Metadata: Preliminary Bedrock Geologic Map of a Portion of the Wilmington 30- by 60-Minute Quadrangle, Southeastern Pennsylvania
The Wilmington 30- x 60-Minute Quadrangle includes rocks that range in age from Middle Proterozoic to Silurian, and in metamorphic grade from lowest greenschist to granulite facies. Tectonic environments represented in the quadrangle include continental basement rocks, shelf sediments, marine sediments spanning rift to drift, a fragment of a magmatic arc, and deep-seated plutons. By way of introduction, a speculative tectonic history for the area follows. This history is compiled from discussion with all the authors whose work contributed to this map, with additional input from Plank and others (2000) and Srogi (2004). As the Iapetus Ocean opened during the latest Precambrian to early Cambrian, a rift margin developed on the eastern side (present coordinates) of Laurentia. Sediments that became the Glenarm Group, Glenarm Wissahickon, and Mt. Cuba Wissahickon were deposited in marine rift basins floored by continental crust represented by the Baltimore Gneiss. In these rift basins, the sediments intermingled with continental rift-related basalts that became the White Clay Creek Amphibolite. The Kennett Square Amphibolite with its ocean-floor geochemical signature, now found associated with the Mt. Cuba Wissahickon, represents the transition from rift to drift. The Peters Creek Schist, with its interlayers of oceanic-affinity Bald Friar Metabasalt, may represent the culmination of sedimentation in the rift basins. As true sea-floor spreading progressed during the Cambrian, a passive margin formed inboard of the rift basins. The transgressive shelf sequence of the Chickies Formation, Antietam and Harpers Formations, and Conestoga Formation through Vintage Formation was deposited at this time over Laurentian continental basement gneiss. The clay-rich sediments that became the Octoraro Formation were deposited on the continental slope and rise. In the early Ordovician, the Wilmington Complex metaigneous rocks were forming as part of a magmatic arc built over an east-dipping subduction zone in the Iapetus Ocean. The boninitic amphibolites in the Rockford Park Gneiss indicate subduction of relatively young and hot lithosphere, possibly even a mid-ocean ridge. Intimate interlayering of Mt. Cuba Wissahickon with Wilmington Complex rocks in Pennsylvania and Delaware suggest that the eastern part of the Mt. Cuba may have been deposited with Wilmington Complex volcanic rocks in a forearc basin, or it may have been tectonically incorporated into the forearc accretionary prism during subduction. The sliver of Mt. Cuba Wissahickon east of the Wilmington Complex exhibits a metamorphic episode contemporaneous with arc magmatism. Evidence of shelf drowning and deformation in the sedimentary rocks of central and eastern Pennsylvania indicates that the Taconic Orogeny reached this area in the late Ordovician. Finding evidence of the same orogeny in the deep-seated rocks of the Pennsylvania Piedmont is more problematic. As yet, no evidence of metamorphic mineral growth during the late Ordovician has been found. However, growth of metamorphic monazite during the early Silurian is common to the Wilmington Complex, Wissahickon Formation (restricted sense, as defined herein), and Mt. Cuba Wissahickon formation at least as far west as the Delaware state line (the western Mt. Cuba and the Glenarm Wissahickon have not been investigated). A metamorphic event of this magnitude and intensity requires tectonic burial, perhaps as the magmatic arc overrode its forearc accretionary complex. Northeast-trending fabrics and major thrust faults in the area date from this period of subduction and collision. The position of the Wissahickon Formation (restricted sense; eastern part of the historic Wissahickon Formation) throughout the preceding period is enigmatic. It must be early Silurian or older in age because it is intruded by the Silurian-aged Springfield Granodiorite. Unlike the adjacent Mt. Cuba Wissahickon, it carries no evidence of stratigraphic interrelation to the Wilmington Complex, or of a metamorphic episode that might be related to Wilmington Complex magmatism. The geochemistry of interlayered amphibolites suggest that it may have been deposited in a back-arc basin with continental crustal input, perhaps in some other part of Iapetus. The Silurian was a period of high heat flow in the Pennsylvania Piedmont, marked by high-temperature metamorphism centered on the Brandywine Blue Gneiss and emplacement of both mafic and felsic plutons. The Arden Plutonic Supersuite, the Springfield Granodiorite, and the Ridley Park Granite were emplaced at this time. Because the trace element geochemistries of Silurian mafic igneous rocks are close to back-arc basin basalts, Plank and others (2001) suggest an extensional environment with high mantle heat flow. Such an environment could have resulted from slab break-off following oblique collision of the Wilmington Complex arc, or from back-arc extension over a newly-established west-dipping subduction zone. Early Devonian metamorphism recorded by monazite growth in the Wissahickon Formation (restricted sense) marks renewed plate convergence. Late fabrics and motion along high-angle ductile shear zones (Rosemont Fault, Cream Valley Fault, zone marked by Peters Creek tectonite) are probably related to the Alleghanian Orogeny. Minor high-angle brittle faults, some of which exhibit undeformed slickensides, probably date from Mesozoic extension.
For use as primary or supplementary data in mapping and/or decision making.
The datasets are not intended to be used for detailed or site-specific analyses, nor are they intended to be used at any scale finer (larger) than 1:50,000 (e.g., use at 1:24,000 scale is inappropriate). This report has not been reviewed for conformity with the publication standards of the Bureau of Topographic and Geologic Survey.
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The data for the geologic units, structures, and other features contained in these maps and shapefiles may be used provided that credit for the original (unaltered) datasets is given to the author, Gale C. Blackmer, and the publisher, Pennsylvania Bureau of Topographic and Geologic Survey, Department of Conservation and Natural Resources. Users shall clearly indicate the source of the data. Users who modify the data in any way are obligated to describe the types of modifications they have made, including modifications to hard-copy maps. Users are not to misrepresent Pennsylvania Bureau of Topographic and Geologic Survey (Pennsylvania Geological Survey, or PaGS) datasets, nor to imply that changes they (users) have made were approved by the PaGS. These datasets are public information and, as such, the data may be used as a reference source and may be interpreted by organizations, agencies, units of government, or others based on needs; however, each user is responsible for the appropriate application of the data. Federal, state, or local regulatory bodies are not to reassign to the Bureau of Topographic and Geologic Survey any authority for the decisions they make using these datasets. The datasets are not meant for site-specific analysis or for use at scales finer (i.e., larger, more detailed) than 1:50,000. Users are not to misrepresent the datasets by presenting them at scales for which they are not intended, nor to imply that presentation at such scales is approved by the PaGS. Not for commercial resale. Please refer also to the section on Distribution_Liability for important information concerning the appropriate use of the datasets.
3240 Schoolhouse Road
Bedrock geology and digital data - Gale C. Blackmer, Bureau of Topographic and Geologic Survey, Middletown, PA 17057.
Map is a compilation of eleven 7.5-minute quadrangle maps. Mapping is as complete as the time frame allowed. Further research and revision is always possible and encouraged. Soils and surficial materials were not mapped and are not included in this report.
Geologic contacts were seldom directly observed in the field. In general, the horizontal accuracy of geologic contacts (which include faults) is no better than the distance between the observations. Contacts were placed using the author's best judgment based on outcrop lithologic observations, observed structural patterns, driller's logs for water wells, and in some cases, topography. Data stations were located in the field on 1:24,000-scale quadrangle maps and using GPS. The final composite map was compiled at 1:50,000 scale.
The vertical positional accuracy of geologic contacts and faults is unknown but is no better than the standard for 1:24,000-scale quadrangle maps with 10-foot topographic contours, which is ±5 feet or half the contour interval for 90 percent or more of vertical points tested.
Map is a compilation of previous geologic mapping (see data sources). The final map was prepared using ESRI ArcView 9.1 software.
Feature geometry.
Internal feature number.
Area of feature in internal units squared.
These data files and documentation are provided "as is," and the user assumes the entire risk as to their quality and performance. The Commonwealth of Pennsylvania makes no guarantee or warranty concerning the accuracy of information contained in the geographic data or accompanying documentation. The Commonwealth of Pennsylvania further makes no warranties, either expressed or implied, as to any other matter whatsoever, including, without limitation, the completeness or condition of the product, or its fitness for any particular purpose. The burden for determining fitness for use lies entirely with the user. Although these data have been processed successfully on a computer system at the Department of Conservation and Natural Resources, the Commonwealth of Pennsylvania makes no warranty, expressed or implied, regarding the use of the data files or accompanying documentation on any other computer system, nor does the act of distribution constitute any such warranty. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Commonwealth of Pennsylvania government or any agency thereof. The user shall save the Commonwealth of Pennsylvania harmless and indemnify it from any suits, claims, or actions arising out of the use of or any defect in the data files or accompanying documentation. The Commonwealth of Pennsylvania assumes no legal liability for the accuracy, completeness, or usefulness of the data files and accompanying documentation. In no event shall the Commonwealth of Pennsylvania have any liability whatsoever for payment of any consequential, incidental, indirect, special, or tort damages of any kind, including, but not limited to, any loss of profits arising out of use of or reliance on the geographic data. Not for commercial resale.
3240 Schoolhouse Road