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:
Petrography is a branch of geoscience focused on the description and classification of rocks, primarily by microscopic study of optical properties of minerals. A thin sliver of rock is cut from a sample, mounted on a glass slide, ground to approximately 30 microns (0.03mm), and viewed under a microscope that uses polarized light. By observing the colors produced as plain polarized light and crossed (90 degrees) polarized light shines through the minerals, petrologists can determine the minerals that comprise the sampled rock.
In Delaware, predominantly a pure, coarsely crystalline, blue-white dolomite marble interlayered with calc-schist. Major minerals in the marble include calcite and dolomite with phlogopite, diopside, olivine, and graphite. Major minerals in the calc-schist are calcite with phlogopite, microcline, diopside, tremolite, quartz, plagioclase, scapolite, and clinozoisite. Pegmatites and pure kaolin deposits and quartz occur locally.
Coarse-grained gabbroic and metagabbroic rocks, variably metamorphosed and deformed. Primary igneous minerals include olivine, clinopyroxene, orthopyroxene, and plagioclase.
Fine- to coarse-grained gabbronorite and minor diorite with subophitic to ophitic textures, variably foliated or lineated. Plagioclase, orthopyroxene, clinopyroxene, and hornblende are major minerals; biotite and olivine locally present. Olivine typically surrounded by corona structures as described for the Bringhurst Gabbro. Contemporaneous with the Ardentown Granitic Suite.
Coarse- to very coarse-grained gabbronoite with subophitic textures. Primary minerals are plagioclase, olivine, clinopyroxene and orthopyroxene. Olivine, where present, is surrounded by an inner corona of orthopyroxene and an outer corona of pargasitic hornblende, both with spinel symplectites. The gabbronorites locally contain abundant xenoliths of mafic Brandywine Blue Gneiss.
Black to very dark green, coarse- to very coarse-grained, uralitized olivine-hypersthene gabbronorite and pyroxenite with subophitic textures. Primary minerals are calcic plagioclase, orthopyroxene, clinopyroxene, and olivine. Amphibole is secondary, a pale blue-green actinolite. Olivine, when present, is surrounded by coronas similar to those in the Bringhurst Gabbro. The gabbronorite is deeply weathered leaving a layer of iron oxides, limonite, goethite, and hematite, mixed with ferruginous jasper. The jasper contains thin seams lined with drusy quartz. Contacts with the Christianstead Gneiss are covered with sediments of the Coastal Plain.
The Delaware Academy of Science has been instrumental in informing Delaware citizens about science and utilization of local resources. Since 1970 the annual meeting of the Delaware Academy of Science has been used as a time for presentation of ongoing research in various areas of science in the Delaware region. The proceedings of these meetings have resulted in publication of transactions of the Delaware Academy of Science. The 1976 annual meeting focused on aspects of the geology of Delaware.