Ardentown Granitic Suite
The Piedmont rock units in Delaware, and bedrock geologic map of Schenck et al. (2000) are revised in this report based on new rock geochemistry, geochronometric data, petrography, and recent detailed mapping. Major revisions include:
In Claymont, DE, the intersection of Marvel Avenue and Parkside Boulevard occurs at Lesher Park, which contains Perkins Run, a creek West of Harvey Road. In the streambed of this creek is an outcrop of Perkins Run Gabbro, which is part of the Arden Plutonic Supersuite. The gabbro displays joints that are oriented 10 degrees west of north. Along these joints, veins of charnockite (orthopyroxene-bearing granite of the Ardentown Granitic Suite) can be found.
Located in Ardentown are a few silicic boulders just on the northwestern side of the railroad bridge that crosses the South Branch of Naaman Creek. These boulders are part of the Ardentown Granitic Suite. Some have very large (several cm) feldspar phenocrysts. Some display contacts between granitic rock and quartz-rich rock, which is probably metasedimentary rock due to the granular nature of quartz.
On the south side of Chestnut Hill is an outcrop of very large boulders in the woods of Hanby Park near Arden, DE. This area of the park seems to be the site of an old quarry. The rocks here are very similar to the rocks found down the South Branch of Naaman Creek (Ardentown and Ardencroft) as they both share the same fine-grained, mafic properties with traces of coarse-grained charnockite.
Running through Knollwood Park in Claymont, DE is the South Branch of Naaman Creek. This stream is laden with fairly mafic, medium to coarse-grained charnockite. Some of the charnockite samples here may be mylonitic. A few boulders contain xenoliths as well. Other gabbro boulders display charnockite veins in a gradational zone over about 1-2 meters.
In the valley of the South Branch of Naaman Creek, through Ardentown, is a group of charnockite boulders and fine-grained mafic rock (probably amphibolitized gabbroid). The mafic rock is mostly non-megacrystic with some coarse-grained and equigranular charnockite. On the ground around the boulders are small pieces that contain a clear example of a contact between coarse-grained and fine-grained rock types.
This report accompanies a new map that revises the original bedrock geologic maps of the Delaware Piedmont compiled by Woodruff and Thompson and published by the Delaware Geological Survey (DGS) in 1972 and 1975. Combined detailed mapping, petrography, geochemistry, and U-Pb geochronology have allowed us to redefine two rock units and formally recognize eleven new units. A section of the Pennsylvania Piedmont is included on the new map to show the entire extent of the Mill Creek Nappe and the Arden Plutonic Supersuite.
Medium- to coarse-grained granitic rocks containing primary orthopyroxene and clinopyroxene; includes quartz norites, quartz monzonorites, opdalites, and charnockites. Feldspar phenocrysts common. Mafic enclaves locally abundant in proximity to gabbronorites.
This map shows the surficial geology of New Castle County, Delaware at a scale of 1:100,000. Maps at this scale are useful for viewing the general geologic framework on a county-wide basis, determining the geology of watersheds, and recognizing the relationship of geology to regional or county-wide environmental or land-use issues. This map, when combined with the subsurface geologic information, provides a basis for locating water supplies, mapping ground-water recharge areas, and protecting ground and surface water.