FIELD MAPPING THE TERRESTRIAL DEVONIAN: A PENNSYLVANIA PERSPECTIVE

Pennsylvania Geological Survey’s geologic field mapping focuses on delineating contacts, collecting structural data, and making maps. Time does not permit for detailed stratigraphic analysis. Field observations not significant to making the map often do not get revisited. At times, it feels like hit-and-run geology, but is necessary to fulfill our mission.

The author has mapped in five counties in south-central, central, and northeast Pennsylvania, which include Upper Devonian Catskill and Huntley Mountain Formations. Typical deposits consist of fine- to medium-grained sandstone, siltstone, claystone, and intraformational conglomerate. Colors range from gray to olive gray and grayish red to reddish brown. Rocks of the south-central region are characterized by mappable members. The lowest member contains marine fossils, an unusual white orthoquartzite, and a conglomerate. The central part of the state has more red beds, no marine fossils, lacks mappable members, and contains three conglomerates. The upper conglomerate marks the end of the Devonian. The outcrops in the northeast are dominated by fine-grained, gray sandstones. Rare well-developed paleosols were noted. Plant fossils, rare in the other areas, are common in the northeast. Marine fossils were not observed. A few curious finds, include a fossilized shrimp, fish plates, and thick paleosols, are ripe for further research. Collaboration with experts in the subdisciplines could also lead to improved sequence stratigraphy and paleoenvironment reconstruction.

To complement field mapping, the Survey has collected several 1,000’-1,700’ drill cores through terrestrial Devonian formations. Drilling was intended to define contacts, but has potential for much more. Cores contrast to the outcrops: exposing more fine-grained and calcareous rocks; providing unweathered material for analysis; and indicating the formations are confoundingly thicker than field work suggests. The cores offer opportunity for detailed sequence stratigraphy, age-dating, isotope analysis, et cetera, and are available for study and analysis in the Survey’s core library.

Mutual benefits can occur with strengthened ties between field geologists and professionals and students in the subdisciplines.