY Robert R. Jordan, State Geologist REPORT OF INVESTIGATIONS NO. 59 BEDROCK GEOLOGY OF THE PIEDMONT OF DELAWARE AND ADJACENT PENNSYLVANIA by Margaret O. Plank1, William S. Schenck1, LeeAnn Srogi2 University of Delaware Newark, Delaware 2000 1Delaware Geological Survey 2West Chester University State of Delaware DELAWARE GEOLOGICAL SURVEY Robert R. Jordan, State Geologist REPORT OF INVESTIGATIONS NO. 59 BEDROCK GEOLOGY OF THE PIEDMONT OF DELAWARE AND ADJACENT PENNSYLVANIA by Margaret O. Plank1 William S. Schenck1 LeeAnn Srogi2 University of Delaware Newark, Delaware 2000 1Delaware Geological Survey 2West Chester University CONTENTS Page Page ABSTRACT . 1 Montchanin Metagabbro. 15 Mill Creek Metagabbro . 16 INTRODUCTION. 1 Rockford Park Gneiss . 16 Purpose and Scope. 1 Brandywine Blue Gneiss. 17 Regional Geologic Setting . 2 Arden Plutonic Supersuite . 18 Acknowledgments. 2 Ardentown Granitic Suite. 19 GEOLOGIC UNITS. 2 Perkins Run Gabbronorite Suite. 20 Baltimore Gneiss . 2 Biotite Tonalite. 21 Setters Formation. 4 Bringhurst Gabbro. 21 Cockeysville Marble. 6 Iron Hill Gabbro. 22 Wissahickon Formation. 7 Diabase Dike . 23 Ultramafic Lens . 10 GEOCHEMISTRY OF THE MAFIC ROCKS . 23 Wilmington Complex. 10 Windy Hills Gneiss . 11 DISCUSSION . 23 Faulkland Gneiss . 12 CONCLUSIONS . 25 Christianstead Gneiss. 13 Barley Mill Gneiss. 14 REFERENCES CITED . 26 APPENDIX Page TYPE AND REFERENCE SECTION LOCATION MAPS AND LITHOLOGIES . 31 Baltimore Gneiss in Delaware . 32 Setters Formation in Pennsylvania. 33 Setters Formation in Delaware. 34 Cockeysville Marble in Delaware. 35 Wissahickon Formation at Brandywine Springs Park, Delaware. 36 Wissahickon Formation at Mt. Cuba, Delaware . 37 Ultramafic Lens . 38 Windy Hills Gneiss. 39 Faulkland Gneiss. 40 Christianstead Gneiss . 41 Barley Mill Gneiss . 42 Montchanin Metagabbro . 43 Mill Creek Metagabbro. 44 Rockford Park Gneiss . 45 Brandywine Blue Gneiss . 46 Arden Plutonic Supersuite Ardentown Granitic Suite . 47 Arden Plutonic Supersuite Perkins Run Gabbronorite Suite . 48 Arden Plutonic Supersuite biotite tonalite lithodeme . 49 Bringhurst Gabbro . 50 Iron Hill Gabbro . 51 Diabase dike . 52 ILLUSTRATIONS Page Figure 1. Map of the Mill Creek Nappe showing the location of the Landenberg and Hockessin-Yorklyn anticlines and Pleasant Hill valley. 3 BEDROCK GEOLOGY OF THE PIEDMONT OF DELAWARE AND ADJACENT PENNSYLVANIA Margaret O. Plank, William S. Schenck, and LeeAnn Srogi ABSTRACT This report accompanies a new map that revises the original bedrock geologic maps of the Delaware Piedmont com- piled by Woodruff and Thompson and published by the Delaware Geological Survey (DGS) in 1972 and 1975. Combined detailed mapping, petrography, geochemistry, and U-Pb.